X-linked Centronuclear Myopathy Research Articles

Inactivating the lipid kinase activity of PI3KC2β is sufficient to rescue myotubular myopathy in mice.

Phosphoinositides (PIs) are membrane lipids that regulate signal transduction and vesicular trafficking. X-linked centronuclear myopathy (XLCNM), also called myotubular myopathy, results from loss-of-function mutations in the MTM1 gene, which encodes the myotubularin phosphatidylinositol 3-phosphate (PtdIns3P) lipid phosphatase. No therapy for this disease is currently available. Previous studies showed that loss of expression of the class II phosphoinositide 3-kinase (PI3K) PI3KC2β (PI3KC2B) protein improved the phenotypes of an XLCNM mouse model. PI3Ks are well known to have extensive scaffolding functions and the importance of the catalytic activity of this PI3K for rescue remains unclear. Here, using PI3KC2β kinase-dead mice, we show that the selective inactivation of PI3KC2β kinase activity is sufficient to fully prevent muscle atrophy and weakness, histopathology, and sarcomere and triad disorganization in Mtm1-knockout mice. This rescue correlates with normalization of PtdIns3P level and mTORC1 activity, a key regulator of protein synthesis and autophagy. Conversely, lack of PI3KC2β kinase activity did not rescue the histopathology of the BIN1 autosomal CNM mouse model. Overall, these findings support the development of specific PI3KC2β kinase inhibitors to cure myotubular myopathy.

Endosomal lipid signaling reshapes the endoplasmic reticulum to control mitochondrial function.

Cells respond to fluctuating nutrient supply by adaptive changes in organelle dynamics and in metabolism. How such changes are orchestrated on a cell-wide scale is unknown. We show that endosomal signaling lipid turnover by MTM1, a phosphatidylinositol 3-phosphate [PI(3)P] 3-phosphatase mutated in X-linked centronuclear myopathy in humans, controls mitochondrial morphology and function by reshaping the endoplasmic reticulum (ER). Starvation-induced endosomal recruitment of MTM1 impairs PI(3)P-dependent contact formation between tubular ER membranes and early endosomes, resulting in the conversion of ER tubules into sheets, the inhibition of mitochondrial fission, and sustained oxidative metabolism. Our results unravel an important role for early endosomal lipid signaling in controlling ER shape and, thereby, mitochondrial form and function to enable cells to adapt to fluctuating nutrient environments.

Antagonistic control of active surface integrins by myotubularin and phosphatidylinositol 3-kinase C2β in a myotubular myopathy model

X-linked centronuclear myopathy (XLCNM) is a severe human disease without existing therapies caused by mutations in the phosphoinositide 3-phosphatase MTM1. Loss of MTM1 function is associated with muscle fiber defects characterized by impaired localization of β-integrins and other components of focal adhesions. Here we show that defective focal adhesions and reduced active β-integrin surface levels in a cellular model of XLCNM are rescued by loss of phosphatidylinositiol 3-kinase C2β (PI3KC2β) function. Inactivation of the Mtm1 gene impaired myoblast differentiation into myotubes and resulted in reduced surface levels of active β1-integrins as well as corresponding defects in focal adhesions. These phenotypes were rescued by concomitant genetic loss of Pik3c2b or pharmacological inhibition of PI3KC2β activity. We further demonstrate that a hitherto unknown role of PI3KC2β in the endocytic trafficking of active β1-integrins rather than rescue of phosphatidylinositol 3-phosphate levels underlies the ability of Pik3c2b to act as a genetic modifier of cellular XLCNM phenotypes. Our findings reveal a crucial antagonistic function of MTM1 and PI3KC2β in the control of active β-integrin surface levels, thereby providing a molecular mechanism for the adhesion and myofiber defects observed in XLCNM. They further suggest specific pharmacological inhibition of PI3KC2β catalysis as a viable treatment option for XLCNM patients.

Can we talk about myoblast fusion?

Can we talk about myoblast fusion?

