Human cDNA clones that modify radiomimetic sensitivity of ataxia-telangiectasia (group A) cells

ATM: genome stability, neuronal development, and cancer cross paths

One of the cornerstones of the web of signaling pathways governing cellular life and differentiation is the DNA damage response. It spans a complex network of pathways, ranging from DNA repair to modulation of numerous processes in the cell. DNA double-strand breaks (DSBs), which are formed as a result of genotoxic stress or normal recombinational processes, are extremely lethal lesions that rapidly mobilize this intricate defense system. The master controller that pilots cellular responses to DSBs is the ATM protein kinase, which turns on this network by phosphorylating key players in its various branches. ATM is the protein product of the gene mutated in the human genetic disorder ataxia-telangiectasia (A-T), which is characterized by neuronal degeneration, immunodeficiency, sterility, genomic instability, cancer predisposition, and radiation sensitivity. The clinical and cellular phenotype of A-T attests to the numerous roles of ATM, on the one hand, and to the link between the DNA damage response and developmental processes on the other hand. Recent studies of this protein and its effectors, combined with a thorough investigation of animal models of A-T, have led to new insights into the mode of action of this master controller of the DNA damage response. The evidence that ATM is involved in signaling pathways other than those related to damage response, particularly ones relating to cellular growth and differentiation, reinforces the multifaceted nature of this protein, in which genome stability, developmental processes, and cancer cross paths.

A critical role for Pin2/TRF1 in ATM-dependent regulation. Inhibition of Pin2/TRF1 function complements telomere shortening, radiosensitivity, and the G(2)/M checkpoint defect of ataxia-telangiectasia cells

Cells derived from patients with the human genetic disorder ataxia-telangiectasia (A-T) display many abnormalities, including telomere shortening, premature senescence, and defects in the activation of S phase and G(2)/M checkpoints in response to double-strand DNA breaks induced by ionizing radiation. We have previously demonstrated that one of the ATM substrates is Pin2/TRF1, a telomeric protein that binds the potent telomerase inhibitor PinX1, negatively regulates telomere elongation, and specifically affects mitotic progression. Following DNA damage, ATM phosphorylates Pin2/TRF1 and suppresses its ability to induce abortive mitosis and apoptosis (Kishi, S., Zhou, X. Z., Nakamura, N., Ziv, Y., Khoo, C., Hill, D. E., Shiloh, Y., and Lu, K. P. (2001) J. Biol. Chem. 276, 29282-29291). However, the functional importance of Pin2/TRF1 in mediating ATM-dependent regulation remains to be established. To address this question, we directly inhibited the function of endogenous Pin2/TRF1 in A-T cells by stable expression of two different dominant-negative Pin2/TRF1 mutants and then examined their effects on telomere length and DNA damage response. Both the Pin2/TRF1 mutants increased telomere length in A-T cells, as shown in other cells. Surprisingly, both the Pin2/TRF1 mutants reduced radiosensitivity and complemented the G(2)/M checkpoint defect without inhibiting Cdc2 activity in A-T cells. In contrast, neither of the Pin2/TRF1 mutants corrected the S phase checkpoint defect in the same cells. These results indicate that inhibition of Pin2/TRF1 in A-T cells is able to bypass the requirement for ATM in specifically restoring telomere shortening, the G(2)/M checkpoint defect, and radiosensitivity and demonstrate a critical role for Pin2/TRF1 in the ATM-dependent regulation of telomeres and DNA damage response.

