Transcriptional co -activator dysfunction in polyglutamine disease

Spinal and bulbar muscular atrophy (SBMA) is caused by a CAG repeat expansion in exon 1 of the androgen receptor (AR). This CAG repeat expansion is translated as a polyglutamine repeat, making SBMA one of the eight known polyglutamine expansion diseases. The polyglutamine expansion confers a novel toxic function to the host protein. A central issue in the field is identification of this novel function and its molecular consequences. This thesis addresses changes in transcriptional regulation caused by nuclear expression of polyglutamine, with emphasis on the sequestration of CREB-binding protein (CBP). CBP distribution is altered in the presence of nuclear accumulation of mutant polyglutamine in cell culture, mouse models, and in patient tissue. When nuclear inclusions of polyglutamine form, CBP redistributes from a diffuse nuclear pattern to the inclusions. Coincident with the redistribution are a decrease in soluble CBP and an increase in CBP mRNA. The ability of CBP function to drive transcription from a reported plasmid is impaired in cells expressing mutant polyglutamine. These data prompted the central hypothesis of this thesis, that CBP sequestration by polyglutamines is a cause of cell dysfunction and death in polyglutamine disease. A prediction of the hypothesis is that over-expression of exogenous CBP should reduce the amount of cell death. The hypothesis was tested by developing a system with transient transfection of mutant polyglutamine targeted to the nucleus. CBP over-expression reduced polyglutamine-induced cell loss, as did expression of fragments containing the amino terminus alone. Since CBP is one of several transcriptional coactivators with histone acetyltransferase activity found in polyglutamine inclusions, the acetylation state of histones was assessed. Nuclear expression of mutant polyglutamine causes decreased histone acetylation. Reversal of this deacetylation, either by CBP over-expression or by addition of deacetylase inhibitors reduces cell death. The findings presented in this thesis support a model for polyglutamine disease in which critical coactivators are sequestered, altering acetylation states in the cells, and triggering cell death. Furthermore, deacetylase inhibitors are identified as potential therapeutic agents