The intragenic microRNA miR199A1 in the dynamin 2 gene contributes to the pathology of X-linked centronuclear myopathy

Mutations in the myotubularin 1 (MTM1) gene can cause the fatal disease X-linked centronuclear myopathy (XLCNM), but the underlying mechanism is incompletely understood. In this report, using an Mtm1 -/y disease model, we found that expression of the intragenic microRNA miR-199a-1 is up-regulated along with that of its host gene, dynamin 2 (Dnm2), in XLCNM skeletal muscle. To assess the role of miR-199a-1 in XLCNM, we crossed miR-199a-1 -/- with Mtm1 -/y mice and found that the resultant miR-199a-1-Mtm1 double-knockout mice display markers of improved health, as evidenced by lifespans prolonged by 30% and improved muscle strength and histology. Mechanistic analyses showed that miR-199a-1 directly targets nonmuscle myosin IIA (NM IIA) expression and, hence, inhibits muscle postnatal development as well as muscle maturation. Further analysis revealed that increased expression and phosphorylation of signal transducer and activator of transcription 3 (STAT3) up-regulates Dnm2/miR-199a-1 expression in XLCNM muscle. Our results suggest that miR-199a-1 has a critical role in XLCNM pathology and imply that this microRNA could be targeted in therapies to manage XLCNM.

Myostatin: a Circulating Biomarker Correlating with Disease in Myotubular Myopathy Mice and Patients

Myotubular myopathy, also called X-linked centronuclear myopathy (XL-CNM), is a severe congenital disease targeted for therapeutic trials. To date, biomarkers to monitor disease progression and therapy efficacy are lacking. The Mtm1−/y mouse is a faithful model for XL-CNM, due to myotubularin 1 (MTM1) loss-of-function mutations. Using both an unbiased approach (RNA sequencing [RNA-seq]) and a directed approach (qRT-PCR and protein level), we identified decreased Mstn levels in Mtm1−/y muscle, leading to low levels of myostatin in muscle and plasma. Myostatin (Mstn or growth differentiation factor 8 [Gdf8]) is a protein released by myocytes and inhibiting muscle growth and differentiation. Decreasing Dnm2 by genetic cross with Dnm2+/− mice or by antisense oligonucleotides blocked or postponed disease progression and resulted in an increase in circulating myostatin. In addition, plasma myostatin levels inversely correlated with disease severity and with Dnm2 mRNA levels in muscles. Altered Mstn levels were associated with a generalized disruption of the myostatin pathway. Importantly, in two different forms of CNMs we identified reduced circulating myostatin levels in plasma from patients. This provides evidence of a blood-based biomarker that may be used to monitor disease state in XL-CNM mice and patients and supports monitoring circulating myostatin during clinical trials for myotubular myopathy.

The Dog Model in the Spotlight: Legacy of a Trustful Cooperation.

Dogs have long been used as a biomedical model system and in particular as a preclinical proof of concept for innovative therapies before translation to humans. A recent example of the utility of this animal model is the promising myotubularin gene delivery in boys affected by X-linked centronuclear myopathy after successful systemic, long-term efficient gene therapy in Labrador retrievers. Mostly, this is due to unique features that make dogs an optimal system. The continuous emergence of spontaneous inherited disorders enables the identification of reliable complementary molecular models for human neuromuscular disorders (NMDs). Dogs’ characteristics including size, lifespan and unprecedented medical care level allow a comprehensive longitudinal description of diseases. Moreover, the highly similar pathogenic mechanisms with human patients yield to translational robustness. Finally, interindividual phenotypic heterogeneity between dogs helps identifying modifiers and anticipates precision medicine issues.This review article summarizes the present list of molecularly characterized dog models for NMDs and provides an exhaustive list of the clinical and paraclinical assays that have been developed. This toolbox offers scientists a sensitive and reliable system to thoroughly evaluate neuromuscular function, as well as efficiency and safety of innovative therapies targeting these NMDs. This review also contextualizes the model by highlighting its unique genetic value, shaped by the long-term coevolution of humans and domesticated dogs. Because the dog is one of the most protected research animal models, there is considerable opposition to include it in preclinical projects, posing a threat to the use of this model. We thus discuss ethical issues, emphasizing that unlike many other models, the dog also benefits from its contribution to comparative biomedical research with a drastic reduction in the prevalence of morbid alleles in the breeding stock and an improvement in medical care.

