The Biology Behind Mouse Mammary Tumor Virus-Like Viral Infection and Human Breast Cancer

In the last half of the 20th century the possibility that human breast cancer might be intimately associated with oncogenic viruses was often suggested. This followed naturally on Bittner’s demonstration (1) that a virus caused breast cancer in mice. Convincing data, however, eluded workers before the molecular biological revolution. Robust evidence has now been presented in a series of papers demonstrating that a sequence highly homologous to a unique 660-bp segment of the mouse mammary tumor virus (MMTV) envelope gene is present in 38% of American women’s breast cancer (2, 3). The sequences were found to be expressed in most of the positive tumors (4). Subsequent polymerase chain reaction amplifications have demonstrated other gene segments homologous to their MMTV counterpart, but not to the endogenous retrovirus K10 most similar to MMTV (5). A 2.7-kb sequence of env and long terminal repeat (LTR) genes has been visualized by fluorescent in situ hybridization inserted into several chromosomes (ref. 6; Fig. 1). This too mimics MMTV, which lacks an oncogene and evokes neoplasia by its impact on genes in the near vicinity of its insertion site. Indeed, we have detected the entire viral genome from fresh breast cancers (6) and from particles isolated from their primary cultures. The virus contains hormoneresponsive elements, which may account for the virulence of breast cancer in pregnancy and lactation; 61% of breast cancers developing during pregnancy and lactation contain the envelope sequence (7). The human isolates also contain superantigen sequences, which are functional and may contribute to pathogenesis (8). We have visualized budding virus from infective breast cancer membranes and presented preliminary data that the virus can be transmitted in vitro (9). Because of its human origin and some pervasive differences from MMTV in its LTRs, we refer to the agent as human mammary tumor virus (HMTV; 10). The viral sequences are found in breast cancer but not in normal breast tissue, thus precluding their germ-line transmission and establishing that horizontal transmission has taken place (11). Our original report of a MMTV-like segment of the env gene in human breast cancer lead Stewart et al. (12) to a provocative possible explanation of the well-known difference of breast cancer incidence in Western and Eastern Europe. West of a narrow north-south swath through central Europe, the exclusive mouse species is Mus domesticus, the common house mouse with abundant MMTV in its genome and a high incidence of breast cancer. East of the swath, Mus musculus and Mus castaneus are the principal indigenous species, with a low content of MMTV. Given the fact that mice are coinhabitants of nearly every human community, this striking correlation between breast cancer incidence and murine viral pools in the presence of a horizontally acquired viral infection in humans suggested trans-species infection. Inuit women, in circumpolar regions where mice do not exist, have a low incidence of breast cancer (12). The incidence data prompted us to examine the env positivity in breast cancers in several countries west of the swath, or those colonized by Western European powers. Italy, the United States, Mexico, Brazil, and Argentina all have 30% to 40% positivity, and Tunisian breast cancer specimens are 74% positive (13). East of the swath, in China and Japan, where breast cancer incidence is known to be low, the positivity is 10% and 12%, respectively (14). Vietnamese breast cancer has been reported to be 0.8% positive (15). The fundamental observation of MMTV-like env sequences in breast cancer has been confirmed by Etkind et al. (16), Ford et al. (15), and Levine et al. (13) and by another laboratory known to us that has not yet published its results. Other workers using different methods have not reproduced our results, but none has then reported a faithful replication of our exact methodology (17–19). A recent report using in situ polymerase chain reaction localized the product and its expression by reverse transcription-polymerase chain reaction to epithelial cells, whereas stromal cells were negative (20). In the original paper by Etkind et al. (16), two patients with breast cancer and coexistent lymphoma were studied, and env gene sequences were found in both tumors. Elsewhere in this issue, an elaboration of their important observation is published indicating that 6 of 12 patients with both breast cancer and non-Hodgkin’s lymphoma had viral sequences in each (21). This extends prior reports that non-Hodgkin’s lymphoma is more common in patients with breast cancer than expected if the diseases were randomly distributed (22) and offers a possible explanation. It is well established that MMTV can evoke lymphomas in mice. Etkind et al. (21) sequenced the env sequences from the six breast cancers and six lymphomas and found that each contained a single nucleotide mutation compared with the env sequence from C3H MMTV. Only the breast cancers each contained one clone of unmutated sequence (21). This observation might represent two independent infections (the mutant virus was previously seen in a male with non-Hodgkin’s lymphoma who did not have breast cancer) or high mutability of this particular strain of HMTV with selective parasitism of lymphoid Received 6/24/04; accepted 7/19/04. Requests for reprints: James F. Holland, Mount Sinai School of Medicine, One Gustave Levy Place, New York, NY 10029. Phone: 212-241-4495; Fax: 212-860-7186; E-mail: james.holland@mssm.edu. 1 S. Melana, J. F. Holland, and Beatriz G. T. Pogo, Isolation of human mammary tumor virus, manuscript in preparation. ©2004 American Association for Cancer Research. 5647 Vol. 10, 5647–5649, September 1, 2004 Clinical Cancer Research