The Virology of Breast Cancer: Viruses as the Potential Causative Agents of Breast Tumorigenesis

Abstract

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 20^th 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 neighbor-hoods 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 increasedbreast-cancer 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.

Breast cancer is the most common form of cancer in women. The cause of sporadic cases is usually difficult to ascertain. A minority, about 30%, are related to a gene, mostly BRCA1/2.

In 1936, John Joseph Bittner identified the mouse mammary tumor virus (MMTV) (Bittner, 1936). 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.

Since Bittner’s discovery, there has been considerable interest in identifying infectious agents that might cause human breast cancer. An infectious origin is inferred, since 16% of all human cancer can be attributed to a micro-organism. Viruses that might be related to breast cancer are human papillomaviruses, herpes viruses, and MMTV, a prime suspect (Gannon et al., 2018; Lawson et al., 2018a).

Mouse Mammary Tumor Virus

MMTV is a lentivirus (lente-, Latin for “slow”), a genus of retroviruses that causes chronic, lethal diseases with long incubation periods in humans and other mammals. The best-known lentivirus is the Human Immunodeficiency Virus (HIV). Lentiviruses can become endogenous, integrating their genome into the host germline genome. The virus or part thereof is then inherited by the host’s offspring.

Nartey et al. (2017) have identified MMTV DNA sequences 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. In addition, they identified MMTV-like DNA sequences that represented a complete provirus; these sequences, known as human mammary tumor virus (HMTV), contain hormone-responsive elements, as does the mouse virus. The sequences have also been found in breast cancer tissues and breast cancer cells, and they produce viral particles. The identification of MMTV (MMTV-like) sequences in breast tissues prior to the development of MMTV positive breast cancer suggests that an MMTV-like virus is involved in human breast cancer (Nartey et al., 2017).

Etkind et al. (2004) have reported HMTV env sequences in breast tumor and lymphoma tissue in patients diagnosed with both malignancies, but not in normal breast tissue. Etkind et al. (2008) subsequently identified HMTV sequences in the breast tumors of an entire household including the husband, the wife, and their adult daughter who lived together for decades in the same home. Although Nartey et al. (2014) have found HMTV sequences in human milk, being breast-fed in infancy does not affect breast cancer incidence in adult life, according to two large studies (Ekbom et al., 1993; Michels et al., 2001).

Lehrer and Rheinstein (2019) have searched for MMTV sequences within the human genome. To compare the MMTV genome to the human genome, they used BLAT, the Blast-Like Alignment Tool of the UCSC (University of California, Santa Cruz) Genome Browser. BLAT can align a user sequence of 25 bases or more to the genome. 60 MMTV sequences were in the human genome. Of 56 sequences from the MMTV POL gene, 36 POL sequences were from the same part of the gene, beginning at viral nucleotide 4800 but of different lengths. Eight viral sequences began at nucleotide ~3430 of the POL gene. Four viral sequences were from GAGdUTPase, encoded by the MMTV PRO gene. dUTPase (deoxyuridine 5′-triphosphate nucleotidohydrolase) is an enzyme present in several major retroviral families. In MMTV, dUTPase may be essential for viral replication. Since BLAT identified no MMTV envelope (env) sequence in the human genome, the env sequences from breast tumors and normal breast tissue found in other studies may have come from an MMTV infection (Lehrer and Rheinstein, 2019).

It is not known for certain how MMTV could enter human cells, since the cells do not have a cellular receptor for MMTV, as do mouse cells. But Indik et al. (2007) have observed the rapid spread of mouse mammary tumor virus in cultured human breast cells.

Humans apparently acquire MMTV infection from one species of mice, Mus domesticus. Areas in which these mice live coincide with locales with the highest rates of breast cancer in humans (Stewart et al., 2000). 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. Foremost are HIV/AIDS, tuberculosis, malaria, and hepatitis C (Hansen and Paintsil, 2016). The association is usually detrimental, but not always. A case in point is polio.

Polio is primarily an enteric disease. When hygiene and sewage systems were primitive or unknown, children were infected with polio during infancy. Humoral immunity passively transferred from the mother protected them from paralytic polio, and they remained immune for life. Polio epidemics were a phenomenon of the 20^th 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 (Oshinsky, 2005). A similar relationship may exist with MMTV.

High income, high socioeconomic status, and affluence increasebreastcancer incidence (Lehrer et al., 2016). Madigan et al. (1995) reported that women in the upper two thirds of the U.S. population by income had an age-adjusted breast cancer risk of 1.7, while in the lower third their risk was not elevated. In Marin County, north of San Francisco, the median per capita income is more than twice as high as that of the U.S. as a whole, and between 1990 and 1999 Marin County breast cancer incidence increased six times more rapidly than in comparison areas (Clarke et al., 2002).

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. 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. MMTV in breast milk would not be immunogenic Impoverished girls living in substandard, mouse-infested housing would be exposed to mice and MMTV in in infants, since the virus exposure is too early. Maternal antibody and poor generation of T-cell and B-cell memory in neonates and infants are known to result in inadequate adaptive immunity from vaccinating this population compared to older children (Pichichero, 2014).

Many MMTV positive human breast cancers have similar histology to MMTV positive mouse mammary tumors. As noted above, MMTV infection in normal breast tissue precedes the development of MMTV positive human breast cancer (Lawson et al., 2018a). Therefore, MMTV may be a cause of some cases of human breast cancer (Brower, 2009).

Human Papilloma Virus

High risk oncogenic human papillomaviruses (HPVs) have been found in breast cancers in 30 studies con ducted in 17 countries and 4 continents. In 10 case-control studies, the prevalence of HPVs in breast cancers was significantly higher than in controls (OR = 3.60). Women with HPV-positive cervical cancer have a higher tendency to later develop HPV-positive breast cancer. However, several groups have not identified HPVs in breast cancer [summarized in (Lawson et al., 2015)]. HPV prevalence in breast cancer in published reports ranges from 0% to 86% [summarized in (Gannon et al., 2018)].

Epstein Barr Virus

Epstein-Barr virus (EBV), also called human herpes virus 4 (HHV-4) is one of eight members of the herpes family. EBV is one of the most widespread viruses in humans. EBV causes infectious mononucleosis. More than 80% of 18-year olds show serological evidence of prior EBV infection. EBV is involved in nasopharyngeal carcinoma, gastric cancer, endemic Burkitt’s lymphoma, and some Hodgkin’s lymphoma. Detection rates of EBV in breast cancer range from 0% to 68% [summarized in (Gannon et al., 2018)].

Bovine Leukemia Virus

Bovine leukemia virus (BLV) is a retrovirus closely related to the human T cell leukemia virus 1, and causes leukemia in dairy cattle. In one study of 114 U.S. breast cancers, 67 (59%) were positive for BLV when compared with 30 (29%) of 104 normal breast controls (odds ratio = 3.07). The source of BLV infection may be cows’ milk (Buehring et al., 2015). BLV has been identified in a similar proportion of Australian breast cancers. One anecdotal report has BLV infecting human neural cells [summarized in (Lawson et al., 2018b)].

Conclusion

No one doubts HPV’s causal role in cervical cancer. If a causal connection between MMTV and breast cancer can be established, new methods of prevention and treatment could be developed.