Luciana Chessa

Splicing Defects in the Ataxia-Telangiectasia Gene, ATM: Underlying Mutations and Consequences

Mutations resulting in defective splicing constitute a significant proportion (30/62 [48%]) of a new series of mutations in the ATM gene in patients with ataxia-telangiectasia (AT) that were detected by the protein-truncation assay followed by sequence analysis of genomic DNA. Fewer than half of the splicing mutations involved the canonical AG splice-acceptor site or GT splice-donor site. A higher percentage of mutations occurred at less stringently conserved sites, including silent mutations of the last nucleotide of exons, mutations in nucleotides other than the conserved AG and GT in the consensus splice sites, and creation of splice-acceptor or splice-donor sites in either introns or exons. These splicing mutations led to a variety of consequences, including exon skipping and, to a lesser degree, intron retention, activation of cryptic splice sites, or creation of new splice sites. In addition, 5 of 12 nonsense mutations and 1 missense mutation were associated with deletion in the cDNA of the exons in which the mutations occurred. No ATM protein was detected by western blotting in any AT cell line in which splicing mutations were identified. Several cases of exon skipping in both normal controls and patients for whom no underlying defect could be found in genomic DNA were also observed, suggesting caution in the interpretation of exon deletions observed in ATM cDNA when there is no accompanying identification of genomic mutations.

Chk2 Activation Dependence on Nbs1 after DNA Damage

ABSTRACT The checkpoint kinase Chk2 has a key role in delaying cell cycle progression in response to DNA damage. Upon activation by low-dose ionizing radiation (IR), which occurs in an ataxia telangiectasia mutated (ATM)-dependent manner, Chk2 can phosphorylate the mitosis-inducing phosphatase Cdc25C on an inhibitory site, blocking entry into mitosis, and p53 on a regulatory site, causing G1 arrest. Here we show that the ATM-dependent activation of Chk2 by γ- radiation requires Nbs1, the gene product involved in the Nijmegen breakage syndrome (NBS), a disorder that shares with AT a variety of phenotypic defects including chromosome fragility, radiosensitivity, and radioresistant DNA synthesis. Thus, whereas in normal cells Chk2 undergoes a time-dependent increased phosphorylation and induction of catalytic activity against Cdc25C, in NBS cells null for Nbs1 protein, Chk2 phosphorylation and activation are both defective. Importantly, these defects in NBS cells can be complemented by reintroduction of wild-type Nbs1, but neither by a carboxy-terminal deletion mutant of Nbs1 at amino acid 590, unable to form a complex with and to transport Mre11 and Rad50 in the nucleus, nor by an Nbs1 mutated at Ser343 (S343A), the ATM phosphorylation site. Chk2 nuclear expression is unaffected in NBS cells, hence excluding a mislocalization as the cause of failed Chk2 activation in Nbs1-null cells. Interestingly, the impaired Chk2 function in NBS cells correlates with the inability, unlike normal cells, to stop entry into mitosis immediately after irradiation, a checkpoint abnormality that can be corrected by introduction of the wild-type but not the S343A mutant form of Nbs1. Altogether, these findings underscore the crucial role of a functional Nbs1 complex in Chk2 activation and suggest that checkpoint defects in NBS cells may result from the inability to activate Chk2.

Ataxia-telangiectasia: Identification and detection of founder-effect mutations in the ATM gene in ethnic populations

To facilitate the evaluation of ATM heterozygotes for susceptibility to other diseases, such as breast cancer, we have attempted to define the most common mutations and their frequencies in ataxia-telangiectasia (A-T) homozygotes from 10 ethnic populations. Both genomic mutations and their effects on cDNA were characterized. Protein-truncation testing of the entire ATM cDNA detected 92 (66%) truncating mutations in 140 mutant alleles screened. The haplotyping of patients with identical mutations indicates that almost all of these represent common ancestry and that very few spontaneously recurring ATM mutations exist. Assays requiring minimal amounts of genomic DNA were designed to allow rapid screening for common ethnic mutations. These rapid assays detected mutations in 76% of Costa Rican patients (3), 50% of Norwegian patients (1), 25% of Polish patients (4), and 14% of Italian patients (1), as well as in patients of Amish/Mennonite and Irish English backgrounds. Additional mutations were observed in Japanese, Utah Mormon, and African American patients. These assays should facilitate screening for A-T heterozygotes in the populations studied.

