Ataxia-telangiectasia: mild neurological presentation despite null ATM mutation and severe cellular phenotype

Ataxia-telangiectasia (A-T) is an autosomal recessive disorder characterized by progressive neurodegeneration, immunodeficiency, susceptibility to cancer, genomic instability, and sensitivity to ionizing radiation. A-T is caused by mutations that eliminate or inactivate the nuclear protein kinase ATM, the chief activator of the cellular response to double strand breaks (DSBs) in the DNA. Mild A-T is usually caused by ATM mutations that leave residual amounts of active ATM. We studied two siblings with mild A-T, as defined by clinical examination and a quantitative A-T neurological index. Surprisingly, no ATM was detected in the patients' cells, and sequence analysis revealed that they were homozygous for a truncating ATM mutation (5653delA) that is expected to lead to the classical, severe neurological presentation. Moreover, the cellular phenotype of these patients was indistinguishable from that of classical A-T: all the tested parameters of the DSB response were severely defective as in typical A-T. This analysis shows that the severity of the neurological component of A-T is determined not only by ATM mutations but also by other influences yet to be found.     

Underexpression and abnormal localization of ATM products in ataxia telangiectasia patients bearing ATM missense mutations

Ataxia telangiectasia (A-T) is a rare autosomal recessive disorder characterized by progressive cerebellar ataxia, oculocutaneous telangiectasia, immune defects and predisposition to malignancies. A-T is caused by biallelic inactivation of the ATM gene, in most cases by frameshift or nonsense mutations. More rarely, ATM missense mutations with unknown consequences on ATM function are found, making definitive diagnosis more challenging. In this study, a series of 15 missense mutations, including 11 not previously reported, were identified in 16 patients with clinical diagnosis of A-T belonging to 14 families and 1 patient with atypical clinical features. ATM function was evaluated in patient lymphoblastoid cell lines by measuring H2AX and KAP1 phosphorylation in response to ionizing radiation, confirming the A-T diagnosis for 16 cases. In accordance with previous studies, we showed that missense mutations associated with A-T often lead to ATM protein underexpression (15 out of 16 cases). In addition, we demonstrated that most missense mutations lead to an abnormal cytoplasmic localization of ATM, correlated with its decreased expression. This new finding highlights ATM mislocalization as a new mechanism of ATM dysfunction, which may lead to therapeutic strategies for missense mutation associated A-T.     

Cancer risk in ATM heterozygotes: a model of phenotypic and mechanistic differences between missense and truncating mutations

The failure to find an increased frequency of ATM mutations in large cancer cohorts, especially breast cancer, is contrary to what was anticipated based on the increased cancer susceptibility of obligate ATM heterozygotes from families with ataxia-telangiectasia (A-T). We hypothesize that this paradox might be resolved if two types of ATM heterozygotes exist and the phenotypes differ, i.e., those with truncating types of mutations (ATM(trunc)), that make no protein, and those with missense types of mutations (ATM(mis)), that make reduced amounts of defective protein. The phenotype of ATM(trunc/trunc) mutations is the A-T syndrome; the phenotype of ATM(mis/mis) mutations, judging from the few homozygous patients that have been documented, appears to include some neurological features and cancer susceptibility but not the A-T syndrome. Evidence is reviewed which suggests that ATM(mis/wt) mutations are technically more difficult to detect than ATM(trunc/wt) mutations. Despite this, most large cancer cohort studies have identified mainly missense mutations and few truncating mutations. This model would require a paradigm shift for cancer risk analyses, to recognize the existence of different allele frequencies for the two types of A-T heterozygotes.     

