Wonder of the World

Welcome back from our holidays on which, we trust, you had as much fun reading the four refresher pieces as I had writing them. Utter nonsense, of course. I’ve never found writing to be an orgasmic activity but, as they say about cod liver oil, it is good for you. However, whilst we were all improving ourselves on our deck-chairs and sun-loungers, the Tide of Science was waiting for no man: the waves of cancer biology have obliterated our sand castles and are fast approaching our toes. So let’s get on – albeit doing our best to make the segue from vacation to vocation as seamless as possible …..

So, on the subject of holidays, newspapers and magazines rather like the theme of ‘places to visit before you die’ – which is OK in that the world is wonderful and we should appreciate it. But there’s a problem in that one of the modern wonders is being able to see magnificent photos and movies of every far-flung nook, cranny and creature without leaving our sofa. So when we finally do get off our rear ends and chug past the Statue of Liberty on the Staten Island Ferry, zoom into Sydney or rock up to the Taj Mahal, the reaction is likely to be ‘That’s nice: looks just like on tv. Where next?’

Fortunately, being blasé has its limits. The only time I’ve made it to the Grand Canyon the mid-winter sun highlighted the colours of the rock striations so they were breathtaking in a way no photograph could quite capture. In the same vein, everyone should take the Trans-Siberian Railway we’re often told. And so you should but not because you will see houses and churches, rivers and trees that you can’t find on the Internet but because only borne by the train do you begin to sense the immensity of Mother Russia. The fact that the scenery is almost entirely birch trees minimizes distraction: all you can do is contemplate vastness – and the harshness that brings – an unvarying obbligato to Russian life.

A Provodnitsa looking after one of her passengers on The Trans-Siberian Railway

A Provodnitsa looking after one of her passengers on The Trans-Siberian Railway

The thrice-weekly freight at Grand Canyon Station, circa 1970

The thrice-weekly freight at Grand Canyon Station, circa 1970

 

 

 

 

 

 

Not Forgetting

All of which brings us to something else that is also truly a wonder of the world – cancer. If it seems a trifle weird to describe thus what’s usually classed as one of man’s greatest blights, consider this. The drive to control cancer has generated research on a scale unmatched in any other field of science. One upshot, not necessarily at the top of the list, is that we now have a breathtakingly detailed picture of the astonishing adaptability of life  – that is of our genetic material, DNA, and how its calisthenics can promote the most incredible behaviour on the part of individual cells. It’s true, you might point out, that we can see this by simply looking at the living world around us. The power of DNA to carry, in effect, limitless information produces the infinite cellular variety underpinning the staggering range of life that has evolved on earth. {Did you spot just the other day that a school field trip discovered 13 new species of spider in Queensland – yes, thirteen – inevitably headlined by The Sun as Creepy Hauly}

In the new world

But in focusing on cancers – what happens at the molecular level as they develop and how they evade our attempts to control them – the fine detail of this nigh-on incomprehensible power has been revealed as in no other way.

You’ll know what’s coming: the biggest single boost to this unveiling has been the arrival in the twenty-first century of methods for sequencing DNA and identifying which genes are expressed in cells at any given time. I know: in umpteen blogs I’ve gone on about its awe-inspiring power – but it is stunning and we’re at that stage when new developments leave one gasping almost on a monthly basis. The point here is that it’s not that the science keeps getting turned on its head. Far from it: the message remains that cells pick up changes to their DNA and, with time, these cumulative effects may drive them to make more of themselves than they should.

That’s cancer. But what is fantastic is the molecular detail that the ’omics revolution continues to lay bare. And that’s important because, as we have come to recognize that every cancer is unique, ideally we need to provide specifically tailored treatments, and we can only think of doing that when we know all the facts – even if taking them in demands a good deal of lying down in darkened rooms!

You could think of the fine molecular detail of cancers as corresponding to musical ornaments – flourishes that don’t change the overall tune but without which the piece would be unrecognizable. These include trills and turns – and all musicians will know their appoggiaturas from their acciaccaturas. They’re tiny embellishments – but just try removing them from almost any piece of music.

Lapping at your toes

So let’s look at three recent papers that have used these fabulous methods to unveil as never before the life history of cancers. The first is another masterful offering from The Sanger Institute on breast cancer: an in-depth analysis of 12 patients in which each tumor was sampled from 8 different locations. In the main the mutation patterns differed between regions of the same tumour. They extended this by looking at samples from four patients with multi-focal disease (‘foci’ being small clumps of tumour cells). As expected, individual foci turned out to be clearly genetically related to their neighbours but they also had many ‘private mutations’ – a term usually meaning a mutation found only in a single family or a small population. Here the ‘family’ are individual foci that must have arisen from a common ancestor, and you could think of them as a cellular diaspora – a localised spreading – which makes them a kind of metastasis. Quite often the mutations acquired in these focal sub-clones included major ‘driver’ genes (e.g., P53, PIK3CA and BRCA2). In general such potent mutations tend to be early events but in these foci they’ve appeared relatively late in tumour development. This doesn’t upend our basic picture: it’s just another example of ‘anything goes’ in cancer – but it does make the point that identifying therapeutic targets requires high-depth sequencing to track how individual cancers have evolved through continual acquisition of new mutations and the expansion of individual clones.

The authors used ‘coxcomb’ plots to portray these goings-on but they are quite tricky to make head or tail of. So, to avoid detail overload, I’ve converted some into genetic wallpaper, the non-repeating patterns illustrating the breathtaking variety that has evolved.

Wallpaper jpegDecorative DNA. The discs are ‘coxcomb’ plots – a variant of a pie chart. Here the colours and the wedge sizes represent mutations in different regions of four primary breast tumours. Every disc is different so that the message from this genetic wallpaper is of mutational variation not only between cancers but across the different samples taken from a single tumour. I trust that Lucy Yates, Peter Campbell and their colleagues will not be too upset at my turning their work into art (and greatly abbreviating the story): you can read the original in all its wondrous glory in Nature Medicine 21, 751–759.

The first person to come up with this very graphic way of conveying information was Florence Nightingale who, whilst working in Turkey during the Crimean War, realized that soldiers were dying in the hospitals not only from their wounds but, in much greater numbers, from preventable causes including infections, malnutrition and poor sanitation. Her meticulous recording and original presentation of hospital death tolls made her a pioneer in applied statistics and established the importance of sanitation in hospitals.

Something for the gentlemen

Two equally powerful onslaughts from Gunes Gundem, Peter Campbell and their colleagues at The Sanger Institute (again!) and Dan Robinson and pals from the University of Michigan Medical School have revealed the corresponding molecular detail of prostate cancer. Here too the picture is of each region of a tumour being unique in DNA terms. Moreover, they showed that metastasis-to-metastasis spread was common, either through the seeding of single clones or by the transfer of multiple tumour clones between metastatic sites.

Even that miserable old sod Lenin might have brightened at such fabulous science, before reverting to Eeyore mode with the inevitable “What’s to be done?” But it’s a good question. For example, as a general strategy should we try to kill the bulk of the tumour cells or aim for clones that, although small, carry very potent mutations.

Aside from the basic science, there is one quite bright ray of sunshine: about 90% of the mutations linked with the spread of prostate cancer are potentially treatable with existing drugs. And that really is encouraging, given that the disease kills 11,000 in the UK and over 30,000 in the USA every year.

prostate dogWe might also be heartened by the skills of German Shepherd dogs that can, apparently, be persuaded to apply one of their favourite pastimes – sniffing – to the detection of prostate cancer. Point them at a urine sample and 90% of the time they come up with the right answer. Given the well-known unreliability of the prostate-specific antigen blood test for prostate cancer, it’s nice to think that man’s best friend is on the job.

References

Yates, L.R., et al. (2015). Subclonal diversification of primary breast cancer revealed by multiregion sequencing. Nature Medicine 21, 751–759.

Robinson, D., et al. (2015). Integrative Clinical Genomics of Advanced Prostate Cancer. Cell 161, 1215–1228.

Gundem, G., et al. (2015). The evolutionary history of lethal metastatic prostate cancer. ICGC Prostate UK Group (2015). Nature 520, 353–357.

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Holiday Reading (4) – Can We Make Resistance Futile?

