Site-specific Mtm1 mutagenesis by an AAV-Cre vector reveals that myotubularin is essential in adult muscle.

Manipulation of the mouse genome by site-specific mutagenesis has been extensively used to study gene function and model human disorders. Mouse models of myotubular myopathy (XLMTM), a severe congenital muscular disorder due to loss-of-function mutations in the MTM1 gene, have been generated by homologous recombination and shown that myotubularin is essential for skeletal muscle. However, since the Mtm1 deletion occurred constitutively or shortly after birth in these mice, it is not known whether myotubularin is required during adulthood, an important issue in the context of not only muscle biology but also therapies. To delete the Mtm1 gene in adult muscle fibers, we constructed a recombinant adeno-associated vector (AAV) that expresses the Cre recombinase under the muscle-specific desmin promoter. We report that a single injection of this vector into muscles of 3-month-old Mtm1 conditional mice leads to a myotubular myopathy phenotype with myofiber atrophy, disorganization of organelle positioning, such as mitochondria and nuclei, T-tubule defects and severe muscle weakness. In addition, our results show that MTM1-related atrophy and dysfunction correlate with abnormalities in satellite cell number and markers of autophagy, protein synthesis and neuromuscular junction transmission. The expression level of atrogenes was also analyzed. Therefore, we provide a valuable tissue model that recapitulates the main features of the disease, and it is useful to study pathogenesis and evaluate therapeutic strategies. We establish the proof-of-concept that myotubularin is required for the proper function of skeletal muscle during adulthood, suggesting that therapies will be required for the entire life of XLMTM patients.

Enzyme replacement therapy rescues weakness and improves muscle pathology in mice with X-linked myotubular myopathy.

No effective treatment exists for patients with X-linked myotubular myopathy (XLMTM), a fatal congenital muscle disease caused by deficiency of the lipid phosphatase, myotubularin. The Mtm1δ4 and Mtm1 p.R69C mice model severely and moderately symptomatic XLMTM, respectively, due to differences in the degree of myotubularin deficiency. Contractile function of intact extensor digitorum longus (EDL) and soleus muscles from Mtm1δ4 mice, which produce no myotubularin, is markedly impaired. Contractile forces generated by chemically skinned single fiber preparations from Mtm1δ4 muscle were largely preserved, indicating that weakness was largely due to impaired excitation contraction coupling. Mtm1 p.R69C mice, which produce small amounts of myotubularin, showed impaired contractile function only in EDL muscles. Short-term replacement of myotubularin with a prototypical targeted protein replacement agent (3E10Fv-MTM1) in Mtm1δ4 mice improved contractile function and muscle pathology. These promising findings suggest that even low levels of myotubularin protein replacement can improve the muscle weakness and reverse the pathology that characterizes XLMTM.

Differential muscle hypertrophy is associated with satellite cell numbers and Akt pathway activation following activin type IIB receptor inhibition in Mtm1 p.R69C mice.

X-linked myotubular myopathy is a congenital myopathy caused by deficiency of myotubularin. Patients often present with severe perinatal weakness, requiring mechanical ventilation to prevent death from respiratory failure. We recently reported that an activin receptor type IIB inhibitor produced hypertrophy of type 2b myofibers and modest increases of strength and life span in the severely myopathic Mtm1δ4 mouse model of X-linked myotubular myopathy. We have now performed a similar study in the less severely symptomatic Mtm1 p.R69C mouse in hopes of finding greater treatment efficacy. Activin receptor type IIB inhibitor treatment of Mtm1 p.R69C animals produced behavioral and histological evidence of hypertrophy in gastrocnemius muscles but not in quadriceps or triceps. The ability of the muscles to respond to activin receptor type IIB inhibitor treatment correlated with treatment-induced increases in satellite cell number and several muscle-specific abnormalities of hypertrophic signaling. Treatment-responsive Mtm1 p.R69C gastrocnemius muscles displayed lower levels of phosphorylated ribosomal protein S6 and higher levels of phosphorylated eukaryotic elongation factor 2 kinase than were observed in Mtm1 p.R69C quadriceps muscle or in muscles from wild-type littermates. Hypertrophy in the Mtm1 p.R69C gastrocnemius muscle was associated with increased levels of phosphorylated ribosomal protein S6. Our findings indicate that muscle-, fiber type-, and mutation-specific factors affect the response to hypertrophic therapies that will be important to assess in future therapeutic trials.

