MTM1 Mutations Research Articles

Expression of the neuropathy-associated MTMR2 gene rescues MTM1-associated myopathy.

Myotubularins (MTMs) are active or dead phosphoinositides phosphatases defining a large protein family conserved through evolution and implicated in different neuromuscular diseases. Loss-of-function mutations in MTM1 cause the severe congenital myopathy called myotubular myopathy (or X-linked centronuclear myopathy) while mutations in the MTM1-related protein MTMR2 cause a recessive Charcot-Marie-Tooth peripheral neuropathy. Here we aimed to determine the functional specificity and redundancy of MTM1 and MTMR2, and to assess their abilities to compensate for a potential therapeutic strategy. Using molecular investigations and heterologous expression of human MTMs in yeast cells and in Mtm1 knockout mice, we characterized several naturally occurring MTMR2 isoforms with different activities. We identified the N-terminal domain as responsible for functional differences between MTM1 and MTMR2. An N-terminal extension observed in MTMR2 is absent in MTM1, and only the short MTMR2 isoform lacking this N-terminal extension behaved similarly to MTM1 in yeast and mice. Moreover, adeno-associated virus-mediated exogenous expression of several MTMR2 isoforms ameliorates the myopathic phenotype owing to MTM1 loss, with increased muscle force, reduced myofiber atrophy, and reduction of the intracellular disorganization hallmarks associated with myotubular myopathy. Noteworthy, the short MTMR2 isoform provided a better rescue when compared with the long MTMR2 isoform. In conclusion, these results point to the molecular basis for MTMs functional specificity. They also provide the proof-of-concept that expression of the neuropathy-associated MTMR2 gene improves the MTM1-associated myopathy, thus identifying MTMR2 as a novel therapeutic target for myotubular myopathy.

Affected female carriers of MTM1 mutations display a wide spectrum of clinical and pathological involvement: delineating diagnostic clues.

X-linked myotubular myopathy (XLMTM), a severe congenital myopathy, is caused by mutations in the MTM1 gene located on the X chromosome. A majority of affected males die in the early postnatal period, whereas female carriers are believed to be usually asymptomatic. Nevertheless, several affected females have been reported. To assess the phenotypic and pathological spectra of carrier females and to delineate diagnostic clues, we characterized 17 new unrelated affected females and performed a detailed comparison with previously reported cases at the clinical, muscle imaging, histological, ultrastructural and molecular levels. Taken together, the analysis of this large cohort of 43 cases highlights a wide spectrum of clinical severity ranging from severe neonatal and generalized weakness, similar to XLMTM male, to milder adult forms. Several females show a decline in respiratory function. Asymmetric weakness is a noteworthy frequent specific feature potentially correlated to an increased prevalence of highly skewed X inactivation. Asymmetry of growth was also noted. Other diagnostic clues include facial weakness, ptosis and ophthalmoplegia, skeletal and joint abnormalities, and histopathological signs that are hallmarks of centronuclear myopathy such as centralized nuclei and necklace fibers. The histopathological findings also demonstrate a general disorganization of muscle structure in addition to these specific hallmarks. Thus, MTM1 mutations in carrier females define a specific myopathy, which may be independent of the presence of an XLMTM male in the family. As several of the reported affected females carry large heterozygous MTM1 deletions not detectable by Sanger sequencing, and as milder phenotypes present as adult-onset limb-girdle myopathy, the prevalence of this myopathy is likely to be greatly underestimated. This report should aid diagnosis and thus the clinical management and genetic counseling of MTM1 carrier females. Furthermore, the clinical and pathological history of this cohort may be useful for therapeutic projects in males with XLMTM, as it illustrates the spectrum of possible evolution of the disease in patients surviving long term.

