Shaun P. Scott

ATM associates with and phosphorylates p53: mapping the region of interaction

The human genetic disorder ataxia-telangiectasia (AT) is characterized by immunodeficiency, progressive cerebellar ataxia, radiosensitivity, cell cycle checkpoint defects and cancer predisposition. The gene mutated in this syndrome, ATM (for AT mutated), encodes a protein containing a phosphatidyl-inositol 3-kinase (PI-3 kinase)-like domain. ATM also contains a proline-rich region and a leucine zipper, both of which implicate this protein in signal transduction. The proline-rich region has been shown to bind to the SH3 domain of c-Abl, which facilitates its phosphorylation and activation by ATM (Refs 4,6). Previous results have demonstrated that AT cells are defective in the G1/S checkpoint activated after radiation damage and that this defect is attributable to a defective p53 signal transduction pathway. We report here direct interaction between ATM and p53 involving two regions in ATM, one at the amino terminus and the other at the carboxy terminus, corresponding to the PI-3 kinase domain. Recombinant ATM protein phosphorylates p53 on serine 15 near the N terminus. Furthermore, ectopic expression of ATM in AT cells restores normal ionizing radiation (IR)-induced phosphorylation of p53, whereas expression of ATM antisense RNA in control cells abrogates the rapid IR-induced phosphorylation of p53 on serine 15. These results demonstrate that ATM can bind p53 directly and is responsible for its serine 15 phosphorylation, thereby contributing to the activation and stabilization of p53 during the IR-induced DNA damage response.

Missense mutations but not allelic variants alter the function of ATM by dominant interference in patients with breast cancer

The human genetic disorder ataxia-telangiectasia (A-T) is characterized by hypersensitivity to ionizing radiation and an elevated risk of malignancy. Epidemiological data support an increased risk for breast and other cancers in A-T heterozygotes. However, screening breast cancer cases for truncating mutations in the ATM (A-T mutated) gene has failed largely to reveal an increased incidence in these patients. It has been hypothesized that ATM missense mutations are implicated in breast cancer, and there is some evidence to support this. The presence of a large variety of rare missense variants in addition to common polymorphisms in ATM makes it difficult to establish such a relationship by association studies. To investigate the functional significance of these changes we have introduced missense substitutions, identified in either A-T or breast cancer patients, into ATM cDNA before establishing stable cell lines to determine their effect on ATM function. Pathogenic missense mutations and neutral missense variants were distinguished initially by their capacity to correct the radiosensitive phenotype in A-T cells. Furthermore missense mutations abolished the radiation-induced kinase activity of ATM in normal control cells, caused chromosome instability, and reduced cell viability in irradiated control cells, whereas neutral variants failed to do so. Mutant ATM was expressed at the same level as endogenous protein, and interference with normal ATM function seemed to be by multimerization. This approach represents a means of identifying genuine ATM mutations and addressing the significance of missense changes in the ATM gene in a variety of cancers including breast cancer.

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 checkpoint control we investigated whether Schizosaccaromyces pombe chk1, a protein kinase implicated in controlling the G2 DNA damage checkpoint, might alter the radiosensitive phenotype in A-T cells. The fission yeast chk1 gene was cloned into an EBV-based vector under the control of a metallothionein promoter and transfected into A-T lymphoblastoid cells. Induction of chk1 enhanced the survival of an A-T cell line in response to radiation exposure as determined by cell viability and reduction of radiation-induced chromosome aberrations. This can be accounted for at least in part by the restoration of the G2 checkpoint to chk1 expressing cells. There was no evidence that chk1 expression corrected either the G1/S checkpoint or radioresistant DNA synthesis in S phase in these cells. These results suggest that chk1 when overexpressed acts downstream from ATM to restore the G2 checkpoint in these cells and correct the radiosensitive phenotype. These data allow us to dissociate individual checkpoint events and relate them to the radiosensitive phenotype in A-T cells.

An anti-sense construct of full-length ATM cDNA imposes a radiosensitive phenotype on normal cells

The cloning of a full-length cDNA for the gene (ATM) mutated in the human genetic disorder ataxia-telangiectasia (A-T) has been described recently. This cDNA, as well as a fragment representing a functional region from ATM, are capable of rescuing various aspects of the radiosensitive phenotype in A-T cells. We have subcloned full-length ATM cDNA in the opposite orientation in an EBV-based vector under the control of an inducible promoter to determine whether this anti-sense construct might sensitize control lymphoblastoid cells to ionizing radiation. The effectiveness of expression of this construct in control cells was monitored by loss of ATM protein which was evident over a period 6–12 h after induction. Under these conditions radiosensitivity was enhanced approximately threefold in control cells, approaching the degree of radiosensitivity observed in A-T cells. Expression of the anti-sense construct also increased the number of radiation-induced chromosomal breaks and led to the appearance of radioresistant DNA synthesis in these cells. Abrogation of the G1/S checkpoint was evident from the loss of the p53 response and that of its downstream effector, p21/WAF1, post-irradiation. The extent of accumulation of transfected cells in G2/M phase at 24 h post-irradiation was similar to that observed in A-T cells and the induction of stress-activated protein kinase by ionizing radiation was prevented by antisense ATM cDNA expression. These data demonstrate that full-length ATM anti-sense cDNA, by reducing the amount of ATM protein, is effective in imposing a series of known defects characteristic of the A-T phenotype. This inducible system provides an experimental model to further investigate mechanisms underlying radiosensitivity and cell cycle control.

