[Show abstract][Hide abstract] ABSTRACT: Nijmegen breakage syndrome (NBS) is a genomic instability disorder caused by hypomorphic mutations in the Nbs1 gene. When Nbs1 is conditionally inactivated in the central nervous system of mice (Nbs1-CNS-Δ), they suffer from severe cerebellar atrophy, ataxia, and white matter damage. Here, we show that conditional inactivation of the murine Nbs1 gene has a profound effect on the integrity and the functionality of the glial cells, which suggests their crucial role in the pathogenesis of NBS. Interestingly, in Nbs1-CNS-Δ mice, the dramatic reduction in the numbers of Purkinje and granule cells was also linked to a reduction of microglial cells but not to astrocytes (GFAP+), suggesting an impairment in astrocytic functionality. Nbs1 levels were dramatically reduced in adult astrocyte isolated from Nbs1-CNS-Δ mice, suggesting a major role in cerebellar pathology. In order to investigate the effect of Nbs1 deletion on astrocyte activity, we investigated glutamine synthetase levels in astrocyte and discovered 40% reduction as compared to WT. Furthermore, we found a significant reduction in the secretion of neurotrophic factors, such as brain-derived neurotrophic factor and neurotrophin 3. Understanding the contribution of malfunctioning astrocytes to the etiology of NBS can elucidate a hitherto unknown aspect of this disorder.