hnRNP L is essential for myogenic differentiation and modulates myotonic dystrophy pathologies

Introduction/aims: RNA binding proteins (RBPs) play an important role in skeletal muscle development and disease by regulating RNA splicing. In myotonic dystrophy type 1 (DM1), the RBP MBNL1 (Muscleblind-like) is sequestered by toxic CUG repeats, leading to mis-splicing of MBNL1 targets. Mounting evidence from the literature has implicated other factors in the pathogenesis of DM1. Here we sought to evaluate the functional role of hnRNP L in normal and DM1 muscle cells. We sought to test if modulation of hnRNP L expression affected DM1 splicing targets and myogenic outcomes. Methods: Co-immunoprecipitation assays using hnRNPL and MBNL1 expression constructs and expression profiling in normal and DM1 muscle cell lines were performed. Zebrafish morpholinos targeting hnrnpl and hnrnpl2 were injected into one-cell zebrafish for developmental and muscle analysis. Ascochlorin administration to DM1 myoblasts was performed and expression of the CUG repeats, DM1 splicing biomarkers, and hnRNP L expression levels were evaluated. Results: Using DM1 patient myoblast cell lines we observed the formation of abnormal hnRNP L nuclear foci within and outside the expanded CUG repeats, further suggesting a role for this factor in DM1 pathology. We showed that the antiviral and antitumorigenic isoprenoid compound ascochlorin increased MBNL1 and hnRNP L expression levels. Drug treatment of DM1 muscle cells with ascochlorin partially rescued mis-splicing of established early biomarkers of DM1 and improved the defective myotube formation displayed by DM1 muscle cells. Discussion: Together, these studies reveal that hnRNP L modulated DM1 pathologies, and is a potential therapeutic target.