The virology of breast cancer: viruses as the potential causative agents of breast tumorigenesis.

Breast cancer is the most common form of cancer in women. The cause of sporadic cases is usually difficult to ascertain. Viruses that might be related to breast cancer are human papillomaviruses and herpes viruses. Mouse mammary tumor virus (MMTV) has also been a suspect. MMTV is a milk-transmitted beta retrovirus, a form of single-stranded positive-sense RNA virus that inserts a copy of its genome into the DNA of a host cell, thus altering the cell's genome. MMTV DNA sequences have been found in 36% of human breast tumor samples and 24% of non-cancerous breast tissue. The sequences were 98% similar to the MMTV envelope (env) gene. But a search for MMTV sequences within the human genome found no env sequences, although there were sequences from the MMTV GAGdUTPase and POL genes. Therefore, env sequences from breast tumors and normal breast tissue identified in other studies may have come from an MMTV infection. Humans apparently acquire MMTV infection from one species of mice, Mus domesticus. MMTV transmission from mice to humans may explain the relationship of breast cancer to high socioeconomic status. Many infectious diseases are associated with low income and poverty. The association is usually detrimental, but not always. During the 20th century, improved sanitation delayed exposure to the poliovirus and onset of infection in middle and upper class children. These children contracted paralytic polio, while children from low-income families living in poor neighborhoods and substandard housing, infected in infancy, were spared. Humoral immunity passively transferred from the mother protected them from paralytic polio, and they remained immune for life. A similar relationship may exist with MMTV. High income and affluence are linked to increasedbreastcancer incidence. Girls of high socioeconomic status living in affluent, clean homes would have delayed exposure to Mus domesticus and MMTV. When infection finally occurred they would be vulnerable to MMTV-induced breast cancer in later life. Impoverished girls living in substandard, mouse-infested housing would be exposed to mice and MMTV in early life. Humoral MMTV immunity passively transferred from the mother would protect them and render them immune to MMTV-induced breast cancer in later life.