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AAV-mediated intramuscular delivery of myotubularin corrects the myotubular myopathy phenotype in targeted murine muscle and suggests a function in plasma membrane homeostasis.

ABSTRACT: Myotubular myopathy (XLMTM, OMIM 310400) is a severe congenital muscular disease due to mutations in the myotubularin gene (MTM1) and characterized by the presence of small myofibers with frequent occurrence of central nuclei. Myotubularin is a ubiquitously expressed phosphoinositide phosphatase with a muscle-specific role in man and mouse that is poorly understood. No specific treatment exists to date for patients with myotubular myopathy. We have constructed an adeno-associated virus (AAV) vector expressing myotubularin in order to test its therapeutic potential in a XLMTM mouse model. We show that a single intramuscular injection of this vector in symptomatic Mtm1-deficient mice ameliorates the pathological phenotype in the targeted muscle. Myotubularin replacement in mice largely corrects nuclei and mitochondria positioning in myofibers and leads to a strong increase in muscle volume and recovery of the contractile force. In addition, we used this AAV vector to overexpress myotubularin in wild-type skeletal muscle and get insight into its localization and function. We show that a substantial proportion of myotubularin associates with the sarcolemma and I band, including triads. Myotubularin overexpression in muscle induces the accumulation of packed membrane saccules and presence of vacuoles that contain markers of sarcolemma and T-tubules, suggesting that myotubularin is involved in plasma membrane homeostasis of myofibers. This study provides a proof-of-principle that local delivery of an AAV vector expressing myotubularin can improve the motor capacities of XLMTM muscle and represents a novel approach to study myotubularin function in skeletal muscle.

SUBMITTER: Buj-Bello A

PROVIDER: S-EPMC2441725 | biostudies-literature |

REPOSITORIES: biostudies-literature

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Project description:Myotubularin is a lipid phosphatase implicated in endosomal trafficking in vitro, but with an unknown function in vivo. Mutations in myotubularin cause myotubular myopathy, a devastating congenital myopathy with unclear pathogenesis and no current therapies. Myotubular myopathy was the first described of a growing list of conditions caused by mutations in proteins implicated in membrane trafficking. To advance the understanding of myotubularin function and disease pathogenesis, we have created a zebrafish model of myotubular myopathy using morpholino antisense technology. Zebrafish with reduced levels of myotubularin have significantly impaired motor function and obvious histopathologic changes in their muscle. These changes include abnormally shaped and positioned nuclei and myofiber hypotrophy. These findings are consistent with those observed in the human disease. We demonstrate for the first time that myotubularin functions to regulate PI3P levels in a vertebrate in vivo, and that homologous myotubularin-related proteins can functionally compensate for the loss of myotubularin. Finally, we identify abnormalities in the tubulo-reticular network in muscle from myotubularin zebrafish morphants and correlate these changes with abnormalities in T-tubule organization in biopsies from patients with myotubular myopathy. In all, we have generated a new model of myotubular myopathy and employed this model to uncover a novel function for myotubularin and a new pathomechanism for the human disease that may explain the weakness associated with the condition (defective excitation-contraction coupling). In addition, our findings of tubuloreticular abnormalities and defective excitation-contraction coupling mechanistically link myotubular myopathy with several other inherited muscle diseases, most notably those due to ryanodine receptor mutations. Based on our findings, we speculate that congenital myopathies, usually considered entities with similar clinical features but very disparate pathomechanisms, may at their root be disorders of calcium homeostasis.

