Domchek, Susan M.

ABSTRACT Reversion mutations (REVs) restore homologous recombination repair (HRR) and confer resistance to PARP inhibitors (PARPi) in HR-deficient cancers. Yet, their prevalence, mechanisms, and biological constraints remain undefined. We analyzed genomic profiling of 609,464 tissue and liquid biopsy samples across multiple cancer types to delineate the pan-cancer landscape of REVs. REVs were identified in eight HRR genes, most frequently BRCA2 and BRCA1 and notably never in ATM or CHEK2 . REVs exclusively impacted truncating pathogenic variants, predominantly through large in-frame and exon-level deletions, associated with repetitive sequences. Conserved functional domains are relatively depleted of REVs. Structural modeling and functional studies support that exon-level deletions preserve critical domain architecture and confer PARPi resistance. The study establishes HRR reversion as a structurally permissive, yet evolutionarily constrained, resistance mechanism with implications on response, monitoring, and therapeutic strategy. One Sentence Summary Homologous recombination reversion evolves through structurally tolerated, microhomology-driven alterations that restore DNA repair under therapeutic selection across cancers.

The NCCN Clinical Practice Guidelines in Oncology (NCCN Guidelines) for Genetic/Familial High-Risk Assessment: Breast, Ovarian, Pancreatic, and Prostate are intended to serve as a resource for health care providers to identify individuals who may benefit from cancer risk assessment and genetic counseling and testing; help guide decisions related to genetic testing; and facilitate a multidisciplinary approach in the comprehensive care of individuals at increased risk for hereditary breast, ovarian, pancreatic, and prostate cancer. The current guidelines focus primarily on assessment of pathogenic and likely pathogenic (P/LP) variants associated with increased risk of breast, ovarian, pancreatic, and prostate cancer and recommended approaches to genetic counseling/testing and care strategies in individuals with these P/LP variants associated with increased risk of these cancers. These NCCN Guidelines Insights summarize the panel’s most recent recommendations regarding screening for prostate cancer and pancreas cancer, as well as testing criteria for nonepithelial ovarian cancer.

Importance Inherited pathogenic variants (PVs) in known predisposition genes can greatly increase breast cancer risk, but the combined impact of PV status, family history, and other factors on breast cancer risk in the general US population has not been well described. Objective To evaluate population-based breast cancer risk estimates for those with established PVs overall and stratified by first-degree family history of breast cancer and other factors. Design, Setting, and Participants This study used pooled data from 13 US-based breast cancer case-control studies participating in the Cancer Risk Estimates Related to Susceptibility (CARRIERS) consortium. Enrollment for individual studies occurred between 1976 and 2013, and results are based on data released March 2023, with analyses conducted from June 2022 to July 2025. Exposures PVs, breast cancer family history, self-reported race and ethnicity, and established risk factors. Main Outcomes and Measures Breast cancer rate ratios for PVs in 7 genes were estimated from the CARRIERS consortium. PV status and incidence and mortality statistics were combined using the Individualized Coherent Absolute Risk Estimation (iCARE) model to estimate conditional cumulative breast cancer risks and 95% CIs, stratified by family history and standardized to the US population. Models that incorporated population-based data and published estimates for established epidemiologic risk factors were also evaluated. Results A total of 67 692 women were studied, including 33 841 who were diagnosed with breast cancer. PVs in ATM , BRCA1 , BRCA2 , CHEK2 , and PALB2 were strongly associated with breast cancer risk, with BRCA1 and PALB2 PVs showing evidence of heterogeneity by family history. In models considering PVs, family history, and established risk factors, the estimated cumulative risks of breast cancer by age 50 years ranged from 2.4% (95% CI, 2.4-2.4) in women with no PVs and no family history to 35.5% (95% CI, 21.6-55.1) in PALB2 PV carriers with a family history. Among women who have not been diagnosed with breast cancer by age 50 years, the cumulative risk of breast cancer by age 80 years ranged from 11.1% (95% CI, 11.0-11.2) in noncarriers with no family history to 70.5% (95% CI, 52.8-83.5) for PALB2 carriers with a family history. PV-specific cumulative risk estimates varied across subgroups defined by race and ethnicity and potentially modifiable epidemiologic risk factors. Conclusions and Relevance In this study, population-based estimates of cumulative breast cancer risk for established PVs, as informed by the CARRIERS case-control sample, varied by family history and potentially modifiable risk factors. These estimates provide guidance for identifying individuals who will most benefit from enhanced screening and prevention strategies.

