Human papillomavirus (HPV), human cytomegalovirus (HCMV), Epstein-Barr, measles, and bovine leukemia viruses all have been found in breast tumors and proposed as possible causes of breast cancer. Viruses appear to be present more frequently in very young breast cancer patients. However, the topic is controversial, in part because of problems with methodology and inconsistent findings. In any case, investigation of the relationship between viruses and breast cancer is at an early stage. Now a new study has reported the presence of a virus very similar to mouse mammary tumor virus (MMTV) in the breast milk of women at heightened risk of breast cancer.

Mouse mammary tumor virus (MMTV)

MMTV causes mammary tumors to develop in mice spontaneously without additional exposure to a mammary carcinogen. It is found in wild mice (i.e., living freely, not in the laboratory) and also in some strains of mice bred specifically to be used in breast cancer studies. MMTV mice reliably develop mammary tumors. MMTV is transmitted from female mice to their pups both through their milk and through normal infection during daily contact.

MMTV has also been a long-standing candidate as a contributing cause of some breast cancers in women. However, in recent years, researchers have discovered subtle differences between mouse MMTV and the MMTV virus found in women, which now has been named human mammary tumor virus (HMTV).

Latest research finds HMTV in breast milk

The study referenced at the beginning of this news story was designed to investigate HMTV in breast milk. Previous research examined the tumor tissue and normal breast tissue of a group of U.S. breast cancer patients. HMTV sequences very similar to MMTV (90 to 95% identical structural features and genes) were found in 38% of the tumors. On the other hand, no HMTV was found in the healthy breast tissue of the same patients. The entire viral structure was described and the virus was designated human mammary tumor virus (HMTV).

Since milk epithelial cells are under hormonal regulation, and hormonally responsive elements are found in HMTV, the authors hypothesized that these cells would be excellent candidates for HMTV sequence detection. To conduct the study, the presence of HMTV was assessed in breast milk samples from two groups of nursing women. The first group (73 women) either had breast cancer or were assumed to have a heightened risk of breast cancer since they previously had undergone breast biopsies (Biopsy Group). The second group (92 women) had never had a breast biopsy (Reference Group).

HMTV was detected in the milk of 20.55% of the Biopsy Group and 7.61% of the Reference Group. Tests were conducted to rule out mouse mitochondrial or genomic DNA contamination. Eight of the 73 Biopsy Group participants had breast cancer, however HTMV was found only in the milk of one of them. On the other hand, HMTV was found in the milk of 14 of the remaining 65 women in the Biopsy Group, nearly three times the rate of the Reference Group (21.54% versus 7.61%). The authors conclude that significance of HMTV in milk and its possible infectivity for infants are important questions deserving study. The similarity of HMTV to MMTV is striking, according to the authors, and suggests a possible avenue for viral transmission of breast cancer in human beings.

Please see our article on HPV for more information.