Ataxia-telangiectasia: chronic activation of damage-responsive functions is reduced by alpha-lipoic acid

Absence of ATM leads to altered NK cell function in mice

Ataxia-telangiectasia (A-T) is a rare disorder caused by genetic defects of A-T mutated (ATM) kinase, a key regulator of stress response, and characterized by neurodegeneration, immunodeficiency, and high incidence of cancer. Here we investigated NK cells in a mouse model of A-T (Atm-/-) showing that they are strongly impaired at killing tumor cells due to a block of early signaling events. On the other hand, in Atm-/- littermates with thymic lymphoma NK cell cytotoxicity is enhanced as compared with ATM-proficient mice, possibly via tumor-produced TNF-α. Results also suggest that expansion of exhausted NKG2D+ NK cells in Atm-/- mice is driven by low-level expression of stress-inducible NKG2D ligands, whereas development of thymoma expressing the high-affinity MULT1 ligand is associated with NKG2D down-regulation on NK cells. These results expand our understanding of immunodeficiency in A-T and encourage exploring NK cell biology in A-T patients in the attempt to identify cancer predictive biomarkers and novel therapeutic targets.

Altered NK-cell compartment and dysfunctional NKG2D/NKG2D-ligand axis in patients with ataxia-telangiectasia

Ataxia-telangiectasia (A-T) is a multisystem disorder caused by biallelic pathogenic variants in the gene encoding A-T mutated (ATM) kinase, a master regulator of the DNA damage response (DDR) pathway. Most A-T patients show cellular and/or humoral immunodeficiency that has been associated with cancer risk and reduced survival, but NK cells have not been thoroughly studied. Here we investigated NK cells of A-T patients with a special focus on the NKG2D receptor that triggers cytotoxicity upon engagement by its ligands (NKG2DLs) commonly induced via the DDR pathway on infected, transformed, and variously stressed cells. Using flow cytometry, we examined the phenotype and function of NK cells in 6 A-T patients as compared with healthy individuals. NKG2D expression was evaluated also by western blotting and RT-qPCR; plasma soluble NKG2DLs (sMICA, sMICB, sULBP1, ULBP2) were measured by ELISA. Results showed that A-T NK cells were skewed towards the CD56^neg anergic phenotype and displayed decreased expression of NKG2D and perforin. NKG2D was reduced at the protein but not at the mRNA level and resulted in impaired NKG2D-mediated cytotoxicity in 4/6 A-T patients. Moreover, in A-T plasma we found 24-fold and 2-fold increase of sMICA and sULBP1, respectively, both inversely correlated with NKG2D expression. Overall, NK cells are disturbed in A-T patients showing reduced NKG2D expression, possibly caused by persistent engagement of its ligands, that may contribute to susceptibility to cancer and infections and represent novel targets for therapeutic interventions.

ATM Kinase-Dependent Regulation of Autophagy: A Key Player in Senescence?

Increasing evidence suggests a strong interplay between autophagy and genomic stability. Recently, several papers have demonstrated a molecular connection between the DNA Damage Response (DDR) and autophagy and have explored how this link influences cell fate and the choice between apoptosis and senescence in response to different stimuli. The aberrant deregulation of this interplay is linked to the development of pathologies, including cancer and neurodegeneration. Ataxia-telangiectasia mutated kinase (ATM) is the product of a gene that is lost in Ataxia-Telangiectasia (A-T), a rare genetic disorder characterized by ataxia and cerebellar neurodegeneration, defects in the immune response, higher incidence of lymphoma development, and premature aging. Importantly, ATM kinase plays a central role in the DDR, and it can finely tune the balance between senescence and apoptosis: activated ATM promotes autophagy and in particular sustains the lysosomal-mitochondrial axis, which in turn promotes senescence and inhibits apoptosis. Therefore, ATM is the key factor that enables cells to escape apoptosis by entering senescence through modulation of autophagy. Importantly, unlike apoptotic cells, senescent cells are viable and have the ability to secrete proinflammatory and mitogenic factors, thus influencing the cellular environment. In this review we aim to summarize recent advances in the understanding of molecular mechanisms linking DDR and autophagy to senescence, pointing out the role of ATM kinase in these cellular responses. The significance of this regulation in the pathogenesis of Ataxia-Telangiectasia will be discussed.

The cerebellar degeneration in ataxia-telangiectasia: A case for genome instability

Research on the molecular pathology of genome instability disorders has advanced our understanding of the complex mechanisms that safeguard genome stability and cellular homeostasis at large. Once the culprit genes and their protein products are identified, an ongoing dialogue develops between the research lab and the clinic in an effort to link specific disease symptoms to the functions of the proteins that are missing in the patients. Ataxi A-T elangiectasia (A-T) is a prominent example of this process. A-T's hallmarks are progressive cerebellar degeneration, immunodeficiency, chronic lung disease, cancer predisposition, endocrine abnormalities, segmental premature aging, chromosomal instability and radiation sensitivity. The disease is caused by absence of the powerful protein kinase, ATM, best known as the mobilizer of the broad signaling network induced by double-strand breaks (DSBs) in the DNA. In parallel, ATM also functions in the maintenance of the cellular redox balance, mitochondrial function and turnover and many other metabolic circuits. An ongoing discussion in the A-T field revolves around the question of which ATM function is the one whose absence is responsible for the most debilitating aspect of A-T - the cerebellar degeneration. This review suggests that it is the absence of a comprehensive role of ATM in responding to ongoing DNA damage induced mainly by endogenous agents. It is the ensuing deterioration and eventual loss of cerebellar Purkinje cells, which are very vulnerable to ATM absence due to a unique combination of physiological features, which kindles the cerebellar decay in A-T.

Lipoic Acid Synergizes with Antineoplastic Drugs in Colorectal Cancer by Targeting p53 for Proteasomal Degradation

Lipoic acid (LA) is a redox-active disulphide compound, which functions as a pivotal co-factor for mitochondrial oxidative decarboxylation. LA and chemical derivatives were shown to target mitochondria in cancer cells with altered energy metabolism, thereby inducing cell death. In this study, the impact of LA on the tumor suppressor protein p53 was analyzed in various colorectal cancer (CRC) cell lines, with a focus on the mechanisms driving p53 degradation. First, LA was demonstrated to trigger the depletion of both wildtype and mutant p53 protein in all CRC cells tested without influencing its gene expression and preceded LA-triggered cytotoxicity. Depletion of p53 coincided with a moderate, LA-dependent ROS production, but was not rescued by antioxidant treatment. LA induced the autophagy receptor p62 and differentially modulated autophagosome formation in CRC cells. However, p53 degradation was not mediated via autophagy as shown by chemical inhibition and genetic abrogation of autophagy. LA treatment also stabilized and activated the transcription factor Nrf2 in CRC cells, which was however dispensable for p53 degradation. Mechanistically, p53 was found to be readily ubiquitinylated and degraded by the proteasomal machinery following LA treatment, which did not involve the E3 ubiquitin ligase MDM2. Intriguingly, the combination of LA and anticancer drugs (doxorubicin, 5-fluorouracil) attenuated p53-mediated stabilization of p21 and resulted in synergistic killing in CRC cells in a p53-dependant manner.

