| BRCA1 is a multi‑domain nuclear tumor suppressor that assembles into the BRCA1–BARD1 heterodimer and larger genome surveillance complexes to coordinate DNA damage sensing, homologous recombination repair, checkpoint control and transcriptional regulation, thereby maintaining genomic stability in proliferating tissues. The protein contains an N‑terminal RING finger domain that forms a stable four‑helix bundle with the RING domain of BARD1 and acts as an E3 ubiquitin ligase, central exons 11–13 that harbor binding sites for PALB2, BRCA2 and RAD51 along with multiple nuclear localization and regulatory motifs, and a C‑terminal tandem BRCT repeat module that recognizes phosphorylated serine motifs on DNA damage response proteins; structural studies show that both the BRCA1–BARD1 RING heterodimer and the BRCT tandem repeat are critical for tumor suppressor function, with many cancer‑linked mutations mapping to zinc‑binding residues, the RING dimer interface, or the BRCT repeat junction where they compromise E3 ligase activity or phospho‑ligand binding and destabilize the composite domain. Mechanistically, the BRCA1–BARD1 complex catalyzes Lys‑6–linked polyubiquitin chain formation at sites of DNA damage and acts upstream in homologous recombination by recruiting and modulating PALB2, which in turn loads BRCA2–RAD51 repair machinery onto resected double‑strand break ends, fine‑tuning strand invasion and synthesis‑dependent strand annealing and contributing to high‑fidelity repair pathways such as SDSA and double‑Holliday junction resolution. BRCA1 also forms complexes with RBBP8 (CtIP) to regulate end resection and CHEK1 activation, controls G2/M checkpoint enforcement, associates with RNA polymerase II and histone deacetylase complexes through its BRCT tail to influence chromatin structure and transcription, and participates in centrosome and microtubule nucleation regulation, illustrating its integration of DNA repair, cell cycle and transcriptional programs. Germline BRCA1 mutations confer high lifetime risks of breast and ovarian cancer, and large portions of hereditary breast/ovarian cancer families carry pathogenic variants that truncate or disrupt the RING or BRCT domains or key interaction surfaces in exons 11–13; functional classification frameworks emphasize that variants of uncertain significance within these domains need assessment in E3 ligase, DNA repair and transcriptional assays to determine impact on BRCA1 activity and cancer risk. Recent work highlights that the BRCA1 C‑terminal BRCT region is particularly vulnerable, with missense and truncating mutations at the BRCT–BRCT interface or phospho‑binding groove reducing proteolytic stability, abrogating phospho‑ligand interactions and promoting tumorigenesis, making this domain a focus of structural and therapeutic studies. BRCA1 also interfaces with emerging immune and antiviral pathways via related noncoding RNAs such as the BRCA1P1 pseudogene, which regulates NF‑κB‑driven antiviral gene expression and antitumor immunity in breast cancer, underscoring that BRCA1‑linked genomic stability networks extend into innate immune modulation and may provide new angles for immunotherapy in BRCA1‑mutant and BRCA1‑regulated tumors. |
