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

POTty training for chromosomes

Our genetic material comes in chunks called chromosomes, the ends of which are capped with repetitive DNA sequences called telomeres. Every time DNA is replicated to make new cells bits of the telomeres are lost – so they get shorter – and eventually this turns on a stress signal that puts a stop to further cell reproduction. So, the older you get the shorter your telomeres become and when that stops you making more cells you conk out. Lurking within our chromosomes is a gene that can stop this happening: it encodes an enzyme called, of course, telomerase that extends chromosome caps. But, you exclaim, a well-known feature of cancer cells is that they are ‘immortal’ – so they must find a way of switching on the telomerase gene that in normal cells is turned off to ensure that we don’t hang around too long. And indeed most of them do – which highlights another of life’s balancing acts: telomerase off = finite life-span, telomerase on = cancer.

Telomeres (red) cap the ends of chromosomes

Telomeres (red) cap the ends of chromosomes

Putting a cap on it

Human telomeres contain thousands of repeats of the 6-base sequence TTAGGG that cap the ends of chromosomes. To prevent these being worn away and enable cells to become ‘immortal’, the genetic mayhem that characterizes tumours usually includes a means of activating telomerase. However, you won’t be surprised to find that extending telomeres is a complicated business and the telomerase enzyme is just one bit of a multi-component molecular machine that does the job. One of the bits is a protein by the name of POT (POT1 to be precise) and a Spanish group have just shown that mutations in POT1 occur in chronic lymphocytic leukemia. Normal POT1 acts as a negative regulator that suppresses telomere extension: mutations in POT1 permit telomere extension and also enable chromosomes to fuse end-to-end with one another – a common type of genetic damage in leukemia. It appears that, although POT1 mutations are quite rare, they occur only in the clinically aggressive subtype of CLL – so they provide not only a new potential drug target but also a prognostic indicator.

Incidentally, despite what you might think, ‘cancer genes’, i.e. genes that by acting abnormally (as a result of suffering some sort of mutation, either in themselves or indirectly) can help to drive cancer development, have names that are very sensible and logical. Thus POT1 stands for protection of telomere – and it’s POT1 just in case a close rello turns up – which would be POT2.

Reference

Ramsay, A.J. et al., (2013). POT1 mutations cause telomere dysfunction in chronic lymphocytic leukemia. Nature Genetics 45, 526–530.

http://www.readcube.com/articles/10.1038/ng.2584?locale=en