Synthesis: BRCA1/2 and homologous recombination deficiency

Evidence grades (GRADE-adapted): A high — multiple well-conducted RCTs or systematic reviews converge. B moderate — single pivotal RCT or consistent observational evidence. C limited — single observational study, mechanistic, or expert consensus. D preclinical / hypothesis-generating.

The HR pathway in one paragraph

Homologous recombination is the high-fidelity DNA double-strand break (DSB) repair pathway used during S and G2 phases of the cell cycle. After a DSB occurs, exonuclease resection produces single-stranded 3' overhangs that are coated with RPA. BRCA1, in complex with PALB2 and BRCA2, recruits the RAD51 recombinase to these overhangs. RAD51 catalyzes strand invasion into the homologous sister chromatid, where DNA polymerases synthesize across the damaged region using the sister as template. Branch migration and resolution complete the repair without error. Cells lacking any of BRCA1, BRCA2, PALB2, RAD51 paralogs, or other HR-pathway components cannot complete this repair correctly; they fall back to error-prone non-homologous end joining (NHEJ), alternative end joining (alt-EJ), or single-strand annealing (SSA), producing characteristic genomic scars: large deletions, complex rearrangements, and signature mutational patterns^[1]A.

BRCA1 and BRCA2 epidemiology in TNBC

BRCA1 is enriched in TNBC; BRCA2 is more evenly distributed:

• Germline BRCA1 carriers develop breast cancers that are triple-negative approximately 70–80% of the time^[2]A. The BRCA1 mutation carrier population is the strongest known genetic predisposition to TNBC; the mechanism is thought to involve preferential loss of luminal differentiation in BRCA1-deficient mammary progenitor cells.
• Germline BRCA2 carriers develop predominantly HR+ breast cancers; only ~10–15% of BRCA2-associated breast cancers are triple-negative.
• In unselected TNBC populations, the germline BRCA1/2 carrier frequency is approximately 10–15%: about 7–10% germline BRCA1 and 3–5% germline BRCA2, with some cohort variation by ancestry and age.

Higher rates appear in specific subgroups:

• TNBC patients aged under 40: approximately 25–30% germline BRCA1/2 carrier rate
• TNBC patients of Ashkenazi Jewish descent: 30–40% (driven by three founder mutations: BRCA1 185delAG, 5382insC; BRCA2 6174delT)
• TNBC patients with family history of breast or ovarian cancer in close relatives: higher than the population baseline; the exact rate depends on family-history depth

NCCN guidelines recommend germline genetic testing for all TNBC patients regardless of age, family history, or ancestry, because the ~10–15% baseline carrier rate exceeds the typical threshold for guideline-driven testing and the result has direct therapeutic implications (PARP inhibitor eligibility) and cascade-testing implications for family members^[3]A.

Somatic BRCA mutations

Somatic (tumor-only) BRCA1 or BRCA2 mutations are detected in approximately 3–5% of TNBC tumors, typically in patients without germline mutations. Biallelic loss in the tumor — one allele mutated, the second allele lost via deletion or methylation — produces the same HR-deficient phenotype as biallelic germline-driven loss. The TBCRC 048 trial demonstrated that somatic-BRCA-mutated TNBC patients responded to olaparib at rates comparable to germline-BRCA-mutated patients^[4]B, supporting tumor BRCA testing in addition to germline testing as part of metastatic-disease workup.

In clinical practice, comprehensive tumor profiling panels (FoundationOne CDx, Tempus xT, Caris MI Profile, others) routinely report BRCA1/2 mutation status and variant type. Distinguishing pathogenic loss-of-function variants from benign polymorphisms requires reference to curated variant databases (ClinVar, BRCA Exchange).

HRD scoring beyond BRCA

Biallelic loss of HR-pathway genes beyond BRCA1/2 should produce a BRCA-like genomic phenotype and therefore should predict similar therapeutic vulnerabilities. The HRD scoring concept attempts to operationalize this insight by measuring the genomic-scar features that BRCA loss produces, agnostic of which specific gene is responsible.