Pro-fluorescent mitochondria-targeted real-time responsive redox probes synthesised from carboxy isoindoline nitroxides: Sensitive probes of mitochondrial redox status in cells

Here we describe new fluorescent probes based on fluorescein and rhodamine that provide reversible, real-time insight into cellular redox status. The new probes incorporate bio-imaging relevant fluorophores derived from fluorescein and rhodamine linked with stable nitroxide radicals such that they cannot be cleaved, either spontaneously or enzymatically by cellular processes. Overall fluorescence emission is determined by reversible reduction and oxidation, hence the steady state emission intensity reflects the balance between redox potentials of critical redox couples within the cell. The permanent positive charge on the rhodamine-based probes leads to their rapid localisation within mitochondria in cells. Reduction and oxidation also leads to marked changes in the fluorophore lifetime, enabling monitoring by fluorescence lifetime imaging microscopy. Finally, we demonstrate that administration of a methyl ester version of the rhodamine-based probe can be used at concentrations as low as 5 nM to generate a readily detected response to redox stress within cells as analysed by flow cytometry.

Inflammation, a significant player of Ataxia-Telangiectasia pathogenesis?

INTRODUCTION

Ataxia-Telangiectasia (A-T) syndrome is an autosomal recessive neurodegenerative disorder characterized by cerebellar ataxia, oculocutaneous telangiectasia, immunodeficiency, chromosome instability, radiosensitivity, and predisposition to malignancy. There is growing evidence that A-T patients suffer from pathologic inflammation that is responsible for many symptoms of this syndrome, including neurodegeneration, autoimmunity, cardiovascular disease, accelerated aging, and insulin resistance. In addition, epidemiological studies have shown A-T heterozygotes, somewhat like deficient patients, are susceptible to ionizing irradiation and have a higher risk of cancers and metabolic disorders.

AREA COVERED

This review summarizes clinical and molecular findings of inflammation in A-T syndrome.

CONCLUSION

Ataxia-Telangiectasia Mutated (ATM), a master regulator of the DNA damage response is the protein known to be associated with A-T and has a complex nuclear and cytoplasmic role. Loss of ATM function may induce immune deregulation and systemic inflammation.

Ataxia-telangiectasia (A-T): An emerging dimension of premature ageing

A-T is a prototype genome instability syndrome and a multifaceted disease. A-T leads to neurodegeneration - primarily cerebellar atrophy, immunodeficiency, oculocutaneous telangiectasia (dilated blood vessels), vestigial thymus and gonads, endocrine abnormalities, cancer predisposition and varying sensitivity to DNA damaging agents, particularly those that induce DNA double-strand breaks. With the recent increase in life expectancy of A-T patients, the premature ageing component of this disease is gaining greater awareness. The complex A-T phenotype reflects the ever growing number of functions assigned to the protein encoded by the responsible gene - the homeostatic protein kinase, ATM. The quest to thoroughly understand the complex A-T phenotype may reveal yet elusive ATM functions.

Ataxia telangiectasia: a review

DEFINITION OF THE DISEASE

Ataxia telangiectasia (A-T) is an autosomal recessive disorder primarily characterized by cerebellar degeneration, telangiectasia, immunodeficiency, cancer susceptibility and radiation sensitivity. A-T is often referred to as a genome instability or DNA damage response syndrome.

EPIDEMIOLOGY

The world-wide prevalence of A-T is estimated to be between 1 in 40,000 and 1 in 100,000 live births.

CLINICAL DESCRIPTION

A-T is a complex disorder with substantial variability in the severity of features between affected individuals, and at different ages. Neurological symptoms most often first appear in early childhood when children begin to sit or walk. They have immunological abnormalities including immunoglobulin and antibody deficiencies and lymphopenia. People with A-T have an increased predisposition for cancers, particularly of lymphoid origin. Pulmonary disease and problems with feeding, swallowing and nutrition are common, and there also may be dermatological and endocrine manifestations.

ETIOLOGY

A-T is caused by mutations in the ATM (Ataxia Telangiectasia, Mutated) gene which encodes a protein of the same name. The primary role of the ATM protein is coordination of cellular signaling pathways in response to DNA double strand breaks, oxidative stress and other genotoxic stress.

DIAGNOSIS

The diagnosis of A-T is usually suspected by the combination of neurologic clinical features (ataxia, abnormal control of eye movement, and postural instability) with one or more of the following which may vary in their appearance: telangiectasia, frequent sinopulmonary infections and specific laboratory abnormalities (e.g. IgA deficiency, lymphopenia especially affecting T lymphocytes and increased alpha-fetoprotein levels). Because certain neurological features may arise later, a diagnosis of A-T should be carefully considered for any ataxic child with an otherwise elusive diagnosis. A diagnosis of A-T can be confirmed by the finding of an absence or deficiency of the ATM protein or its kinase activity in cultured cell lines, and/or identification of the pathological mutations in the ATM gene.

DIFFERENTIAL DIAGNOSIS

There are several other neurologic and rare disorders that physicians must consider when diagnosing A-T and that can be confused with A-T. Differentiation of these various disorders is often possible with clinical features and selected laboratory tests, including gene sequencing.

ANTENATAL DIAGNOSIS

Antenatal diagnosis can be performed if the pathological ATM mutations in that family have been identified in an affected child. In the absence of identifying mutations, antenatal diagnosis can be made by haplotype analysis if an unambiguous diagnosis of the affected child has been made through clinical and laboratory findings and/or ATM protein analysis.

GENETIC COUNSELING

Genetic counseling can help family members of a patient with A-T understand when genetic testing for A-T is feasible, and how the test results should be interpreted.

MANAGEMENT AND PROGNOSIS

Treatment of the neurologic problems associated with A-T is symptomatic and supportive, as there are no treatments known to slow or stop the neurodegeneration. However, other manifestations of A-T, e.g. immunodeficiency, pulmonary disease, failure to thrive and diabetes can be treated effectively.

The disulfide compound α-lipoic acid and its derivatives: A novel class of anticancer agents targeting mitochondria

The endogenous disulfide α-lipoic acid (LA) is an essential mitochondrial co-factor. In addition, LA and its reduced counterpart dihydro lipoic acid form a potent redox couple with antioxidative functions, for which it is used as dietary supplement and therapeutic. Recently, it has gained attention due to its cytotoxic effects in cancer cells, which is the key aspect of this review. We initially recapitulate the dietary occurrence, gastrointestinal absorption and pharmacokinetics of LA, illustrating its diverse antioxidative mechanisms. We then focus on its mode of action in cancer cells, in which it triggers primarily the mitochondrial pathway of apoptosis, whereas non-transformed primary cells are hardly affected. Furthermore, LA impairs oncogenic signaling and displays anti-metastatic potential. Novel LA derivatives such as CPI-613, which target mitochondrial energy metabolism, are described and recent pre-clinical studies are presented, which demonstrate that LA and its derivatives exert antitumor activity in vivo. Finally, we highlight clinical studies currently performed with the LA analog CPI-613. In summary, LA and its derivatives are promising candidates to complement the arsenal of established anticancer drugs due to their mitochondria-targeted mode of action and non-genotoxic properties.

The discovery and development of new potential antioxidant agents for the treatment of neurodegenerative diseases

INTRODUCTION

Several neurodegenerative disorders (NDs) including Alzheimer's and Huntington's diseases have had associations with the oxidative process and free radical damage. Consequently, in past decades, several natural and synthetic antioxidants have been assessed as therapeutic agents but have shown limitations in bioavailability, metabolic susceptibility and permeability to the blood brain barrier. Given these issues, medicinal chemists are hard at work to modify/improve the chemical structures of these antioxidants, thereby improving their efficacy.

