A 35-year-old woman with a family history of early-onset breast cancer (mother diagnosed at age 38) undergoes genetic counseling. Pedigree analysis suggests autosomal dominant inheritance. Which investigation is most appropriate to confirm a germline BRCA1/BRCA2 mutation?

## Germline Mutation Detection in Hereditary Cancer Syndromes **Key Point:** Whole-genome sequencing (or targeted NGS of BRCA1/BRCA2) of constitutional (blood) DNA is the gold standard for identifying germline mutations that confer hereditary cancer predisposition. ### Rationale for WGS / Targeted Sequencing 1. **Detects all mutation types:** Point mutations, insertions, deletions, large rearrangements, and copy number changes in BRCA1/BRCA2 2. **Constitutional DNA:** Analyzes germline DNA from blood, not somatic tumor DNA 3. **High sensitivity and specificity:** >99% for pathogenic variants 4. **Clinical utility:** Results guide surveillance, prophylactic surgery, and family screening 5. **Actionable:** Identifies BRCA1/BRCA2 carriers eligible for PARP inhibitors (olaparib, rucaparib) and platinum-based chemotherapy ### Why Other Investigations Fail | Investigation | Problem | |---|---| | **IHC for BRCA1** | Detects protein expression in tumor, not germline mutations; loss of BRCA1 expression is indirect and non-specific | | **CGH** | Analyzes somatic tumor DNA; cannot identify germline mutations; designed for copy number changes, not point mutations | | **Karyotyping** | Detects chromosomal aberrations (trisomy, translocations); cannot identify single-gene mutations | **High-Yield:** BRCA1/BRCA2 mutations account for ~5–10% of breast cancers and confer 45–87% lifetime risk of breast cancer. Genetic testing is recommended for: - Personal history of breast cancer <45 years - Family history of early-onset breast or ovarian cancer - Ashkenazi Jewish ancestry - Male breast cancer **Mnemonic: BRCA — Breast cancer susceptibility genes (BRCA1, BRCA2) encode DNA repair proteins; mutations impair homologous recombination repair.** **Clinical Pearl:** BRCA1/BRCA2 carriers benefit from enhanced surveillance, risk-reducing surgery (mastectomy, oophorectomy), and PARP inhibitor therapy in metastatic disease.