Humans are not the only creatures plagued by breast cancer. Mice get it too--and as scientists discovered several decades ago, they get it from a virus. The so-called mouse mammary tumor virus (MMTV) spreads from mothers to their offspring through breast milk. It integrates its genes permanently into the chromosomes of the cells it infects. And when the viral genes are activated, they can cause host cells to divide uncontrollably. Could something similar be happening in people? There is no evidence that breast-fed babies become cancer-prone adults, yet recent studies suggest that viruses may play some role in human breast cancer. Two American teams have recently found evidence of an MMTV-like virus in human breast tumors. And a new report from France suggests that the Epstein-Barr virus--a germ already linked to several human cancers--may play a role in breast cancer as well.
Most of us contract Epstein-Barr during childhood and carry it for life without ever suffering symptoms. But the virus causes mononucleosis in people who first encounter it as adolescents or adults. And when combined with other risk factors, it may promote cancers of the throat, stomach and lymph system. (Though it's widely suspected of causing chronic fatigue syndrome, the evidence implicating it remains sparse.) Past efforts to link EBV to breast cancer have brought mixed results, but the new study suggests a clear connection.
Writing in the Journal of the National Cancer Institute last week, researchers led by Dr. Irene Joab reported they had detected Epstein-Barr DNA in 51 of the 100 breast tumors they sampled--but in only 3 of 30 samples of healthy nearby tissue. What's more, the most aggressive tumors were the most likely to harbor the virus. The researchers don't claim to know what role EBV plays in breast cancer. It may help spawn tumors, or simply colonize them after they form. In either case, the virus could provide a useful target. In theory, at least, a drug or a vaccine aimed at the virus might also succeed at killing cancerous tissue.
Epstein-Barr is not the first virus to turn up suspiciously in cancerous breast tumors. Four years ago, researchers at New York's Mt. Sinai School of Medicine found that 39 percent of the breast tumors they analyzed contained a gene closely resembling one from the mouse virus, MMTV. In subsequent studies, the New York group has detected variants of all MMTV's genes in human breast tumors. Now researchers from Tulane University are reporting similar findings. Speaking at a scientific meeting this month, Tulane virologist Robert Garry reported that he and his colleagues had spotted an MMTV-like gene in up to 80 percent of the breast tumors they analyzed. The two teams may be homing in on different viruses--the New York group has found viral markers only in tumor cells, while the Tulane group has found them in healthy tissue as well--but both are calling their discoveries the "human mammary tumor virus," or HMTV. Alas, it now appears that several bugs may be worthy of that name.