Mutations in ATXN2 cause spinocerebellar ataxia type 2 (SCA2). The autosomal dominant cerebellar ataxias (ADCA) are a heterogeneous group of neurodegenerative disorders characterized by progressive degeneration of the cerebellum, brain stem and spinal cord. Clinically, ADCA has been divided into three groups: ADCA types I-III. ADCAI is genetically heterogeneous, with five genetic loci, designated spinocerebellar ataxia (SCA) 1, 2, 3, 4 and 6, being assigned to five different chromosomes. ADCAII, which always presents with retinal degeneration (SCA7), and ADCAIII often referred to as the `pure' cerebellar syndrome (SCA5), are most likely homogeneous disorders. Several SCA genes have been cloned and shown to contain CAG repeats in their coding regions. SCA2 is caused by the expansion of a CAG repeat in the coding region of the ATXN2 gene producing an elongated polyglutamine tract in the corresponding protein. The expanded repeats are variable in size and unstable, usually increasing in size when transmitted to successive generations. The function of the ataxins is not known. This locus has been mapped to chromosome 12, and it has been determined that the disease allele usually contains 34-52 CAG repeats, but can contain as few as 32 or more than 100. Normal alleles usually have 22 or 23 repeats, but can contain up to 31 repeats. A potential transcript variant, missing an internal coding exon, has been described; however, its full-length nature is not certain.
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