A novel recessive mutation affecting DNAJB6a causes myofibrillar myopathy

Mitochondrial abnormalities contribute to muscle weakness in a Dnajb6 deficient zebrafish model

Mutations in DNAJB6 are a well-established cause of limb girdle muscular dystrophy type D1 (LGMD D1). Patients with LGMD D1 develop progressive muscle weakness with histology showing fibre damage, autophagic vacuoles, and aggregates. Whilst there are many reports of LGMD D1 patients, the role of DNAJB6 in the muscle is still unclear. In this study, we developed a loss of function zebrafish model in order to investigate the role of Dnajb6. Using a double dnajb6a and dnajb6b mutant model, we show that loss of Dnajb6 leads to a late onset muscle weakness. Interestingly, we find that adult fish lacking Dnajb6 do not have autophagy or myofibril defects, however, they do show mitochondrial changes and damage. This study demonstrates that loss of Dnajb6 causes mitochondrial defects and suggests that this contributes to muscle weakness in LGMD D1. These findings expand our knowledge of the role of Dnajb6 in the muscle and provides a model to screen novel therapies for LGMD D1.

DNAJB6 isoform specific knockdown: Therapeutic potential for limb girdle muscular dystrophy D1

Dominant missense mutations in DNAJB6, a co-chaperone of HSP70, 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 that DNAJB6b is primarily responsible for disease pathogenesis. 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 LGMDD1 myoblasts, as well as in vivo in mouse skeletal muscle. To assess isoform specific knockdown in LGMDD1, we created primary myotube cultures from a knockin 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 toward wild type levels. Additional in vivo functional data is required to determine if selective reduction of DNAJB6b is a viable therapeutic target for LGMDD1.

Frameshift mutation in SQSTM1 causes proximal myopathy with rimmed vacuoles: A case report

p62/Sequestosome-1 (SQSTM1) is a stress-inducible scaffold protein involved in multiple cellular processes, including apoptosis, inflammation, cell survival, and selective autophagy. SQSTM1 mutations are associated with a spectrum of multisystem proteinopathy, including Paget disease of the bone, amyotrophic lateral sclerosis, frontotemporal dementia, and distal myopathy with rimmed vacuoles (MRV). Herein, we report a new phenotype of SQSTM1-associated proteinopathy, a novel frameshift mutation in SQSTM1 causing proximal MRV. A 44-year-old Chinese patient presented with progressive limb–girdle weakness. She had asymmetric proximal limb weakness and myopathic features on electromyography. The magnetic resonance images showed fatty infiltration into muscles, predominantly in the thighs and medial gastrocnemius, sparing the tibialis anterior. Muscle histopathology revealed abnormal protein deposition, p62/SQSTM1-positive inclusions, and rimmed vacuoles. Next-generation sequencing showed a novel pathogenic SQSTM1 frameshift mutation, c.542_549delACAGCCGC (p. H181Lfs*66). We expanded the pathogenic genotype of SQSTM1 to include a new, related phenotype: proximal MRV. We suggest that SQSTM1 variations should be screened in cases of proximal MRV.

Case Report: A Novel Splice-Site Mutation in DNAJB6 Associated With Juvenile-Onset Proximal–Distal Myopathy in a Chinese Patient

DNAJB6 was identified as the causative gene of limb-girdle muscular dystrophy type 1D. In recent years, the phenotypic and molecular spectrum of DNAJB6-myopathy has been expanded, and several mutations of DNAJB6 have been identified in Europe, North America, and Asia. Interestingly, almost all identified mutations in previous reports were point mutations, and most of them were clustered in exon 5, which encodes the G/F domain of DNAJB6. The so-far unique splice site mutation eliminating the entire G/F domain was reported to cause a severe, early-onset phenotype. Here, we report a juvenile-onset Chinese patient who presented with proximal–distal myopathy as well as esotropia and facial weakness. Muscle pathology showed rimmed vacuolation and myofibrillar disarrangement. A novel splice-site mutation NM_058246:c.236-1_240delGGTGGA of the DNAJB6 gene was identified by targeted exome sequencing, which results in a severe defect of the G/F domain. This rare mutation type expands the molecular spectrum of DNAJB6-myopathy and further underlines the importance of the G/F region.

A female carrier of spinal and bulbar muscular atrophy diagnosed with DNAJB6-related distal myopathy

Mutations in the DNAJB6 gene cause limb girdle muscular dystrophy D1 (LGMD D1) and distal myopathy with rimmed vacuoles. With the discovery of new mutations, the phenotypic spectrum of DNAJB6-related myopathy has been extended, making the diagnosis more complicated. In this study, we describe a female carrier of spinal and bulbar muscular atrophy (SBMA) diagnosed with DNAJB6-related distal myopathy. The c.292_294delGAT (p. Asp98del) mutation in the DNAJB6 gene and a 49 CAG repeat expansion in the androgen receptor (AR) gene were identified. According to the clinical manifestations of distal-dominant lower limb involvement, a myogenic pattern in the electrophysiological study, and rimmed vacuoles on muscle pathology, the patient was ultimately diagnosed with DNAJB6-related distal myopathy. A functional study in a zebrafish model indicated that the c.292_294delGAT (p. Asp98del) mutation contributed to muscle structure defects. This study offers useful insights for the differential diagnosis of a condition in which patients carry pathogenic variants in different genes.