For those with a fondness for happy endings we should note that, despite the shortcomings of available drugs, the prospects for patients with a range of cancers have increased significantly over the latter part of the twentieth century. The overall 5-year survival rate for white Americans diagnosed between 1996 and 2004 with breast cancer was 91%; for prostate cancer and non-Hodgkin’s lymphoma the figures were 99% and 66%, respectively. These figures are part of a long-term trend of increasingly effective cancer treatment and there is no doubt that the advances in chemotherapy summarised in the earlier Holiday Readings are contributory factor. Nonetheless, the precise contribution of drug treatments remains controversial and impossible to disentangle quantitatively from other significant factors, notably earlier detection and improved surgical and radiotherapeutic methods.

Peering into the future there is no question that the gradual introduction of new anti-cancer drugs will continue and that those coming into use will be more specific and therefore less unpleasant to use. By developing combinations of drugs that can simultaneously poke the blancmange at several points it may be possible to confront tumor cells with a multiple challenge that even their nimbleness can’t evade, thereby eliminating the problem of drug resistance. Perhaps, therefore, in 20 years time we will have a drug cabinet sufficiently well stocked with cocktails that the major cancers can be tackled at key stages in their evolution, as defined by their genetic signature.

However, on the cautionary side we should note that in the limited number of studies thus far the effect of drug combinations on remission times has not been startling, being measured in months rather than years or decades. Having noted the durability of cancer cells we should not be surprised by this and the concern, of course, is that, however ingenious our efforts to develop drug cocktails, we may always come second to the adaptability of nature.

Equally perturbing is the fact that over 90% of cancer deaths arise from primary tumors spreading to other sites around the body. For this phenomenon, called metastasis, there are currently very few treatment options available and the magnitude of this problem is reflected in the fact that for metastatic breast cancer there has been little change in the survival rates over the past forty years.

Metastasis therefore remains one of the key cancer challenges. It’s over 125 years since the London physician Stepen Paget asked the critical question: ‘What is it that allows tumour cells to spread around the body?’ and it’s a daunting fact that only very recently have we made much progress towards an answer – and thus perhaps a way of controlling it. To the fore in this pursuit has been David Lyden and his colleagues at Weill Cornell Medical College in New York. Their most astonishing finding is that cells in the primary tumour release messengers into the circulation and these, in effect, tag what will become landing points for wandering cells. Astonishing because it means that these sites are determined before any tumour cells actually set foot outside the confines of the primary tumour. Lyden has christened this ‘bookmarking’ cancer. That is a quite remarkable finding – but, as ever in science, it merely shifts the question to ‘OK but what’s the messenger?’

A ray of sunshine

It might appear somewhat churlish, especially after all that funding, to end on a note of defeatist gloom so let’s finish with my ray of sunshine that represents a radical approach to the problem. It relies on the fact that small numbers of cells break away from tumors and pass into the circulation. In addition, tumours can release both DNA and small sacs – like little cells – that contain DNA, proteins and RNAs (nucleic acids closely related to DNA). These small, secreted vesicles are called exosomes – a special form of messengers, communicating with other cells by fusing to them. By transferring molecules between cells, exosomes may play a role in mediating the immune response and they are now recognized as key regulators of tumour growth and metastasis.

Step forward Lyden and friends once more who have just shown in a mouse model of pancreatic cancer that exosomes found their way to the liver during the tumour’s earliest stages. Exosomes are taken up by some of the liver cells and this sets off a chain of cell-to-cell signals that eventually cause the accumulation of a kind of molecular glue (fibronectin). This is the critical ingredient in a microenvironment that attracts tumour cells and promotes their growth as a metastasis (secondary growth). So you can think of exosomes as a kind of environmental educator.

Exosome Fig

Exosomes released from primary tumours can mark a niche for metastasis.

The small sacs of goodies called exosomes are carried to the liver where they fuse with some cells, setting off a chain reaction that produces a sticky protein – fibronectin – a kind of glue for immune cells and tumour cells. (from Costa-Silva, B., Lyden, D. et al., Nature Cell Biology 17, 816–826, 2015).

The recent, remarkable technical advances that permit the isolation of exosomes also make it possible to fish out circulating tumour cells and tumour DNA from a mere teaspoonful of blood.

Circulating tumour cells have already been used to monitor patient responses to chemotherapy – when a treatment works the numbers drop: a gradual rise is the earliest indicator of the treatment failing. Even more exciting, this approach offers the possibility of detecting the presence of cancers years, perhaps decades, earlier than can presently be achieved. Coupling this to the capacity to sequence the DNA of the isolated cells to yield a genetic signature of the individual tumor can provide the basis for drug treatment. There are still considerable reservations attached to this approach but if it does drastically shift the stage at which we can detect tumors it may also transpire that their more naïve forms, in which fewer mutations have accumulated, are more susceptible to inhibitory drugs. If that were to be the case then even our currently rather bare, though slowly expanding, drug cabinet may turn out to be quite powerful.

The Hay Festival

According to the Hay Festival  a recording of my talk ‘Demystifying Cancer’ on Wednesday 28th May should be available on their web site shortly and it can also be heard on the university site. However, I thought it might be helpful to post a version, not least for the for the rather breathless lady who arrived at the book signing session apologising for missing the lecture because she’d got stuck in mud. So for her and perhaps for many others I had the privilege of chatting to afterwards, read on …

 The Amazing World of Cells, Molecules … and CancerOpening pic

One of the biggest influences on my early years was the composer and conductor Antony Hopkins, who died a few days ago. Most of what I knew about music by the time I was 15 came from his wonderfully clear dissections of compositions in the series Talking About Music broadcast by the BBC Third Programme. When he was axed by the Beeb in 1992 for being ‘too elitist’ – yes, they talked that sort of drivel even then – Hopkins might have wished he’d been a biologist. After all, biology must be the easiest subject in the world to talk about. Your audience is hooked from the outset because they know it’s about them – if not directly then because all living things on the planet are interlinked – so even the BBC would struggle to make an ‘elitism’ charge stick. They know too that it’s beautiful, astonishing and often funny – both from what they see around them and also, of course, courtesy of David Attenborough. So it’s not a surprise when you show them that the micro-world of cells and molecules is every bit as wonderful.

The secret of life

What does come as a bit of a shock to most non-scientists is when you explain the secret of life. No, that’s not handing round pots of an immortalization elixir – much better, it’s outlining what’s sometimes rather ponderously called the central dogma of molecular biology – the fact that our genetic material (aka DNA) is made from only four basic units (most easily remembered by their initials: A, C, G and T – humans have over three thousand million of these stuck together). This is our ‘genome’ and the ‘genetic code’ enshrined in the DNA sequence makes us what we are – with small variations giving rise to the differences between individuals. The genetic code carries instructions for glueing together another set of small chemicals to make proteins. There are 20 of these (amino acids) and they can be assembled in any order to make proteins that can be thousands or even tens of thousands of amino acids long. These assemblies fold up into 3D shapes that give them specific activities. Proteins make living things what they are – they’re ‘the machines of life’ – and their infinite variety is responsible for all the different species to have appeared on earth. Can the basis of life really be so simple?

The paradox of cancer

Turning to cancer, a three word definition of ‘cells behaving badly’ would do fine. A more scientific version would be ‘cells proliferating abnormally.’ That is, cells reproducing either when they shouldn’t, or more rapidly than normal, or doing so in the wrong place. The cause of this unfriendly behavior is damaged DNA, that is, alteration in the genetic code – any such change being a ‘mutation’. If a mutation affects a protein so that it becomes, say, hyperactive at making cells proliferate (i.e. dividing to make more cells), you have a potential cancer ‘driver’. So at heart cancer’s very simple: it’s driven by mutations in DNA that affect proteins controlling proliferation. That’s true even of the 20% or so of cancers caused by chronic infection – because that provokes inflammation, which in turn leads to DNA damage.

The complexity of cancer arises because, in contrast to several thousand other genetic diseases in which just a single gene is abnormal (e.g., cystic fibrosis), tumour cells accumulate lots of mutations. Within this genetic mayhem, relatively small groups of potent mutations (half a dozen or so) emerge that do the ‘driving’. Though only a few ‘driver mutations’ are required, an almost limitless number of combinations can arise.