ATM mutations in patients with ataxia telangiectasia screened by a hierarchical strategy

ATM has been identified as a gene that is responsible for ataxia telangiectasia (AT), a pleiotropic disorder of autosomal recessive inheritance. While many mutations of this gene in AT patients of various ethnicities have been reported, data on Japanese patients are scarce. In this report, we present the results of a thorough survey of ATM mutations in 14 unrelated AT patients, with an emphasis on Japanese subjects. We used a hierarchical strategy in which we extensively analyzed the entire coding region of the cDNA. In the first stage, point mutations were sought by PCR-SSCP in short patches. In the second and third stages, the products of medium- and long-patch PCR, each covering the entire region, were examined by agarose gel electrophoresis to search for length changes. We found a total of 15 mutations (including 12 new) and 4 polymorphisms. Abnormal splicing of ATM was frequent among Japanese, and no hotspot was obvious, suggesting no strong founder effects in this ethnic group. Eleven patients carried either one homozygous or two compound heterozygous mutations, one patient carried only one detectable heterozygous mutation, and no mutation was found in two patients. Overall, mutations were found in at least 75% of the different ATM alleles examined. Possible reasons for the inability to detect mutations in some patients are discussed.

Requirement of sequences outside the conserved kinase domain of fission yeast Rad3p for checkpoint control

The fission yeast Rad3p checkpoint protein is a member of the phosphatidylinositol 3-kinase-related family of protein kinases, which includes human ATMp. Mutation of the ATM gene is responsible for the disease ataxia-telangiectasia. The kinase domain of Rad3p has previously been shown to be essential for function. Here, we show that although this domain is necessary, it is not sufficient, because the isolated kinase domain does not have kinase activity in vitro and cannot complement a rad3 deletion strain. Using dominant negative alleles of rad3, we have identified two sites N-terminal to the conserved kinase domain that are essential for Rad3p function. One of these sites is the putative leucine zipper, which is conserved in other phosphatidylinositol 3-kinase-related family members. The other is a novel motif, which may also mediate Rad3p protein-protein interactions.

Is Fanconi anemia caused by a defect in the processing of DNA damage?

Fanconi anemia (FA) is an autosomal genetic disease characterized by a complex array of developmental disorders, a high predisposition to bone marrow failure and to acute myelogenous leukemia. The chromosomal instability and the hypersensitivity to DNA cross-linking agents led to its classification with the DNA repair disorders. This review aimed at establishing whether it is still appropriate to consider 1/approximately FA within a DNA repair framework taking into account the recently discovered genetic heterogeneity characteristics of the defect (eight complementation groups). We discuss the possibility that the FA proteins interact to form a complex which may control different functions, including the processing of specific DNA lesions. Such a complex may act as a sensor to initiate protective systems as well as transcription of specific genes specifying, among others proteins, growth factors. Such steps may be organized as a linear cascade or more likely under the form of a web network.

Genotype-phenotype relationships in ataxia-telangiectasia and variants

Ataxia-telangiectasia (A-T) is an autosomal recessive disorder characterized by cerebellar degeneration, immunodeficiency, chromosomal instability, radiosensitivity, and cancer predisposition. A-T cells are sensitive to ionizing radiation and radiomimetic chemicals and fail to activate cell-cycle checkpoints after treatment with these agents. The responsible gene, ATM, encodes a large protein kinase with a phosphatidylinositol 3-kinase-like domain. The typical A-T phenotype is caused, in most cases, by null ATM alleles that truncate or severely destabilize the ATM protein. Rare patients with milder manifestations of the clinical or cellular characteristics of the disease have been reported and have been designated "A-T variants." A special variant form of A-T is A-TFresno, which combines a typical A-T phenotype with microcephaly and mental retardation. The possible association of these syndromes with ATM is both important for understanding their molecular basis and essential for counseling and diagnostic purposes. We quantified ATM-protein levels in six A-T variants, and we searched their ATM genes for mutations. Cell lines from these patients exhibited considerable variability in radiosensitivity while showing the typical radioresistant DNA synthesis of A-T cells. Unlike classical A-T patients, these patients exhibited 1%-17% of the normal level of ATM. The underlying ATM genotypes were either homozygous for mutations expected to produce mild phenotypes or compound heterozygotes for a mild and a severe mutation. An A-TFresno cell line was found devoid of the ATM protein and homozygous for a severe ATM mutation. We conclude that certain "A-T variant" phenotypes represent ATM mutations, including some of those without telangiectasia. Our findings extend the range of phenotypes associated with ATM mutations.

