A growing number of human neurodegenerative diseases result from the expansion of a glutamine repeat in the protein that causes the disease. Spinocerebellar ataxia type 1 (SCA1) is one such disease-caused by expansion of a polyglutamine tract in the protein ataxin-1. To elucidate the genetic pathways and molecular mechanisms underlying neuronal degeneration in this group of diseases, we have created a model system for SCA1 by expressing the full-length human SCA1 gene in Drosophila. Here we show that high levels of wild-type ataxin-1 can cause degenerative phenotypes similar to those caused by the expanded protein. We conducted genetic screens to identify genes that modify SCA1-induced neurodegeneration. Several modifiers highlight the role of protein folding and protein clearance in the development of SCA1. Furthermore, new mechanisms of polyglutamine pathogenesis were revealed by the discovery of modifiers that are involved in RNA processing, transcriptional regulation and cellular detoxification. These findings may be relevant to the treatment of polyglutamine diseases and, perhaps, to other neurodegenerative diseases, such as Alzheimer's and Parkinson's disease.

The hypoxanthine phosphoribosyltransferase (HPRT) gene is constitutively expressed at low levels in all tissues but at higher levels in the brain; the significance and mechanism of this differential expression are unknown. We previously identified a 182-bp element (hHPRT-NE) within the 5-flanking region of the human HPRT (hHPRT) gene, which is involved not only in conferring neuronal specificity but also in repressing gene expression in nonneuronal tissues. Here we report that this element interacts with different nuclear proteins, some of which are present specifically in neuronal cells (complex I) and others of which are present in cells showing constitutive expression of the gene (complex II). In addition, we found that complex I factors are expressed in human NT2/D1 cells following induction of neuronal differentiation by retinoic acid. This finding correlates with an increase of HPRT gene transcription following neuronal differentiation. We also mapped the binding sites for both complexes to a 60-bp region (Ff; positions ؊510 to ؊451) which, when analyzed in transfection assays, functioned as a repressor element analogous to the full-length hHPRT-NE sequence. Methylation interference footprintings revealed a minimal unique DNA motif, 5-GGAAGCC-3, as the binding site for nuclear proteins from both neuronal and nonneuronal sources. However, site-directed mutagenesis of the footprinted region indicated that different nucleotides are essential for the associations of these two complexes. Moreover, UV cross-linking experiments showed that both complexes are formed by the association of several different proteins. Taken together, these data suggest that differential interaction of DNA-binding factors with this regulatory element plays a crucial role in the brain-preferential expression of the gene, and they should lead to the isolation of transcriptional regulators important in neuronal expression of the HPRT gene.Hypoxanthine phosphoribosyltransferase (HPRT) (IMP:pyrophosphate phosphoribosyltransferase; EC 2.4.2.8) catalyzes one of the first steps in the metabolic salvage of the purine bases hypoxanthine and guanine in mammalian cells (57). In humans, HPRT deficiency is associated with two clinical disorders: a partial deficiency results in gouty arthritis (25), whereas a total deficiency causes a devastating and incurable neurological disorder, Lesch-Nyhan syndrome (29,56).HPRT, a soluble cytoplasmic enzyme of approximately 24.5 kDa, is composed of 217 amino acids and is highly conserved among humans, mice, and hamsters (7). The HPRT genes of these species have been isolated and characterized (26,35,41,50). The 5Ј-flanking regions of these genes lack TATA and CAAT elements (21, 37, 48), are extremely GC rich, display several Sp1 consensus sequences (22), and contain multiple transcription initiation sites, all of which are features of genes with housekeeping functions. The coding sequences and untranslated regions of mouse and human HPRT mRNAs exhibit approximately 95 and 80% homology, respectively. Although th...