Accumulating mutations takes time, which is why cancers are predominantly diseases of old age. Even so, we should be aware that life is a game of genetic roulette in which each individual has to deal with the dice thrown by their parents. The genetic cards we’re dealt at birth may combine with mutations that we pick up all the time (due to radiation from the sun and the ground, from some foods and as a result of chemical reactions going on inside us) to cause cancers and, albeit rarely, in unlucky individuals these can arise at an early age. However, aside from what Mother Nature endows, humans are prone to giving things a helping hand through self-destructive life-style choices – the major culprits, of course, being tobacco, alcohol and poor diets, the latter being linked to becoming overweight and obese. Despite these appalling habits we’re living longer (twice as long as at the beginning of the twentieth century) which means that cancer incidence will inevitably rise as we have more time to pick up the necessary mutations. Nevertheless, if we could ban cigarettes, drastically reduce alcohol consumption and eat sensibly we could reduce the incidence of cancers by well over a half.

How are we doing?

Some readers may recall that forty-odd years ago in 1971 President Nixon famously committed the intellectual and technological might of the USA to a ‘War on Cancer’ saying, in effect, let’s give the boffins pots of money to sort it out pronto. Amazing discoveries and improved treatments have emerged in the wake of that dramatic challenge (not all from Uncle Sam, by the way!) but, had we used the first grant money to make a time machine from which we were able to report back that in 2013 nearly six hundred thousand Americans died from cancer, that the global death toll was over eight million people a year and will rise to more than 13 million by 2030 (according to the Union for International Cancer Control), rather less cash might subsequently have been doled out. Don’t get me wrong: Tricky Dicky was spot on to do what he did and scientists are wonderful – clever, dedicated, incredibly hard-working, totally uninterested in personal gain and almost always handsome and charming. But the point here is that, well, sometimes scientific questions are a little bit more difficult than they look.

Notwithstanding, there have been fantastic advances. The five year survival rates for breast and prostate cancers have gone from below 50% to around 90% – improvements to which many factors have contributed including greater public awareness (increasing the take-up of screening services), improved surgical and radiology methods and, of course, new drugs. But for all the inspiration, perspiration and fiscal lubrication, cancer still kills over one third of all people in what we like to refer to as the “developed” world, globally breast cancer killed over half a million in 2012 and for many types of cancer almost no impact has been made on the survival figures. In the light of that rather gloomy summary we might ask whether there is any light at the end of the tunnel.

The Greatest Revolution

From one perspective it’s surprising we’ve made much progress at all because until just a few years ago we had little idea about the molecular events that drive cancers and most of the advances in drug treatment have come about empirically, as the scientists say – in plain language by trial and error. But in 2003 there occurred one of the great moments in science – arguably the most influential event in the entire history of medical science – the unveiling of the first complete DNA sequence of a human genome. This was the product of a miraculous feat of international collaboration called The Human Genome Project that determined the order of the four units (A, C, G and T) that make up human DNA (i.e. the sequence). Set up in 1990, the project was completed by 2003, two years ahead of schedule and under budget.

If the human genome project was one of the most sensational triumphs in the history of science what has happened in the ensuing 10 years is perhaps even more dazzling. Quite breathtaking technical advances now mean that DNA can be sequenced on a truly industrial scale and it is possible to obtain the complete sequence of a human genome in a day or so at a cost of about $1,000.

These developments represent the greatest revolution because they are already having an impact on every facet of biological science: food production, microbiology and pesticides, biofuels – and medicine. But no field has been more dramatically affected by this technological broadside than cancer and already thousands of genomes have been sequenced from a wide range of tumours. The most striking result has been to reveal the full detail of the astonishing genetic mayhem that characterizes cancer cells. Tens of thousands or even hundreds of thousands of mutations featuring every kind of molecular gymnastics imaginable occur in a typical tumour cell, creating a landscape of stunning complexity. At first sight this makes the therapeutic challenge seem daunting, but all may not be lost because the vast majority of this genetic damage plays no role in cancer development (they’re ‘passenger’ mutations) and the power of sequencing now means they can be sifted from the much smaller hand of ‘driver’ mutations. From this distillation have emerged sets of ‘mutational signatures’ for most of the major types of cancers. This is a seismic shift from the traditional method of assessing tumours – looking directly at the cells after treating them with markers to highlight particular features – and this genetic approach, providing for the first time a rigorous molecular basis for classifying tumours, is already affecting clinical practice through its prognostic potential and informing decisions about treatment.

A new era

One of the first applications of genomics to cancer, was undertaken by a group at The Wellcome Trust Sanger Institute near Cambridge (where the UK part of the Human Genome Project had been carried out), who screened samples of the skin cancer known as malignant melanoma. This is now the fifth most common UK cancer – in young people (aged 15 to 34) it’s the second most common – and it killed over 2,200 in 2012. Remarkably, about half the tumours were found to have a hyperactivating mutation in a gene called BRAF, the effect being to switch on a signal pathway so that it drives cell proliferation continuously. It was a remarkable finding because up until then virtually nothing was known about the molecular biology of this cancer. Even more amazingly, within a few years it had lead to the development of drugs that caused substantial regression of melanomas that had spread to secondary sites (metastasized).

This was an early example of what has become known as personalized medicine – the concept that molecular analysis will permit treatment regimens to be tailored to the stage of development of an individual’s cancer. And maybe, at some distant time, the era of personalized medicine will truly come about. At the moment, however, we have very few drugs that are specific for cancer cells – and even when drugs work initially, patients almost invariably relapse as tumours become resistant and the cancer returns – one of the major challenges for cancer biology.

It behoves us therefore to think laterally, of impersonal medicine if you like, and one alternative approach to trying to hit the almost limitless range of targets revealed by genomics is to ask: do tumour cells have a molecular jugular – a master regulator through which all the signals telling it to proliferate have to pass. There’s an obvious candidate – a protein called MYC that is essential for cells to proliferate. The problem with stopping MYC working is that humans make about one million new cells a second, just to maintain the status quo – so informed opinion says that blocking MYC will kill so many cells the animal will die – which would certainly fix cancer but not quite in the way we’re aiming for. Astoundingly, it turns out in mice at least it doesn’t work like that. Normal cells tolerate attenuation of MYC activity pretty well but the tumour cells die. What a result!! We should, of course, bear in mind that the highway of cancer therapy is littered with successful mouse treatments that simply didn’t work in us – but maybe this time we’ll get lucky.

An Achilles’ heel?

In defining cancers we noted the possibility that tumour cells might proliferate in the wrong place. So important is this capacity that most cancer patients die as a result of tumour cells spreading around the body and founding secondary colonies at new sites – a phenomenon called metastasis. Well over 100 years ago a clever London physician by the name of Stephen Paget drew a parallel between the growth of tumours and plants: ‘When a plant goes to seed, its seeds are carried in all directions; but they can only live and grow if they fall on congenial soil.’ From this emerged the “seed and soil” theory as at least a step to explaining metastasis. Thus have things languished until very recent findings have begun to lift the metastatic veil. Quite unexpectedly, in mouse models, primary tumours dispatch chemical messengers into the blood stream long before any of their cells set sail. These protein news-bearers essentially tag a landing site within the circulatory system on which the tumour cells touch down. Which sites are tagged depends on the type of tumour – consistent with the fact that human cancers show different preferences in metastatic targets.

These revelations have been matched by stunning new video methods that permit tumour cells to be tracked inside live mice. For the first time this has shone a light on the mystery of how tumour cells get into the circulation – the first step in metastasis. Astonishingly tumour cells attach themselves to a type of normal cell, macrophages, whose usual job is to engulf and digest cellular debris and bugs. The upshot of this embrace is that the macrophages cause the cells that line blood vessels to lose contact with each other, creating gaps in the vessel wall through which tumour cells squeeze to make their escape. This extraordinary hijacking has prognostic value and is being used to develop a test for the risk of metastasis in breast cancers.

The very fact that cancers manifest their most devastating effects by spreading to other sites may lay bare an Achilles’ heel. Other remarkable technical developments mean that it’s now possible to fish out cancer cells (or DNA they’ve released) from a teaspoonful of circulating blood (that’s a pretty neat trick in itself, given we’re talking about fewer than 100 tumour cells in a sea of several billion cells for every cubic millimeter of blood). Coupling this to genome sequencing has already permitted the response of patients to drug therapy to be monitored but an even more exciting prospect is that through these methods we may be moving towards cancer detection perhaps years earlier than is possible by current techniques.

As we’ve seen, practically every aspect of cancer biology is now dominated by genomics. Last picIt’s so trendy that anyone can join in. Songs have been written about DNA and you can even make a musical of your own genetic code, French physicist Joel Sternheimer having come up with a new genre – protein music – in which sequence information is converted to musical notes. Antony Hopkins, ever receptive to new ideas, would have been enthralled and, with characteristic enthusiasm, been only too happy to devote an episode of Talking About Music to making tunes from nature.

