Relationship of polyglutamine expansion and loss of ataxin-1 function in neurological diseases

Ataxin-1 (ATXN1) was originally identified as a gene in which abnormal expansion of glutamine encoding CAG repeats causes inherited neurodegenerative disease spinocerebellar ataxia type 1. Spinocerebellar ataxia type 1 is characterized by alterations in movement, cognition, and mood, with severe pathology in the cerebellum and brain stem. Studies in spinocerebellar ataxia type 1 mouse models indicated that CAG expansion predominantly causes pathogenic ATXN1 gain-of-function in the cerebellum. Conversely, mice lacking ATXN1 are severely impaired in cognitive tests and exhibit abnormalities in cortical functions. Subsequent genetic and functional studies have associated ATXN1 with intelligence and several neurological conditions, including Alzheimer's disease, amyotrophic lateral sclerosis, schizophrenia, and multiple sclerosis. In some conditions, association is found with CAG expansions in ATXN1 and in others with the loss of ATXN1 expression. Additionally, recent studies indicate the role of ATXN1 in glial cells. Here, we review current genetic and functional evidence of how ATXN1 CAG expansion and loss of ATXN1 function contribute to brain dysfunction in spinocerebellar ataxia type 1, Alzheimer's disease, and multiple sclerosis, with an emphasis on glial cells.