Ataxia-telangiectasia and the Nijmegen breakage syndrome: related disorders but genes apart

Gene mutations provide valuable clues to cellular metabolism. In humans such insights come mainly from genetic disorders. Ataxia-telangiectasia (A-T) and Nijmegen breakage syndrome (NBS) are two distinct but closely related, single gene disorders that highlight a complex junction of several signal transduction pathways. These pathways appear to control defense mechanisms against specific types of damage to cellular macromolecules, and probably regulate the processing of certain types of DNA damage or normal intermediates of DNA metabolism. A-T is characterized primarily by cerebellar degeneration, immunodeficiency, genome instability, clinical radiosensitivity, and cancer predisposition. NBS shares all these features except cerebellar deterioration. The cellular phenotypes of A-T and NBS are almost indistinguishable, however, and include chromosomal instability, radiosensitivity, and defects in cell cycle checkpoints normally induced by ionizing radiation. The recent identification of the gene responsible for A-T, ATM, has revealed its product to be a large, constitutively expressed phosphoprotein with a carboxy-terminal region similar to the catalytic domain of phosphatidylinositol 3-kinases (PI 3-kinases). ATM is a member of a family of proteins identified in various organisms, which share the PI 3-kinase domain and are involved in regulation of cell cycle progression and response to genotoxic agents. Some of these proteins, most notably the DNA-dependent protein kinase, have an associated protein kinase activity, and preliminary data indicate this activity in ATM as well. Mutations in A-T patients are null alleles that truncate or destabilize the ATM protein. Atm-deficient mice recapitulate the human phenotype with slower nervous-system degeneration. Two ATM interactors, c-Abl and p53, underscore its role in cellular responses to genotoxic stress. The complexity of ATM's structure and mode of action make it a paradigm of multifaceted signal transduction proteins involved in many physiological pathways via multiple protein-protein interactions. The as yet unknown NBS protein may be a component in an ATM-based complex, with a key role in sensing and processing specific DNA damage or intermediates and signaling their presence to the cell cycle machinery.

Phenotypic correction of ataxia-telangiectasia cellular defect by exogenously introduced human or mouse subchromosomal fragments

A human-mouse hybrid containing a human 11q22-23 fragment including the ATM locus was used to examine its capability to correct the cellular defect of ataxia-telangiectasia (A-T). Examination of 21 A-T-derived hybrids indicated that the acquired radioresistance was observed in the clones where the 11q22-23 fragment was transferred intact, but not in those where donor-derived 11q segment was lost. In one exceptional clone, the ATM locus was deleted from the transferred fragment, while it was still partially radioresistant. This partially radioresistant clone was found to include the mouse-derived fragment containing the Atm gene, the mouse homologue of human ATM gene. Similar association of partial radioresistance with the presence of mouse Atm gene was observed in three additional hybrids. The results indicate that the cellular A-T defect can be corrected by the mouse subchromosomal fragment containing the Atm gene as well as by the human 11q22-23 fragment containing the ATM gene, but apparently to a lesser extent in the former.

Isolation of full-length ATM cDNA and correction of the ataxia-telangiectasia cellular phenotype

A gene mutated in the human genetic disorder ataxia-telangiectasia (A-T), ATM, was recently identified by positional cloning. ATM is a member of the phosphatidylinositol-3-kinase superfamily, some of which are protein kinases and appear to have important roles in cell cycle control and radiation signal transduction. We describe herein, to our knowledge, for the first time, the cloning of a full-length cDNA for ATM and correction of multiple aspects of the radio-sensitive phenotype of A-T cells by transfection with this cDNA. Overexpression of ATM cDNA in A-T cells enhanced the survival of these cells in response to radiation exposure, decreased radiation-induced chromosome aberrations, reduced radio-resistant DNA synthesis, and partially corrected defective cell cycle checkpoints and induction of stress-activated protein kinase. This correction of the defects in A-T cells provides further evidence of the multiplicity of effector functions of the ATM protein and suggests possible approaches to gene therapy.