Insights from genotype–phenotype correlations by novel SPEG mutations causing centronuclear myopathy

Centronuclear myopathies (CNM) are a clinically and genetically heterogeneous group of congenital myopathies, defined histologically by increased number of fibres with centrally located nuclei, and type I fibre predominance in muscle biopsy. Myotubular myopathy, the X-linked form of CNM caused by mutations in the phosphoinositide phosphatase MTM1, is histologically characteristic since muscle fibres resemble myotubes.Here we present two unrelated patients with CNM and typical myotubular fibres in the muscle biopsy caused by mutations in striated muscle preferentially expressed protein kinase (SPEG). Next generation sequencing revealed novel biallelic homozygous mutations in SPEG in both cases. Patient 1 showed the c.1627_1628insA (p.Thr544Aspfs*48) mutation and patient 2 the c.9586C>T (p.Arg3196*) mutation. The clinical phenotype was distinctive in the two patients since patient 2 developed a dilated cardiomyopathy with milder myopathy features, while patient 1 showed only myopathic features without cardiac involvement. These findings expand the genotype–phenotype correlations after the initial report. Additionally, we describe whole body muscle MRI of patient 2 and we argue on the different SPEG isoforms in skeletal muscle and heart as the possible explanation leading to variable phenotypes of SPEG mutations.

Phenotypes, genotypes, and prevalence of congenital myopathies older than 5 years in Denmark.

Objective:Congenital myopathy as a nosologic entity has long been recognized, but knowledge of overall and subtype prevalence and phenotype-genotype relationship is scarce, especially in the adult population.Methods:A national cohort of 107 patients ≥5 years diagnosed with congenital myopathy were prospectively assessed clinically, histologically, and genetically.Results:Twenty-five patients were excluded because of atypical features or alternative etiologies. The remaining 82 were on average 28 years old. Histologic examination revealed 14 (17%) with core disease, 15 (18%) centronuclear myopathy, 12 (15%) nemaline rods, 27 (33%) congenital fiber-type disproportion or type I predominance, and 14 (17%) nonspecific myopathic changes. Genetic etiology was identified in 46 patients (56.1%); 22.0% were heterozygous or compound heterozygous for mutations in RYR1, 7.3% had DNM2 mutations, and 7.3% NEB mutations. Less than 5% had mutations in ACTA1, TPM2/3, MTM1, TTN, SEPN1, or SC4NA. A genetic cause was established in 83% with specific histology (cores/rods/centronuclear myopathy) vs 29% with unspecific histology. The detailed clinical examination found gene-dependent discrepancies in the pattern of muscle affection and walking ability. Although walking ability was delayed in patients with ACTA1, TPM2/3, and RYR1 mutations, it was within normal limits in patients with NEB and DNM2 mutations.Conclusions:We found that overall, genetic and histologic prevalence of congenital myopathy in Denmark differs from previous retrospective reports. Less RYR1 and more DNM2 and NEB mutations and less core histology were present in our cohort. These differences may be explained by our prospective design, the older cohort of patients, and by differences in genetic background.

Cellular, Biochemical and Molecular Changes in Muscles from Patients with X-Linked Myotubular Myopathy due to MTM1 Mutations

Cellular, Biochemical and Molecular Changes in Muscles from Patients with X-Linked Myotubular Myopathy due to MTM1 Mutations

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.

PIK3C2B inhibition improves function and prolongs survival in myotubular myopathy animal models.

Myotubular myopathy (MTM) is a devastating pediatric neuromuscular disorder of phosphoinositide (PIP) metabolism resulting from mutations of the PIP phosphatase MTM1 for which there are no treatments. We have previously shown phosphatidylinositol-3-phosphate (PI3P) accumulation in animal models of MTM. Here, we tested the hypothesis that lowering PI3P levels may prevent or reverse the MTM disease process. To test this, we targeted class II and III PI3 kinases (PI3Ks) in an MTM1-deficient mouse model. Muscle-specific ablation of Pik3c2b, but not Pik3c3, resulted in complete prevention of the MTM phenotype, and postsymptomatic targeting promoted a striking rescue of disease. We confirmed this genetic interaction in zebrafish, and additionally showed that certain PI3K inhibitors prevented development of the zebrafish mtm phenotype. Finally, the PI3K inhibitor wortmannin improved motor function and prolonged lifespan of the Mtm1-deficient mice. In all, we have identified Pik3c2b as a genetic modifier of Mtm1 mutation and demonstrated that PIK3C2B inhibition is a potential treatment strategy for MTM. In addition, we set the groundwork for similar reciprocal inhibition approaches for treating other PIP metabolic disorders and highlight the importance of modifier gene pathways as therapeutic targets.