Abstract Purpose: To determine the relationship between germline pathogenic variants (PV) in cancer predisposition genes and the risk of ductal carcinoma in situ (DCIS). Experimental Design: Germline PV frequencies in breast cancer predisposition genes (ATM, BARD1, BRCA1, BRCA2, CDH1, CHEK2, PALB2, RAD51C, and RAD51D) were compared between DCIS cases and unaffected controls and between DCIS and invasive ductal breast cancer (IDC) cases from a clinical testing cohort (n = 9,887), a population-based cohort (n = 3,876), and the UK Biobank (n = 2,421). The risk of contralateral breast cancer (CBC) for DCIS cases with PV was estimated in the population-based cohort. Results: Germline PV were observed in 6.5% and 4.6% of women with DCIS in the clinical testing and population-based cohorts, respectively. BRCA1, BRCA2, and PALB2 PV frequencies were significantly lower among women with DCIS than those with IDC (clinical cohort: 2.8% vs. 5.7%; population-based cohort: 1.7% vs. 3.7%), whereas the PV frequencies for ATM and CHEK2 were similar. ATM, BRCA1, BRCA2, CHEK2, and PALB2 PV were significantly associated with an increased risk of DCIS (OR > 2.0), but only BRCA2 PV were associated with high risk (OR > 4) in both cohorts. The cumulative incidence of CBC among carriers of PV in high-penetrance genes with DCIS was 23% over 15 years. Conclusions: The enrichment of PV in ATM, BRCA1, BRCA2, CHEK2, and PALB2 among women with DCIS suggests that multigene panel testing may be appropriate for women with DCIS. Elevated risks of CBC in carriers of PV in high-penetrance genes with DCIS confirmed the utility of testing for surgical decision-making.

Abstract Background: Breast cancer has been associated with monogenic, polygenic, and epidemiologic (clinical, reproductive, and lifestyle) risk factors, but studies evaluating the combined effects of these factors have been limited. Methods: We extended previous work in breast cancer risk modeling, incorporating pathogenic variants (PV) in six breast cancer predisposition genes and a 105-SNP polygenic risk score (PRS), to include an epidemiologic risk score (ERS) in a sample of non-Hispanic White women drawn from prospective cohorts and population-based case–control studies, with 23,518 cases and 22,832 controls, from the Cancer Risk Estimates Related to Susceptibility (CARRIERS) Consortium. Results: The model predicts 4.4-fold higher risk of breast cancer for postmenopausal women with no predisposition PV and median PRS, but with the highest versus lowest ERS. Overall, women with CHEK2 PVs had >20% lifetime risk of breast cancer. However, 15.6% of women with CHEK2 PVs and a family history of breast cancer, and 45.1% of women with CHEK2 PVs but without a family history of breast cancer, had low (<20%) predicted lifetime risk and thus were below the threshold for MRI screening. CHEK2 PV carriers at the 10th percentile of the joint distribution of ERS and PRS, without a family history of breast cancer, had a predicted lifetime risk similar to the general population. Conclusions: These results illustrate that an ERS, alone and combined with the PRS, can contribute to clinically relevant risk stratification. Impact: Integrating monogenic, polygenic, and epidemiologic risk factors in breast cancer risk prediction models may inform personalized screening and prevention efforts.

10511 Background: Germline BRCA2 loss-of function (LOF) variants identified by clinical genetic testing predispose to breast, ovarian, prostate and pancreatic cancer. However, variants of uncertain significance (VUS) (n > 4000) limit the clinical use of testing results. Thus, there is an urgent need for functional characterization and clinical classification of all BRCA2 variants. Here we report on comprehensive saturation genome editing (SGE)-based functional characterization of 97% of all possible single nucleotide variants (SNVs) in the BRCA2 DNA Binding Domain hotspot encoded by exons 15 to 26 for pathogenic missense variants. Methods: SGE of exons 15 to 26 of BRCA2(MANE transcript ENST00000380152.8) was performed in the haploid human HAP1 cell line. Each exon along with 10bp flanking intronic sequence was studied as an independent target region. The most efficient sgRNA for each target region was selected and cloned into a Cas9 expressing plasmid. Site-saturation mutagenesis (SSM) libraries for each of the 14 target regions were designed to target every nucleotide and were generated by site directed mutagenesis using NNN-tailed PCR primers. HAP1 cells were transfected and harvested at Day5 and 14. Cell survival in response to each variant was assessed based on deep sequence analysis of the surviving endogenously targeted HAP1 cells at Day 5 and Day 14. Results: A total of 7013 SNVs were characterized as functionally abnormal (n = 955), intermediate/uncertain, or functionally normal (n = 5224) based on 95% precision for ClinVar known pathogenic and benign standards. Results were validated relative to batches of nonsense and synonymous variants and variants evaluated using a homology directed repair (HDR) functional assay. Breast cancer case-control association studies showed that pooled SNVs encoding functionally abnormal missense variants were associated with increased risk of breast cancer (odds ratio (OR) 3.89, 95%CI: 2.77-5.51). The functional data were added to other sources of information in a ClinGen/ACMG/AMP-like model and 700 functionally abnormal SNVs, including 220 missense SNVs, were classified as pathogenic or likely pathogenic, while 4862 functionally normal SNVs, including 3084 missense SNVs, were classified as benign or likely benign. Conclusions: The production of functional maps for 97% of all SNVs in the BRCA2 DBD allowed for separation of nucleotide/protein-level functional aberrations. When coupled with genetic and clinical sources of evidence this study led to clinical classification of 5500 individual variants. These data will prove useful in the future, through integration with other datasets, for characterization and clinical classification of all variants in this region in individuals from all racial and ethnic backgrounds and for all BRCA2 associated forms of cancer.

