ATM signaling

Ataxia-telangiectasia (A-T) is a highly pleiotropic, autosomal recessive disease that leads to multisystem defects and has an intricate cellular phenotype, all linked to the functional inactivation of a single gene. Extensive research on the phenotype and the recent discovery and cloning of the responsible gene point to a defect as a central biochemical locus which links several signal transduction pathways that operate under stress as well as in normal physiological conditions. Ataxia is the first symptom in all patients and is predominantly truncal, first manifested in swaying of the head and trunk on standing and even sitting. Truncal ataxia precedes appendicular cerebellar disease. In the first years of life, certain manifestations are present such as dysarthria, muscular hypotonia, the slow initiation and performance of all voluntary movements, characteristic hypotonic facies and postures, and drooling. Dyssynergia and intention tremor of the upper extremities become a major feature after the fifth year of life. The tendon reflexes are diminished or lost, but may be normal or even hyperactive in the early stages. All these observations show a clear ataxia of cerebellar type, initially of station and gait, and later of intention. Early observations of brains from patients with A-T showed neurodegenerative changes, particularly in the Purkinje and granular cells of the cerebellum. Neuronal degeneration is also present in the brainstem, and dentate and olivary nuclei atrophy. Neuronal loss occurs in the substantial nigra and oculomotor nuclei, dorsal root ganglia, and degenerative changes are evident in spinal motor neurons, and dorsal root and sympathetic motor neurons. Moreover, multiple abnormalities in Purkinje cell development have been observed in an Atm-deficient mouse model. Misplaced Purkinje cells have been observed in both the granular and molecular cell layers. In addition, Purkinje cell dendrites tend to grow laterally instead of extending towards the surface of the cerebellum. ATM (for Ataxia-telangiectasia mutated) has been located by restriction-fragment length polymorphism in the chromosome 11, location: 108,093,211-108,239,829. Interestingly, the site of ATM is the same or adjacent to the region occupied by CD3 (Antigen, Delta subunit), THY1 (T-Cell antigen), and NCAM (Cell Adhesion Molecule, Neural, 1) genes, all of which are members of the immunoglobulin-gene superfamily and consequently may be subject to the same defect that afflicts the T-cell receptor and immunoglobulin molecules in A-T. The ATM gene presents an open reading frame (ORF) of 9,165 kb cDNA and is constituted by 66 exons spread over 150 kb of genomic DNA which has a transcript of 12 kb. The ORF of this transcript predicts a 370-kDa protein composed of 3056 amino acids. Over 300 mutations have been found in A-T patients, distributed across the full length (150 kb of genomic DNA) of the ATM gene. Sequence homology indicates that the atm gene product falls into a family of proteins that are related to the catalytic subunit of phosphatidylinositol 3-kinase (PI 3-kinase). This family includes TEL1, MEC1, TOR1, and TOR2 of the budding yeast Saccharomyces cerevisiae, RAD3 of the fission yeast Schizosaccharomyces pombe, and MEI-41 of Drosophila melanogaster. The mammalian family member most closely related to ATM is the ATR/FRP1 protein and, like its yeast homologs, it mediates cellular responses to unreplicated or damaged DNA. In humans the PI 3-kinase family includes the catalytic subunit of the DNA-dependent protein kinase (DNA-PKcs) and FRAP. These sequence homologies appear to reflect functional homology because many of the PI 3-kinase family members are involved in DNA repair, recombination and cell cycle control. Despite the resemblance to lipid kinases, members of this family, including ATM, possess a serine/threonine protein kinase activity, which is wortmannin sensitive. ATM phosphoprotein is ubiquitously expressed and predominantly found in nuclei of proliferating cells, but subcellular fractionation and immunofluorescence revealed that 10-20% of the protein is present in cytoplasmic vesicles, including peroxisomes and endosomes and a prominent cytoplasmic fraction in mouse oocytes. ATM is endosome-bound in mouse neurons, suggesting molecular sorting of the protein occurs in the cytoplasm. In Purkinje cells, distribution of ATM protein is primarily in cytoplasm, and this may be related to the differentiation state of the cells. ATM mRNA is present in all human and mouse tissues. In situ hybridization shows that ATM mRNA is expressed throughout the whole mouse embryo. Furthermore, ATM has been associated with beta-adaptin in lymphoblast vesicles indicating that it may play a role in intracellular vesicle and/or protein transport mechanisms. No obvious nuclear localization signals have been detected in ATM. Neither an ectopically expressed N-terminal fragment of the protein nor a C-terminal fragment is capable of entering the nucleus.

