An alternative mode of translation permits production of a variant NBS1 protein from the common Nijmegen breakage syndrome allele

Abstract

Nijmegen breakage syndrome (NBS) is a rare chromosomal-instability syndrome associated with cancer predisposition, radiosensitivity and radioresistant DNA synthesis–S phase checkpoint deficiency, which results in the failure to suppress DNA replication origins following DNA damage. Approximately 90% of NBS patients are homozygous for the 657del5 allele^1,2, a truncating mutation of NBS1 that causes premature termination at codon 219. Because null mutations in MRE11 and RAD50, which encode binding partners of NBS1, are lethal in vertebrates^3,4,5, and mouse Nbs1-null mutants are inviable⁶, we tested the hypothesis that the NBS1 657del5 mutation was a hypomorphic defect. We showed that NBS cells contain the predicted 26-kD amino-terminal protein fragment, NBS1^p26, and a 70-kD NBS1 protein (NBS1^p70) lacking the native N terminus. The NBS^p26 protein is not physically associated with the MRE11 complex, whereas the p70 species is physically associated with it. NBS1^p70 is produced by internal translation initiation within the NBS1 mRNA using an open reading frame generated by the 657del5 frameshift. We propose that the common NBS1 allele encodes a partially functional protein that diminishes the severity of the NBS phenotype.