Amphiphysin 2 modulation rescues myotubular myopathy and prevents focal adhesion defects in mice.

Centronuclear myopathies (CNMs) are severe diseases characterized by muscle weakness and myofiber atrophy. Currently, there are no approved treatments for these disorders. Mutations in the phosphoinositide 3-phosphatase myotubularin (MTM1) are responsible for X-linked CNM (XLCNM), also called myotubular myopathy, whereas mutations in the membrane remodeling Bin/amphiphysin/Rvs protein amphiphysin 2 [bridging integrator 1 (BIN1)] are responsible for an autosomal form of the disease. Here, we investigated the functional relationship between MTM1 and BIN1 in healthy skeletal muscle and in the physiopathology of CNM. Genetic overexpression of human BIN1 efficiently rescued the muscle weakness and life span in a mouse model of XLCNM. Exogenous human BIN1 expression with adeno-associated virus after birth also prevented the progression of the disease, suggesting that human BIN1 overexpression can compensate for the lack of MTM1 expression in this mouse model. Our results showed that MTM1 controls cell adhesion and integrin localization in mammalian muscle. Alterations in this pathway in Mtm1 -/y mice were associated with defects in myofiber shape and size. BIN1 expression rescued integrin and laminin alterations and restored myofiber integrity, supporting the idea that MTM1 and BIN1 are functionally linked and necessary for focal adhesions in skeletal muscle. The results suggest that BIN1 modulation might be an effective strategy for treating XLCNM.

Centronuclear myopathies under attack: A plethora of therapeutic targets.

Centronuclear myopathies are a group of congenital myopathies characterized by severe muscle weakness, genetic heterogeneity, and defects in the structural organization of muscle fibers. Their names are derived from the central position of nuclei on biopsies, while they are at the fiber periphery under normal conditions. No specific therapy exists yet for these debilitating diseases. Mutations in the myotubularin phosphoinositides phosphatase, the GTPase dynamin 2, or amphiphysin 2 have been identified to cause respectively X-linked centronuclear myopathies (also called myotubular myopathy) or autosomal dominant and recessive forms. Mutations in additional genes, as RYR1, TTN, SPEG or CACNA1S, were linked to phenotypes that can overlap with centronuclear myopathies. Numerous animal models of centronuclear myopathies have been studied over the last 15 years, ranging from invertebrate to large mammalian models. Their characterization led to a partial understanding of the pathomechanisms of these diseases and allowed the recent validation of therapeutic proof-of-concepts. Here, we review the different therapeutic strategies that have been tested so far for centronuclear myopathies, some of which may be translated to patients.

CONGENITAL MYOPATHIES (CNM): P.147Novel SPEG mutations in congenital myopathy without centralized nuclei

Congenital myopathies are clinically and genetically heterogeneous disorders characterized by distinctive morphological abnormalities in skeletal muscle fibers. Several genes have already been linked to congenital myopathies. However, about half of the patients do not have a genetic diagnosis. Within the 'Myocapture' project, a French consortium with the aim to better characterize the genetic data of patients, we studied a patient with molecularly unexplained muscle weakness. At birth, the patient presented with hypotonia and poor sucking. Motor milestones were delayed and cognitive involvement was normal. Reduced myocardial contraction appeared at age 5 and normalized with treatment. At 9 years of age, the patient presented with proximal and distal muscle weakness and facial weakness without ptosis. Morphological studies of muscle biopsy revealed mild fiber size heterogeneity as main histological change. Biallelic truncating SPEG mutations were identified by exome sequencing. SPEG encodes the striated muscle preferentially expressed protein kinase, that interacts with myotubularin (MTM1), a protein mutated in X-linked centronuclear myopathy (CNM). Biallelic SPEG mutations were recently described as mutated in patients with CNM associated in most cases with dilated cardiomyopathy. Conversely, in our patient, muscle biopsy did not reveal internalized nuclei and our patient did not develop dilated cardiomyopathy. Overall, we describe two novel SPEG mutations in a patient with a mild myopathy characterized by muscle weakness and fiber size heterogeneity on muscle biopsy. This study expands the spectrum of diseases associated with mutations in SPEG, previously described in patients with CNM. Specific SPEG mutations can induce a severe CNM-like or, as described in our study, a rather moderate congenital myopathy without central nuclei.