A Refresher from the BBC

Regular readers will probably feel they know all this stuff but if you’re interested in a spirited and wide-ranging conversation about cancer with the wonderful Jeremy Vine on his BBC Radio 2 show yesterday you can find it at:

http://www.bbc.co.uk/programmes/b03yn0jd about 1 hour 10 min from the beginning.

BBC Radio 4As ever, any arising thoughts, questions or comments appreciated – apart, of course, from the below the belt: “Judging by the photo it’s a good job it was radio not t.v.”

 

Spinning Out In Control

In Signs of Resistance and Seeing the Invisible we emphasized two things well known to the interested, namely that most cancer deaths occur because cells spread from the original (primary) to secondary sites (metastases) where they are very difficult to treat, and that this places massive importance on early detection. Many will also be familiar with the currently used methods for tumor detection – X-ray based imaging (as in mammography and CT scans) and PET that detects injected radioactive tracers. The problem is that these are not sensitive enough to detect growths smaller than about 1 cm in diameter – and by that point there are several hundred million cells in the tumor and some may already have metastasized.

Tumor cells spread around the body by detaching from the primary and getting into the circulatory system and it’s beginning to look as though quite literally tapping into the circulation may revolutionize cancer detection. Seeing the Invisible showed how silicon chip technology can be used to retrieve circulating tumor cells (CTCs) by getting them to stick to targets anchored in a flow cell. Although this is hugely promising, another very recent advance may be even more effective. This uses centrifugal force to separate cells in blood on the basis of their size – that’s the one that pushes outwards on objects rotating about an axis. Because force is proportional to mass and tumor cells are larger than red blood cells and most white cells, this effect can be used to extract CTCs from fluid being pumped around a spiral microchannel. The spirals are made from a silicon-based polymer (the same stuff that’s used for contact lenses) stuck on glass slides and they have two outlet channels. Their shape creates two-counter rotating vortices in the fluid that exert a drag force on the cells so that bigger (heavier) tumor cells can be selectively directed to one of the outlets. Typically red blood cells are about 6 microns (one-millionth of a metre), white cells 8-14 microns and CTCs 16-25 microns in diameter.

The vortices are named after a Cambridge chappie, William Dean, who worked on flow patterns in curved pipes and channels and you can look up Dean vortices on the internet for images of these in action.

MCF7s right, rest left

In this picture of the two exits from a spiral microchannel breast cancer cells are carried to the right (yellow arrows) whilst all the other types of blood cell funnel left.

This method appears to be remarkably efficient in that over 90% of tumor cells (10-100 cells per ml of blood) can be separated from 99.99% of red cells (5,000,000 per ml) and 99.6% of white cells (10,000 per ml).

References

Hou, H.W., Warkiani, M. E., Khoo, B.L., Li, Z.R., Soo, R.A., Tan, D.S.-W., Lim, W.-T., Bhagat, A.A.S., and Lim, C.T. (2013). Isolation and retrieval of circulating tumor cells using centrifugal forces. Scientific Reports 3, Article Number: 1259. DOI: 10.1038/srep01259.

Bhagat, A.A.S. et al., 15th International Conference on Miniaturized Systems for Chemistry and Life Sciences October 2-6, 2011, Seattle, Washington, USA

Seeing the Invisible: A Cancer Early Warning System?

Sherlock Holmes enthusiasts who also follow this column may, in a contemplative moment, have asked themselves whether their hero would have made a good cancer detective. Answer perhaps ‘yes’ in that he was obsessive about sticking to the facts and not guessing and would probably have said that, when tracking down a secretive quarry, you need to be as open-minded as possible in looking for clues. One of his most celebrated efforts at marrying observation with knowledge was his greeting upon first meeting Dr. Watson: “How are you? You have been in Afghanistan, I perceive”. Watson was suitably astonished by this apparent clairvoyance although its basis was in fact rather mundane and only beyond him because, as Sherlock kindly explained, “You see, but you do not observe.”

Holmes-Image-Loupe

Dr. Holmes perchance?

If Watson had paused to wonder whether Holmes’ combination of superiority complex and investigative genius would have fitted him for a career in the medical fraternity, he might have reflected that indeed many internal afflictions do manifest external signs – much as the furtive body language of a felon on a job might mark him out to the observant eye in the throng of bodies pressing into Baker Street underground station. So perhaps the ’tec turned doc could make it in infectious diseases or become a consultant in rheumatoid arthritis. But would he have steered clear of oncology, reasoning that most cancers are without symptoms during their early development and that even he could not observe the invisible?

Lithograph of Baker Street Station   Baker Street Station on the Metropolitan Railway in 1863 (London Transport Museum collection)

Probably, but before taking that decision he would have asked for a tutorial – perhaps from that bright fellow Stephen Paget, who would have explained that cancers are unusual lumps of cells that can often be cut out by surgeons such as himself. But he’d have highlighted the problem that similar growths commonly turn up later at other, secondary, sites in the body – they are what kills most cancer patients and no one has a clue how this happens or what to do about it. Holmes would doubtless have taken a deep suck on his pipe, commented that, as no one appeared to disagree with William Harvey’s 250 year old finding that blood is passed to every nook and cranny of the body by the circulatory system, it scarcely required his giant intellect to deduce that to be the most probable way of spreading tumours. Further observing that cancers develop very slowly, he would have pointed out that it is highly likely that within the body there might be clues – molecular signs that something is amiss – long before overt disease appears. All that was required was a biological magnifying glass and tweezers to spot and pick out rogue cells and molecules. Muttering ‘Elementary’ he would then have asked to be excused to return to the really tricky problem of outsmarting Professor Moriarty.

An Achilles’ heel?

Well, as we have just reviewed in Scattering the Bad Seed, some 130 years after that imaginary encounter the ‘elementary’ way in which tumours spread to form metastases is just beginning to be revealed and, of course, the hope is that eventually this knowledge will lead to ways of treating disseminated cancers or even preventing them. That’s a wonderful prospect but even more exciting are technical advances enabling us to exploit what Sherlock had spotted as something of a cancer Achilles’ heel – namely that, if tumour cells spread via the bloodstream, we need only the right tools (magnifying glass and tweezers) to detect secondary growths almost before they’ve started to form. As most people know, the earlier cancers are caught the more likely they are to be cured, the most critical intervention being before they have spread to form metastases that are the major cause of death.

The things you find in blood

In fact, quite apart from intact tumour cells migrating around the circulation, it’s been known for 40 years that most types of cell in our bodies have the rather odd quirk of releasing short bits of their DNA into the circulation. Cancer cells do this too and these chromosome fragments reflect the genetic mayhem that is their hallmark. How DNA gets out of the nucleus and then across the outer membrane of the cell isn’t known but it does – and the bits of nucleic acid act as messengers, being taken up by other cells that respond by changing their behaviour. In Beware of Greeks we saw that DNA fragments released by leukemia cells can help those cells escape from the bone marrow into circulating blood.

There’s yet another sort of cellular garbage swishing around in our circulation: small sacs like little cells that contain proteins and RNAs (nucleic acids closely related to DNA). These small, secreted vesicles are called exosomes and in fact they’re not at all rubbish but are also messengers, communicating with other cells by fusing and transferring their contents. So exosomes are another form of environmental educator.

Going fishing

The problem has been that until very recently it has not been possible to fish out tumour cells or DNA from the vast number of cells in blood (we’ve each got over 20 trillion red blood cells in our five litres or so). However, an exciting new development has been the application of silicon chip technology to the detection of circulating tumour cells (CTCs). The chips, which are the size of a microscope slide (10 x 2 cm), have about 80,000 microscopic columns etched on their surface that are coated with an array of antibodies that stick to molecules expressed on the surface of CTCs. By incorporating the chips into small flow cells it’s possible to capture about 100 CTCs from a teaspoon of blood – that’s pulling out one tumour cell from a background of a billion (109) normal cells.

CTC CHIP

Tumour cell isolation from whole blood by a CTC-chip. Whole blood is circulated through a flow cell containing the capture columns (Stott et al., 2010)

This microfluidics approach can also be used to isolate tumour cell DNA. For this the coatings are short stretches of artificial DNA of different sequences: these bind to free DNA in the same way that two strands of DNA stick together to make the double helix.

