Mystery Rays from Outer Space

Clonal evolution during in situ to invasive breast carcinoma progression1

What’s a tumor?

In some ways, that’s a bad question (never mind the answer) because it implies that a tumor is a single thing. But we know that’s not true. A tumor, by the time we can detect it, is a collection of many cells, at least billions of them, and those cells are not all the same. I’m not even talking about the normal cell types that are incorporated into a tumor (things like blood vessels and support cells). Even cells that are unambiguously cancerous are very different within a tumor. And of course, that’s important for the things we’re most interested in, prognosis and treatment, because it’s not the average tumor cell that we’re most concerned about, it’s the subset of tumor cells that are most resistant to treatment, or that are most aggressive.

The development of this variation is really fundamental to how we understand tumor formation and tumor growth. Cancerous cells don’t just appear, fully ready to metastasize and grow. What happens is that a normal cell mutates slightly and gains a little advantage. Most of its progeny stay like that, but one of them mutates again and changes a little more, and then one of that cell’s progeny mutates again, and so on. It probably takes at least a half-dozen mutations, over many cell generations, before a normal cell has progressed through to a detectably cancerous cell.  (I’ve talked about this before, here.)

Also, since truly normal cells simply don’t mutate that many times — there are too many checks and repair systems to allow a half-dozen mutations to accumulate in a single human’s² lifetime — one of the mutations is probably in the check/repair system, turning the cancerous pathway into a mutator pathway as well.

So we expect tumors to be made up of many different cell types, and this is indeed what we see:

With rare exceptions, human malignancies are thought to originate from a single cell, yet by the time of diagnosis, most tumors display startling heterogeneity in cell morphology, proliferation rates, angiogenic and metastatic potential, and expression of cell surface molecules. ¹

So how diverse are tumors?

That’s been a hard question to answer, because you’d need tools to look at individual cells, and you’d also need some way of expressing that diversity. A recent paper¹ looked at diversity in breast cancer using some individual-cell tools, which I’m not going to discuss, and took an interesting approach to describing the variability:

… we applied diversity measures from the ecology and evolution sciences to our copy number data. These diversity measures estimate the number and distribution of species in a certain geographical area or environmental niche. In our context, a species is a cancer cell population … Hence, a region of a tumor containing cancer cells with 3 different copy number ratios is interpreted to contain 3 distinct “species.” ¹

They suggest that this way of describing tumors could be a useful aid to prognosis and to predicting response to therapy, offering a quantitative description of tumor variability (which might correlate with the tumor’s potential for spread and escaping treatment).

I hadn’t thought of tumors as ecosystems before, but I wonder if the analogy could be taken further by considering, say,cytotoxic T lymphocytes as predators …

1. Park, S., Gönen, M., Kim, H., Michor, F., & Polyak, K. (2010). Cellular and genetic diversity in the progression of in situ human breast carcinomas to an invasive phenotype Journal of Clinical Investigation DOI: 10.1172/JCI40724[↩][↩][↩][↩]
2. let alone a mouse’s lifetime[↩]

We’re just dipping our toes into the oceans of information from large-scale genome sequencing. We’re at the point now where sequencing a human genome is, not routine, but not extraordinary. The most recent examples of this are two groups who sequenced the genome of a cancer (one group did a lung cancer, the other did a melanoma), and compared to the person’s normal cells. ¹   This lets you see where the cancer cells are mutated.

How many mutations are there in a cancer? We already know that cancer is a multi-step process, involving probably at least 7 or 8 distinct stages. We also know that cancer cells have far more mutations than are needed for these minimals steps. How many is “more”?

• Over 20,000 mutations – 23,000 mutations in the lung cancer, 33,000 in the skin cancer.

Where did these mutations come from? What drives mutagenesis in a cancer cell?

• Cigarettes and UV light. They can point out the typical kinds of mutagenesis for each and show that the lung cancer mutations are tobacco-induced, the skin cancer mutations are UV-induced.

How often do cigarettes cause mutations?

• “… an average of one mutation for every 15 cigarettes smoked.”

(I question this figure, or rather, question whether the implied causation is that direct. But it’s not impossible, given their data.)  From an immunological viewpoint, the 20,000 mutations is interesting because it suggests that cancers should have lots of targets for the immune system. This was already pretty clear, but this helps nail it down.

(By the way, the poster at the top, like the research in question, comes from the Wellcome Trust Institute.)