CXorf6 is a causative gene for hypospadias.

46,XY disorders of sex development (DSD) refer to a wide range of abnormal genitalia, including hypospadias, which affects approximately 0.5% of male newborns. We identified three different nonsense mutations of CXorf6 in individuals with hypospadias and found that its mouse homolog was specifically expressed in fetal Sertoli and Leydig cells around the critical period for sex development. These data imply that CXorf6 is a causative gene for hypospadias.

Phosphoinositide substrates of myotubularin affect voltage-activated Ca²âº release in skeletal muscle.

Skeletal muscle excitation­contraction (E­C) coupling is altered in several models of phosphatidylinositol phosphate (PtdInsP) phosphatase deficiency and ryanodine receptor activity measured in vitro was reported to be affected by certain PtdInsPs, thus prompting investigation of the physiological role of PtdInsPs in E­C coupling. We measured intracellular Ca2+ transients in voltage-clamped mouse muscle fibres microinjected with a solution containing a PtdInsP substrate (PtdIns(3,5)P2 or PtdIns(3)P) or product (PtdIns(5)P or PtdIns) of the myotubularin phosphatase MTM1. No significant change was observed in the presence of either PtdIns(5)P or PtdIns but peak SR Ca2+ release was depressed by ~30% and 50% in fibres injected with PtdIns(3,5)P2 and PtdIns(3)P, respectively, with no concurrent alteration in the membrane current signals associated with the DHPR function as well as in the voltage dependence of Ca2+ release inactivation. In permeabilized muscle fibres, the frequency of spontaneous Ca2+ release events was depressed in the presence of the three tested phosphorylated forms of PtdInsP with PtdIns(3,5)P2 being the most effective, leading to an almost complete disappearance of Ca2+ release events. Results support the possibility that pathological accumulation of MTM1 substrates may acutely depress ryanodine receptor-mediated Ca2+ release. Overexpression of a mCherry-tagged form of MTM1 in muscle fibres revealed a striated pattern consistent with the triadic area. Ca2+ release remained although unaffected by MTM1 overexpression and was also unaffected by the PtdIns-3-kinase inhibitor LY2940002, suggesting that the 3-phosphorylated PtdIns lipids active on voltage-activated Ca2+ release are inherently maintained at a low level, inefficient on Ca2+ release in normal conditions.

Modeling the human MTM1 p.R69C mutation in murine Mtm1 results in exon 4 skipping and a less severe myotubular myopathy phenotype.

X-linked myotubular myopathy (MTM) is a severe neuromuscular disease of infancy caused by mutations of MTM1, which encodes the phosphoinositide lipid phosphatase, myotubularin. The Mtm1 knockout (KO) mouse has a severe phenotype and its short lifespan (8 weeks) makes it a challenge to use as a model in the testing of certain preclinical therapeutics. Many MTM patients succumb early in life, but some have a more favorable prognosis. We used human genotype-phenotype correlation data to develop a myotubularin-deficient mouse model with a less severe phenotype than is seen in Mtm1 KO mice. We modeled the human c.205C>T point mutation in Mtm1 exon 4, which is predicted to introduce the p.R69C missense change in myotubularin. Hemizygous male Mtm1 p.R69C mice develop early muscle atrophy prior to the onset of weakness at 2 months. The median survival period is 66 weeks. Histopathology shows small myofibers with centrally placed nuclei. Myotubularin protein is undetectably low because the introduced c.205C>T base change induced exon 4 skipping in most mRNAs, leading to premature termination of myotubularin translation. Some full-length Mtm1 mRNA bearing the mutation is present, which provides enough myotubularin activity to account for the relatively mild phenotype, as Mtm1 KO and Mtm1 p.R69C mice have similar muscle phosphatidylinositol 3-phosphate levels. These data explain the basis for phenotypic variability among human patients with MTM1 p.R69C mutations and establish the Mtm1 p.R69C mouse as a valuable model for the disease, as its less severe phenotype will expand the scope of testable preclinical therapies.

Ultrasound assessment of the diaphragm: Preliminary study of a canine model of X-linked myotubular myopathy.

