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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., Graphical Abstract, X-linked centronuclear myopathy (XL-CNM) is a severe congenital disease targeted for therapeutic trials. The authors identified decreased plasma myostatin levels in XL-CNM patients and mice. Decreasing Dnm2 in mice blocked or postponed disease progression and resulted in an increase in circulating myostatin, supporting monitoring circulating myostatin in XL-CNM clinical trials.

Omics analyses are powerful methods to obtain an integrated view of complex biological processes, disease progression, or therapy efficiency. However, few studies have compared different disease forms and different therapy strategies to define the common molecular signatures representing the most significant implicated pathways. In this study, we used RNA sequencing and mass spectrometry to profile the transcriptomes and proteomes of mouse models for three forms of centronuclear myopathies (CNMs), untreated or treated with either a drug (tamoxifen), antisense oligonucleotides reducing the level of dynamin 2 (DNM2), or following modulation of DNM2 or amphiphysin 2 (BIN1) through genetic crosses. Unsupervised analysis and differential gene and protein expression were performed to retrieve CNM molecular signatures. Longitudinal studies before, at, and after disease onset highlighted potential disease causes and consequences. Main pathways in the common CNM disease signature include muscle contraction, regeneration and inflammation. The common therapy signature revealed novel potential therapeutic targets, including the calcium regulator sarcolipin. We identified several novel biomarkers validated in muscle and/or plasma through RNA quantification, western blotting, and enzyme-linked immunosorbent assay (ELISA) assays, including ANXA2 and IGFBP2. This study validates the concept of using multi-omics approaches to identify molecular signatures common to different disease forms and therapeutic strategies.

Classical dynamins are large GTPases regulating membrane and cytoskeleton dynamics, and they are linked to different pathological conditions ranging from neuromuscular diseases to encephalopathy and cancer. Dominant dynamin 2 (DNM2) mutations lead to either mild adult onset or severe autosomal dominant centronuclear myopathy (ADCNM). Our objectives were to better understand the pathomechanism of severe ADCNM and test a potential therapy. Here, we created the Dnm2SL/+ mouse line harboring the common S619L mutation found in patients with severe ADCNM and impairing the conformational switch regulating dynamin self-assembly and membrane remodeling. The Dnm2SL/+ mouse faithfully reproduces severe ADCNM hallmarks with early impaired muscle function and force, together with myofiber hypotrophy. It revealed swollen mitochondria lacking cristae as the main ultrastructural defect and potential cause of the disease. Patient analysis confirmed this structural hallmark. In addition, DNM2 reduction with antisense oligonucleotides after disease onset efficiently reverted locomotor and force defects after only 3 weeks of treatment. Most histological defects including mitochondria alteration were partially or fully rescued. Overall, this study highlights an efficient approach to revert the severe form of dynamin-related centronuclear myopathy. These data also reveal that the dynamin conformational switch is key for muscle function and should be targeted for future therapeutic developments., The dynamin 2 S619L mouse model displays defects in skeletal muscle that are rescued by reducing dynamin 2 protein levels with antisense oligonucleotide treatment.

Skeletal muscle development and regeneration are tightly regulated processes. How the intracellular organization of muscle fibers is achieved during these steps is unclear. Here, we focus on the cellular and physiological roles of amphiphysin 2 (BIN1), a membrane remodeling protein mutated in both congenital and adult centronuclear myopathies (CNM), that is ubiquitously expressed and has skeletal muscle-specific isoforms. We created and characterized constitutive muscle-specific and inducible Bin1 homozygous and heterozygous knockout mice targeting either ubiquitous or muscle-specific isoforms. Constitutive Bin1-deficient mice died at birth from lack of feeding due to a skeletal muscle defect. T-tubules and other organelles were misplaced and altered, supporting a general early role for BIN1 in intracellular organization, in addition to membrane remodeling. Although restricted deletion of Bin1 in unchallenged adult muscles had no impact, the forced switch from the muscle-specific isoforms to the ubiquitous isoforms through deletion of the in-frame muscle-specific exon delayed muscle regeneration. Thus, ubiquitous BIN1 function is necessary for muscle development and function, whereas its muscle-specific isoforms fine tune muscle regeneration in adulthood, supporting that BIN1 CNM with congenital onset are due to developmental defects, whereas later onset may be due to regeneration defects., Summary: The BIN1 gene, mutated in centronuclear myopathies, expresses ubiquitous and muscle-specific isoforms. It is shown here that ubiquitous isoforms are necessary for muscle development, whereas muscle-specific isoforms fine-tune muscle regeneration.

