Mystery Rays from Outer Space

Meddling with things mankind is not meant to understand. Also, pictures of my kids

August 21st, 2009

Vertical transmission of tumors

Pregnant woman (Ivory Coast)
Pregnant woman (Ivory Coast, West Africa)

Recently I’ve mentioned a few cases of transmissible tumors — that is, cases where tumors actually spread from their original host, to other individuals. The two most dramatic transmissible tumors are Canine Transmissible Venereal Tumor (CTVT) and Tasmanian Devil Facial Tumor (TDFT), where the original tumor can spread widely throughout the entire species. (See this post, this one, and this one, for more detail.) There’s also at least one case of a tumor that accomplished a single transmission, from the original patient to the surgeon who operated on him. 1

Tumors aren’t supposed to be able to spread in this way, because they’re essentially foreign transplants — they should be rapidly rejected, as if they were, say, a skin graft between two random people. In this post I talked about how CVTV and TDFT might have arisen. (There are also a number of cases of tumors that spread to immuno-suppressed individuals, such as after organ transplants, but those cases are easier to understand from an immunologic viewpoint.)

When checking up on references for the last post, I ran across a set of transmissible tumors I hadn’t known about: Vertically transmitted tumors, in which tumors spread from a pregnant mother to the fetus in utero.2

This also, I’m glad to say, seems to be very rare, as you’d expect. Even though mother and child are partially tissue-matched, and even though pregnancy is a very special situation, immunologically, the parent and her child are not genetically identical, and should reject grafts from each other pretty efficiently. (Transplants from parent to child still require immune suppressive treatment.) The review I ran across lists a total of 14 cases of vertical spread of tumors, from 18663 to 2002.4 Although they do note that:

Given the lag time between birth and diagnosis in several of the infants, cases of maternal–fetal transmission may not be as rare as the literature would suggest, and the number of cases could be higher as the detection of metastatic tumor in the fetus may go undetected in cases of abortion or maternal–fetal demise. 2

Malignancy during pregnancy isn’t all that uncommon (0.1% of pregnancies, it says here), so the handful of cases with actual spread of the tumor to the fetus are “numerically inconsequential”. What was different about these 14 cases? We don’t really know, in general. Almost all of the described cases are earlier than 1965,5 predating the molecular era of medicine. Perhaps some, or many, of the infants were immune compromised, as the authors note:

Fetuses with a congenital immunodeficiency are likely to be at an even higher risk for the engraftment of such tumor cells.6 Other factors that may affect the likelihood of tumor cells entering the fetal circulation include maternal homozygosity for one of the fetal HLA haplotypes,7 metastatic potential of the maternal tumor, and a high maternal blood and/or placental tumor load. 2

The outcome of this transmission was very poor; only 3 of the 14 children survived the disease.

I don’t really have any lesson to draw from these cases. Without an extensive molecular workup that isn’t available for almost all of these cases, I don’t know that we can learn much about tumor transmission. Still, these stories are worth keeping in mind when thinking about mechanisms of tumor transmission.

  1. Gartner HV, Seidl Ch, Luckenbach C, et al. Genetic analysis of a sarcoma accidentally transplanted from patient to a surgeon. N Engl J Med 1996;335:1494–1496.[]
  2. Tolar J, & Neglia JP (2003). Transplacental and other routes of cancer transmission between individuals. Journal of pediatric hematology/oncology : official journal of the American Society of Pediatric Hematology/Oncology, 25 (6), 430-4 PMID: 12794519[][][]
  3. Friedreich N. Beitrage zur pathologie des Krebses. Virchows Arch 1866; 36:465–477.[]
  4. Tolar J, Coad JE, Neglia JP. Transplacental transfer of small cell carcinoma of the lung. N Engl J Med 2002; 346:1501–1502.[]
  5. Not saying the molecular medicine abruptly switched on in 1965, it’s just a convenient cutoff[]
  6. Pollack MS, Kirkpatrick D, Kapoor N, et al. Identification by HLA typing of intrauterine-derived maternal T cells in four patients with severe combined immunodeficiency. N Engl J Med 1982; 307:662–666.[]
  7. Osada S, Horibe K, Oiwa K, et al. A case of infantile acute monocytic leukemia caused by vertical transmission of the mother’s leukemic cells. Cancer 1990; 65:1146–1149.[]
August 14th, 2009

On cancer mortality

[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

Cancer mortality by birth cohort

All-site cancer rates in successive birth cohorts by age of death.
Mortality rates for decadal birth cohorts between 1925 and 2004 are plotted by age at death.