1. Pleasance, E., Stephens, P., O’Meara, S., McBride, D., Meynert, A., Jones, D., Lin, M., Beare, D., Lau, K., Greenman, C., Varela, I., Nik-Zainal, S., Davies, H., Ordoñez, G., Mudie, L., Latimer, C., Edkins, S., Stebbings, L., Chen, L., Jia, M., Leroy, C., Marshall, J., Menzies, A., Butler, A., Teague, J., Mangion, J., Sun, Y., McLaughlin, S., Peckham, H., Tsung, E., Costa, G., Lee, C., Minna, J., Gazdar, A., Birney, E., Rhodes, M., McKernan, K., Stratton, M., Futreal, P., & Campbell, P. (2009). A small-cell lung cancer genome with complex signatures of tobacco exposure Nature DOI: 10.1038/nature08629

   Pleasance, E., Cheetham, R., Stephens, P., McBride, D., Humphray, S., Greenman, C., Varela, I., Lin, M., Ordóñez, G., Bignell, G., Ye, K., Alipaz, J., Bauer, M., Beare, D., Butler, A., Carter, R., Chen, L., Cox, A., Edkins, S., Kokko-Gonzales, P., Gormley, N., Grocock, R., Haudenschild, C., Hims, M., James, T., Jia, M., Kingsbury, Z., Leroy, C., Marshall, J., Menzies, A., Mudie, L., Ning, Z., Royce, T., Schulz-Trieglaff, O., Spiridou, A., Stebbings, L., Szajkowski, L., Teague, J., Williamson, D., Chin, L., Ross, M., Campbell, P., Bentley, D., Futreal, P., & Stratton, M. (2009). A comprehensive catalogue of somatic mutations from a human cancer genome Nature DOI: 10.1038/nature08658 [↩]

TRegs infiltrate into a tumor

One of the reasons the immune system doesn’t destroy tumors is the presence of regulatory T cells (TRegs) that actively shut down the anti-tumor response.  For once, there’s a little bit of encouraging news on that front.

TRegs are normal parts of the immune system.  They actively prevent other T cells (and so on) from attacking their target. ¹  What’s more, TRegs are antigen-specific.  That is, they recognize a specific target, just as do other T cells, but instead of responding by, say, destroying the cells (like  cytotoxic T lymphocyte) or by releasing interferon (like a T helper cell) a TReg’s response to antigen is to prevent other T cells from doing anything in response to that antigen.  In other words, TRegs cause an antigen-specific inhibition of the conventional immune response. ²

Back to tumors.  We know that immune responses don’t routinely eliminate tumors by the time they’re detectable.  There is some evidence that lots of very small, proto-tumors, are in fact destroyed by the immune system very early on, before they’re clinically detectable, but those tumors that survive that attack seem to be pretty resistant to immune control.  At least part of that resistance is because TRegs get co-opted into the tumor’s control (see here, and references therein, for more on that).

So if TRegs are antigen-specific, and TRegs control immune responses to the tumor, what are the tumor antigens that are driving the TRegs?

I would have assumed that TRegs are looking at many, many tumor antigens, including both normal self antigens³ as well as classical tumor antigens.⁴  But a recent paper⁵ suggests, to my surprise, that this assumption is wrong.  Instead, “Tregs in tumor patients were highly specific for a distinct set of only a few tumor antigens“. ⁵ What’s more, eliminating TRegs cranked up the functional immune response, but only to those antigens TRegs recognized — as you’d expect, if the suppression is indeed antigen specific.

This is interesting for several reasons.  If TRegs can be specific for tumor antigens, then at least in theory ((In practice, we don’t quite have the tools yet, I think) it should be possible to turn off these TRegs while leaving the bulk of TRegs intact (and therefore not precipitating violent autoimmunity).  It also suggests that if the TRegs are only suppressing a subset of effector T cells, there’s something else preventing most effector T cells from, well, effecting.  Maybe those are antigen non-specific TRegs, or maybe there’s something else we need to know about.

I’d like to see this sort of study replicated, and a little more fine-tuning on identifying the TReg’s targets (the readout was intentionally fairly coarse here, in order to identify as many as possible).  Still, it’s an unexpected, and potentially very useful, observation.

1. It’s still not quite clear how they do this[↩]
2. There are also antigen-nonspecific TRegs, but we will ignore them for now.  They’re not as effective as the antigen-specific sort, anyway.[↩]
3. Because TRegs, unlike most immune cells, can be stimulated by normal self antigens[↩]
4. That is, antigens that are mutated, or dysregulated, and that therefore act as standard targets for immune cells[↩]
5. Bonertz, A., Weitz, J., Pietsch, D., Rahbari, N., Schlude, C., Ge, Y., Juenger, S., Vlodavsky, I., Khazaie, K., Jaeger, D., Reissfelder, C., Antolovic, D., Aigner, M., Koch, M., & Beckhove, P. (2009). Antigen-specific Tregs control T cell responses against a limited repertoire of tumor antigens in patients with colorectal carcinoma Journal of Clinical Investigation DOI: 10.1172/JCI39608[↩][↩]

TRegs in normal skin

Tumors are supposed to be destroyed by our immune system. So how come we still see tumors?

