We have previously reported sequences homologous to the env gene of Mouse Mammary Tumor Virus (MMTV) but not to human endogenous retroviruses in 38% of the human breast cancers studied, yet they were absent from others human tissues (Wang et al, Cancer Research 1995, 55: 5173). We have also found that normal breast tissue from env positive breast cancer patients was negative, suggesting that the sequences are of exogenous origin (Melana et al, Cl. Cancer Research 2001, 7:283). The sequences were also expressed in most of the positive breast cancer specimens as detected by RT PCR (Wang et al, Cl. Cancer Research 1998, 4:2565).Finally, the complete 9.9 Kb proviral sequence of an MMTV-like agent has been amplified and sequenced in two breast cancers (Liu, B. et al, Cancer Res 2001, 61:1754). Structural features of this provirus suggest that it is functional. We have now looked for the presence of viral particles in primary cultures (MSSM cells) of env sequence positive tumors. The cells expressed Env proteins as shown by immunofluorescence and western blot. FACS analysis indicated that between 10 to 20 % of the cells expressed the surface protein. Retroviral particles budding from cells as well as in particulate fractions from culture media were observed by electron microscopy. Particulate fractions also display reverse transcriptase (RT) activity and the presence of all viral genes as detected by RT-PCR. The RT activity peaked at densities characteristic of retroviruses in sucrose gradients. None of these properties were observed in similar studies with normal Human Mammary Epithelial Cells (HMEC) primary cultures or established normal breast cell line (MCF10F). Co-culture of MSSM virus-producing cells with HMEC, MCF10F, and B cells (Ramos) demonstrated transfer of viral sequences, as detected by PCR, and expression of Env proteins by western blot and FACS analysis in the recipient cells. Taken together, these findings support the identification of a human mammary tumor virus (HMTV) similar to MMTV in human breast cancer, which is able to infect human cells. Morphological cell transformation and tumorigenesis experiments are being performed.

[Proc Amer Assoc Cancer Res, Volume 47, 2006]