By Joseph Longo
Breast cancer is the most common type of cancer diagnosed among Canadian women. Several known factors contribute to a women’s risk of developing breast cancer, including family history of the disease, obesity and breast density . However, little is understood about the role that genetics plays in breast cancer risk. The most well-known examples of genetic mutations that increase a woman’s risk of breast cancer are mutations in the BRCA1 and BRCA2 genes. These genes are known as “tumour suppressor” genes because they prevent cells from growing uncontrollably. Mutations that stop these genes from working properly increase the chance of developing cancer. Women with inherited mutations in BRCA1 or BRCA2 have up to an 85% chance of developing breast cancer in their lifetime, and are at a higher risk of developing breast cancer at a younger age . Furthermore, these women tend to develop more aggressive breast cancers, including those of the estrogen receptor (ER)-negative subtype . Routine screening for mutations in these genes could therefore be informative to help guide breast cancer treatment. For example, some women may decide to undergo a preventative mastectomy (removal of the breast) if they are known carriers of a BRCA1 or BRCA2 mutation, and therefore at a high risk of developing breast cancer. Hence, a better overall understanding of the genetic factors that predict breast cancer risk is important to improve breast cancer screening and prevention for all women.
Through a recent collaborative international effort that included a number of Canadian researchers, over 250,000 women (approximately half of which were diagnosed with breast cancer) were tested for genetic variants that might be associated with breast cancer risk. When comparing the genetic differences of women diagnosed with breast cancer to those without breast cancer, the researchers identified 65 new genetic regions that were associated with overall risk of developing breast cancer . In a similar study involving over 20,000 women with ER-negative breast cancer, 10 genetic variants were identified to be associated with this difficult-to-treat subtype of breast cancer .
Included in the study by Michailidou and colleagues  were over 14,000 breast cancer patients of East Asian ancestry. Interestingly, the majority of genetic regions found to be associated with breast cancer risk in women of East Asian ancestry were shared with those of European ancestry.
With the exception of a few studies, the majority of studies conducted to date that have tried to better define breast cancer risk have focused on women of European ancestry. While these studies have been fruitful in identifying new genetic regions that are associated with breast cancer risk, significant racial disparities exist in the incidence and outcome of breast cancer that are not represented in these studies. For example, the incidence of breast cancer is lower in women of East Asian ancestry compared to women of European ancestry . On the other hand, women of African ancestry are more likely to be diagnosed with breast cancer at a younger age and tend to be diagnosed with more advanced and aggressive forms of the disease compared to women of European ancestry [5-6]. Despite these observations, little is known about the genetic factors that account for these racial disparities. Identifying the genetic risk factors of breast cancer in women of difference races will therefore be an important step towards improving breast cancer screening and prevention world-wide.
 Canadian Cancer Society. Risk factors for breast cancer. http://www.cancer.ca/en/cancer-information/cancer-type/breast/risks/?region=bc
 Milne et al. (2017). Identification of ten variants associated with risk of estrogen-receptor-negative breast cancer. Nature Genetics 49: 1767-1778.
 Michailidou et al. (2017). Association analysis identifies 65 new breast cancer risk loci. Nature 551:92-94.
 Perry et al. (2009). Risk factors for breast cancer in East Asian women relative to women in the West. Asia-Pacific Journal of Clinical Oncology 5:219-231.
 Carey et al. (2006). Race, breast cancer subtypes, and survival in the Carolina Breast Cancer Study. JAMA 295:2492–2502.
 Dietze et al. (2015). Triple-negative breast cancer in African American women: disparities versus biology. Nature Reviews Cancer 15:248–254.