A novel mutation and novel features in Nijmegen breakage

Nijmegen breakage syndrome (NBS) is a rare autosomal recessive disorder, characterised by microcephaly, bird-like face, growth retardation, immunodeficiency, cytogenetic abnormalities, increased radiosensitivity, and high susceptibility to lymphoid malignancy. The NBS1 gene, mapped on chromosome 8q21 and recently cloned, codes for nibrin, a member of the hMre11/hRAD50 protein complex, involved in DNA double strand break repair. The NBS Registry in Nijmegen includes 55 patients. The majority of them are of eastern European origin and share a common haplotype, suggesting a founder effect, and a mutation consisting of a truncating 5 bp deletion in exon 6, 657-661 del ACAAA. Five further mutations have been found in six patients with different haplotypes and of various ethnic origins. We found a new mutation of the NBS1 gene in a 2 year old girl from Morocco. The patient, a girl born at term in August 1997 , is the third child of apparently non-consanguineous parents; the two brothers, aged 12 and 6 years, are healthy. With the aim of characterising this new NBS mutation at the cellular level, chromosomal sensitivity, cell cycle disturbances, and induction of p53 and p21 (WAF1/Cip1) were evaluated after treatment with DNA damage inducing agents and compared with those of normal and ataxia-telangiectasia (AT) cells. Normal cells were strongly impaired in their progression from G2 to M phase in a dose related manner after X irradiation. However, irradiated NBS and AT cells sustained less reduction, in particular at the lowest dose. Our data on DNA damage inducing agents indicate a close similarity between NBS and AT cells in the G2 chromosomal response after irradiation. This is in agreement with the few published data available on the radiation sensitivity of LCL established from NBS patients. According to Carney et al and Petrini et al, nibrin, the product of the NBS1 gene, is a member of the hMre11/hRad50 complex involved in dsb repair, by transducing a signal originating from the sites of DNA damage. According to our and other published data, the relationship between DNA damage and cell cycle delay is not so straightforward. In fact, in spite of their similar chromosomal radiosensitivity, NBS and AT display strong differences in the percentage of G2 accumulated cells after irradiation. Ionising radiation induced accumulation of p53 and p21, as evaluated at two and four hours after 400 cGy x rays, appears in this NBS line to be similar to that observed in normal cells. These observations are in close agreement with recent reports showing that NBS cells, from this point of view, are more similar to normal cells than to AT ones.