498. Prolonged Benefit from Systemic rAAV8 in a Canine Model of Myotubular Myopathy

Body: Mutations in MTM1 result in X-linked myotubular myopathy (XLMTM), a rare neuromuscular disease that causes devastating muscular weakness in affected children, often fatal in the first years of life. A naturally occurring MTM1 mutation in Labrador retrievers provides unprecedented opportunity to assess MTM1 gene replacement strategies. Here, we report clinical and physiological outcomes more than 3 years after a single intravenous infusion of AAV8-MTM1 in two dogs. Readouts include neurological assessment, limb strength, gait, respiratory function, and histology. Results (Fig 1Fig 1) are shown in comparison to age-matched littermates, and indicate that a single infusion of AAV8-MTM1 can markedly prolong life and achieve near normal levels of neurological function, strength, gait, and respiratory function for more than 3 years in MTM1 mutant dogs.View Large Image | Download PowerPoint Slide

MG-132 Next-generation sequencing in the neonatal intensive care unit: Pilot data from 12 newborns

Background Rare disease can present in the first days and weeks of life and is associated with significant morbidity and mortality. The genetic and clinical heterogeneity of these conditions can pose a significant challenge for diagnosis. Objective To evaluate the diagnostic utility of the Illumina TruSightTM One panel, we performed Next-generation sequencing (NGS) for a series of 20 newborns presenting with features suggestive of a Mendelian disease in the Neonatal Intensive Care Unit (NICU). Design/methods Twenty patients were recruited from the NICU at the Children’s Hospital of Eastern Ontario. Inclusion criteria required a complex medical presentation (congenital anomalies, abnormalities in growth and/or neurological features). We used a family-based trio approach. Target enrichment was performed with the Illumina TruSightTM One Sequencing Panel kit. This panel targets 4,813 genes that are deemed clinically-relevant and referred to as the “clinome”. Sequencing was performed on the Illumina MiSeq. NextGene software (v2.3.4.4) was used for analyses. Time from sample acquisition to data analysis was possible in 7 working days. Results/conclusion To date, 13 trios were sequenced and analysed. A molecular diagnosis was made in 5 of 13 patients, comparable to the rate obtained by whole-exome sequencing from the literature. Positive cases included bi-allelic mutations in WDR19 and ACE, an X-linked mutation in MTM1, homozygous mutations in FTO, and a de novo mutation in SCN1A. Clinome sequencing has the potential to improve our ability to efficiently diagnose rare diseases in the NICU by providing a cost-effective tool to evaluate the clinically relevant portion of the genome in a week’s time.

Frequency and Phenotype of Myotubular Myopathy Amongst Danish Patients with Congenital Myopathy Older than 5 Years.

Background: Centronuclear myopathy (CNM) is one of four main subtypes of congenital myopathy. X-linked myotubular myopathy (XLMTM) is considered one of the most severe forms, but survivors past infancy have been described. However, detailed information on XLMTM phenotypes in patients who survive infancy is scarce.Objective: The aim of the study was to report the genetic findings in patients with a predominant centronuclear finding on muscle biopsy and describe the prevalence, phenotypes and the course of the disease in patients with XLMTM in a Danish cohort of patients with congenital myopathies older than five years.Methods: Ninety-four out of 119 invited patients older than five years were included in the study and assessed by muscle tests, functional tests, muscle biopsy, plasma creatine kinase levels and genetic testing. Genes related to CNM were sequenced in patients who had centronuclear findings on muscle histology. In patients with MTM1 mutations, medical records from local hospitals were reviewed to obtain information on birth history and course of disease.Results: Sixteen of 94 patients had CNM on muscle biopsy; three male patients, aged 14–25 years, carried a pathogenic MTM1 mutation, six patients carried a pathogenic DNM2 mutation and two carried pathogenic RYR1 mutations. The mutations have all been described before to cause CNM. The MTM phenotypes ranged from severe (classical) to mild; one patient had always been non-ambulant, one had lost ambulation, and one was still ambulant at 25 years.Conclusions: Our findings show that CNM caused by DNM2 mutations is the most common form of CNM in Danish patients older than 5 years, but XLMTM is not negligible even past age 5 years, and the phenotype may be much milder than generally described - also in patients with the classically described infantile form of the disease.

