Spectrum of double heterozygosity in individuals diagnosed with hereditary breast and ovarian cancer.

(Likely) pathogenic variants (pVs) in hereditary breast and/or ovarian cancer (HBOC) genes increase cancer risk, but the clinical implications of multiple pVs remain insufficiently understood. Using data from the prospective nationwide German Consortium for HBOC registry, we conducted a case-control study of genotype-phenotype associations, matching female carriers to compare cancer risk and disease severity between multiple and single heterozygous (SH) pV carriers. Among 26,983 carriers of single or multiple HBOC-associated pVs, 409 individuals had multiple heterozygosities, including 400 double heterozygous (DH) carriers. In a subcohort tested for 13 core genes, 98/3407 (2.9%) individuals had multiple HBOC-associated pVs. Breast cancers (BCs) in DH women with pVs in BRCA1 and another gene were predominantly triple-negative, even when the second, non-BRCA1 pV was typically associated with hormone receptor (HR)-positive BC. By contrast, BRCA1/CHEK2 DH carriers showed more HR-positive BCs than BRCA1 SH women. The median age at first BC was similar in female ATM/CHEK2 DH carriers and BRCA1 SH carriers (41.5 [IQR 14.0] vs. 41.3 [14.1] years). Matched BRCA1 and BRCA2 SH carriers were less likely to present with higher disease severity than BRCA1/BRCA2 DH carriers (BRCA1: OR 0.396 [0.180-0.87]; BRCA2: OR 0.224 [0.100-0.50]). Multiple heterozygosity in HBOC core genes is more frequent than previously assumed. Our data indicate that gene-gene constellations can reshape tumour phenotype and clinical severity rather than following single-gene expectations. Integrating multiple pVs into risk assessment and counselling may enhance personalised surveillance and risk-reducing strategies for DH individuals and their families in the era of widespread multigene testing.