AREAS COVERED

In this review, the authors critically analyze several biological mechanisms involved in the generation of free radicals. Additionally, they analyze free radicals' role in the generation of oxidative stress and in the progression of many NDs. Further, the authors review a collection of natural and synthetic antioxidants, their role as free radical scavengers along with their mechanisms of action and their potential for preventing neurodegenerative diseases.

EXPERT OPINION

So far, preclinical studies on several antioxidants have shown promise for treating NDs, despite their limitations. The authors do highlight the lack of the adequate animal models for preclinical assessment and this does hinder further progression into clinical trials. Further studies are necessary to fully investigate the potential of these antioxidants as ND therapeutic options.

The Aspergillus nidulans ATM kinase regulates mitochondrial function, glucose uptake and the carbon starvation response

Mitochondria supply cellular energy and also perform a role in the adaptation to metabolic stress. In mammals, the ataxia-telangiectasia mutated (ATM) kinase acts as a redox sensor controlling mitochondrial function. Subsequently, transcriptomic and genetic studies were utilized to elucidate the role played by a fungal ATM homolog during carbon starvation. In Aspergillus nidulans, AtmA was shown to control mitochondrial function and glucose uptake. Carbon starvation responses that are regulated by target of rapamycin (TOR) were shown to be AtmA-dependent, including autophagy and hydrolytic enzyme secretion. AtmA also regulated a p53-like transcription factor, XprG, inhibiting starvation-induced XprG-dependent protease secretion and cell death. Thus, AtmA possibly represents a direct or indirect link between mitochondrial stress, metabolism, and growth through the influence of TOR and XprG function. The coordination of cell growth and division with nutrient availability is crucial for all microorganisms to successfully proliferate in a heterogeneous environment. Mitochondria supply cellular energy but also perform a role in the adaptation to metabolic stress and the cross-talk between prosurvival and prodeath pathways. The present study of Aspergillus nidulans demonstrated that AtmA also controlled mitochondrial mass, function, and oxidative phosphorylation, which directly or indirectly influenced glucose uptake. Carbon starvation responses, including autophagy, shifting metabolism to the glyoxylate cycle, and the secretion of carbon scavenging enzymes were AtmA-dependent. Transcriptomic profiling of the carbon starvation response demonstrated how TOR signaling and the retrograde response, which signals mitochondrial dysfunction, were directly or indirectly influenced by AtmA. The AtmA kinase was also shown to influence a p53-like transcription factor, inhibiting starvation-induced XprG-dependent protease secretion and cell death. Therefore, in response to metabolic stress, AtmA appears to perform a role in the regulation of TOR signaling, involving the retrograde and SnfA pathways. Thus, AtmA may represent a link between mitochondrial function and cell cycle or growth, possibly through the influence of the TOR and XprG function.

The appropriateness of the mouse model for ataxia-telangiectasia: neurological defects but no neurodegeneration

Patients with ataxia-telangiectasia (A-T) are characterised by genome instability, cancer predisposition and a progressive neurodegeneration. A number of model systems have been developed for A-T but none recapitulate all the phenotype. The majority of these models have been generated in mice. While Atm deficient mouse models exhibit much of the phenotype described in patients with A-T, the broad consensus is that they do not display the most debilitating aspect of A-T, i.e. neurodegeneration. Cerebellar atrophy is one of the neuronal characteristics of A-T patients due to defects in neuronal development and progressive loss of Purkinje and granule cells. This is not evident in Atm-deficient mutants but there are multiple reports on neurological abnormalities in these mice. The focus of this review is to evaluate the appropriateness of Atm mutant mouse models for A-T, particularly with reference to neurological abnormalities and how they might relate to neurodegeneration.

Oxidative stress as a cofactor in spinocerebellar ataxia type 2

Spinocerebellar ataxia type 2 (SCA2) is a redox-sensitive neurodegenerative disease affecting the cerebellum, fibre connections in the cerebellum, the peripheral nervous system, and extracerebellar central pathways. Currently, Cuba has the highest reported global rate for this disease. The aim of this review article is to summarize and discuss the current knowledge about evidence of oxidative stress during SCA2. Recent reports have suggested that ataxin 2 and other related factors contribute to the redox imbalance in this disease. It is important to recognize and clarify the molecular mechanisms associated with the redox imbalance to consider ataxias innovative approaches to counteract oxidative stress-induced tissue damage, through alternative therapeutic or nutritional intervention in SCA2 and related diseases.

All stressed out without ATM kinase

Ataxia-telangiectasia (A-T) is a rare, neurodegenerative, inherited disease arising from mutations in the kinase A-T mutated (ATM), which promotes cell cycle checkpoints and DNA double-strand break repair. Puzzlingly, these ATM activities fail to fully explain A-T neuropathologies, which instead have links to stress induced by reactive oxygen species (ROS). However, a landmark discovery reveals an unexpected intersection of ROS and kinase signaling: ATM can be directly activated by oxidation to form a disulfide-linked dimer in a mechanism distinct from DNA damage activation. When combined with notable structural-based insights into the ATM homolog DNA-PK (DNA-protein kinase) and mTOR (mammalian target of rapamycin), these results suggest conformation and assembly mechanisms to signal oxidative stress through an ATM nodal point. These findings fundamentally affect our understanding of ROS and ATM signaling and of the A-T phenotype, with implications for altering signaling in cancer cells to increase sensitivities to current therapeutic interventions.

Iron loading and oxidative stress in the Atm-/- mouse liver

Ataxia-Telangiectasia (A-T) is an autosomal recessive disorder resulting in a myriad of abnormalities, including progressive neurodegeneration and cancer predisposition. At the cellular level, A-T is a disease of chronic oxidative stress (OS) causing damage to proteins, lipids, and DNA. OS is contributed to by pro-oxidative transition metals such as iron that catalyze the conversion of weakly reactive oxygen species to highly reactive hydroxyl radicals. Iron-associated OS has been linked to neurodegeneration in Alzheimer's and Parkinson's diseases and development of lymphoid tumors (which afflict ∼30% of A-T patients). To investigate iron regulation in A-T, iron indexes, regulatory genes, and OS markers were studied in livers of wild-type and Ataxia telangiectasia mutated (Atm) null mice on control or high-iron diets. Atm(-/-) mice had increased serum iron, hepatic iron, and ferritin and significantly higher Hepcidin compared with wild-type mice. When challenged with the high-iron diet, Bmp6 and Hfe expression was significantly increased. Atm(-/-) mice had increased protein tyrosine nitration and significantly higher Heme Oxygenase (decycling) 1 levels that were substantially increased by a high-iron diet. Ferroportin gene expression was significantly increased; however, protein levels were unchanged. We demonstrate that Atm(-/-) mice have a propensity to accumulate iron that is associated with a significant increase in hepatic OS. The iron-induced increase in hepcidin peptide in turn suppresses ferroportin protein levels, thus nullifying the upregulation of mRNA expression in response to increased OS. Our results suggest that increased iron status may contribute to the chronic OS seen in A-T patients and development of disease pathology.

