[The importance of genetic factors for development of breast cancer].

It is well documented that breast cancer aggregates in certain families suggesting a genetic etiology for this cancer. Numerous epidemiologic studies have shown that women with a first degree relative (mother, sister) with breast cancer have double the risk of developing this cancer when compared to women in the general population. More recent studies have shown that the magnitude of the risk of developing breast cancer is dependent on the age at diagnosis and the number of relatives affected. Aggregation of breast cancer in families in itself does not clarify the true nature of the underlying factors which could be genetic or due to familial resemblance in other risk factors. Complex segregation analyses of breast cancer families suggest that breast cancer susceptibility is inherited in some families as an autosomal dominant trait. Recently, a breast cancer susceptibility gene, BRCA1, has been mapped to chromosome 17q12-q21 through linkage analyses. BRCA1 appears to play a role in families with a large number of breast cancer cases who have developed breast cancer before the age 45, and/or who have breast and ovarian cancer cases. The risk of cancer for female carriers of the BRCA1 mutation has been estimated to be 87% for breast cancer and 44% for ovarian cancer by the age of 70. BRCA1 seems to play a role in only a proportion of affected families and it is likely that other genes are also involved. In the majority of breast cancer families with two or three cases, BRCA1 appears to play a small role.