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Janet Gray, Ph.D.
Janet Gray, Ph.D.

As author of our 2008 and 2010 State of the Evidence reports, Dr. Gray drives the science behind all our work.

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Genetics and Family History

Genetics and family history can be impacted by environmental factors

In addition to the two primary genes with mutations linked to breast cancer — BRCA1 and BRCA2 — other genes may be related to the likelihood of breast cancer. On top of that, environmental exposures may interact with genes to increase the risk.

Even without a strong known genetic link to the disease, family history can influence a woman’s risk of developing breast cancer. In addition to genetic similarity, family members share common environments. This means that factors such as air and water quality, household and outdoor chemicals, and food sources will be similar for all individuals within the family. 


Exposure to some chemicals can turn genes on or off, complicating the link between genetics and disease.

To make things even more complicated, genetic and environmental factors can interact to increase individuals’ risk for diseases, including breast cancer. In other words, some genetic profiles may make individuals more susceptible to negative effects from environmental exposures

In the mid-1990s, researchers discovered two genes that can have mutations linked to breast cancer: BRCA1 and BRCA2. These two genes are known as tumor-suppressor genes. This means that in their normal state they help repair DNA and make sure that cells in the body create healthy replicas when they reproduce, instead of cells that can turn into tumors (Oldenburg, 2007). Mutations in these genes interfere with the job of making sure new cells are healthy. The mutated versions can be inherited from one’s parents and passed from generation to generation in much the same ways as genes regulating other family characteristics. 

While an individual with these mutations has a greatly increased risk of developing breast cancer, evidence suggests that women born after 1940 with these mutations have a higher risk of developing breast cancer than those born before 1940 (King, 2003). Damage from environmental factors including exposure to toxic chemicals and radiation combined with these mutations are likely contributing to this increased risk. A recent study found that diagnostic radiation exposure before age 30 increased risk of breast cancer among women with BRCA mutations, with higher radiation exposures resulting in higher risk (Pifpe, 2012).

Other genes may influence the risk for developing breast cancer, although the effects of these genes are more complicated. Genes that manage the way the body makes and breaks down hormones, as well as other genes that affect DNA repair and other cell processes, may also be involved in a person’s vulnerability to developing breast cancer. Indeed, a range of genes working in concert may affect susceptibility to breast cancer (Ghoussaini, 2009). This complex of multiple genes may make some individuals more susceptible than others to environmental, hormonal and lifestyle factors. 

In addition, a newer field of research called epigenetics has demonstrated that other factors besides direct mutations can alter the effects of genes on cell activity and thus influence the likelihood of developing a disease like breast cancer. An epigenetic process is one that can essentially turn genes “on” and “off,” and therefore subtly affect certain processes related to health and disease (Dworkin, 2009). These epigenetic effects can add up over a lifetime. A number of environmental toxicants, including heavy metals, several organic solvents and endocrine-disrupting compounds, have been shown to lead to epigenetic changes in gene activity that increase the risk for later development of disease (Chiam, 2009). Not all epigenetic changes are bad; some may be protective against breast cancer. For example, it has been shown that pregnancy-induced epigenetic changes in the mammary DNA, which remain even after menopause, result in breast cells with lower proliferation rates and decreased sensitivity to hormonal and other chemical factors (Russo, 2012).