TOP ＞ シンポジウム

シンポジウム Molecular pathogenesis of non-coding microsatellite repeat expansion disorders 1S5-1 An overview of non-coding microsatellite repeat expansion disorders Ikeda Yoshio Department of Neurology, Gunma University Graduate School of Medicine Members of hereditary neuromuscular disorders caused by the expansions of various microsatellite repeat motifs are still growing after the first identification of respective mutations in spinal and bulbar muscular atrophy and fragile X syndrome in 1991. They are classified into two categories based on the location of repeat expansions in the genes, one is the expansions in the coding region, and the other is that in the non-coding region. The most frequent mutation in the coding region is the CAG trinucleotide expansion that is translated into the polyglutamine stretch in spinocerebellar ataxias such as SCA1, SCA2, Machado-Joseph disease/SCA3, Huntington disease, and SBMA, etc. The pathogenesis of this type of mutation falls into the toxic gain-of-function mechanism due to polyglutamine-mediated toxicity in neurons. In contrast, the microsatellite repeat motifs of non-coding expansions are more variable and complicated such as（CTG）n in DM1 and SCA8, （CGG）n in FXTAS and FXS, （GAA）n in Friedreich ataxia, （CCTG）n in DM2, （ATTCT）n in SCA10, （TGGAA）n in SCA31, （GGCCTG）n in SCA36/Asidan, and（GGGGCC）n in C9-linked ALS/FTD. The common pathological hallmark of a subset of these disorders is found in the affected cells as RNA foci that are derived from an accumulation of the expanded repeat transcripts. Several lines of evidence suggested that the molecular mechanism associated with RNA foci underlies RNA gain-of-function. However, loss-of-function mechanism is also suggested in some non-coding expansion mutations. Surprisingly, in some disorders it is revealed that the non-coding expanded repeat transcripts can express homopolymeric proteins by the mechanism called as repeat-associated non-ATG（RAN）translation. In this presentation, the hereditary neuromuscular disorders caused by the coding and the non-coding microsatellite repeat expansions are overviewed. 1S5-2 Myotonic dystrophy-toxic RNA and spliceopathy Nakamori Masayuki Department of Neurology, Osaka University Graduate School of Medicine Myotonic dystrophy（DM）is the most common type of muscular dystrophy in adults, caused by unstable genomic expansions of simple tandem repeats. Myotonic dystrophy type 1（DM1）results from expansion of a CTG repeat in the 3´untranslated region of DMPK. In myotonic dystrophy type 2（DM2）, the expanded repeat is a CCTG tetramer in intron 1 of CNBP/ZNF9. The transcripts containing the expanded repeat form ribonuclear inclusions, thereby retained in the nucleus. The mutant RNA gives rise to a toxic gain-of-function by perturbing splicing factors, leading to misregulation of alternative pre-mRNA splicing. The misregulated splicing is thought to be responsible for multisystemic symptoms in DM, such as myotonia, cardiac conduction defects, and glucose intolerance. 1S5-3 A Novel ALS/SCA Crossroad Mutation Asidan Yamashita Toru1，Ikeda Yoshio2，Abe Koji1 1Department of Neurology, Okayama University Graduate School of Medicine, Dentistry and Pharmaceutical Sciences，2Department of Neurology, Gunma University Graduate School of Medicine Recently we found intronic hexanucleotide GGCCTG gene expansion in NOP56 gene as the causative mutation（=SCA36；nicknamed“Asidan”）in nine unrelated Japanese familial SCA. In the nine families, 14 patients were clinically examined and genetically confirmed to Asidan. The age at onset of ataxia was 53.1±3.4 years, with the most frequent symptoms being truncal ataxia（100% of patients）, ataxic dysarthria（100%）, limb ataxia（93%）, and hyperreflexia（79%）. Tongue fasciculation and subsequent atrophy were found in 71% of cases, particularly in those of long duration. Skeletal muscle fasciculation and atrophy of the limbs and trunk were found in 57% of cases. Lower motor involvement was confirmed by EMG and muscle biopsy. The neuropathologic study revealed significant cerebellar Purkinje cell degeneration with obvious loss of lower