INTRODUCTION: We tested the feasibility of using neuromuscular ultrasound for non-invasive real-time assessment of diaphragmatic structure and function in a canine model of X-linked myotubular myopathy (XLMTM). METHODS: Ultrasound images in 3 dogs [wild-type (WT), n=1; XLMTM untreated, n=1; XLMTM post-AAV8-mediated MTM1 gene replacement, n=1] were analyzed for diaphragm thickness, change in thickness with respiration, muscle echogenicity, and diaphragm excursion amplitude during spontaneous breathing. RESULTS: Quantitative parameters of diaphragm structure were different among the animals. WT diaphragm was thicker and less echogenic than the XLMTM control, whereas the diaphragm measurements of the MTM1-treated XLMTM dog were comparable to those of the WT dog. CONCLUSIONS: This pilot study demonstrates the feasibility of using ultrasound for quantitative assessment of the diaphragm in a canine model. In the future, ultrasonography may replace invasive measures of diaphragm function in canine models and in humans for non-invasive respiratory monitoring and evaluation of neuromuscular disease.

Myotubularin-deficient myoblasts display increased apoptosis, delayed proliferation, and poor cell engraftment.

X-linked myotubular myopathy is a severe congenital myopathy caused by deficiency of the lipid phosphatase, myotubularin. Recent studies of human tissue and animal models have discovered structural and physiological abnormalities in myotubularin-deficient muscle, but the impact of myotubularin deficiency on myogenic stem cells within muscles is unclear. In the present study, we evaluated the viability, proliferative capacity, and in vivo engraftment of myogenic cells obtained from severely symptomatic (Mtm1δ4) myotubularin-deficient mice. Mtm1δ4 muscle contains fewer myogenic cells than wild-type (WT) littermates, and the number of myogenic cells decreases with age. The behavior of Mtm1δ4 myoblasts is also abnormal, because they engraft poorly into C57BL/6/Rag1null/mdx5cv mice and display decreased proliferation and increased apoptosis compared with WT myoblasts. Evaluation of Mtm1δ4 animals at 21 and 42 days of life detected fewer satellite cells in Mtm1δ4 muscle compared with WT littermates, and the decrease in satellite cells correlated with progression of disease. In addition, analysis of WT and Mtm1δ4 regeneration after injury detected similar abnormalities of satellite cell function, with fewer satellite cells, fewer dividing cells, and increased apoptotic cells in Mtm1δ4 muscle. These studies demonstrate specific abnormalities in myogenic cell number and behavior that may relate to the progression of disease in myotubularin deficiency, and may also be used to develop in vitro assays by which novel treatment strategies can be assessed.

MTM1 mutation associated with X-linked myotubular myopathy in Labrador Retrievers.

Mutations in the MTM1 gene encoding myotubularin cause X-linked myotubular myopathy (XLMTM), a well-defined subtype of human centronuclear myopathy. Seven male Labrador Retrievers, age 14-26 wk, were clinically evaluated for generalized weakness and muscle atrophy. Muscle biopsies showed variability in fiber size, centrally placed nuclei resembling fetal myotubes, and subsarcolemmal ringed and central dense areas highlighted with mitochondrial specific reactions. Ultrastructural studies confirmed the centrally located nuclei, abnormal perinuclear structure, and mitochondrial accumulations. Wild-type triads were infrequent, with most exhibiting an abnormal orientation of T tubules. MTM1 gene sequencing revealed a unique exon 7 variant in all seven affected males, causing a nonconservative missense change, p.N155K, which haplotype data suggest derives from a recent founder in the local population. Analysis of a worldwide panel of 237 unaffected Labrador Retrievers and 59 additional control dogs from 25 other breeds failed to identify this variant, supporting it as the pathogenic mutation. Myotubularin protein levels and localization were abnormal in muscles from affected dogs, and expression of GFP-MTM1 p.N155K in COS-1 cells showed that the mutant protein was sequestered in proteasomes, where it was presumably misfolded and prematurely degraded. These data demonstrate that XLMTM in Labrador Retrievers is a faithful genetic model of the human condition.

Inhibition of activin receptor type IIB increases strength and lifespan in myotubularin-deficient mice.