脊髓小脑性共济失调（A-T）是一种高度多态性的常染色体隐性遗传疾病，导致多系统缺陷，并具有复杂的细胞表型，这些均与单个基因的功能失活密切相关。对表型的研究以及对致病基因的近期发现与克隆，均指向一个作为中心生化位点的基本缺陷，该缺陷连接了多种在应激条件下以及正常生理状态下运作的信号转导途径。共济失调是所有患者的首发症状，主要表现为躯干共济失调，首先在站立和静坐时头部和躯干的摇摆中显现。躯干共济失调先于周围小脑疾病。在生命的最初几年，某些症状如构音障碍、肌张力低下、所有自愿运动的缓慢起始和执行、特征性的肌张力低下面容和姿势，以及流涎等症状会出现。在五岁之后，上肢的运动不协调和意向性震颤成为主要特征。腱反射减弱或消失，但在早期阶段可能正常甚至过度活跃。所有这些观察结果均表明明显的脑干型共济失调，最初是站立和步态，后来是意向性共济失调。对A-T患者脑部的早期研究显示神经退行性变化，尤其是在小脑的浦肯野细胞和颗粒细胞中。神经元退行性变也存在于脑干、齿状核和橄榄核中。在黑质和动眼神经核、背根神经节中发生神经元丢失，并且在脊髓运动神经元、背根和交感神经神经元中存在退行性改变。此外，在Atm缺陷小鼠模型中观察到浦肯野细胞发育的多种异常。在颗粒层和分子层中观察到浦肯野细胞定位不当。此外，浦肯野细胞的树突倾向于横向生长而不是向小脑表面延伸。ATM（Ataxia-telangiectasia mutated）通过限制性片段长度多态性定位在11号染色体上，位置：108,093,211-108,239,829。有趣的是，ATM的位置与CD3（抗原，Delta亚单位）、THY1（T细胞抗原）和NCAM（细胞粘附分子，神经，1）基因的区域相同或相邻，所有这些基因都是免疫球蛋白基因超家族的成员，因此可能受到与A-T中T细胞受体和免疫球蛋白分子相同的缺陷的影响。ATM基因呈现一个9,165 kb的开放阅读框（ORF）cDNA，由66个外显子组成，这些外显子分布在150 kb的基因组DNA上，转录本长度为12 kb。该转录本的ORF预测了一个由3056个氨基酸组成的370-kDa蛋白。在A-T患者中发现了超过300种突变，分布在ATM基因的全长（150 kb的基因组DNA）上。序列同源性表明，atm基因产物属于与磷脂酰肌醇3-激酶（PI 3-kinase）催化亚单位相关的蛋白质家族。该家族包括酵母菌Saccharomyces cerevisiae的TEL1、MEC1、TOR1和TOR2，裂殖酵母Schizosaccharomyces pombe的RAD3，以及果蝇Drosophila melanogaster的MEI-41。与ATM最接近的哺乳动物家族成员是ATR/FRP1蛋白，类似于其酵母同源物，它介导细胞对未复制或受损DNA的响应。在人类中，PI 3-激酶家族包括DNA依赖性蛋白激酶（DNA-PKcs）和FRAP的催化亚单位。这些序列同源性似乎反映了功能同源性，因为PI 3-激酶家族的许多成员都参与DNA修复、重组和细胞周期控制。尽管与脂质激酶相似，但该家族的成员，包括ATM，都拥有丝氨酸/苏氨酸蛋白激酶活性，对wortmannin敏感。ATM磷酸蛋白在细胞中普遍表达，主要存在于增殖细胞的细胞核中，但亚细胞分离和免疫荧光揭示，10-20%的蛋白存在于细胞质囊泡中，包括过氧化物酶体和内体，以及小鼠卵母细胞中的显著细胞质部分。在鼠标神经元中，ATM与内体结合，表明蛋白质在细胞质中的分子排序。在浦肯野细胞中，ATM蛋白的分布主要在细胞质中，这可能与其细胞的分化状态有关。ATM mRNA存在于所有人类和鼠标组织中。原位杂交显示，ATM mRNA在整个小鼠胚胎中表达。此外，ATM与淋巴细胞泡中的beta-适应素相关联，表明它可能在细胞内囊泡和/或蛋白质转运机制中发挥作用。在ATM中没有检测到明显的核定位信号。蛋白质N端片段的异位表达或C端片段均不能进入细胞核。