Centronuclear myopathies (CNM) are a group of severe muscle diseases for which no effective therapy is currently available. We have previously shown that reduction of the large GTPase DNM2 in a mouse model of the X-linked form, due to loss of myotubularin phosphatase MTM1, prevents the development of the skeletal muscle pathophysiology. As DNM2 is mutated in autosomal dominant forms, here we tested whether DNM2 reduction can rescue DNM2-related CNM in a knock-in mouse harboring the p.R465W mutation (Dnm2(RW/+)) and displaying a mild CNM phenotype similar to patients with the same mutation. A single intramuscular injection of adeno-associated virus-shRNA targeting Dnm2 resulted in reduction in protein levels 5 wk post injection, with a corresponding improvement in muscle mass and fiber size distribution, as well as an improvement in histopathological CNM features. To establish a systemic treatment, weekly i.p. injections of antisense oligonucleotides targeting Dnm2 were administered to Dnm2(RW/+)mice for 5 wk. While muscle mass, histopathology, and muscle ultrastructure were perturbed in Dnm2(RW/+)mice compared with wild-type mice, these features were indistinguishable from wild-type mice after reducing DNM2. Therefore, DNM2 knockdown via two different strategies can efficiently correct the myopathy due to DNM2 mutations, and it provides a common therapeutic strategy for several forms of centronuclear myopathy. Furthermore, we provide an example of treating a dominant disease by targeting both alleles, suggesting that this strategy may be applied to other dominant diseases.

Skeletal muscle development and regeneration are tightly regulated processes. How the intracellular organization of muscle fibers is achieved during these steps is unclear. Here we focus on the cellular and physiological roles of amphiphysin 2 (BIN1), a membrane remodeling protein mutated in both congenital and adult centronuclear myopathies, that is ubiquitously expressed and has skeletal muscle-specific isoforms. We created and characterized constitutive, muscle-specific and inducible Bin1 homozygous and heterozygous knockout mice targeting either ubiquitous or muscle-specific isoforms. Constitutive Bin1-deficient mice died at birth from lack of feeding due to a skeletal muscle defect. T-tubules and other organelles were misplaced and altered, supporting a general early role of BIN1 on intracellular organization in addition to membrane remodeling. Whereas restricted deletion of Bin1 in unchallenged adult muscles had no impact, the forced switch from the muscle-specific isoforms to the ubiquitous isoforms through deletion of the in-frame muscle–specific exon delayed muscle regeneration. Thus, BIN1 ubiquitous function is necessary for muscle development and function while its muscle-specific isoforms fine-tune muscle regeneration in adulthood, supporting that BIN1 centronuclear myopathy with congenital onset are due to developmental defects while later onset may be due to regeneration defects.

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 in vitro and in vivo several short hairpin RNA (shRNA) sequences that efficiently target Dnm2 mRNA. A single 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.

International audience; Regulation of skeletal muscle development and organization is a complex process that is not fully understood. Here, we focused on amphiphysin 2 (BIN1, also known as bridging integrator-1) and dynamin 2 (DNM2), two ubiquitous proteins implicated in membrane remodeling and mutated in centronuclear myopathies (CNMs). We generated Bin1-/- Dnm2+/- mice to decipher the physiological interplay between BIN1 and DNM2. While Bin1-/- mice die perinatally from a skeletal muscle defect, Bin1-/- Dnm2+/- mice survived at least 18 months, and had normal muscle force and intracellular organization of muscle fibers, supporting BIN1 as a negative regulator of DNM2. We next characterized muscle-specific isoforms of BIN1 and DNM2. While BIN1 colocalized with and partially inhibited DNM2 activity during muscle maturation, BIN1 had no effect on the isoform of DNM2 found in adult muscle. Together, these results indicate that BIN1 and DNM2 regulate muscle development and organization, function through a common pathway, and define BIN1 as a negative regulator of DNM2 in vitro and in vivo during muscle maturation. Our data suggest that DNM2 modulation has potential as a therapeutic approach for patients with CNM and BIN1 defects. As BIN1 is implicated in cancers, arrhythmia, and late-onset Alzheimer disease, these findings may trigger research directions and therapeutic development for these common diseases.

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.

