Ataxia-telangiectasia and the ATM gene: linking neurodegeneration, immunodeficiency, and cancer to cell cycle checkpoints

How do neurons live long and healthy? The mechanism of neuronal genome integrity

Genome DNA of neurons in the brain is unstable, and mutations caused by inaccurate repair can lead to neurodevelopmental and neurodegenerative disorders. Damage to the neuronal genome is induced both exogenously and endogenously. Rapid cell proliferation of neural stem cells during embryonic brain development can lead to errors in genome duplication. Electrical excitations and drastic changes in gene expression in functional neurons cause risks of damaging genomic DNA. The precise repair of DNA damages caused by events making genomic DNA unstable maintains neuronal functions. The maintenance of the DNA sequence and structure of the genome is known as genomic integrity. Molecular mechanisms that maintain genomic integrity are critical for healthy neuronal function. In this review, we describe recent progress in understanding the genome integrity in functional neurons referring to their disruptions reported in neurological diseases.

Shared brain transcriptomic signature in TDP-43 type A FTLD patients with or without GRN mutations

Frontotemporal lobar degeneration with TDP-43 inclusions (FTLD-TDP) is a complex heterogeneous neurodegenerative disorder for which mechanisms are poorly understood. To explore transcriptional changes underlying FTLD-TDP, we performed RNA-sequencing on 66 genetically unexplained FTLD-TDP patients, 24 FTLD-TDP patients with GRN mutations and 24 control participants. Using principal component analysis, hierarchical clustering, differential expression and coexpression network analyses, we showed that GRN mutation carriers and FTLD-TDP-A patients without a known mutation shared a common transcriptional signature that is independent of GRN loss-of-function. After combining both groups, differential expression as compared to the control group and coexpression analyses revealed alteration of processes related to immune response, synaptic transmission, RNA metabolism, angiogenesis and vesicle-mediated transport. Deconvolution of the data highlighted strong cellular alterations that were similar in FTLD-TDP-A and GRN mutation carriers with NSF as a potentially important player in both groups. We propose several potentially druggable pathways such as the GABAergic, GDNF and sphingolipid pathways. Our findings underline new disease mechanisms and strongly suggest that affected pathways in GRN mutation carriers extend beyond GRN and contribute to genetically unexplained forms of FTLD-TDP-A.

R-loop Mediated DNA Damage and Impaired DNA Repair in Spinal Muscular Atrophy

Defects in DNA repair pathways are a major cause of DNA damage accumulation leading to genomic instability and neurodegeneration. Efficient DNA damage repair is critical to maintain genomicstability and support cell function and viability. DNA damage results in the activation of cell death pathways, causing neuronal death in an expanding spectrum of neurological disorders, such as amyotrophic lateral sclerosis (ALS), Parkinson’s disease (PD), Alzheimer’s disease (AD), and spinal muscular atrophy (SMA). SMA is a neurodegenerative disorder caused by mutations in the Survival Motor Neuron 1 (SMN1) gene. SMA is characterized by the degeneration of spinal cord motor neurons due to low levels of the SMN protein. The molecular mechanism of selective motor neuron degeneration in SMA was unclear for about 20 years. However, several studies have identified biochemical and molecular mechanisms that may contribute to the predominant degeneration of motor neurons in SMA, including the RhoA/ROCK, the c-Jun NH₂-terminal kinase (JNK), and p53-mediated pathways, which are involved in mediating DNA damage-dependent cell death. Recent studies provided insight into selective degeneration of motor neurons, which might be caused by accumulation of R-loop-mediated DNA damage and impaired non-homologous end joining (NHEJ) DNA repair pathway leading to genomic instability. Here, we review the latest findings involving R-loop-mediated DNA damage and defects in neuron-specific DNA repair mechanisms in SMA and discuss these findings in the context of other neurodegenerative disorders linked to DNA damage.

Progressive choreodystonia in X-linked hyper-IgM immunodeficiency: a rare but recurrent presentation

An association between movement disorders and immune‐system dysfunction has been described in the context of rare genetic diseases such as ataxia telangiectasia as well as infectious encephalopathies. We encountered a male patient who presented immunodeficiency of unknown etiology since childhood. A medication‐refractory, progressive choreodystonic movement disorder emerged at the age of 42 years and prompted an exome‐wide molecular testing approach. This revealed a pathogenic hemizygous variant in CD40LG, the gene implicated in X‐linked hyper‐IgM syndrome. Only two prior reports have specifically suggested a causal relationship between CD40LG mutations and involuntary hyperkinetic movements. Our findings thus confirm the existence of a particular CD40LG‐related condition, combining features of compromised immunity with neurodegenerative movement abnormalities. Establishing the diagnosis is crucial because of potential life‐threatening immunological complications.

From yeast to humans: Understanding the biology of DNA Damage Response (DDR) kinases

The DNA Damage Response (DDR) is a complex network of biological processes that protect cells from accumulating aberrant DNA structures, thereby maintaining genomic stability and, as a consequence, preventing the development of cancer and other diseases. The DDR pathway is coordinated by a signaling cascade mediated by the PI3K-like kinases (PIKK) ATM and ATR and by their downstream kinases CHK2 and CHK1, respectively. Together, these kinases regulate several aspects of the cellular program in response to genomic stress. Much of our understanding of these kinases came from studies performed in the 1990s using yeast as a model organism. The purpose of this review is to present a historical perspective on the discovery of the DDR kinases in yeast and the importance of this model for the identification and functional understanding of their mammalian orthologues.

Type I interferon (IFN)-inducible Absent in Melanoma 2 proteins in neuroinflammation: implications for Alzheimer's disease

Cumulative evidence indicates that activation of innate immune responses in the central nervous system (CNS) induces the expression of type 1 interferons (T1 IFNs), a family of cytokines. The T1 IFNs (IFN-α/β), through activation of the JAK/STAT-signaling in microglia, astrocytes, and neurons, induce the expression of IFN-inducible proteins, which mediate the pro- and anti-inflammatory functions of IFNs. Accordingly, T1 IFN-inducible Absent in Melanoma 2 proteins (murine Aim2 and human AIM2) negatively regulate the expression of TI IFNs and, upon sensing higher levels of cytosolic DNA, assemble the Aim2/AIM2 inflammasome, resulting in activation of caspase-1, pyroptosis, and the secretion of pro-inflammatory cytokines (e.g., IL-1β and IL-18). Of interest, studies have indicated a role for the Aim2/AIM2 proteins in neuroinflammation and neurodegenerative diseases, including Alzheimer's disease (AD). The ability of Aim2/AIM2 proteins to exert pro- and anti-inflammatory effects in CNS may depend upon age, sex hormones, cell-types, and the expression of species-specific negative regulators of the Aim2/AIM2 inflammasome. Therefore, we discuss the role of Aim2/AIM2 proteins in the development of AD. An improved understanding of the role of Absent in Melanoma 2 proteins in AD could identify new approaches to treat patients.

