Heir of the Dog

I’ve probably in the past owned up to causing generations of students to do that raised eyebrow thing, familiar to all parents of teenagers, that, far more pointedly than words, says ‘The old boy’s finally lost it.’ Indeed I may well have a bit of a causative repertoire but one that unfailingly works is revealing that, even after a life in science, I still get ‘Wow’ moments every couple of months or so when I read or hear of some new discovery, method or insight that brings home yet again the wonder of Nature – or has you asking ‘Why didn’t I think of that?’ (The response to that one’s easy, by the way, so please don’t write in).

A common question

The most recent of these jaw-dropping events relates to a question often asked about cancer: ‘Can you catch it from someone else?’ In other words, can cancers be passed from one person to another by infection, much as happens with ’flu? The answer’s ‘No’ but, as usual in this field, even the firmest statement can do with a little explanation. The first point is that the ‘No’ is true even for 20% or so of cancers that are actually started by microbial infection – what you might call ‘bugs’ – bacteria, fungi, and viruses. One such, the bacterium Helicobacter pylori, can cause stomach ulcers that may lead to cancer. Those even smaller bugbears, viruses (typically one one-hundredth the size of a bacterium), are responsible for much of the cervical and liver cancer burden world-wide. Oh, and there’s a little, single-cell parasite (Trichomonas vaginalis), the most common non-viral, sexually transmitted infection in the world that, in men, can cause prostate cancer. But these infections are not cancers even though they may be an underlying cause – bacteria through prolonged inflammation and effects on the immune system and viruses by making proteins that affect how cells behave. Only when these perturbations cause genetic damage – i.e. DNA mutations – do you have a cancer. Which is why the answer to the original question is ‘No.’

There’s always one

Well, two in this case – and, given that we’re talking about cancer, you won’t be surprised that there are some oddities. They’re not exceptions to the ‘No’ answer because they occur in other animals – not in humans – but, in each, tumour cells are directly transferred from one creature to another – so it is cancer by infection. One such contagious tumour occurs in the Tasmanian devil. It’s transmitted by biting, an activity popular with these little chaps, and it gives rise to a particularly virulent facial tumor, eventually fatal because it prevents eating. To counter the probability that Tasmanian devils will become extinct in their native habitat, a number of Australian sanctuaries have breeding programmes aimed at setting up a disease-free colony on Kangaroo Island, South Australia.

TDs

Tas D

 

 

 

 

 

Tasmanian devils – cancer-free – Lone Pine Koala Sanctuary, Brisbane

A very similar condition in dogs known as canine transmissible venereal tumour (CTVT: also called Sticker’s sarcoma), mainly affects the external genitalia. First spotted in the nineteenth century by a Russian vet, it too is spread either by licking or biting and also through coitus. Dogs with CTVT can now be found on five continents and, from DNA analysis, we’ve known for some time that – remarkably – all their cancers are descended from a single, original tumour cell that appeared many years ago. They’re like one of those cell lines grown in labs all over the world, except they’ve been going far longer than any lab – with man’s best friend doing the cultivating.

So what is new?

Elizabeth Murchison and colleagues at The Wellcome Trust Sanger Institute, Cambridge have just produced the first whole-genome sequences of two of these tumours – from Australia and Brazil (an Aboriginal camp dog and a purebred American cocker spaniel). These confirmed that all CTVTs descend from a single ancestor who, they estimated, was trotting around about 11,000 years ago. The last common relative of the two dogs whose tumours were sequenced lived about 500 years ago, before his descendants went walkies to different continents.

And the ‘Wow’?

We already had a pretty good idea of how CTVTs have been handed down. In this paper the really amazing bit came in the detail. The authors estimated roughly how many mutations were present in each tumour. Answer: a staggering 1.9 million. And it’s staggering partly because it’s only slightly less than a change every 1,000 units (bases) in dog DNA but it’s truly awesome when you note that it’s several hundred times more than you find in most human cancers. We’re getting used to the idea of thousands or tens of thousands of mutations turning up in human cancer cells with associated gross disruptions of individual chromosomes. But these canine cancers display genetic mayhem on a massive scale – perhaps best visualized by comparing their chromosomes with those of a normal dog using a method that labels each with a different colour. A glance at the two pictures tells the story: all the cancer chromosomes from one of the tumour-bearing dogs (on the right) have been shuffled as if in some molecular card game. The full range of colours can still be seen, but of the normal pattern of 39 pairs of identical segments of DNA (left) there is no sign.

Two dogs chromos

Dog chromosomes. Left: normal; right: CTVT

(from Murchison, E.P. et al. (2014) Science 343, 437-440)

It seems incredible that cells can survive such a shattering of their genetic material – a state called ‘genetic instability’ because, once DNA damage sets in, mutations usually continue to accumulate. These cancers are uniquely bizarre, however, because although their genomes have been blown to smithereens, not only do the cells survive but they’ve continued suspended in this surreal state for centuries. They’re genetically stable – it really is the cellular equivalent of balancing an elephant on a pin.

‘Wow’ Indeed – but so what?