Requirement of the ATM/p53 tumor suppressor pathway for glucose homeostasis

Ataxia telangiectasia (A-T) patients can develop multiple clinical pathologies, including neuronal degeneration, an elevated risk of cancer, telangiectasias, and growth retardation. Patients with A-T can also exhibit an increased risk of insulin resistance and type 2 diabetes. The ATM protein kinase, the product of the gene mutated in A-T patients (Atm), has been implicated in metabolic disease, which is characterized by insulin resistance and increased cholesterol and lipid levels, blood pressure, and atherosclerosis. ATM phosphorylates the p53 tumor suppressor on a site (Ser15) that regulates transcription activity. To test whether the ATM pathway that regulates insulin resistance is mediated by p53 phosphorylation, we examined insulin sensitivity in mice with a germ line mutation that replaces the p53 phosphorylation site with alanine. The loss of p53 Ser18 (murine Ser15) led to increased metabolic stress, including severe defects in glucose homeostasis. The mice developed glucose intolerance and insulin resistance. The insulin resistance correlated with the loss of antioxidant gene expression and decreased insulin signaling. N-Acetyl cysteine (NAC) treatment restored insulin signaling in late-passage primary fibroblasts. The addition of an antioxidant in the diet rendered the p53 Ser18-deficient mice glucose tolerant. This analysis demonstrates that p53 phosphorylation on an ATM site is an important mechanism in the physiological regulation of glucose homeostasis.

45S rDNA regions are chromosome fragile sites expressed as gaps in vitro on metaphase chromosomes of root-tip meristematic cells in Lolium spp

BACKGROUND

In humans, chromosome fragile sites are regions that are especially prone to forming non-staining gaps, constrictions or breaks in one or both of the chromatids on metaphase chromosomes either spontaneously or following partial inhibition of DNA synthesis and have been well identified. So far, no plant chromosome fragile sites similar to those in human chromosomes have been reported.

METHODS AND RESULTS

During the course of cytological mapping of rDNA on ryegrass chromosomes, we found that the number of chromosomes plus chromosome fragments was often more than the expected 14 in most cells for Lolium perenne L. cv. Player by close cytological examination using a routine chromosome preparation procedure. Further fluorescent in situ hybridization (FISH) using 45S rDNA as a probe indicated that the root-tip cells having more than a 14-chromosome plus chromosome fragment count were a result of chromosome breakage or gap formation in vitro (referred to as chromosome lesions) at 45S rDNA sites, and 86% of the cells exhibited chromosome breaks or gaps and all occurred at the sites of 45S rDNA in Lolium perenne L. cv. Player, as well as in L. multiflorum Lam. cv. Top One. Chromatin depletion or decondensation occurred at various locations within the 45S rDNA regions, suggesting heterogeneity of lesions of 45S rDNA sites with respect to their position within the rDNA region.

CONCLUSIONS

The chromosome lesions observed in this study are very similar cytologically to that of fragile sites observed in human chromosomes, and thus we conclude that the high frequency of chromosome lesions in vitro in Lolium species is the result of the expression of 45S rDNA fragile sites. Possible causes for the spontaneous expression of fragile sites and their potential biological significance are discussed.

The neurological phenotype of ataxia-telangiectasia: solving a persistent puzzle

Human genomic instability syndromes affect the nervous system to different degrees of severity, attesting to the vulnerability of the CNS to perturbations of genomic integrity and the DNA damage response (DDR). Ataxia-telangiectasia (A-T) is a typical genomic instability syndrome whose major characteristic is progressive neuronal degeneration but is also associated with immunodeficiency, cancer predisposition and acute sensitivity to ionizing radiation and radiomimetic chemicals. A-T is caused by loss or inactivation of the ATM protein kinase, which mobilizes the complex, multi-branched cellular response to double strand breaks in the DNA by phosphorylating numerous DDR players. The link between ATM's function in the DDR and the neuronal demise in A-T has been questioned in the past. However, recent studies of the ATM-mediated DDR in neurons suggest that the neurological phenotype in A-T is indeed caused by deficiency in this function, similar to other features of the disease. Still, major issues concerning this phenotype remain open, including the presumed differences between the DDR in post-mitotic neurons and proliferating cells, the nature of the damage that accumulates in the DNA of ATM-deficient neurons under normal life conditions, the mode of death of ATM-deficient neurons, and the lack of a major neuronal phenotype in the mouse model of A-T. A-T remains a prototype disease for the study of the DDR's role in CNS development and maintenance.

Intrinsic mitochondrial dysfunction in ATM-deficient lymphoblastoid cells

One of the characteristic features of cells from patients with ataxia telangiectasia (A-T) is that they are in a state of continuous oxidative stress and exhibit constitutive activation of pathways that normally respond to oxidative damage. In this report, we investigated whether the oxidative stress phenotype of A-T cells might be a reflection of an intrinsic mitochondrial dysfunction. Mitotracker Red staining showed that the structural organization of mitochondria in A-T cells was abnormal compared to wild-type. Moreover, A-T cells harbored a much larger population of mitochondria with decreased membrane potential (DeltaPsi) than control cells. In addition, the basal expression levels of several nuclear DNA-encoded oxidative damage responsive genes whose proteins are targeted to the mitochondria--polymerase gamma, mitochondrial topoisomerase I, peroxiredoxin 3 and manganese superoxide dismutase--are elevated in A-T cells. Consistent with these results, we found that overall mitochondrial respiratory activity was diminished in A-T compared to wild-type cells. Treating A-T cells with the antioxidant, alpha lipoic acid (ALA), restored mitochondrial respiration rates to levels approaching those of wild-type. When wild-type cells were transfected with ATM-targeted siRNA, we observed a small but significant reduction in the respiration rates of mitochondria. Moreover, mitochondria in A-T cells induced to stably express full-length ATM, exhibited respiration rates approaching those of wild-type cells. Taken together, our results provide evidence for an intrinsic mitochondrial dysfunction in A-T cells, and implicate a requirement for ATM in the regulation of mitochondrial function.

Glutathione levels in blood from ataxia telangiectasia patients suggest in vivo adaptive mechanisms to oxidative stress

OBJECTIVE

To evaluate an in vivo pro-oxidant state in patients with ataxia telangiectasia (AT).

METHODS

A set of oxidative stress endpoints were measured in 9 AT homozygotes, 16 AT heterozygotes (parents) and 83 controls (grouped in age ranges as for patients and parents, respectively). The following analytes were measured: (a) leukocyte 8-hydroxy-2'-deoxyguanosine (8-OHdG); (b) blood glutathione (GSSG and GSH); and (c) plasma levels of glyoxal (Glx) and methylglyoxal (MGlx).

RESULTS

AT patients displayed a significant decrease in blood GSSG (p=0.012) and in MGlx plasma concentrations (p=0.012). A non-significant decrease in the GSSG:GSH ratio (p=0.1) and a non-significant increase in 8-OHdG and Glx levels were observed in AT patients vs. young controls (age range 4-35 years). AT heterozygotes failed to display any significant changes vs. adult controls (age range 36-68 years).

CONCLUSION

No significant increase in oxidative stress biomarkers was detected in blood from AT patients. The decrease in GSSG and MGlx levels in AT patients may suggest an adaptive response to a pro-oxidant state in AT-related target organs.