The Major Tegument Protein of Bovine Herpesvirus 1, VP8, Interacts with DNA Damage Response Proteins and Induces Apoptosis

VP8, the U_L47 gene product in bovine herpesvirus-1 (BoHV-1), is a major tegument protein that is essential for virus replication in vivo The major DNA damage response protein, ataxia telangiectasia mutated (ATM), phosphorylates Nijmegen breakage syndrome (NBS1) and structural maintenance of chromosome-1 (SMC1) proteins during the DNA damage response. VP8 was found to interact with ATM and NBS1 during transfection and BoHV-1 infection. However, VP8 did not interfere with phosphorylation of ATM in transfected or BoHV-1-infected cells. In contrast, VP8 inhibited phosphorylation of both NBS1 and SMC1 in transfected cells, as well as in BoHV-1-infected cells, but not in cells infected with a VP8 deletion mutant (BoHV-1ΔU_L47). Inhibition of NBS1 and SMC1 phosphorylation was observed at 4 h postinfection by nuclear VP8. Furthermore, UV light-induced cyclobutane pyrimidine dimer (CPD) repair was reduced in the presence of VP8, and VP8 in fact enhanced etoposide or UV-induced apoptosis. This suggests that VP8 blocks the ATM/NBS1/SMC1 pathway and inhibits DNA repair. VP8 induced apoptosis in VP8-transfected cells through caspase-3 activation. The fact that BoHV-1 is known to induce apoptosis through caspase-3 activation is in agreement with this observation. The role of VP8 was confirmed by the observation that BoHV-1 induced significantly more apoptosis than BoHV-1ΔU_L47. These data reveal a potential role of VP8 in the modulation of the DNA damage response pathway and induction of apoptosis during BoHV-1 infection.IMPORTANCE To our knowledge, the effect of BoHV-1 infection on the DNA damage response has not been characterized. Since BoHV-1ΔU_L47 was previously shown to be avirulent in vivo, VP8 is critical for the progression of viral infection. We demonstrated that VP8 interacts with DNA damage response proteins and disrupts the ATM-NBS1-SMC1 pathway by inhibiting phosphorylation of DNA repair proteins NBS1 and SMC1. Furthermore, interference of VP8 with DNA repair was correlated with decreased cell viability and increased DNA damage-induced apoptosis. These data show that BoHV-1 VP8 developed a novel strategy to interrupt the ATM signaling pathway and to promote apoptosis. These results further enhance our understanding of the functions of VP8 during BoHV-1 infection and provide an additional explanation for the reduced virulence of BoHV-1ΔU_L47.

RAD52 is required for RNA-templated recombination repair in post-mitotic neurons

It has been long assumed that post-mitotic neurons only utilize the error-prone non-homologous end-joining pathway to repair double-strand breaks (DSBs) associated with oxidative damage to DNA, given the inability of non-replicating neuronal DNA to utilize a sister chromatid template in the less error-prone homologous recombination (HR) repair pathway. However, we and others have found recently that active transcription triggers a replication-independent recombinational repair mechanism in G₀/G₁ phase of the cell cycle. Here we observed that the HR repair protein RAD52 is recruited to sites of DNA DSBs in terminally differentiated, post-mitotic neurons. This recruitment is dependent on the presence of a nascent mRNA generated during active transcription, providing evidence that an RNA-templated HR repair mechanism exists in non-dividing, terminally differentiated neurons. This recruitment of RAD52 in neurons is decreased by transcription inhibition. Importantly, we found that high concentrations of amyloid β, a toxic protein associated with Alzheimer's disease, inhibits the expression and DNA damage response of RAD52, potentially leading to a defect in the error-free, RNA-templated HR repair mechanism. This study shows a novel RNA-dependent repair mechanism of DSBs in post-mitotic neurons and demonstrates that defects in this pathway may contribute to neuronal genomic instability and consequent neurodegenerative phenotypes such as those seen in Alzheimer's disease.

Topoisomerase I in Human Disease Pathogenesis and Treatments

Mammalian topoisomerase 1 (TOP1) is an essential enzyme for normal development. TOP1 relaxes supercoiled DNA to remove helical constraints that can otherwise hinder DNA replication and transcription and thus block cell growth. Unfortunately, this exact activity can covalently trap TOP1 on the DNA that could lead to cell death or mutagenesis, a precursor for tumorigenesis. It is therefore important for cells to find a proper balance between the utilization of the TOP1 catalytic activity to maintain DNA topology and the risk of accumulating the toxic DNA damages due to TOP1 trapping that prevents normal cell growth. In an apparent contradiction to the negative attribute of the TOP1 activity to genome stability, the detrimental effect of the TOP1-induced DNA lesions on cell survival has made this enzyme a prime target for cancer therapies to kill fast-growing cancer cells. In addition, cumulative evidence supports a direct role of TOP1 in promoting transcriptional progression independent of its topoisomerase activity. The involvement of TOP1 in transcriptional regulation has recently become a focus in developing potential new treatments for a subtype of autism spectrum disorders. Clearly, the impact of TOP1 on human health is multifold. In this review, we will summarize our current understandings on how TOP1 contributes to human diseases and how its activity is targeted for disease treatments.

Neurons in Vulnerable Regions of the Alzheimer's Disease Brain Display Reduced ATM Signaling

Ataxia telangiectasia (A-T) is a multisystemic disease caused by mutations in the ATM (A-T mutated) gene. It strikes before 5 years of age and leads to dysfunctions in many tissues, including the CNS, where it leads to neurodegeneration, primarily in cerebellum. Alzheimer's disease (AD), by contrast, is a largely sporadic neurodegenerative disorder that rarely strikes before the 7th decade of life with primary neuronal losses in hippocampus, frontal cortex, and certain subcortical nuclei. Despite these differences, we present data supporting the hypothesis that a failure of ATM signaling is involved in the neuronal death in individuals with AD. In both, partially ATM-deficient mice and AD mouse models, neurons show evidence for a loss of ATM. In human AD, three independent indices of reduced ATM function-nuclear translocation of histone deacetylase 4, trimethylation of histone H3, and the presence of cell cycle activity-appear coordinately in neurons in regions where degeneration is prevalent. These same neurons also show reduced ATM protein levels. And though they represent only a fraction of the total neurons in each affected region, their numbers significantly correlate with disease stage. This previously unknown role for the ATM kinase in AD pathogenesis suggests that the failure of ATM function may be an important contributor to the death of neurons in AD individuals.

Dysregulation of the DNA Damage Response and KMT2A Rearrangement in Fetal Liver Hematopoietic Cells

Etoposide, a topoisomerase 2 (TOP2) inhibitor, is associated with the development of KMT2A (MLL)-rearranged infant leukemia. An epidemiological study suggested that in utero exposure to TOP2 inhibitors may be involved in generation of KMT2A (MLL) rearrangement. The present study examined the mechanism underlying the development of KMT2A (MLL)-rearranged infant leukemia in response to in utero exposure to etoposide in a mouse model. Fetal liver hematopoietic stem cells were more susceptible to etoposide than maternal bone marrow mononuclear cells. Etoposide-induced Kmt2a breakage was detected in fetal liver hematopoietic stem cells using a newly developed chromatin immunoprecipitation (ChIP) assay. Assessment of etoposide-induced chromosomal translocation by next-generation RNA sequencing (RNA-seq) identified several chimeric fusion messenger RNAs that were generated by etoposide treatment. However, Kmt2a (Mll)-rearranged fusion mRNA was detected in Atm-knockout mice, which are defective in the DNA damage response, but not in wild-type mice. The present findings suggest that in utero exposure to TOP2 inhibitors induces Kmt2a rearrangement when the DNA damage response is defective.

