A 32-year-old BRCA1 mutation carrier undergoes surveillance breast MRI which detects a 1.2 cm mass. Excision biopsy shows high-grade triple-negative invasive ductal carcinoma (ER–, PR–, HER2–) with pushing borders, central necrosis, and tumor-infiltrating lymphocytes. What is the molecular basis for the characteristic triple-negative, high-grade, basal-like phenotype seen in BRCA1-associated breast cancers?

• A BRCA1 protein directly upregulates ESR1 (estrogen receptor gene) transcription; its loss causes ER silencing and shifts differentiation toward a basal-like lineage
• B BRCA1 mutations cause constitutive JAK-STAT activation, directly suppressing HER2 amplification and ER expression
• C BRCA1 is required for homologous recombination DNA repair; its loss causes genomic instability with accumulation of chromosomal aberrations, promoting dedifferentiation toward a basal-like (CK5/6+, EGFR+) progenitor phenotype, and the resulting tumor lacks the hormonal signaling molecules (ER, PR) characteristic of luminal differentiation ✓
• D BRCA1 loss triggers methylation of HER2 promoter, preventing HER2 amplification but not affecting ER/PR expression

Explanation

BRCA1 functions in homologous recombination (HR) repair via its BRCT domain interaction with DNA repair complexes including PALB2-BRCA2. Loss of BRCA1 leads to HR deficiency and chromosomal instability, causing accumulation of copy number changes and widespread genomic rearrangements. The resulting cancers characteristically originate from an ER-negative luminal progenitor/basal-like cell of origin and display a basal-like molecular subtype (CK5/6+, EGFR+, p53 mutated) that lacks ER, PR, and HER2 amplification. The triple-negative, high-grade morphology with TILs is thus a direct consequence of the genomic instability and dedifferentiated basal-like origin. These tumors are sensitive to platinum-based agents and PARP inhibitors.

Reference: Robbins & Cotran Pathologic Basis of Disease, 10th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.