Chk1 complements the G2/M checkpoint defect and radiosensitivity of ataxia-telangiectasia cells

Cells from patients with the human genetic disorder ataxia-telangiectasia ( A-T) are defective in the activation of cell cycle checkpoints in response to ionizing radiation damage. In order to understand the role of ATM in che ckpoint control we investigated whether Schizosaccaromyces pombe chk1, a pr otein kinase implicated in controlling the G2 DNA damage checkpoint, might alter the radiosensitive phenotype in A-T cells. The fission yeast chk1 gen e was cloned into an EBV-based vector under the control of a metallothionei n promoter and transfected into A-T lymphoblastoid cells, Induction of chk1 enhanced the survival of an A-T cell line in response to radiation exposur e as determined by cell viability and reduction of radiation-induced chromo some aberrations. This can be accounted for at least in part by the restora tion of the G2 checkpoint to chk1 expressing cells. There was no evidence t hat chk1 expression corrected either the G1/S checkpoint or radioresistant DNA synthesis in S phase in these cells. These results suggest that chk1 wh en overexpressed acts downstream from ATM to restore the G2 checkpoint in t hese cells and correct the radiosensitive phenotype, These data allow us to dissociate individual checkpoint events and relate them to the radiosensit ive phenotype in A-T cells.