DNAJB6 Isoform Specific Knockdown: Therapeutic Potential for LGMDD1

Dominant missense mutations in DNAJB6, an HSP40 co-chaperone, cause limb girdle muscular dystrophy (LGMD) D1. No treatments are currently available. Two isoforms exist, DNAJB6a and DNAJB6b, each with distinct localizations in muscle. Mutations reside in both isoforms, yet evidence suggests only DNAJB6b is responsible for disease pathogenesis. Mechanistic data supports either a toxic gain of function, a dominant negative mechanism, or a combination of both. Knockdown treatment strategies involving both isoforms carry risk as DNAJB6 knockout is embryonic lethal. We therefore developed an isoform specific knockdown approach using morpholinos. Selective reduction of each isoform was achieved in-vitro in primary mouse myotubes and human myoblasts, as well as in-vivo in mouse skeletal muscle. To assess isoform specific knockdown in LGMDD1, we created primary myotube cultures from a knock-in LGMDD1 mouse model. Using mass spectrometry, we identified an LGMDD1 protein signature related to protein homeostasis and myofibril structure. Selective reduction of DNAJB6b levels in LGMDD1 myotubes corrected much of the proteomic disease signature towards wild type levels. While additional in-vivo functional data is required, these findings suggest selective reduction of DNAJB6b may be a viable therapeutic target for LGMDD1.