Critical Involvement of the ATM-Dependent DNA Damage Response in the Apoptotic Demise of HIV-1-Elicited Syncytia

DNA damage can activate the oncosuppressor protein ataxia telangiectasia mutated (ATM), which phosphorylates the histone H2AX within characteristic DNA damage foci. Here, we show that ATM undergoes an activating phosphorylation in syncytia elicited by the envelope glycoprotein complex (Env) of human immunodeficiency virus-1 (HIV-1) in vitro. This was accompanied by aggregation of ATM in discrete nuclear foci that also contained phospho-histone H2AX. DNA damage foci containing phosphorylated ATM and H2AX were detectable in syncytia present in the brain or lymph nodes from patients with HIV-1 infection, as well as in a fraction of blood leukocytes, correlating with viral status. Knockdown of ATM or of its obligate activating factor NBS1 (Nijmegen breakage syndrome 1 protein), as well as pharmacological inhibition of ATM with KU-55933, inhibited H2AX phosphorylation and prevented Env-elicited syncytia from undergoing apoptosis. ATM was found indispensable for the activation of MAP kinase p38, which catalyzes the activating phosphorylation of p53 on serine 46, thereby causing p53 dependent apoptosis. Both wild type HIV-1 and an HIV-1 mutant lacking integrase activity induced syncytial apoptosis, which could be suppressed by inhibiting ATM. HIV-1-infected T lymphoblasts from patients with inactivating ATM or NBS1 mutations also exhibited reduced syncytial apoptosis. Altogether these results indicate that apoptosis induced by a fusogenic HIV-1 Env follows a pro-apoptotic pathway involving the sequential activation of ATM, p38MAPK and p53.

Activation of ATM and Chk2 kinases in relation to the amount of DNA strand breaks

The diverse checkpoint responses to DNA damage may reflect differential sensitivities by molecular components of the damage-signalling network to the type and amount of lesions. Here, we determined the kinetics of activation of the checkpoint kinases ATM and Chk2 (the latter substrate of ATM) in relation to the initial yield of genomic DNA single-strand (SSBs) and double-strand breaks (DSBs). We show that doses of γ-radiation (IR) as low as 0.25 Gy, which generate vast numbers of SSBs but only a few DSBs per cell (<8), promptly activate ATM kinase and induce the phosphorylation of the ATM substrates p53–Ser15, Nbs1–Ser343 and Chk2–Thr68. The full activation of Chk2 kinase, however, is triggered by treatments inflicting >19 DSBs per cell (e.g. 1 Gy), which cause Chk2 autophosphorylation on Thr387, Chk2-dependent accumulation of p21waf1 and checkpoint arrest in the S phase. Our results indicate that, in contrast to ATM, Chk2 activity is triggered by a greater number of DSBs, implying that, below a certain threshold level of lesions (<19 DSBs), DNA repair can occur through ATM, without enforcing Chk2-dependent checkpoints.

Intra-Erythrocyte Infusion of Dexamethasone Reduces Neurological Symptoms in Ataxia Teleangiectasia Patients: Results of a Phase 2 Trial