A novel tumor suppressor function of Kindlin-3 in solid cancer

Kindlin-3 (FERMT-3) is known to be central in hemostasis and thrombosis control and its deficiency disrupts platelet aggregation and causes Leukocyte Adhesion Deficiency disease. Here we report that Kindlin-3 has a tumor suppressive role in solid cancer. Our present genetic and functional data show that Kindlin-3 is downregulated in several solid tumors by a mechanism involving gene hypermethylation and deletions. In vivo experiments demonstrated that Kindlin-3 knockdown in 2 tumor cell models (breast cancer and melanoma) markedly increases metastasis formation, in accord with the in vitro increase of tumor cell malignant properties. The metastatic phenotype was supported by a mechanism involving alteration in β3-integrin activation including decreased phosphorylation, interaction with talin and the internalization of its active form leading to less cell attachment and more migration/invasion. These data uncover a novel and unexpected tumor suppressor role of Kindin-3 which can influence integrins targeted therapies development.

Ataxia-telangiectasia: future prospects

Ataxia-telangiectasia (A-T) is an autosomal recessive multi-system disorder caused by mutation in the ataxia-telangiectasia mutated gene (ATM). ATM is a large serine/threonine protein kinase, a member of the phosphoinositide 3-kinase-related protein kinase (PIKK) family whose best-studied function is as master controller of signal transduction for the DNA damage response (DDR) in the event of double strand breaks (DSBs). The DDR rapidly recognizes DNA lesions and initiates the appropriate cellular programs to maintain genome integrity. This includes the coordination of cell-cycle checkpoints, transcription, translation, DNA repair, metabolism, and cell fate decisions, such as apoptosis or senescence. DSBs can be generated by exposure to ionizing radiation (IR) or various chemical compounds, such as topoisomerase inhibitors, or can be part of programmed generation and repair of DSBs via cellular enzymes needed for the generation of the antibody repertoire as well as the maturation of germ cells. AT patients have immunodeficiency, and are sterile with gonadal dysgenesis as a result of defect in meiotic recombination. In the cells of nervous system ATM has additional role in vesicle dynamics as well as in the maintenance of the epigenetic code of histone modifications. Moderate levels of ATM are associated with prolonged lifespan through resistance to oxidative stress. ATM inhibitors are being viewed as potential radiosensitizers as part of cancer radiotherapy. Though there is no cure for the disease at present, glucocorticoids have been shown to induce alternate splicing site in the gene for ATM partly restoring its activity, but their most effective timing in the disease natural history is not yet known. Gene therapy is promising but large size of the gene makes it technically difficult to be delivered across the blood-brain barrier at present. As of now, apart from glucocorticoids, use of histone deacetylase inhibitors/EZH2 to minimize effect of the absence of ATM, looks more promising.

Cell cycle checkpoint abnormalities during dementia: A plausible association with the loss of protection against oxidative stress in Alzheimer's disease [corrected]

Background

Increasing evidence suggests an association between neuronal cell cycle (CCL) events and the processes that underlie neurodegeneration in Alzheimer’s disease (AD). Elevated levels of oxidative stress markers and mitochondrial dysfunction are also among early events in AD. Recent studies have reported the role of CCL checkpoint proteins and tumor suppressors, such as ATM and p53 in the control of glycolysis and oxidative metabolism in cancer, but their involvement in AD remains uncertain.

Methods and Findings

In this postmortem study, we measured gene expression levels of eight CCL checkpoint proteins in the superior temporal cortex (STC) of persons with varying severities of AD dementia and compare them to those of cognitively normal controls. To assess whether the CCL changes associated with cognitive impairment in AD are specific to dementia, gene expression of the same proteins was also measured in STC of persons with schizophrenia (SZ), which is also characterized by mitochondrial dysfunction. The expression of CCL-checkpoint and DNA damage response genes: MDM4, ATM and ATR was strongly upregulated and associated with progression of dementia (cognitive dementia rating, CDR), appearing as early as questionable or mild dementia (CDRs 0.5–1). In addition to gene expression changes, the downstream target of ATM-p53 signaling - TIGAR, a p53-inducible protein, the activation of which can regulate energy metabolism and protect against oxidative stress was progressively decreased as severity of dementia evolved, but it was unaffected in subjects with SZ. In contrast to AD, different CCL checkpoint proteins, which include p53, CHEK1 and BRCA1 were significantly downregulated in SZ.

Conclusions

These results support the activation of an ATM signaling and DNA damage response network during the progression of AD dementia, while the progressive decrease in the levels of TIGAR suggests loss of protection initiated by ATM-p53 signaling against intensifying oxidative stress in AD.

ISG15 deregulates autophagy in genotoxin-treated ataxia telangiectasia cells

Ataxia-telangiectasia (A-T) is a cerebellar neurodegenerative disorder; however, the basis for the neurodegeneration in A-T is not well established. Lesions in the ubiquitin and autophagy pathways are speculated to contribute to the neurodegeneration in other neurological diseases and may have a role in A-T neurodegeneration. Our recent studies revealed that the constitutively elevated ISG15 pathway impairs targeted proteasome-mediated protein degradation in A-T cells. Here, we demonstrate that the basal autophagy pathway is activated in the ubiquitin pathway-compromised A-T cells. We also show that genotoxic stress triggers aberrant degradation of the proteasome and autophagy substrates (autophagic flux) in A-T cells. Inhibition of autophagy at an early stage using 3-methyladenine blocked UV-induced autophagic flux in A-T cells. On the other hand, bafilomycin A1, which inhibits autophagy at a late stage, failed to block UV-induced autophagic flux, suggesting that overinduction of autophagy may underlie aberrant autophagic flux in A-T cells. The ISG15-specific shRNA that restored proteasome function restores autophagic function in A-T cells. These findings suggest that autophagy compensates for the ISG15-dependent ablation of proteasome-mediated protein degradation in A-T cells. Genotoxic stress overactivates this compensatory mechanism, triggering aberrant autophagic flux in A-T cells. Supporting the model, we show that autophagy is activated in the brain tissues of human A-T patients. This highlights a plausible causal contribution of a novel "ISG15 proteinopathy" in A-T neuronal cell death.

