A 48-year-old woman from Mumbai with a family history of early-onset breast cancer (mother diagnosed at age 38) presents with a 2 cm palpable mass in the upper outer quadrant of the right breast. Imaging and biopsy confirm invasive ductal carcinoma. Genetic testing reveals a heterozygous germline mutation in the BRCA1 gene. Which of the following best explains why this patient developed cancer despite inheriting only one mutated BRCA1 allele?

Explanation

## Knudson's Two-Hit Hypothesis and BRCA1 Tumor Suppression ### The Two-Hit Model in Hereditary Breast Cancer Although the patient inherited a **germline heterozygous BRCA1 mutation** (one mutant allele, one wild-type), cancer develops only when the **wild-type allele is lost somatically** in a single cell, leading to complete loss of BRCA1 function in that cell. **Key Point:** Tumor suppressors like BRCA1 require **biallelic inactivation** for malignant transformation. One functional copy is usually sufficient for normal DNA repair; loss of both copies creates a "mutator phenotype." ### Mechanism of BRCA1 Loss and Cancer Development 1. **Germline hit (inherited):** One BRCA1 allele is mutated in all cells 2. **Somatic hit (acquired):** The wild-type allele is lost in a breast epithelial cell via: - Deletion - Mutation - Epigenetic silencing (rare for BRCA1) 3. **Result:** Complete loss of BRCA1 → impaired homologous recombination (HR) repair → genomic instability → malignant transformation **High-Yield:** This explains why: - Heterozygous carriers have **increased lifetime cancer risk** (up to 70% for breast cancer) - Cancer is **clonal** (arises from a single cell with both alleles inactivated) - Multiple independent cancers can occur (different cells lose the wild-type allele independently) ### BRCA1 Function and DNA Repair | Function | Role in Carcinogenesis | |----------|------------------------| | **Homologous recombination (HR) repair** | Repairs double-strand breaks (DSBs) accurately; loss → genomic instability | | **Transcriptional regulation** | Regulates p53 and other tumor suppressors | | **Cell cycle checkpoint control** | Activates G1/S and G2/M checkpoints | | **Protein stability** | Maintains genomic integrity | **Clinical Pearl:** BRCA1-deficient tumors are **HER2-negative, ER-negative** (triple-negative breast cancer) and show a characteristic "medullary" histology with high mitotic rate and lymphocytic infiltration. ### Knudson's Two-Hit Hypothesis Illustrated ```mermaid flowchart TD A[Inherited BRCA1 Mutation]:::outcome --> B[Heterozygous in all cells] B --> C[Most cells retain wild-type allele]:::action C --> D[Normal DNA repair capacity]:::outcome B --> E[Rare cell loses wild-type allele]:::urgent E --> F[Homozygous BRCA1 inactivation]:::outcome F --> G[Loss of HR repair]:::urgent G --> H[Genomic instability]:::urgent H --> I[Malignant transformation]:::urgent I --> J[Breast cancer develops]:::outcome ``` **Mnemonic:** **Two Hits = Two Alleles = Tumor.** First hit is inherited; second hit is somatic. Both must be lost for tumor suppressor function to fail.