Atm and cellular response to DNA damage.

Most human cancers display a myriad of genetic changes, a characteristic often attributed to genome instability. Cytogenetic studies have long identified chromosomal aberrations as a hallmark of human tumours, but the causes and consequences of genomic defects in tumours still remain to be fully understood. In particular, the role of genome instability in the development of human cancers as well as its relevance to treatment paradigms continue to evoke intense debate. To address these critical issues it is clearly important to understand the mechanisms that give rise to genome instability. This book reviews both genetic and biochemical data on the origin of genome instability and its impact on carcinogenesis. Reflecting recent discoveries and ongoing research, it discusses DNA repair mechanisms and hereditary cancer syndromes, as well as checkpoint mechanisms operating to safeguard chromosome integrity during cell cycle progression. Moreover, it summarises our current understanding of the various defects that may allow cancer cells to rapidly accumulate critical mutations and evolve, through processes reminiscent of Darwinian selection, an increasingly aggressive behaviour. Hopefully, this book will stimulate thought, discussion and experimentation, and serve as a rich source of information for a wide audience, including advanced students, researchers and clinical oncologists.

Approaches to sensitizing glioblastoma to radiotherapy: Use of lentiviral vectors

Glioblastoma multiforme (GBM) is the most common primary brain tumour and extirpation followed by radio- and chemotherapy has had minimal impact on the median survival of patients which is still less than one year. Hence, a novel therapeutic modality is required if the survival of patients with this disease is to be improved. ATM, mutated in the human genetic disorder ataxia-telangiectasia (A-T), plays a central role in the response to DNA double strand breaks and patients with this disorder are characterised by extreme sensitivity to radiation, increased risk of cancer and neurodegeneration. Thus, ATM represents a potential target for radiosensitization of brain tumour cells. A safe, non-replicating lentivirus is used to abrogate ATM in GBM through the antisense and RNAi approaches for radiosensitization. With either techniques, ATM protein was reduced by >90% and there was a 3‑fold sensitization of GBM cells to radiation. ATM protein activation as well as ATM pS1981 foci formation were defective and downstream signalling determined by Ser15 phosphorylation on p53 was reduced. Success in the approaches provides a novel and exciting strategy for the treatment of GBM and thus improving the survival of patients with these tumours.

Analyzing the regulation and function of ATM.

We describe here the cloning of full-length ataxia-telangiectasia mutated (ATM) cDNA and characterization of its activity. Full-length ATM cDNA is cloned into an inducible EBV-based vector (pMEP4) and its expression analyzed in a stably transfected cell line. ATM protein induction is monitored by immunoblotting with antibodies against both ATM and a FLAG sequence tag in the recombinant protein. Extracts from irradiated cells are immunoprecipitated with anti-ATM antibodies, and protein kinase activity is measured using p53(1-44)-specific substrate or by immunoblotting extracts with an anti-phosphoserine 15 p53-specific antibody. Missense mutations affecting ATM kinase activity are detected using in vitro mutagenesis of ATM cDNA followed by the procedures outlined above.

Role of ATM in radiation signal transduction

ATM is a member of a family of proteins that share a phosphatidylinositol 3-kinase (PI3K) domain. This group includes the catalytic subunit of DNA-dependent protein kinase (DNA-PKcs), A-T and rad3-related protein (ATR), and proteins in other organisms responsible for DNA damage recognition or cell cycle control. ATM kinase is rapidly activated by ionizing radiation to phosphorylate a series of substrates involved in radiation signaling. However, evidence also exists to show that ATM can be regulated at both the transcriptional and translational levels. A more widespread role for ATM in events other than DNA damage recognition exists including receptor signaling, cellular proliferation, K + channel activity, and insulin signaling pathways. It is possible that ATM plays a direct role in these processes, or that in its absence cellular homeostasis is altered by oxidative stress or some other form of perturbation, leading to the myriad of defects described in A-T cells. This chapter focuses on the function of ATM; its role in DNA damage recognition and its relationship to other DNA damage recognition systems; intermediates phosphorylated; and pathways activated to help coordinate the cellular response to radiation. A possible role in more general signaling is discussed later.