CHK2 KINASE REGULATION AND SIGNALLING IN AT-VARIANT AND NBS SYNDROMES

To ensure that genetic information is accurately transmitted, cells have evolved surveillance mechanisms that monitor the genome for the presence of DNA damage that can arise during normal metabolic activity or as result of exposure to enviromental agents. Two key components of the surveillance system are Atm and and Nbs1, two proteins involved in sensing DNA damage and activating cell cycle arrest checkpoints. Inactivation of the genes encoded by these proteins cause Ataxia telangiectasia (AT) and Nijmegen Breakage Syndrome (NBS), two disorders that share chromosomal instability, cell cycle checkpoint defects, immunodeficiency and neurological lesions. One effector of this pathway is Chk2, a kinase whose phosphorylation, which correlates with its activation, occurs in an ATM-dependent manner following DNA damage. Activation of Chk2 causes arrest in G2/M and contribute to G1 arrest. We have recently found that the enzymatic activity of Chk2 in response to DNA damage is defective in NBS cells. Significantly, this defect can be corrected by re-introduction in NBS cells of the wild type Nbs1 . These results imply that Nbs1 is a regulator of Chk2 activity, but how this is accomplished remains unknown. We have additionally observed defects in Chk2 activation among AT variant cases with normal ATM and NBS1 genes, suggesting alterations affecting the Atm/Nbs1 signalling pathway. On the bases of these observations, we propose: i)To assess the activation of Chk2 in AT variant cases available through the Italian Registry for Ataxia Telangiectasia. ii)To determine the Nbs1-dependent Chk2 phosphorylation sites, and to identify, by biochemical and genetic approaches, new Chk2-interacting proteins. iii) To establish the role of Chk2 in S-phase checkpoint arrest.