X-linked centronuclear myopathy (XLMTM), also called myotubular myopathy, is a severe congenital myopathy characterized by generalized hypotonia and weakness at birth and the typical histological finding of centralization of myo-nuclei. It is caused by mutations in the MTM1 gene encoding the 3-phosphoinositides phosphatase myotubularin. Mutations in dynamin 2 and amphiphysin 2 genes lead to autosomal forms of centronuclear myopathy (CNM). While XLMTM is the most frequent and severe form of CNM, no mutations are found in about 30% of patients by sequencing all MTM1 exons. Moreover, the impact of MTM1 sequence variants is sometimes difficult to assess. It is thus important to devise a complete molecular diagnostic strategy that includes analysis of the myotubularin transcript and protein expression. We therefore developed novel antibodies against human myotubularin and showed that they are able to detect the endogenous protein by direct Western blot from muscle samples and from cultured cells. In conjunction with RT-PCR analysis we validated the consequences of missense and splice mutations on transcript integrity and protein level. We also detected and characterized a novel deep intronic mutation consisting of a single nucleotide change that induces exonisation of a conserved intronic sequence. Patients with centronuclear myopathy and no molecular diagnosis should be investigated for MTM1 defects at the cDNA and protein level.

International audience; 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.

International audience; Centronuclear myopathies (CNM) are congenital disorders associated with muscle weakness and abnormally located nuclei in skeletal muscle. An autosomal dominant form of CNM results from mutations in the gene encoding dynamin 2 (DNM2), and loss-of-function mutations in the gene encoding myotubularin (MTM1) result in X-linked CNM (XLCNM, also called myotubular myopathy), which promotes severe neonatal hypo-tonia and early death. Currently, no effective treatments exist for XLCNM. Here, we found increased DNM2 levels in XLCNM patients and a mouse model of XLCNM (Mtm1 –/y). Generation of Mtm1 –/y mice that were heterozygous for Dnm2 revealed that reduction of DNM2 in XLCNM mice restored life span, whole-body strength, and diaphragm function and increased muscle strength. Additionally, classic CNM-associated his-tological features, including fiber atrophy and nuclei mispositioning, were absent or reduced. Ultrastructur-al analysis revealed improvement of sarcomere organization and triad structures. Skeletal muscle–specific decrease of Dnm2 during embryogenesis or in young mice after disease onset revealed that the rescue associated with downregulation of Dnm2 is cell autonomous and is able to stop and potentially revert XLCNM progression. These data indicate that MTM1 and DNM2 regulate muscle organization and force through a common pathway. Furthermore, despite DNM2 being a key mechanoenzyme, its reduction is beneficial for XLCNM and represents a potential therapeutic approach for patients.

X-linked myotubular myopathy (XLMTM) is a severe congenital muscle disorder due to mutations in the MTM1 gene. The corresponding protein, myotubularin, contains the consensus active site of tyrosine phosphatases (PTP) but otherwise shows no homology to other phosphatases. Myotubularin is able to hydrolyze a synthetic analogue of tyrosine phosphate, in a reaction inhibited by orthovanadate, and was recently shown to act on both phosphotyrosine and phosphoserine. This gene is conserved down to yeast and strong homologies were found with human ESTs, thus defining a new dual specificity phosphatase (DSP) family. We report the presence of novel members of the MTM gene family in Schizosaccharomyces pombe, Caenorhabditis elegans, zebrafish, Drosophila, mouse and man. This represents the largest family of DSPs described to date. Eight MTM-related genes were found in the human genome and we determined the chromosomal localization and expression pattern for most of them. A subclass of the myotubularin homologues lacks a functional PTP active site. Missense mutations found in XLMTM patients affect residues conserved in a Drosophila homologue. Comparison of the various genes allowed construction of a phylogenetic tree and reveals conserved residues which may be essential for function. These genes may be good candidates for other genetic diseases.

