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26 March 2011
This week’s question was submitted by Katie Funk of San Francisco, California:
Can cancer be caused by viruses?
Yes. Cancers can be caused by viruses. However, most cases are not, especially if you live in a first-world country – this is because most types of virus-caused cancer are preventable via vaccination or easily detectable in early stages.
Late-stage Hepatitis B and C infections can cause liver cirrhosis, which in many parts of the word is a primary cause of liver cancer (hepatocellular carcinoma). Vaccination against Hepatitis is mandatory for schoolchildren in most states in the US.
In the last few years, Human Papilloma Virus (HPV) has gained a lot of attention. There are approximately fifty known sexually transmitted HPV strains, most American adults are infected with at least one, and most are benign. There are two strains in particular of HPV that are known to cause cervical cancer, which used to be a leading cause of cancer mortality in the US. However, the rise in pap smears in recent years has lead to much earlier detection, better treatments, and increased survival rates. HPV vaccines are now available, and there are many elegant ethical debates concerning vaccinating young boys and girls against a sexually transmitted cancer-causing virus.
Right: a computer-generated rendition of the HPV virion.
There was a time when the prevailing theory among cancer biologists was that cancer viruses were common. Indeed, many of the early oncogenes (cancer-causing genes) were discovered via studying tumor viruses in birds. Peyton Rous, winner of the 1966 Nobel Prize in Physiology or Medicine, championed this theory. It turns out that tumor viruses are much more common in birds and rodents and apes and monkeys than in humans. While there are human viruses that can cause cancer, most cancer cases in humans are caused by carcinogenesis. In a classic blunder of brilliant hubris, Peyton Rous was famously wrong and vehemently opposed the carcinogenesis theory of acquired mutations over time. It is this history that I believe is partially to blame for the reverberating over-emphasis on viruses causing cancer.
Years later, Harold Varmus discovered that pre-cancer causing genes (known as proto-oncogenes) are present in the genomes of all of us. It was not until after Varmus’ discovery that the field fully backed the carcinogenesis model.
Right: Chronic Hepatitis infection can cause liver cirrhosis, which is a major source of liver cancer worldwide.
Bottom line: if you’re not vaccinated against Hepatitis B and C, get vaccinated. As for the HPV vaccines, they can be fairly expensive, but there are many indicators that suggest that the price will drop in the coming years, and many health insurance providers will cover much of the cost. Ask your physician about the current options. Regardless, pap smears allow for the detection of cervical cancer years before it becomes a malignant disease. When caught early, most cancers are treatable or even curable. The problem is that most of the time cancers are not caught early.
The best option is to not get cancer at all! Get vaccinated against Hepatitis and HPV if possible. Next week’s article will go into more detail about mitigating your risk of getting cancer with various prevention techniques.
Thanks for reading! Please feel free to submit your questions for topics.