Malignant melanoma, breast cancer and other cancers in patients with Parkinson's disease

Previous studies report an atypical cancer pattern among patients with Parkinson's disease. Here, we evaluate the cancer pattern among people diagnosed with Parkinson's disease in an extension of our previous cohort study. For this Danish population-based cohort study, we identified 20,000 people with Parkinson's disease diagnosed in 1977-2006, from the National Danish Hospital Register. Cohort members were followed up for cancer in the Danish Cancer Registry until December 31, 2008, and their incidence rates of cancer were compared to age-, sex- and calendar period-specific rates in the general population as standardized incidence rate ratios (SIRs). In subanalyses, we estimated the risk for cancer among patients with early onset Parkinson's disease and we also compared breast tumor characteristics among women with Parkinson's disease to that of a control group. The overall cancer risk in our cohort was decreased [SIR = 0.86; 95% confidence interval (CI) = 0.83-0.90], as were those for smoking-related (SIR = 0.65; 95% CI = 0.60-0.70) and nonsmoking-related cancers (SIR = 0.79; 95% CI = 0.71-0.86). The cohort had increased risks for malignant melanoma (SIR = 1.41; 95% CI = 1.09-1.80), nonmelanoma skin cancer (SIR = 1.29; 95% CI = 1.18-1.39) and female breast cancer (SIR = 1.17; 95% CI = 1.02-1.34). Among patients with early onset Parkinson's disease, the risk for cancer was comparable to that of the general population. Of breast tumor characteristics, only grade of malignancy differed between Parkinson's disease women and controls. This study confirms a lower cancer risk among people with Parkinson's disease. Increased risks for malignant melanoma, nonmelanoma skin cancer and breast cancer might be due to shared risk factors with Parkinson's disease.

Cancer and neurodegeneration: between the devil and the deep blue sea

Cancer and neurodegeneration are often thought of as disease mechanisms at opposite ends of a spectrum; one due to enhanced resistance to cell death and the other due to premature cell death. There is now accumulating evidence to link these two disparate processes. An increasing number of genetic studies add weight to epidemiological evidence suggesting that sufferers of a neurodegenerative disorder have a reduced incidence for most cancers, but an increased risk for other cancers. Many of the genes associated with either cancer and/or neurodegeneration play a central role in cell cycle control, DNA repair, and kinase signalling. However, the links between these two families of diseases remain to be proven. In this review, we discuss recent and sometimes as yet incomplete genetic discoveries that highlight the overlap of molecular pathways implicated in cancer and neurodegeneration.

Somatic mutations of the Parkinson's disease-associated gene PARK2 in glioblastoma and other human malignancies

Mutation of the gene PARK2, which encodes an E3 ubiquitin ligase, is the most common cause of early-onset Parkinson's disease. In a search for multisite tumor suppressors, we identified PARK2 as a frequently targeted gene on chromosome 6q25.2-q27 in cancer. Here we describe inactivating somatic mutations and frequent intragenic deletions of PARK2 in human malignancies. The PARK2 mutations in cancer occur in the same domains, and sometimes at the same residues, as the germline mutations causing familial Parkinson's disease. Cancer-specific mutations abrogate the growth-suppressive effects of the PARK2 protein. PARK2 mutations in cancer decrease PARK2's E3 ligase activity, compromising its ability to ubiquitinate cyclin E and resulting in mitotic instability. These data strongly point to PARK2 as a tumor suppressor on 6q25.2-q27. Thus, PARK2, a gene that causes neuronal dysfunction when mutated in the germline, may instead contribute to oncogenesis when altered in non-neuronal somatic cells.

Oxidative stress is linked to ERK1/2-p16 signaling-mediated growth defect in ATM-deficient astrocytes

The gene that encodes the ATM protein kinase is mutated in ataxia-telangiectasia (A-T). One of the prominent features of A-T is progressive neurodegeneration. We have previously reported that primary astrocytes isolated from Atm(-/-) mice grow slowly and die earlier than control cells in culture. However, the mechanisms for this remain unclear. We show here that intrinsic elevated intracellular levels of reactive oxygen species (ROS) are associated with the senescence-like growth defect of Atm(-/-) astrocytes. This condition is accompanied by constitutively higher levels of ERK1/2 phosphorylation and p16(Ink4a) in Atm(-/-) astrocytes. We also observe that ROS-induced up-regulation of p16(Ink4a) occurs correlatively with ERK1/2-dependent down-regulation and subsequent dissociation from chromatin of Bmi-1. Furthermore, both mitogen-activated protein kinase (MAPK)/ERK inhibitor PD98059 and antioxidant N-acetyl-l-cysteine restored normal proliferation of Atm(-/-) astrocytes. These results suggest that ATM is required for normal astrocyte growth through its ability to stabilize intracellular redox status and that the inability to control ROS is the molecular basis of limited cell growth of Atm(-/-) astrocytes. This defect may be mediated by a mechanism involving ERK1/2 activation and Bmi-1 derepression of p16(Ink4a). These data identify new potential targets for therapeutic intervention in A-T neurodegeneration.

Regulation of reactive oxygen species by Atm is essential for proper response to DNA double-strand breaks in lymphocytes

The ataxia telangiectasia-mutated (ATM) gene plays a pivotal role in the maintenance of genomic stability. Although it has been recently shown that antioxidative agents inhibited lymphomagenesis in Atm(-/-) mice, the mechanisms remain unclear. In this study, we intensively investigated the roles of reactive oxygen species (ROS) in phenotypes of Atm(-/-) mice. Reduction of ROS by the antioxidant N-acetyl-l-cysteine (NAC) prevented the emergence of senescent phenotypes in Atm(-/-) mouse embryonic fibroblasts, hypersensitivity to total body irradiation, and thymic lymphomagenesis in Atm(-/-) mice. To understand the mechanisms for prevention of lymphomagenesis, we analyzed development of pretumor lymphocytes in Atm(-/-) mice. Impairment of Ig class switch recombination seen in Atm(-/-) mice was mitigated by NAC, indicating that ROS elevation leads to abnormal response to programmed double-strand breaks in vivo. Significantly, in vivo administration of NAC to Atm(-/-) mice restored normal T cell development and inhibited aberrant V(D)J recombination. We conclude that Atm-mediated ROS regulation is essential for proper DNA recombination, preventing immunodeficiency, and lymphomagenesis.

Activation of DNA damage signaling

Cells respond to DNA damage by activating DNA repair and DNA damage signaling pathways. While DNA repair proteins directly interact with DNA lesions, activation of DNA damage signaling pathways may be triggered by the effect the DNA lesions have on replication, transcription or chromatin topology. This review will focus on the potential mechanisms of the activation of DNA damage-induced signal transduction pathways.

Functional interaction of H2AX, NBS1, and p53 in ATM-dependent DNA damage responses and tumor suppression

Ataxia-telangiectasia (A-T) mutated (ATM) kinase signals all three cell cycle checkpoints after DNA double-stranded break (DSB) damage. H2AX, NBS1, and p53 are substrates of ATM kinase and are involved in ATM-dependent DNA damage responses. We show here that H2AX is dispensable for the activation of ATM and p53 responses after DNA DSB damage. Therefore, H2AX functions primarily as a downstream mediator of ATM functions in the parallel pathway of p53. NBS1 appears to function both as an activator of ATM and as an adapter to mediate ATM activities after DNA DSB damage. Phosphorylation of ATM and H2AX induced by DNA DSB damage is normal in NBS1 mutant/mutant (NBS1m/m) mice that express an N-terminally truncated NBS1 at lower levels. Therefore, the pleiotropic A-T-related systemic and cellular defects observed in NBS1m/m mice are due to the disruption of the adapter function of NBS1 in mediating ATM activities. While H2AX is required for the irradiation-induced focus formation of NBS1, our findings indicate that NBS1 and H2AX have distinct roles in DNA damage responses. ATM-dependent phosphorylation of p53 and p53 responses are largely normal in NBS1m/m mice after DNA DSB damage, and p53 deficiency greatly facilitates tumorigenesis in NBS1m/m mice. Therefore, NBS1, H2AX, and p53 play synergistic roles in ATM-dependent DNA damage responses and tumor suppression.

