Third of five student guest posts by Dana Lowry
In 1911, Peyton Rous first discovered viruses can cause cancer. A chicken with a lump in her breast had been brought to Rous by a farmer. Rous prepared an extract that eliminated bacteria and tumor cells and injected this extract into other chickens—tumors grew. Rous suggested “a minute parasitic organism” was causing the tumor growth, which is now known to be a virus. However, Rous’ discovery remained very controversial, and it wasn’t until 1966 that he was awarded a Nobel Prize for his discovery. Since Rous’s discovery, researchers have found an estimated 15 percent of all cancers worldwide are associated with viruses. Some common virus and cancer associations are: human papilloma virus (HPV) and cervical cancer, hepatitis B and liver cancer and human T lymphotropic virus type 1 (HTLV-1) and T-cell leukemia.
Epstein-Barr virus (EBV), a member of the herpesvirus family, is one of the most common viruses worldwide. Among 35 to 40 year olds in the U.S., up to 95% have been infected with EBV. Oftentimes, children infected with EBV have no clinical signs or symptoms; however, 30% to 50% of adolescents and young adults exposed to EBV for the first time will develop infectious mononucleosis, commonly known as mono. In the U.S., individuals are usually exposed to EBV in adolescence or young adulthood compared to developing countries, where oftentimes individuals are exposed as infants or young children. EBV usually remains dormant in the body throughout an individual’s lifetime, similar to the varicella-zoster virus, the virus responsible for the chicken pox. EBV is known to play a role in Burkitt ’s lymphoma (cancer of the immune cells), nasopharyngeal cancer (cancer of the upper throat) and Hodgkin’s lymphoma (cancer of the lymphatic system), but can EBV also play a role in breast cancer?
In 2010, James Lawson and Benjamin Heng reviewed 27 papers concerning EBV and breast cancer associations. EBV infections are universal in high and low risk breast cancer groups, making it unlikely that EBV is the sole contributor to forms of breast cancer . However, the age at which EBV is contracted seems to play a role in the risk of developing breast cancer. Women in Western countries are at higher risk of developing breast cancer and tend to be infected with EBV during adolescence or young adulthood, whereas women from non-Western countries have a lower risk for developing breast cancer and tend be infected during infancy or early childhood. Hodgkin’s lymphoma shares a similar correlation with higher rates in Western countries . Although there seems to be a relationship between age of EBV infections and risk of breast cancer, potential confounders need to be considered. Women in developing countries tend to have more children, have children at a younger age and breastfeed their children for longer periods of time. Breastfeeding, having more children and having children earlier in life all seem to be protective factors against breast cancer.
Beyond epidemiological evidence, lies biological evidence. Twenty two of the studies Lawson and Heng reviewed were based on polymerase chain reaction (PCR) techniques. Issues have been found with standard PCR procedures, but it is becoming widely accepted that EBV can be identified in breast cancer tissue through specific PCR techniques . EBV genes have been found in breast cancers through polymerase chain reaction (PCR) analyses. EBV has not only been shown to shed in human breast milk , but it has also been shown to stimulate growth of human breast-milk cells . The mechanism by which EBV actually causes cell alterations is not known, but it is thought to be different from the mechanisms used in lymphomas and nasopharyngeal cancer .
It is unlikely that we can actually “catch” breast cancer, as EBV doesn’t seem to be the sole cause of breast cancer. EBV may contribute to breast cancer by altering genes in the breast cells which eventually leads the uncontrolled cell division, known as cancer. More importantly, it seems the age an individual is infected with EBV may play an even bigger role in the outcome of disease. An EBV vaccination is in the works that will hopefully prevent infectious mononucleosis and EBV-associated cancers. However, the vaccination may not prevent the EBV infection itself; it is targeted towards the most abundant protein on the virus and on virus-infected cells. If the vaccination proves to be successful, it will be interesting to see if a reduction in breast cancer rates will follow, along with the known cancers associated with EBV. Only time will tell.
- Lawson, J. and Heng, B. (2010). Viruses and Breast Cancer. Cancers 2010, 2(2), 752-772; doi: 10.3390/cancers2020752.
- Yasui et al. (2001). Breast cancer risk and “delayed” primary Epstein-Barr virus infection. Cancer Epidemiology, Biomarkers & Prevention, 10:9-16. http://cebp.aacrjournals.org/content/ 10/1/9.long.
- Junker et al. Epstein-Barr virus shedding in breast milk. (1991). The American Journal of the Medical Sciences, 302: 220–223. http://www.ncbi.nlm.nih.gov/pubmed/1656752.
- Xue et al. (2003). Epstein-Barr virus gene expression in human breast cancer: protagonist or passenger?. British Journal of Cancer, 89:113–119. http://www.nature.com/bjc/journal/ v89/n1/full/6601027a.html
Does growth in human breast-milk cells promote production of milk? If so, does this confer more over-all fitness, to be selected? If also so, does this mean that we've co-evolved with EBV?