This remarkable technology may offer both the most promising way to early tumour detection and of determining responses to drugs. It also provides a bridge between proteomic and genomic technologies because DNA, captured directly or extracted from isolated cells, can be used for whole genome sequencing. If this system is able to capture cells from most major types of tumour it will indeed provide a rapid route from early detection through genomic analysis to tailored chemotherapy without the requirement for tumour biopsies. In Signs of Resistance we noted that it’s possible to track the response of secondary tumours (metastases) to drug treatment (chemotherapy) using this method of pulling out tumor DNA from blood and sequencing it.

The really optimistic view is that chip isolation of DNA or tumour cells may be a means to cancer detection years, perhaps decades, before any other test would show its presence. By following up with the power of sequencing, the hope is that appropriate drug cocktails can be devised to, so to speak, nip the tumour in the bud.

Wizard’s secret

By the way, Conan Doyle eventually revealed the method behind Sherlock’s wizardry: Watson was a medical man but walked with a military bearing: the skin on his wrists was fair but his face tanned and haggard and he held his left arm in a stiff and unnatural manner. So here was a British army doctor who had served in the tropics (or somewhere equally hot) and been wounded. In 1886 where would that have been? Oh yes, of course. Afghanistan.

Reference

Stott, S.L., Hsu, C.-H., Tsukrov, D.I., Yu, M., Miyamoto, D.T., Waltman, B.A., Rothenberg, M.S., Shah, A.M., Smas, M.E., Korir, G.K., Floyd, Jr., F.P., Gilman, A.J., Lord, J.B., Winokur, D., Springer, S., Irimia, D., Nagrath, S., Sequist, L.V., Lee, R.J., Isselbacher, K.J., Maheswaran, S., Haber, D.A. and Toner, M. (2010). Isolation of circulating tumour cells using a microvortex-generating herringbone-chip. Proceedings of the National Academy of Sciences of the United States of America 107, 18392-18397.

Scattering the Bad Seed

Cancers are very peculiar diseases. One of their fairly well-known oddities is that, by and large, it’s not the initial tumour that does the damage – rather that the vast majority of fatalities arise from its offshoots, secondary growths formed by cells escaping from the primary and spreading around the body, a diaspora called metastasis. That ‘vast majority’ is actually over 90% – so you might suppose most research effort would be focussed on how cells disseminate and what can be done to stop them in their tracks, whilst leaving the surgeons to deal with the primaries. But like many other things in life, logic plays a limited part in research strategy and to a great extent the boffins do what they fancy – or, to make it sound a bit more rigorous, what they feel is possible given the available tools. Which is perfectly reasonable: launching a project to build a radio would have been a bit perverse before Michael Faraday discovered electricity. In short, scientific research is all about practicalities – it’s what that great science communicator (and Nobel Prize winner) Peter Medawar called The Art of the Soluble.

Metastasis on the move

We recently recounted the emergence of the notion that cancers could spread around the body and how, by the end of the 19th century, this had led to the idea of ‘seed and soil’ – that cells cast off from primary tumours could drift around the circulation until they found somewhere congenial to drop anchor and set up a new home. That was in Keeping Cancer Catatonic and it was prompted by the fact that for rather more than 100 years metastasis seemed so difficult to get at, so impossible to model, there was virtually no progress and it is only now in the last few years that this critical cancer niche is once again on the move. The really exciting, and surprising, finding has been that, in mouse models, primary tumours dispatch chemical messengers into the blood stream long before any cells set sail. These protein news-bearers essentially tag a landing site within the circulatory system for the tumour cells to follow. And which sites are tagged depends on the type of tumour – consistent with the fact that human cancers show different preferences in metastatic targets.

A further twist is that even if tumour cells manage to follow this complicated guidance system and seed a new site, it’s not a disaster because their growth is suppressed by proteins released from nearby blood vessels. This presumably reflects the fact that tissues have systems to maintain the normal balance – to ensure that unusual things don’t happen – which means that everything is fine until that control is overwhelmed. When that happens other signals convert the dormant tumour into an expanding metastasis.

These very recent discoveries show that, at long last, our ignorance of how tumours spread is beginning to be chipped away and, because metastasis is the critical issue in cancer, this is a timely moment to do one of our crystal clear, simple summaries of what we know – which is relatively easy and will take much less time than if we reviewed our ignorance.

BOOKMARKING copy

Bookmarking cancer: Primary tumours mark sites around the body to which they will spread (metastasize) by sending out chemical signals that create sticky ‘landing sites’ (red protein A) on target cells. Cells released from the bone marrow carry proteins B and C. B attaches to A and tumour cells ‘land’ on C. Cells may remain quiescent in a new site for years or decades, their growth suppressed by signals (e.g., TSP-1) released from nearby blood vessels. Only when appropriate activating signals dominate (e.g., TGF beta) is secondary tumour growth switched on (see Keeping Cancer Catatonic for more details).

So what do we know?

Tumours arise from the accumulation of (essentially) random mutations and these drive the expansion of a family of cells to the point where they make their presence felt. From that, if the bearer is unlucky, emerges a sub-set of cells with the wanderlust. Cells in which the mutational hand they have acquired confer the ability to escape from the family bosom, chew through surrounding tissue, burrow into nearby blood vessels and thus voyage to distant places around the body. Some of these adventurous fellows may find landing sites where they can stick and, in effect, reverse their escape routine by squeezing through the vessel wall and chomping their way to a new niche in which to set up home. This process is sometimes called ‘colonization’ and it’s a pretty vivid description, evoking images of brave chaps taking on the elements to find a new world in which to prosper. The upshot is a malignant tumour.

I’m sorry for pulling a sciency trick back there by inserting ‘essentially’ – in brackets to persuade you to skim over it as if it was a mild hallucination. We’ll come back to the rivetting explanation of why I’d feel uncomfortable about just saying ‘random mutations’ another day but for the moment just stick with the idea that changes in DNA make cancers.

Tumour cells are not very bright

This sequence is so convoluted that it sounds like the product of some devilish mastermind but in fact we know that the metastatic cell is incapable of thought because otherwise it would have stayed at home. Metastasis is a process so inefficient that it’s almost always fatal for the cell that tries it. Tumour cells that get into the circulation may be damaged in the rush-hour scrum that is cellular life in the bloodstream and be gobbled up by scavenger cells. Even if they do finally squeeze through a space in the wall – feeling they’ve made it – they may have suffered so much stress they’re just not up to producing a family in a new environment that mayn’t be entirely welcoming. So even after reaching a new home they may not survive any longer or just manage to form a small cluster of cells that hang on as a ‘dormant’ tumour – an indolent little outpost that represents no threat to the carrier, even though it may persist for decades. So, despite metastasis being the most life-threatening facet of cancer, the odds are strongly weighted against escaping tumour cells: even after they’ve made it into the circulation, only about one in every ten thousand makes it to a compatible site where it forms an embryonic colony.

How does it kick off?

Given that tumours are products of evolution – albeit on the hugely accelerated time-scale of an individual lifetime rather than the geological frame within which new species emerge – you might suppose that metastases are merely a potent end-product. A tumour cell continues to pick up mutations until eventually it has the required toolkit to burrow and squeeze, float and drift, touch down  on sticky patches, squeeze and burrow again and eventually thrive in a new home. In the best traditions of cancer, however, it turns out not to be like that – at least, as far as is known, no set of mutations defines cells as having acquired the tools of the spreading trade. In short, there’s no ‘genetic signature’ that uniquely marks a metastatic cell. Nevertheless, they are different: only a fraction of primary tumour cells acquire the ability to spread – so if it isn’t simply by picking up an escape kit of changes in DNA, how do they do it?

Making an escape kit

One of the things that does mark metastatic cells is a change in the genes expressed compared to their relatives in the rest of the tumour. That is they alter the pattern of proteins that they make. This switch reorganises the cell’s shape and helps it to move and, most notably, includes enzymes released into the environment that cut a path for the cell to invade its local surroundings en route to the circulation.  As you might guess, this switch in protein production appears to be reversed once a cell has found a new niche. But if this transition into an invasive (i.e. malignant) cell isn’t driven by specific mutations, how does it come about?

