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. 

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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.

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Policing DNA

In A Sinister Side to Sequencing we noted that, wondrous though the advances in sequencing DNA have been, every silver lining …, so to speak. This was in the context of our now being able to determine whether babies will be born with genetic defects, raising the prospect that parents may opt not to have afflicted children. Because we feel that the avoidance of politics has such beneficial effects on the tone of these pieces, we did not mention another problem raised by the ready availability and sophistication of current DNA analysis methods, namely use of the data by police forces, specifically by the procedure usually called DNA fingerprinting. But, needs must …

First of all, what’s DNA fingerprinting?

DNA profiling (or fingerprinting) methods, pioneered by Sir Alec Jeffreys at the University of Leicester, have undergone substantial refinement since they were first used as a police forensic test in 1986 to identify the rapist and killer of two teenagers. Regardless of detail, the essential point is that that the genetic code of individuals is compared using DNA that can be extracted from most tissues or body fluids (e.g., blood, semen, cheek cells, etc.). From the samples to be compared short lengths of DNA are generated and separated by size in a gel, stained so that the DNA fragments show up as bands. Each of us has a unique pattern. In the picture the pattern from suspect 2 is identical to that of DNA taken from the crime scene – so he dunnit!

DNA patterns

 

 

 

Short lengths of DNA samples obtained from a crime scene and from the tissues of three suspects separated in a gel and stained to show as black bands

 

 

 

 

Why is it in the news?

The Supreme Court of the United States has just ruled that the police should be permitted to take DNA samples from an arrested individual, as Maryland officers had done from a character who’d been waving a shotgun only to find that he’d committed an unsolved rape case in 2003 from which they were still holding a DNA sample. An excellent result, you might think. Perhaps, but the ruling has nevertheless got the liberals up in arms, as represented by Antonin Scalia, an Associate Justice of the Supreme Court. Put briefly, his point is that DNA sampling is following the path of fingerprinting, things will go headlong downhill and before long we’ll have to submit to it if we want to get on a plane or play for a school sports team. This would clearly be an unacceptable invasion and so, inevitably, The Fourth Amendment to the United States Constitution is invoked – for outsiders that’s the thing purporting to protect citizens from ‘unreasonable’ actions by the powers that be, whatever that means to any administration that happens to be in charge.

No one sane is arguing that the police should not be permitted to use DNA in the pursuit of villains. The problem is how to keep the lid on Pandora’s box. So, without for a moment implying that the good denizens of the US of A might be a touch parochial, let’s take the drastic step of casting a glance beyond the limits of sea to shining sea.

In the rest of the world?

What better place to start than with the mother of modern democracy and what was the land of the free before the US of A was invented? Shock horror: it emerges that little old England has more DNA samples per head of population in the hands of its police than any other country for which information is available! How can this have come about? Well, the Police and Criminal Evidence Act of 1984 permitted the police to take fingerprints and body samples without consent from people charged with, or convicted of, a recordable offence (these include begging, being drunk and disorderly and taking part in an illegal demonstration – ‘illegal’ having been interpreted somewhat flexibly in recent times). However, those powers were extended in 2004 to permit samples being taken from anyone arrested on suspicion of any recordable offence. You will recall that this was during the Premiership of Mr. Blair, a chap with little interest in civil liberties and none at all in parliamentary democracy.

Back in 1949, when a parliamentary democracy seemed an immovable feature of British life, the United Kingdom (UK) became a founder member of something called The Council of Europe – designed to promote cooperation over matters relating to the law and human rights. Bear in mind that this is quite distinct from the European Union (EU) – the organization that lives in decadent style in Brussels to which the mother of parliaments has for some time ceded control of its affairs. EU members include Poland, Hungary and Estonia and it’s largely run by the Germans and the French. Funny how things turn out. Shock horror number 2 is that the UK is the only Council of Europe member that allows retention of biological samples from people who have been acquitted of charges or against whom criminal charges have been dropped. The Council of Europe has a sort of sub-body called The European Court of Human Rights before which was recently brought a UK case about whether the retention of DNA and fingerprints from innocent people is consistent with human rights law. In short, they don’t think it is and it’s worth quoting a key phrase in their conclusions: “… retention … constitutes a disproportionate interference with the applicants’ right to respect for private life and cannot be regarded as necessary in a democratic society.

So, what’s happened?

Well, for once let us rejoice in being interfered with by those pesky Europeans because, as of May 2012, the UK now has a Protection of Freedoms Act covering the operation of the UK Police National DNA Database. A critical feature is that DNA and fingerprint records of over a million innocent individuals will be deleted and the DNA samples destroyed. It should be added that seemingly Maryland law also requires destruction of DNA samples taken in cases that do not lead to conviction.

What will be done?

The problem with protecting freedoms is that laws are OK but someone needs to do the protecting. No one who has reviewed the activities of the British police that have come to public attention over the last twenty years would have any confidence either in their morality or their competence, never mind their inclination to police themselves. The fact that, according to GeneWatch UK, companies have been permitted to use the police DNA Database for research purposes without, of course, individual consent would confirm your doubts.

Re-crossing the Atlantic it would also seem sensible to ensure first that reasonable legal safeguards are in place across all states to control police activity  but after that to keep Mr. Scalia’s concerns in mind and remain vigilant in ensuring that those provisions are adhered to.

Finally, all that having been said and with due regard to the seriousness of this matter, one might observe that countries prone to promoting themselves as models of human rights that nevertheless pass laws permitting indefinite detention of citizens without trial and are not averse to the use of torture have somewhat weightier matters to resolve than what the fuzz do with a few cheek swabs.

References

https://cancerforall.wordpress.com/2012/07/01/a-sinister-side-to-sequencing/

http://www.thedailybeast.com/articles/2013/06/04/big-brother-invades-your-genes.html

http://www.genewatch.org/sub-539478

National Defense Authorization Act, 2012.