Abstract Background: The 21-gene recurrence score assay (Oncotype DX, Genomic Health Inc., CA) is prognostic, quantifies predicted benefit of adjuvant chemotherapy, and informs clinical decisions for patients with early-stage, hormone receptor-positive (HR+), human epidermal growth factor receptor 2-negative (HER2-) breast cancer (BC). Differences in Oncotype recurrence scores (RS) in individuals with and without BRCA1 and BRCA2 (BRCA1/2) pathogenic variants (PV) have been demonstrated, but the results of this assay in patients with BC and germline PALB2 PV have not previously been described. This is of particular relevance as PALB2 BC has demonstrated biologic features similar to BRCA1/2 BC such as poly(adenosine diphosphate ribose) polymerase inhibitor (PARPi) sensitivity. Methods: Patients with early-stage, HR+, HER2-, invasive breast cancer and a germline PALB2 PV were ascertained from academic cancer center databases and the Prospective Registry of MultiPlex Testing (PROMPT), an online research registry for individuals who have had multigene panel testing for inherited cancer susceptibility. Study participants (pts) were categorized by age < 50 and ≥50. Oncotype DX RS were retrospectively reviewed and categorized as those that would and would not warrant consideration of chemotherapy (≥16 and < 16, respectively, for pts < 50; ≥26 and < 26, respectively, for pts ≥50). Yates corrected Chi-square tests were used to compare RS distributions between carriers and a previously published genetically unselected reference population. Results: Oncotype DX reports were reviewed for 20 pts with BC and germline PALB2 PV. Median RS was 19 (range 6-41) overall. Four patients were diagnosed under the age of 50 (range 33-48); median RS for these individuals was 19 (range 12-41). Sixteen patients were diagnosed at age 50 or greater (median 59, range 50-70); median RS for these individuals was also 19 (range 6-31). Compared to the genetically unselected reference population, Oncotype DX RS in pts with PALB2-associated BC demonstrated a numerical but not statistically significant shift in distribution towards categories in which chemotherapy would be considered. Among pts under the age of 50, 75% had Oncotype DX RS for which chemotherapy would be considered and 25% had Oncotype DX RS for which endocrine therapy alone would likely be recommended (p=0.726 compared to reference). Among pts aged 50 and older, 25% had Oncotype DX RS for which chemotherapy would be considered and 75% had Oncotype DX RS for which endocrine therapy alone would likely be recommended (p=0.579 compared to reference). Conclusions: Compared to data from the published genetically unselected commercial database, BC associated with PALB2 PV may have higher Oncotype DX RS reflective of intrinsically less favorable biology and greater therapeutic sensitivity. Larger datasets are forthcoming to confirm presence of significant differences in Oncotype DX RS distributions between BC with and without PALB2 PV. Table 1. Oncotype DX RS distribution by age among pts with gPALB2 PV and genetically unselected reference *Jakubowski et al., J Surg Oncol 2020. PMID 3249731 Citation Format: Payal Shah, Siddhartha Yadav, Jamie Brower, Jada Hamilton, Erica Simmons, Judy Garber, Kenneth Offit, Mark Robson, Fergus Couch, Susan Domchek. Twenty-one gene recurrence scores in individuals with breast cancer associated with PALB2 germline pathogenic variants [abstract]. In: Proceedings of the 2023 San Antonio Breast Cancer Symposium; 2023 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2024;84(9 Suppl):Abstract nr PO2-09-03.