A big part of the answer is probably that our immune system is very good at destroying proto-tumors, but is not so good at handling those that manage to sneak through and grow to the point of detectability. That splits the first question into two questions: Why do some proto-tumors manage to sneak through, not being eliminated by the immune system? And why is it that detectable tumors are not effectively handled?

The first part, I think, may often be related to cell-intrinsic immune escape mutations. That is, pre-cancerous cells are constantly being attacked by the immune system; in turn (if they survive long enough) they constantly mutate, doing things like damaging the antigen-presentation pathway that makes them recognizable by the immune system. Eventually, they find some configuration that reduces the rate at which they’re killed. Once cancer cell replication is even fractionally greater than destruction,¹ a tumor can begin to grow.

So that’s probably the earliest stage of tumor growth. But once tumors reach a certain size, a second factor kicks in. Chronic immune responses are dangerous; after all, the whole point of the immune system is to kill things. The chronic immune response against the growing tumor is now shut down. This has been understood for quite a while — the immune system often becomes “tolerant” of a tumor. More recently, it’s become clear that it’s not merely “tolerance” (which implies that the immune system is simply benignly ignoring the tumor); the presence of a tumor actively forces the immune system to shut itself down, slamming on the brakes rather than just peacefully coasting by.

Brakes are a fundamental part of an active immune response. If you look at diagrams of normal immune responses, they show inverted “U” shaped curves (in here and here, for example), where the response is triggered, rapidly ramps up, hopefully does its thing, and then just as rapidly shuts down to near-background levels once again. There used to be a sort of general feeling that this was a rather passive thing — pathogen stimulates response, response destroys pathogen, no more stimulus, response goes away — but now we understand that the shut-down phase is just as active and dynamic as the upward curve. Just as with the upward phase, there are all kinds of different mechanisms to control the response; one of the most important is the “Regulatory T cell” (TReg).  And it’s pretty clear that TRegs are involved in controlling the immune response to tumors (I talked about that here, and links therein).

TRegs have been known for a while (I gave a brief history, including the I-J fiasco, here). The usual description of a TReg includes a number of markers;² one of the most basic is CD4. CD4 T cells used to be lumped together as “T Helper” cells, but now we have multiple sub-specialties in the CD4 category, and TRegs are one of those specialities.

More recently, TRegs — or at least cells that function the same way as TRegs — have been described in the CD8 population of T cells.³ CD8 T cells are traditionally called “Cytotoxic T lymphocytes” (CTL) (although it’s been increasingly clear that cytotoxicity is just one of many functions a CD8 T cell can offer), but it seems that these variants of CD8s can actively shut down an ongoing immune response, in a specific and targeted way. There seems to be a trend to calling these cells “suppressor cells” rather than “TRegs”. “Suppressor T cells” is an older term that was out of favor for a while, but it’s probably useful to bring it back and distinguish between the natural TRegs and some of the other cells that can do something similar but that have different sources and origins.

At least some of the CD8 suppressor T cells can arise from apparently-conventional CD8 T cells. That is, you can pull CD8 T cells out of a normal mouse’s spleen, and depending on what those cells see and are exposed to, they could progress to being conventional CTL — killing tumor cells, producing interferon and other cytokines, generally being a destructive force — or they could become suppressor CD8 T cells, and actively prevent that destruction from happening.

It turns out that one of the forces that can drive a CD8 T cell into being a suppressor T cell is a tumor. A recent paper from Arthur Hurwitz’s lab⁴ shows this quite clearly. They had shown previously that transferring specific CD8 T cells into a tumor-bearing mouse resulted in what they called “tolerance”.⁵ But now they demonstrate that it’s more than that; the transferred CD8s are converted into suppressor T cells that actively shut down immune responses.

Tumor-infiltrating TcR-I cells suppressed the in vitro proliferation of both melanoma Ag-specific CD8+ (37B7) T cells and OVA-specific CD4+ (OT-II) T cells. … Even at a ratio of one TcR-I cell to four responder T cells, we observed 30% suppression of proliferation. ⁴

This isn’t the only way that tumors escape immune recognition, but (at least for some tumors) it may be an important one. It’s clearly an important consideration for things like tumor vaccines and immune therapy, because it suggests that immunizing with tumor antigens (and thereby generating lots of tumor-specific CD8 T cells) may actually increase the suppressive effect of the tumor.