Several commercial and research-grade HRD assays are in use:

Myriad myChoice CDx

The Myriad myChoice CDx assay (FDA-cleared for olaparib selection in ovarian cancer) measures three orthogonal features:

• Loss of heterozygosity (LOH) — large regions of allele-specific copy-number reduction
• Telomeric allelic imbalance (TAI) — allele-specific imbalance extending to the chromosome ends
• Large-scale state transitions (LST) — chromosomal breakpoints between regions of different copy number

The three are summed into a composite "Genomic Instability Score" (GIS); GIS ≥ 42 plus pathogenic BRCA1/2 mutation is the FDA-cleared HRD-positive call for ovarian cancer. In breast cancer, the HRD score has been associated with platinum response in TNBC (Telli 2016)^[5]B.

FoundationFocus CDx / Foundation One CDx

Foundation Medicine's HRD measurement combines LOH percentage with tumor BRCA mutation calling. Used as a companion diagnostic for PARP inhibitors in prostate and ovarian cancer indications. Breast cancer use is investigational.

COSMIC mutational Signature 3

Mutational signature analysis decomposes the trinucleotide context of single-base substitutions in a tumor's mutational profile into combinations of known process-specific signatures. Signature 3 is the BRCA-deficient signature, characterized by C>T and C>A transitions with specific flanking-base preferences and an elevated mutation burden. The HRDetect classifier (Davies 2017) trains a signature-based predictor that classifies tumors as HR-deficient or HR-proficient^[6]B. HRDetect performance is excellent in identifying germline-BRCA-mutated tumors; performance in non-BRCA HR-deficient tumors has been more variable.

Clinical implications

Early-stage (adjuvant) TNBC

• Germline BRCA1/2 carriers with high-risk early-stage disease — adjuvant olaparib 1 year per OlympiA (see PARP synthesis for OlympiA detail). 3-year iDFS HR 0.58; OS benefit confirmed at 4 years.
• Germline BRCA1/2 carriers undergoing surgery — contralateral risk-reducing mastectomy is an option; risk-reducing salpingo-oophorectomy is recommended at age 35–40 for ovarian-cancer risk reduction.
• HRD-positive non-BRCA early-stage TNBC — no current adjuvant trials are specifically targeting this group; whether OlympiA-style benefit extends here is unknown.

Metastatic TNBC

• Germline OR somatic BRCA1/2 — first-line PARP inhibitor (olaparib per OlympiAD, talazoparib per EMBRACA) or platinum chemotherapy are both reasonable.
• Platinum sensitivity — the TNT trial showed platinum benefit specifically in germline-BRCA-mutated TNBC vs taxane; broader-HRD selection has been less definitive (see PARP synthesis).
• PALB2 mutations — the TALA-PALB2 single-arm trial reported ~56% response rate to talazoparib in PALB2-mutated metastatic breast cancer^[7]B.
• Other HRR-pathway genes — TBCRC 048 reported responses to olaparib in PALB2-mutated patients but minimal response in other HRR-mutated patients (ATM, CHEK2, RAD51 family), suggesting these alterations don't reliably produce a BRCA-like vulnerability.

Mechanisms of HR restoration / acquired resistance

Acquired resistance to PARP inhibitors and to platinum involves restoration of HR competence in tumors that initially lacked it. Documented mechanisms^[8]B:

• BRCA reversion mutations — secondary mutations within the BRCA gene that restore the open reading frame. Detectable in ctDNA in approximately 20–30% of resistant patients.
• Loss of 53BP1, REV7, or shieldin complex components — reactivates end resection independent of BRCA1, restoring HR.
• BRCA promoter methylation reversal — in tumors where the BRCA inactivation mechanism was methylation, demethylation can restore expression.
• Increased drug efflux — ABCB1 (P-glycoprotein) upregulation pumps PARP inhibitor out of tumor cells.

