No abstract

Caveolae are invaginations of the plasma membrane involved in many cellular processes, including clathrinindependent endocytosis, cholesterol transport, and signal transduction. They are characterized by the presence of caveolin proteins. Mutations that cause deficiency in caveolin-3, which is expressed exclusively in skeletal and cardiac muscle, have been linked to muscular dystrophy. Polymerase I and transcript release factor (PTRF; also known as cavin) is a caveolar-associated protein suggested to play an essential role in the formation of caveolae and the stabilization of caveolins. Here, we identified PTRF mutations in 5 nonconsanguineous patients who presented with both generalized lipodystrophy and muscular dystrophy. Muscle hypertrophy, muscle mounding, mild metabolic complications, and elevated serum creatine kinase levels were observed in these patients. Skeletal muscle biopsies revealed chronic dystrophic changes, deficiency and mislocalization of all 3 caveolin family members, and reduction of caveolae structure. We generated expression constructs recapitulating the human mutations; upon overexpression in myoblasts, these mutations resulted in PTRF mislocalization and disrupted physical interaction with caveolins. Our data confirm that PTRF is essential for formation of caveolae and proper localization of caveolins in human cells and suggest that clinical features observed in the patients with PTRF mutations are associated with a secondary deficiency of caveolins.

Facioscapulohumeral dystrophy (FSHD) is associated with somatic chromatin relaxation of the D4Z4 repeat array and derepression of the D4Z4-encoded DUX4 retrogene coding for a germline transcription factor. Somatic DUX4 derepression is caused either by a 1-10 unit repeat-array contraction (FSHD1) or by mutations in SMCHD1, which encodes a chromatin repressor that binds to D4Z4 (FSHD2). Here, we show that heterozygous mutations in DNA methyltransferase 3B (DNMT3B) are a likely cause of D4Z4 derepression associated with low levels of DUX4 expression from the D4Z4 repeat and increased penetrance of FSHD. Recessive mutations in DNMT3B were previously shown to cause immunodeficiency, centromeric instability, and facial anomalies (ICF) syndrome. This study suggests that transcription of DUX4 in somatic cells is modified by variations in its epigenetic state and provides a basis for understanding the reduced penetrance of FSHD within families.

In patients with Duchenne muscular dystrophy (DMD), the absence of a functional dystrophin protein results in sarcolemmal instability, abnormal calcium signaling, cardiomyopathy, and skeletal muscle degeneration. Using the dystrophin-deficient sapje zebrafish model, we have identified microRNAs (miRNAs) that, in comparison to our previous findings in human DMD muscle biopsies, are uniquely dysregulated in dystrophic muscle across vertebrate species. MiR-199a-5p is dysregulated in dystrophindeficient zebrafish, mdx 5cv mice, and human muscle biopsies. MiR-199a-5p mature miRNA sequences are transcribed from stem loop precursor miRNAs that are found within the introns of the dynamin-2 and dynamin-3 loci. The miR-199a-2 stem loop precursor transcript that gives rise to the miR-199a-5p mature transcript was found to be elevated in human dystrophic muscle. The levels of expression of miR-199a-5p are regulated in a serum response factor (SRF)-dependent manner along with myocardin-related transcription factors. Inhibition of SRF-signaling reduces miR-199a-5p transcript levels during myogenic differentiation. Manipulation of miR-199a-5p expression in human primary myoblasts and myotubes resulted in dramatic changes in cellular size, proliferation, and differentiation. MiR-199a-5p targets several myogenic cell proliferation and differentiation regulatory factors within the WNT signaling pathway, including FZD4, JAG1, and WNT2. Overexpression of miR-199a-5p in the muscles of transgenic zebrafish resulted in abnormal myofiber disruption and sarcolemmal membrane detachment, pericardial edema, and lethality. Together, these studies identify miR-199a-5p as a potential regulator of myogenesis through suppression of WNT-signaling factors that act to balance myogenic cell proliferation and differentiation.

Congenital muscular dystrophy is a heterogeneous group of inherited muscle diseases characterized clinically by muscle weakness and hypotonia in early infancy. A number of genes harboring causative mutations have been identified, but several cases of congenital muscular dystrophy remain molecularly unresolved. We examined 15 individuals with a congenital muscular dystrophy characterized by early-onset muscle wasting, mental retardation, and peculiar enlarged mitochondria that are prevalent toward the periphery of the fibers but are sparse in the center on muscle biopsy, and we have identified homozygous or compound heterozygous mutations in the gene encoding choline kinase beta (CHKB). This is the first enzymatic step in a biosynthetic pathway for phosphatidylcholine, the most abundant phospholipid in eukaryotes. In muscle of three affected individuals with nonsense mutations, choline kinase activities were undetectable, and phosphatidylcholine levels were decreased. We identified the human disease caused by disruption of a phospholipid de novo biosynthetic pathway, demonstrating the pivotal role of phosphatidylcholine in muscle and brain.