Rapid and efficient ATM mutation detection by fluorescent chemical cleavage of mismatch: identification of four novel mutations

Mutations in the Ataxia Telangiectasia Mutated (ATM) gene are responsible for the autosomal recessive disease Ataxia Telangiectasia (A-T). A wide variety of mutations scattered across the entire coding region (9168bp) of ATM have been found, which presents a challenge in developing an efficient mutation screening strategy for detecting unknown mutations. Fluorescent chemical cleavage of mismatch (FCCM) is an ideal mutation screening method, offering a non-radioactive alternative to other techniques such as restriction endonuclease fingerprinting (REF). Using FCCM, we have developed an efficient, accurate and sensitive mutation detection method for screening RT-PCR products for ATM mutations. We have identified seven ATM mutations in five A-T families, four of which are previously unknown. We quantified ATM protein expression in four of the families and found variable ATM protein expression (0-6.4%), further evidence for mutant ATM protein expression in both classic and variant A-T patients. We conclude that FCCM offers a robust ATM mutation detection method and can be used to screen for ATM mutations in cancer-prone populations.     

The complete sequence of the coding region of the ATM gene reveals similarity to cell cycle regulators in different species

Ataxia-telangiectasia (A-T) is an autosomal recessive disorderinvolving cerebellar degeneration, immunodeficiency, radiationsensitivity, and cancer predisposition. A-T heterozygotes aremoderately cancer prone. The A-T gene, designated ATM, was recentlyidentified in our laboratory by positional cloning, and a partialcDNA clone was found to encode a polypeptide with a PI-3 kinasedomain. We report here the molecular cloning of a cDNA contigspanning the complete open reading frame of the ATM gene. Thepredicted protein of 3056 amino acids shows significant sequencesimilarities to several large proteins in yeast, Drosophilaand mammals, all of which share the PI-3 kinase domain. Manyof these proteins are involved in the detection of DNA damageand the control of cell cycle progression. Mutations in theirgenes confer a variety of phenotypes with features similar tothose observed in human A-T cells. The complete sequence ofthe ATM gene product provides useful clues to the function ofthis protein, and furthers understanding of the pleiotropicnature of the A-T mutations.     

Ataxia-telangiectasia: phenotype/genotype studies of ATM protein expression, mutations, and radiosensitivity

Previous studies on a limited number of ataxia-telangiectasia (A-T) patients with detectable levels of intracellular ATM protein have suggested a genotype/phenotype correlation. We sought to elucidate this possible correlation by comparing ATM protein levels with mutation types, radiosensitivity, and clinical phenotype. In this study, Western blot analysis was used to measure ATM protein in lysates of lymphoblastoid cell lines (LCLs) from 123 unrelated A-T patients, 10 A-T heterozygotes, and 10 patients with phenotypes similar to A-T. Our Western blot protocol can detect the presence of ATM protein in as little as 1 microg of total protein; at least 25 microg of protein was tested for each individual. ATM protein was absent in 105 of the 123 patients (85%); most of these patients had truncating mutations. The remaining subset of 18 patients (15%) had reduced levels of normal-sized ATM protein; missense mutations were more common in this subset. We used a colony survival assay to characterize the phenotypic response of the LCLs to radiation exposure; patients with or without detectable ATM protein were typically radiosensitive. Nine of 10 A-T heterozygotes also had reduced expression of ATM, indicating that both alleles contribute to ATM protein production. These data suggest that although ATM-specific mRNA is abundant in A-T cells, the abnormal ATM protein is unstable and is quickly targeted for degradation. We found little correlation between level of ATM protein and the type of underlying mutation, the clinical phenotype, or the radiophenotype.     

Ataxia-telangiectasia: founder effect among north African Jews

The ATM gene is responsible for the autosomal recessive disorder ataxia- telangiectasia (A-T), characterized by cerebellar degeneration, immunodeficiency and cancer predisposition. A-T carriers were reported to be moderately cancer-prone. A wide variety of A-T mutations, most of which are unique to single families, were identified in various ethnic groups, precluding carrier screening with mutation-specific assays. However, a single mutation was observed in 32/33 defective ATM alleles in Jewish A-T families of North African origin, coming from various regions of Morocco and Tunisia. This mutation, 103C-->T, results in a stop codon at position 35 of the ATM protein. In keeping with the nature of this mutation, various antibodies directed against the ATM protein failed to defect this protein in patient cells. A rapid carrier detection assay detected this mutation in three out of 488 ATM alleles of Jewish Moroccan or Tunisian origin. This founder effect provides a unique opportunity for population-based screening for A-T carriers in a large Jewish community.      

