Abstract
Double pathogenic mutations occurring in an individual are considered a rare event. The introduction of a multiple-gene panel at Hong Kong Hereditary Breast Cancer Family Registry has allowed the identification of pathogenic variants in multiple genes, providing more information on clinical management and surveillance to the proband and their family members. Breast cancer patients who are double heterozygous (DH) for different hereditary breast and ovarian cancer syndrome (HBCO)-related genes were identified from a cohort of 3649 Chinese patients. Nine patients (0.25%) were observed to have germline DH mutations in ATM, BRCA1, BRCA2, BRIP1, CDH1, CHEK2, MSH6, PALB2, and TP53. Three probands were diagnosed with unilateral breast cancer, two patients were diagnosed with bilateral breast cancer, and four patients had multiple primary cancers. The median age for breast cancer diagnosis was an early age of 36 years. Chinese DH carriers did not show worse phenotypes or have a significantly downhill clinical presentation. However, seven out of nine (77.8%) of our DH carriers harbored a BRCA1 mutation, and four of them (44.4%) developed bilateral breast cancer, suggesting Chinese DH individuals may have a higher chance of having bilateral breast cancer than other populations (p = 0.0237).