The majority of the genetic variants underlying susceptibility to breast cancer are yet to be discovered. Genome-wide linkage analyses, utilizing anonymous genetic markers, have been successful at identifying two genes, BRCA1 and BRCA2, which account for a sizeable fraction of the clearly inherited forms of breast cancer occurring in autosomal dominant Mendellian patterns. Mutations in these genes, however, account for less than
of the total genetic component of breast cancer susceptibility and the remaining component is likely to be polygenic in nature – the interaction of mutations in multiple genes, each with a weak effect, in combination with environmental influences. The identification of these polygenes will likely require the study of large numbers of unrelated subjects with and without breast cancer in case-control association studies. Until the time when complete genome sequence information is feasible for all study subjects, interim strategies involving a subset of all common genetic variation (primarily single nucleotide polymorphisms, SNPs) are being planned. Assuming the common disease-common variant theory of complex disease susceptibility, studying a very dense set of genetic markers is likely to identify breast cancer susceptibility genes by virtue of including either the biologically relevant variant, or one closely correlated with it due to linkage disequilibrium.
