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

 

 

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The Creation of Cancer

Where do cancers come from?’ One of those dreaded childish questions – so best to get your thinking in first, rather than trying to answer on the hoof in the face of that unblinking stare of expectation. In the beginning, as you might say, we need a hand-wavy word on how DNA ‘makes proteins’, why they’re important (‘Proteins R Us’, in short) and what can go wrong with them.

DNA double-helix

The double helix of DNA

In 1953 Watson and Crick worked out the structure of DNA. It holds, of course, the secret of life and you might observe that it has the appropriate shape of a spiral staircase to nowhere. The ‘genetic code’ is the order of thousands of small bits that are linked together to make the very long molecules of DNA. These bits contain smaller bits called bases – four of them (A, C, G and T) – and they’re firmly stuck together so that each DNA molecule is pretty stable. In addition, bases in one DNA can stick to those in a second strand – hence the double-helix.

Protein

DNA encodes proteins

The essence of life is the transformation of the genetic code into the corresponding sequence of the building blocks that make proteins. The blocks are amino acids, stitched together to make proteins in much the same way as DNA is built from its base-containing units. There are 20 different types that can be glued together in any order, a typical protein containing a thousand amino acids. They tell the protein how to fold up into its final shape – a 3D structure unique for each protein. Many proteins are blobs (like balls of string) but, as you’d guess given that they do everything, they come in all shapes and sizes—cables, sheets, coils, bridges, etc. The idea then is fairly simple: flexible protein chains fold themselves into their working shape – and individual shapes enable proteins to do specific jobs. A simple sum can show that a limitless variety of proteins can be made: they are the machines of life that make all living things work and they have created all the species of life on earth.

Mutations

Proteins make life possible because the exquisite choreography that generates their shape creates localised regions (sticky bits, clefts, cavities, etc.) for interactions with other molecules. These confer amazing versatility: proteins can ‘talk’ to each other and form relay teams that transmit information from one part of a cell to another, they can generate movement (as in muscles), and bring molecules together (e.g., when they act as enzymes driving chemical reactions that otherwise would not occur). But, as we all know, mistakes can happen even in the best-run enterprises. Mistakes in proteins arise from mutations – changes in the DNA code. Many diseases result from single base alterations: if that changes an amino acid the result can be a protein with dramatically altered function. A well-known example is cystic fibrosis: a protein made in the lung has one abnormal amino acid: the effect on its activity causes a build-up of mucus that makes breathing difficult and is a target for fatal infections.

Mutations and cancer

Cancers are also caused by mutations but they’re a bit more complicated, being driven by groups of mutations, rather than by one event. For most cancers these are picked up as we go through life – so the creation of a cancer is a slow process. Most don’t appear until we are over 60 years of age – collecting a suitable hand of mutations takes time. Because several critical mutations are required you’d guess that what tumour cells are up to is evolving a number of tactics for outsmarting their normal counterparts on the survival front. Indeed they are. They multiply in an unregulated way (because they ignore signals that control normal cells), side-step protective mechanisms that usually kill abnormal cells, divert nutrients from normal tissue to themselves, and make new blood vessels for the delivery of food and oxygen. Perhaps most amazingly of all, they seduce and subvert cells of the immune system: these begin by trying to eliminate the tumour but end up playing a key role in its growth – a sort of co-operative corruption.

All this is why cancer needs several mutations, and these are part of a wider genetic mayhem that will kill most cells – because essential survival genes are damaged. The cells that emerge as tumour precursors are molecular freaks in that they’ve both survived and picked up a bag of dirty tricks with which to out-compete their normal brethren. So, molecularly speaking, cancers are rare events. What’s more, there’s no forethought, no premeditation at work here. If the expression ‘unintelligent design’ conveys random chance in a game of genetic roulette then it’s an excellent descriptor of cancer evolution.

Stop me if you’ve heard it

If all this is beginning to sound familiar, so it should. It’s a completely undirected process that usually fails – but when it succeeds represents an extraordinary triumph of the flexibility of DNA and hence the adaptability of cells. Familiar, of course, because it’s a form of evolution that parallels the emergence of new species.

Tree of life

In the revolution started by unveiling the structure of DNA, the biggest advance has been finding a way to work out the order of bases – the genetic code. The first complete human DNA sequence came in 2003. Since then astonishing technical advances have led to thousands of tumours and hundreds of different species being sequenced. From this you can estimate when new species arose and draw a map of the evolution of all major forms of life on earth from a single, common ancestor. The time scale is incomprehensibly vast, but the picture is stunning in its simplicity, showing how everything is related – bugs, plants, fungi and humans – and how that family has emerged over nearly four billion years. This would have delighted Charles Darwin who, in 1859, was able to define evolution by natural selection only on the basis of what he could see. Molecular biology has now revealed its foundations.

Cancer evolution

In many ways tumours do indeed behave like new species: through the acquisition of mutations they out-compete normal neighbours and establish new niches in which to survive and prosper. But tumours are not new organisms: they’re normal cells that have gone off the rails – been hijacked, if you will, by delinquent genes. The big difference is the brief time scale over which tumours develop compared with the almost infinitely slow, step-wise testing of novel genetic variants in species evolution. So becoming a tumour is a very chancy business – but it’s a lot less fraught than making a new form of life. They take any short-term growth advantage conferred by a mutation without concern for the consequences.

Short trials and lots of errors

When a cell picks up its first growth-promoting mutation it has taken an irreversible step towards a life of crime. It’s become a high roller in the cellular casino, addicted to roulette of the Russian variety, and no amount of genetic counselling will reform it. If only it could think, how our tumour cell would long for a guiding hand – a more knowing form of life that could steer its orgies of DNA destruction toward survival. Alas! Like every other life form, tumours are in thrall to the random creator called chemistry. In a tiny few the dice fall favourably and they grow to rule their kingdom – briefly. Oh for an intelligent brain to design them not to kill their life-support system! Like cellular spaceships seeking immortality in the celestial wastes without the know-how to reach escape velocity, they can only burn brightly before crashing. Tumours are indeed a microcosm of evolution, working on an abbreviated time-scale – they’re dynamic Darwinism.