Ataxia telangiectasia (AT) is a rare human autosomal recessive disorder with pleiotropic phenotypes, including neuronal degeneration, immune dysfunction, premature ageing and increased cancer risk. The gene mutated in AT, ATM, encodes a putative lipid or protein kinase. Most of the human AT patient phenotypes are recapitulated in Atm-deficient mice. Cells derived from Atm-/- mice, like those from AT patients, exhibit abnormal response to ionizing radiation. One of the known responses to ionizing radiation is the activation of a nuclear tyrosine kinase encoded by the c-abl proto-oncogene. Ionizing radiation does not activate c-Abl in cells from AT patients or in thymocytes or fibroblasts from the Atm-deficient mice. Ectopic expression of a functional ATM kinase domain corrects this defect, as it phosphorylates the c-Abl tyrosine kinase in vitro at Ser 465, leading to the activation of c-Abl. A mutant c-Abl with Ser 465 changed to Ala 465 is not activated by ionizing radiation or ATM kinase in vivo. These findings identify the c-Abl tyrosine kinase as a downstream target of phosphorylation and activation by the ATM kinase in the cellular response to ionizing radiation.

No abstract

The ATM protein has been implicated in pathways controlling cell cycle checkpoints, radiosensitivity, genetic instability, and aging. Expression of ATM fragments containing a leucine zipper motif in a human tumor cell line abrogated the S-phase checkpoint after ionizing irradiation and enhanced radiosensitivity and chromosomal breakage. These fragments did not abrogate irradiation-induced G 1 or G 2 checkpoints, suggesting that cell cycle checkpoint defects alone cannot account for chromosomal instability in ataxia telangiectasia (AT) cells. Expression of the carboxy-terminal portion of ATM, which contains the PI-3 kinase domain, complemented radiosensitivity and the S-phase checkpoint and reduced chromosomal breakage after irradiation in AT cells. These observations suggest that ATM function is dependent on interactions with itself or other proteins through the leucine zipper region and that the PI-3 kinase domain contains much of the significant activity of ATM.Ataxia telangiectasia (AT) is a rare human autosomal recessive disorder with a wide variety of phenotypic manifestations. AT patients exhibit progressive cerebellar ataxia with degeneration of Purkinje cells, oculocutaneous telangiectases, hypersensitivity to ionizing radiation (IR) and radiomimetic drugs, thymic hypoplasia with variable cellular and humoral immune dysfunction, hypogonadism, growth retardation, and premature aging (20,55). AT homozygotes have an approximately fivefold increased risk of developing leukemia or lymphoblastic lymphomas and usually die by the second or third decade of life (22,59). Epidemiologic studies have suggested that AT heterozygotes (approximately 1% of the population) may be at a three-to fivefold increased risk of developing cancer (58, 59), although this has not yet been confirmed by population-based studies of AT carriers.Cells derived from AT patients, such as fibroblasts and lymphoblasts, exhibit a variety of abnormalities in culture such as cytoskeletal defects, hypersensitivity to IR, higher requirements for serum growth factors, and a reduced life span in culture (12,36,42). AT cells have elevated frequencies of spontaneous and induced chromosomal aberrations, including defects in immune gene rearrangements and abnormally high spontaneous rates of intrachromosomal recombinations (30,34,37,44,47). AT cells also appear to have suboptimal G 1 , S, and G 2 cell cycle checkpoints induced by DNA damage. The lack of an S-phase checkpoint in AT cells was noted by Painter and colleagues as the continued synthesis of DNA in AT cells at early time points following irradiation (known as radioresistant DNA synthesis [RDS]) (26,43). Abnormalities in the irradiation-induced G 1 checkpoint response are manifested as suboptimal induction of p53 protein and the p53-dependent gene products, p21

The ATM gene product, which is defective in the cancerprone disorder ataxia telangiectasia, has been implicated in mitogenic signal transduction, chromosome condensation, meiotic recombination and cell cycle control. The ATM gene has homology with the TEL1 gene of yeast, mutations of which lead to shortened telomeres. To test the hypothesis that the ATM gene product is involved in telomere metabolism, we examined telomeric associations (TA), telomere length, and telomerase activity in human cells expressing either dominant-negative or complementing fragments of the ATM gene. The phenotype of RKO colorectal tumor cells expressing ATM fragments containing a leucine zipper (LZ) motif mimics that of ataxia telangiectasia (A-T) cells. These transfected RKO cells relative to transfected controls had a higher frequency of cells with TA and shortened telomeres, but no detectable change in telomerase activity. In addition, the percentage of cells with TA after gamma irradiation was higher in the transfected RKO cells with dominant negative activity of the ATM gene, compared to control cells. SV40 transformed ®broblasts derived from an A-T patient and transfected with a complementing carboxyl terminal kinase region of the ATM gene had a reduced frequency of cells with TA, with no eect on the telomere length or telomerase activity. The present studies using isogenic cells with manipulated ATM function demonstrate a role for the ATM gene product in telomere metabolism.

No abstract