BRCA1 Hereditary Breast Ovarian Cancer Syndrome

A 34-year-old woman of Ashkenazi Jewish ancestry presents with a 2 cm left breast mass. Core biopsy reveals triple-negative invasive ductal carcinoma with medullary features. Family history is significant: mother died of bilateral breast cancer at age 42, maternal aunt had high-grade serous ovarian cancer at 48, and maternal grandmother had contralateral breast cancer at 39. Germline multi-gene panel testing identifies a pathogenic frameshift mutation in the BRCA1 gene located at the locus marked **B** in the diagram. Which of the following best describes the molecular consequence of BRCA1 loss of function in this patient's tumor cells?