Functional domain disruption to define the molecular and clinical spectrum of germline <i>RAD51</i> family variants in hereditary breast cancer (BC).

572 Background: RAD51 paralogs (RAD51C, RAD51D, XRCC2) and RAD50 are crucial in DNA repair. Germline pathogenic variants (PVs) increase hereditary breast/ovarian cancer risk, but domain-specific impacts remain unclear. Methods: Retrospective cohort study (2019-2025) of breast cancer patients undergoing multigene testing. PVs were mapped to functional domains. Tumor features included histology, stage, grade, TILs, and molecular subtype. Associations were assessed using Fisher’s exact test. Results: Among 495 patients, 23 (4.6%) had RAD51 family PVs (RAD51C = 16, RAD51D = 4, RAD50 = 1, XRCC2 = 1). Median diagnosis age was 49. Synchronous malignancies were significantly more frequent in RAD51 carriers vs. BRCA1 carriers (34.8% vs.; OR 3.51, 95% CI 1.09–11.26). Most variants (22/23) were truncating (11 nonsense, 9 frameshift, 1 splice-site, 1 exon deletion). Domain mapping revealed disruption of critical regions: RAD51-like core domain truncations (RAD51D p.Arg186 ; RAD51C p.Arg193 ), RAD51C C-terminal truncations removing essential regions for complex integrity, and early RAD50 truncation predicted to abolish MRN complex function. Tumors were predominantly early-stage (65.2% stage I). Histology showed invasive carcinoma of no special type (69%) and invasive lobular carcinoma (25%), with strong enrichment of lobular histology in RAD51C carriers (p &lt; 0.001). RAD51C tumors were basal-like or luminal A, while the RAD50 tumor was luminal B HER2-positive. RAD51C tumors often had high nuclear grade (77.3%) but low TILs (71%). Conclusions: RAD51 family PVs are primarily truncating, disrupting key HRR domains and supporting loss-of-function as the main mechanism. Domain location correlates with distinct clinicopathological features, refining genotype–phenotype correlations for improved risk assessment.