DNAJB6 isoform specific knockdown: mechanistic insights and therapeutic potential for LGMD-D1

Abstract

Dominant missense mutations in DNAJB6 gene, encoding a ubiquitously transcribed HSP40 co-chaperone cause limb girdle muscular dystrophy (LGMD) D1, an adult onset myopathy characterized by vacuolar and myofibrillar pathology. No treatments are currently available. DNAJB6 has two isoforms, DNAJB6a which localizes to myonuclei, and DNAJB6b, which localizes to both myonuclei and myofibrillar structures in muscle. Absence of both isoforms is embryonic lethal in mouse models. Disease causing mutations reside within regions shared by both isoforms, yet several lines of evidence suggest DNAJB6b may be solely responsible for disease pathogenesis. Clarifying each isoforms’ pathomechanistic role is critical to therapeutic strategy development. We therefore created an approach using antisense oligonucleotides to selectively reduce levels of each DNAJB6 isoform in either WT or knock-in LGMD-D1 primary mouse myotubes, followed by tandem mass-spectrometry, to characterize the proteomic signature of each condition. We found selective reduction of DNAJB6b but not DNAJB6a created a similar proteomic profile to LGMD-D1 myotubes. Additionally, reducing levels of DNAJB6b, but not DNAJB6a in LGMD-D1 myotubes normalized approximately 50% of the disease proteomic profile to WT levels. These findings support DNAJB6b's involvement in LGMD-D1 pathogenesis, and suggest selective reduction of the DNAJB6b isoform may be a viable therapeutic strategy for LGMD-D1.