Gene therapy in myotubular myopathy: promising progress and future directions.

X-linked myotubular myopathy (MTM; OMIM 310400) is a severe congenital myopathy that is classified as one of the centronuclear myopathies (1). MTM is estimated to affect 1/50,000 live born males and is due to loss of function mutations in MTM1 , which encodes myotubularin, a ubiquitously expressed lipid phosphatase (2,3). The function of myotubularin is still not entirely clear, but a picture is starting to emerge in which myotubularin is a critical maintenance protein of the myofiber sarcotubular system and has a role in excitation-contraction coupling (4-7).

X-linked myotubular myopathy in Rottweiler dogs is caused by a missense mutation in Exon 11 of the MTM1 gene.

BackgroundCongenital and inherited myopathies in dogs are faithful models of human muscle diseases and are being recognized with increasing frequency. In fact, canine models of dystrophin deficient muscular dystrophy and X-linked myotubular myopathy are of tremendous value in the translation of new and promising therapies for the treatment of these diseases. We have recently identified a family of Australian Rottweilers in which male puppies were clinically affected with severe muscle weakness and atrophy that resulted in early euthanasia or death. X-linked myotubular myopathy was suspected based on the early and severe clinical presentation and histopathological changes within muscle biopsies. The aim of this study was to determine the genetic basis for myopathy in these dogs and compare and contrast the clinical presentation, histopathology, ultrastructure, and mutation in this family of Rottweiler dogs with the previously described myotubular myopathy in Labrador retrievers.ResultsHistopathology, histochemistry, and ultrastructural examination of muscle biopsies from affected Rottweiler puppies were consistent with an X-linked myotubular myopathy. An unusual finding that differed from the previously reported Labradors and similar human cases was the presence of excessive autophagy and prominent autophagic vacuoles. Molecular investigations confirmed a missense mutation in exon 11 of MTM1 that was predicted to result in a non-functional phosphatase activity. Although the clinical presentations and histopathology were similar, the MTM1 p.(Q384P) mutation is different from the p.(N155K) mutation in exon 7 affecting Labrador retrievers with X-linked myotubular myopathy.ConclusionsHere we describe a second pathogenic mutation in MTM1 causing X-linked myotubular myopathy in dogs. Our findings suggest a variety of MTM1 mutations in dogs as seen in human patients. The number of MTM1 mutations resulting in similar severe and progressive clinical myopathy and histopathological changes are likely to increase as canine myopathies are further characterized.

Congenital myopathies: Natural history of a large pediatric cohort.

To assess the natural history of congenital myopathies (CMs) due to different genotypes. Retrospective cross-sectional study based on case-note review of 125 patients affected by CM, followed at a single pediatric neuromuscular center, between 1984 and 2012. Genetic characterization was achieved in 99 of 125 cases (79.2%), with RYR1 most frequently implicated (44/125). Neonatal/infantile onset was observed in 76%. At birth, 30.4% required respiratory support, and 25.2% nasogastric feeding. Twelve percent died, mainly within the first year, associated with mutations in ACTA1, MTM1, or KLHL40. All RYR1-mutated cases survived and did not require long-term ventilator support including those with severe neonatal onset; however, recessive cases were more likely to require gastrostomy insertion (p = 0.0028) compared with dominant cases. Independent ambulation was achieved in 74.1% of all patients; 62.9% were late walkers. Among ambulant patients, 9% eventually became wheelchair-dependent. Scoliosis of variable severity was reported in 40%, with 1/3 of (both ambulant and nonambulant) patients requiring surgery. Bulbar involvement was present in 46.4% and required gastrostomy placement in 28.8% (at a mean age of 2.7 years). Respiratory impairment of variable severity was a feature in 64.1%; approximately half of these patients required nocturnal noninvasive ventilation due to respiratory failure (at a mean age of 8.5 years). We describe the long-term outcome of a large cohort of patients with CMs. While overall course is stable, we demonstrate a wide clinical spectrum with motor deterioration in a subset of cases. Severity in the neonatal/infantile period is critical for survival, with clear genotype-phenotype correlations that may inform future counseling.

