Spinocerebellar Ataxia type 7 (SCA7) is one of 9 neurodegenerative diseases caused by polyglutamine (polyQ) expansions in target proteins. The length of the polyQ expansions are inversely correlated with the age of onset and directly correlated with the severity of the disease. Although the affected protein is different in each of the polyQ diseases, common cellular and molecular changes are associated with the expansions, including abnormal gene expression patterns, the presence of nuclear inclusions, and altered ubiquitin levels. The mechanistic basis for these changes, or how these features give rise to progressive ataxias, is not clear. Clues to these questions may come from studies of Atxn7, the protein affected in SCA7. Atxn7 is part of the SAGA complex, which houses both histone acetyltransferase (HAT) and deubiquitinase (DUB) activity. Both of these activities are essential for the role of SAGA in gene regulation, and disruption of these functions by polyQ-Atxn7 might directly contribute to the disease phenotype. Recent studies indicate that the SAGA DUB module, which includes Atxn7, has nonhistone substrates, raising the possibility that loss or misdirection of the DUB module might contribute to the abnormal accumulation of ubiquitin observed in SCA7 cells. Here we explore this idea and present a model for how polyQ-Atxn7 might act in a dominant manner over time to disrupt SAGA functions, disturbing both transcriptional and non-transcriptional processes, ultimately leading to development of SCA7.
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