Spinocerebellar ataxia type 1 (SCA1) is an autosomal dominant neurodegenerative disorder characterized by progressive ataxia, motor disturbance, and bulbar dysfunction. The disease is typically manifested in middle age and causes death 10-15 years after onset. Cases with juvenile onset and more severe disease course have been reported in the most recent generations of some SCA1 families, suggesting anticipation. Neuropathologic studies in SCA1 reveal neuronal loss in the cerebellar cortex, dentate nuclei, and brain stem. We have recently identified the SCA1 gene and determined that it contains a highly polymorphic CAG repeat which is unstable and expanded in affected individuals. The repeat is present in a 10-11 kb mRNA transcript that is expressed in a variety of tissues. We have cloned and sequenced 9.5 kb of cDNA from this transcript and identified a single open reading frame of 2448 bp to predict an 816 amino acid gene product. The CAG repeat is within the coding region and encodes a polyglutamine tract. The SCA1 protein, ataxin-1, appears to be a new protein which does not share any homology with previously identified molecules. The goals of this project are to characterize ataxin-1, to investigate its normal function, and to elucidate the pathogenetic mechanism in SCA1. To accomplish these goals we will use genetic and biochemical approaches. To begin with, we will complete the characterization of the structure of the human SCA1 gene, including analysis of the promoter region and investigations of alternative splicing. To identify potential functional domains in ataxin-1 we will characterize and sequence the murine and Drosophila homologues. Polyclonal antibodies against ataxin-1 will be developed and used to determine the cellular and subcellular localization of this protein in man and mouse. The expression patterns and properties of ataxin- l in tissues from SCA1 patients will be examined. There are various hypotheses regarding the pathogenetic mechanisms in SCA1. The polyglutamine expansion may change the conformation of ataxin-1 rendering it vulnerable to aggregation, or alternatively it may cause the protein to interact aberrantly with other proteins. The possible aberrant interaction of the mutant ataxin- 1 with itself or with other proteins will be investigated. A murine model for SCA1 will be generated by introducing an expanded polyglutamine tract into the murine gene using homologous recombination. The phenotypic, neuropathologic, and physiologic features of these mice will be analyzed in detail. To investigate the normal role of ataxin-1 in development and physiology, we will mutate the murine gene in embryonic stem cells by homologous recombination to generate null mutants. These studies should enhance our understanding of pathogenetic mechanisms in SCA1 and other neurodegenerative disorders.
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