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

Defects in regulation of the cellular life cycle may lead to premature cellular death or malignant transformation. Most of the proteins known to be involved in these processes are mediators of mitogenic signals or components of the cell cycle machinery. It has recently become evident, however, that systems responsible for ensuring genome stability and integrity are no less important in maintaining the normal life cycle of the cell. These systems include DNA repair enzymes and a recently emerging group of proteins that alert growth regulating mechanisms to the presence of DNA damage. These signals slow down the cell cycle while DNA repair ensues. Ataxia-telangiectasia (A-T) is a genetic disorder whose clinical and cellular phenotype points to a defect in such a signaling system. A-T is characterized by neurodegeneration, immunodeficiency, radiosensitivity, cancer predisposition, and defective cell cycle checkpoints. The responsible gene, ATM, was recently cloned and sequenced. ATM encodes a large protein with a region highly similar to the catalytic domain of PI 3-kinases. The ATM protein is similar to a group of proteins in various organisms which are directly involved in the cell cycle response to DNA damage. It is expected to be part of a protein complex that responds to a specific type of DNA strand break by conveying a regulatory signal to other proteins. Interestingly, the immune and nervous systems, which differ markedly in their proliferation rates, are particularly sensitive to the absence of ATM function. The identification of the ATM gene highlights the growing importance of signal transduction initiated in the nucleus rather than in the external environment, for normal cellular growth.     

Astrocytoma linked to familial ataxia-telangiectasia

Summary A 7-year and 11 month-old girl with cerebellar astrocytoma linked to familial ataxia-telangiectasia (AT) is presented. She was born as the 7th girl of a woman with aortic arch syndrome. Two elder sisters of the patient have ataxia telangiectasia. She had immunodeficiency, and cerebellar ataxia, but had no oculocutaneous telangiectasia. The risk of cancer developing in AT patients is about 1,200 times greater than that in age-matched controls. With regard to central nervous system tumours, seven primary tumours have been reported, such as 3 cases of medulloblastoma and 4 cases of glioma. Members of AT families who were under the age of 45 had a risk of dying of a malignant neoplasm five times greater than in the general population. However, there were no reports of glioma in AT families. In this case, it is suggested that IgA deficiency linked to familial AT may have contributed to the development of astrocytoma.     

B-cell subsets imbalance and reduced expression of CD40 in ataxia-telangiectasia patients

Background

Ataxia-telangiectasia (AT) is a well-known primary immunodeficiency with recurrent sinopulmonary infections and variable abnormalities in both the humoral and cellular immune system. Dysfunctions in immunoglobulin production, reduced number of B cells, and B-cell receptor excision circles copies have been reported. We aimed to understand the immunological mechanisms involving the humoral compartment in AT patients by analysing peripheral blood B cells subsets, B-T lymphocyte cooperation through the expression of CD40 and CD40 ligand (CD40L), and cytokines involved in class-switch recombination production.

Neurological and cytogenetic study in early-onset ataxia-telangiectasia patients

The clinical diagnosis of ataxia-telangiectasia (AT) is difficult before the age of 4 years. We report clinical and cytogenetic data on three early-onset, early-diagnosed AT patients at the age of 12, 18 and 22 months, respectively. Postural instability of the trunk, characterized by motor impersistence, was the earliest neurological sign detected as early as 1 year of life. Dystonic movements and postures of arms and trunk and a subtle disorder of eye movement (blinking before gaze changing, increased latency and dysmetry of saccades) were observed during the 2nd year of life. All patients exhibited an unusual temper tantrum. We also observed an increased bleomycin-induced chromosomal instability in patient's cells in the early stages of the disease before all the clinical hallmarks were apparent. Our data suggest that detection of clinical indications, leading to early laboratory confirmation of AT, can reduce the age at diagnosis.     