Project description:Centronuclear myopathy (CNM) is an inherited neuromuscular disorder characterised by clinical features of a congenital myopathy and centrally placed nuclei on muscle biopsy.The incidence of X-linked myotubular myopathy is estimated at 2/100000 male births but epidemiological data for other forms are not currently available.The clinical picture is highly variable. The X-linked form usually gives rise to a severe phenotype in males presenting at birth with marked weakness and hypotonia, external ophthalmoplegia and respiratory failure. Signs of antenatal onset comprise reduced foetal movements, polyhydramnios and thinning of the ribs on chest radiographs; birth asphyxia may be the present. Affected infants are often macrosomic, with length above the 90th centile and large head circumference. Testes are frequently undescended. Both autosomal-recessive (AR) and autosomal-dominant (AD) forms differ from the X-linked form regarding age at onset, severity, clinical characteristics and prognosis. In general, AD forms have a later onset and milder course than the X-linked form, and the AR form is intermediate in both respects.Mutations in the myotubularin (MTM1) gene on chromosome Xq28 have been identified in the majority of patients with the X-linked recessive form, whilst AD and AR forms have been associated with mutations in the dynamin 2 (DNM2) gene on chromosome 19p13.2 and the amphiphysin 2 (BIN1) gene on chromosome 2q14, respectively. Single cases with features of CNM have been associated with mutations in the skeletal muscle ryanodine receptor (RYR1) and the hJUMPY (MTMR14) genes.Diagnosis is based on typical histopathological findings on muscle biopsy in combination with suggestive clinical features; muscle magnetic resonance imaging may complement clinical assessment and inform genetic testing in cases with equivocal features. Genetic counselling should be offered to all patients and families in whom a diagnosis of CNM has been made.The main differential diagnoses include congenital myotonic dystrophy and other conditions with severe neonatal hypotonia.Management of CNM is mainly supportive, based on a multidisciplinary approach. Whereas the X-linked form due to MTM1 mutations is often fatal in infancy, dominant forms due to DNM2 mutations and some cases of the recessive BIN1-related form appear to be associated with an overall more favourable prognosis.

Project description:Purpose: The goal of this study was to investigate the transcriptional profile of genes expressed in the quadriceps muscle of myotubularin deficient (Mtm1-/-) mice, and determine whether improvements in the pathophysiology of tamoxifen treated Mtm1-/- mice are a consequence of ER-dependent transcriptional modulation. Methods: Muscle mRNA profiles from quadriceps of 36-day-old wild-type (WT), myotubularin deficient (Mtm1-/-) mice, and tamoxifen treated WT and Mtm1-/- mice were generated by deep sequencing, in triplicate or quadruplicate, using Illumina HiSeq 2500. Read sets from each of the 14 samples across the four conditions were aligned to the reference genome (GRCm38/MM10 version of the Mus musculus genome) and transcriptome using STAR (Dobin et al. Bioinformatics 2013 PMID: 23104886) two-step alignment to generate a Binary Alignment Map file (BAM file). Coordinate sorted BAM files were used to quantify transcript abundance (count data) using HTSeq (Anders et al. Bioinformatics 2014, PMID: 25260700). Raw read counts generated were used as input for differential gene expression analysis, carried out using both DESeq (Anders and Huber 2010) and edgeR (Robinson, McCarthy, and Smyth 2010) R/Bioconductor packages. This was carried out in a pair-wise manner between any two conditions that were considered. FDR adjusted p-values from both edgeR and DESeq was used to determine genes that are significantly differentially expressed between the conditions tested. Finally, we used the GoSeq R/Bioconductor package (Young MD, et al Genome Biology, 11, pp. R14) to identify pathways that were significantly enriched for differentially expressed genes between any two conditions. Results: Using an established data analysis workflow, 65 million reads were aligned per sample (>90% exonic). We observed 849 differentially expressed transcripts in untreated mtm1-/-mice (p< 0.01) in comparison to wild-type (WT) controls. Furthermore, we identified 29 differentially expressed genes (p<0.01) between mtm1-/- mice and their TAM-treated mtm1-/- mice. In particular, we observed that the transcriptional profiles of mtm1 -/- mice and tamoxifen-treated mtm1-/- mice cluster together and are distinct from wild-type contros. Moreover, no significant changes were observed in the expression of estrogen-response genes that are known gene targets of tamoxifen; no significant transcriptional changes were observed in well-known MTM-related genes.This strongly indicates that, contrary to its well-established function as a transcriptional modulator, tamoxifen improves the pathophysiology of myotubular myopathy in a post-transcriptional manner. Concusions: Our study represents the first comprehensive analysis of the whole transcriptome of the Mtm1-/- and tamoxifen treated Mtm1-/- mouse, generated by RNA-seq technology. The data reported here provides a framework a comparative investigation of the expression profiles in Mtm1-/- mice and tamoxifen treated Mtm1-/- mice. Importantly, our results demonstrate that in our MTM murine model, tamoxifen does not appear to alter the transcriptional profile of many estrogen-receptor (ER) related genes, and moreover does not appear to alter the transcriptional profile of genes that are abnormally expressed in Mtm1-/- mice alone. This suggests that tamoxifen is acting to improve the pathophysiology of the MTM mouse model in a transcriptionally-indepedent manner.

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AAV-mediated intramuscular delivery of myotubularin corrects the myotubular myopathy phenotype in targeted murine muscle and suggests a function in plasma membrane homeostasis.

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