X-linked myotubular myopathy (XLMTM) is a congenital disorder caused by deficiency of the lipid phosphatase, myotubularin. Patients with XLMTM often have severe perinatal weakness that requires mechanical ventilation to prevent death from respiratory failure. Muscle biopsy specimens from patients with XLMTM exhibit small myofibers with central nuclei and central aggregations of organelles in many cells. It was postulated that therapeutically increasing muscle fiber size would cause symptomatic improvement in myotubularin deficiency. Recent studies have elucidated an important role for the activin-receptor type IIB (ActRIIB) in regulation of muscle growth and have demonstrated that ActRIIB inhibition results in significant muscle hypertrophy. To evaluate whether promoting muscle hypertrophy can attenuate symptoms resulting from myotubularin deficiency, the effect of ActRIIB-mFC treatment was determined in myotubularin-deficient (Mtm1δ4) mice. Compared with wild-type mice, untreated Mtm1δ4 mice have decreased body weight, skeletal muscle hypotrophy, and reduced survival. Treatment of Mtm1δ4 mice with ActRIIB-mFC produced a 17% extension of lifespan, with transient increases in weight, forelimb grip strength, and myofiber size. Pathologic analysis of Mtm1δ4 mice during treatment revealed that ActRIIB-mFC produced marked hypertrophy restricted to type 2b myofibers, which suggests that oxidative fibers in Mtm1δ4 animals are incapable of a hypertrophic response in this setting. These results support ActRIIB-mFC as an effective treatment for the weakness observed in myotubularin deficiency.

T-tubule disorganization and defective excitation-contraction coupling in muscle fibers lacking myotubularin lipid phosphatase.

Skeletal muscle contraction is triggered by the excitation-contraction (E-C) coupling machinery residing at the triad, a membrane structure formed by the juxtaposition of T-tubules and sarcoplasmic reticulum (SR) cisternae. The formation and maintenance of this structure is key for muscle function but is not well characterized. We have investigated the mechanisms leading to X-linked myotubular myopathy (XLMTM), a severe congenital disorder due to loss of function mutations in the MTM1 gene, encoding myotubularin, a phosphoinositide phosphatase thought to have a role in plasma membrane homeostasis and endocytosis. Using a mouse model of the disease, we report that Mtm1-deficient muscle fibers have a decreased number of triads and abnormal longitudinally oriented T-tubules. In addition, SR Ca(2+) release elicited by voltage-clamp depolarizations is strongly depressed in myotubularin-deficient muscle fibers, with myoplasmic Ca(2+) removal and SR Ca(2+) content essentially unaffected. At the molecular level, Mtm1-deficient myofibers exhibit a 3-fold reduction in type 1 ryanodine receptor (RyR1) protein level. These data reveal a critical role of myotubularin in the proper organization and function of the E-C coupling machinery and strongly suggest that defective RyR1-mediated SR Ca(2+) release is responsible for the failure of muscle function in myotubular myopathy.

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

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.

Vici syndrome associated with sensorineural hearing loss and evidence of neuromuscular involvement on muscle biopsy.

Vici syndrome is a rare, genetically unresolved congenital multisystem disorder comprising agenesis of the corpus callosum, cataracts, immunodeficiency, cardiomyopathy, and hypopigmentation. An associated neuromuscular phenotype has not previously been described in detail. We report on an infant with clinical features suggestive of Vici syndrome and additional sensorineural hearing loss. Muscle biopsy revealed several changes including markedly increased variability in fiber size, increased internal nuclei, and abnormalities on Gomori trichrome and oxidative stains, raising a wide differential diagnosis including neurogenic atrophy, centronuclear myopathy (CNM) or a metabolic (mitochondrial) cytopathy. Respiratory chain enzyme studies, however, were normal and sequencing of common CNM-associated genes did not reveal any mutations. This case expands the clinical spectrum of Vici syndrome and indicates that muscle biopsy ought to be considered in infants presenting with suggestive clinical features. In addition, we suggest that Vici syndrome is considered in the differential diagnosis of infants presenting with congenital callosal agenesis and that additional investigation has to address the possibility of associated ocular, auditory, cardiac, and immunologic involvement when this radiologic finding is present.

Centronuclear myopathy due to a de novo dominant mutation in the skeletal muscle ryanodine receptor (RYR1) gene.

