From protein to RNA: Study of SCA3 polyglutamine disease in Drosophila

Spinocerebellar ataxia type 3 (SCA3) is one of nine human nucleotide repeat expansion diseases due to expansion of a CAG repeat encoding glutamine within the open reading frame of the respective genes. Current data indicate that the expanded polyglutamine (polyQ) protein confers pathogenicity. Protein domains with high percentage of glutamine and/or asparagine (Q/N-rich domain) is also associated with another phenomenon: yeast prion. Prion is a class of protein elements which regulate gene expression through self-propagating conformational changes. For yeast prion, as for pathogenic polyQ protein, the protein domain rich in glutamine is critical for protein conformations. Using Drosophila model of SCA3, we investigated the potential relationship between pathogenic polyQ protein and a yeast prion domain, Sup35N. Our analysis revealed that Sup35N formed accumulations and suppressed polyQ toxicity in the fly. In addition, suppression required the oligopeptide repeat region of the Sup35 prion domain, which is critical for Sup35 prion activity. These results suggest a direct interaction between the pathogenic polyQ protein and Sup35N. With the expansion in the understanding of polyQ diseases, other observations implicate that there are toxic interactions beyond those conferred by polyQ protein. Increasing evidence suggests that simple nucleotide repeat RNAs, such as CUG or CGG repeats, can exert dominant toxicity and induce neurological symptoms. Using Drosophila, we now reveal a pathogenic role of the CAG repeat RNA in polyQ toxicity. Muscleblind, a gene previously implicated in CUG repeat RNA toxicity, was identified as an enhancement modifier of polyQ toxicity in a screen for modifiers of SCA3-induced degenerative phenotypes. Our analysis indicated that mbl enhances polyQ toxicity by acting at the level of the RNA. Further studies revealed that altering the CAG repeat sequence to an interrupted CAACAG repeat dramatically mitigated polyQ-induced neurodegeneration in Drosophila. Moreover, expression of an untranslated CAG repeat conferred neural dysfunction. These studies reveal a role for the RNA in polyQ toxicity, highlighting a potential common component of RNA-based and protein-based trinucleotide expansion diseases