Scientists are not certain what causes breast cancer, though there appears to be some connection with hormone levels. Estrogen is a part of breast cell growth and division, and this applies to tumor cells as well. In fact, some tumors feed off estrogen to sustain themselves. High estrogen isn’t a guarantee of cancer; breast cancer is known to be tied with genetics, with the genetics-breast cancer link one of the most firmly established links between genetic factors and a form of cancer. Although fewer than 10 percent of breast cancer is hereditary, having a family history of the disease is a major risk factor. About a quarter of inherited breast cancers associated with mutated forms of the genes BRCA1 and BRCA2, though there are other genes associated with the disease as well.
Now researchers have added four more to that list. Certain mutations in genes identified as RINT1, MRE11A, RAD50, and NBN have been confirmed to heighten susceptibility to breast cancer. These are "intermediate-risk" genes, not as strongly associated with cancer as dangerous forms of BRCA1 and BRCA2, but still more than doubling cancer risk, and so at a higher level than the genes labeled modest-risk. These genes were being studied for a breast cancer link. The protein produced by RINT1 is linked with, and those produced by the other three form, a protein complex called the MRN complex, which readies DNA broken in replication for repair by other proteins.
"Many genes responsible for a strong increase in cancer risk at one or two sites in the body are also connected with lesser increases in risk at other sites," David Goldgar, PhD, a lead author of the RINT1 study, said in a statement. Indeed, other researchers have found connections between BRCA2 mutations, including some that raise the risk of breast cancer, and an increased risk of lung cancer in people who smoke. Smokers who have the BRCA2 mutations, they found are more prone to lung cancer than smokers who do not, though all smokers are at higher risk for lung cancer than non-smokers are.
Scientists are also looking into factors other than genetics that might cause breast cancer. Environmental exposure, for example, plays an important role, particularly for the estimated 80 percent of patients who have no family history of breast cancer at all. In a recent study, researchers identified 17 types of chemicals that may be linked with breast cancer development. The most common was gasoline and combustion chemicals such as butadine. Not everyone exposed to these chemicals will get sick, but they raise the risk.