BRCA1 Gene Function
BRCA1 DNA Repair Associated
Overview
Germline BRCA1 mutations confer ~70% lifetime breast cancer risk and ~44% ovarian cancer risk, underpinning the most well-characterised hereditary cancer syndrome. BRCA1 orchestrates homologous recombination repair by bridging the MRN damage sensor complex to RAD51 loading via PALB2–BRCA2. Loss generates a distinctive genomic scar (Signature 3: large deletions, tandem duplications) detectable via HRD genomic assays, enabling biomarker-directed therapy even without direct BRCA1 sequencing. PARP inhibitors (olaparib, niraparib) exploit synthetic lethality in BRCA1-deficient tumours; resistance emerges through BRCA1 reversion mutations, 53BP1/Shieldin complex loss, and PARP1 mutations — all detectable by liquid biopsy before clinical progression.
Molecular Mechanism
Mechanism Summary
Germline BRCA1 mutations eliminate the primary homologous recombination repair pathway, creating synthetic lethality with PARP inhibitors that trap PARP1 at single-strand break sites — generating the lethal DSBs that BRCA1-deficient cells cannot repair. BRCA1-deficient tumours accumulate a distinctive genomic scar — Signature 3 (large deletions, tandem duplications, inter-chromosomal rearrangements) — detectable even without direct BRCA1 sequencing via genome-wide homologous recombination deficiency (HRD) scores. Approximately 15–25% of BRCA1-mutant tumours treated with PARP inhibitors acquire BRCA1 reversion mutations or Shieldin complex loss that restore HR and confer resistance, detectable by liquid biopsy before clinical progression.
Step-by-Step Mechanism
ATM-mediated phosphorylation of histone H2AX (γH2AX) at DSBs recruits MDC1, which amplifies ATM activation and recruits RNF8/RNF168 E3 ubiquitin ligases.
RNF8/RNF168 ubiquitinate histone H2A at Lys13/Lys15, creating a ubiquitin mark recognised by RAP80, which recruits the BRCA1-A complex (BRCA1–BARD1–ABRAXAS–RAP80) to the damage site.
BRCA1–BARD1 heterodimer, stabilised by BARD1 interaction, possesses E3 ubiquitin ligase activity and promotes DNA end resection by antagonising 53BP1-mediated NHEJ to favour HR.
BRCA1 directly binds PALB2 via its coiled-coil domain. PALB2 then bridges BRCA1 to BRCA2, positioning the BRCA2–RAD51 complex at the resected 3′ single-stranded DNA overhang.
BRCA2 (assisted by DSS1) displaces RPA from ssDNA and loads RAD51 as a nucleoprotein filament. This RAD51 filament catalyses strand invasion of the homologous sister chromatid template.
DNA synthesis from the intact template followed by branch migration and Holiday junction resolution restores the original sequence with high fidelity, completing homologous recombination repair.
Upstream Regulators
Downstream Targets
RAD51 filament loading for strand invasion and HR repair
DNA end resection to generate 3′ ssDNA for HR
Blocks NHEJ; enforces HR in S/G2 phase
Key Post-Translational Modifications
Checkpoint activation; interaction with BACH1 helicase
Required for full HR proficiency
Substrate ubiquitination promotes HR pathway choice
Disease Mechanism
Loss of BRCA1 abolishes high-fidelity HR, forcing DSB repair through error-prone NHEJ and MMEJ. The resulting genomic instability — characteristic deletions, translocations, tandem duplications — accumulates over decades and drives breast and ovarian tumourigenesis. BRCA1-deficient cells retain absolute dependence on PARP-mediated single-strand break repair, creating synthetic lethality exploited by olaparib and niraparib. Why PARP inhibitor resistance develops: (1) BRCA1 reversion mutations — secondary intragenic mutations that restore the reading frame of truncating BRCA1 variants — are the most common resistance mechanism (~25–30% of resistant tumours), partially restoring HR function sufficient for tumour cell survival. (2) Loss of 53BP1 or Shieldin complex components (SHLD1/2/3) shifts DSB repair balance toward HR-like pathways even in BRCA1-deficient cells, bypassing the synthetic lethality that PARP inhibitors depend on. (3) PARP1 point mutations or decreased PARP1 expression reduce PARP1 trapping at single-strand break sites, eliminating the mechanism through which PARP inhibitors generate cytotoxic DSBs. (4) RAD51 paralog upregulation (RAD51C, RAD51D) provides independent HR backup without restoring BRCA1 itself. Reversion mutations are detectable in circulating tumour DNA before clinical progression, enabling earlier therapy switching to platinum-based chemotherapy, which shares the same HRD dependency but acts through a distinct mechanism.
