Expression of ATM in ataxia telangiectasia fibroblasts rescues defects in DNA double-strand break repair in nuclear extracts

Ataxia telangiectasia (A-T) is cc human genetic disorder characterized by p rogressive cerebellar degeneration, hypersensitivity to ionizing radiation (IR), immunodeficiency, and high cancer risk. At the cellular level, IR sen sitivity and increased frequency of spontaneous and IR-induced chromosomal breakage and rearrangements are the hallmarks of A-T. The ATM gene, mutated in this syndrome, has been cloned and codes for a protein sharing homology with DNA-PKcs, a protein kinase involved in DNA double-strand break (DSB) repair and DNA damage responses. The characteristics of the A-T cellular ph enotypes and ATM gene suggest that ATM may play a role similar to that of D NA-PKcs in DSB repair and that there is a primary DNA repair defect in A-T cells. In the current study, the Function of ATM in DNA DSB repair was eval uated in an in vitro system using two plasmids, carrying either an EcoRI-in duced DSB within the lacZ alpha gene or various endonuclease-induced DSB in the SupF suppressor tRNA gene. We Found that the DSB repair efficiency in A-T nuclear extracts was comparable to, if not higher than, that in normal nuclear extracts. However, the repair fidelity in A-T nuclear extracts was decreased when repairing DSB with short 5 ' and 3 ' overhangs (<4 base pair s (bp)) or blunt ends, but not 5<prime> 4-bp overhangs. Sequencing of the m utant plasmids revealed that deletions involving 1-6 nucleotide microhomolo gies were the major class of mutations in both A-T and normal extracts. How ever, the size of the deletions in plasmids from A-T nuclear extracts was l arger than that from normal nuclear extracts. Expression of the ATM protein in A-T cells corrected the defect in DSB repair in A-T nuclear extracts. T hese results suggest that ATM ploys a role in maintaining genomic stability by preventing the repair of DSB from an error-prone pathway. Environ. (C) 2001 Wiley-Liss, Inc.