Induction of base damages representing a high risk site for double-strand DNA break formation in genomic DNA by exposure of cells to DNA damaging agents

DNA repair is known as a defense mechanism against genotoxic insults. However, the most lethal type of DNA damages, double-strand DNA breaks (DSBs), can be produced by DNA repair. We have previously demonstrated that when long patch base excision repair attempts to repair a synthetic substrate containing two uracils, the repair produces DSBs (Vispe, S. and Satoh, M. S. (2000) J. Biol. Chem. 275, 27386-27392 and Vispe, S., Ho, E. L., Yung, T. M., and Satoh, M. S. (2003) J. Biol. Chem. 278, 35279-35285). In this synthetic substrate, the two uracils are located on the opposite DNA strands (separated by an intervening sequence stable at 37 degrees C) and represent a high risk site for DSB formation. It is not clear, however, whether similar high risk sites are also induced in genomic DNA by exposure to DNA damaging agents. Thus, to investigate the mechanisms of DSB formation, we have modified the DSB formation assay developed previously and demonstrated that high risk sites for DSB formation are indeed generated in genomic DNA by exposure of cells to alkylating agents. In fact, genomic DNA containing alkylated base damages, which could represent high risk sites, are converted into DSBs by enzymes present in extracts prepared from cells derived from clinically normal individuals. Furthermore, DSBs are also produced by extracts from cells derived from ataxia-telangiectasia patients who show cancer proneness due to an impaired response to DSBs. These results suggest the presence of a novel link between base damage formation and DSBs and between long patch base excision repair and human diseases that occur due to an impaired response to DSB.

α-Lipoic acid and glutathione protect against the prooxidant activity of SOD/catalase mimetic manganese salen derivatives

Manganese(III) N,N'-ethylenebis(salicylideneiminato) chloride (Mn-salen chloride) and manganese(III) N,N'-ethylenebis(3-methoxysalicylideneiminato) chloride (Mn-(3,3'-MeO)salen chloride) are in vitro superoxide dismutase and catalase mimetics. They protect against free radical-related disease in animals, but Mn-salen can also be a potent prooxidant, damaging free DNA. Mn-salen protects human fibroblast DNA against hydrogen peroxide damage, however, damage to free DNA was confirmed by the comet assay. The DNA-damaging activity was dramatically reduced by co-administration with glutathione with the combination being less damaging to free DNA than either molecule alone. alpha-Lipoic acid, an antioxidant disulfide commonly used as a dietary supplement, also prevented Mn-salen prooxidant activity. Mn-(3,3'-MeO)salen protected fibroblasts against hydrogen peroxide as efficiently as Mn-salen and showed little damaging activity against free DNA. Protection was invested by both complexes in the presence and in the absence of EDTA, a potential competing chelator. Stabilities of the complexes with respect to decomposition and inactivation were studied by spectroscopic and electrochemical techniques. The complexes' binding to, and cleavage of, DNA was measured using a quartz crystal resonant sensor. Mn-salen was shown to bind strongly to DNA, prior to cleaving it; Mn-(3,3'-MeO)salen bound weakly and left DNA intact. Co-administration of either glutathione or alpha-lipoic acid appears to inhibit binding by Mn-salen thus preventing DNA-cleavage.

Modifications of nuclear architecture and chromatin organization in ataxia telangiectasia cells are coupled to changes of gene transcription

Ataxia telangiectasia (AT) is a rare genetic disorder caused by mutations of ATM gene. ATM kinase is a "master controller" of DNA-damage response and signal transducer of external stimuli. The complex role of ATM may explain the pleiotropic phenotype characteristic of AT syndrome, only partially. In our hypothesis, the multi-faceted phenotype of AT patients might depend on specific chromatin reorganization, which then reflects on the cellular transcription. We analyzed three lymphoblastoid cell-lines isolated from AT patients and one healthy control. The three-dimensional reconstruction disclosed marked changes of nuclear morphology and architecture in AT cells. When chromatin condensation was analyzed by differential scanning calorimetry, a remodeling was observed at the level of fiber folding and nucleosome conformation. Despite the structural differences, chromatin did not exhibit modifications of the average acetylation status in comparison to the control. Moreover, AT cells presented significant alterations in the transcription of genes involved in cell-cycle regulation and stress response. In AT3RM cells, the average chromatin decondensation went with the upregulation of c-fos, c-jun, and c-myc and downregulation of metallothioneins, p21 and p53. AT9RM and AT44RM cells were instead characterized by an increased chromatin condensation and presented a different transcription unbalance. Whereas in AT44RM all the considered genes were downregulated, in AT3RM the three oncogenes and metallothioneins were upregulated, but p53 and p21 were downregulated.

Ataxia-Telangiectasia-Mutated-Dependent Activation of Ku in Human Fibroblasts Exposed to Hydrogen Peroxide

DNA is damaged in cells during cell replication, by infection, or by various environmental stresses. The damaged cells stop cell cycle, repair damaged DNA, and when repaired progress into the next cell cycle stage. But when the attempt to repair the damage fails, the cells undergo apoptosis. The most deleterious damage of all is double-strand DNA breaks (DSBs), where ATM (ataxia-telangiectasia-mutated) serves as a sensor. The ATM pathway culminates in DNA repair through nonhomologous end-joining or through homologous recombination. Upon DNA damage, the DNA repair protein Ku70/80 translocates into the nucleus, which may be mediated by ATM. Previously, we found that pancreatic acinar cells undergo apoptosis upon oxidative stress, and the cell death stems from nuclear loss of Ku70/80. This study aims to investigate whether ATM has a role in Ku activation and prevention of cell death induced by oxidative stress (hydrogen peroxide) using A-T fibroblasts stably transfected with human full-length ATM cDNA or empty vector. As a result, hydrogen peroxide-induced cell death was augmented in A-T cells transfected with empty vector while cell death was prevented in A-T fibroblasts stably transfected with human full-length ATM cDNA. Ku DNA-binding activity induced by hydrogen peroxide treatment was increased in the A-T fibroblasts stably transfected with human full-length ATM cDNA compared to that in A-T cells transfected with empty vector. The results suggest that ATM may be essential for Ku activation to repair DNA damage from oxidative stress and prevent cell death caused by oxidative stress.

Characterization of the breast cancer associated ATM 7271T>G (V2424G) mutation by gene expression profiling

Mutations in ATM are responsible for the autosomal recessive disorder ataxia telangiectasia. Heterozygous mutations in ATM have been associated with an elevated risk of breast cancer. We previously reported one breast cancer family in which ATM 7271T>G (V2424G) segregated with disease, and apparently acted in a dominant negative manner. We now report the screening of 782 multiple-case breast cancer families that identified two additional index cases with ATM 7271T>G. Phylogenetic sequence analysis showed that V2424 is a highly conserved residue, and that the 2424G variant is likely to interfere with function. To elucidate the consequences of this mutation, we expression profiled wild-type, heterozygous, and homozygous lymphoblastoid cell lines (LCLs) from Scottish and Australian families using an oligonucleotide microarray. Cluster analysis revealed 77 genes that were differentially expressed in homozygous and heterozygous V2424G cells (compared to wild-type) and 11 genes differentially expressed in the homozygous cells. We also evaluated the profiles of LCLs after exposure to ionizing radiation (IR) and identified 77 genes that were differentially expressed in wild-type cells, but not in homozygous or heterozygous V2424G cells. We validated the expression differences by RT-PCR in additional heterozygous V2424G LCLs from another breast cancer family. We found no consistent cytotoxicity or abrogation of ATM kinase activity after IR in seven heterozygous V2424G LCLs, compared to wild-type LCLs, but did find an increase in the number of chromosomal aberrations. These data suggest that the V2424G missense mutation acts largely as a dominant negative in terms of the associated expression profiles.