Adult-onset autosomal dominant centronuclear myopathy due to BIN1 mutations.

Centronuclear myopathies are congenital muscle disorders characterized by type I myofibre predominance and an increased number of muscle fibres with nuclear centralization. The severe neonatal X-linked form is due to mutations in MTM1, autosomal recessive centronuclear myopathy with neonatal or childhood onset results from mutations in BIN1 (amphiphysin 2), and dominant cases were previously associated to mutations in DNM2 (dynamin 2). Our aim was to determine the genetic basis and physiopathology of patients with mild dominant centronuclear myopathy without mutations in DNM2. We hence established and characterized a homogeneous cohort of nine patients from five families with a progressive adult-onset centronuclear myopathy without facial weakness, including three sporadic cases and two families with dominant disease inheritance. All patients had similar histological and ultrastructural features involving type I fibre predominance and hypotrophy, as well as prominent nuclear centralization and clustering. We identified heterozygous BIN1 mutations in all patients and the molecular diagnosis was complemented by functional analyses. Two mutations in the N-terminal amphipathic helix strongly decreased the membrane-deforming properties of amphiphysin 2 and three stop-loss mutations resulted in a stable protein containing 52 supernumerary amino acids. Immunolabelling experiments revealed abnormal central accumulation of dynamin 2, caveolin-3, and the autophagic marker p62, and general membrane alterations of the triad, the sarcolemma, and the basal lamina as potential pathological mechanisms. In conclusion, we identified BIN1 as the second gene for dominant centronuclear myopathy. Our data provide the evidence that specific BIN1 mutations can cause either recessive or dominant centronuclear myopathy and that both disorders involve different pathomechanisms.

G.P.38: Adult-onset autosomal dominant centronuclear myopathy due to BIN1 mutations

Centronuclear myopathies (CNM) are congenital muscle disorders characterized by an increased number of muscle fibers with central nuclei. The severe neonatal X-linked form is due to mutations in MTM1, autosomal recessive CNM with neonatal or childhood onset results from mutations in BIN1 (amphiphysin 2), and dominant cases were previously associated to mutations in DNM2 (dynamin 2). However, the genetic basis of the disease is not known for all cases. Here we characterize a novel CNM entity. We established a homogeneous cohort of nine patients from five families with dominant CNM without mutations in DNM2. All patients presented with a mildly progressive adult-onset myopathy and had similar histological and ultrastructural features involving type I fiber predominance and hypotrophy, as well as prominent nuclear centralization and clustering. We identified heterozygous BIN1 mutations in all five families: two missense mutations in the N-terminal amphipathic helix, and three single nucleotide deletions involving the loss of the stop codon. Amphiphysin 2 (encoded by BIN1) induces membrane tubulation and is implicated in excitation-contraction coupling (ECC) at the T-tubules. We confirmed mutation segregation with the disease in the dominant families, and we complemented the molecular diagnosis by functional analyses. Both missense mutations in the N-terminal amphipathic helix strongly decreased the membrane-deforming properties of amphiphysin 2 and three stop-loss mutations resulted in a stable protein containing 52 supernumerary amino acids. Additional immunolabeling experiments revealed abnormal central accumulation of dynamin 2 and caveolin-3, and general membrane alterations of the triad, the sarcolemma, and the basal lamina. In conclusion, we identified BIN1 as a novel gene for dominant adult-onset CNM. Our data provide the evidence that specific BIN1 mutations can cause either recessive or dominant CNM and that both disorders involve different pathomechanisms.