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[][][]
August 13th, 2009

Why aren’t most tumors transmissible?

Canine Venereal Tumor phylogeny
Canine Venereal Tumor phylogeny

Bayman commented, after reading this post:

So isn’t the real question why can’t all tumors be transmissible? If you believe the tumor immunologists, all tumors should be capable of avoiding T cell attack…no??

I don’t have answers, but I can speculate a little. 1

Very quick background: In general, tumors are unique. They arise independently each time, and when their host dies, the tumor dies too. That’s in contrast to pathogens, whic may or may not kill their hosts, but which survive and are transmitted to a new host; pathogen infections are not unique, they have a long evolutionary history reaching back through many individual hosts. Tumors can’t do this, for the same reason that skin grafts are rejected by unrelated animals — tumors are essentially unrelated grafts, and should be very rapidly rejected by the new host.

But in very rare circumstances — there are two known instances, and a couple of other possible ones — tumors have arisen that can be transmitted from one host to another.  The two cases are canine transmissible venereal tumor, and Tasmanian Devil facial tumor.  There have been suggestions that these tumors are unique in some immunological way, but I am not convinced by those arguments: See this post and this one for more background.  That’s not to say that these tumors have no ways of evading the immune system; what I am saying, is that virtually all tumors have some way of evading the immune system, and the functions that have been convincingly described for the transmissible tumors don’t seem all that exceptional for tumors in general.

So, if these tumors can be transmitted, and they aren’t all that extraordinary immunologically, what does make them extraordinary?  As I say, I don’t know, but based on tumor immunology as I understand it, I can make some guesses.

The most important factor, I suspect, has nothing to do with immunology.  These tumors are unusual in that they have a built-in way of contacting new hosts. TDFT is spread through bites, CTVT is spread sexually.  There’s no similar way that, say, a liver tumor, or a brain tumor, could be spread.  So that immediately rules out the vast majority of tumors; even if they could survive after transmission, there’s no chance of a transmission chain. 2  But still, most tumors would be rejected even if they did manage to be transmitted.

The Three E's of tumor immunity
The Three E’s of tumor immunity

What seems to happen with most tumors3 is that proto-tumors appear quite early, but are controlled by the immune system – perhaps for years — and never become detectable.  Many such proto-tumors are completely eliminated by the immune system, and we have no way of telling that they even existed.  Many more are controlled at the half-dozen cell stage, much too small to detect; they aren’t eliminated by the immune system, but they can’t escape and grow either.  A very small percentage of these equilibrium tumors, though, eventually find a way of at least partially escaping from immune control, and begin to grow. (Perhaps the immune system kills 99% of the new cells, but a 1.01% growth rate compounds itself fast enough to be eventually detectable.)  This is the “Three E’s” theory of tumor growth (discussed more here and here) — “Elimination, Equilibrium, Escape”.

Regulatory T cells
Regulatory T cells and cancer

The Three E’s apply to the very small proto-tumors. But there is probably another factor that kicks in once the tumor becomes larger.  Tumors are themselves immunosuppressive — they shut down immunity throughout the entire body, to some extent, but they shut down immunity to themselves very powerfully.  The immune system has powerful safeguards that prevent it from attacking its own body; broadly speaking, tumors are their own body, and in many cases tumors probably also have been selected to massively amplify the normal protective signals. 4 (See this post, and this one, for more on that.)

So here5 is my speculation.  We suspect that the ability to be transmitted is present in several tumors, but they never get the opportunity to transmit.  Of those rarities that do get transmitted, most are rapidly rejected, as foreign grafts.  But a tiny minority of this minority may be able to survive because they have powerful immune suppression abilities on top of their common immune evasion abilities.

Tasmanian Devil crossing
Why did the Tasmanian Devil cross the road?