Pituitary Changes in Ataxia-Telangiectasia Syndrome: An Immunocytochemical,In Situ Hybridization,and DNA Cytometric Study of Three Cases

Ataxia-telangiectasia (AT) syndrome (cerebellar ataxia, oculocutaneous telangiectasias, immunodeficiency, susceptibility to infections, and neoplasia) is associated with cyto- and nucleomegaly in several organ systems. Our aim was to determine (1) whether such cellular abnormalities in the pituitary selectively involve specific cell types, and (2) the proliferation and DNA ploidy status of such cells. Three AT autopsy pituitaries were studied by histology, immunohistochemistry (pituitary hormones, MIB-1, p53 protein),in situ hybridization (pituitary hormones), and Feulgen stain image analysis for ploidy. Results indicated that, in adenohypophyses the scattered pleomorphic, bizarre nuclei were mainly those of somatotrophs and corticotrophs, growth hormone (GH), or adrenocorticotropic hormone (ACTH) immunoreactive and expressing the GH or ACTH gene, respectively. Cyto-and nucleomegaly were less frequent in other secretory cells but were also noted in pituicytes of the posterior lobe. Affected cells were immunonegative for MIB-1 and for p53 protein. Image morphometric DNA analysis showed the bizarre cells to be aneuploid with complex histogram patterns, including many nuclei with DNA contents >8 n. No adenomas were found. We conclude that in AT adenohypophyseal cells with cyto- and nucleomegaly, as well as pleomorphism, synthesize and store adenohypophyseal hormones, mainly GH or ACTH. They and affected pituicytes are nonproliferative and are aneuploid.     

靶向沉默ATM基因对人肺腺癌A549细胞放射敏感性影响

目的 探讨质粒介导RNA干扰抑制ATM基因表达对人肺腺癌A₅₄₉细胞放射敏感性影响。方法 构建ATM基因小分子干扰RNA (siRNA)真核表达质粒pSilencer2.1-ATM并转染A₅₄₉细胞(阳性组),转染pSilencer2.1-nonspecific质粒为阴性组,未转染为对照组。RT-PCR和蛋白印迹法分别检测ATM基因mRNA及蛋白表达,细胞克隆形成实验观察细胞放射敏感性变化,流式细胞仪检测细胞周期和细胞凋亡。结果 成功构建了ATM基因siRNA真核表达质粒。RT-PCR和蛋白印迹法证实阳性组细胞ATM基因表达下调,阳性组、阴性组的放射增敏比(D₀值比)分别为1.50、1.01,流式细胞仪检测显示阳性组G₁、G₂+M期细胞比例减少[51.27%∶61.85%(P=0.012)、6.34%∶10.91%(P=0.008)],细胞凋亡率则高于对照组、阴性组[49.31%∶13.58%(P=0.000)、49.31%∶13.17%(P=0.000)]。结论 ATM基因沉默可增加A₅₄₉细胞的放射敏感性,其机制可能与细胞周期变化及凋亡有关。     

The hallmarks of aging in Ataxia-Telangiectasia

Ataxia-telangiectasia (A-T) is caused by absence of the catalytic activity of ATM, a protein kinase that plays a central role in the DNA damage response, many branches of cellular metabolism, redox and mitochondrial homeostasis, and cell cycle regulation. A-T is a complex disorder characterized mainly by progressive cerebellar degeneration, immunodeficiency, radiation sensitivity, genome instability, and predisposition to cancer. It is increasingly recognized that the premature aging component of A-T is an important driver of this disease, and A-T is therefore an attractive model to study the aging process. This review outlines the current state of knowledge pertaining to the molecular and cellular signatures of aging in A-T and proposes how these new insights can guide novel therapeutic approaches for A-T.     

Hepatitis C virus NS3/4A protein interacts with ATM, impairs DNA repair and enhances sensitivity to ionizing radiation

Hepatitis C virus (HCV) infection is frequently associated with the development of hepatocellular carcinomas and non-Hodgkin's B-cell lymphomas. Nonstructural protein 3 (NS3) of HCV possesses serine protease, nucleoside triphosphatase, and helicase activities, while NS4A functions as a cofactor for the NS3 serine protease. Here, we show that HCV NS3/4A interacts with the ATM (ataxia-telangiectasia mutated), a cellular protein essential for cellular response to irradiation. The expression of NS3/4A caused cytoplasmic translocation of either endogenous or exogenous ATM and delayed dephosphorylation of the phosphorylated ATM and gamma-H2AX following ionizing irradiation. As a result, the irradiation-induced gamma-H2AX foci persisted longer in the NS3/4A-expressing cells. Furthermore, these cells showed increased comet tail moment in single-cell electrophoresis assay, indicating increased double-strand DNA breaks. The cells harboring an HCV replicon also exhibited cytoplasmic localization of ATM and increased sensitivity to irradiation. These results demonstrate that NS3/4A impairs the efficiency of DNA repair by interacting with ATM and renders the cells more sensitive to DNA damage. This effect may contribute to HCV oncogenesis.