Regulation of oxidative stress by ATM is required for self-renewal of haematopoietic stem cells

The 'ataxia telangiectasia mutated' (Atm) gene maintains genomic stability by activating a key cell-cycle checkpoint in response to DNA damage, telomeric instability or oxidative stress. Mutational inactivation of the gene causes an autosomal recessive disorder, ataxia-telangiectasia, characterized by immunodeficiency, progressive cerebellar ataxia, oculocutaneous telangiectasia, defective spermatogenesis, premature ageing and a high incidence of lymphoma. Here we show that ATM has an essential function in the reconstitutive capacity of haematopoietic stem cells (HSCs) but is not as important for the proliferation or differentiation of progenitors, in a telomere-independent manner. Atm-/- mice older than 24 weeks showed progressive bone marrow failure resulting from a defect in HSC function that was associated with elevated reactive oxygen species. Treatment with anti-oxidative agents restored the reconstitutive capacity of Atm-/- HSCs, resulting in the prevention of bone marrow failure. Activation of the p16(INK4a)-retinoblastoma (Rb) gene product pathway in response to elevated reactive oxygen species led to the failure of Atm-/- HSCs. These results show that the self-renewal capacity of HSCs depends on ATM-mediated inhibition of oxidative stress.

Pharmacogenomics and breast cancer

Germline variants can be used to study breast cancer susceptibility as well as the variable response to both drug and radiation therapy used in the treatment of breast cancer. In addition to germline high-penetrance mutations important in familial and hereditary breast cancer, a substantial component of breast cancer risk can be attributed to the combined effect of many low-risk germline polymorphisms involved in relevant pathways like those of DNA repair, adhesion, carcinogen and estrogen metabolism. Additionally, the identification of sequence variants in genes involved in response to chemotherapy and radiation treatment, has created the opportunity to apply genomics to individualized treatment. The continued insight into the molecular pathways involved in drug and radiation response has enabled progress in tailoring therapies in such a way as to both maximize efficacy and minimize toxicity. Polymorphisms in genes encoding drug-metabolizing enzymes, drug transporters and drug targets can be used to predict toxicity and response to pharmacologic agents used in breast cancer treatment. Similarly, germline variants in genes involved in DNA repair, radiation-induced fibrosis and reactive oxygen species may be used to predict response to radiation therapy. As a result, pharmacogenomics is rapidly evolving to affect the entire spectrum of breast cancer management, influencing both prevention and treatment choices.

Ataxia telangiectasia-mutated protein can regulate p53 and neuronal death independent of Chk2 in response to DNA damage

DNA damage is a key initiator of neuronal death. We have previously shown that the tumor suppressor p53, in conjunction with cyclin-dependent kinases (CDKs), regulates the mitochondrial pathway of death in neurons exposed to genotoxic agents. However, the mechanisms by which p53 is regulated is unclear. Presently, we show that p53 is phosphorylated on Ser-15 following DNA damage and this occurs independently of the CDK pathway. Instead, we show that p53 phosphorylation, stability, as well as neuronal death is regulated, in part, by the ataxia telangiectasia-mutated (ATM) protein. Previous reports have suggested that ATM regulation of p53 occurs through Chk2. However, in our present paradigms, we show that ATM functions separately from Chk2 to regulate p53 stability and neuronal death. Chk2 deficiency does not affect p53 stability or neuronal death induced by Topoisomerase I or II inhibition. Taken together, our results provide a model by which DNA damage can activate an ATM-dependent, Chk2-independent pathway of p53-mediated neuronal death.

Suppression of Tousled-like kinase activity after DNA damage or replication block requires ATM, NBS1 and Chk1

The human Tousled-like kinases 1 and 2 (TLK) have been shown to be active during S phase of the cell cycle. TLK activity is rapidly suppressed by DNA damage and by inhibitors of replication. Here we report that the signal transduction pathway, which leads to transient suppression of TLK activity after the induction of double-strand breaks (DSBs) in the DNA, is dependent on the presence of a functional ataxia-telangiectasia-mutated kinase (ATM). Interestingly, we have discovered that rapid suppression of TLK activity after low doses of ultraviolet (UV) irradiation or aphidicolin-induced replication block is also ATM-dependent. The nature of the signal that triggers ATM-dependent downregulation of TLK activity after UVC and replication block remains unknown, but it is not due exclusively to DSBs in the DNA. We also demonstrate that TLK suppression is dependent on the presence of a functional Nijmegan Breakage Syndrome protein (NBS1). ATM-dependent phosphorylation of NBS1 is required for the suppression of TLK activity, indicating a role for NBS1 as an adaptor or scaffold in the ATM/TLK pathway. ATM does not phosphorylate TLK directly to regulate its activity, but Chk1 does phosphorylate TLK1 GST-fusion proteins in vitro. Using Chk1 siRNAs, we show that Chk1 is essential for the suppression of TLK activity after replication block, but that ATR, Chk2 and BRCA1 are dispensable for TLK suppression. Overall, we propose that ATM activation is not linked solely to DSBs and that ATM participates in initiating signaling pathways in response to replication block and UV-induced DNA damage.

SMC protein complexes and the maintenance of chromosome integrity

Structural maintenance of chromosomes (SMC) family proteins have attracted much attention for their unique protein structure and critical roles in mitotic chromosome organization. Elegant genetic and biochemical studies in yeast and Xenopus identified two different SMC heterodimers in two conserved multiprotein complexes termed 'condensin' and 'cohesin'. These complexes are required for mitotic chromosome condensation and sister chromatid cohesion, respectively, both of which are prerequisite to accurate segregation of chromosomes. Although structurally similar, the SMC proteins in condensin and cohesin appear to have distinct functions, whose specificity and cell cycle regulation are critically determined by their interactions with unique sets of associated proteins. Recent studies of subcellular localization of SMC proteins and SMC-containing complexes, identification of their interactions with other cellular factors, and discovery of new SMC family members have uncovered unexpected roles for SMC proteins and SMC-containing complexes in different aspects of genome functions and chromosome organization beyond mitosis, all of which are critical for the maintenance of chromosome integrity.

