Abstract PD4-04: Impact of Polygenic Risk Scores on Breast Cancer Risk Assessment and Clinical Decision Making in Carriers of Germline Pathogenic Variants in ATM, BRCA1, BRCA2, CHEK2, and PALB2: Results from the Prospective Multisite GENRE-2 Clinical

Abstract Background: Incorporation of Polygenic Risk Scores (PRS) can refine traditional breast cancer (BC) risk assessment models to provide precise estimates of BC risk. However, the impact of such an integrated model on BC risk estimates and clinical decision-making related to BC screening and preventive strategies in germline pathogenic variant (PV) carriers is not fully understood. Methods: The GENRE-2 is a prospective single-arm multisite clinical trial (NCT04474834) incorporating 313-PRS into standard BC risk assessment models to determine the impact of PRS on risk estimates and clinical decisions on prevention and screening. All women received pre- and post-PRS BC risk estimates and completed surveys on their screening and cancer prevention decisions before and after integrating PRS estimates. Herein, we report on the impact of integrating PRS into CanRisk-based BC risk estimates among women enrolled in GENRE-2 due to PVs in ATM, BRCA1, BRCA2, CHEK2, or PALB2. Women were categorized into average risk (<20%), moderate risk (20-40%), or high risk (>40%) for BC based on clinically relevant lifetime risk thresholds for supplemental MRI screening and risk-reducing bilateral mastectomies. Changes in categories of lifetime BC risk estimates were then evaluated before and after integration of PRS. Impact of PRS on patient decisions to undertake screening or preventive action (medication or prophylactic mastectomy) was evaluated based on pre- and post-PRS surveys. Results: Between 2020 and 2025, 933 women were enrolled, 633 received PRS results and completed a survey to date. Of those who received PRS results, 214 (33.8%) were PV carriers (ATM: 24, BRCA1: 54, BRCA2: 73, CHEK2: 54 and PALB2: 9). Among PV carriers, the CanRisk-based median 10-year and lifetime pre-PRS risk estimates were 6.0% and 23.9% for ATM, 22.5% and 77.5% for BRCA1, 17.9% and 77.6% for BRCA2, 7.1% and 26.6% for CHEK2, and 16.6% and 42.4% for PALB2, respectively. A total of 7 (29.2%) ATM PV carriers had a shift in lifetime BC risk categories after integration of PRS, with 1 (4.2%) shifting from average to moderate-risk, 3 (12.5%) shifting from moderate to average-risk category, and 3 (12.5%) shifting from moderate to high-risk category. Among CHEK2 PV carriers, 9 (16.7%) had a shift in lifetime risk categories, with 4 (7.4%) shifting from moderate to average-risk, 2 (3.7%) shifting from moderate to high-risk, and 3 (5.6%) shifting from high to moderate-risk. None of the BRCA1 and 4.1% of BRCA2 PV carriers had a shift in lifetime risk categories after PRS integration. Only 1 (11.1%) PALB2 PV carrier had a change in lifetime risk category after integration of PRS, although this was limited by a small sample. A significantly higher proportion of PV carriers with a high lifetime risk intended to take preventative action compared to the PV carriers with average lifetime risk (41.4% vs. 20.0%, p=0.007). Conclusions: Integration of PRS into BC lifetime risk estimates led to a clinically meaningful change in risk categories in 29.2% of ATM PV carriers and 16.7% of CHEK2 PV carriers, but in <5% of BRCA1 or BRCA2 PV carriers. These findings suggest that PRS may be helpful for BC risk stratification in moderate-risk genes such as ATM and CHEK2 but may have limited clinical utility in high-risk genes such as BRCA1 or BRCA2. Citation Format: S. Yadav, C. Klassen, D. Stan, J. Fraker, S. Sahni, S. A. Khan, L. Khatri, C. Vachon, D. Schaid, J. P. Sinnwell, E. Carlson, L. Hasadsri, F. J. Couch, S. Pruthi. Impact of Polygenic Risk Scores on Breast Cancer Risk Assessment and Clinical Decision Making in Carriers of Germline Pathogenic Variants in ATM, BRCA1, BRCA2, CHEK2, and PALB2: Results from the Prospective Multisite GENRE-2 Clinical Trial [abstract]. In: Proceedings of the San Antonio Breast Cancer Symposium 2025; 2025 Dec 9-12; San Antonio, TX. Philadelphia (PA): AACR; Clin Cancer Res 2026;32(4 Suppl):Abstract nr PD4-04.