Here is everything that is known about why breast cancer develops on the cellular level: Nothing.
This is very different from asking what risk factors contribute to the development the disease. Here are some: family history, obesity, alcohol consumption, and genetics mutation of either of the BRCA genes (which afflicted the actress Angelina Jolie).
Yet, breast cancers can also occur in the absence of all of these factors. Why is that?
There's a new theory, which upon first glance appears to be nothing more than material for a bad joke, in that breast cancer may be caused, in part, by a cow virus called bovine leukemia virus (BLV). However, upon closer inspection, this theory may not be as outlandish as it may seem. Because a new study in PLOS ONE examines just this, and the findings are intriguing.
Lead author, Dr. Gertrude Case Buehring, from the Division of Infectious Diseases and Vaccinology, at the School of Public Health at Berkeley, and her colleagues, posed a fascinating question: Can it be possible that a virus is one cause of breast cancer?
If so, it would not the first time. The following viruses are known to cause cancers (cancer-causing viruses are called oncoviruses):
hepatitis B and C (liver)
HTLV (human T-lymphotropic virus, lymphoma and leukemia)
human papilloma virus (HPV, cervical, others)
HHV-8 (herpes human virus number 8, Kaposi's Sarcoma)
In fact, a 2006 paper in the International Journal of Cancer, estimated that almost 18 percent of all cancers worldwide can be attributed to an infectious pathogen.
So, the idea of a virus playing a part in the development of breast cancer is not really far-fetched after all.
How good is the evidence? Not half bad.
Preserved breast tissue samples collected from 239 donors between 2002 2008 were analyzed. About half of the samples were from women who had breast cancer; the remaining half, which made up the control group, were not afflicted.
The presence of BLV was determined by PCR (polymerase chain reaction) detection of DNA from BLV a surrogate biomarker from the virus. The PCR method of detection is extremely sensitive.
Despite the small sample size, it appears the researchers may have found something real. For the samples that came from cancerous breasts, BLV DNA was detected 59 percent of the time. For non-cancerous samples, the frequency was 29 percent. The adjusted odds ratio was 3.0 (P = 0.0004 excellent). In other words, BLV was found three times more often in cancerous tissue samples than in those that were non-cancerous. This is worth paying attention to.
Another bit of support for this relationship is that BLV belongs to a very small subset of viruses called retroviruses (HIV being the best known). Retroviruses function in the same manner as normal viruses with one exception: they contain an enzyme called integrase.
The integrase enzyme's function is to cut the DNA of the host cell, and insert itself into the DNA. In a sense, once this happens, you are not you anymore. The genes of your infected cells are different than the rest. This is an obvious mutation, and, given the current emphasis on the connection between gene mutation and cancer, the Berkeley group may have made an important discovery.
Given this, what should you do differently? Nothing. Don't stop drinking milk or eating cheese based on this study. In the future, if this all holds up, screening of cattle for the virus may become useful, but we are a long way from there.
Go ahead. Have that yogurt.