MYOTONIC DYSTROPHY: EP.233 The FORCETM platform achieves durable knockdown of toxic human nuclear DMPK RNA and correction of splicing in the hTFR1/DMSXL mouse model

Abstract

Myotonic dystrophy type 1 (DM1) is a debilitating neuromuscular disease caused by expansion of CUG repeats in the 3’-untranslated region of the DMPK mRNA. The disease-causing DMPK is confined to the nucleus, where it forms hairpin-loop structures that sequester muscle blind-like (MBNL) splicing factors. These events inhibit MBNL function and result in splicing defects that drive DM1 progression in muscles and ultimately cause early death. Antisense oligonucleotides (ASO) designed to knockdown DMPK and administered as naked oligonucleotides have shown promise as DM1 treatments in pre-clinical models; however, translation to the clinic has failed because of limited delivery to muscle. We have developed the FORCE platform which consists of three components: a Fab targeting TfR1 to enable muscle targeted delivery, a clinically-validated Val-Cit linker, and a therapeutic payload rationally selected to target the genetic basis of disease, in this case an ASO targeting human DMPK. To evaluate the ability of Dyne's lead candidate to knock down toxic human DMPK mRNA in vivo, we generated an innovative hTfR1/DMSXL mouse model that expresses human TfR1 and a human DMPK with more than 1,000 CUG repeats, which represents a severe DM1 phenotype. Administration of the FORCE DM1 lead candidate to hTfR1/DMSXL mice significantly knocked down the human toxic nuclear DMPK mRNA in the gastrocnemius, tibialis anterior, diaphragm, and heart. The effect was durable, as it lasted for several weeks. Subcellular fractionation followed by analysis of gene expression in nuclear fractions provided direct evidence that the FORCE DM1 lead candidate delivered the ASO to the nucleus and reduced accumulation of toxic human DMPK mRNA trapped therein. In conclusion, in vivo studies demonstrated that the FORCE DM1 lead candidate targets toxic DMPK in the nucleus and is well-tolerated with a long-lasting effect.