Mutations in the phosphoinositide phos- phatase myotubularin (MTM1) results in X-linked myo- tubular/centronuclear myopathy (XLMTM), character- ized by a severe decrease in muscle mass and strength in patients and murine models. However, the molecular mechanism involved in the muscle hypotrophy is un- clear. Here we show that the IGF1R/Akt pathway is affected in Mtm1-deficient murine muscles, character- ized by an increase in IGF1 receptor and Akt levels in both the presymptomatic and symptomatic phases. Moreover, up-regulation of atrogenes was observed in the presymptomatic phase of the myopathy, supporting overactivation of the ubiquitin-proteasome pathway. In parallel, the autophagy machinery was affected as indi- cated by the increase in the number of autophagosomes and of autophagy markers, such as LC3 and P62. However, phosphorylation of FOXO3a and mTOR were abnormal at late but not at early stages of the disease, suggesting that myotubularin acts both up- stream in the IGF1R/Akt pathway and downstream on the balance between the autophagy and ubiquitin-pro- teasome pathways in vivo. Adeno-associated virus-medi- ated delivery of Mtm1 into Mtm1-null muscles rescued muscle mass and normalized the expression levels of IGF1 receptor, the ubiquitin-proteasome pathway, and autophagy markers. These data support the hypothesis that the unbalanced regulation of the ubiquitin protea- some pathway and the autophagy machinery is a pri- mary cause of the XLMTM pathogenesis.—Al-Qusairi, L., Prokic, I., Amoasii, L., Kretz, C., Messaddeq, N., Mandel, J.-L., Laporte, J. Lack of myotubularin (MTM1) leads to muscle hypotrophy through unbal- anced regulation of the autophagy and ubiquitin-pro- teasome pathways. FASEB J. 27, 000 - 000 (2013). www.fasebj.org

X-linked recessive myotubular myopathy (MTM1) is characterized by severe hypotonia and generalized muscle weakness, with impaired maturation of muscle fibres. We have restricted the candidate region to 280 kb and characterized two candidate genes using positional cloning strategies. The presence of frameshift or missense mutations (of which two are new mutations) in seven patients proved that one of these genes is indeed implicated in MTM1. The protein encoded by the MTM1 gene is highly conserved in yeast, which is surprising for a muscle specific disease. The protein contains the consensus sequence for the active site of tyrosine phosphatases, a wide class of proteins involved in signal transduction. At least three other genes, one located within 100 kb distal from the MTM1 gene, encode proteins with very high sequence similarities and define, together with the MTM1 gene, a new family of putative tyrosine phosphatases in man.

Adrenoleukodystrophy (ALD), the most frequent peroxisomal disorder, is a severe neurodegenerative disease associated with an impairment of very long chain fatty acids β-oxidation. We have recently identified by positional cloning the gene responsible for ALD, located in Xq28. It encodes a new member of the "ABC" superfamily of membrane-associated transporters that shows, in particular, significant homology to the 70-kDa peroxisomal membrane protein (PMP70). We report here a detailed characterization of the ALD gene structure. It extends over 21 kb and consists of 10 exons. To facilitate the detection of mutations in ALD patients, we have determined the intronic sequences flanking the exons as well as the sequence of the 3′ untranslated region and of the immediate 5′ promoter region. Sequences present in distal exons cross-hybridize strongly to additional sequences in the human genome. The ALD gene has been positioned on a pulsed-field map between DXS15 and the L1CAM gene, about 650 kb upstream from the color pigment genes. The frequent occurrence of color vision anomalies observed in patients with adrenomyeloneuropathy (the adult onset form of ALD) thus does not represent a contiguous gene syndrome but a secondary manifestation of ALD.

Adrenoleukodystrophy is a severe genetic demyelinating disease associated with an impairment of beta-oxidation of very long chain fatty acids (VLCFA) in peroxisomes. Earlier studies had suggested that a deficiency in VLCFA CoA synthetase was the primary defect. A candidate adrenoleukodystrophy gene has recently been cloned and was found unexpectedly to encode a putative ATP-binding cassette transporter. We have raised monoclonal antibodies against this protein, that detect a 75kDa band. This protein was absent in several patients with adrenoleukodystrophy. Immunofluorescence and immunoelectron microscopy showed that the adrenoleukodystrophy protein (ALDP) is associated with the peroxisomal membrane. Distinct immunofluorescence patterns were observed in cell lines from patients with Zellweger syndrome (a peroxisomal biogenesis disorder) belonging to different complementation groups.

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.

International audience; An 8-year old boy with genetically confirmed X-linked myotubular myopathy developed progressively worsening dementia and subclinical seizures at age 5-6 years. Previously, seizures or dementia have been noted in only a small number of myotubular myopathy patients, and only in association with significant metabolic disturbances. This patient had no evidence of hypoxemia or other metabolic disturbance. The present case suggests that the clinical spectrum of X-linked myotubular myopathy is broader than previously considered and may include mutation-dependent central nervous system disease.