The answer seems to lie in a subtle fine-tuning of cell behaviour, rather than dramatic changes caused by mutations in DNA. In other words, cells emerge from the morass of mutations within a tumour with critical signal systems that are just that little bit more active than those of their companions. It’s less a tall poppy syndrome than the odd blade of grass that’s missed the mower and can see a wider world. If this still seems a bit far-fetched, recall that every cell is unique: however identical two cells may be, there will be tiny differences in the signals that control their level of response.  The minuscule edge that can give one cell over another is enough. Given time, it will reproduce to make a clone with the gymnastic ability and stamina required to embark on the fraught experience of founding a metastatic colony.

Spreading variety

One of the fascinating things about cancer is that there seems to be no absolute rules. For every generalization there’s a renegade – a piece of molecular or cellular jiggery-pokery that does it in a different way, often in a breath-taking example of Nature’s flexibility. So it is with metastasis in that, as we noted, different cancers show widely variable behaviour.  Some major types have usually spread by the time they are detected (lung, pancreatic) whereas generally breast and prostate tumours have not. Some forms of brain tumour usually invade locally and are rarely found at distant sites whilst others often metastasize. Sometimes secondary growths are found when the primary source can’t de detected at all – so they’re ‘cancers of unknown primary’ and they’re not uncommon, coming in the top 10% of diagnoses.

Equally bemusing is the range of favoured targets for dissemination. Prostate cancer cells commonly home in on bone whereas bone and muscle tumours often spread to the lungs. Others, however, are much more promiscuous and go for multiple sites (e.g., triple-negative breast cancer, skin melanoma and tumours originating in the lung and kidney). We have little idea what’s behind this variability though it may be a combination of different circulation patterns, capacity to slip through vessel walls and how well-equipped the cell is to survive in new terrain.

Making friends with the neighbours

In Cooperative Cancer Groupies we talked about one of the most recent evolutions in cancer thinking – the notion that tumours are not just made up of clumps of abnormal cells but that their locale becomes flooded with a variety of normal cells as the host mounts first an inflammatory response and then attempts to kill off the intruder through its immune system. When this defence fails and the tumour begins to develop it has succeeded in corrupting the groupies in the microenvironment so that now they send out signals that actively promote tumour growth. This type of local support is similarly critical in determining whether metastases take root, so to speak. Moreover, variation in the precise signals from normal cells between different tissues contributes to target preference for malignant cells.

Not like you see on t.v.

In the currently popular Danish political drama television series called Borgen there’s a scene in which a tabloid newspaper editor is offered a piece by a reputable journalist about the European Union that he rejects. “Don’t try to give me a story about the EU: it’s not sexy and it’s too complicated for our readers to understand.” We will have no truck with such patronising here, despite the fact that nobody ever accused metastasis of being sexy. Moreover, as no one ‘understands’ it, we take the view that we’re all in this together and, because it’s infinitely more important and fascinating than political stories, we have belaboured you with the foregoing! Just to make sure that the little we do know is clear, let us summarise in nine (more or less) one-liners:

  1. Tumor cells signal to potential secondary sites.
  2. They escape, burrow, circulate, lodge at landing sites and colonize.
  3. They change the pattern of proteins they make to permit escape.
  4. They change the pattern again when they colonize.
  5. No genetic signature (set of mutations) is known that indicates capacity to metastasize.
  6. The process is very inefficient – i.e. most tumor cells never form a colony.
  7. Despite the low success rate, metastasis is responsible for >90% of cancer deaths.
  8. Once colonization starts at secondary site, tumor cells recruit help from adjacent normal cells (as they do in primary tumors).
  9. Normal cells can also colonize – that is, non-tumour cells injected into the bloodstream of mice have been shown to form colonies in the lungs. 

364

This beautiful picture taken by Bettina Weigelin and Peter Friedl, UMC St Radboud Nijmegen, shows the remarkable plasticity of cells. The tumour cells (green) are invading normal mouse skin (orange) that also contains nerve fibers (blue) and collagen (grey). Cells may invade singly or as clusters. Their flexibility in wiggling through skin is similar to what happens when they cross the walls of blood vessels. http://www.cell.com/Cell_Picture_Show

Perhaps the most surprising item is the one we slipped in at Number 9 – that metastasis, or at least the capacity to colonize secondary sites, is not an exclusively property of some tumour cells but that normal cells can do it too. For sure we assume tumour cells are better at it – not least because they can send out advance signals giving them a better chance of a happy landing. And, of course, once a colony has been founded, tumour cells already carry mutated genes that can act as ‘drivers’ for further expansion of the secondary growth. Even so, the fact that normal cells can pass from the blood to a niche in lung tissue shows that colony foundation is not a unique property of tumour cells. Lung colonization by normal cells may be down to mechanics. Your lungs, which of course fit inside your chest, resemble a sponge – a mass of fine tubes linked to 300 million air sacs (called alveoli): spread them out and they’d cover a tennis court. The alveoli are surrounded by the most intricate network of blood vessels (called capillaries) and it is here that oxygen is transferred to blood. The fine capillaries may simply be a very effective trap – cells may become stuck without the requirement for any specific markers.

And the outlook?

We have therefore a dim picture of what is involved in metastasis but the presumption is that it may rapidly brighten. It’s not hard to see why metastasis is the culprit in the overwhelming majority of cancer deaths. By spreading to new sites cancers increase enormously the difficulty of detecting them, they become almost impossible to treat by surgery and the only strategy remaining is to use drugs (chemotherapy). Currently there are hardly any treatment options available for tumours that have metastasized and even when drugs do work their effects are short lived and tumours recur. The unveiling of every new facet of the amazing puzzle that is metastasis refines our thinking about the problem and carries with it the possibility of new targets and strategies for its blockade. The end is nowhere in sight but we are, at long last, making a significant beginning.

References

Ghajar, C.M. et al. (2013). The perivascular niche regulates breast tumour dormancy. Nature Cell Biology 15, 807–817.

Brabletz, T., Lyden, D., Steeg, P.S. and Werb, Z. (2013). Roadblocks to translational advances on metastasis research. Nature Medicine 19, 1104-1109.

Keeping Cancer Catatonic

Over a century ago there lived in London an astute physician by the name of Stephen Paget. He was one of those who may or may not be envied in being part of a super-talented family. His Dad, Sir James Paget, was pals with Charles Darwin and, together with Rudolph Virchow, laid the foundations of modern pathology, though today medical students usually encounter his infinitesimal immortality through several diseases that bear his name. These include a rare condition, Paget’s disease of the breast, in which malignant cells form in the skin of the nipple creating an itchy rash, usually treatable by surgery. His Uncle George had been Regius Professor of Physic at Cambridge and he had several brothers, two of whom became bishops. Fortunately Stephen continued the medical thread of the family and Paget’s passion became breast cancer.

A Key Question

Paget had that invaluable scientific gift of being able to pinpoint a key question – in his case ‘What is it that allows tumour cells to spread around the body?’ – and it was such a good question that to this day we don’t have a complete answer. That it happens had been known long before the appearance of Paget Junior. René-Théophile-Hyacinthe Laënnec, French of course, in the early years of the 19th century described how skin cancer could spread to the lungs before he went on to invent the stethoscope in 1816. The mother of this invention was a young lady whom he described as having a ‘great degree of fatness’ that made her heartbeat inaudible by the then conventional method of placing ear to chest. Using a piece of paper rolled into a tube as a bridge, Laënnec was somewhat taken aback that the beat was more distinct than he’d ever heard before. Needless to say, medicine being a somewhat reactionary profession, not all its practitioners had ears tuned to receive this advance with glee but in the end, of course, it caught on and we can therefore award Laënnec first prize in reducing human cumulative embarrassment. It was another French surgeon, Joseph Récamier, who subsequently coined the term metastasis, (to be precise ‘métastase’) to describe the formation of secondary growths derived from a primary tumour.

Early Ideas about Metastasis

The notion that primary tumours could give rise to a diaspora gradually took root but it was not until 1840 that the Munich-born surgeon Karl Thiersch showed that it was actually cells – malignant cells – that wandered off and found new homes. Rudolf Virchow had come up with the idea that spreading was via a ‘juice’ released by primaries that somehow converted normal cells at other sites into tumours. As Virchow was jolly famous, having not only made the study of disease into a science but also discovered leukemia, it took a while for Thiersch to triumph, notwithstanding the evidence of Laënnec and others. Funnily enough, and as quite often happens in scientific arguments, it now looks as though both were right if for ‘juice’ you substitute ‘messengers’ – that is, chemicals dispatched by tumour cells – as we shall see.