Were CTVT and TDFT just lucky — just happened to have the right immune suppressive abilities?  I don’t think so.  I think they were in the right place at the right time.  They were tumors that had a mechanism for transmission, and that had some ability to immune suppress, but they would normally have been rejected as foreign grafts.  Except that both of these tumors, I think, arose at a time and place where their population was highly inbred.  CTVT arose, we speculate, as dogs were becoming domesticated; probably a small, inbred, closely-related population.  Tasmanian Devils in general may not be closely related, but I suspect there are sub-populations6 that were closely related and that would not have rapidly rejected skin grafts.  The early version of the respective tumors would not have been rapidly rejected by these closely-related new hosts, giving them a chance to establish their own immune suppressive regime.

Now we have the chance for natural selection of the tumors.  Variants with more powerful immune suppression could spread to a wider range of hosts; variants with standard immune suppression died out with their victims.  In dogs, this natural selection could occur over time; as dogs became gradually more variable, there would be continuous new selection for new tumors that could keep up with the dogs. 7 With the Devils, the selection would be over space: The tumors would be selected for their ability to spread within new sub-populations of the Devils, perhaps through gradually more distantly-related subgroups. Eventually, we see the tumors as being capable of transmission and growth throughout the entire population, but the original tumor might not have had this ability.

Channel Island Fox
Rapid MHC diversity in Channel Island Foxes

This model suggests that humans are probably not at great risk of having a transmissible tumor spread in us; and the same is true for most species.  You need the combination of an inbred sub-population with a mechanism of tumor spread and the right kind of tumor. And inbred populations are usually a transient thing; MHC becomes diverse very rapidly, and then the window for tumor establishment is closed.

But this is just a guess, so don’t be too comforted.

  1. And by the way, I disagree with Bayman’s suggestion here (“Tumor Immunology Is A Waste of Time”) that he “find[s] it impossible to believe that effective therapy will ever achieved by artificially stimulating the immune system to attack weak and largely self antigens.” But this post is already too long, so I’ll save my answer for another time.[]
  2. There’s at least one case of a surgeon who apparently contracted a patient’s tumor after cutting himself during surgery — given the option, perhaps many more tumors could be transmissible, but don’t get the chance.[]
  3. Not necessarily those induced artificially, with high doses of carcinogens or with powerful oncogenes, but with those that arise naturally, in older individuals[]
  4. I suspect that tumors have many ways of achieving this localized immune suppression.  I also suspect that different tumors have different dependence on this localized immune suppression.  Those tumors that were highly successful as proto-tumors might already be very good at avoiding immunity — for example, they may secrete tons of TGF?, or otherwise have very powerful TReg-inducing abilities — and only need to shut down a little.  Those that barely squeaked by as proto-tumors, may have very potent immune suppression.  I don’t think the mechanisms for this tumor-based immune suppression are very well understood, though over the next couple years they probably will be. []
  5. Finally![]
  6. Subpopulations that are now, probably, extinct, because of the tumors[]
  7. I’m told that CTVT is eliminated faster or slower in different dogs.  It would be very interesting to correlate this with MHC types, to see if there’s still some effect of rejection even after 50,000 years of selection on these tumors.[]
August 5th, 2009

How many human cancers are caused by viruses?

Merkel cell carcinomaWe know that viruses cause a significant minority of human cancers, but we don’t know quite how many, or which, cancers are viral. It’s not as easy as you might think to tell.

The link between viruses and cancer was one of the major breakthroughs in cancer biology, but you could also make a case that that link set cancer research back several years. (Cancer viruses were shown, in chickens, in 19111, but it wasn’t until the 1960s that interest in the concept took off, with Epstein’s demonstration of Epstein-Barr virus (EBV)2 in some human tumors. ) Studying viral oncogenesis has led to huge advances in our understanding of the fundamental biology of cancers. The problem was that there was a general assumption, in the 1960s and 1970s, that viruses were directly responsible for the vast majority of human tumors. If so, all we needed to do was to identify the viruses, develop vaccines against them, and voila! No more cancer!

Of course, it wasn’t that easy. For all the fundamental advances from this concept, it’s been relatively unproductive as far as the bedside is concerned. Most human tumors are not caused by viruses,3 and even those that are, have been really resistant to treatment via direct anti-viral approaches.4 The research units that were established in the 1970s to look at the virus/tumor connection have mostly either disbanded, or taken a different direction now.