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Centronuclear myopathies are characterized by muscle weakness and abnormal centralization of nuclei in muscle fibers not secondary to regeneration. The severe neonatal X-linked form (myotubular myopathy) is due to mutations in the phosphoinositide phosphatase myotubularin (MTM1), whereas mutations in dynamin 2 (DNM2) have been found in some autosomal dominant cases. By direct sequencing of functional candidate genes, we identified homozygous mutations in amphiphysin 2 (BIN1) in three families with autosomal recessive inheritance. Two missense mutations affecting the BAR (Bin1/amphiphysin/RVS167) domain disrupt its membrane tubulation properties in transfected cells, and a partial truncation of the C-terminal SH3 domain abrogates the interaction with DNM2 and its recruitment to the membrane tubules. Our results suggest that mutations in BIN1 cause centronuclear myopathy by interfering with remodeling of T tubules and/or endocytic membranes, and that the functional interaction between BIN1 and DNM2 is necessary for normal muscle function and positioning of nuclei.

International audience; 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.

International audience; In eukaryotic cells, phosphoinositides are lipid second messengers important for many cellular processes and have been found dysregulated in several human diseases. X-linked myotubular (centronuclear) myopathy is a severe congenital myopathy caused by mutations in a phosphatidylinositol 3-phosphate (PtdIns3P) phosphatase called myotubularin, and mutations in dominant centronuclear myopathy (CNM) cases were identified in the dynamin 2 gene. The genes mutated in autosomal recessive cases of CNMs have not been found. We have identified a novel phosphoinositide phosphatase (hJUMPY) conserved through evolution, which dephosphorylates the same substrates as myotubularin, PtdIns3P and PtdIns(3,5)P(2), in vitro and ex vivo. We found, in sporadic cases of CNMs, two missense variants that affect the enzymatic function. One of these appeared de novo in a patient also carrying a de novo mutation in the dynamin 2 gene. The other missense (R336Q) found in another patient changes the catalytic arginine residue of the core phosphatase signature present in protein tyrosine/dual-specificity phosphatases and in phosphoinositide phosphatases and drastically reduces the enzymatic activity both in vitro and in transfected cells. The inheritance of the phenotype with regard to this variant is still unclear and could be either recessive with an undetected second allele or digenic. We propose that impairment of hJUMPY function is implicated in some cases of autosomal CNM and that hJUMPY cooperates with myotubularin to regulate the level of phosphoinositides in skeletal muscle.

Dynamins are large GTPases implicated in membrane remodeling and cytoskeletal dynamics. Heterozygous mutations in dynamin 2 (DNM2) have been linked to autosomal dominant centronuclear myopathy (ADCNM) and Charcot–Marie–Tooth peripheral neuropathy (CMT), two discrete progressive neuromuscular disorders, highlighting the importance of dynamin 2 for normal axonal and muscle maintenance. We have now identified the first homozygous mutation in a dynamin protein, DNM2, leading to a congenital syndrome associating fetal hypokinesia and early death. Patient fibroblasts displayed reduced transferrin uptake, and this mutation impacted on DNM2 in vitro membrane tubulation function. Skeletal muscles were strongly affected in these patients, as in patients with dominant ADCNM myopathy, suggesting a key role for DNM2 in this tissue. To gain insight into the function of DNM2 in skeletal muscle, we first exogenously expressed wild-type DNM2 or R465W DNM2, the most common ADCNM mutation, into adult wild-type mouse skeletal muscle by intramuscular Adeno-Associated Virus (AAV) injections. Expression of R465W DNM2 led to the development of a centronuclear myopathy phenotype with abnormal organelle positioning. In addition, expression of both constructs reduced the specific maximal muscle force, suggesting that the ADCNM disease arises from an increased DNM2 function. As adult muscles were injected, it also suggested that DNM2 has an important role in the maintenance of muscle structure. To further decipher the function of DNM2 in skeletal muscle, we created DNM2 knockout mice. Complete knockout of DNM2 in all tissues was lethal before embryonic day 12. Therefore we developed a muscle-specific knockout mouse and have characterized muscle-specific isoforms of DNM2. KO mice were viable to birth and muscle defects led to death within the first weeks of life. Taken together, we conclude DNM2 plays important roles in both development and maintenance of skeletal muscle.