Independent mutational events are rare in the ATM gene: haplotype prescreening enhances mutation detection rate

Mutations in the ATM gene are responsible for the autosomal recessive disorder ataxia-telangiectasia (A-T). Many different mutations have been identified using various techniques, with detection efficiencies ranging from 57 to 85%. In this study, we employed short tandem repeat (STR) haplotypes to enhance mutation identification in 55 unrelated A-T families of Iberian origin (20 Spanish, 17 Brazilian, and 18 Hispanic-American); we were able to identify 95% of the expected mutations. Allelic sizes were standardized based on a reference sample (CEPH 1347-2). Subsequent mutation screening was performed by PTT, SSCP, and DHPLC, and abnormal regions were sequenced. Many STR haplotypes were found within each population and six haplotypes were observed across several of these populations. Single nucleotide polymorphism (SNP) haplotypes further suggested that most of these common mutations are ancestrally related, and not hot spots. However, two mutations (8977C>T and 8264_8268delATAAG) may indeed be recurring mutational events. Common haplotypes were present in 13 of 20 Spanish A-T families (65%), in 11 of 17 Brazilian A-T families (65%), and, in contrast, in only eight of 18 Hispanic-American families (44%). Three mutations were identified that would be missed by conventional screening strategies. In all, 62 different mutations (28 not previously reported) were identified and their associated haplotypes defined, thereby establishing a new database for Iberian A-T families, and extending the spectrum of worldwide ATM mutations.     

ATM germline mutations in classical ataxia-telangiectasia patients in the Dutch population

Germline mutations in the ATM gene are responsible for the autosomal recessive disorder ataxia-telangiectasia (A-T). In our study, we have determined the ATM mutation spectrum in 19 classical A-T patients, including some immigrant populations, as well as 12 of Dutch ethnic origin. Both the protein truncation test (PTT) and the restriction endonuclease fingerprinting (REF) method were used and compared for their detection efficiency, identifying 76% and 60% of the mutations, respectively. Most patients were found to be compound heterozygote. Seventeen mutations were distinct, of which 10 were not reported previously. Mutations are small deletions or point mutations frequently affecting splice sites. Moreover, a 16.7-kb genomic deletion of the 3′ end of the gene, most likely a result of recombination between two LINE elements, was identified. The most frequently found mutation, identified in three unrelated Turkish A-T individuals, was previously described to be a Turkish A-T founder mutation. The presence of a founder mutation among relatively small ethnic population groups in Western Europe could indicate a high carrier frequency in such communities. In patients of Dutch ethnic origin, however, no significant founder effect could be identified. The observed genetic heterogeneity including the relative high percentage of splice-site mutations had no reflection on the phenotype. All patients manifested classical A-T and increased cellular radioresistant DNA synthesis. Hum Mutat 12:330–337, 1998.© 1998 Wiley-Liss, Inc.     

ATM protein synthesis patterns in sporadic breast cancer.

AIMS: The gene mutated in ataxia-telangiectasia (A-T), designated ATM (for "A-T mutated"), is believed to be associated with an increased risk of developing breast cancer. Most patients with A-T have null mutations of the ATM gene that appear to give rise to a truncated nonfunctional ATM protein. Therefore, the increased risk of breast cancer reported in A-T heterozygotes appears to be the result of haplo-insufficiency of ATM in breast tissues. This study aimed to determine whether reduced synthesis of ATM was also an important factor in sporadic breast cancer. METHODS: Paraffin wax embedded tissues from patients with breast invasive ductal carcinoma (IDC) (n = 42), patients with ductal carcinoma in situ (DCIS) (n = 17), and others with lymph node metastases (n = 14) were studied. A streptavidin-biotin-peroxidase system was used to stain tissue sections for the ATM protein using the ATM-4BA and CT-1 polyclonal and monoclonal antibodies, respectively. The protein truncation test was used to screen for mutations in the ATM gene in those patients who had greatly reduced ATM protein immunoreactivity in the primary carcinoma (n = 3). RESULTS: Most metastatic breast carcinomas in lymph nodes (71%) had greatly reduced or absent ATM protein synthesis, which was significant when compared with that observed in non-metastatic invasive breast carcinomas (p = 0.029; chi 2 test). Although not significant (p = 0.045; chi 2 test), some sporadic breast carcinomas (14 of 42) also had reduced or absent ATM protein immunoreactivity. The protein truncation test did not reveal any gross ATM gene abnormality in the cases tested, indicating that the patients were not A-T heterozygotes, who are predisposed to breast cancer. CONCLUSIONS: A reduction in immunohistochemically detectable ATM protein in sporadic breast carcinoma implicates ATM in the progression of the disease.     

