PRMT5 is required for full-lengthHTTexpression by repressing multiple proximal intronic polyadenylation sites

Abstract

AbstractExpansion of the CAG trinucleotide repeat tract in exon 1 of theHuntingtin(HTT) gene above a threshold of ∼36 repeats causes Huntington’s disease (HD) through the expression of a polyglutamine-expanded form of the HTT protein. This mutation triggers wide-ranging cellular and biochemical pathologies leading to cognitive, motor, and psychiatric symptoms in HD patients. As accurate splicing is required to produce the full-length HTT protein of ∼348 kDa, targetingHTTsplicing with small molecule drugs is a compelling approach to lower HTT protein levels to treat HD, and splice modulators are being tested in the clinic. Here, we identify PRMT5 as a novel regulator ofHTTmRNA splicing and alternative polyadenylation. PRMT5 inhibition disrupts the splicing ofHTTintrons 9 and 10, leading to activation of multiple proximal intronic polyadenylation sites within these introns and promoting premature termination, cleavage and polyadenylation (PCPA) of theHTTmRNA, thus lowering total HTT protein levels. We also detected increasing levels of these truncated, intron-containingHTTtranscripts across a series of neuronal differentiation samples which correlated with lower PRMT5 expression. Notably, PRMT5 inhibition in glioblastoma (GBM) stem cells potently induced neuronal differentiation. We posit that PRMT5-mediated regulation of intronic polyadenylation, premature termination and cleavage of theHTTmRNA modulates HTT expression and plays an important role during embryonic development and neuronal differentiation.