Iron chelators reduce chromosomal breaks in ataxia-telangiectasia cells

Ataxia-telangiectasia (A-T) is characterized by ataxia, genomic instability, and increased cancer incidence. Previously, iron chelator concentrations which suppressed normal cell colony formation increased A-T cell colony formation. Similarly, iron chelators preferentially increased A-T cell colony formation following peroxide exposure compared to normal cells. Last, A-T cells exhibited increased short-term sensitivity to labile iron exposure compared to normal cells, an event corrected by recombinant ATM (rATM) expression. Since chromosomal damage is important in A-T pathology and iron chelators exert beneficial effects on A-T cells, we hypothesized that iron chelators would reduce A-T cell chromosomal breaks. We treated A-T, normal, and A-T cells expressing rATM with labile iron, iron chelators, antioxidants, and t-butyl hydroperoxide, and examined chromosomal breaks and ATM activation. Additionally, the effect of ATM-deficiency on transferrin receptor (TfR) expression and TfR activity blockage in A-T and syngeneic A-T cells expressing rATM was examined. We report that (1) iron chelators and iron-free media reduce spontaneous and t-butyl hydroperoxide-induced chromosomal breaks in A-T, but not normal, or A-T cells expressing rATM; (2) labile iron exposure induces A-T cell chromosomal breaks, an event lessened with rATM expression; (3) desferal, labile iron, and copper activate ATM; (4) A-T cell TfR expression is lowered with rATM expression and (5) blocking TfR activity with anti-TfR antibodies increases A-T cell colony formation, while lowering chromosomal breaks. ATM therefore functions in iron responses and the maintenance of genomic stability following labile iron exposure.

The role of double-strand break repair - insights from human genetics

The efficient repair of DNA double-strand breaks is crucial in safeguarding the genomic integrity of organisms. Responses to double-strand breaks include complex signal-transduction, cell-cycle-checkpoint and repair pathways. Defects in these pathways lead to several human disorders with pleiotropic clinical features. Dissection of the molecular basis that underlies the diverse clinical features is enhancing our understanding of the damage-response mechanisms and their role in development, and might ultimately facilitate treatment.

Iron chelators increase the resistance of Ataxia telangeictasia cells to oxidative stress

Ataxia telangeictasia (A-T) is an autosomal recessive disorder characterized by immune dysfunction, genomic instability, chronic oxidative damage, and increased cancer incidence. Previously, desferal was found to increase the resistance of A-T, but not normal cells to exogenous oxidative stress in the colony forming-efficiency assay, suggesting that iron metabolism is dysregulated in A-T. Since desferal both chelates iron and modulates gene expression, we tested the effects of apoferritin and the iron chelating flavonoid quercetin on A-T cell colony-forming ability. We demonstrate that apoferritin and quercetin increase the ability of A-T cells to form colonies. We also show that labile iron levels are significantly elevated in Atm-deficient mouse sera compared to syngeniec wild type mice. Our findings support a role for labile iron acting as a Fenton catalyst in A-T, contributing to the chronic oxidative stress seen in this disease. Our findings further suggest that iron chelators might promote the survival of A-T cells and hence, individuals with A-T.

The impact of carboxy nitroxide antioxidants on irradiated ataxia telangiectasia cells

Three water-soluble carboxy nitroxide antioxidants, 5-carboxy-1,1,3,3-tetramethylisoindolin-2-yloxyl, 4-carboxy-2,2,6,6-tetramethylpiperidin-1-yloxyl, and 3-carboxy-2,2,5,5-tetramethylpyrrolidin-1-yloxyl, show significant impact on the postirradiation survival rates of ataxia telangiectasia (A-T) cells compared to normal cells, an assay which represents a model for understanding the impact of ROS damage on the A-T phenotype. The effects of these antioxidants are much more significant than those of vitamin E or Trolox (a water-soluble vitamin E analog), studied using the same cell survival model.

Symptomatic and Disease-Modifying Therapy for the Progressive Ataxias

BACKGROUND

The progressive ataxias are a diverse group of neurologic diseases that share features of degeneration of the cerebellum and its inflow/outflow pathways but differ in etiology, course, and associated noncerebellar system involvement. Some will have treatable causes, but for most, the pathophysiology is incompletely known.

REVIEW SUMMARY

Treatment strategies will include (1) definitive therapy when available, (2) symptomatic treatment and prevention of complications, and (3) rehabilitation and support resources. The physician will have to decide whether to introduce or approve the use of therapies based on as yet-unproven mechanisms or the use of complementary medicine approaches.

CONCLUSIONS

There are as yet no drugs that have been approved by the Food and Drug Administration for the treatment of the progressive ataxias and relatively few disease-modifying therapies, but symptomatic and rehabilitation interventions can greatly improve the quality of life of individuals with these disabling neurodegenerative disorders.

Degradation of p53, not telomerase activation, by E6 is required for bypass of crisis and immortalization by human papillomavirus type 16 E6/E7

Bypass of two arrest points is essential in the process of cellular immortalization, one of the components of the transformation process. Expression of human papillomavirus type 16 E6 and E7 together can escape both senescence and crisis, processes which normally limit the proliferative capacity of primary human keratinocytes. Crisis is thought to be mediated by telomere shortening. Because E6 stimulates telomerase activity and exogenous expression of the TERT gene with E7 can immortalize keratinocytes, this function is thought to be important for E6 to cooperate with E7 to bypass crisis. However, it has also been reported that E6 dissociates increased telomerase activity from maintenance of telomere length and that a dominant-negative p53 molecule can substitute for E6 in cooperative immortalization of keratinocytes with E7. Thus, to determine which functions of E6 are required to allow bypass of crisis and immortalization of keratinocytes with E7, immortalization assays were performed using specific mutants of E6, in tandem with E7. In these experiments, every clone expressing an E6 mutant capable of degrading p53 was able to bypass crisis and immortalize, regardless of telomerase induction. All clones containing E6 mutants incapable of degrading p53 died at crisis. These results suggest that the ability of E6 to induce degradation of p53 compensates for continued telomere shortening in E6/E7 cells and demonstrate that degradation of p53 is required for immortalization by E6/E7, while increased telomerase activity is dispensable.

Stable siRNA-mediated silencing of ATM alters the transcriptional profile of HeLa cells

The ATM protein, which is mutated in the inherited disease ataxia telangiectasia (AT), is a key regulator of the cells' DNA damage response. AT cells also exhibit constitutive activation of transcriptional regulators such as p53, E2F, AP1, and NFkappaB. Inactivation of ATM may therefore alter the cells' transcriptional profile. ATM expression in HeLa cells was silenced with siRNA expressed from a plasmid based vector, generating a stable cell line, HeLaATM601. HeLaATM601 cells displayed minimal levels of ATM protein and had a 10-fold increase in sensitivity to ionizing radiation. DNA microarray analysis demonstrated that 35 genes were upregulated and five genes were downregulated in HeLaATM601 cells. Genes upregulated in the absence of ATM included interferon-response proteins, cell cycle regulators, integral membrane proteins, and adhesion and extracellular matrix proteins. Using real-time PCR, these genes were also upregulated in cells derived from AT patients. Inactivation of the ATM protein therefore has a significant impact on the transcriptional profile of the cell.