Expression of ATM in ataxia telangiectasia fibroblasts rescues defects in DNA double-strand break repair in nuclear extracts

Ataxia telangiectasia (A-T) is a human genetic disorder characterized by progressive cerebellar degeneration, hypersensitivity to ionizing radiation (IR), immunodeficiency, and high cancer risk. At the cellular level, IR sensitivity and increased frequency of spontaneous and IR-induced chromosomal breakage and rearrangements are the hallmarks of A-T. The ATM gene, mutated in this syndrome, has been cloned and codes for a protein sharing homology with DNA-PKcs, a protein kinase involved in DNA double-strand break (DSB) repair and DNA damage responses. The characteristics of the A-T cellular phenotypes and ATM gene suggest that ATM may play a role similar to that of DNA-PKcs in DSB repair and that there is a primary DNA repair defect in A-T cells. In the current study, the function of ATM in DNA DSB repair was evaluated in an in vitro system using two plasmids, carrying either an EcoRI-induced DSB within the lacZalpha gene or various endonuclease-induced DSB in the SupF suppressor tRNA gene. We found that the DSB repair efficiency in A-T nuclear extracts was comparable to, if not higher than, that in normal nuclear extracts. However, the repair fidelity in A-T nuclear extracts was decreased when repairing DSB with short 5' and 3' overhangs (<4 base pairs (bp)) or blunt ends, but not 5' 4-bp overhangs. Sequencing of the mutant plasmids revealed that deletions involving 1-6 nucleotide microhomologies were the major class of mutations in both A-T and normal extracts. However, the size of the deletions in plasmids from A-T nuclear extracts was larger than that from normal nuclear extracts. Expression of the ATM protein in A-T cells corrected the defect in DSB repair in A-T nuclear extracts. These results suggest that ATM plays a role in maintaining genomic stability by preventing the repair of DSB from an error-prone pathway.     

ATM protein synthesis patterns in sporadic breast cancer.

AIMS: The gene mutated in ataxia-telangiectasia (A-T), designated ATM (for "A-T mutated"), is believed to be associated with an increased risk of developing breast cancer. Most patients with A-T have null mutations of the ATM gene that appear to give rise to a truncated nonfunctional ATM protein. Therefore, the increased risk of breast cancer reported in A-T heterozygotes appears to be the result of haplo-insufficiency of ATM in breast tissues. This study aimed to determine whether reduced synthesis of ATM was also an important factor in sporadic breast cancer. METHODS: Paraffin wax embedded tissues from patients with breast invasive ductal carcinoma (IDC) (n = 42), patients with ductal carcinoma in situ (DCIS) (n = 17), and others with lymph node metastases (n = 14) were studied. A streptavidin-biotin-peroxidase system was used to stain tissue sections for the ATM protein using the ATM-4BA and CT-1 polyclonal and monoclonal antibodies, respectively. The protein truncation test was used to screen for mutations in the ATM gene in those patients who had greatly reduced ATM protein immunoreactivity in the primary carcinoma (n = 3). RESULTS: Most metastatic breast carcinomas in lymph nodes (71%) had greatly reduced or absent ATM protein synthesis, which was significant when compared with that observed in non-metastatic invasive breast carcinomas (p = 0.029; chi 2 test). Although not significant (p = 0.045; chi 2 test), some sporadic breast carcinomas (14 of 42) also had reduced or absent ATM protein immunoreactivity. The protein truncation test did not reveal any gross ATM gene abnormality in the cases tested, indicating that the patients were not A-T heterozygotes, who are predisposed to breast cancer. CONCLUSIONS: A reduction in immunohistochemically detectable ATM protein in sporadic breast carcinoma implicates ATM in the progression of the disease.     