G.O.22: Reducing dynamin 2 rescues centronuclear myopathy

Centronuclear myopathies (CNM) are associated with muscle weakness and abnormally located nuclei in skeletal muscle. They can be due to mutations in the MTM1 gene encoding myotubularin (X-linked centronuclear myopathy or myotubular myopathy), in the DNM2 gene encoding dynamin 2 (dominant CNM), or in BIN1 (amphiphysin 2), RYR1 or TTN for typical or atypical autosomal forms. Currently, no effective treatments exist for centronuclear myopathies. We and others showed that overexpression of wildtype DNM2 in muscle cause a CNM-like phenotype. We thus hypothesized MTM1 and DNM2 function in a common pathway, where either MTM1 loss-of-function or DNM2 gain-of-function lead to the CNM phenotype. To test this hypothesis, we reduced the expression of Dnm2 in Mtm1-/y mice that reproduce a CNM phenotype with a progressive myopathy leading to death by 6–12 weeks. Mtm1-/yDnm2+/− mice survived up to 2 years. CNM histological features including fiber atrophy and nuclei mispositioning were prevented or strongly delayed and reduced, and muscle strength was increased. Downregulation of Dnm2 selectively in skeletal muscle during embryogenesis or in young Mtm1-/y mice showed the rescue is cell autonomous and that downregulation of Dnm2 can stop and potentially revert the progression of the phenotype. Thus, we identified MTM1 and DNM2 are in a common pathway regulating muscle organization and force. We introduce the original concept of ”cross-therapy” where one form of the disease (X-linked CNM, MTM1) can be rescued by decreasing expression of another gene mutated in CNM (DNM2). Preliminary data will be presented supporting downregulation of DNM2 can also rescue a myopathy not due to MTM1 mutations, enlarging the spectrum of myopathies that could be rescued by this strategy. While DNM2 is a key mechanoenzyme for important cellular processes, its reduction is strongly beneficial for several myopathies and represents a novel potential therapeutic approach.

SPEG Interacts with Myotubularin, and Its Deficiency Causes Centronuclear Myopathy with Dilated Cardiomyopathy

Centronuclear myopathies (CNMs) are characterized by muscle weakness and increased numbers of central nuclei within myofibers. X-linked myotubular myopathy, the most common severe form of CNM, is caused by mutations in MTM1, encoding myotubularin (MTM1), a lipid phosphatase. To increase our understanding of MTM1 function, we conducted a yeast two-hybrid screen to identify MTM1-interacting proteins. Striated muscle preferentially expressed protein kinase (SPEG), the product of SPEG complex locus (SPEG), was identified as an MTM1-interacting protein, confirmed by immunoprecipitation and immunofluorescence studies. SPEG knockout has been previously associated with severe dilated cardiomyopathy in a mouse model. Using whole-exome sequencing, we identified three unrelated CNM-affected probands, including two with documented dilated cardiomyopathy, carrying homozygous or compound-heterozygous SPEG mutations. SPEG was markedly reduced or absent in two individuals whose muscle was available for immunofluorescence and immunoblot studies. Examination of muscle samples from Speg-knockout mice revealed an increased frequency of central nuclei, as seen in human subjects. SPEG localizes in a double line, flanking desmin over the Z lines, and is apparently in alignment with the terminal cisternae of the sarcoplasmic reticulum. Examination of human and murine MTM1-deficient muscles revealed similar abnormalities in staining patterns for both desmin and SPEG. Our results suggest that mutations in SPEG, encoding SPEG, cause a CNM phenotype as a result of its interaction with MTM1. SPEG is present in cardiac muscle, where it plays a critical role; therefore, individuals with SPEG mutations additionally present with dilated cardiomyopathy.

Conditional Knockout of Pik3c3 Causes a Murine Muscular Dystrophy

Abnormalities in phosphoinositide metabolism are an emerging theme in human neurodegenerative disease. Myotubular myopathy is a prototypical disorder of phosphoinositide dysregulation that is characterized by profound muscle pathology and weakness and that is caused by mutations in MTM1, which encodes a phosphatase that targets 3-position phosphoinositides, including phosphatidylinositol 3-phosphate. Although the association between MTM1 and muscle disease has become increasingly clarified, the normal role(s) of phosphatidylinositol 3-phosphate metabolism in muscle development and homeostasis remain poorly understood. To begin to address the function of phosphatidylinositol 3-phosphate in skeletal muscle, we focused on the primary kinase responsible for its production, and created a muscle-specific conditional knockout of the class III phosphatidylinositol 3-kinase, Pik3c3. Muscle-specific deletion of Pik3c3 did not disturb embryogenesis or early postnatal development, but resulted in progressive disease characterized by reduced activity and death by 2 months of age. Histopathological analysis demonstrated changes consistent with a murine muscular dystrophy. Examination for cellular mechanism(s) responsible for the dystrophic phenotype revealed significant alterations in the autophagolysosomal pathway with mislocation of known dystrophy proteins to the lysosomal compartment. In all, we present the first analysis of Pik3c3 in skeletal muscle, and report a novel association between deletion of Pik3c3 and muscular dystrophy.

