Targeted transcript analysis in muscles from patients with genetically diverse congenital myopathies.

Congenital myopathies are a group of early onset muscle diseases of variable severity often with characteristic muscle biopsy findings and involvement of specific muscle types. The clinical diagnosis of patients typically relies on histopathological findings and is confirmed by genetic analysis. The most commonly mutated genes encode proteins involved in skeletal muscle excitation-contraction coupling, calcium regulation, sarcomeric proteins and thin-thick filament interaction. However, mutations in genes encoding proteins involved in other physiological functions (for example mutations in SELENON and MTM1, which encode for ubiquitously expressed proteins of low tissue specificity) have also been identified. This intriguing observation indicates that the presence of a genetic mutation impacts the expression of other genes whose product is important for skeletal muscle function. The aim of the present investigation was to verify if there are common changes in transcript and microRNA expression in muscles from patients with genetically heterogeneous congenital myopathies, focusing on genes encoding proteins involved in excitation-contraction coupling and calcium homeostasis, sarcomeric proteins, transcription factors and epigenetic enzymes. Our results identify RYR1, ATPB2B and miRNA-22 as common transcripts whose expression is decreased in muscles from congenital myopathy patients. The resulting protein deficiency may contribute to the muscle weakness observed in these patients. This study also provides information regarding potential biomarkers for monitoring disease progression and response to pharmacological treatments in patients with congenital myopathies.

STAC3 variants cause a congenital myopathy with distinctive dysmorphic features and malignant hyperthermia susceptibility.

SH3 and cysteine-rich domain-containing protein 3 (STAC3) is an essential component of the skeletal muscle excitation-contraction coupling (ECC) machinery, though its role and function are not yet completely understood. Here, we report 18 patients carrying a homozygous p.(Trp284Ser) STAC3 variant in addition to a patient compound heterozygous for the p.(Trp284Ser) and a novel splice site change (c.997-1G > T). Clinical severity ranged from prenatal onset with severe features at birth, to a milder and slowly progressive congenital myopathy phenotype. A malignant hyperthermia (MH)-like reaction had occurred in several patients. The functional analysis demonstrated impaired ECC. In particular, KCl-induced membrane depolarization resulted in significantly reduced sarcoplasmic reticulum Ca2+ release. Co-immunoprecipitation of STAC3 with CaV 1.1 in patients and control muscle samples showed that the protein interaction between STAC3 and CaV 1.1 was not significantly affected by the STAC3 variants. This study demonstrates that STAC3 gene analysis should be included in the diagnostic work up of patients of any ethnicity presenting with congenital myopathy, in particular if a history of MH-like episodes is reported. While the precise pathomechanism remains to be elucidated, our functional characterization of STAC3 variants revealed that defective ECC is not a result of CaV 1.1 sarcolemma mislocalization or impaired STAC3-CaV 1.1 interaction.

Cellular, biochemical and molecular changes in muscles from patients with X-linked myotubular myopathy due to MTM1 mutations.

Centronuclear myopathies are early-onset muscle diseases caused by mutations in several genes including MTM1, DNM2, BIN1, RYR1 and TTN. The most severe and often fatal X-linked form of myotubular myopathy (XLMTM) is caused by mutations in the gene encoding the ubiquitous lipid phosphatase myotubularin, an enzyme specifically dephosphorylating phosphatidylinositol-3-phosphate and phosphatidylinositol-3,5-bisphosphate. Because XLMTM patients have a predominantly muscle-specific phenotype a number of pathogenic mechanisms have been proposed, including a direct effect of the accumulated lipid on the skeletal muscle calcium channel ryanodine receptor 1, a negative effect on the structure of intracellular organelles and defective autophagy. Animal models knocked out for MTM1 show severe reduction of ryanodine receptor 1 mediated calcium release but, since knocking out genes in animal models does not necessarily replicate the human phenotype, we considered it important to study directly the effect of MTM1 mutations on patient muscle cells. The results of the present study show that at the level of myotubes MTM1 mutations do not dramatically affect calcium homeostasis and calcium release mediated through the ryanodine receptor 1, though they do affect myotube size and nuclear content. On the other hand, mature muscles such as those obtained from patient muscle biopsies exhibit a significant decrease in expression of the ryanodine receptor 1, a decrease in muscle-specific microRNAs and a considerable up-regulation of histone deacetylase-4. We hypothesize that the latter events consequent to the primary genetic mutation, are the cause of the severe decrease in muscle strength that characterizes these patients.