Small Molecule Screening Identifies HSP90 as a Modifier of RNA Foci in Myotonic Dystrophy Type 1

Myotonic dystrophy type 1 (DM1) is a multisystemic disorder caused by a CTG triplet repeat expansion within the 3' untranslated region of the <i>DMPK</i> gene. Expression of the expanded allele generates RNA containing long tracts of CUG repeats (CUGexp RNA) that form hairpin structures and accumulate in nuclear RNA foci; however, the factors that control <i>DMPK</i> expression and the formation of CUGexp RNA foci remain largely unknown. We performed an unbiased small molecule screen in an immortalized human DM1 skeletal muscle myoblast cell line and identified HSP90 as a modifier of endogenous RNA foci. Small molecule inhibition of HSP90 leads to enhancement of RNA foci and upregulation of <i>DMPK</i> mRNA levels. Knockdown and overexpression of HSP90 in undifferentiated DM1 myoblasts validated the impact of HSP90 with upregulation and downregulation of <i>DMPK</i> mRNA, respectively. Furthermore, we identified p-STAT3 as a downstream mediator of HSP90 impacting levels of <i>DMPK</i> mRNA and RNA foci. Interestingly, differentiated cells exhibited an opposite effect of HSP90 inhibition displaying downregulation of <i>DMPK</i> mRNA through a mechanism independent of p-STAT3 involvement. This study has revealed a novel mediator for <i>DMPK</i> mRNA and foci regulation in DM1 cells with the potential to identify targets for future therapeutic intervention.