Oxidative stress is responsible for deficient survival and dendritogenesis in purkinje neurons from ataxia-telangiectasia mutated mutant mice

Atm gene-disrupted mice recapitulate the majority of characteristics observed in patients with the genetic disorder ataxia-telangiectasia (A-T). However, although they exhibit defects in neuromotor function and a distinct neurological phenotype, they do not show the progressive neurodegeneration seen in human patients, but there is evidence that ataxia-telangiectasia mutated (Atm)-deficient animals have elevated levels of oxidized macromolecules and some neuropathology. We report here that in vitro survival of cerebellar Purkinje cells from both Atm "knock-out" and Atm "knock-in" mice was significantly reduced compared with their wild-type littermates. Although most of the Purkinje neurons from wild-type mice exhibited extensive dendritic elongation and branching under these conditions, most neurons from Atm-deficient mice had dramatically reduced dendritic branching. An antioxidant (isoindoline nitroxide) prevented Purkinje cell death in Atm-deficient mice and enhanced dendritogenesis to wild-type levels. Furthermore, administration of the antioxidant throughout pregnancy had a small enhancing effect on Purkinje neuron survival in Atm gene-disrupted animals and protected against oxidative stress in older animals. These data provide strong evidence for a defect in the cerebellum of Atm-deficient mice and suggest that oxidative stress contributes to this phenotype.

Requirement of the MRN complex for ATM activation by DNA damage

The ATM protein kinase is a primary activator of the cellular response to DNA double-strand breaks (DSBs). In response to DSBs, ATM is activated and phosphorylates key players in various branches of the DNA damage response network. ATM deficiency causes the genetic disorder ataxia-telangiectasia (A-T), characterized by cerebellar degeneration, immunodeficiency, radiation sensitivity, chromosomal instability and cancer predisposition. The MRN complex, whose core contains the Mre11, Rad50 and Nbs1 proteins, is involved in the initial processing of DSBs. Hypomorphic mutations in the NBS1 and MRE11 genes lead to two other genomic instability disorders: the Nijmegen breakage syndrome (NBS) and A-T like disease (A-TLD), respectively. The order in which ATM and MRN act in the early phase of the DSB response is unclear. Here we show that functional MRN is required for ATM activation, and consequently for timely activation of ATM-mediated pathways. Collectively, these and previous results assign to components of the MRN complex roles upstream and downstream of ATM in the DNA damage response pathway and explain the clinical resemblance between A-T and A-TLD.

Slow progression of ataxia-telangiectasia with double missense and in frame splice mutations

Ataxia-telangiectasia (A-T) is caused by mutations of the ATM gene, the product of which is involved in the regulation of cellular responses to radiation damage. Ataxia usually starts in early childhood but a delayed age at onset and slower rate of neurological deterioration has been found for some patients with variant A-T. Only few patients have been documented to survive into the 4th decade. We report on a patient with an attenuated form of A-T who was diagnosed as having A-T by the age of 52 years and died by the age of 60 years. He was found to be a compound heterozygote for a double missense mutation (D2625E and A2626P) and a novel splicing mutation (496 + 5G --> A) of the ATM gene. Cytogenetic studies of the patient's lymphoblastoid cells revealed modest levels of bleomycin-induced chromosomal instability. Residual ATM protein was found at a level of 10-20% of wildtype. Low residual ATM kinase activity could be demonstrated towards p53, whereas it was poorly detectable towards nibrin. Our results corroborate the view that the clinical variability of A-T is partly determined by the mutation type and indicate that A-T can extend to late adulthood disease.

Linoleic acid and antioxidants protect against DNA damage and apoptosis induced by palmitic acid

Polyunsaturated fats are the main target for lipid peroxidation and subsequent formation of mutagenic metabolites, but diets high in saturated fats are more strongly associated with adverse health effects. We show that the common saturated fatty acid, palmitic acid, is a potent inducer of DNA damage in an insulin-secreting cell line, and in primary human fibroblasts. Damage is not associated with upregulation of inducible nitric oxide synthase, but is prevented by two different antioxidants, alpha-lipoic acid and 3,3'-methoxysalenMn(III) (EUK134), which also partly prevent palmitic acid-induced apoptosis and growth inhibition. Since mutagenic metabolites can be formed from peroxidation of polyunsaturated fatty acids, co-administration of palmitic and a polyunsaturated fatty acid might be particularly harmful. Palmitic acid-induced DNA damage is instead prevented by linoleic acid, which is acting here as a protective agent against oxidative stress, rather than as a source of mutagenic metabolites. These results illustrate the complexity of the relationship of dietary fat intake to genotoxicity.

Spinocerebellar degeneration

The spinocerebellar degenerations/ataxias (SCAs) are a diverse group of rare, slowly progressive, neurological diseases, often inherited but of incompletely understood pathophysiology, which affect the cerebellum and its related pathways. They have few animal models and share no reliable biomarkers. They have, as yet, no universally validated rating scale for use in clinical trials. In the past 25 years, there have been, at most, 18 controlled (Class 1) trials for ataxia, which have focused on neurotransmitter mechanisms. There is currently only one National Institute of Neurological Disorders and Stroke-sponsored drug trial for ataxia (Phase I study of idebenone in Friedreich's ataxia). There are, as yet, no FDA-approved drugs for SCA. Current treatment practices encompass rehabilitation interventions and off-label use of symptomatic medications [1,2].

Oxidative stress in ataxia telangiectasia

Ataxia telangiectasia is one of a group of recessive hereditary genomic instability disorders and is characterized by progressive neurodegeneration, immunodeficiency and cancer susceptibility. Heterozygotes for the mutated gene are more susceptible to cancer and to ischaemic heart disease. The affected gene, ATM (ataxia telangiectasia mutated), has been cloned and codes for a protein kinase (ATM), which orchestrates the cellular response to DNA double-strand breaks after ionising radiation. An underlying feature of ataxia telangiectasia is oxidative stress and there is chronic activation of stress response pathways in tissues showing pathology such as the cerebellum, but not in the cerebrum or liver. ATM has also been shown to be activated by insulin and to have a wider role in signal transduction and cell growth. Many, but not all, aspects of the phenotype can be attributed to a defective DNA damage response. The oxidative stress may result directly from accumulated DNA damage in affected tissues or ATM may have an additional role in sensing/modulating redox homeostasis. The basis for the observed tissue specificity of the oxidative damage in ataxia telangiectasia is not clear.

Ataxia-telangiectasia: diagnosis and treatment

Much progress has been made in the early diagnosis of ataxia-telangiectasia since the gene was cloned in 1995, A clinical diagnosis can now be confirmed by radiosensitivity testing (colony survival assay), immunoblotting, and mutation detection. The diagnostic value of serum alpha-fetoprotein levels and radiosensitivity has been reevaluated using patients with diagnoses based on the presence of mutations in the ATM gene and the absence of ATM protein in nuclear extracts. Little progress has been made in treating the progressive ataxia.

