Global ribosome profiling reveals that mutant huntingtin stalls ribosomes and represses protein synthesis independent of fragile X mental retardation protein

The regulators that stall ribosome translocation are poorly understood. We find that polyglutamine-expanded mutant Huntingtin (mHtt), the Huntington’s disease (HD) causing protein, promotes ribosome stalling and physiologically suppresses protein synthesis. A comprehensive, genome-wide analysis of ribosome footprint profiling (Ribo-Seq) revealed widespread ribosome stalling on mRNA transcripts and a shift in the distribution of ribosomes toward the 5’ end, with single-codon unique pauses on selected mRNAs in HD cells. In Ribo-Seq, we found fragile X mental retardation protein (FMRP), a known regulator of ribosome stalling, translationally upregulated and it co-immunoprecipitated with mHtt in HD cells and postmortem brain. Depletion of FMRP gene, Fmr1 , however, did not affect the mHtt-mediated suppression of protein synthesis or ribosome stalling in HD cells. Consistent with this, heterozygous deletion of Fmr1 in Q175FDN-Het mouse model, Q175FDN-Het; Fmr1 +/– , showed no discernable phenotype, but a subtle deficit in motor skill learning. On the other hand, depletion of mHtt, which binds directly to ribosomes in an RNase-sensitive manner, enhanced global protein synthesis, increased ribosome translocation and decreased stalling. This mechanistic knowledge advances our understanding of the inhibitory role of mHtt in ribosome translocation and may lead to novel target(s) identification and therapeutic approaches that modulate ribosome stalling in HD. One Sentence Summary Huntington’s disease (HD) protein, mHtt, binds to ribosomes and affects their translocation and promotes stalling independent of the fragile X mental retardation protein.