Mouse polyomavirus
Mouse polyomavirus

In spite of that, there’s still new and exciting stuff coming out. Most recently,5 Yuan Chang and Patrick Moore identified a new cancer-causing virus of humans. (This was their second breakthrough virus; in 19946 they also found the second-most recent cancer-causing virus of humans, Kaposi’s Sarcoma Herpesvirus.)) This brings to six the number of clearly-linked human cancer viruses: papillomaviruses, HTLV-1, hepatitis B virus, EBV, Kaposi’s Sarcoma Herpesvirus, and the new one, Merkel cell polyomavirus.

I keep meaning to talk about the discovery of Merkel cell polyomavirus, but I’ll set that aside for now. Very briefly, last fall Chang and Moore showed that the presence of the virus is linked to a fairly rare tumor, Merkel cell carcinoma, and they and others have now confirmed the epidemiological association and demonstrated a mechanism for causing tumors. We’re at the stage now of trying to understand the normal biology of the virus. As with most (and all human) cancer viruses, the virus is much more widespread than the tumor. How widespread it is? How does it spread? Does it cause other disease — in particular, is it involved in other tumors?

That last is a particularly interesting question, because although Merkel cell tumors are rare, there are a number of common tumors that could, conceivably, also be caused by the virus; and if the virus causes even a subset of those, then it could be a common cause of cancer rather than an unusual one. So far, though, I think the evidence suggests that the virus is fairly limited in the damage it causes; quite a few groups have looked for the virus in other types of cancer, and although it may be occasionally found7, that most likely reflects general background infection with the virus rather than a causative role.

One exception is a recent paper8 which suggested that the virus might be associated with a subset of squamous cell carcinomas. SCC are a pretty common form of skin tumor, so if MCPyV is a cause of even a subset of SCC it might be a significant cause of cancer.

The problem is that the other studies on MCPyV have shown that it’s out there even in normal, non-diseased humans9 — as you’d expect; the virus must be able to spread and circulate within the human population somehow, and the version of the virus found in Merkel cell tumors is damaged and likely can’t spread, so there must be virus replicating in normal tissues. In this paper, the authors find the virus in a subset of SCC cancers, but I would like to know how many they’d find in normal human skin using the same techniques —  the 15% of positive tumor samples may or may not be significantly different. On the other hand, the SCC-associated virus did show a similar molecular signature to that found in Merkel cell cancer,10 suggesting a causative role. Right now, I’m not completely convinced, but am definitely intrigued.

One really interesting point to add is that — if MCPyV really does cause a significant number of tumors — then it’s been missed all these years, despite high interest in searching for human cancer viruses, until new techniques were applied to the right samples in the right way. Are there other human cancer viruses out there, waiting for the right technique? Is it still possible that most human cancers are caused by viruses? I think that’s pretty unlikely, but the door is still open a crack.

  1. A sarcoma of the fowl transmissible by an agent separable from the tumor cells. Rous, P. 1911. J. Exp. Med. 13:397–411.[]
  2. Virus particles in cultured lymphoblasts from Burkitt’s lymphoma. MA Epstein, BG Achong and YM Barr. Lancet 1 (1964), pp. 702–703.[]
  3. The usual educated guess is 20%[]
  4. One major exception, of course, being papilloma virus vaccines, which are a direct outcome of this line of research. But it’s taken a long time to come to fruition. You could also point to vaccination against hepatitis B virus, and a number of veterinary vaccines, as clinical advances arising from the virus/cancer research. But I think it’s fair to say that the energy put into this has been fundamentally, but not clinically, well spent.[]
  5. Feng, H., Shuda, M., Chang, Y., & Moore, P. (2008). Clonal Integration of a Polyomavirus in Human Merkel Cell Carcinoma Science, 319 (5866), 1096-1100 DOI: 10.1126/science.1152586[]
  6. Identification of herpesvirus-like DNA sequences in AIDS-associated Kaposi’s sarcoma.
    Chang Y, Cesarman E, Pessin MS, Lee F, Culpepper J, Knowles DM, Moore PS.
    Science. 1994 Dec 16;266(5192):1865-9.[]
  7. For example, Human Merkel cell polyomavirus infection I. MCV T antigen expression in Merkel cell carcinoma, lymphoid tissues and lymphoid tumors.
    Shuda M, Arora R, Kwun HJ, Feng H, Sarid R, Fernández-Figueras MT, Tolstov Y, Gjoerup O, Mansukhani MM, Swerdlow SH, Chaudhary PM, Kirkwood JM, Nalesnik MA, Kant JA, Weiss LM, Moore PS, Chang Y.
    Int J Cancer. 2009 Sep 15;125(6):1243-9[]
  8. Dworkin, A., Tseng, S., Allain, D., Iwenofu, O., Peters, S., & Toland, A. (2009). Merkel Cell Polyomavirus in Cutaneous Squamous Cell Carcinoma of Immunocompetent Individuals Journal of Investigative Dermatology DOI: 10.1038/jid.2009.183[]
  9. Human Merkel cell polyomavirus infection II. MCV is a common human infection that can be detected by conformational capsid epitope immunoassays.
    Tolstov YL, Pastrana DV, Feng H, Becker JC, Jenkins FJ, Moschos S, Chang Y, Buck CB, Moore PS.
    Int J Cancer. 2009 Sep 15;125(6):1250-6[]
  10. Shuda, M., Feng, H., Kwun, H., Rosen, S., Gjoerup, O., Moore, P., & Chang, Y. (2008). T antigen mutations are a human tumor-specific signature for Merkel cell polyomavirus Proceedings of the National Academy of Sciences, 105 (42), 16272-16277 DOI: 10.1073/pnas.0806526105 []
July 15th, 2009

