| p95/NBS1 (nibrin, NBN) is a phosphoprotein component of the MRE11–RAD50–NBS1 (MRN) complex that couples DNA double‑strand break (DSB) recognition to ATM activation, checkpoint signaling, and high‑fidelity repair, thereby acting as a key organizer of the DNA damage response and a guardian of chromosomal integrity. The N‑terminal region contains an FHA domain followed by tandem BRCT repeats that bind phosphorylated epitopes on mediator proteins such as MDC1 and γH2AX, and this phospho‑recognition module is essential for recruiting and retaining the MRN complex at DSB sites and for proper G2/M checkpoint control, while the C‑terminal part mediates interaction with MRE11, RAD50, and additional partners, including ATM and the p110α subunit of PI3K. Upon DSB formation, NBS1 directs MRN accumulation to chromatin via γH2AX binding and positions the complex to stimulate ATM activation; ATM phosphorylates NBS1 at defined serines in an S‑phase checkpoint module, and NBS1 in turn amplifies ATM signaling by concentrating ATM and its substrates at damage foci, resulting in coordinated phosphorylation of downstream effectors such as CHK2, p53, and SMC1 that enforce S‑phase slowing, G2/M arrest, and transcriptional responses while repair proceeds. The MRN–NBS1 assembly also promotes DNA end tethering, processing, and channeling into homologous recombination and, to a lesser extent, modulates non‑homologous end joining, and NBS1 has additional roles in suppressing aberrant inter‑chromosomal recombination and maintaining telomere stability through effects on TRF2 and ATM‑dependent telomere damage signaling, so that NBS1 deficiency leads to impaired checkpoint activation, defective DSB repair, accelerated telomere erosion, telomere fusions, and aneuploidy. NBS1 can act as an adaptor for growth signaling: a conserved C‑terminal motif directly binds the N‑terminal region of p110α and stimulates class I PI3K activity, and NBS1 overexpression activates PI3K–Akt signaling and induces cellular transformation. Deregulated NBS1 can couple DNA damage pathways to oncogenic survival and proliferation circuits. Germline loss‑of‑function mutations in NBS1 cause Nijmegen breakage syndrome, a recessive chromosomal instability disorder characterized by microcephaly, growth retardation, immunodeficiency, radiosensitivity, and a high incidence of lymphoid malignancies, and heterozygous carriers of recurrent mutations such as 657del5 show increased breast cancer risk, reflecting the dose‑dependent requirement of NBS1 for genome maintenance. |
