Cancer incidence in Nijmegen breakage syndrome is modulated by the amount of a variant NBS protein

Abstract

The human genetic disorder, Nijmegen breakage syndrome (NBS), is characterized by radiosensitivity, immunodeficiency and an increased risk for cancer, particularly B-cell non-Hodgkin lymphoma. The NBS1 gene codes for a protein, nibrin, involved in the processing/repair of DNA double-strand breaks and in cell cycle checkpoints. The majority of patients are homozygous for a founder mutation, a 5 bp deletion. This mutation is actually hypomorphic, since a functionally relevant truncated protein, of approximately 70 kDa, is produced by alternative translation. Null mutation of the homologous gene in mice is lethal; however, null-mutant murine cells can be rescued by a human NBS1 cDNA carrying the founder mutation. Clearly, the truncated p70-nibrin is able to sustain vital cellular functions of the full-length protein. We have used semi-quantitative immunoprecipitation to examine a panel of 26 lymphoblastoid B-cell lines from NBS patients for their level of p70-nibrin expression and correlate this with details of clinical phenotype provided by the two contributing centres. We find considerable variation in the amount of p70-nibrin in cell lines from different patients. Examination of clinical history indicated a clear and statistically significant correlation between p70-nibrin expression levels and lymphoma incidence. The variation in p70-nibrin levels between patients probably reflects the susceptibility of the alternative translation process to other genetic and non-genetic factors. Patients whose cells are able to maintain particularly high levels of the truncated p70-nibrin protein are at a lower risk for lymphoma than those patients with low levels of p70-nibrin in their cells.

Cleavage of the BRCT tandem domains of nibrin by the 657del5 mutation affects the DNA damage response less than the Arg215Trp mutation

“…This result agrees with data indicating that the NBN fragment comprising amino acids 401-754 is able to interact with MRE11 (32,39). the 70-kDa NBN fragment levels and the lymphoma risk (40). Remarkably, the Arg215Trp mutation determines a defective interaction between the Arg215Trp NBN mutant and c-H2AX, thus supporting the notion that the integrity of the tandem BRCT domains of NBN is fundamental for the proper localization of NBN at the DSB (see 27 and present data).…”

Deficient G2-M and S Checkpoints are Associated with Increased Lung Cancer Risk: A Case-Control Analysis

“…Olivieri and Micheli (29) also showed that cells with deficient radiation-induced G 2 arrest displayed a high level of induced chromosome aberrations. In addition, it has been shown that individuals with inherited mutations in genes involved in the G 2 -M checkpoint pathways such as ATR, ATM, NBS1, BRAC1, and 14-3-3e were more susceptible to the development of various cancers (30)(31)(32)(33)(34). All these lines of evidence support the hypothesis that attenuated function of the G 2 -M checkpoint may result in genomic instability, and therefore, an elevated cancer risk.…”

Clinical variability and expression of the NBN c.657del5 allele in Nijmegen Breakage Syndrome

“…The method has been published previously (Krüger et al, 2007). Basically, whole cell lysates were prepared and incubated with protein G-coated magnetic beads (Dynabeads) preloaded with equal quantities of polyclonal rabbit antibodies directed against MRE11 (Novus Biologicals) and nibrin (Novus Biologicals).…”

“…We used the same semi-quantitative immunoprecipitation procedure we have previously employed on cell lines established from NBS patients to examine cell lines from NBN heterozygotes (Krüger et al, 2007). As shown in Fig.…”

“…We have previously reported that the steady state levels of p70-nibrin vary in lymphoblastoid cell lines derived from different NBS patients and that low levels correlate with cancer occurrence in those patients (Krüger et al, 2007). Clearly understanding the basis for the variation in p70-nibrin expression would be an important goal in identifying modifiers of NBS.…”