Presence of ATM protein and residual kinase activity correlates with the phenotype in ataxia-telangiectasia: a genotype-phenotype study

Ataxia-telangiectasia (A-T) is an autosomal recessive neurodegenerative disorder with multisystem involvement and cancer predisposition, caused by mutations in the A-T mutated (ATM) gene. To study genotype-phenotype correlations, we evaluated the clinical and laboratory data of 51 genetically proven A-T patients, and additionally measured ATM protein expression and kinase activity. Patients without ATM kinase activity showed the classical phenotype. The presence of ATM protein, correlated with slightly better immunological function. Residual kinase activity correlated with a milder and essentially different neurological phenotype, absence of telangiectasia, normal endocrine and pulmonary function, normal immunoglobulins, significantly lower X-ray hypersensitivity in lymphocytes, and extended lifespan. In these patients, cancer occurred later in life and generally consisted of solid instead of lymphoid malignancies. The genotypes of severely affected patients generally included truncating mutations resulting in total absence of ATM kinase activity, while patients with milder phenotypes harbored at least one missense or splice site mutation resulting in expression of ATM with some kinase activity. Overall, the phenotypic manifestations in A-T show a continuous spectrum from severe classical childhood-onset A-T to a relatively mild adult-onset disorder, depending on the presence of ATM protein and kinase activity. Each patient is left with a tremendously increased cancer risk.     

Improved diagnostic testing for ataxia-telangiectasia by immunoblotting of nuclear lysates for ATM protein expression

The laboratory diagnosis of ataxia-telangiectasia (A-T) currently relies upon measurement of serum alphafetoprotein (AFP) and cellular sensitivity to ionizing radiation. A previous report suggests that immunoblotting of whole cell lysates from lymphoblastoid cell lines (LCLs) might be informative for diagnosis. To further evaluate this possibility, and improve sensitivity, we performed immunoblotting for ATM protein on nuclear lysates of 71 consecutive radiosensitive LCLs that were established from patients with clinical features suggestive of A-T. Fifty-two LCLs (73%) contained no detectable ATM protein, with a representative sample (N=25) testing negative for ATM kinase activity, having at least one ATM mutation, and having elevated AFP levels; these results confirmed the diagnosis. Seventeen LCLs (24%) expressed intermediate or normal levels of ATM protein and exhibited normal ATM kinase activity; follow-up studies failed to detect ATM mutations and AFP levels were normal in all but three. Of the remaining two radiosensitive LCLs, one had 35% of normal protein with normal kinase activity and no ATM mutations. The other LCL had 9% of normal protein, with intermediate levels of kinase activity, a homozygous missense ATM mutation, and elevated AFP. Our data suggest that it is very uncommon to encounter bonafide A-T patients with more than trace amounts of ATM protein. We conclude that immunoblotting for ATM protein is of higher specificity for diagnosing A-T than radiosensitivity testing. In addition, we have documented in vitro radiosensitivity in other patients who share some clinical features with A-T.     

ATM mutations, haplotype analysis, and immunological status of Russian patients with ataxia telangiectasia

Mutations in the ATM gene are responsible for the autosomal recessive disorder, ataxia telangiectasia (A-T). Mutations in different ethnic groups are distributed along the entire length of the large, 66 exon ATM gene. In this study, A-T patients from 16 Russian families were assessed for immunological status and ATM haplotype analysis, and screened for ATM mutations. Haplotype analysis was performed to enhance the efficiency of mutation detection. Mutations predicted to cause disease were identified in 19 of 32 alleles (59%), including a truncating mutation (c.5932G>T) that was identified in 8/32 (25%) alleles both by haplotype analysis and mutation screening. This mutation has been found in low abundance in other European A-T cohorts suggesting that this founder-effect mutation may be of Russian origin. The abundance of this mutation may allow for large-scale screening of cancer patients to help clarify the role of ATM in breast and other cancers. Nine of the remaining mutations were previously unreported, and add to the multitude of unique mutations found throughout the gene.     