The conversion of CD8+ effector T cells into suppressor cells may be one mechanism by which tumors restrict the immune response from effectively controlling tumor growth. As subsequent effectors infiltrate the tumor, either following peripheral sensitization or as a result of adoptive transfer therapy, the induced regulatory cells may suppress these new effectors and reduce their ability to confer tumor immunity. This cyclic suppressive process may contribute to the profound loss of antitumor responses following adoptive immunotherapy. ⁴

(My emphasis.) On the other hand, if this is a common mechanism, then overriding it — which should be possible, using cytokines, specific T cell subsets, and/or targeted receptor ligands — may switch the suppressive population abruptly back into an effector group, turning the brainwashed traitors into resistance fighters.

1. Destruction would include far more than immune destruction, of course — it would include cells that become differentiated and no longer replicated, cells that outgrow their oxygen supply, cells that undergo apoptosis, and so on[↩]
2. FoxP3, CD25, and so on[↩]
3. I’m not sure who made the first identification; this looks as if it’s one of those fields where there were incremental advances, hinting more and more strongly at the presence of these cells, but with no single clearcut starting point. Papers in the early 2000s start to point at regulatory CD8s, and by 2004 a handful of relatively high-profile papers fairly solidly identified them. A 2004 review paper is
   Zimring, J., & Kapp, J. (2004). Identification and Characterization of CD8+ Suppressor T Cells Immunologic Research, 29 (1-3), 303-312 DOI: 10.1385/IR:29:1-3:303[↩]
4. Shafer-Weaver, K., Anderson, M., Stagliano, K., Malyguine, A., Greenberg, N., & Hurwitz, A. (2009). Cutting Edge: Tumor-Specific CD8+ T Cells Infiltrating Prostatic Tumors Are Induced to Become Suppressor Cells The Journal of Immunology, 183 (8), 4848-4852 DOI: 10.4049/jimmunol.0900848[↩][↩][↩]
5. Anderson MJ, Shafer-Weaver K, Greenberg NM, & Hurwitz AA (2007). Tolerization of tumor-specific T cells despite efficient initial priming in a primary murine model of prostate cancer. Journal of immunology (Baltimore, Md. : 1950), 178 (3), 1268-76 PMID: 17237372[↩]

A new technique called optical frequency domain imaging (OFDI) provides amazing images of tumors (especially their blood vessels) in situ.

“(a) OFDI images of representative control and treated tumors 5 d after initiation of antiangiogenic VEGFR-2. The lymphatic vascular networks are also presented (blue) for both tumors. (b) Quantification of tumor volume and vascular geometry and morphology in response to VEGFR-2 blockade.”

– Vakoc, B., Lanning, R., Tyrrell, J., Padera, T., Bartlett, L., Stylianopoulos, T., Munn, L., Tearney, G., Fukumura, D., Jain, R., & Bouma, B. (2009). Three-dimensional microscopy of the tumor microenvironment in vivo using optical frequency domain imaging Nature Medicine DOI: 10.1038/nm.1971

[Cancer] mortality has been systematically decreasing among younger individuals for many decades. … the cancer mortality rates for 30 to 59 year olds born between 1945 and 1954 was 29% lower than for people of the same age born three decades earlier.  … substantial changes in cancer mortality risk across the life span have been developing over the past half century in the United States. … this analysis suggests that efforts in prevention, early detection, and/or treatment have significantly affected our society’s experience of cancer risk.¹

But:

The mortality decline we describe in this paper cannot therefore be attributed to an overall decline in cancer incidence. Rather, the net improvement in cancer mortality in birth cohorts born since 1925 seems to reflect a succession of public health and medical care efforts. ¹

They point to reduction in smoking, effective treatment of childhood leukemias and lymphomas and testicular cancers of young adulthood, and “increasingly successful screening programs for breast, prostate, and colon cancer” as important factors, and add “We are optimistic that ongoing efforts in very early cancer prevention (such as use of HB and human papillomavirus vaccines), as well as ongoing clinical trials of targeted therapies, will preserve the downward trend of cancer mortality“.

1. Kort, E., Paneth, N., & Vande Woude, G. (2009). The Decline in U.S. Cancer Mortality in People Born since 1925 Cancer Research, 69 (16), 6500-6505 DOI: 10.1158/0008-5472.CAN-09-0357[↩][↩][↩]

A year or so ago, the first time I mentioned transmissible tumors, I dismissed Tasmanian Devil Facial Tumor disease as not particularly surprising. As I said more recently, I’ve changed my mind about that, almost entirely because of a chat with Elizabeth Murchison at the Origins of Cancer symposium last month, who told me a couple of things I hadn’t known about the situation.