Evidence table

Open questions and active investigation

• HRD beyond BRCA — will it ever be clinically reliable? Despite over a decade of investigation, no HRD-based selection assay has reliably identified non-BRCA TNBC patients who benefit from PARP inhibition at rates comparable to BRCA-mutated patients. Whether this reflects insufficient assay sensitivity, true biological heterogeneity of "HR deficiency," or the impracticality of capturing functional HR state from genomic-scar measurements is unresolved.
• Functional HR assays. Direct measurement of HR competence (RAD51 foci formation after irradiation, for example) is technically demanding but provides functional rather than indirect HRD assessment. Several groups are working on clinically-deployable functional HR assays; success would change the field.
• Combination strategies for non-BRCA HRD. If single-agent PARP inhibition isn't sufficient for non-BRCA HRD patients, combinations (PARP + WEE1, PARP + ATR, PARP + IO) might rescue activity in this subset. Multiple trials ongoing.
• POLθ inhibitors. POLθ-mediated alt-EJ is a backup repair pathway that BRCA-deficient cells use; POLθ inhibition is synthetic-lethal with HR deficiency in preclinical models. First-in-class POLQ inhibitors are in early-phase trials in BRCA-mutated breast and ovarian cancer.
• ctDNA-based monitoring for reversion mutations. Serial ctDNA monitoring could detect BRCA reversion mutations before clinical progression, enabling proactive treatment switching. Whether this improves outcomes is being tested.
• Standardization of HRD reporting. The proliferation of HRD assays (Myriad, Foundation, Caris, academic centers) with different cutoffs and somewhat different feature definitions has made cross-trial and cross-institution interpretation difficult. International harmonization efforts (analogous to the TILs Working Group) are needed.

For the PARP inhibitor clinical trials, see the PARP inhibitor synthesis. For platinum chemotherapy in the metastatic decision tree, see the first-line metastatic synthesis. For early-stage adjuvant therapy options, see the (forthcoming) adjuvant-residual-disease synthesis.

References

Each citation links to the original publication via DOI. The same records are searchable in the evidence library by title or DOI.

1. Roy R, Chun J, Powell SN. BRCA1 and BRCA2: different roles in a common pathway of genome protection. Nat Rev Cancer. 2011;12(1):68–78. doi:10.1038/nrc3181. ↩
2. Foulkes WD, Smith IE, Reis-Filho JS. Triple-Negative Breast Cancer. N Engl J Med. 2010;363(20):1938–1948. doi:10.1056/NEJMra1001389. ↩
3. Daly MB, Pal T, Berry MP, et al. NCCN Guidelines Insights: Genetic/Familial High-Risk Assessment: Breast, Ovarian, and Pancreatic, Version 2.2021. J Natl Compr Canc Netw. 2021;19(1):77–102. doi:10.6004/jnccn.2021.0001. ↩
4. Tung NM, Robson ME, Ventz S, et al. TBCRC 048: Phase II Study of Olaparib for Metastatic Breast Cancer and Mutations in Homologous Recombination-Related Genes. J Clin Oncol. 2020;38(36):4274–4282. doi:10.1200/JCO.20.02151. ↩
5. Telli ML, Timms KM, Reid J, et al. Homologous Recombination Deficiency (HRD) Score Predicts Response to Platinum-Containing Neoadjuvant Chemotherapy in Patients with Triple-Negative Breast Cancer. Clin Cancer Res. 2016;22(15):3764–3773. doi:10.1158/1078-0432.CCR-15-2477. ↩
6. Davies H, Glodzik D, Morganella S, et al. HRDetect is a predictor of BRCA1 and BRCA2 deficiency based on mutational signatures. Nat Med. 2017;23(4):517–525. doi:10.1038/nm.4292. ↩
7. Gruber JJ, Afghahi A, Timms K, et al. A phase II study of talazoparib monotherapy in patients with wild-type BRCA1 and BRCA2 with a mutation in other homologous recombination genes (TALA-PALB2). Nat Cancer. 2022;3(10):1181–1191. doi:10.1038/s43018-022-00439-1. ↩
8. Noordermeer SM, van Attikum H. PARP Inhibitor Resistance: A Tug-of-War in BRCA-Mutated Cells. Trends Cell Biol. 2019;29(10):820–834. doi:10.1016/j.tcb.2019.07.008. ↩

Last reviewed: 2026-06-04. Researcher-layer synthesis page. Evidence grades follow the GRADE-adapted rubric defined at the top of this page.