NATURE OF G1 S CELL-CYCLE CHECKPOINT DEFECT IN ATAXIA-TELANGIECTASIA/

We have previously demonstrated that cells from patients with ataxia-t elangiectasia (A-T) fail to show initial delay at several cell cycle c heckpoints post-irradiation. In addition a defect in the induction of p53 by ionizing radiation was evident. We demonstrate here that the ra diation signal transduction pathway operating through p53, its target gene WAF1, cyclin-dependent kinases and the retinoblastoma (Rb) protei n is defective in A-T cells. The defective p53 induction after ionizin g radiation, observed previously in A-T cells, was also reflected at t he functional level using p53-DNA binding activity, transactivation an d transfection with wild type p53. Correction of the defect at the G1/ S checkpoint was observed when wild type p53 was constitutively expres sed in A-T cells. Exposure of control cells to radiation gave rise to p53 induction and as a consequence increased expression of WAF1 mRNA a nd protein, but A-T cells were defective in this response. As expected the WAF1 response in irradiated control cells resulted in an inhibiti on of cyclin-dependent kinase activity including cyclin E-cdk2, which plays an important role in the transition from G1 to S phase. No inhib ition of cyclin-dependent kinase activity was observed in A-T cells co rrelating with the delayed WAF1 response. On the contrary an enhanceme nt of cyclin-dependent kinase activity was seen in A-T cells post-irra diation. An accumulation of the hypophosphorylated form of Rb protein occurred in irradiated control cells compatible with the G1/S phase de lay observed in these cells after exposure to radiation. In unirradiat ed A-T cells the amount of Rb protein was much higher compared to cont rols and it was mainly in the hyperphosphorylated (functionally inacti ve) form. In addition, accumulation of the hypophosphorylated form of Rb in A-T cells postirradiation was defective, consistent with the lac k of cell cycle arrest. Thus the failure of the G1/S checkpoint in A-T cells after exposure to ionizing radiation is consistent with a defec tive radiation signal transduction pathway operating through p53.