Fragments of ATM which have dominant-negative or complementing activity

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 G1 or G2 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.

The genetic defect in ataxia-telangiectasia

The autosomal recessive human disorder ataxia-telangiectasia (A-T) was first described as a separate disease entity 40 years ago. It is a multisystem disease characterized by progressive cerebellar ataxia, oculocutaneous telangiectasia, radiosensitivity, predisposition to lymphoid malignancies and immunodeficiency, with defects in both cellular and humoral immunity. The pleiotropic nature of the clinical and cellular phenotype suggests that the gene product involved is important in maintaining stability of the genome but also plays a more general role in signal transduction. The chromosomal instability and radiosensitivity so characteristic of this disease appear to be related to defective activation of cell cycle checkpoints. Greater insight into the nature of the defect in A-T has been provided by the recent identification, by positional cloning, of the responsible gene, ATM. The ATM gene is related to a family of genes involved in cellular responses to DNA damage and/or cell cycle control. These genes encode large proteins containing a phosphatidylinositol 3-kinase domain, some of which have protein kinase activity. The mutations causing A-T completely inactivate or eliminate the ATM protein. This protein has been detected and localized to different subcellular compartments.

Generation of a panel of radiation-reduced hybrids containing human 11q22-23 fragments bearing a HPRT selective marker: identification of hybrids carrying various subregions around the ataxia-telangiectasia locus

A human-mouse monochromosomal hybrid that contains a human t(X;11) translocated chromosome carrying pter-->q23 segment of chromosome 11 was used to construct a panel of radiation-reduced hybrids. The hypoxhanthine phosphoribosyltransferase (HPRT) gene located close to the translocation breakpoint was used as a marker to select for the hybrids that preferentially retain the 11q22-23 region. Twenty-three HAT-resistant hybrids were isolated and screened by polymerase chain reaction (PCR) for the retention of 31 loci on 11q22-23 region. Among the 14 hybrids that had breakpoints within the 11q22-23 region, 6 hybrids contained fragments that extend either from centromere or telomere to the 5-Mb region spanned by GRIA4 and FDX, carrying various breakpoints within the region. This subpanel could be a potential resource to analyze the ataxia-telangiectasia disease locus and its neighboring region.

Ataxia-telangiectasia: a multifaceted genetic disorder associated with defective signal transduction

The gene responsible for the defect in the human genetic disorder ataxia-telangiectasia, ATM, was cloned recently. The part of the gene coding for a phosphatidylinositol 3-kinase domain showed it to be related to a family of genes involved in signal transduction, cell cycle control and the response to DNA damage. The elucidation of the role of the ATM gene product will provide valuable insight into the radiosensitivity, cancer predisposition, immunodeficiency and neuropathology that characterize this syndrome.

A single ataxia telangiectasia gene with a product similar to PI-3 kinase

A gene, ATM, that is mutated in the autosomal recessive disorder ataxia telangiectasia (AT) was identified by positional cloning on chromosome 11q22-23. AT is characterized by cerebellar degeneration, immunodeficiency, chromosomal instability, cancer predisposition, radiation sensitivity, and cell cycle abnormalities. The disease is genetically heterogeneous, with four complementation groups that have been suspected to represent different genes. ATM, which has a transcript of 12 kilobases, was found to be mutated in AT patients from all complementation groups, indicating that it is probably the sole gene responsible for this disorder. A partial ATM complementary DNA clone of 5.9 kilobases encoded a putative protein that is similar to several yeast and mammalian phosphatidylinositol-3' kinases that are involved in mitogenic signal transduction, meiotic recombination, and cell cycle control. The discovery of ATM should enhance understanding of AT and related syndromes and may allow the identification of AT heterozygotes, who are at increased risk of cancer.