Transmissible tumors: Similar after all?

Tasmanian Devil road signA 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.)

Orphan Tasmanian DevilCancers 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.

Tasmanian Devil skeletonThe 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.[]
July 10th, 2009

On cancer immunogenicity

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)

July 6th, 2009

Origins of an infectious cancer

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

Dog and wolf phylogenyThe 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[]
June 24th, 2009

Humans as models of human disease

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

CTL attacking a tumor cell
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
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[]
May 26th, 2009

Why (some) similar tumors are similar

Caco2 colon carcinoma cells
Caco2 colon carcinoma cells

One of my long-standing questions now has at least a partial answer, or maybe a pathway toward a partial answer.

My question was, “Why are different tumors the same?” That is, why do tumors of the same type often seem to have similar immunological changes?

Viruses of the same kind (all herpes simplex viruses, say) all avoid immunity in the same way because they all have a common ancestor from which they inherited their immune evasion molecules. But tumors have no common ancestor; each tumor has to grope around and find its own solution to common problems independently. So why should tumors of a particular tissue converge on common solutions? What would make a certain pathway a simple solution for colon carcinomas, but not for bladder tumors? And yet, apparently that’s what happens. Even though many different tumor types become non-immunogenic, tumors of a particular tissue type often reach non-immunogenicity by a similar path. For example:

… distinct molecular events underlie HLA class I loss, depending on the aetiology of the tumours; Lynch syndrome-related cancers presented with mutations in the β2-m molecule, while sporadic microsatellite-unstable tumours mainly showed alterations in the antigen-processing machinery components 1

When I posed this question a couple of months ago, I didn’t have an answer. I said

Part of the answer may be that the particular oncogenes associated with different tumor types lead to particular transcriptional hot-spots, and being a transcriptional hot-spot makes the region a mutational hot-spot as well, but at least as I understand it that’s not enough to account for the trends.

EH, in the comments, suggested a couple more possibilities: Perhaps destruction of a particular pathway is “a nice side benefit of destroying some yet undiscovered tumor suppressor important for melanoma or colon cancer? Or maybe the loss of certain repair genes common to a cancer type (like BRCA) leads to selective chromosomal instability in parts of chromosome 6?

It turns out, unsurprisingly, that I’m not the only one to ask the question. There is no general answer, but apparently a partial answer has been kicking around for a while now, and Hans Morreau’s group is starting to put some of the pieces together.2 As their model, Morreau’s group looked at MUTYH-associated polyposis (“MAP”)-associated tumors. (MAP is a heritable defect in DNA repair, so MAP patients have high rates of mutagenesis and usually develop colon carcinomas.) They reasoned that

MAP tumours could be more prone to stimulate a cytotoxic T-cell-mediated immune response, due to their frequent generation of aberrant peptides. Hence, these tumours could also be subjected to a strong selective pressure favouring the outgrowth of cancer cells that acquire an immune evasive phenotype.

DNA Repair
DNA Repair

In other words, the argument is that the group of tumors that lose (or reduce) DNA repair are much more likely to throw out mutant proteins. These mutant proteins are targets for the immune response (because they’re no longer “self” antigens), so to survive the immune attack the tumor has a strong selection for loss of immunogenicity (and also has the high mutation rate that allows them to rapidly mutate away from immunogenicity).