motor neurons. Immunohistochemical analysis showed that NOP56 was localized to the nuclei of various neurons. Cytoplasmic or intranuclear inclusion staining of NOP56, TDP-43, and ataxin-2 was not observed in the remaining neurons.Taken together, these patients showed unique clinical features of cerebellar ataxia and motor neuron disease（MND）, locating on the crossroad of these two diseases. In this symposium, we would like to introduce the clinical features of Asidan, and discuss the possible mechanism of hexanucleotide GGCCTG expansion leading to both Purkinje cell degeneration and motor neuron loss. 1S5-4 Cytotoxic properties of dipeptide repeat proteins generated by repeat-associated, non-ATG（RAN）translation on c9ALS/FTD Ito Daisuke Department of Neurology, Keio University School of Medicine Advances in genetics and pathological studies have revealed that amyotrophic lateral sclerosis（ALS）and frontotemporal dementia（FTD）constitute a disease spectrum that shares a common molecular basis. The expansion of the GGGGCC hexanucleotide repeat in the non-coding region of the chromosome 9 open reading frame 72（C9orf72）gene is the most common cause of ALS and FTD（c9ALS/FTD）. Recently, it was reported that an unconventional mechanism of repeat-associated non-ATG（RAN）translation arises from C9orf72 expansion. Sense and anti-sense transcripts of the expanded C9orf72 repeat, i.e., the dipeptide repeat protein（DRP）of glycine-alanine（poly-GA）, glycine-proline（poly-GP）, glycine-arginine（poly-GR）, proline-arginine（poly-PR）, and proline-alanine（poly-PA）are deposited in the brains of patients with c9ALS/FTD. However, the pathological significance of RAN-translated peptides remains unknown. To elucidate the impact of individual DRP, we generated synthetic cDNAs encoding 100 repeats of poly-GA, -GP, -GR, -PR, and -PA with start codon, avoiding GGGGCC repeats and evaluated the effects of these proteins on cultured cells and cortical neurons in vivo. Our results revealed that the poly-GA protein formed highly aggregated ubiquitin/p62-positive inclusion bodies in neuronal cells. In contrast, the highly basic proteins poly-GR and PR also formed unique ubiquitin/p62-negative cytoplasmic inclusions, which colocalized with the components of RNA granules. The evaluation of cytotoxicity revealed that overexpressed poly-GA, -GP, and -GR increased the substrates of the ubiquitin-proteasome system（UPS）, including TDP-43, and enhanced the sensitivity to a proteasome inhibitor, indicating that these DRPs are cytotoxic, possibly via UPS dysfunction. These findings demonstrate that DRP affect the protein quality control system, resulting in cytotoxicity and are potential therapeutic targets for c9FTD/ALS. 1S5-5 Mechanism and Function of RAN translation in FXTAS and related microsatellite expansion disorders Todd Peter K. Department of Neurology, University of Michigan Fragile X-associated Tremor Ataxia Syndrome（FXTAS）is a neurodegenerative disorder caused by a CGG trinucleotide repeat expansion in the 5’UTR of the Fragile X gene, FMR1. FXTAS is thought to arise primarily from an RNA gain-of-function toxicity mechanism. However, recent studies demonstrate that the repeat also elicits production of a toxic polyglycine protein, FMRpolyG, via Repeat-Associated Non-AUG（RAN）translation- an atypical form of translational initiation that occurs in association with a variety of pathologic repeats in the absence of an AUG start codon. Here we describe how different RAN translation products from CGG repeats contribute to toxicity in model systems and provide insights into the mechanisms by which RAN translation occurs in different reading frames. Our findings demonstrate that the FMR1 5’UTR supports RAN translation across multiple repeat reading frames, follows some but not all canonical eukaryotic translational initiation steps, and contributes to both the normal translational control of Fragile X mRNA and FXTAS disease pathogenesis. These findings provide a model for Repeat associated Non-AUG（RAN）translation and shed light on the roles of unconventional translational initiation in both normal and disease states.