Viruses found in lung tumoursApril 24, 2008
Researchers have found evidence that two common viruses may be lurking behind some cases of lung cancer: human papilloma virus (HPV), already recognized as a cause of cervical cancer, and the measles virus.
The results, which will be presented today at the European Lung Cancer Conference in Geneva, Switzerland, are preliminary: while viruses have been found associated with lung tumours, there is no direct evidence that the viruses are actually causing the cancer. But the notion that a virus could contribute to some cases of the disease is a plausible one, says Denise Galloway, a virologist at the Fred Hutchinson Cancer Research Center in Seattle, Washington, who was not affiliated with the new studies.
As much as 20% of the world's cancers have been linked to infections. In addition to the connection between HPVs and cervical cancer, chronic infections by hepatitis-B and -C viruses contribute to liver cancer, and the bacterium Helicobacter pylori has been associated with stomach cancer.
In February, researchers reported viral genome sequences found in an aggressive form of skin cancer called Merkel cell carcinoma1, although it remains to be seen precisely how the virus contributes to skin cancer, if at all. And some have proposed that a virus similar to the 'mouse mammary tumor virus' - which causes breast cancer in mice - could also be associated with breast cancer in humans2.
"There's reason to believe that there are more tumours that have a viral origin," says Galloway. "I think there's growing interest in this field."
Determining whether there is an infectious cause of a cancer can have an effect on treatment and prevention strategies. Widespread use of a vaccine against HPV, for example, has been proposed to lower rates of cervical cancer. Though smoking is clearly still the main cause of lung cancer, identifying other contributors to the disease could help keep incidence down.
Samuel Ariad of the Soroka Medical Center in Beer Sheva, Israel, and his colleagues began by analyzing tumours taken from 65 lung cancer patients. They found evidence of measles virus proteins in about half of their samples.
The work provides the first hint that the measles virus may contribute to lung cancer, and follows on Ariad's previous studies showing a possible association between the measles virus and a cancer of the lymph tissue called Hodgkin's disease3.
In addition, Arash Rezazadeh of the University of Louisville in Kentucky and his colleagues tested 23 lung cancer tumours for HPVs. In five cases, the samples tested positive for the virus's DNA. Others have previously shown a possible link between the virus and lung cancer, but, as in this case, have relied on small sample sizes.
The cancer connection
The studies leave a number of unanswered questions. Ariad speculates that the measles virus may not be the root cause of lung cancer, but may somehow facilitate progression of the disease once it has already started. But it is difficult to understand how the virus would achieve this and why it would act in the lungs, notes Galloway.
Nevertheless, the risk of lung cancer rises dramatically in patients whose immune systems are not functioning well: a hallmark that Galloway looks for when gauging whether a cancer may be caused by infectious agents. Although overall cancer risk rises when the immune system fails, the link is stronger with some cancers - including skin, lung and cervical cancer.
The studies are preliminary, but warrant additional follow-up, says Dusty Miller, a virologist at the Fred Hutchinson Cancer Research Center in Seattle, Washington. "The problem is that these are associations," he says. "And there's no way that you're going to put those viruses into people to test them." Establishing a causal link between a virus and cancer - if there is one - can take decades.