Sure enough, the tumors did frequently (72%) have defects in antigen presentation. (In fact, because of the way they measured HLA expression — by immunohistochemistry rather than sequencing — I would bet that the rate of functional defects was actually much higher than that.) They conclude that this “provides additional evidence that tumours carrying defects in DNA base repair mechanisms are more prone to undergo immune escape mechanisms.2  Since they don’t formally compare to other tumor types, I don’t think they can really say “more prone” — you’d have to use the same techniques to look at tumors from non-MAP patients to be able to say that. Still, I do think that is a significantly higher rate than has been turned up in previous studies using similar techniques,3 so I’ll tentatively accept that conclusion.

This doesn’t really explain why similar tumors target similar components, but it’s at least a conceptual connection between different tumor types and an underlying pathway. I’d be interested in a more large-scale screen, looking at cancers with stronger and weaker mutator phenotypes to see if common pathways emerge. One may already have popped up, since the authors note here that expression of β2-m was frequently lost4 in several tumors with DNA repair defects:

Although speculative, it is interesting to underline that carcinomas derived from both MAP and Lynch syndromes preferentially lose β2-m expression coupled to HLA class I deficiencies. A functional explanation for these observations remains elusive, but perhaps distinct reactions (both qualitative and quantitative) by the immune system, depending on the age of onset of the tumours, could condition the type of mechanisms that lead to HLA class I expression deficiencies. 2

Again, we would need to compare to other tumor types to see if this really is more frequent, but overall it feels as if there’s a hint of a pathway here. At least there are some specific questions that can be asked.

  1. de Miranda, N., Nielsen, M., Pereira, D., van Puijenbroek, M., Vasen, H., Hes, F., van Wezel, T., & Morreau, H. (2009). MUTYH-associated polyposis carcinomas frequently lose HLA class I expression-a common event amongst DNA-repair-deficient colorectal cancers The Journal of Pathology DOI: 10.1002/path.2569
    Dierssen JWF, de Miranda NFCC, Ferrone S, van Puijenbroek M, Cornelisse CJ, Fleuren GJ, et al. HNPCC versus sporadic microsatellite-unstable colon cancers follow different routes toward loss of HLA class I expression. BMC Cancer 2007; 7: 33[]
  2. de Miranda, N., Nielsen, M., Pereira, D., van Puijenbroek, M., Vasen, H., Hes, F., van Wezel, T., & Morreau, H. (2009). MUTYH-associated polyposis carcinomas frequently lose HLA class I expression-a common event amongst DNA-repair-deficient colorectal cancers The Journal of Pathology DOI: 10.1002/path.2569[][][]
  3. Other studies, especially those of Soldano Ferrone, have turned up much higher rates of functional HLA class I defects, but they’ve used more focused, and more difficult and expensive, techniques to reach that conclusion.[]
  4. β2-m is a physical component of the MHC class I complex[]
April 14th, 2009

Tumor immunity: The Goldilocks approach

GoldilocksWe know that the immune system can destroy tumors. We also know, unfortunately, that by the time we see a tumor, immunity probably won’t destroy the tumor. There are lots of reasons for that. One is that tumors are essentially part of the normal body, so it’s normal for the immune system to ignore them. It looks as if you need to have immunity that’s just right to get rid of a tumor.

Tumors arise from normal self cells,1 that the immune response has been programmed to ignore. Now, the process of becoming a tumor is not normal, and so tumors are not entirely normal self any more — meaning that there are likely to be some targets in most if not all tumors. But in all but the most reckless tumors the differences between abnormal and normal are relatively small, compared to, say, a virus-infected cell that contains many potential targets.

There’s actually a long list of known tumor antigens; the T-cell tumor peptide database lists many hundreds of them. But most are not truly specific for the tumor.  The’re actually normal self antigens; they’re derived from proteins that are overexpressed in tumors, or that are differentiation antigens or “cancer-germline” antigens that are normally also found in self tissues. What’s more, these normal self antigens are the most interesting tumor antigens, as far as clinical utility is concerned. Mutations can make brand-new, non-self targets for the immune system, but they’re going to be sporadic targets, often unique to individual tumors — not something you can prepare for. The normal antigens, though, are likely to be predictable, common targets; it’s conceivable that tumor vaccines can be prepared in advance.

