Debunking myths on genetics and DNA

We're all chimeras: roughly 10% of our DNA is made of ancestral viral sequences.
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Monday, February 20, 2012

Is cancer contagious? Sometimes. But it may not be a bad thing.


About 15% of all cancers worldwide are caused by infectious pathogens such as viruses, bacteria, or parasites [1]. Viruses that are capable of inducing cancer are called oncoviruses -- HPV is an example. The pathogen is transmitted from a donor to a recipient, starts the infection, and the infection eventually causes the cancer. But did you know there existed such a thing as a transmissible cancer? In this case, it's not the pathogen, but the cancer cell line itself that gets transmitted from one individual to another.

Yes, it's scary, but there are some good news.

For one thing, "relatively common" cases have been observed in animals only. Canine transmissible venereal tumor (CTVT) is quite common in dogs. It's transmitted during mating and eventually rejected by the host dog who then acquires lifelong immunity.
"In man, only scattered case reports exist about such communicable cancers, most often in the setting of organ or hematopoietic stem cell transplants and cancers arising during pregnancy that are transmitted to the fetus. In about one third of cases, transplant recipients develop cancers from donor organs from individuals who were found to harbor malignancies after the transplantation. The fact that two thirds of the time cancer does not develop, along with the fact that cancer very rarely is transmitted from person to person, supports the notion that natural immunity prevents such cancers from taking hold in man. These observations might hold invaluable clues to the immunobiology and possible immunotherapy of cancer [1]."
CTVT is particularly interesting to study because it has evolved some ingenious mechanisms to escape the immune system. Every nucleated cell has a class of molecules, called MHC, which have the function to display fragments of proteins that are "flags" as to whether the cell is healthy or harbors some pathogen. Once in the host, CTVT
"downregulates its MHC I expression, thereby reducing its initial visibility to the host's immune system. This allows it to not only to escape T-cell mediated immunity (which would occur if MHC I were fully expressed) but also natural killer cells (which would eradicate the cells were they completely devoid of MHC I)."
Despite this type of "defense", eventually the dog's immune system recognizes the pathogen and clears it, and understanding this mechanism is of interest for a possible cancer vaccine (I talked about cancer vaccines here).

A recent study published in Science [2] looked at two regions in the mitochondrial genome (mtDNA) from 37 CTVT samples, and compared them with sequences from the mtDNA of 15 hosts. Through phylogenetic analysis Rebbeck et al. showed a high variability in the sequenced regions, suggesting that CTVT periodically acquires mtDNA from infected hosts. The reason for this, the researchers hypothesize, is that CTVT mitochondria, due to a high metabolic rate, tend to accumulate deleterious mutations and therefore, transfers of mtDNA from the host may have the benefit of restoring CTVT mitochondrial function.

[1] Welsh, J. (2011). Contagious Cancer The Oncologist, 16 (1), 1-4 DOI: 10.1634/theoncologist.2010-0301

[2] Rebbeck, C., Leroi, A., & Burt, A. (2011). Mitochondrial Capture by a Transmissible Cancer Science, 331 (6015), 303-303 DOI: 10.1126/science.1197696

ResearchBlogging.org

6 comments:

  1. antisocialbutterflieFebruary 21, 2012 at 5:47 PM

    I'm pretty well versed in the role of chronic inflammation and tumorigenesis, the mechanism thought to induce cancer in infected patients. This I've never heard of before. That is pretty awesome. It looks like I have some reading to do.

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  2. thank you; and feel free to add if you feel like it; I was at a conference the past few days so this last post came out rather... sketchy... (sorry!)

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  3. So, is cancer contagious, or not? An old friend visited yesterday and told me he has prostrate cancer, then asked to use my bathroom(toilet). Then he told me his doctors think he has cancer of the esophagus. Then we shared lunch... I'm worried that I should disinfect my house and never invite my friend again. Please advise.

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  4. Sharing a toilet or lunch with a cancer patient will not give you cancer.

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  5. What about kissing, or having sex with someone who has cancer? o.o

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  6. I'm not a medical doctor and you should probably consult a medical doctor if you are concerned. What I do know is that human papilloma virus can be passed via kissing or sex and that's one of the viruses that can induce cancer. However, these days, a vaccine is available.

    Cancers in general are NOT transmissible via sex or kissing, though many viruses are.

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