Centronuclear myopathy is a genetically heterogeneous congenital myopathy. Whilst mutations in the myotubularin (MTM1) gene are implicated in the X-linked variant, mutations in the dynamin 2 (DNM2) gene have been recently associated with dominant inheritance. We report a 16-year-old girl with clinical features of a congenital myopathy and external ophthalmoplegia. Multiple central nuclei affecting up to 50% of fibres and central accumulation of oxidative enzyme stains were the most prominent findings on muscle biopsy obtained at 1 year. However, some core-like areas appeared on repeat biopsy 8 years later; in addition, muscle MRI was compatible with the pattern we previously reported in patients with mutations in the skeletal muscle ryanodine receptor (RYR1) gene. Mutational analysis identified a de novo dominant RYR1 missense mutation (c.12335C>T; Ser4112Leu) affecting a highly conserved domain of the protein. Our findings expand the phenotypical spectrum associated with RYR1 mutations and indicate that RYR1 screening should be considered in centronuclear myopathy patients without MTM1 or DNM2 mutations; muscle MRI may aid selection of appropriate genetic testing.

Genotype-phenotype correlations in X-linked myotubular myopathy.

X-linked myotubular myopathy is a severe congenital myopathy that presents in the neonatal period with profound hypotonia and an inability to establish spontaneous respiration. Usually death occurs in infancy from respiratory failure. However, there is phenotypic variability; a number of affected boys have achieved respiratory independence and become ambulatory. Disease-causing mutations have been identified throughout the MTM1 gene on Xq28. MTM1 encodes the protein myotubularin, which is expressed ubiquitously. The main objectives of this study were to establish whether the nature or site of the mutation in the MTM1 gene could predict severity of the disease and to investigate whether early intensive clinical intervention facilitated survival until spontaneous improvement occurred. An association was demonstrated between the presence of a non-truncating mutation of the MTM1 gene and the mild phenotype. However, many non-truncating mutations were also seen in association with the severe phenotype and these were not confined to recognized functional domains of the protein. This suggests that the use of mutation analysis to predict prognosis in the early period following diagnosis is limited. Unexpectedly, over 50 patients surviving for more than 1 year were identified in this study. Further information obtained on 40 of these cases revealed that 50% were receiving 24-h ventilatory support, while 27% were ventilated at night only. The high survival rate for this disorder therefore reflects intensive medical intervention without which the majority of these boys would not survive.

Gait characteristics in a canine model of X-linked myotubular myopathy.

X-linked myotubular myopathy (XLMTM) is a fatal pediatric disease where affected boys display profound weakness of the skeletal muscles. Possible therapies are under development but robust outcome measures in animal models are required for effective translation to human patients. We established a naturally-occurring canine model, where XLMTM dogs display clinical symptoms similar to those observed in humans. The aim of this study was to determine potential endpoints for the assessment of future treatments in this model. Video-based gait analysis was selected, as it is a well-established method of assessing limb function in neuromuscular disease and measures have been correlated to the patient's quality of life. XLMTM dogs (N = 3) and their true littermate wild type controls (N = 3) were assessed at 4-5 time points, beginning at 10 weeks and continuing through 17 weeks. Motion capture and an instrumented carpet were used separately to evaluate spatiotemporal and kinematic changes over time. XLMTM dogs walk more slowly and with shorter stride lengths than wild type dogs, and these differences became greater over time. However, there was no clear difference in angular measures between affected and unaffected dogs. These data demonstrate that spatiotemporal parameters capture functional changes in gait in an XLMTM canine model and support their utility in future therapeutic trials.

Gene therapy prolongs survival and restores function in murine and canine models of myotubular myopathy.

Loss-of-function mutations in the myotubularin gene (MTM1) cause X-linked myotubular myopathy (XLMTM), a fatal, congenital pediatric disease that affects the entire skeletal musculature. Systemic administration of a single dose of a recombinant serotype 8 adeno-associated virus (AAV8) vector expressing murine myotubularin to Mtm1-deficient knockout mice at the onset or at late stages of the disease resulted in robust improvement in motor activity and contractile force, corrected muscle pathology, and prolonged survival throughout a 6-month study. Similarly, single-dose intravascular delivery of a canine AAV8-MTM1 vector in XLMTM dogs markedly improved severe muscle weakness and respiratory impairment, and prolonged life span to more than 1 year in the absence of toxicity or a humoral or cell-mediated immune response. These results demonstrate the therapeutic efficacy of AAV-mediated gene therapy for myotubular myopathy in small- and large-animal models, and provide proof of concept for future clinical trials in XLMTM patients.

Myotubular myopathy and the neuromuscular junction: a novel therapeutic approach from mouse models.