Dynamin 2 the rescue for centronuclear myopathy

Centronuclear myopathy is a lethal muscle disease. The most severe form of the disease, X-linked centronuclear myopathy, is due to mutations in the gene encoding myotubularin (MTM1), while mutations in dynamin 2 (DNM2) and amphiphysin 2/BIN1 (AMPH2) cause milder forms of myopathy. MTM1 is a lipid phosphatase, and mutations that disrupt this activity cause severe muscle wasting. In this issue of the JCI, Cowling and colleagues report on their finding of increased DNM2 levels in human and mouse muscle with MTM1 mutations. Partial reduction of Dnm2 in mice harboring Mtm1 mutations remarkably rescued muscle wasting and lethality, and this effect was muscle specific. DNM2 regulates membrane trafficking through vesicular scission, and it is presumed that reducing this activity accounts for improved outcome in X-linked centronuclear myopathy.

P.4.8 X-linked benign form of myotubular myopathy (XLMTM) with necklace fibres: A case report

X-linked myotubular myopathy (XLMTM) is usually a severe condition caused by mutations in myotubularin gen MTM1, (Xq28), a lipid inositide phosphatase acting in remodeling of membranes. Most of the affected patients present a severe neonatal form with hypotonia, weakness, facial diplegia, difficulties in sucking in addition to other symptoms dying within the first months. The biopsy features are consistent with the presence of central nuclei in most fibres. However, a more benign phenotype with a distinctive hallmark has recently been described. Herein, we present a four year old boy affected by this benign form. Patient was born by urgent caesarean section due to foetal bradycardia. He weighted 3.030 g. Presented axial hypotonic, scarce movements and respiratory distress, needing CPAP for few days. No tube feeding was required. Motor development was delayed. He had always difficulties in getting up from the floor. At 4 years he showed, facial weakness, weak cry, axial and head flexor muscles weakess. Cough was ineffective but no hypercapnia was detected. CK was normal. EMG was myopathic. Muscle biopsy showed two population of fibres. Most of the smaller one, revealed a “necklace” appearance. Genetic studies confirmed c.605T > C in exon 8 leading to p.I202S mutation in MTM1 gene. Necklace fibres constitute a hallmark for suspect diagnosis of this benign phenotype of XLMTM associated with mutation in the MTM1 gene. X-linked myotubular myopathy (XLMTM) is usually a severe condition caused by mutations in myotubularin gen MTM1, (Xq28), a lipid inositide phosphatase acting in remodeling of membranes. Most of the affected patients present a severe neonatal form with hypotonia, weakness, facial diplegia, difficulties in sucking in addition to other symptoms dying within the first months. The biopsy features are consistent with the presence of central nuclei in most fibres. However, a more benign phenotype with a distinctive hallmark has recently been described. Herein, we present a four year old boy affected by this benign form. Patient was born by urgent caesarean section due to foetal bradycardia. He weighted 3.030 g. Presented axial hypotonic, scarce movements and respiratory distress, needing CPAP for few days. No tube feeding was required. Motor development was delayed. He had always difficulties in getting up from the floor. At 4 years he showed, facial weakness, weak cry, axial and head flexor muscles weakess. Cough was ineffective but no hypercapnia was detected. CK was normal. EMG was myopathic. Muscle biopsy showed two population of fibres. Most of the smaller one, revealed a “necklace” appearance. Genetic studies confirmed c.605T > C in exon 8 leading to p.I202S mutation in MTM1 gene. Necklace fibres constitute a hallmark for suspect diagnosis of this benign phenotype of XLMTM associated with mutation in the MTM1 gene.