Paget’s attention had been drawn to this subject through his observations on breast cancer, and he’d taking as a starting point the most obvious question: ‘How do tumour cells know where to stick?’ Or, as he elegantly phrased it in a landmark paper of 1889: ‘What is it that decides what organs shall suffer in a case of disseminated cancer?’ The simplest answer would be that it just depends on anatomy: when cells leave a tumour and get into the circulation they stick to the first tissue they meet. But in looking at over 700 cases he’d found this just didn’t happen and that secondary growths often appeared in the lungs, kidneys, spleen and bone. Paget acknowledged the uncommonly prescient suggestion a few years earlier by Ernst Fuchs that certain organs may be ‘predisposed’ for secondary cancer and concluded that ‘the distribution of secondary growths was not a matter of chance.’ This led him to a botanical analogy for tumour metastasis: ‘When a plant goes to seed, its seeds are carried in all directions; but they can only live and grow if they fall on congenial soil.’ From this, then, emerged the ‘seed and soil’ theory of metastasis, its great strength being the image of interplay between tumour cells and normal cells, their actions collectively determining the outcome. Rather charmingly, Paget concluded his paper with: ‘The best work in the pathology of cancer is now done by those who are studying the nature of the seed. They are like scientific botanists; and he who turns over the records of cases of cancer is only a ploughman, but his observation of the properties of the soil may also be useful.’

BOOKMARKING

Bookmarking cancer: Primary tumours mark sites around the body to which they will spread (metastasize) by sending out chemical signals that create sticky ‘landing sites’ (red protein A) on target cells. Cells released from the bone marrow carry proteins B and C. B attaches to A and tumour cells ‘land’ on C. Cells may remain quiescent in a new site for years or decades, their growth suppressed by signals (e.g., TSP-1) released from nearby blood vessels. Only when appropriate activating signals dominate (e.g., TGFbeta) is secondary tumour growth switched on.

Finding a Landing Strip

For well over a century Paget’s aphorism of  ‘seed and soil’ pretty well summed up our knowledge of metastasis. It’s obvious that before any rational therapy can be designed we need to unravel the molecular detail but we’ve had to wait until the twenty-first century for any further significant insight into the process. As so often in science, the hold-up has been largely due to waiting for the appropriate combination of methods to be developed – in this case fluorescently tagged antibodies to detect specific proteins in cells and tissues and genetically modified mice.

In the forefront of this pursuit has been David Lyden and his colleagues at Weill Cornell Medical College and other centers and their most extraordinary finding is that cells in the primary tumour release proteins into the circulation and these, in effect, tag what will become landing points for wandering cells. Extraordinary because it means that these sites are determined before any tumour cells actually set foot outside the confines of the primary tumour. These are chemical messengers rather equivalent to Virchow’s ‘juice’: they don’t change normal cells into tumour cells but they do direct operations. However, it’s a bit more complicated because, in addition to sending out a target marker, tumours also release proteins that signal to the bone marrow. This is the place where the cells that circulate in our bodies (red cells, white cells, etc.) are made from stem cells. The arrival of signals from the tumour causes some cells to be released into the circulation; these carry two protein markers on their surface: one sticks to the pre-marked landing site, the other to tumour cells once they appear in the circulation. It’s a double-tagging process: the first messenger makes a sticky patch for bone marrow cells that appear courtesy of another messenger, and they become the tumour cell target. It’s molecular Velcro: David Lyden calls it ‘cellular bookmarking.’

Controlling Metastatic Takeoff

Tumour cells that find a new home in this way, after they’ve burrowed out of the circulation, could in principle then take off, growing and expanding as a ‘secondary.’ However, and perhaps surprisingly, generally they do the exact opposite: they go into a state of hibernation, remaining dormant for months or years until some trigger finally sets them off. The same group has now modeled this ‘pre-metastatic niche’ for human breast cancer cells, showing that the switch between dormancy and take-off is controlled by proteins released by nearby blood vessels. The critical protein that locks tumour cells into hibernation appears to be TSP-1 (thrombospondin-1). As long as TSP-1 is made by the blood vessel cells metastatic growth is suppressed. This effect is overridden by stimuli that turn on new vessel growth and in so doing switch secretion from TSP-1 to TGFB (transforming growth factor beta). Now proliferation of the disseminated tumour cells is activated and the micro-metastasis becomes fully malignant. It should be said that this is a model system and may possibly bear little relation to what goes on in real tumours. However, the fact that specific proteins that are, moreover, highly plausible candidates, can control such a switch strongly suggests its relevance and also highlights potential targets for therapeutic manipulation.

Stranger Than Fiction

The system for directing tumour cells to a target seems extraordinarily elaborate. Given that tumour cells cannot evolve in the sense of getting better at being metastatic – they just have to go with what they’ve got – how on earth might it have come about? We don’t know, but the most likely explanation is that they are taking advantage of natural defense mechanisms. Although tumours start from normal cells, the first reaction of the body is to see them as ‘foreign’ – much as it does bugs that get into a cut – and the response is to switch on inflammation and an immune response to eliminate the ‘invader.’

Perhaps what is happening in these mouse models is that the proteins released by the tumour cells are just a by-product of the genetic disruption in cancer cells. Nevertheless, they may signal ‘damage somewhere in the body’. That at least would explain why the bone marrow decides to release cells that are, in effect, a response to the tumour. The second question is trickier: Why should tumours release proteins that mark specific sites? We’ve known since Dr. P’s studies that cells from different tumours do indeed head for different places and it may just be that the messengers arising in the genetic mayhem happen to reflect the tissue of origin. The mouse models, encouragingly, show that the target changes with tumour type (e.g., swap from breast to skin and the cells go somewhere else). In other words, tumours send out their own protein messengers that set up sticky landing strips in different places around the circulation.

As for take-off, it may be that newly arrived tumour cells simply adapt to the style of their neighborhood. By and large, the blood vessels are pretty static structures: they don’t go in for cell proliferation unless told to do so by specific signals, as happens when you get injured and need to repair the damage. TSP-1 appears to be a ‘quiescence’ signal, telling cells to sit tight. The switch to proliferation comes when that signal is overcome by TGFB, activating both blood vessels and tumour. All of which would delight Paget: not only is our expanding picture consistent with ‘seed and soil’ but the control by local signals over what happens next makes his rider that ‘observation of the properties of the soil may also be useful’ spot on.

References

Kaplan, R.N., Riba, R.D., Zacharoulis, S., Bramley, A.H., Vincent, L., Costa, C., MacDonald, D.D., Jin, D.K., Shido, K., Kerns, S.A., Zhu, Z., Hicklin, D., Wu, Y., Port, J.L., Altork, N., Port, E.R., Ruggero, D., Shmelkov, S.V., Jensen, K.K., Rafii, S. and Lyden, D. (2005). VEGFR1-positive haematopoietic bone marrow progenitors initiate the pre-metastatic niche. Nature 438, 820-827.

http://www.ncbi.nlm.nih.gov/pubmed/16341007

Ghajar, C.M. et al. (2013). The perivascular niche regulates breast tumour dormancy. Nature Cell Biology 15, 807–817.

http://www.readcube.com/articles/10.1038/ncb2767

 

 

Molecular Mosaics

Piazza Armerina is a little spot more or less in the centre of Sicily. It is in other words, with due respect to our Sicilian readers, pretty well the middle of nowhere. But you really should go there, off the beaten track though it is, because in the 4th century the local Roman landowner built himself a house. Unfortunately it was later buried by a landslide and remained thus until the 20th century when an excavation revealed that the villa contained the most extensive set of mosaics anywhere in the world. I should tell you that it takes a fair bit to get me excited about archaeological remains but I defy anyone not to be reduced to dropped jaws and softly exhaled ‘wows’ by the Villa Romana del Casale. The mosaics are vast and the scenes depicted span many aspects of the life of the times, from hunting to circuses and, although they won’t tell you who invented the bikini (and of course you’ve wondered!), the images of the ten ‘Bikini Girls’ reveal, so to speak, that such a garment, or lack of it, enlivened the Sicilian summer back in the days when Constantine & Co. were running the show.

Bikini girls

Bikini girls

How big is ‘vast’?

Mosaics seem to me unique among human creative endeavours because you may readily recognize the technical craft and bathe in the history or beauty displayed, as with other art forms, but after a bit you can’t ignore the feeling of sore knees and a twinge of back ache as your body goes into sympathetic spasms with the poor blighters who toiled away for decades to make the things. And how! In the Villa Romana there are 37 million colored tiles – each unique if examined closely enough, each set in position with loving care, each making its minute contributing to the whole.