Abstract Many germline BRCA2 missense variants of uncertain significance (VUS) remain to be clinically classified as pathogenic or benign. Here we identify 70 missense VUS in the DNA binding domain (DBD) of BRCA2 that partially reduced homologous recombination DNA repair activity of the protein in a homology directed DNA repair cell-based assay. CRISPR/cas9-based knockin of these variants reduced cell survival of haploid cells and conferred increased sensitivity to the PARP inhibitor Olaparib. Sequence alignment analysis found that the variants were highly conserved across species from pufferfish to human. Structural analysis showed that these variants are located predominantly in unstructured regions of the DBD. Case-control association studies showed that the hypomorphic variants conferred moderate risks of breast (OR:2.85; 95%CI:1.43-5.88) and ovarian cancer (OR:3.37; 95%CI:1.30-8.54) that were significantly different from risks associated with known pathogenic and benign variants. Cumulative lifetime risks of breast and ovarian cancer among carriers were estimated at 25% and 4.7%, respectively. Age at diagnosis of breast cancer among hypomorphs carriers was 55 years of age as opposed to 48 years of age for carriers of pathogenic variants. In addition, the combination of functional and other genetic data in an ACMG/AMP-like variant classification model reclassified 39 variants as likely pathogenic. This novel group of low penetrance missense variants further calibrate BRCA2 variant classification methods and have significant implications for the management of variant carriers. Citation Format: Huaizhi (Gilbert) Huang, Chunling Hu, Jie Na, Mohamed Abozaid, Amik Munankarmy, Tara Rao, Carolyn A. Dunn Lumby, Paulo Cilas Morais Lyra, Ronan E. Couch, Benjamin R. Persons, Eric C. Polley, Rachid Karam, Tina Pesaran, Siddhartha Yadav, Alvaro N.A. Monteiro, Nicholas J. Boddicker, Susan M. Domchek, Marcy E. Richardson, Fergus J. Couch. Functional and clinical characterization of hypomorphic missense variants in the BRCA2 cancer predisposition gene [abstract]. In: Proceedings of the AACR Special Conference in Cancer Research: Advances in Breast Cancer Research; 2023 Oct 19-22; San Diego, California. Philadelphia (PA): AACR; Cancer Res 2024;84(3 Suppl_1):Abstract nr PR09.

Abstract Germline BRCA2 loss-of function (LOF) variants identified by clinical genetic testing predispose to breast, ovarian, prostate and pancreatic cancer. However, variants of uncertain significance (VUS) (n>4000) limit the clinical use of testing results. Thus, there is an urgent need for functional characterization and clinical classification of all BRCA2 variants. Here we report on comprehensive saturation genome editing-based functional characterization of 97% of all possible single nucleotide variants (SNVs) in the BRCA2 DNA Binding Domain hotspot for pathogenic missense variants that is encoded by exons 15 to 26. The assay was based on deep sequence analysis of surviving endogenously targeted haploid cells. A total of 7013 SNVs were characterized as functionally abnormal (n=955), intermediate/uncertain, or functionally normal (n=5224) based on 95% agreement with ClinVar known pathogenic and benign standards. Results were validated relative to batches of nonsense and synonymous variants and variants evaluated using a homology directed repair (HDR) functional assay. Breast cancer case-control association studies showed that pooled SNVs encoding functionally abnormal missense variants were associated with increased risks of breast and ovarian cancer (odds ratio (OR) 3.89, 95%CI: 2.77-5.51). In addition, 86% of tumors associated with abnormal missense SNVs displayed loss of heterozygosity (LOH), whereas 26% of tumors with normal variants had LOH. The functional data were added to other sources of information in a ClinGen/ACMG/AMP-like model and 700 functionally abnormal SNVs, including 220 missense SNVs, were classified as pathogenic or likely pathogenic, while 4862 functionally normal SNVs, including 3084 missense SNVs, were classified as benign or likely benign. These classified variants can now be used for risk assessment and clinical care of variant carriers and the remaining functional scores can be used directly for clinical classification and interpretation of many additional variants.

The NCCN Guidelines for Genetic/Familial High-Risk Assessment: Breast, Ovarian, and Pancreatic focus primarily on assessment of pathogenic/likely pathogenic (P/LP) variants associated with increased risk of breast, ovarian, pancreatic, and prostate cancer, including BRCA1 , BRCA2 , CDH1 , PALB2 , PTEN , and TP53 , and recommended approaches to genetic counseling/testing and care strategies in individuals with these P/LP variants. These NCCN Guidelines Insights summarize important updates regarding: (1) a new section for transgender, nonbinary and gender diverse people who have a hereditary predisposition to cancer focused on risk reduction strategies for ovarian cancer, uterine cancer, prostate cancer, and breast cancer; and (2) testing criteria and management associated with TP53 P/LP variants and Li-Fraumeni syndrome.