BioEd Online

The humble house mouse could be more dangerous than we thought, according to a study that suggests a rodent virus plays a role in the development of breast cancer. But the finding is contentious and reignites a long-standing wrangle about the potential causes of the disease.

US researchers found fragments of virus DNA in up to three-quarters of biopsy samples taken from women with breast cancer.

"This doesn't mean that the virus causes the cancer but it means there may be a link," says Stella Melana from Mount Sinai School of Medicine, New York, who co-authored the study. The DNA sequences were almost identical to those of a virus called MMTV, which causes breast cancer in mice.

The team looked for MMTV-like viruses in breast tumour biopsies taken from women living in six different countries. Their results are reported in Cancer online¹.

Over 70% of Tunisian samples contained fragments of the virus, compared with around 35% of samples from the United States, Italy, Australia and Argentina, and less than 1% of samples from Vietnam.

The researchers say that this roughly corresponds to the distribution of the virus-carrying house mouse, Mus domesticus, which is extremely common in North Africa but less so in the United States and elsewhere. In a previous study comparing healthy and cancerous breast tissue in the same patients, the researchers showed that the virus was only present in the cancerous cells.

Melana believes this could mean that humans acquire MMTV-like viruses from mice and go on to develop breast cancer.

But she is a long way from proving it. "I don't think you can draw valid conclusions from this study," says cancer expert Rob Newton of the Cancer Research UK epidemiology unit in Oxford.

The study only looked at 38 Tunisian patients, he points out, and further proof is needed to confirm that the virus is the cancer-causing culprit. It is important to spot whole virus particles, rather than fragments, inside the breast cancer samples. Then the virus needs to be isolated and proven to infect animals and cultured human breast cells.

Viral link?

Viruses have been linked to many types of tumour, such as those in cervical cancer and some types of leukaemia. But a connection with breast cancer remains controversial.

Environmental and genetic factors are thought to contribute to the disease, which affects one in nine women. Age is the most significant risk factor. But other factors, such as having children late in life, taking the contraceptive pill and drinking heavily, are thought to raise the risk of developing breast cancer. Faulty genes cause fewer than one in twenty cases.

Researchers postulated the virus link over 60 years ago, when they claimed to have found a mouse virus in breast tissue. Subsequent studies failed to back the finding, and the debate has rumbled on.