Myotubular myopathy (MTM) is a severe congenital muscle disease characterized by profound weakness, early respiratory failure and premature lethality. MTM is defined by muscle biopsy findings that include centralized nuclei and disorganization of perinuclear organelles. No treatments currently exist for MTM. We hypothesized that aberrant neuromuscular junction (NMJ) transmission is an important and potentially treatable aspect of the disease pathogenesis. We tested this hypothesis in two murine models of MTM. In both models we uncovered evidence of a disorder of NMJ transmission: fatigable weakness, improved strength with neostigmine, and electrodecrement with repetitive nerve stimulation. Histopathological analysis revealed abnormalities in the organization, appearance and size of individual NMJs, abnormalities that correlated with changes in acetylcholine receptor gene expression and subcellular localization. We additionally determined the ability of pyridostigmine, an acetylcholinesterase inhibitor, to ameliorate aspects of the behavioral phenotype related to NMJ dysfunction. Pyridostigmine treatment resulted in significant improvement in fatigable weakness and treadmill endurance. In all, these results describe a newly identified pathological abnormality in MTM, and uncover a potential disease-modifying therapy for this devastating disorder.

Muscle-specific alternative splicing of myotubularin-related 1 gene is impaired in DM1 muscle cells.

The myotubularin-related 1 (MTMR1) gene belongs to a highly conserved family of eucaryotic phosphatases, with at least 11 members in humans. The founder member of this gene family, MTM1, is mutated in X-linked myotubular myopathy, a severe congenital disorder that affects skeletal muscle, and codes for myotubularin, a specific phosphatidylinositol 3-phosphate [PI(3)P] phosphatase. MTM1 and MTMR1 are adjacent on the X chromosome, and the corresponding proteins share 59% sequence identity. In the present study, we investigated the putative role of MTMR1 in myogenesis by analysing its expression pattern in muscle cells during differentiation and in skeletal muscle throughout development. We have identified three novel coding exons in the MTMR1 intron 2 that are conserved between mouse and human, are alternatively spliced, and give rise to six mRNA isoforms. One of the transcripts is muscle-specific and is induced during myogenesis both in vitro and in vivo, and represents the major isoform in adult skeletal muscle. We show that the two main MTMR1 protein muscular isoforms, like myotubularin, efficiently dephosphorylate PI(3)P in vitro. We have also analysed whether MTMR1 alternative splicing is affected in skeletal muscle cells derived from patients with congenital myotonic dystrophy (cDM1), in which mRNA splicing disturbances of specific genes are thought to constitute an important pathogenic mechanism. We found a striking reduction in the level of the muscle-specific isoform and the appearance of an abnormal MTMR1 transcript in differentiated cDM1 muscle cells in culture and in skeletal muscle from cDM1 patients. Our results suggest that MTMR1 plays a role in muscle formation and represents a novel target for abnormal mRNA splicing in myotonic dystrophy.

The lipid phosphatase myotubularin is essential for skeletal muscle maintenance but not for myogenesis in mice.

Myotubularin is a ubiquitously expressed phosphatase that acts on phosphatidylinositol 3-monophosphate [PI(3)P], a lipid implicated in intracellular vesicle trafficking and autophagy. It is encoded by the MTM1 gene, which is mutated in X-linked myotubular myopathy (XLMTM), a muscular disorder characterized by generalized hypotonia and muscle weakness at birth leading to early death of most affected males. The disease was proposed to result from an arrest in myogenesis, as the skeletal muscle from patients contains hypotrophic fibers with centrally located nuclei that resemble fetal myotubes. To understand the physiopathological mechanism of XLMTM, we have generated mice lacking myotubularin by homologous recombination. These mice are viable, but their lifespan is severely reduced. They develop a generalized and progressive myopathy starting at around 4 weeks of age, with amyotrophy and accumulation of central nuclei in skeletal muscle fibers leading to death at 6-14 weeks. Contrary to expectations, we show that muscle differentiation in knockout mice occurs normally. We provide evidence that fibers with centralized myonuclei originate mainly from a structural maintenance defect affecting myotubularin-deficient muscle rather than a regenerative process. In addition, we demonstrate, through a conditional gene-targeting approach, that skeletal muscle is the primary target of murine XLMTM pathology. These mutant mice represent animal models for the human disease and will be a valuable tool for understanding the physiological role of myotubularin.