Catching up – and overtaking – the Romans

It’s taken us seventeen centuries to come up with anything to match the complexity of such creations but the astonishing advances in sequencing our genetic code are now laying bare the mosaicism of tumours. The basic idea is simple to grasp: chip out little bits from different regions of a primary tumour and from secondary growths (metastases) that have seeded from the primary. Then throw the full power of DNA sequencing at them.

The picture that emerges is also simple to describe. Different regions of a tumour – as well as any secondary growths (formed when cells migrate through the circulation to lodge in other sites around the body, called metastases) – differ from each other. They have different ‘DNA signatures’. In other words, although the whole thing starts from a common ancestor (a starter cell if you like), tumours diverge as they grow. So you can think of them like evolving species – and draw family trees just like the ones that show how long it was since you were a chimp.

Kidney tumor muts.006Kidney Tumor tree

Dissecting a tumour (left) and its evolutionary tree.  R1-6: regions of a primary tumour; M: a secondary metastasis. The length of the lines between the kinks is proportional to the number of mutations picked up – thousands of them – on that stretch of the journey.

So tumours are a mosaic …

Indeed. Things are actually even more complicated than that because the sequence obtained for each bit is an average – it’s the predominant mutation pattern. That’s because even a small piece of tissue, say a millimetre in diameter, contains billions of cells (yes, that’s a thousand million) and, if you looked closely enough, you’d find that each individual cell has quirks in its DNA sequence that are all its own. Each is unique – like the tiles of the Villa Romana.

So what?

The molecular complexity, even within one tumour, is utterly mind boggling, but there are two good reasons why we should know what is being unearthed and make an effort to come to terms with it.

The first is that, although cancers are an aberration, the cellular variety they embody is a wonder to behold and the adaptability – mutability if you like – that they reveal gives us a new vantage point from which to contemplate the breathtaking variety of the natural world.

The second is more prosaic: if we’re going to reduce cancers to readily treatable conditions, we must understand the nature of the challenge – what makes a cancer cell tick. However stupefying the picture, that is what we are tackling and the more we know the more rational we can be in dealing with it.

So that’s good news?

Indeed. We’re approaching the point where it would be possible to offer this sort of screen to all new cancer patients and one benefit would be that we’d stop giving people drugs that won’t do any good – because they won’t hit the mutations carried by that individual. Such a programme would make a sizeable hole in the NHS budget but let’s follow the governmental example and not worry about money because we’ve got enough problems with the science. Although the huge number of mutations is daunting, a cheering point is that we can ignore most of them because we know that what’s important is a relatively small set of ‘driver’ mutations – the ones that force the abnormal cell proliferation that is the key feature of cancers – the rest are ‘passengers’, collateral DNA damage if you like, that surf along for the ride.

So we aren’t looking for drugs that can reverse all mutational effects – just enough to hit the drivers.

And the bad news?

We don’t have them. That is, there’s a huge number of drugs that are used against cancers but hardly any of them are ‘specific’ – actually target a molecular defect. Most are like a kind of shotgun that may affect cancer cells but does a lot of collateral damage. Cancers are driven by combinations of driver mutations, of which there are perhaps 20 or so that can come into play, and it’s the specific combinations of those in a given tumour that we need to target.

All of which means that there are two huge challenges: (1) to produce new drugs with good specificity that can hit the major drivers and (2) to be able to deploy them in combinations that give us a chance of outwitting the defences of cancer cells – namely mutating to use other drivers and getting rid of drugs by pumping them out. So, the biology is stunning but, despite the outrageous items that occasionally appear in our newspapers, it is difficult to be optimistic on anything other than a very long time-scale.

If, back in 330 AD, that Roman gent had said to his builder “So here’s the deal, chief: we’re aiming to decorate this place with about 40 million tiles” there would for sure have followed a good deal of stylus sucking as a prelude to “Can’t be done, Magister. Not with all the slaves in Rome” But it was. It just took a very long time and a lot of effort.

Reference

Gerlinger, M. et al., (2012). Intratumor Heterogeneity and Branched Evolution Revealed by Multiregion Sequencing. The New England Journal of Medicine 366;10 nejm.org March 8.

http://www.telegraph.co.uk/health/healthnews/9832535/DNA-map-offers-hope-on-cancer-treatments.html

Slip-Slop-Slap Is Not Enough

I’ve always credited Richie Benaud, the wonderful Australian cricketer and commentator, with the Slip-Slop-Slap slogan (I know – it was really thought up by some bright spark in Cancer Council Victoria who got Sid Seagull to sing it). But Mr. B ingrained it into cricket lovers world-wide as a mantra for preventing them getting skin cancer (slip on long sleeved clothing, slop on sunscreen and slap on a hat, you see) – the main preventable cause of melanoma being excessive exposure to ultraviolet (u.v.) radiation, most of which comes from the sun. And maybe it worked for the cricket fraternity who are, by definition, very smart cookies. Unfortunately in Australia the number of new cases of melanoma – the most lethal form of skin cancer – goes on increasing (over 11,000 in 2008) and it’s a serious UK problem too with 13,000 new cases in 2012 (the second most common cancer in 15 to 35 year olds) and over 2,200 deaths.

     

Avoid sunburn …                     Play cricket …                     … for Cambridge!!

Finding a major melanoma mutation

In 2003 sequencing of the human genome was completed and it wasn’t long before Mike Stratton and his colleagues at The Sanger Centre near Cambridge had applied the knowledge and methods of that great triumph to melanoma – a form of cancer about which virtually nothing was known in terms of its molecular biology. They made the remarkable finding that a mutation in a gene called BRAF switched on a signal pathway that drove cell proliferation in about two-thirds of melanomas. Remarkable because that gene had not previously been associated with any cancer. Even more amazingly, within a few years drugs had been developed that targeted BRAF and caused substantial regression of tumours that had spread (metastasized) in patients.

Like all cancers, melanoma is not caused by one mutation and other ‘drivers’ have since been identified – which was a bit of a relief because a perplexing thing about BRAF is that the mutation that turns it on is not the kind of genetic change caused by u.v. radiation. So the link between radiation, BRAF and melanoma is explained in most cases by u.v. exposure damaging a variety of genes which then act together with mutant BRAF to promote the disease.

Black, red & white mice

A different complexion

Clear so far? Good – but you will know that cricket and cancer have in common the fact that they are both a deal more complicated than they appear to the uninitiated. The first quirk of melanoma is that folk of fair complexion or with red hair or freckles are more at risk. The wide variation in human colouration is controlled by two forms of a pigment called melanin (pheomelanin and eumelanin). A key regulator of the balance between the two is a signalling system – a messenger talks to its receptor on the cell surface telling the cell to make more eumelanin. Upset this system and the balance is disturbed – you get redder because you make relatively more pheomelanin.

Mouse models

It’s possible to make a mouse model of human redheads (you shouldn’t be surprised: remember mice have more or less the same number of genes as us, including one that makes the receptor that controls redness). So some bright sparks in the US of A have done just that by mutating the mouse receptor – with interesting results. When the ‘red’ mice were bred with animals carrying the mutated BRAF gene associated with melanoma in humans, many develop exactly the same type of cancer – whereas black mice (lots of eumelanin) and white mice (who can make neither of the pigments) have very low rates of melanoma. And, of course, it was important in these experiments that the mice weren’t allowed to nip off to sunbathe and watch cricket – i.e. they were kept in a u.v.-free environment.

What this shows is that red mice (and by extension, their human counterparts) can get melanoma by some means that doesn’t involve u.v. It may be that eumelanin protects DNA from two forms of assault: it not only absorbs sunlight but also limits the effect of chemicals produced within the body that can mutilate our DNA.

Whatever the mechanism, Richie is right as usual: we should continue to Slip-Slop-Slap because too much catching the rays can cause melanoma – especially in fair, freckled red-heads. But that isn’t the full story and the imperfections of our bodies mean that we can develop this cancer from within as well as without.

Reference

Mitra, D., Luo, X., Morgan, A., Wang, J., Hoang, M.P. (2012). An ultraviolet-radiation-independent pathway to melanoma carcinogenesis in the red hair/fair skin background. Nature 491, 449–453.

http://www.nature.com/nature/journal/v491/n7424/full/nature11624.html