- NBS1, also known as nibrin or NBN, is the regulatory subunit of the MRN complex, the master sensor of DNA double‑strand breaks (DSBs) and a central guardian of genome stability. While MRE11 provides nuclease activity and RAD50 supplies structural tethering, NBS1 functions as the signalling hub that recruits ATM to sites of DNA damage, activates checkpoint pathways and coordinates repair through homologous recombination or non‑homologous end joining. Because DSBs are among the most lethal forms of DNA damage, NBS1’s regulatory functions are indispensable for maintaining chromosomal integrity and preventing genomic instability.
- NBS1 contains several functional domains that enable precise control of DNA‑damage signalling. Its FHA and BRCT domains recognise phosphorylated proteins at damage sites, allowing NBS1 to anchor the MRN complex to chromatin. Through its C‑terminal region, NBS1 interacts directly with ATM, facilitating ATM recruitment and activation. Once ATM is activated, it phosphorylates numerous substrates—including H2AX, CHK2, BRCA1 and p53—triggering cell‑cycle arrest, chromatin remodelling and DNA repair. If damage is irreparable, ATM signalling promotes apoptosis or other forms of programmed cell death to prevent propagation of genomic instability.
- NBS1 plays a central role in homologous recombination by coordinating DNA end resection. After MRE11 and CtIP initiate short‑range resection, NBS1 helps recruit additional factors required for long‑range resection and RAD51 loading. Through this mechanism, NBS1 ensures that homologous recombination proceeds efficiently during S and G2 phases of the cell cycle, when sister chromatids are available as templates. NBS1 also contributes to non‑homologous end joining by stabilising the MRN complex at DNA ends and facilitating interactions with Ku70/Ku80 and DNA‑PKcs. This dual functionality allows NBS1 to support repair pathway choice depending on cell‑cycle stage and chromatin context.
- Beyond its role in DSB repair, NBS1 is essential for replication fork stability. Stalled replication forks are vulnerable to collapse, which can generate lethal breaks. NBS1 stabilises these forks, coordinates their restart and prevents excessive genomic instability during replication stress. NBS1 also participates in telomere maintenance by regulating TRF2 and preventing chromosome end‑to‑end fusions. These functions link NBS1 to broader genome‑maintenance networks, including the proteostasis network and the oxidative stress response, which influence DNA repair capacity and cellular stress tolerance.
- Mutations in NBS1 cause Nijmegen breakage syndrome (NBS), a severe genomic‑instability disorder characterised by microcephaly, immunodeficiency, radiosensitivity and cancer predisposition. NBS1 deficiency disrupts ATM activation, impairs homologous recombination and destabilises replication forks, leading to widespread chromosomal instability. In cancer, NBS1 dysfunction promotes replication stress, chromosomal rearrangements and therapy resistance. Tumours with defective NBS1 often show hypersensitivity to radiotherapy, PARP inhibitors and DNA‑damaging chemotherapeutics, making NBS1 status clinically relevant for treatment planning.
- In summary, NBS1 is the regulatory arm of the MRN complex and a central coordinator of ATM‑mediated DNA‑damage signalling. Through its roles in ATM recruitment, homologous recombination, non‑homologous end joining, replication fork stability and telomere maintenance, NBS1 safeguards genome integrity and prevents catastrophic chromosomal damage. Its essential functions in DNA repair make NBS1 a key determinant of cellular survival and a major focus of research in genome stability, cancer biology and therapeutic development.