TDFT is a transmissible tumor, a cancer that arose in one Tasmanian Devil some time ago (at least a decade ago, and maybe much longer) and that is now spreading throughout the Tasmanian Devil population — killing a vast number of them. It’s a tumor that’s spread when these highly territorial animals¹ bite each other on the face; the tumor then grows on the bitten animal’s face and eventually makes it unable to eat properly. Nearly 90% of Tasmanian Devils, in affected areas, have died. This is a catastrophe.

(The good news, such as it is, may be that the Devils as a population are adapting to the tumor to some extent — not by resisting the tumor, though, but through rapid selection for much earlier breeding. ² By reproducing earlier, the population may be maintained even though the Devils are dying as adults. Of course, the population that “survives” won’t be the same as those the existed earlier — they’ll be younger, they’ll have all kinds of different characteristics because they’re no longer being selected for adult survival — but at least it might open a window to save the species.)

Cancers are not, in general, transmissible.  Each new cancer is a new event, that arises from normal cells of the affected individual.  A transmissible cancer would be like an organ transplant — it would be rapidly rejected, because individuals in a population have different MHC molecules, and the immune system rapidly and aggressively rejects cells with the wrong MHC. So there are two possible reasons why a tumor could, in fact, become transmissible. Either the individuals in a population do not have different MHC molecules on their cells, or the tumor can transmit between individual in spite of different MHC molecules.

The explanation for the spread of TDFT seemed to be the former: It was claimed that Tasmanian Devils have very little MHC diversity, so that they can’t reject the tumors³. Even though MHC is normally highly diverse in a population, there are certainly populations — especially small, island, threatened populations, like Tasmanian Devils — that have very limited MHC diversity, so this seemed reasonably plausible. ⁴ This would be a fairly well-understood mechanism of tumor spread, which is why I said it wasn’t that interesting.

The other possibility is that the tumor could spread between individuals in spite of them having different MHC molecules. This is not supposed to happen, according to our understanding of MHC and organ transplants; but in fact, it’s the way the only other extant transmissible tumor (Canine Transmissible Venereal Tumor) spreads.  We have no idea why that happens, which makes it interesting. (A number of ways have been proposed by which CTVT avoids rejection. I won’t go into them in any detail here: none of them, as far as I can see, are unique to CTVT, but rather are very common molecular changes in tumors of all kinds, and yet other tumors are not transmissible; so I don’t consider any of these suggestions to explain the unique feature of CTVT.)

Anyway, as I say, the explanation for TDFT was simple enough: The tumor spread because Tasmanian Devils aren’t genetically diverse. But there are three major arguments, I’ve learned, that say that explanation is wrong.

First: Tasmanian Devils are not, in fact, spectacularly genetically homogenous. ⁵ They’re not as diverse as you’d like to see, but they’re not completely homogenous. In particular, there are two clear, genetically distinct, populations of Devils in Tasmania, one in the East, and another in the Northwest.

Second: Murchison told me that Tasmanian Devils — even those in the same sub-population — vigorously reject each others’ skin grafts. This is what’s supposed to happen with skin grafts, of course. It implies that the Devils do not, in fact, have the same MHC; and in my opinion it’s a much stronger experiment than those in the original homogenous-MHC paper. ³ If Devils reject skin grafts from each other, then they ought to reject tumors from each other — in other words, even if the tumor can take in one individual, then it should be rejected in another, so the tumor should not spread throughout the population. The skin graft finding hasn’t, as far as I know, been published, but if it holds up, it’s a strong argument against homogenous MHC.

Third: Murchison also told me that the tumor is spreading into Devils in the Northwest. ⁶ As I said, the Northwestern and Eastern Tasmanian Devils are clearly distinct populations, with different genetic characteristics. ⁷  If the tumor can spread between them, then the tumor isn’t relying on genetic homogeneity for its transmission.

So the evidence for MHC homogeneity is not good, the evidence that the tumor requires MHC homogeneity is not good, and the only precedent we know of, CTVT, does not require MHC homogeneity for its transmission.

It seems that the two transmissible tumors we know of may be much more similar than I had thought.