Dysregulation of DNA repair pathways in a transforming growth factor alpha/c-myc transgenic mouse model of accelerated hepatocarcinogenesis

Previous work from our laboratory has implicated oxidative DNA damage and genetic instability in the etiology of transforming growth factor-alpha (TGFalpha)/c-myc-associated hepatocarcinogenesis. In contrast, oxidative DNA damage was lower in c-myc single-transgenic mice, consistent with less chromosomal damage and with later and more benign tumor formation. We examined whether defects in the DNA repair pathways contribute to the acceleration of liver cancer in TGFalpha/c-myc mice. A cDNA expression array containing 140 known genes and multiplex RT-PCR were used to compare the basal levels of expression of DNA repair genes at the dysplastic stage. Thirty-five percent (8/23) and 43% (10/23) of DNA repair genes were constitutively up-regulated in 10-week-old TGFalpha/c-myc and c-myc transgenic livers, respectively, compared with wild-type controls. The commonly up-regulated genes were OGG1 and NTH1 of base excision repair; ERCC5, RAD23A, and RAD23B of nucleotide excision repair; and RAD50, RAD52, and RAD54 involved in DNA strand break repair. Additional treatment with a peroxisome proliferator, Wy-14,643, known to increase the level of oxidants in the liver, failed to induce a further increase in the expression level of DNA repair enzymes in TGFalpha/c-myc but not in c-myc or wild-type livers. Moreover, expression of several genes, including Ku80, PMS2, and ATM, was decreased in TGFalpha/c-myc livers, suggesting a fault or inefficient activation of the DNA repair pathway upon induction of oxidative stress. Together, the results show that DNA damage response is attenuated in TGFalpha/c-myc mice, creating a condition that may contribute to acceleration of liver cancer in this model.

Identification of a novel protein kinase mediating Akt survival signaling to the ATM protein

We identified a novel human AMP-activated protein kinase (AMPK) family member, designated ARK5, encoding 661 amino acids with an estimated molecular mass of 74 kDa. The putative amino acid sequence reveals 47, 45.8, 42.4, and 55% homology to AMPK-alpha1, AMPK-alpha2, MELK, and SNARK, respectively, suggesting that it is a new member of the AMPK family. It has a putative Akt phosphorylation motif at amino acids 595-600, and Ser(600) was found to be phosphorylated by active Akt resulting in the activation of kinase activity toward the SAMS peptide, a consensus AMPK substrate. During nutrient starvation, ARK5 supported the survival of cells in an Akt-dependent manner. In addition, we also demonstrated that ARK5, when activated by Akt, phosphorylated the ATM protein that is mutated in the human genetic disorder ataxia-telangiectasia and also induced the phosphorylation of p53. On the basis of our current findings, we propose that a novel AMPK family member, ARK5, is the tumor cell survival factor activated by Akt and acts as an ATM kinase under the conditions of nutrient starvation.

Upregulation of FasL and apoptosis in thymic lymphomas in Atm knock-in mice

Several members of the phosphatidylinositol 3-kinase family play key roles in recognising and responding to damage in DNA, induced by a variety of chemicals and other agents. One of these, ATM, the product of the gene mutated in the human genetic disorder ataxia-telangiectasia (A-T), recognises double strand breaks in DNA caused by ionizing radiation and radiomimetic chemicals. In order to study DNA damage recognition and the abnormalities of genome instability and cancer predisposition that occur in A-T patients, we generated a mouse model expressing a mutant form of Atm corresponding to a common human mutation. In this model, a 9 nucleotide in-frame deletion was introduced into the Atm gene and has been designated Atm-Delta SRI. These animals had a longer lifespan than Atm gene disrupted mice (Atm(-/-)) and they developed less thymic lymphomas. A characteristic of the lymphomas appearing in Atm-Delta SRI mice was an increased rate of apoptosis compared to the corresponding tumours in Atm(-/-) mice. Increased expression of FasL in these tumours may account for the higher levels of apoptosis. These results demonstrate that expression of mutant Atm in mice gives rise to phenotypic differences compared to Atm(-/-) mice and has implications for heterogeneity described in the human syndrome.

Regulation of the IRF-1 tumour modifier during the response to genotoxic stress involves an ATM-dependent signalling pathway

The mechanism by which genotoxic stress induces IRF-1 and the signalling components upstream of this anti-oncogenic transcription factor during the response to DNA damage are not known. We demonstrate that IRF-1 and the tumour suppressor protein p53 are coordinately up-regulated during the response to DNA damage in an ATM-dependent manner. Induction of IRF-1 protein by either ionizing radiation (IR) or etoposide occurs through a concerted mechanism involving increased IRF-1 expression/synthesis and an increase in the half-life of the IRF-1 protein. A striking defect in the induction of both IRF-1 mRNA and IRF-1 protein was observed in ATM deficient cells. Although ATM deficient cells failed to increase IRF-1 in response to genotoxic stress, the induction of IRF-1 in response to viral mimetics remained intact. Re-expression of the ATM kinase in AT cells restored the DNA damage inducibility of IRF-1, whilst the PI-3 kinase inhibitor wortmannin inhibited IRF-1 induction by DNA damage in ATM-positive cells. The data highlight a role for the ATM kinase in orchestrating the coordinated induction and transcriptional cooperation of IRF-1 and p53 to regulate p21 expression. Thus, IRF-1 is controlled by two distinct signalling pathways; a JAK/STAT-signalling pathway in viral infected cells and an ATM-signalling pathway in DNA damaged cells.

Radiation Research Society. 1952-2002. Historical and current highlights in radiation biology: has anything important been learned by irradiating cells?

Around 30 years ago, a very prominent molecular biologist confidently proclaimed that nothing of fundamental importance has ever been learned by irradiating cells! The poor man obviously did not know about discoveries such as DNA repair, mutagenesis, connections between mutagenesis and carcinogenesis, genomic instability, transposable genetic elements, cell cycle checkpoints, or lines of evidence historically linking the genetic material with nucleic acids, or origins of the subject of oxidative stress in organisms, to name a few things of fundamental importance learned by irradiating cells that were well known even at that time. Early radiation studies were, quite naturally, phenomenological. They led to the realization that radiations could cause pronounced biological effects. This was followed by an accelerating expansion of investigations of the nature of these radiobiological phenomena, the beginnings of studies aimed toward better understanding the underlying mechanisms, and a better appreciation of the far-reaching implications for biology, and for society in general. Areas of principal importance included acute tissue and tumor responses for applications in medicine, whole-body radiation effects in plants and animals, radiation genetics and cytogenetics, mutagenesis, carcinogenesis, cellular radiation responses including cell reproductive death, cell cycle effects and checkpoint responses, underlying molecular targets leading to biological effects, DNA repair, and the genetic control of radiosensitivity. This review summarizes some of the highlights in these areas, and points to numerous examples where indeed, many things of considerable fundamental importance have been learned by irradiating cells.

Cellular response to oxidative stress: signaling for suicide and survival

Reactive oxygen species (ROS), whether produced endogenously as a consequence of normal cell functions or derived from external sources, pose a constant threat to cells living in an aerobic environment as they can result in severe damage to DNA, protein, and lipids. The importance of oxidative damage to the pathogenesis of many diseases as well as to degenerative processes of aging has becoming increasingly apparent over the past few years. Cells contain a number of antioxidant defenses to minimize fluctuations in ROS, but ROS generation often exceeds the cell's antioxidant capacity, resulting in a condition termed oxidative stress. Host survival depends upon the ability of cells and tissues to adapt to or resist the stress, and repair or remove damaged molecules or cells. Numerous stress response mechanisms have evolved for these purposes, and they are rapidly activated in response to oxidative insults. Some of the pathways are preferentially linked to enhanced survival, while others are more frequently associated with cell death. Still others have been implicated in both extremes depending on the particular circumstances. In this review, we discuss the various signaling pathways known to be activated in response to oxidative stress in mammalian cells, the mechanisms leading to their activation, and their roles in influencing cell survival. These pathways constitute important avenues for therapeutic interventions aimed at limiting oxidative damage or attenuating its sequelae.