Retroviral expression of the NBS1 gene in cultured Nijmegen breakage syndrome cells restores normal radiation sensitivity and nuclear focus formation

The majority of cases of the autosomal recessive disorder Nijmegen breakage syndrome (NBS) are associated with null mutations in the NBS1 gene, which encodes a 95 kDa protein, nibrin, Cell lines established from NBS patients fail to express nibrin and display hypersensitivity to ionizing radiation a nd dysregulation of the nuclear localization of two key proteins involved i n DNA repair, Mre11 and Rad50. Conclusive proof that mutations in the NBS1 gene are responsible for NBS requires that re-expression of normal nibrin i n NBS cells complements these phenotypes, In the current study, retroviral expression vectors containing a normal copy of the NBS1 gene or a mutated f orm derived from a NBS patient were introduced into a well-characterized NB S cell line. Introduction of a normal copy of the NBS1 gene, but not the mu tant form, resulted in robust expression of nibrin that displayed correct n uclear localization. Expression of nibrin also restored the ability of nibr in, Mre11 and Rad50 to complex and to redistribute within the nucleus in re sponse to ionizing radiation. Radiation sensitivity of NBS cells expressing wild-type nibrin was restored to normal levels. Hence, introduction of the NBS1 gene can correct the phenotypes observed in NBS cells.