F40. Myotonic discharges in a cohort of patients with centronuclear myopathies

Electromyography (EMG) is a useful ancillary test in the diagnosis of neuromuscular disorders. EMG patterns may be helpful to identify specific subtypes of inherited muscle disease. There are few studies looking at the EMG profile in congenital myopathies, and centronuclear myopathy (CNM) in particular. The objective of this study is to describe clinical and EMG findings in patients with CNM from a tertiary Brazilian university hospital. We selected all patients with pathological confirmation (muscle biopsy) of CNM and regularly followed at UNICAMP (State University of Campinas), from 2016 to 2017. Individuals with X-linked CNM were not included. For each patient, we reviewed nerve conduction studies (NCS) and EMG findings at rest and during contraction of upper and lower limbs. We looked specifically at the presence of myotonic discharges. The total score of motor function measure scale (MFM32) was employed as a clinical examination marker of disease severity. Eleven patients were enrolled in this study. There were six children, seven females, mean age 18.1 years when first examined in our service; a boy with 4-years-old, was the youngest and a 54-year-old lady was the oldest. The mean/median of MFM32 total score was 65.0%.The motor and sensory NCS were normal for all nerves tested. Regarding EMG and the occurrence of abnormal spontaneous activity: the myotonic discharges, when present, predominated in distal muscles: tibialis anterior and abductor pollicis brevis, and all occurred at rest. During activation, all muscles presented short duration and polyphasic motor unit action potentials without proximal or distal preferential involvement. None of the patients had clinical myotonia (either percussion or action), but electrical myotonia was found in 6 out of the 11 patients (54%). Five women and one men had clinical phenotype similar to juvenile or classical Steinert disease (DM1). Two patients had negative genetic test for the disease, and in one of the was found a mutation in the DNM2 gene. We do highlight that two patients were followed up for decades as DM1 until properly diagnosed as CNM at our hospital. CNM must be included in the differential diagnosis of a patient with electrical, but not clinical myotonia. Clinical neurophysiologists must be aware of this profile to assist in the recognition of CNM. The frequency of abnormal spontaneous activity at EMG is similar to other studies.

Single Intramuscular Injection of AAV-shRNA Reduces DNM2 and Prevents Myotubular Myopathy in Mice.

Myotubular myopathy, or X-linked centronuclear myopathy, is a severe muscle disorder representing a significant burden for patients and their families. It is clinically characterized by neonatal and severe muscle weakness and atrophy. Mutations in the myotubularin (MTM1) gene cause myotubular myopathy, and no specific curative treatment is available. We previously found that dynamin 2 (DNM2) is upregulated in both Mtm1 knockout and patient muscle samples, whereas its reduction through antisense oligonucleotides rescues the clinical and histopathological features of this myopathy in mice. Here, we propose a novel approach targeting Dnm2 mRNA. We screened and validated invitro and invivo several short hairpin RNA (shRNA) sequences that efficiently target Dnm2 mRNA. Asingle intramuscular injection of AAV-shDnm2 resulted in long-term reduction of DNM2 protein level and restored muscle force, mass, histology, and myofiber ultrastructure and prevented molecular defects linked to the disease. Our results demonstrate a robust DNM2 knockdown and provide an alternative strategy based on reduction of DNM2 to treat myotubular myopathy.

TH.O.19 - Tamoxifen increases survival, improves motor function and reduces levels of BIN1 and DNM2 in a mouse model of X-linked centronuclear (myotubular) myopathy