BackgroundAtaxia Teleangiectasia [AT] is a rare neurodegenerative disease characterized by early onset ataxia, oculocutaneous teleangiectasias, immunodeficiency, recurrent infections, radiosensitivity and proneness to cancer. No therapies are available for this devastating disease. Recent observational studies in few patients showed beneficial effects of short term treatment with betamethasone. To avoid the characteristic side effects of long-term administration of steroids we developed a method for encapsulation of dexamethasone sodium phosphate (DSP) into autologous erythrocytes (EryDex) allowing slow release of dexamethasone for up to one month after dosing. Aims of the study were: the assessment of the effect of EryDex in improving neurological symptoms and adaptive behaviour of AT patients; the safety and tolerability of the therapy.MethodsTwenty two patients (F:M = 1; mean age 11.2 ± 3.5) with a confirmed diagnosis of AT and a preserved or partially supported gait were enrolled for the study. The subjects underwent for six months a monthly infusion of EryDex. Ataxia was assessed by the International Cooperative Ataxia Rating Scale (ICARS) and the adaptive behavior by Vineland Adaptive Behavior Scales (VABS). Clinical evaluations were performed at baseline and 1, 3, and 6 months.ResultsAn improvement in ICARS (reduction of the score) was detected in the intention-to-treat (ITT) population (n = 22; p = 0.02) as well as in patients completing the study (per protocol PP) (n = 18; p = 0.01), with a mean reduction of 4 points (ITT) or 5.2 points (PP). When compared to baseline, a significant improvement were also found in VABS (increase of the score) (p < 0.0001, ITT, RMANOVA), with statistically significant increases at 3 and 6 months (p < 0.0001). A large inter-patient variability in the incorporation of DSP into erythrocytes was observed, with an evident positive effect of higher infusion dose on ICARS score decline. Moreover a more marked improvement was found in less neurologically impaired patients. Finally, a 19 month-extension study involving a subgroup of patients suggested that Erydex treatment can possibly delay the natural progression of the disease.EryDex was well tolerated; the most frequent side effects were common AT pathologies.ConclusionsEryDex treatment led to a significant improvement in neurological symptoms, without association with the typical steroid side effects.Trial registrationCurrent Controlled Trial2010-022315-19SpA

ATM protein and p53-serine 15 phosphorylation in ataxia-telangiectasia (AT) patients and at heterozygotes

ATM (ataxia-telangiectasia mutated) gene plays a central role in the DNA-damage response pathway. We characterized the ATM protein expression in immortalized cells from AT and AT-variant patients, and heterozygotes and correlated it with two ATM-dependent radiation responses, G1 checkpoint arrest and p53-Ser 15 phosphorylation. On Western blots, the full-length ATM protein was detected in eight of 18 AT cases, albeit at 1–32% of the normal levels, whereas a truncated ATM protein was detected in a single case, despite the prevalence among cases of truncation mutations. Of two ataxia without telangiectasia [A-(T)] cases, one expressed 20% and the other ~70% of the normal ATM levels. Noteworthy, among ten asymptomatic heterozygous carriers for AT, normal amounts of ATM protein were found in one and reduced by 40–50% in the remaining cases. The radiation-induced phosphorylation of p53 protein at serine 15, largely mediated by ATM kinase, was defective in AT, A(-T) and in 2/4 heterozygous carriers, while the G1 cell cycle checkpoint was disrupted in all AT and A(-T) cases, and in 3/10 AT heterozygotes. Altogether, our study shows that AT and A(-T) cases bearing truncation mutations of the ATM gene can produce modest amounts of full-length (and only rarely truncated) ATM protein. However, this limited expression of ATM protein provides no benefit regarding the ATM-dependent responses related to G1 arrest and p53-ser15 phosphorylation. Our study additionally shows that the majority of AT heterozygotes express almost halved levels of ATM protein, sufficient in most cases to normally regulate the ATM-dependent DNA damage-response pathway.

Ataxia-telangiectasia-mutated dependent phosphorylation of Artemis in response to DNA damage.

