An emerging connection between Nbs1 and Sonic Hedgehog (SHH) pathway is essential for cerebellar development and carcinogenesis

Genome integrity needs to be preserved for the propagation of genetic information. Inactivation of proteins involved in DNA damage responses (DDR) are often associated with cancer and/or developmental disorders of the nervous system, which appear particularly vulnerable to DNA distress. The Nijmegen Breakage syndrome (NBS), due to NBS1 gene mutations, is characterized by microcephaly, facial dysmorphisms and cancer predisposition. The neurological features of NBS patients have been modeled in the Nbs1CNS-Δ mouse, a Nbs1 CNS-restricted knock-out which shows microcephaly, severe ataxia and dramatically impaired cerebellar development. Strikingly, a very similar phenotype is also observed in mice with SHH/MYCN conditional KO, suggesting that Nbs1 and SHH/MYCN might be functionally linked. Prompted by this hypothesis, we generated new mouse models with CNS-restricted inactivation of Nbs1 in a transgenic SmoA1 cancer-prone context and new GCPs cultures for in vitro studies. The new SmoA1/Nbs1CNS-Δ mouse model showed that the absence of Nbs1 completely blocks the SmoA1-dependent tumor phenotype and causes severe cerebellar defects during postnatal development. This suggests an epistatic role of Nbs1 on the SHH pathway. The dramatic and degenerated phenotype of this model did not allow an appropriate analysis of the molecular interactions between Nbs1 and the SHH pathway. Therefore, we generated a new Nbs1GCP-Δ mouse model with the specific deletion of Nbs1 in cerebellar granule progenitors (GCPs). This model showed that the absence of Nbs1 caused defects in cerebellar development and this was associated with downregulation of the SHH pathway both in vivo and ex vivo. This was also confirmed in an inducible cell autonomous context. Given that the primary cilium is an essential structure for SHH signaling and given the emergent link between DDR proteins, centrosomes and ciliogenesis, we examined whether Nbs1 might be required for primary cilia formation and regulation, which in turn could affect SHH signaling. Accordingly, we provided strong evidence that loss of Nbs1 determines severe defects in ciliogenesis. Moreover, we showed that either the ciliary phenotype or the associated downregulation of the SHH pathway are rescued by loss of p53. In conclusion, our data support a novel function for Nbs1 in cilia-dependent SHH signaling and raise the possibility that the Nbs1-p53 axis, in addition to their conventional role in DNA damage, may regulate ciliogenesis both in physiological and pathological conditions in the cerebellum.