http://www.ncbi.nlm.nih.gov/pubmed/20388779
"Despite the substantial evidence that MMTV-like virus may have a role in human breast cancer, the development of conclusive evidence has been elusive (2). The reasons include the difficulty in detecting the low levels of MMTV in human breast cancers and concern that the main investigative tool (PCR) may be confounded by false-positive and false-negative results due to sequence variations that affect primer or probe binding"
"Seventy percent of the complete MMTV-like virus genome, identified in human breast cancer specimens and viral particles from human breast metastases, have been sequenced and shown to display 91% to 99% homology to MMTV from mouse mammary tumors. In a recent study, env and long terminal repeat sequences with >98% homology to those of MMTV have been identified in breast cancers that had occurred in a mother, father, and daughter of the same family, living under the same roof "
"A phylogenetic tree was constructed using the sequence data that was obtained (Fig. 3). The phylogenetic tree shows that the MMTV-like virus env gene sequences from human breast cancer, liver diseases, and MMTV from inbred and wild mice interspersed with each other and did not group separately. This confirms the very high homology between MMTV env viral sequences identified in wild and inbred laboratory mice, human liver diseases, and human breast cancer from the United States, Australia, Italy, and Mexico. This indicates that the same MMTV-like virus env sequences are present in mice and humans"
Histologic assessments are also subjective and any similarities may be due to other causes and not necessarily related to MMTV. Again on the other hand, some MMTV-positive IDC breast cancer specimens are so similar to MMTV-positive mouse mammary tumors that at a cellular level, it is not possible to distinguish between the human and mouse tumors. In our opinion, the homology comparisons are probably valid because of the identification of nearly identical MMTV env sequences in breast tumors in women from many different populations with the PCR analyses conducted in different laboratories.
The phylogenetic analyses are in accord with those of Etkind and colleagues (7) who has clearly showed that MMTV-like long terminal repeat nucleotide sequences identified in human breast tumors, human nonHodgkin's lymphomas, and human primary biliary cirrhosis tissues do not cluster as two distinct MMTV species. However, full-length MMTV-like virus sequences from humans have to be compared with full-length MMTV in mice before definitive conclusions can be made about the two viruses being the same, although the envelope genes of retroviruses are known to show the most sequence divergence.
Our current observations in DCIS and IDC breast cancer specimens validate the previous experimental evidence that MMTV can infect, multiply, and randomly integrate into the DNA of normal human breast epithelial cells (810). Together, these data suggest a possible causal role for MMTV-like virus in some human breast cancers. A minority (25%) of invasive human breast tumors have very high levels of similarity to both human medullary type breast cancers and mouse mammary tumors. Therefore, it is possible that MMTV-like virus is associated with this small proportion of human breast tumors despite the consistent finding that MMTV-like virus sequences are present in ?40% of breast tumors."