1. Incidentally, when I was looking for images of Tasmanian Devils, I found it interesting that the great majority of images I found using Google Image Search showed an open-mouthed, angry Devil, about to take a chomp out of something.  I was going to comment on that, and then I went to Flickr and did the same search, finding instead thousands of pictures of peaceful, timid, close-mouthed Devils.  I think the popular images, put by Google at the top of the list, as so popular because they reinforce the Devil stereotype, and the Flickr photos are a more accurate reflection of their true nature.[↩]
2. Jones, M., Cockburn, A., Hamede, R., Hawkins, C., Hesterman, H., Lachish, S., Mann, D., McCallum, H., & Pemberton, D. (2008). Life-history change in disease-ravaged Tasmanian devil populations Proceedings of the National Academy of Sciences, 105 (29), 10023-10027 DOI: 10.1073/pnas.0711236105[↩]
3. Siddle, H. V., Kreiss, A., Eldridge, M. D., Noonan, E., Clarke, C. J., Pyecroft, S., Woods, G. M., and Belov, K. (2007). Transmission of a fatal clonal tumor by biting occurs due to depleted MHC diversity in a threatened carnivorous marsupial. Proc Natl Acad Sci U S A 104:16221-16226[↩][↩]
4. Olivia Judson had a very good summary of the argument in her blog; she also made the important point that MHC diversity, or lack of it, is clearly not the only factor involved in the tumor spread.[↩]
5. JONES, M., PAETKAU, D., GEFFEN, E., & MORITZ, C. (2004). Genetic diversity and population structure of Tasmanian devils, the largest marsupial carnivore Molecular Ecology, 13 (8), 2197-2209 DOI: 10.1111/j.1365-294X.2004.02239.x[↩]
6. Again, as far as I know this isn’t published yet.[↩]
7. The authors of the 2004 paper didn’t specifically look at MHC diversity, though; so it remains possible, if unlikely, that the MHC is relatively homogenous while the rest of the genome is diverse.  This is completely the opposite of the usual situation, so I think it’s not likely.[↩]

As the overwhelming majority of the mutations in cancer cells are unrelated to malignancy, the mutation-generated epitopes shall be specific for each individual tumor, and constitute the antigenic fingerprint of each tumor. These calculations highlight the benefits for personalization of immunotherapy of human cancer, and in view of the substantial pre-existing antigenic repertoire of tumors, emphasize the enormous potential of therapies that modulate the anti-cancer immune response by liberating it from inhibitory influences.

–Srivastava N, Srivastava PK (2009)
Modeling the Repertoire of True Tumor-Specific MHC I Epitopes in a Human Tumor.
PLoS ONE 4(7): e6094. doi:10.1371/journal.pone.0006094

(My emphasis)

(See many of my previous posts here for more information)

Dogs & Wolf (Gotthilf Heinrich von Schubert, 1872) Naturgeschichte der Säugethiere: mit colorirten Abbildungen zum Anschauugs-Unterricht für die Jugend. (Esslingen : Schreiber, 1872)

Cancer is a creepy disease. Your own cells turn on you, mindlessly and blindly destroying themselves — because the only way a cancer can survive is for its host to survive; unlike viruses, cancers don’t spread from their original carrier to new hosts. Each cancer is a new and unique event, and each cancer is a terminal event, with no future, no children, no transmission. ¹ Cancers don’t spread between individuals, because they would essentially be tissue transplants; if you try to transplant skin (or whatever) between two animals, the skin will be rapidly rejected because of mismatched MHC alleles, and the same is true for tumors.

In theory, tumors could spread from one identical twin to another (I don’t know of cases where this has occurred, though). Tumors might also theoretically spread within a population that has little MHC diversity. Cheetahs, for example, notoriously don’t have much MHC diversity,² probably due to a genetic bottleneck some 10,000 years ago,³ and tolerate random skin grafts very well;⁴ it’s conceivable that a tumor that arose in one cheetah could spread throughout the population. But MHC is generally diverse, and becomes diverse very rapidly, so these sort of low-diversity populations are unusual (though they may be more common than generally realized). And so, as I say, tumors are dead ends and don’t spread from their original host.

But there are always exceptions, and we know of a couple exceptions for this principle as well. Tasmanian Devil Facial Tumor is one; canine transmissible venereal tumor (CTVT) is the other. The last time I talked about this, I said that there wasn’t anything particularly surprising about TDFT. I’ve changed my mind about that, for reasons I’ll talk about later. Here I want to give a quick update on CTVT, which is still even more interesting than TDFT.

CTVT is just what it sounds like: an infectious tumor of dogs. It can be transmitted from one dog to the next, mainly during sex. There’s no sign of a viral agent, and in fact the tumor cells are distinct from the host — that is, the tumor cells are not host cells that have been transformed by, say, a virus, but are foreign agents altogether, cells from an ancient animal that have been passed along in this way, generation after generation, spreading throughout the world. Cells from a CTVT on a German Shepherd in Italy are virtually identical to those from a CTVT on a poodle in Brazil, but the tumor’s cells are very different from their hosts’: “… the differences in genome content between dog and CTVT are substantially greater than those between different CTVT samples.“ ⁵

The last time I talked about this I cited a paper⁶ that showed that the original host of CTVT was actually very ancient: “Our analysis of divergence of microsatellites indicates that the tumor arose between 200 and 2500 years ago.” ⁶ Amazingly enough,  it seemed that the tumor had been passed between dogs perhaps for a couple of thousand years.