Ionizing radiation induces ataxia telangiectasia mutated kinase (ATM)-mediated phosphorylation of LKB1/STK11 at Thr-366

The serine/threonine protein kinase LKB1 functions as a tumour suppressor, and mutations in this enzyme lead to the inherited Peutz-Jeghers cancer syndrome. We previously found that LKB1 was phosphorylated at Thr-366 in vivo, a residue conserved in mammalian, Xenopus and Drosophila LKB1, located on a C-terminal non-catalytic moiety of the enzyme. Mutation of Thr-366 to Ala or Asp partially inhibited the ability of LKB1 to suppress growth of G361 melanoma cells, but did not affect LKB1 activity in vitro or LKB1 localization in vivo. As a first step in exploring the role of this phosphorylation further, we have generated a phosphospecific antibody specifically recognizing LKB1 phosphorylated at Thr-366 and demonstrate that exposure of cells to ionizing radiation (IR) induced a marked phosphorylation of LKB1 at Thr-366 in the nucleus. Thr-366 lies in an optimal phosphorylation motif for the phosphoinositide 3-kinase-like kinases DNA-dependent protein kinase (DNA-PK), ataxia telangiectasia mutated kinase (ATM) and ataxia telangiectasia-related kinase (ATR), which function as sensors for DNA damage in cells and mediate cellular responses to DNA damage. We demonstrate that both DNA-PK and ATM efficiently phosphorylate LKB1 at Thr-366 in vitro and provide evidence that ATM mediates this phosphorylation in vivo. This is based on the finding that LKB1 is not phosphorylated in a cell line lacking ATM in response to IR, and that agents which induce cellular responses via ATR in preference to ATM poorly induce phosphorylation of LKB1 at Thr-366. These observations provide the first link between ATM and LKB1 and suggest that ATM could regulate LKB1.

Detection of ATM gene mutation in human glioma cell line M059J by a rapid frameshift/stop codon assay in yeast

A yeast-based frameshift/stop codon assay for examining ATM (ataxia telangiectasia mutated) mutations was established. Each of six fragments of a PCR-amplified coding sequence for ATM is inserted in frame by homologous recombination into a yeast URA3 fusion protein gene, and the transformants are assayed for growth in the absence of uracil. The usefulness of this assay was verified in a panel of cell lines derived from individuals with homozygous and heterozygous ATM mutations. The assay was also shown to distinguish between specimens with wild-type alleles and those with truncating mutations: a frameshift mutation or an inserted stop codon. Using this assay M059J cells, which fail to express the catalytic subunit of DNA-dependent protein kinase (PRKDC, also known as DNA-PKcs) and are hypersensitive to ionizing radiation, were found to express two different aberrant ATM transcripts: one characterized by 4776 del 133, which corresponds to the deletion of exon 33, and the other by 4909 ins 116. Subsequent analysis of the intron sequences revealed that 4909 ins 116 is comprised of a nucleotide sequence corresponding to 84013-84128 in intron 33 with a cryptic splice site. Thus the radiosensitive phenotype of M059J cells appears to be due to a defect in PRKDC and a truncating ATM mutation.

SMC1 is a downstream effector in the ATM/NBS1 branch of the human S-phase checkpoint

Structural maintenance of chromosomes (SMC) proteins (SMC1, SMC3) are evolutionarily conserved chromosomal proteins that are components of the cohesin complex, necessary for sister chromatid cohesion. These proteins may also function in DNA repair. Here we report that SMC1 is a component of the DNA damage response network that functions as an effector in the ATM/NBS1-dependent S-phase checkpoint pathway. SMC1 associates with BRCA1 and is phosphorylated in response to IR in an ATM- and NBS1-dependent manner. Using mass spectrometry, we established that ATM phosphorylates S957 and S966 of SMC1 in vivo. Phosphorylation of S957 and/or S966 of SMC1 is required for activation of the S-phase checkpoint in response to IR. We also discovered that the phosphorylation of NBS1 by ATM is required for the phosphorylation of SMC1, establishing the role of NBS1 as an adaptor in the ATM/NBS1/SMC1 pathway. The ATM/CHK2/CDC25A pathway is also involved in the S-phase checkpoint activation, but this pathway is intact in NBS cells. Our results indicate that the ATM/NBS1/SMC1 pathway is a separate branch of the S-phase checkpoint pathway, distinct from the ATM/CHK2/CDC25A branch. Therefore, this work establishes the ATM/NBS1/SMC1 branch, and provides a molecular basis for the S-phase checkpoint defect in NBS cells.

CHK2 kinase--a busy messenger

Checkpoint kinase 2 (Chk2) is emerging as a key mediator of diverse cellular responses to genotoxic stress, guarding the integrity of the genome throughout eukaryotic evolution. Recent studies show the fundamental role of Chk2 in the network of genome-surveillance pathways that coordinate cell-cycle progression with DNA repair and cell survival or death. Defects in Chk2 contribute to the development of both hereditary and sporadic human cancers, and earmark this kinase as a candidate tumour suppressor and an attractive target for drug discovery.

Lymphocytic interstitial pneumonitis, elevated IgM concentration, and hepatosplenomegaly in ataxia-telangiectasia

An 8-year-old girl developed ataxia-telangiectasia. Western blotting of lysate revealed absence of the ATM protein, and 2 mutations in the ATM gene were found. Subsequently, the patient developed increased respiratory symptoms. Open lung biopsy revealed lymphocytic interstitial pneumonitis, which is not characteristic of ataxia-telangiectasia. There was a therapeutic response to glucocorticosteroid treatment.

Novel mutations and defective protein kinase C activation of T-lymphocytes in ataxia telangiectasia

Three ataxia telangiectasia (AT) patients have been characterized immunologically and molecularly. Patient 1 presents two nondescribed splicing mutations which affect exons 15 and 21 of the ATM gene. The maternal defect consists of a G > A transition in the first nucleotide of the intron 21 donor splicing site which results in a complete deletion of exon 21. The paternal mutation consists of an A > C transversion in the intron 14 acceptor splicing site which produces a partial skipping of exon 15. Two abnormal alternative transcripts were found, respectively, 17 and 41 nucleotides shorter. Patient 2 presents a homozygous genomic deletion of 28 nucleotides in the last exon of the gene. This deletion changes the normal reading frame after residue 3003 of the protein and introduces a premature stop codon at residue 3008 that could originate a truncated ATM protein. Patient 3, a compound heterozygote, presents a defect which consists of a G > A transition in the first nucleotide of intron 62 donor splicing site which results in a complete deletion of exon 62. The results obtained during a three year period in the proliferation assays show an impaired PMA (phorbol myristate acetate) activation in specific T lymphocyte activation pathways (CD69, CD26, CD28, CD3, PHA, PWM and Con A mediated) but not in others (CD2, ionomycin, and Ig surface receptor). The possible link among specific ATM mutations and abnormal immune responses is unknown.

Cell-cycle checkpoint kinases: checking in on the cell cycle

In response to DNA damage, cell-cycle checkpoints integrate cell-cycle control with DNA repair. The idea that checkpoint controls are an integral component of normal cell-cycle progression has arisen as a result of studies in Drosophila and mice. In addition, an appreciation that DNA damage arises as a natural consequence of cellular metabolism, including DNA replication itself, has influenced thinking regarding the nature of checkpoint pathways.

