A new study has reported that mouse mammary tumor virus (MMTV) sequences in human breast cancer may have cancer-related activities. Mouse mammary tumor virus sequences have been found to be present in some human breast cancers, but it has not been determined whether they have any causal role. MMTV promotes the development of mammary tumors in mice — insertion of MMTV proviral DNA in Wnt oncogenes lead to their activation.

In the study, the authors examined the status of Wnt-1 in human breast cancers harboring mouse mammary tumor virus-like sequences encoding viral envelope (env) genes. The protein encoded by Wnt-1 is almost identical in human beings and mice. The authors found env sequences in the nucleus of cells from human breast cancer specimens that are similar in appearance to MMTV env sequences. Wnt-1 expression was also found to be higher in specimens of env-positive ductal carcinoma in situ (DCIS) and invasive ductal carcinoma compared with env-negative specimens. The authors comment that the study findings extend the evidence that MMTV sequences found in naturally occurring mouse mammary tumors can be found in some human breast cancers, indicating that further evaluation of possible causal roles is warranted.