But it turns out that it’s even more amazing than that: The 2500 year estimate turns out to be an underestimate. “The estimated time of origin of CTVT is 6500–65,000 years ago. … The cycle of infection currently takes about six months; if this has been true for, say, 15,000 years, then there have been 30,000 transmission events.”⁵ That spans the age where dogs were domesticated and diverged from wolves , about 10,000-15,000 years ago; and in fact, based on genome analysis of the tumors, the tumors did arise in wolves, not dogs. ⁷

This raises a fascinating scenario (which I didn’t see spelled out in Reddeck et al, though I think they were hinting at it). It’s still mysterious how CTVT spreads between hosts. As I said, tumors should act like tissue grafts, which are very rapidly rejected if they’re not MHC matched (as is the case with the CTVT and modern dogs). But tumors could, theoretically, spread within a population that has minimal MHC diversity — that is, an inbred population, that has recently undergone a genetic bottleneck. Such a bottleneck very likely happened during the domestication of dogs, as they evolved from wolves. Was the original CTVT a tumor of one of the very first domesticated proto-dogs — an inbred proto-dog,⁸ a member of a small and inbred population — a spontaneous, fairly ordinary tumor that then spread to the first host’s MHC-matched neighbours in the small and inbred population? That would not be so mysterious; we know tumors can be transplanted between genetically identical hosts, that’s done with lab mice all the time. Then, as the domestic dog population expanded and spread over the world, the tumor would expand and spread along with it, and would have the opportunity to undergo natural selection, adapting to the newly-diverse MHC and to the immune responses that arose after the tumor did. We don’t know, in molecular terms, how the tumor has adapted — we don’t know why it’s able to spread in grow in spite of host immune responses. But at least we can now imagine how the tumor could have reached this point. Thirty thousand transmission events — a couple of million cell divisions — is a long time for a cancer to adapt.

1. Even most viral cancers are probably dead ends, because many viral cancers represent abortive viral replication.[↩]
2. DNA variation of the mammalian major histocompatibility complex reflects genomic diversity and population history.
   Yuhki N, O’Brien SJ
   Proc Natl Acad Sci USA (1990) 87:836–840.[↩]
3. Molecular Genetic Insights on Cheetah (Acinonyx jubatus) Ecology and Conservation in Namibia
   Laurie L. Marker, Alison J. Pearks Wilkerson, Ronald J. Sarno, Janice Martenson, Christian Breitenmoser-Würsten, Stephen J. O’Brien, and Warren E. Johnson
   Journal of Heredity 2008 99(1):2-13; doi:10.1093/jhered/esm081[↩]
4. Genetic basis for species vulnerability in the cheetah.
   O’Brien SJ, Roelke ME, Marker L, Newman A, Winkler CA, Meltzer D, Colly L, Evermann JF, Bush M, Wildt DE.
   Science. 1985 Mar 22;227(4693):1428-34

   It was suggested at the time, and is widely believed, that cheetahs are particularly susceptible to infectious diseases for this reason, but I think the evidence for this is very weak. Cheetahs have high mortality when infected with feline infectious peritonitis virus, but it’s not unusual for viruses infecting new species to have a high mortality — think SARS and avian influenza in hunans, canine distemper in seals, and feline (or something) parvovirus in dogs, all of which had high mortality in their new, MHC-diverse, host[↩]

5. Rebbeck, C., Thomas, R., Breen, M., Leroi, A., & Burt, A. (2009). ORIGINS AND EVOLUTION OF A TRANSMISSIBLE CANCER Evolution DOI: 10.1111/j.1558-5646.2009.00724.x[↩][↩]
6. Murgia, C., Pritchard, J. K., Kim, S. Y., Fassati, A., and Weiss, R. A. (2006). Clonal origin and evolution of a transmissible cancer. Cell 126, 477-487 .[↩][↩]
7. To be fair to Murgia, they pointed out that their 250-2500 year estimate was for the divergence of the tumor, not its origin, and they specifically raised the possibility that the tumor originated earlier than dogs.[↩]
8. Reddeck et al. point out that the tumor did, in fact, arise from an inbred animal[↩]

