Limb-girdle muscular dystrophy type 1D (LGMD1D) was linked to 7q36 over a decade ago1, but its genetic cause has remained elusive. We have studied nine LGMD families from Finland, the U.S., and Italy, and identified four dominant missense mutations leading to p.Phe93Leu or p.Phe89Ile changes in the ubiquitously expressed co-chaperone DNAJB6. Functional testing in vivo showed that the mutations have a dominant toxic effect mediated specifically by the cytoplasmic isoform of DNAJB6. In vitro studies demonstrated that the mutations increase the half-life of DNAJB6, extending this effect to the wild-type protein, and reduce its protective anti-aggregation effect. Further, we show that DNAJB6 interacts with members of the CASA complex, including the myofibrillar-myopathy-causing protein BAG3. Our data provide the genetic cause of LGMD1D, suggest that the pathogenesis is mediated by defective chaperone function, and highlight how mutations expressed ubiquitously can exert their effect in a tissue-, cellular compartment-, and isoform-specific manner.

WDM is caused by mutated TIA1 through a dominant pathomechanism probably involving altered stress granule dynamics.

Tibial muscular dystrophy (TMD) is the first described human titinopathy. It is a mild adult-onset slowly progressive myopathy causing weakness and atrophy in the anterior lower leg muscles. TMD is caused by mutations in the last two exons, Mex5 and Mex6, of the titin gene (TTN). The first reported TMD mutations were dominant, but the Finnish founder mutation FINmaj, an 11-bp insertion/deletion in Mex6, in homozygosity caused a completely different severe early-onset limb-girdle muscular dystrophy 2J (LGMD2J). Later, we reported that not all TMD mutations cause LGMD when homozygous or compound heterozygous with truncating mutation, but some of them rather cause a more severe TMD-like distal disease. We have now performed targeted next-generation sequencing of myopathy-related genes on seven families from Albania, Bosnia, Iran, Tunisia, Belgium, and Spain with juvenile or early adult onset recessive distal myopathy. Novel mutations in TTN Mex5, Mex6 and A-band exon 340 were identified in homozygosity or compound heterozygosity with a frameshift or nonsense mutation in TTN I- or A-band region. Family members having only one of these TTN mutations were healthy. Our results add yet another entity to the list of distal myopathies: juvenile or early adult onset recessive distal titinopathy.

BACKGROUND: Extensive genetic screening results in the identification of thousands of rare variants that are difficult to interpret. Because of its sheer size, rare variants in the titin gene (TTN) are detected frequently in any individual. Unambiguous interpretation of molecular findings is almost impossible in many patients with myopathies or cardiomyopathies. OBJECTIVE: To refine the current classification framework for TTN-associated skeletal muscle disorders and standardize the interpretation of TTN variants. METHODS: We used the guidelines issued by the American College of Medical Genetics and Genomics (ACMG) and the Association for Molecular Pathology (AMP) to re-analyze TTN genetic findings from our patient cohort. RESULTS: We identified in the classification guidelines three rules that are not applicable to titin-related skeletal muscle disorders; six rules that require disease-/gene-specific adjustments and four rules requiring quantitative thresholds for a proper use. In three cases, the rule strength need to be modified. CONCLUSIONS:We suggest adjustments are made to the guidelines. We provide frequency thresholds to facilitate filtering of candidate causative variants and guidance for the use and interpretation of functional data and co-segregation evidence. We expect that the variant classification framework for TTN-related skeletal muscle disorders will be further improved along with a better understanding of these diseases.

ObjectiveTo identify the genetic defect causing a distal calf myopathy with cores.MethodsFamilies with a genetically undetermined calf-predominant myopathy underwent detailed clinical evaluation, including EMG/nerve conduction studies, muscle biopsy, laboratory investigations, and muscle MRI. Next-generation sequencing and targeted Sanger sequencing were used to identify the causative genetic defect in each family.ResultsA novel deletion-insertion mutation in ryanodine receptor 1 (RYR1) was found in the proband of the index family and segregated with the disease in 6 affected relatives. Subsequently, we found 2 more families with a similar calf-predominant myopathy segregating with unique RYR1-mutated alleles. All patients showed a very slowly progressive myopathy without episodes of malignant hyperthermia or rhabdomyolysis. Muscle biopsy showed cores or core-like changes in all families.ConclusionsOur findings expand the spectrum of RYR1-related disorders to include a calf-predominant myopathy with core pathology and autosomal dominant inheritance. Two families had unique and previously unreported RYR1 mutations, while affected persons in the third family carried 2 previously known mutations in the same dominant allele.