Congenital myopathies and related disorders

New Compound Heterozygous Splice Site Mutations of the Skeletal Muscle Ryanodine Receptor (RYR1) Gene Manifest Fetal Akinesia: A Linkage with Congenital Myopathies

Mutations in the skeletal muscle ryanodine receptor (RYR1) gene have been linked to malignant hyperthermia susceptibility, central core disease, and minicore myopathy with external ophthalmoplegia. RYR1 is an intracellular calcium release channel and plays a crucial role in the sarcoplasmic reticulum and transverse tubule connection. Here, we report 2 fetuses from the same parents with compound heterozygous mutations in the RYR1 gene (c.10347+1G>A and c.10456-2Α>G) who presented with fetal akinesia and polyhydramnios at 27 and 19 weeks of gestation with intrauterine growth restriction in the third pregnancy. The prospective parents of the fetuses were heterozygous carriers for c.10456-2Α>G (mother) and c.10347+1G>A (father). Both mutations affect splice sites resulting in dysfunctional protein forms probably missing crucial domains of the C-terminus. Our findings reveal a new RYR1 splice site mutation (c.10456-2Α>G) that may be associated with the clinical features of myopathies, expanding the RYR1 spectrum related to these pathologies.

Validity of a Neurological Scoring System for Canine X-Linked Myotubular Myopathy

A simple clinical neurological test was developed to evaluate response to gene therapy in a preclinical canine model of X-linked myotubular myopathy (XLMTM). This devastating congenital myopathy is caused by mutation in the myotubularin (MTM1) gene. Clinical signs include muscle weakness, early respiratory failure, and ventilator dependence. A spontaneously occurring canine model has a similar clinical picture and histological abnormalities on muscle biopsy compared with patients. We developed a neuromuscular assessment score, graded on a scale from 10 (normal) to 1 (unable to maintain sternal recumbency). We hypothesize that this neurological assessment score correlates with genotype and established measures of disease severity and is reliable when performed by an independent observer. At 17 weeks of age, there was strong correlation between neurological assessment scores and established methods of severity testing. The neurological severity score correctly differentiated between XLMTM and wild-type dogs with good interobserver reliability, on the basis of strong agreement between neurological scores assigned by independent observers. Together, these data indicate that the neurological scoring system developed for this canine congenital neuromuscular disorder is reliable and valid. This scoring system may be helpful in evaluating response to therapy in preclinical testing in this disease model, such as response to gene therapy.

Multiminicore disease with respiratory failure

Multiminicore disease is a rare form of slowly progressive or nonprogressive myopathy, characterized by multiple cores within the muscle fibers. Respiratory failure in multiminicore disease rarely occurs. We describe a 5-year-old girl with multiminicore disease and early-onset respiratory failure after an episode of bronchopneumonia. The child received mechanical ventilation for 280 days and was discharged on home ventilation. The relevant literature is reviewed.

Clinical and histologic changes in the follow-up of a congenital myopathy

A 19-year-old woman was born with congenital hypotonia, generalized weakness, and dysmorphic features. A muscle biopsy performed at age 18 months found that type I fibers were smaller and more numerous than type II fibers, and she was diagnosed with congenital fiber type disproportion. She grew up with moderate motor impairment, but after a stationary period her weakness progressed gradually and she developed a severe ophthalmoplegia. When she was 18 years old a second muscle biopsy still indicated the predominance of type I fibers but also the presence of central nuclei and strong oxidative enzyme activity in the center of most of the fibers; this was compatible with centronuclear myopathy. The diagnostic reconsideration raises questions about the pathogenesis of these diseases and the recognition of congenital fiber type disproportion as a distinct nosologic entity.

Intracellular Ca2+ dynamics in malignant hyperthermia and central core disease: established concepts, new cellular mechanisms involved

Malignant hyperthermia (MH) and central core disease (CCD) are inherited human disorders of skeletal muscle Ca2+ homeostasis. Both MH and CCD are linked to mutations and/or deletions in the gene encoding the skeletal muscle ryanodine receptor (RyR1), the intracellular Ca2+ release channel, which is essential to excitation-contraction (EC) coupling. Our knowledge on how mutations in RyR1 disrupt intracellular Ca2+ homeostasis and EC coupling, eventually leading to MH and CCD has been recently improved, thanks to multidisciplinary studies ranging from clinical, single channel recordings, patch-clamp experiments, and molecular biology. This review presents a brief historical perspective, on how pioneer studies resulted in associating MH and CCD to RyR1. The review is also focused on discussing novel results in regard to pathophysiological consequences of specific MH/CCD RyR1 mutant proteins, which are representative of the different cellular mechanisms that are linked to either phenotype.

General anesthesia in an infant with X-linked myotubular myopathy

We present a 20-week-old infant with the X-linked form of myotubular myopathy who required anesthesia for a Nissen fundoplication procedure where the response to nondepolarizing neuromuscular blockade was evaluated.

Distinct effects on Ca2+ handling caused by malignant hyperthermia and central core disease mutations in RyR1

Malignant hyperthermia (MH) and central core disease (CCD) are disorders of skeletal muscle Ca2+ homeostasis that are linked to mutations in the type 1 ryanodine receptor (RyR1). Certain RyR1 mutations result in an MH-selective phenotype (MH-only), whereas others result in a mixed phenotype (MH + CCD). We characterized effects on Ca2+ handling and excitation-contraction (EC) coupling of MH-only and MH + CCD mutations in RyR1 after expression in skeletal myotubes derived from RyR1-null (dyspedic) mice. Compared to wild-type RyR1-expressing myotubes, MH + CCD- and MH-only-expressing myotubes exhibited voltage-gated Ca2+ release (VGCR) that activated at more negative potentials and displayed a significantly higher incidence of spontaneous Ca2+ oscillations. However, maximal VGCR was reduced only for MH + CCD mutants (Y4795C, R2435L, and R2163H) in which spontaneous Ca2+ oscillations occurred with significantly longer duration (Y4795C and R2435L) or higher frequency (R2163H). Notably, myotubes expressing these MH + CCD mutations in RyR1 exhibited both increased [Ca2+]i and reduced sarcoplasmic reticulum (SR) Ca2+ content. We conclude that MH-only mutations modestly increase basal release-channel activity in a manner insufficient to alter net SR Ca2+ content ("compensated leak"), whereas the mixed MH + CCD phenotype arises from mutations that enhance basal activity to a level sufficient to promote SR Ca2+ depletion, elevate [Ca2+]i, and reduce maximal VGCR ("decompensated leak").

Congenital myopathies

The congenital myopathies encompass a group of neuromuscular disorders with characteristic morphologic abnormalities in skeletal muscle, including nemaline myopathy, central core disease, multi-minicore disease, and myotubular myopathy. Giant steps have been made in our understanding of the molecular bases of these disorders, all of which show remarkable genetic heterogeneity. This review of congenital myopathies examines progress in defining clinical diagnostic criteria and novel genetic advances that have provided important clues regarding their pathogeneses.