A 48-year-old woman from Mumbai with a family history of early-onset breast cancer (mother diagnosed at age 38) presents with a palpable left breast mass. Genetic testing reveals a heterozygous germline mutation in the BRCA1 gene (nonsense mutation causing premature termination). Immunohistochemistry on the biopsy shows loss of BRCA1 protein expression in the tumor cells, while normal surrounding tissue retains BRCA1 expression. Which molecular mechanism best explains the loss of BRCA1 function in the tumor?