X-Linked centronuclear myopathy (XLCNM) is a rare and severe congenital myopathy characterised by generalised muscle weakness and abnormal nuclei positioning. Most affected boys die in their first year of life and survivors fail to achieve independent ambulation. It is caused by mutations in the Mtm1 gene encoding myotubularin, a ubiquitously expressed phosphoinositide phosphatase. No cure exists and very few pharmacological avenues are being explored. Here, we treated Mtm1-null mice with tamoxifen (TAM), a drug that modulates estrogen actions and that we have shown earlier to be efficacious in dystrophic (mdx5Cv) mice, a model of Duchenne muscular dystrophy (DMD). We report that TAM is also effective in Mtm1-null mice, a model of XLCNM. Wild type and Mtm1-null mice were given normal chow or a TAM-supplemented chow starting at weaning. Non-treated Mtm1-null mice died at around 40 days. By contrast, about half of the Mtm1-null mice treated with clinically relevant doses of TAM survived beyond 365 days of age. Clinical scoring showed that the motor function of the affected mice was markedly improved. In vivo force recordings performed at D40 and D80 revealed that the force of treated Mtm1-null was significantly improved after only 3 weeks of treatment. Histological and electron microscopy analyses show partial rescue of muscle structure and triads, consistent with improved calcium homeostasis in FDB fibres. Quantitative PCR and western blots demonstrate reduction of BIN1 and DNM2, which act downstream of MTM1. In conclusion, we found that tamoxifen extends the lifespan of Mtm1-null mice up to 10-fold and rescues their motor skills. Collectively, these findings suggest that estrogen signalling is a key pathway that modifies disease severity in unrelated myopathies as diverse as DMD and XLCNM. Tamoxifen is safe and readily available. We believe that it deserves clinical evaluation for XLCNM.

P.253 - Clinical orphan patient pool methodology estimates current patient pool in centronuclear myopathy

Centronuclear myopathy (CNM) is a rare disease of inherited neuromuscular disorders, including X-linked CNM (XLCNM), autosomal dominant and recessive CNM, RYR1-related CNM. Review of the epidemiologic reports yielded limited incidence, prevalence and survival rate data. High variability between geographic regions and per disease etiology and severity resulted in limited insights in the current patient population. Currently, an incidence of 1 in 50000 births has been reported for severe, male XLCNM, and pediatric point prevalence of CNM has been estimated to be 1 in 100000, both known to underestimate the current pool. In support of clinical trial development, an integrated model linking the independent reports is required to obtain a better estimate of CNM patient numbers in function of age, etiology, and severity. To build the model, first an adapted CNM incidence estimate was calculated including moderate and mild XLCNM, female XLCNM patients and other etiologies (e.g. DNM2, BIN1, RYR1). Second, based on the analysis of the observed differences between the limited epidemiologic data, an estimate of current survival rates for each subpopulation was postulated. Next, the model integrates the specific relationships of reported incidence and prevalence data resulting in a patient flow based on the estimated survival for each subpopulation. The model was validated with reported subgroup prevalence and survival data. This methodology resulted in a model generating an estimate of the CNM pool per age, disease etiology and severity and per geography. The model estimates the incidence of the entire CNM population to be twofold the current reports on severe XLCNM alone and current patient numbers (2500 to 3000 in US and 1000 to 1500 in EU). In an orphan disease setting, this modelling methodology allows for a comprehensive assessment of the current CNM patient population, clinical trial development based on unmet need and fuels further epidemiologic discussion

P.251 - Expression of the neuropathy-associated MTMR2 gene rescues MTM1-associated myopathy

Myotubularins 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 myotubularin (MTM1) cause the severe congenital myopathy called myotubular myopathy (or X-linked centronuclear myopathy) while mutations in the myotubularin-related protein MTMR2 cause Charcot-Marie-Tooth peripheral neuropathy (CMT4B1). 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 myotubularins 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 as MTM1 in yeast and mice. Moreover, adeno-associated virus (AAV)-mediated exogenous expression of several MTMR2 isoforms ameliorates the myopathic phenotype due to MTM1 loss, with increased muscle force, reduced myofiber atrophy, and reduction of the intracellular disorganization hallmarks associated to myotubular myopathy. Noteworthy, the short MTMR2 isoform provided a better rescue when compared to the long MTMR2 isoform. In conclusion, these results point to the molecular basis for myotubularins 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.

