Rare variant burden complements polygenic risk in breast cancer and is associated with earlier diagnosis

Breast cancer affects approximately one in eight US women during their lifetime, and about 10% of new cases occur before age 45. Polygenic risk scores (PRS) are associated with breast cancer risk, and previous studies have evaluated PRS alongside pathogenic-variant carrier status. Whether a continuous, gene-effect-weighted rare-variant burden adds information beyond PRS, and whether the two genetic measures relate similarly to diagnosis age, remain unclear. We studied these questions using whole-genome sequencing and electronic health record data from roughly 8,000 breast cancer cases and 134,727 controls of genetically inferred European ancestry in the All of Us Research Program. In joint case-status models, increase in PRS and rare-variant burden (Burdenrare) was associated with higher odds of breast cancer (odds ratios, 1.55 and 1.27, respectively). Adding both scores and their interaction to covariates increased the cross-validated area under the receiver operating characteristic curve from 0.745 to 0.782. The PRS odds ratio differed between noncarriers and carriers (1.56 versus 1.25), suggesting that their effects are not simply multiplicative. We then examined whether the same genetic measures were associated with recorded age at diagnosis among cases. In eligible cases, each standard deviation increase in Burdenrare was associated with diagnosis 1.82 years earlier, compared with 0.43 years earlier for PRS. The proportion carrying a qualifying rare variant decreased from 26.5% among cases diagnosed before age 35 to 3.2% among those diagnosed at age 70 or older. Higher breast cancer risk can itself produce an earlier mean diagnosis age among cases, so we compared observed age associations with those expected from age-scale risk models under age-constant hazard ratios. Notably, the PRS association was consistent with its risk-based expectation, whereas the Burdenrare association was stronger than expected. In this common whole-genome sequencing framework, the weighted rare-variant burden adds information beyond PRS for breast cancer case status and shows a more pronounced association with early diagnosis, guiding future studies of risk and diagnosis timing.