A 38-year-old woman presents with a palpable mass in the right breast discovered during self-examination. Mammography reveals a 2.5 cm irregular lesion with microcalcifications. Core needle biopsy confirms invasive ductal carcinoma, grade 2. On detailed family history, her mother was diagnosed with breast cancer at age 42, and her maternal grandmother had ovarian cancer at age 55. Genetic testing reveals a pathogenic mutation in the BRCA1 gene. Which of the following best explains the increased cancer susceptibility in this family?

Explanation

## Tumor Suppressor Gene Inactivation: The Two-Hit Hypothesis **Key Point:** BRCA1 is a classic tumor suppressor gene that follows Knudson's two-hit hypothesis. Individuals who inherit a germline mutation carry one defective copy in all cells; cancer develops when the remaining wild-type allele is lost in a somatic cell. ### BRCA1 Function and Mechanism **High-Yield:** BRCA1 encodes a protein essential for homologous recombination repair of DNA double-strand breaks. Loss of both copies (or functional inactivation) impairs DNA repair capacity, leading to accumulation of mutations and genomic instability. ### Two-Hit Model in This Case 1. **Germline hit:** Patient inherited a mutant BRCA1 allele from her mother 2. **Somatic hit:** In the breast epithelial cells that formed the tumor, the wild-type BRCA1 allele was lost (deletion, mutation, or epigenetic silencing) 3. **Result:** Complete loss of BRCA1 function → defective DNA repair → malignant transformation ### Why This Family Pattern? **Clinical Pearl:** Families with inherited BRCA1 mutations show: - Earlier age of onset (mother at 42, grandmother at 55) - Multiple affected individuals across generations - Increased risk of both breast AND ovarian cancer (BRCA1 spectrum) - Autosomal dominant inheritance with high penetrance ### Comparison with Other Mechanisms | Feature | BRCA1 (TSG) | Oncogene | Haploinsufficiency | |---------|-----------|---------|-------------------| | Germline mutation alone | Not sufficient | Sufficient | Sufficient | | Requires second hit | Yes | No | No | | Both alleles lost in cancer | Yes | Only one needs mutation | Heterozygous sufficient | | Mechanism | Loss of function | Gain of function | Reduced dosage | **Mnemonic:** **"Two Hits to Lose" (TSG)** — Tumor suppressors need both alleles inactivated; oncogenes need only one activated. ### Why Not the Other Options? - **Option 0 (Haploinsufficiency):** BRCA1 haploinsufficiency does NOT cause cancer; one functional copy is usually sufficient. Cancer requires loss of both copies. - **Option 2 (Constitutive activation):** This describes oncogenes (e.g., RAS, MYC), not tumor suppressors. BRCA1 loss is loss-of-function, not gain-of-function. - **Option 3 (Single hit causes cancer):** Incorrect. A single germline mutation predisposes but does not directly cause cancer without a somatic second hit. [cite:Robbins 10e Ch 7]