Recent genetic studies have identified common variation in susceptibility loci that stratify lifetime risks of
breast cancer and may inform prevention and screening strategies. However, whether these loci have similar implications for women treated with
tamoxifen or
raloxifene (
SERMs) is unknown. We conducted a matched case-control study of 592 cases who developed
breast cancer and 1,171 unaffected women from 32,859 participants on
SERM therapy enrolled on NSABP P-1 and
P-2 breast cancer prevention trials. We formed a quantitative
polygenic risk score (PRS) using genotypes of 75
breast cancer-associated single nucleotide polymorphisms and examined the PRS as a risk factor for
breast cancer among women treated with
SERMs. The PRS ranged from 3.98 to 7.74, with a one-unit change associated with a 42 % increase in
breast cancer (OR = 1.42; P = 0.0002). The PRS had a stronger association with
breast cancer among high-risk women with no first-degree family history (OR = 1.62) compared to those with a positive family history (OR = 1.32) (P intx = 0.04). There was also suggestion that PRS was a stronger risk factor for ER-positive (OR = 1.59, P = 0.0002) than ER-negative (OR = 1.05, P = 0.84)
breast cancer (P intx = 0.10). Associations did not differ by
tamoxifen or
raloxifene treatment, age at trial entry, 5-year predicted Gail model risk or other clinical variables. The PRS is a strong risk factor for ER-positive
breast cancer in moderate to high-risk individuals treated with either
tamoxifen or
raloxifene for
cancer prevention. These data suggest that common genetic variation informs risk of
breast cancer in women receiving
SERMs.