Facioscapulohumeral muscular dystrophy (FSHD) is a common form of muscular dystrophy characterized by an asymmetric progressive weakness and wasting of the facial, shoulder and upper arm muscles, frequently accompanied by hearing loss and retinal vasculopathy. FSHD is an autosomal dominant disease linked to chromosome 4q35, but the causative gene remains controversial. DUX4 is a leading candidate gene as causative of FSHD. However, DUX4 expression is extremely low in FSHD muscle, and there is no DUX4 animal model that mirrors the pathology in human FSHD. Here, we show that the misexpression of very low levels of human DUX4 in zebrafish development recapitulates the phenotypes seen in human FSHD patients. Microinjection of small amounts of human full-length DUX4 (DUX4-fl) mRNA into fertilized zebrafish eggs caused asymmetric abnormalities such as less pigmentation of the eyes, altered morphology of ears, developmental abnormality of fin muscle, disorganization of facial musculature and/or degeneration of trunk muscle later in development. Moreover, DUX4-fl expression caused aberrant localization of myogenic cells marked with α-actin promoter-driven enhanced green fluorescent protein outside somite boundary, especially in head region. These abnormalities were rescued by coinjection of the short form of DUX4 (DUX4-s). Our results suggest that the misexpression of DUX4-fl, even at extremely low level, can recapitulate the phenotype observed in FSHD patients in a vertebrate model. These results strongly support the current hypothesis for a role of DUX4 in FSHD pathogenesis. We also propose that DUX4 expression during development is important for the pathogenesis of FSHD.

We ascertained a nuclear family in which three of four siblings were affected with an unclassified autosomal recessive myopathy characterized by severe weakness, respiratory impairment, scoliosis, joint contractures, and an unusual combination of dystrophic and myopathic features on muscle biopsy. Whole genome sequence from one affected subject was filtered using linkage data and variant databases. A single gene, MEGF10, contained nonsynonymous mutations that co-segregated with the phenotype. Affected subjects were compound heterozygous for missense mutations c.976T > C (p.C326R) and c.2320T > C (p.C774R). Screening the MEGF10 open reading frame in 190 patients with genetically unexplained myopathies revealed a heterozygous mutation, c.211C > T (p.R71W), in one additional subject with a similar clinical and histological presentation as the discovery family. All three mutations were absent from at least 645 genotyped unaffected control subjects. MEGF10 contains 17 atypical epidermal growth factor-like domains, each of which contains eight cysteine residues that likely form disulfide bonds. Both the p.C326R and p.C774R mutations alter one of these residues, which are completely conserved in vertebrates. Previous work showed that murine Megf10 is required for preserving the undifferentiated, proliferative potential of satellite cells, myogenic precursors that regenerate skeletal muscle in response to injury or disease. Here, knockdown of megf10 in zebrafish by four different morpholinos resulted in abnormal phenotypes including unhatched eggs, curved tails, impaired motility, and disorganized muscle tissue, corroborating the pathogenicity of the human mutations. Our data establish the importance of MEGF10 in human skeletal muscle and suggest satellite cell dysfunction as a novel myopathic mechanism.Electronic supplementary materialThe online version of this article (doi:10.1007/s10048-012-0315-z) contains supplementary material, which is available to authorized users.

We developed a portable system for 16S rDNA analyses consisting of a nanopore technology-based sequencer, the MinION, and laptop computers, and assessed its potential ability to determine bacterial compositions rapidly. We tested our protocols using a mock bacterial community that contained equimolar 16S rDNA and a pleural effusion from a patient with empyema, for time effectiveness and accuracy. MinION sequencing targeting 16S rDNA detected all 20 of the bacterial species present in the mock bacterial community. Time course analysis indicated that the sequence data obtained during the first 5 minutes of sequencing (1,379 bacterial reads) were enough to detect all 20 bacteria in the mock sample and to determine species composition, consistent with results of those obtained from 4 hours of sequencing (24,202 reads). Additionally, using a clinical sample extracted from the empyema patient’s pleural effusion, we could identify major bacterial pathogens in that effusion using our rapid sequencing and analysis protocol. All results are comparable to conventional 16S rDNA sequencing results using an IonPGM sequencer. Our results suggest that rapid sequencing and bacterial composition determination are possible within 2 hours after obtaining a DNA sample.