HER2-positive breast cancer in a germline BRCA1 gene large deletion carrier.

A majority of breast cancer (BC) molecular subtype in BRCA1 variants carriers is triple-negative type. In contrast, human epidermal growth factor 2 (HER2)-positive BC among carriers of BRCA1 variants is rarely reported. A 42-year-old woman who previously received adjuvant endocrine therapy against left BC developed a left BC relapse and a right new primary BC. Her mother had BC and ovary cancer, and her cousin had BC. Genetic testing revealed a pathogenic large deletion of exons 1-8 in BRCA1. She was diagnosed with hereditary breast and ovary cancer and underwent bilateral mastectomy. The molecular subtypes of her right and left primary BC were HER2-enriched type and luminal-HER2 type, respectively. After completion of adjuvant therapy for right BC, risk-reducing salpingo-oophorectomy (RRSO) is planned. The present case makes us consider the frequency of BRCA1 large rearrangements in Japanese, the association between HER2 amplification and BRCA1 variants, and the optimal timing of RRSO in patients receiving adjuvant therapy for BC. © The Japan Society of Clinical Oncology 2021.