Melanoma cell (Eva-Maria Schnäker, University of Münster)
Human melanoma cell

If these antigens were common (which they are, in some tumor types — like melanoma), and they were good targets for the immune system, then we wouldn’t see much cancer. We do see melanomas quite often, and part of the reason may be that the immune system generally responds quite weakly to these antigens.  Why is that? And, more to the point, how can we make the immune system respond more strongly? A recent paper in the Journal of Experimental Medicine2 offers answers for both of these questions.

From work in the past couple of years, we now have decent estimates of how many T cells there are that can react with any particular target. (See here and here for my discussion of the earlier papers.) A reasonably strong immune response to a non-self epitope might originate from maybe 100 or so precursor T cells. There’s a rather wide range of frequency for these precursor cells, say from 20 to 1000; and to some extent, the fewer T cells there are the weaker (the less immunodominant) the immune response.

We expect T cells against normal self targets to be less common, because they should be eliminated as they mature in the thymus. Some may survive, though, and we would count on these survivors to attack the normal (albeit overexpressed, or abnormally present) target in the cancer cells. But just how rare are they?

Rizzuto et al say they’re really rare (this was in mice, by the way); at least ten times less abundant than T cells against non-self antigens.  If you look at the range I gave for “normal” precursors, that could mean there are fewer than 5 or 10 precursors.  If the average is “fewer than five”, then quite possibly some mice have only two, or one, or no precursors.  You can’t have much of a response with no precursors.

So there’s a weak anti-tumor response because there aren’t many T cells in the body that can respond to the normal self targets in the tumor. That’s not really a surprise, but it does raise the question, What if there were more of the T cells? To ask that question, Rizzuto et al. tried transferring more of these precursor T cells into tumor-bearing mice — starting at around the normal level for a precursor to non-self antigen, and going up from there — and then vaccinating with the appropriate target.

The effects were pretty dramatic. With no supplemental T cells (that is, with the natural, very low, level of T cell precursors) the mice all died of the tumor quickly. At the middle of the range, almost all of the mice rejected the tumor. And at the highest levels of transfers? The mice all died again. Having enough T cells to respond was protective, but putting in too many made them useless.

These results identify vaccine-specific CD8+ precursor frequency as a remarkably significant predictor of treatment and side-effect outcome. Paradoxically, above a certain threshold there is an inverse relationship between pmel-1 clonal frequency and vaccine-induced tumor rejection.2

Melanoma cell
Mouse melanoma cell

(My emphasis) This paradoxical effect is probably because the numerous T cells started to compete with each other so that none of them were properly activated; they only saw effective-looking polyfunctional T cells at the lower transfer levels.

In other words, if you’re going to transfer T cells to try to eliminate a tumor, more is not necessarily better. Quality and quantity are both important factors, and quantity helps determine quality.

One question I have is how this relates to tumor immune evasion. Many tumor types  acquire mutations, as they develop, that block presentation of antigen to T cells. Are these mutations perhaps only partially effective — giving the tumors sufficient protection against the tiny handful of natural precursors they “expect” to deal with, but not against a larger attack after, say, vaccination — or are they more complete, and protective even if the optimal number of T cells are transfered? I’d guess that it would depend on the tumor, but it looks as if it might be a relevant question and it would be nice to have more than a guess.

Our results show that combining lymphodepletion with physiologically relevant numbers of naive tumor-specific CD8+ cells and in vivo administration of an effective vaccine generates a high-quality, antitumor response in mice. This approach requires strikingly low numbers of naive tumor-specific cells, making it a new and truly potent treatment strategy.   2

  1. I’m ignoring here crazy things like the contagious tumors of Tasmanian Devils and dogs[]
  2. Rizzuto, G., Merghoub, T., Hirschhorn-Cymerman, D., Liu, C., Lesokhin, A., Sahawneh, D., Zhong, H., Panageas, K., Perales, M., Altan-Bonnet, G., Wolchok, J., & Houghton, A. (2009). Self-antigen-specific CD8+ T cell precursor frequency determines the quality of the antitumor immune response Journal of Experimental Medicine, 206 (4), 849-866 DOI: 10.1084/jem.20081382[][][]