So like me you’ve been blown away by these discoveries but you may be asking, apart from the excitement, what’s in it for us humans? Well, there’s one other very strange thing about these dog cancers. Infected animals do indeed develop the most unpleasant, large tumours – but most of them are eventually rejected by the host dog. That is, its immune system gets to work to eliminate them – and after that the dog is immune to further infection. We are only just beginning to find ways of boosting the human immune system so that it can attack cancers and maybe, just maybe, we can extract from the stable chaos of the CTVT genome the secret of how they provoke rejection – and maybe that will guide human treatments.

Reference

Murchison, E.P. et al. (2014). Transmissible Dog Cancer Genome Reveals the Origin and History of an Ancient Cell Lineage. Science 343, 437-440.

Advertisements

Spray Painting Cancer

I’m pretty certain that anyone reading this will be fully aware that one of the biggest problems in cancer is spotting the blighters. We have, of course, X-ray detection (as in mammography), CTs and MRI scans, all so familiar we need not bother to define them, and there’s also a variety of sampling methods for specific cancers (e.g., the Pap test for cervical cancer). But, useful though all these are, the plain fact of the matter is that none are ideal and in particular the pictures created by imaging methods are very limited in sensitivity. Put another way, they won’t pick something up until it is quite large – a centimeter in diameter – meaning that the abnormal growth is already quite advanced.

Cunning Chemistry

Needless to say, much inspiration and perspiration is being applied to this matter and what has been really exciting over the last ten years or so is the way very smart chemists are collaborating with clinicians to come up with new ways of looking at the problem. One of these clever tactics is being developed in the University of Tokyo using a different type of imaging ‘reporter’ that signals its presence by fluorescing. Fluorescence occurs when a molecule absorbs light and becomes ‘excited’ before relaxing back to its ‘ground state’ by giving off a photon. Fluorescent molecules (fluorophores) are much used in biology because the background signal is often very low so the high signal-to-noise ratio gives excellent sensitivity.

Spray Paint scheme

The cell-surface enzyme GGT converts the small molecule  gGlu-HMRG to a fluorescent form (HMRG) that is then taken up by the cell. GGT is only found on tumor cells so they light up and normal cells do not

Fortunately we don’t need to know how the chemists did it – merely to say that Yasuteru Urano and his colleagues came up with a small molecule (called gGlu-HMRG for short) that does not give off light until a small fragment is chopped off its end, whereupon it changes shape: this flips the switch that turns on fluorescence. The cutting step needs an enzyme that is found on the surface of various cancer cells but not in normal tissue (GGT for short).

Joining Forces

To show that there was real mileage in their idea they followed the time-honored blue-print of cancer research, showing first that it works on tumor cells grown in the lab (and, equally important, that it doesn’t highlight normal cells), before moving to mouse models of ovarian tumors. The later is where chemists meet clinicians because an endoscope is required (quite a small one) – a flexible tube for looking inside the body – devices now so sophisticated that they can incorporate a fluorescence camera.

In the final synthetic step the cunning chemists formulated a spray-on version of their probe molecule so that it can be dispensed during endoscopy or surgery – a bit like an underarm deodorant. Now it’s easy: find suspect tissue, give it a squirt of gGlu-HMRG, wait a few minutes and see if it lights up. The answer is, of course, that in their ovarian cancer model the spray-on graffiti lights up within 10 minutes of sticking to a tumor cell and can detect clumps of cells as small as 1 millimeter in diameter – a terrific advance in terms of sensitivity. The brief time taken for the signal to be visible after the probe has been applied means that within the same procedure it could be used to guide surgeons in removing small tumor masses.

The Tokyo system is not the only one under development. My colleague Andre Neves at the Cambridge Cancer Centre, another of these fiendishly clever chemists, is working on a parallel line using different fluorophores that can be topically applied to the lining of the intestine. The goal here is, of course, the early detection of colon tumors. Yet other approaches use molecules that accumulate preferentially in tumor cells and respond to light in the near-infrared region of the spectrum (800 nm to 2500 nm wavelength, compared to just under 500 nm for gGlu-HMRG), giving an even better signal-to-noise ratio.

This is, as Mr. Churchill might have pointed out, not even the beginning of the end of this story. But it is one more small and innovative step forward. Not all cancers even of the same type will be detectable by a given probe because they vary so much in the genes they express but the ingenuity of the chemists gives hope that a substantial panel of ever more sensitive reporters will emerge. It is also true that endoscopy is unlikely to gain widespread popularity as a routine screening method. However, these advances, moving us to detection at ever earlier stages may become very powerful as a follow-up test, combined with the capacity for simultaneous treatment, when tumor cells have been detected in more comfortable screens, for example as circulating cells in small blood samples, an immensely exciting prospect to which we will return in a later episode.

 References

Urano, Y., Masayo Sakabe, Nobuyuki Kosaka, Mikako Ogawa, Makoto Mitsunaga, Daisuke Asanuma, Mako Kamiya, Matthew R. Young, Tetsuo Nagano, Peter L. Choyke, and Kobayashi, H. (2011). Rapid Cancer Detection by Topically Spraying a γ-Glutamyltranspeptidase–Activated Fluorescent Probe. Science Translational Medicine 3, 110ra119.

http://www.ncbi.nlm.nih.gov/pubmed/22116934

Shi, C. (2012). Comment on “Rapid Cancer Detection by Topically Spraying a γ-Glutamyltranspeptidase–Activated Fluorescent Probe. Science Translational Medicine 4, 121le1.

http://stm.sciencemag.org/content/4/121/121le1.long