You can go to the most prestigious medical center in the world and ask “How is my immune system?” and, after a short period of eye rolling and looks of amused incomprehension, you might (if they don’t just throw you out) be offered a white blood cell count (which you should probably decline). … How did we arrive at this state of affairs? A good case can be made that the mouse has been so successful at uncovering basic immunologic mechanisms that now many immunologists rely on it to answer every question. … Well, except that mice are lousy models for clinical studies. This is readily apparent in autoimmunity (von Herrath and Nepom, 2005) and in cancer immunotherapy (Ostrand-Rosenberg, 2004), where of dozens (if not hundreds) of protocols that work well in mice, very few have been successful in humans. ¹

Cytotoxic T lymphocyte attacking a tumor cell

The above (emphasis added) is from a recent manifesto from immunology giant Mark Davis.¹ (He isn’t the first to make this point, of course, but when Mark Davis speaks, immunologists listen.)  Davis’s suggested solution was, among other things, to start using humans as their own models, taking advantage of the large numbers of humans who are routinely screened and overcoming the lack of experimental control by taking a “systems” approach, including large-scale data collection from healthy and ill people and large-scale informatics as part of the analysis.² (He also comments on the “humanized” mouse approach that I mentioned briefly the other day.)

Here’s an example of the power of human models, though it’s not exactly what Davis is describing.³  I’ve mentioned before the evidence that cancers in mice are controlled by the immune system (for example, here and links therein).  In those experiments, mutant mice, lacking one or more components of the immune response, were shown to be predisposed to cancer.  There are also a couple of human studies that indicate the same thing; people on long-term immunosuppression (as in transplant recipients) are somewhat more likely to get certain kinds of cancer, for example.

CTL attacking a tumor cell

One advantage of using humans as models of their own diseases is that there are an amazing number of well-documented mutations and disease-associated genes in humans.  Human disease is taken very seriously, it’s well funded (at least in comparison to, say, dog and cat disease); even diseases that are very rare can be identified in humans and the gene variant identified.  One such disease is Type II familial hemophagocytic lymphohistiocytosis (FHL), a rare, rapidly fatal disease caused by mutations in the perforin gene. ⁴  Perforin is important in cytotoxic T lymphocyte function, and it’s one of the immune genes that has been shown to be important in preventing cancers in mice.⁵

Are patients with Type II FHL at risk of developing cancer, like mice with targeted perforin mutations?  It’s not an easy question to ask, because most patients die relatively young.  But because these are humans, this very rare disease is nevertheless well documented, and the authors were able to search the literature for a suitable subset of patients:

… we identified a subgroup of individuals from nonconsanguineous families who possessed 2 mutated PRF1 alleles but whose onset of FHL was markedly delayed (the age at onset of 10 years or older) or even abolished. A total of only 23 such cases could be identified in the entire literature …  Ten of the individuals (Patients 14–23 inTable 1) developed manifestations of FHL without any other significant infectious or neoplastic sequelae reported. … Remarkably, in 11 of these 13 individuals (or 48% of the entire cohort of 23), the primary clinical presentation was with either B or T cell lymphoma or acute or chronic leukemia of lymphoid origin. … The very high frequency of hematological cancers in this 23-patient cohort …  is vastly in excess of that in the general population.³

There’s a lot of other interesting stuff in the paper, but this is enough to make the point: Just as in mice, perforin in humans (and therefore, the immune system) is important in preventing cancer.

I’ll leave with this now-familiar observation from the paper:

It is clearly problematic to extrapolate experimental data from inbred mouse strains to an outbred human setting where such evidence is far more difficult to gather.³

1. DAVIS, M. (2008). A Prescription for Human Immunology Immunity, 29 (6), 835-838 DOI: 10.1016/j.immuni.2008.12.003[↩][↩]
2. Again, of course, Davis isn’t the first to advocate this approach.[↩]
3. Chia, J., Yeo, K., Whisstock, J., Dunstone, M., Trapani, J., & Voskoboinik, I. (2009). Temperature sensitivity of human perforin mutants unmasks subtotal loss of cytotoxicity, delayed FHL, and a predisposition to cancer Proceedings of the National Academy of Sciences, 106 (24), 9809-9814 DOI: 10.1073/pnas.0903815106[↩][↩][↩]
4. Familial hemophagocytic lymphohistiocytosis. Primary hemophagocytic lymphohistiocytosis.
   Henter JI, Aricò M, Elinder G, Imashuku S, Janka G.
   Hematol Oncol Clin North Am. 1998 Apr;12(2):417-33[↩]
5. Perforin-mediated Cytotoxicity Is Critical for Surveillance of Spontaneous Lymphoma.
   Mark J. Smyth, Kevin Y.T. Thia, Shayna E.A. Street, Duncan MacGregor, Dale I. Godfrey, and Joseph A. Trapani.
   The Journal of Experimental Medicine, Volume 192, Number 5, September 5, 2000 755-760[↩]