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Concept
Nibrin
Summary
Nibrin, also known as NBN or NBS1, is a protein which in humans is encoded by the NBN gene.
Nibrin is a protein associated with the repair of double strand breaks (DSBs) which pose serious damage to a genome. It is a 754 amino acid protein identified as a member of the NBS1/hMre11/RAD50(N/M/R, more commonly referred to as MRN) double strand DNA break repair complex. This complex recognizes DNA damage and rapidly relocates to DSB sites and forms nuclear foci. It also has a role in regulation of N/M/R (MRN) protein complex activity which includes end-processing of both physiological and mutagenic DNA double strand breaks (DSBs).
Cellular response is performed by damage sensors, effectors of lesion repair and signal transduction. The central role is carried out by ataxia telangiectasia mutated (ATM) by activating the DSB signaling cascade, phosphorylating downstream substrates such as histone H2AX and NBS1. NBS1 relocates to DSB sites by interaction of FHA/BRCT domains with phosphorylated histone H2AX. Once it interacts with nibrin c-terminal hMre11-binding domain, hMre11 and hRad50 relocate from the cytoplasm to the nucleus then to sites of DSBs. They finally relocate to N/M/R where they form the foci at the site of damage.
DSBs occur during V(D)J recombination during early B and T cell development. This is at the point when the cells of the immune system are developing and the DSBs affect the development of lymphoid cells. DSBs also occur in immunoglobulin class switch in mature B cells. More frequently, however, DSBs are caused by mutagenic agents like radiomimetic chemicals and ionizing radiation(IR).
As mentioned, DSBs cause extreme damage to DNA. Mutations that cause defective repair of DSBs tend to accumulate un-repaired DSBs. One such mutation is associated with Nijmegen breakage syndrome (NBS), a radiation hyper-sensitive disease. It is a rare inherited autosomal recessive condition of chrosomal instability. It has been linked to mutations within exons 6–10 in the NBS1 gene which results in a truncated protein.
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