Identification of ATM mutations using extended RT-PCR and restriction endonuclease fingerprinting, and elucidation of the repertoire of A-T mutations in Israel

Ataxia-telangiectasia (A-T) is an autosomal recessive disorder characterized by neurodegeneration, immunodeficiency, cancer predisposition, and radiation sensitivity. The responsible gene, ATM, has an extensive genomic structure and encodes a large transcript with a 9.2 kb open reading frame (ORF). A-T mutations are extremely variable and most of them are private. We streamlined a high throughput protocol for the search for ATM mutations. The entire ATM ORF is amplified in a single RT-PCR step requiring a minimal amount of RNA. The product can serve for numerous nested PCRs in which overlapping portions of the ORF are further amplified and subjected to restriction endonuclease fingerprinting (REF) analysis. Splicing errors are readily detectable during the initial amplification of each portion. Using this protocol, we identified 5 novel A-T mutations and completed the elucidation of the molecular basis of A-T in the Israeli population.

Cancer risk and the ATM gene: a continuing debate

Deficiencies in the ability of cells to sense and repair damage in individuals with rare genetic instability syndromes increase the risk of developing cancer. Ataxia-telangiectasia (A-T), such a condition, is associated with a high incidence of leukemia and lymphoma that develop in childhood. Although A-T is an autosomal recessive disorder, some penetrance appears in individuals with one mutated ATM gene (A-T carriers), namely, an increased risk of developing breast cancer. The gene mutated in A-T, designated ATM, is homologous to several DNA damage recognition and cell cycle checkpoint control genes from other organisms. Recent studies suggest that ATM is activated primarily in response to double-strand breaks, the major cytotoxic lesion caused by ionizing radiation, and can directly bind to and phosphorylate c-Abl, p53, and replication protein A (RPA). Analysis of ATM mutations in patients with A-T or with sporadic non-A-T cancers has suggested the existence of two classes of ATM mutation: null mutations leading to A-T and dominant negative missense mutations predisposing to cancer in the heterozygous state. Studies with A-T mouse models have helped determine the basis of lymphoid tumorigenesis in A-T and have shown that ATM plays a critical role in maintaining genetic stability by ensuring high-fidelity execution of chromosomal events. Thus, ATM appears to act as a caretaker of the genome.

BASC, a super complex of BRCA1-associated proteins involved in the recognition and repair of aberrant DNA structures

We report the identities of the members of a group of proteins that associate with BRCA1 to form a large complex that we have named BASC (BRCA1-associated genomesurveillance complex). This complex includes tumor suppressors and DNA damage repair proteins MSH2, MSH6, MLH1, ATM, BLM, and the RAD50–MRE11–NBS1 protein complex. In addition, DNA replication factor C (RFC), a protein complex that facilitates the loading of PCNA onto DNA, is also part of BASC. We find that BRCA1, the BLM helicase, and the RAD50–MRE11–NBS1 complex colocalize to large nuclear foci that contain PCNA when cells are treated with agents that interfere with DNA synthesis. The association of BRCA1 with MSH2 and MSH6, which are required for transcription-coupled repair, provides a possible explanation for the role of BRCA1 in this pathway. Strikingly, all members of this complex have roles in recognition of abnormal DNA structures or damaged DNA, suggesting that BASC may serve as a sensor for DNA damage. Several of these proteins also have roles in DNA replication-associated repair. Collectively, these results suggest that BRCA1 may function as a coordinator of multiple activities required for maintenance of genomic integrity during the process of DNA replication and point to a central role for BRCA1 in DNA repair.

Cell-surface expression of transrearranged Vgamma-cbeta T-cell receptor chains in healthy donors and in ataxia telangiectasia patients

Transrearrangements between the T-cell receptor (TCR) VgammaI family and JbetaCbeta loci occur at increased frequencies in patients with ataxia telangiectasia (AT). We have used an optimized reverse transcriptase polymerase chain reaction (RT-PCR) approach to investigate the occurrence of TCRVgamma-JbetaCbeta transrearrangements involving the dominantly used Vgamma element in peripheral blood gammadelta T cells, i.e. Vgamma9. We detected in frame transcripts of Vgamma9-JbetaCbeta transrearrangements in 4/16 AT patients and in 3/13 healthy control donors. A panel of monoclonal antibodies (mAb) against all expressed TCRVgamma elements was used to monitor cell-surface expression of transrearranged TCR. A very low proportion (< 1%) of peripheral blood TCRalphabeta cells expressed Vgamma instead of Vbeta elements. For the first time, we have isolated and molecularly characterized alphabeta T cells with a Vgamma9-JbetaCbeta transrearrangement from two AT patients and we have shown that such TCR are functional. We conclude that functional TCR transrearrangements can also involve Vgamma9, the dominant Vgamma element in peripheral blood gammadelta T cells.

Spontaneous and oxidative stress-induced programmed cell death in lymphocytes from patients with ataxia telangiectasia (AT)

T cell lymphopenia in the peripheral blood lymphocytes (PBL) of patients with AT is mainly caused by a decrease of naive CD45RA+/CD4+ cells followed by a predominance of memory CD45RO+ lymphocytes. To relate these findings to the regulation of programmed cell death, we investigated the activation state and apoptotic level of PBL in 12 patients and healthy controls by flow cytometry. In accordance with previous investigations, the number of naive CD4+/CD45RA+ cells was significantly decreased in patients compared with healthy controls. This disturbed balance of CD45RA and CD45RO was also reflected in higher amounts of activated HLA-DR and CD95 expressing cells, with a concomitant decrease of Bcl-2 protected lymphocytes in the T cell population. With regard to its role in preventing oxidative-induced cell death, we analysed Bcl-2 expression and apoptosis in the presence of oxidative stress. In culture, cells of patients are more susceptible to spontaneous programmed cell death. However, in our stress-inducing system (hypoxanthine/xanthine oxidase system) the number of cells undergoing apoptosis was lower in patients' cell populations compared with controls. In addition, preliminary results suggest that Bcl-2 expression and level of spontaneous apoptosis in patients can be modified by IL-2 and interferon-gamma.

Rescue of defective T cell development and function in Atm-/- mice by a functional TCR alpha beta transgene

The Atm-/- mice recapitulate most of the defects observed in ataxia-telangiectasia (A-T) patients, including a high incidence of lymphoid tumors and immune defects characterized by defective T cell differentiation, thymus hypoplasia, and defective T-dependent immune responses. To understand the basis of the T cell developmental defects in Atm-/- mice, a functional TCR alpha beta transgene was introduced into these mutant mice. Analysis of the Atm-/-TCR alpha beta+ mice indicated that the transgenic TCR alpha beta can rescue the defective T cell differentiation and partially rescue the thymus hypoplasia in Atm-/- mice, indicating that thymocyte positive selection is normal in the Atm-/- mice. In addition, cell cycle analysis of the thymocytes derived from Atm-/-TCR alpha beta+ and control mice suggested that Atm is involved in the thymocyte expansion. Finally, evaluation of the T-dependent immune responses in Atm-/-TCR alpha beta+ mice indicated that Atm is dispensable for normal T cell function. Therefore, the defective T-dependent immune responses in Atm-/- mice must be secondary to greatly reduced T cell numbers in these mutant mice.