Artemis plays a crucial role in the hairpin‐opening step of antigen receptor VDJ gene recombination in the presence of catalytic subunit of deoxyribonucleic acid (DNA)‐dependent protein kinase (DNA‐PKcs). A defect in Artemis causes human radiosensitive‐severe combined immunodeficiency. Cells from Artemis‐deficient patients and mice display increased chromosomal instability, but the precise function of this factor in the response to DNA damage remains to be elucidate. In this study, we show that Artemis is hyperphosphorylated in an Ataxia‐telangiectasia‐mutated (ATM)‐ and Nijmegen breakage syndrome 1 (Nbs1)‐dependent manner in response to ionizing radiation (IR), and that S645 is an SQ/TQ site that contributes to retarded mobility of Artemis upon IR. The hyperphosphorylation of Artemis is markedly reduced in ATM‐ and Nbs1‐null cells. Reintroduction of wild‐type ATM or Nbs1 reconstituted Artemis hyperphosphorylation in ATM‐ or Nbs1‐deficient cells, respectively. In support of this functional link, hyperphosphorylated Artemis was found to physically associate with the Mre11/Rad50/Nbs1 complex in an ATM‐dependent manner in response to IR‐induced DNA double strand breaks (DSB). Since deficiency of either DNA‐Pkcs or ATM leads to defective repair of IR‐induced DSB, our finding places Artemis at the signaling crossroads downstream of DNA‐PKcs and ATM in IR‐induced DSB repair. (Cancer Sci 2005; 96: 134–141)

A randomized trial of oral betamethasone to reduce ataxia symptoms in ataxia telangiectasia.

No controlled studies exist regarding the pharmaceutical reduction of ataxia symptoms in ataxia telangiectasia (A‐T). In a multicenter, double‐blind, randomized, placebo‐controlled crossover trial, oral betamethasone (BETA) and placebo were compared in terms of their reduction of ataxia symptoms as assessed with the International Cooperative Ataxia Rating Scale (ICARS). In this study of 13 A‐T children, betamethasone reduced the ICARS total score by a median of 13 points in the intent‐to‐treat population and 16 points in the per‐protocol population (ie, median percent decreases of ataxia symptoms of 28% and 31%, respectively). In conclusion, Oral betamethasone could be a promising therapy to relieve ataxia symptoms in A‐T patients; however, long‐term effectiveness and safety must be established. (Current Controlled Trials, number ISRCTN08774933.)

ATM and the DNA damage response

The 2005 International Workshop on Ataxia‐Telangiectasia, ATM and the DNA Damage Response took place between 8 and 11 June 2005 on the banks of Lake Maggiore, Italy. The workshop was organized by L. Chessa and D. Delia. ![][1] This workshop on ataxia‐telangiectasia marked the tenth anniversary of the discovery of the gene that is defective in this syndrome—that is, ataxia‐telangiectasia mutated ( ATM ; Savitsky et al , 1995). At the meeting, several important developments were reported, including: an expansion of the substrate repertoire of the ATM kinase; the use of animal models to analyse the signalling pathways controlled by ATM and the functional consequences of disrupting these pathways; new insights into cell‐cycle control and the maintenance of genome stability; the influence of modifier genes on ATM function; and approaches for correcting the progressive neurodegeneration that is a part of this syndrome. Ataxia‐telangiectasia is an autosomal recessive disorder characterized by neurodegeneration, immunodeficiency, hypogonadism and susceptibility to cancer. At the cellular level, it is marked by genomic instability, which is due to a defective response to double‐stranded breaks (DSBs) in DNA. This is manifested by hypersensitivity to ionizing radiation (IR) and radiomimetic compounds, and by a decreased ability to activate the DNA‐damage‐response network, which includes the cell‐cycle checkpoints (Lavin & Shiloh, 1997; Chun & Gatti, 2004). The protein product of the ATM gene is present in the nucleus as an inactive dimer or oligomer, and is activated in response to DSBs in a process that involves autophosphorylation on serine (Ser) 1,981. This causes a dissociation of the dimer to form active monomeric forms, which are able to initiate the phosphorylation of many intermediates, such as p53 and the checkpoint kinase Chk2, which are involved in DNA repair and cell‐cycle control (Bakkenist & Kastan, 2003). However, ATM is not solely responsible for initiating this … [1]: /embed/graphic-1.gif