P.262 - Antisense oligonucleotide-mediated Dnm2 knockdown delays myotubular myopathy in mice after a single injection

Centronuclear myopathies (CNM) are non-dystrophic muscle diseases for which no effective therapy is currently available. The most severe form, X-linked CNM, is caused by myotubularin 1 (MTM1) loss-of-function mutations, while the main autosomal dominant form is due to dynamin2 (DNM2) mutations. We previously showed that genetic reduction of DNM2 expression in Mtm1 knockout (Mtm1KO) mice prevents development of muscle pathology. Most recently we have shown that weekly systemic delivery of Dnm2 antisense oligonucleotides (ASOs) into Mtm1KO mice efficiently reduces DNM2 protein level in muscle and prevents the myopathy from developing. In addition, systemic weekly ASO injection into severely affected mice leads to reversal of muscle pathology within two weeks. Here we perform a single injection of ASO into 3-week-old Mtm1KO mice, and complete clinical, histological and molecular analysis of the duration of the effects after a single injection. A single injection of 25 mg/kg of ASO targeting DNM2 increased the lifespan, whole body strength, and reduced disease severity in Mtm1KO mice compared to untreated controls. Despite these results, a single injection alone was not sufficient to rescue the disease for the lifespan of the mice, suggesting repeated treatments will be required. Thus, ASO-mediated DNM2 knockdown can efficiently correct muscle defects due to loss of MTM1, providing an attractive therapeutic strategy for this disease.

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.

Antisense oligonucleotide-mediated Dnm2 knockdown prevents and reverts myotubular myopathy in mice

Centronuclear myopathies (CNM) are non-dystrophic muscle diseases for which no effective therapy is currently available. The most severe form, X-linked CNM, is caused by myotubularin 1 (MTM1) loss-of-function mutations, while the main autosomal dominant form is due to dynamin2 (DNM2) mutations. We previously showed that genetic reduction of DNM2 expression in Mtm1 knockout (Mtm1KO) mice prevents development of muscle pathology. Here we show that systemic delivery of Dnm2 antisense oligonucleotides (ASOs) into Mtm1KO mice efficiently reduces DNM2 protein level in muscle and prevents the myopathy from developing. Moreover, systemic ASO injection into severely affected mice leads to reversal of muscle pathology within 2 weeks. Thus, ASO-mediated DNM2 knockdown can efficiently correct muscle defects due to loss of MTM1, providing an attractive therapeutic strategy for this disease.

Exome sequencing is a valuable approach in critically ill patients with suspected monogenic disease: Diagnosis of X-linked centronuclear myopathy in preterm twins

Exome sequencing is a valuable approach in critically ill patients with suspected monogenic disease: Diagnosis of X-linked centronuclear myopathy in preterm twins

Expression of myotubularins in blood platelets: Characterization and potential diagnostic of X-linked myotubular myopathy

Phosphoinositides play a key role in the spatiotemporal control of central intracellular processes and several specific kinases and phosphatases regulating the level of these lipids are implicated in human diseases. Myotubularins are a family of 3-phosphatases acting specifically on phosphatidylinositol 3-monophosphate and phosphatidylinositol 3,5 bisphosphate. Members of this family are mutated in genetic diseases including myotubularin 1 (MTM1) and myotubularin-related protein 2 (MTMR2) which mutations are responsible of X-linked centronuclear myopathy and Charcot-Marie-Tooth neuropathy, respectively. Here we show that MTM1 is expressed in blood platelets and that hundred microliters of blood is sufficient to detect the protein by western blotting. Since the most severe cases of pathogenic mutations of MTM1 lead to loss of expression of the protein, we propose that a minimal amount of blood can allow a rapid diagnostic test of X-linked myotubular myopathy, which is currently based on histopathology of muscle biopsy and molecular genetic testing. In platelets, MTM1 is a highly active 3-phosphatase mainly associated to membranes and found on the dense granules and to a lesser extent on alpha-granules. However, deletion of MTM1 in mouse had no significant effect on platelet count and on platelet secretion and aggregation induced by thrombin or collagen stimulation. Potential compensation by other members of the myotubularin family is conceivable since MTMR2 was easily detectable by western blotting and the mRNA of several members of the family increased during in vitro differentiation of human megakaryocytes and MEG-01 cells. In conclusion, we show the presence of several myotubularins in platelets and propose that minimal amounts of blood can be used to develop a rapid diagnostic test for genetic pathologies linked to loss of expression of these phosphatases.