We have recently refined the localization of the myotubular myopathy (MTM1) gene to a 430-kb region between DXS304 and DXS1345 in proximal Xq28. We report two new polymorphic microsatellite markers, DXS8377 and DXS7423, that were physically mapped within the critical interval. A recombination event in a family segregating for MTM1 placed the disease gene telomeric to the trinucleotide polymorphism DXS8377. Together with the recent mapping of two microdeletions associated with MTM1, the recombination refines the critical region to 280 kb. A second recombination event was observed distal to the tetranucleotide repeat DXS7423. This recombination has occurred in the off-spring of a female with a more than 67% probability of being a carrier and very likely restricts the MTM1 gene to a 130-kb region. This physical refinement is significant for positional cloning of the disease gene. The highly polymorphic markers and the precise localization of the MTM1 gene will facilitate genetic diagnosis of the disorder.

Myotonic dystrophy (DM) is caused by the expansion of a CTG trinucleotide repeat. The mutation is in complete linkage disequilibrium with a nearly two-allele insertion/deletion polymorphism, suggesting a single origin for the mutation or predisposing mutation. To trace this-ancestral event, we have studied the association of CTG repeat alleles in a normal population to alleles of the insertion/deletion polymorphism and of a (CA)n repeat marker 90 kilobases from the DM mutation. The results strongly suggest that the initial predisposing event(s) consisted of a transition from a (CTG)5 allele to an allele with 19 to 30 repeats. The heterogeneous class of (CTG)19-30 alleles which has an overall frequency of about 10%, may constitute a reservoir for recurrent DM mutations.

The fragile X syndrome, the most common form of inherited mental retardation, is caused by mutations that increase the size of a specific DNA fragment of the X chromosome (in Xq27.3). Affected persons have both a full mutation and abnormal DNA methylation. Persons with a smaller increase in the size of this DNA fragment (a premutation) have little or no risk of retardation but are at high risk of having affected children or grandchildren. The passage from premutation to full-mutation status occurs only with transmission from the mother. We have devised a method of identifying carriers of these mutations by direct DNA analysis.We studied 511 persons from 63 families with the fragile X syndrome. Mutations and abnormal methylation were detected by Southern blotting with a probe adjacent to the mutation target. Analysis of EcoRI and EagI digests of DNA distinguished clearly in a single test between the normal genotype, the premutation, and the full mutation.DNA analysis unambiguously established the genetic status at the fragile X locus for all samples tested. This method was much more powerful and reliable than cytogenetic testing or segregation studies with closely linked polymorphic markers. The frequency of mental retardation in persons with premutations was similar to that in the general population, whereas all 103 males and 31 of 59 females with full mutations had mental retardation. About 15 percent of those with full mutations had some cells carrying only the premutation. All the mothers of affected children were carriers of either a premutation or a full mutation.Direct diagnosis by DNA analysis is now an efficient and reliable primary test for the diagnosis of the fragile X syndrome after birth, as well as for prenatal diagnosis and genetic counseling.

Nature Genetics 38, 1369–1371 (2006); published online 5 November; corrected after print 7 December 2006. In the version of this article initially published, the name of the sixth author (Agneta Nordenskjold) was missing. Agneta Nordenskjold is in the Department of Molecular Medicine and Surgery, Karolinska University Hospital, Stockholm, Sweden.

The fragile X syndrome, a common cause of inherited mental retardation, is characterized by an unusual mode of inheritance. Phenotypic expression has been linked to abnormal cytosine methylation of a single CpG island, at or very near the fragile site. Probes adjacent to this island detected very localized DNA rearrangements that constituted the fragile X mutations, and whose target was a 550-base pair GC-rich fragment. Normal transmitting males had a 150- to 400-base pair insertion that was inherited by their daughters either unchanged, or with small differences in size. Fragile X-positive individuals in the next generation had much larger fragments that differed among siblings and showed a generally heterogeneous pattern indicating somatic mutation. The mutated allele appeared unmethylated in normal transmitting males, methylated only on the inactive X chromosome in their daughters, and totally methylated in most fragile X males. However, some males had a mosaic pattern. Expression of the fragile X syndrome thus appears to result from a two-step mutation as well as a highly localized methylation. Carriers of the fragile X mutation can easily be detected regardless of sex or phenotypic expression, and rare apparent false negatives may result from genetic heterogeneity or misdiagnosis.