Functional characterization and targeted correction of ATM mutations identified in Japanese patients with ataxia-telangiectasia

A recent challenge for investigators studying the progressive neurological disease ataxia-telangiectasia (A-T) is to identify mutations whose effects might be alleviated by mutation-targeted therapies. We studied ATM mutations in eight families of Japanese A-T patients (JPAT) and were able to identify all 16 mutations. The probands were compound heterozygotes in seven families, and one (JPAT2) was homozygous for a frameshift mutation. All mutations--four frameshift, two nonsense, four large genomic deletions, and six affecting splicing--were novel except for c.748C>T found in family JPAT6 and c.2639-384A>G found in family JPAT11/12. Using an established lymphoblastoid cell line (LCL) of patient JPAT11, ATM protein was restored to levels approaching wild type by exposure to an antisense morpholino oligonucleotide designed to correct a pseudoexon splicing mutation. In addition, in an LCL from patient JPAT8/9, a heterozygous carrier of a nonsense mutation, ATM levels could also be partially restored by exposure to readthrough compounds (RTCs): an aminoglycoside, G418, and a novel small molecule identified in our laboratory, RTC13. Taken together, our results suggest that screening and functional characterization of the various sorts of mutations affecting the ATM gene can lead to better identification of A-T patients who are most likely to benefit from rapidly developing mutation-targeted therapeutic technologies.     

Ataxia-telangiectasia, cancer and the pathobiology of the ATM gene

Ataxia-telangiectasia (A-T) is a pleiotropic inherited disease characterized by neurodegeneration, cancer, immunodeficiencies, radiation sensitivity, and genetic instability. Although A-T homozygotes are rare, the A-T gene may play a role in sporadic breast cancer and leukemia. ATM, the gene responsible for A-T, is homologous to several cell cycle checkpoint genes from other organisms. ATM is thought to play a crucial role in a signal transduction network that modulates cell cycle checkpoints, genetic recombination, apoptosis, and other cellular responses to DNA damage. New insights into the pathobiology of A-T have been provided by the creation of Atm-/- mice and by in vitro studies of ATM function. Analyses of ATM mutations in A-T patients and in sporadic tumors suggest the existence of two classes of ATM mutation: null mutations that lead to A-T and dominant negative missense mutations that may predispose to cancer in the heterozygous state.     

Mutations in String/CDC25 inhibit cell cycle re-entry and neurodegeneration in a Drosophila model of Ataxia telangiectasia

Mutations in ATM (Ataxia telangiectasia mutated) result in Ataxia telangiectasia (A-T), a disorder characterized by progressive neurodegeneration. Despite advances in understanding how ATM signals cell cycle arrest, DNA repair, and apoptosis in response to DNA damage, it remains unclear why loss of ATM causes degeneration of post-mitotic neurons and why the neurological phenotype of ATM-null individuals varies in severity. To address these issues, we generated a Drosophila model of A-T. RNAi knockdown of ATM in the eye caused progressive degeneration of adult neurons in the absence of exogenously induced DNA damage. Heterozygous mutations in select genes modified the neurodegeneration phenotype, suggesting that genetic background underlies variable neurodegeneration in A-T. The neuroprotective activity of ATM may be negatively regulated by deacetylation since mutations in a protein deacetylase gene, RPD3, suppressed neurodegeneration, and a human homolog of RPD3, histone deacetylase 2, bound ATM and abrogated ATM activation in cell culture. Moreover, knockdown of ATM in post-mitotic neurons caused cell cycle re-entry, and heterozygous mutations in the cell cycle activator gene String/CDC25 inhibited cell cycle re-entry and neurodegeneration. Thus, we hypothesize that ATM performs a cell cycle checkpoint function to protect post-mitotic neurons from degeneration and that cell cycle re-entry causes neurodegeneration in A-T.