Genetic Screens in Yeast to Identify BRCA1 Modifiers

Mutations in the BRCA1 checkpoint gene results in aneuploidy and an increased risk of breast cancer. However, the age of onset and type of cancer can vary among BRCA1 mutation carriers and this difference is partially attributed to unlinked modifier loci. The yeast RAD9 protein has similar functions and sequence motifs as BRCA1 and we proposed to identify candidate modifier loci by identifying haploinsufficient mutations at a second locus that alters the chromosome loss rate of our rad9-/- diploid strains. To complete the screen we created a rad9-/- yeast strain carrying two different chromosome loss markers. To date, we have screened 7500 insertional mutants (25% genome coverage) for alteration in genomic stability and qualitatively identified 10 candidate insertions. By fluctuation analysis, five mutant strains have at least a ten fold increase in chromosome loss rate. The insertions include disruption of uncharacterized yeast open reading frames as well as a gene implicated in chromatin silencing. All mutants will be further characterized for alterations in radiation sensitivity, DNA damage checkpoint function and recombination. Thus, the screening strategy is capable of identifying mutations that alter genomic stability of a rad9-/- strain which can then serve as candidate modifiers for BRCA1 mutation carriers.