BREAST CANCER — GENETIC TESTING

Breast Cancer Genetic Testing — BRCA1 and BRCA2 are the most recognized hereditary breast cancer genes, but PALB2, CHEK2, ATM, TP53, and others collectively account for an equal number of hereditary cases. Comprehensive genetic testing identifies them all.

Whole genome sequencing evaluates all known hereditary breast cancer genes — BRCA1, BRCA2, PALB2, CHEK2, ATM, TP53, CDH1, RAD51C, RAD51D, BARD1, and others — plus structural variants and deep intronic mutations that standard gene panels miss.

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About this condition

Breast Cancer — Genetic Testing

Breast cancer is the most common cancer in women, affecting approximately 1 in 8 (13%) over a lifetime. Approximately 5-10% of breast cancers are hereditary — caused by pathogenic variants in high-penetrance genes (BRCA1, BRCA2, TP53, CDH1, PALB2 — lifetime breast cancer risk 40-85%) and moderate-penetrance genes (CHEK2, ATM, BARD1, RAD51C, RAD51D — lifetime risk 20-40%). BRCA1 and BRCA2 are the most widely tested genes, but they account for only approximately 25% of hereditary breast cancer. Multi-gene panel testing identifies 30-50% more actionable variants than BRCA-only testing.

Genetic diagnosis of hereditary breast cancer has direct therapeutic implications. PARP inhibitors (olaparib, talazoparib) are FDA-approved for BRCA1/2-positive breast cancer and for germline PALB2-positive breast cancer. Platinum chemotherapy is preferentially effective in BRCA-deficient tumors. TP53 carriers (Li-Fraumeni syndrome) should avoid radiation therapy when possible due to radiation-induced secondary malignancy risk. CDH1 carriers are candidates for risk-reducing gastrectomy (for diffuse gastric cancer risk) in addition to breast cancer surveillance. These gene-specific management differences make molecular diagnosis essential.

Beyond treatment, genetic testing guides cancer surveillance intensity and risk-reducing interventions. BRCA1/2 carriers are offered enhanced breast MRI screening starting at age 25, risk-reducing mastectomy, and risk-reducing salpingo-oophorectomy. PALB2 carriers follow similar (though slightly modified) surveillance protocols. CHEK2 and ATM carriers receive enhanced screening but have lower absolute risk, affecting the risk-reduction surgery discussion. All high-penetrance variant carriers benefit from cascade family testing — identifying additional at-risk relatives who may benefit from early surveillance or prevention.

PALB2 is now recognized as a high-penetrance breast cancer gene — comparable to BRCA2 in risk. PARP inhibitors are approved for PALB2+ breast cancer. PALB2 is NOT included in BRCA-only testing — multi-gene evaluation is essential.

Gene locus
BRCA1 (17q21.31), BRCA2 (13q13.1), PALB2 (16p12.2), CHEK2 (22q12.1), ATM (11q22.3), TP53 (17p13.1), CDH1 (16q22.1)

BRCA-only testing misses ~50% of hereditary breast cancer. Multi-gene evaluation identifies PALB2, CHEK2, ATM, TP53, and other actionable genes. WGS is the most comprehensive option — evaluating all genes including those not yet on commercial panels.

PARP inhibitors work for BRCA1/2 AND PALB2 — patients with PALB2 variants have a targeted therapy that BRCA-only testing would never identify

Olaparib (Lynparza) received expanded FDA approval for germline PALB2-positive metastatic breast cancer in 2023 — based on the same homologous recombination deficiency mechanism that makes BRCA-mutant tumors sensitive to PARP inhibition. A patient tested only for BRCA1/2 who carries a PALB2 variant would never be identified as eligible for PARP inhibitor therapy. WGS evaluates PALB2 alongside BRCA1/2 and all additional breast cancer genes, ensuring no actionable variant is missed.

Large genomic rearrangements in BRCA1 account for 8-15% of pathogenic BRCA1 alleles — sequencing-only panels miss them

Approximately 8-15% of pathogenic BRCA1 alleles are large genomic rearrangements (multi-exon deletions or duplications) that are not detectable by standard Sanger sequencing or basic NGS sequencing-only approaches. These structural variants require copy number analysis (MLPA) or comprehensive sequencing with deletion/duplication detection. WGS inherently detects both sequence variants and copy number variants from the same data — identifying the complete spectrum of BRCA1 pathogenic alleles without requiring separate MLPA testing.

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