HPV, or human papillomavirus, is most often thought of as the cervical cancer virus, but HPV is far more than that. There are over 150 types of HPV, more than 40 of which can be sexually transmitted, and these viruses cause everything from common skin warts to genital warts to cancer.
The number of cancers that have been found to be associated with HPV is on the rise. Sexually transmitted HPV infections have been shown to cause not just cervical cancers, but penile cancers , anal cancers, and throat cancers. Recent studies have also shown that there may be an association between HPV and lung cancer.
Smoking, of course, is a major cause of lung cancer in the Western world, but it is not the only cause of lung cancer. Asbestos, radon, and other inhaled toxins have also been linked to lung cancers, as have several infectious diseases. Furthermore, genetic, behavioral, and other factors also play a role in the development of lung cancer.
HPV & Lung Cancer
It should come as no surprise that scientists have hypothesized that HPV might be associated with at least a fraction of lung tumors. Not only is HPV known to cause cancer, but HPV-related cancers have been found in tissues that are adjacent to the lung -- such as the throat and tonsils. In other words, HPV both has access to lung cells and can make cells cancerous.
In fact, a number of studies have demonstrated an association between HPV and lung cancer, although the link remains highly controversial as the studies of various lung cancers have failed to show HPV DNA in their tumor samples. Although these results may seem contradictory, and make people question whether they should trust in science, there are several ways that this seeming controversy could be explained.
- HPV is more likely to be associated with lung cancer in certain parts of the world than in others.
This explanation is highly plausible as the prevalence of HPV types varies strongly by region, as does the prevalence of other cancers that have shown a much stronger relationship with HPV infection -- such as cervical cancer. It is also supported by the data from meta-analyses that have found that the results of HPV and lung cancer studies vary strongly by region but seem more consistent in particular areas. HPV-related lung cancers, as such, appear to be more common in Asia than in Europe. - The studies that have found HPV in lung cancer tumors have suffered from contamination with viral DNA.
Although this is certainly possible in some cases, the large number of studies that have found associations between HPV and lung cancer make a consistent problem with contamination unlikely. - The studies that have found no HPV in lung cancer tumors are not looking for HPV in the right way.
If scientists were looking for specific types of HPV in tumors and chose the wrong types to look for, or chose inefficient tests for the virus, this could explain how HPV could be missed in lung cancer samples. Similarly, since only a subset of lung cancers are likely to be HPV-associated, choosing the wrong cases to examine could also explain why no virus was found.
In all likelihood, it is the first explanation that will be shown to be true. Unlike with cervical cancer, where the vast majority of cases are caused by HPV infection, the studies that have shown an association between lung cancer and HPV have only found the virus in a fraction of tumors. The number of lung cancers that are associated with HPV varies strongly geographically and by type of tumor, but even the studies that show a strong association between lung cancer and HPV have rarely found the virus in more than 10-20 percent of samples.
This is an important reminder that most types of cancer can have a variety of different causes and a variety of different outcomes. While smoking remains prevalent across the globe, it will likely remain the predominant cause of lung cancer. However if, over time, fewer and fewer people smoke, then we might see that a larger percentage of the lung cancers that remain are associated with other causes -- including HPV.
That sort of causative shift has already been seen in throat and oral cancers, where a greater percentage each year seem to be associated with viral infection. Fortunately, that shift has also been associated with an increase in throat cancer survival, since HPV-related throat cancers appear to be less deadly than their tobacco-related counterparts. Whether a similar survival difference might also be true for HPV-related lung cancers remains to be seen.
Sources:
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