Spt4 Is Selectively Required for Transcription of Extended Trinucleotide Repeats

Lengthy trinucleotide repeats encoding polyglutamine (polyQ) stretches characterize the variant proteins of Huntington's disease and certain other inherited neurological disorders. Using a phenotypic screen to identify events that restore functionality to polyQ proteins in S. cerevisiae, we discovered that transcription elongation factor Spt4 is required to transcribe long trinucleotide repeats located either in ORFs or nonprotein-coding regions of DNA templates. Mutation of SPT4 selectively decreased synthesis of and restored enzymatic activity to expanded polyQ protein without affecting protein lacking long-polyQ stretches. RNA-seq analysis revealed limited effects of Spt4 on overall gene expression. Inhibition of Supt4h, the mammalian ortholog of Spt4, reduced mutant huntingtin protein in neuronal cells and decreased its aggregation and toxicity while not altering overall cellular mRNA synthesis. Our findings identify a cellular mechanism for transcription through repeated trinucleotides and a potential target for countermeasures against neurological disorders attributable to expanded trinucleotide regions. [Display omitted] ► Spt4 is selectively required for transcription through extended trinucleotide repeats ► Reduced synthesis of polyQ protein restores function in yeast and neurons ► Nonaggregated expanded polyQ proteins retain enzymatic activity ► Supt4h inhibition decreases synthesis and aggregation of mutant huntingtin The transcription elongation protein Spt4 and its mammalian ortholog, Supt4h, promote the synthesis of mRNAs containing lengthy trinucleotide repeats, revealing a potential target for the treatment of neurological disorders caused by expanded trinucleotide regions.