Anti-oxidative capacity in patients with ataxia telangiectasia

Highly reactive oxygen species (ROS) are involved in T-cell activation and in the defense against environmental pathogens. An imbalance of ROS generation and detoxifying scavenger enzymes could contribute to the increased susceptibility to cancer and infections in ataxia telangiectasia. We studied oxidative status, i.e. plasma total antioxidant capacity (TEAC), retinol, alpha-tocopherol, ubiquinol, and the number of activated T cells in 10 patients with ataxia telangiectasia (AT) compared to age-matched healthy controls. As expected, patients showed significantly increased levels of activated human leukocyte antigen-DR and CD45RO expressing T cells. TEAC levels as well as the exogenous antioxidants retinol and alpha-tocopherol were significantly reduced in patients. In addition, patients showed slightly reduced plasma levels of the endogenous ROS scavenger enzyme ubiquinol (Q10). Although no correlation between number of activated T-cells and antioxidant capacity could be demonstrated, an increase in ROS and a diminished reactive oxygen scavenger capacity may be involved in the disease process of patients with AT.

ATM heterozygote cells exhibit intermediate levels of apoptosis in response to streptonigrin and etoposide

Ataxia-telangiectasia (A-T) is a rare recessive disease characterised by cerebellar ataxia, immunodeficiency, sensitivity to ionising radiation and increased cancer risk. Heterozygotes have an increased risk of cancer and may comprise 1% of the population. In vitro, A-T heterozygote cell lines show radiosensitivity intermediate between normal and A-T homozygotes. Furthermore, in A-T homozygotes, hypersensitivity to chemical agents which cause DNA damage, similar to that produced by ionising radiation, has been observed. To investigate the chemosensitivity of A-T heterozygote cell lines, we used TUNEL to analyse the level of apoptosis after drug treatment with etoposide and streptonigrin. Our samples included four normal, eight A-T heterozygote and 10 A-T homozygote lymphoblastoid cell lines. All cell lines were exposed to drugs for 24 h, then cultivated in fresh media for 0 and 72 h. The levels of apoptosis increased significantly in all cell lines, with the greatest increase in homozygote cells and an intermediate increase in heterozygote cells (P values of < 0.01 for etoposide treatment and < 0.02 for streptonigrin treatment were obtained using the Kruskal-Wallis H-test). Our results indicate that A-T heterozygotes express intermediate sensitivity to etoposide and streptonigrin similar to that observed in response to ionising radiation.

Induction and characterization of human glioma clones with different radiosensitivities

To facilitate investigation of the molecular mechanisms of tumor cell radiosensitivities, we have generated a set of clones with different radiosensitivities from a human glioma cell line U-251 MG-Ho. Forty-four colonies were isolated by subjecting parent cells to the mutagen N-methylnitrosourea and then irradiating these cells with increasing doses of x-rays. About half of the clones displayed different radiosensitivities than the parent cells. We selected one of the most sensitive clones (X3i) and one of the most resistant clones (Y6) for further study. Isoeffective doses for these two clones differed by about a factor of 1.7; the relative radiosensitivities of both clones were stable for at least 30 cell culture passages. These two clones do not differ significantly in either the induction or repair of radiation-induced DNA double-strand breaks as measured by pulsed field gel electrophoresis. Radiation-induced apoptosis measured by terminal deoxynucleotide transferase-mediated dUTP nick end labeling assay and micronucleus formation were similar in both clones. However, potentially lethal damage repair was greater in the radioresistant Y6 clone than in the radiosensitive X3i clone as determined by colony-forming efficiency assay.

MEC1-dependent redistribution of the Sir3 silencing protein from telomeres to DNA double-strand breaks

The yeast Sir2/3/4p complex is found in abundance at telomeres, where it participates in the formation of silent heterochromatin and telomere maintenance. Here, we show that Sir3p is released from telomeres in response to DNA double-strand breaks (DSBs), binds to DSBs, and mediates their repair, independent of cell mating type. Sir3p relocalization is S phase specific and, importantly, requires the DNA damage checkpoint genes MEC1 and RAD9. MEC1 is a homolog of ATM, mutations in which cause ataxia telangiectasia (A-T), a disease characterized by various neurologic and immunologic abnormalities, a predisposition for cancer, and a cellular defect in repair of DSBs. This novel mode by which preformed DNA repair machinery is mobilized by DNA damage sensors may have implications for human diseases resulting from defective DSB repair.

Tumor suppressor genes and breast cancer

The genetic determinants for most breast cancer cases remain elusive. However, a mutation in a tumor suppressor gene, such as p53, BRCA1, BRCA2, or ATM, has been determined to be one mechanism of breast carcinogenesis. It has been established that inherited mutations in p53, BRCA1, and BRCA2 significantly contribute to breast cancer risk, although the importance of an inherited ATM mutation is controversial. Sporadic mutations in p53 are also common in breast cancer cells. The precise deficiencies that result from these genetic mutations have yet to be fully described. Although the functions of these genes are different, they are all involved in the maintenance of genomic stability after DNA damage. Mutations that impair the function of these four genes may adversely affect the manner in which DNA damage is processed. It is likely that the risk of breast cancer development is increased through this mechanism. In this article, we review the relevancy of p53, BRCA1, BRCA2, and ATM mutations to breast cancer development, and review the in vitro, in vivo, and clinical data exploring the mechanisms by which these mutations affect genomic integrity and DNA damage repair.

Critical role for Atm in suppressing V(D)J recombination-driven thymic lymphoma

Chromosome translocations involving T cell receptor (TCR) loci have been found in tumors from Ataxia telangiectasia (AT) patients and in mouse Atm-/- thymoma, suggesting the involvement of V(D)J recombination in these malignancies. By introducing a RAG-1 deficiency into Atm-/- mice in the presence of a TCR transgene, we show that V(D)J recombination is critical for thymoma development in these mice. Therefore, aberrant V(D)J recombination, normally suppressed by Atm, facilitates tumorigenic events leading to cancer. Because V(D)J recombination is dispensable for lymphomagenesis upon p53 deficiency, this study also indicates that Atm and p53 function by distinct mechanisms in suppressing thymoma.

Neurodegeneration in ataxia-telangiectasia is caused by horror autotoxicus

Ataxia-telangiectasia (A-T) is a pleiotropic, multi-system disorder with manifestations that include immune deficiency, sensitivity to ionizing radiation and neoplasms. Many of these manifestations are understood in principle since the identification in A-T patients of mutations in a gene encoding a protein kinase that plays a key role in signaling and repair of DNA damage. However, the cause of the neurodegeneration that afflicts patients with A-T for at least a decade before they succumb to overwhelming infections or malignancy remains mysterious. Based on our work in a mouse model of A-T and previous evidence of extra-neural autoimmune disorders in A-T, we postulate that the neurodegenerative process in A-T is not due to a function for A-T mutated (ATM) essential for the postnatal brain, but to an autoimmune process (hence 'horror autotoxicus', Paul Ehrlich's term for autoimmune disorder). This hypothetical mechanism may be analogous to that in the so-called 'paraneoplastic' neurodegenerative syndromes in patients with various malignancies. Thus, alterations in the balance between cellular and humoral immunity in A-T probably result in autoantibodies to cerebral epitopes shared with cells of the immune system. This hypothesis has important implications for the understanding and development of effective palliative and even preventative strategies for A-T, and probably for other so far relentlessly progressive neurodegenerative disorders.