Database References
Key Pathways
- ·Homologous recombination
- ·DNA damage checkpoint
- ·Fanconi anemia pathway
- ·Ubiquitin-mediated proteolysis
- ·PARP inhibitor synthetic lethality
Disease Associations
- ·Hereditary breast cancer
- ·Hereditary ovarian cancer
- ·Triple-negative breast cancer (sporadic BRCA1)
Research Activity
BRCA1 is an actively studied target: about 100+ clinical trials that mention it are currently recruiting on ClinicalTrials.gov. Trial activity reflects research interest, not proven benefit — designs, endpoints and populations vary widely.
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Functional Partners
Common Questions About BRCA1
What does BRCA1 do in DNA repair?
BRCA1 is essential for homologous recombination (HR), the high-fidelity pathway for repairing DNA double-strand breaks. It nucleates a repair complex including PALB2, BRCA2, and RAD51 at damage sites, using the intact sister chromatid as a template for accurate reconstruction of the broken sequence.
Does a BRCA1 mutation mean you will get cancer?
A BRCA1 mutation significantly raises cancer risk but does not guarantee it. Carriers face approximately 50–70% lifetime breast cancer risk and 40–45% ovarian cancer risk, compared to 12% and 1.2% respectively in the general population — a meaningful difference but not a certainty.
How do PARP inhibitors exploit BRCA1 mutations?
PARP inhibitors (olaparib, niraparib) exploit synthetic lethality: BRCA1-deficient cells are already unable to perform homologous recombination, and PARP inhibition blocks the backup single-strand break repair pathway. Tumour cells accumulate lethal DNA damage they cannot repair, while normal cells with intact HR survive.
What is the difference between BRCA1 and BRCA2?
Both proteins function in homologous recombination, but BRCA1 primarily coordinates damage signalling and early repair steps, while BRCA2 directly loads the RAD51 recombinase onto single-stranded DNA at the break site. Clinically, BRCA1 mutations carry higher ovarian cancer risk than BRCA2.
What is homologous recombination deficiency (HRD) and how is it used clinically?
HRD describes the inability of tumour cells to repair DNA double-strand breaks by homologous recombination. While BRCA1/2 mutations are the best-known cause, HRD also arises from mutations in PALB2, RAD51C, RAD51D, BRIP1, and other HR genes. Genomic HRD assays (Myriad myChoice CDx, FoundationOne CDx) measure three genomic scar signatures: loss of heterozygosity (LOH), telomeric allelic imbalance (TAI), and large-scale state transitions (LST). These scars accumulate because HRD-deficient cells cannot repair DSBs faithfully. HRD-positive status — defined as a composite genomic instability score above a threshold — predicts PARP inhibitor benefit even in patients without detectable BRCA1/2 mutations, expanding eligibility for olaparib and niraparib in ovarian cancer.
What is BRCA1 Signature 3 and why does it matter?
Mutational Signature 3 (COSMIC classification) is characterised by a specific pattern of substitutions and indels — predominantly C>T transitions and a high proportion of large deletions — that accumulates in cells deficient in homologous recombination. It is generated when HR-deficient cells resort to error-prone repair pathways (NHEJ, MMEJ) for DSBs. Signature 3 is detectable in tumours with BRCA1 or BRCA2 mutations, but also in HRD tumours caused by other HR gene alterations, and can be present even when the causative BRCA mutation is undetected by sequencing (e.g., due to epigenetic silencing of BRCA1). Clinically, Signature 3 status is increasingly used as a surrogate biomarker for platinum and PARP inhibitor sensitivity.
How does PARP inhibitor resistance develop in BRCA1-mutant cancer, and what are the options afterwards?
PARP inhibitor resistance in BRCA1-mutant tumours primarily arises through HR restoration: BRCA1 reversion mutations (~25–30% of cases) create intragenic deletions or substitutions that restore the BRCA1 reading frame and partial HR function; loss of 53BP1 or Shieldin complex components shifts DSB repair back toward HR-like pathways without restoring BRCA1; PARP1 mutations reduce PARP trapping. After PARP inhibitor failure, options include: platinum-based chemotherapy (exploits the same HRD but through a different mechanism, showing activity in ~40% of BRCA1-mutant PARP inhibitor-resistant cases), CDK12 inhibitors (being investigated), and clinical trials targeting specific resistance mechanisms detected in liquid biopsy.