Differing effects of breast cancer 1, early onset (BRCA1) and ataxia‐telangiectasia mutated (ATM) mutations on cellular responses to ionizing radiation

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Abstract
Purpose: The ataxia‐telangiectasia mutated (ATM) gene encodes a protein kinase, the activation of which is an early event in the cellular response to ionizing radiation. One of the many substrates of ATM is BRCA1 (breast cancer 1, early onset gene), which has been associated with susceptibility to breast and ovarian cancer, and has been implicated in DNA repair processes. Various cellular responses to radiation were analysed in cells with mutations in ATM or BRCA1 in an attempt to clarify which effects of ATM can be mediated through BRCA1. Materials and methods: The response to radiation of cells with mutations in ATM or BRCA1 was examined, as were BRCA1‐mutant tumour cells transfected with an exogenous wild‐type BRCA1 allele. Assays included cell‐survival curves, studies of potentially lethal damage repair, measurement of chromosomal aberrations and of G1 arrest, and Western blot analysis of lysates of irradiated cells to determine the phosphorylation of the product of the human Mdm2 gene (HDM2). Results: Both ATM and BRCA1 mutations were associated with sensitivity to ionizing radiation, deficient repair of potentially lethal damage and markedly increased chromosomal aberrations. A BRCA1‐mutated tumour cell line HCC1937, like ATM mutant cells, did not exhibit a normal G1 arrest but, unlike ATM mutant cells, did exhibit phosphorylation of HDM2. Expression of wild‐type BRCA1 in HCC1937 cells partially restored radioresistance, restored repair of potentially lethal damage and markedly reduced radiation‐induced chromosomal aberrations. G1 arrest, however, was not restored by expression of BRCA1. Conclusions: The results are consistent with a model in which ATM phosphorylation of BRCA1 regulates DNA repair functions, particularly those involved in potentially lethal damage repair and chromosomal integrity, but not other aspects of the cellular response to radiation such as G1 cell cycle arrest. To the authors' knowledge, this is the first demonstration of the ability of exogenously expressed BRCA1 to restore the ability to perform potentially lethal damage repair and maintain chromosomal integrity in irradiated cells.