Myotubular myopathy: arrest of morphogenesis of myofibres associated with persistence of fetal vimentin and desmin. Four cases compared with fetal and neonatal muscle

Common Pathogenic Mechanisms in Centronuclear and Myotubular Myopathies and Latest Treatment Advances

Centronuclear myopathies (CNM) are rare congenital disorders characterized by muscle weakness and structural defects including fiber hypotrophy and organelle mispositioning. The main CNM forms are caused by mutations in: the MTM1 gene encoding the phosphoinositide phosphatase myotubularin (myotubular myopathy), the DNM2 gene encoding the mechanoenzyme dynamin 2, the BIN1 gene encoding the membrane curvature sensing amphiphysin 2, and the RYR1 gene encoding the skeletal muscle calcium release channel/ryanodine receptor. MTM1, BIN1, and DNM2 proteins are involved in membrane remodeling and trafficking, while RyR1 directly regulates excitation-contraction coupling (ECC). Several CNM animal models have been generated or identified, which confirm shared pathological anomalies in T-tubule remodeling, ECC, organelle mispositioning, protein homeostasis, neuromuscular junction, and muscle regeneration. Dynamin 2 plays a crucial role in CNM physiopathology and has been validated as a common therapeutic target for three CNM forms. Indeed, the promising results in preclinical models set up the basis for ongoing clinical trials. Another two clinical trials to treat myotubular myopathy by MTM1 gene therapy or tamoxifen repurposing are also ongoing. Here, we review the contribution of the different CNM models to understanding physiopathology and therapy development with a focus on the commonly dysregulated pathways and current therapeutic targets.

Tamoxifen prolongs survival and alleviates symptoms in mice with fatal X-linked myotubular myopathy

X-linked myotubular myopathy (XLMTM, also known as XLCNM) is a severe congenital muscular disorder due to mutations in the myotubularin gene, MTM1. It is characterized by generalized hypotonia, leading to neonatal death of most patients. No specific treatment exists. Here, we show that tamoxifen, a well-known drug used against breast cancer, rescues the phenotype of Mtm1-deficient mice. Tamoxifen increases lifespan several-fold while improving overall motor function and preventing disease progression including lower limb paralysis. Tamoxifen corrects functional, histological and molecular hallmarks of XLMTM, with improved force output, myonuclei positioning, myofibrillar structure, triad number, and excitation-contraction coupling. Tamoxifen normalizes the expression level of the XLMTM disease modifiers DNM2 and PI3KC2B, likely contributing to the phenotypic rescue. Our findings demonstrate that tamoxifen is a promising candidate for clinical evaluation in XLMTM patients.

Cellular, biochemical and molecular changes in muscles from patients with X-linked myotubular myopathy due to MTM1 mutations

Centronuclear myopathies are early-onset muscle diseases caused by mutations in several genes including MTM1, DNM2, BIN1, RYR1 and TTN. The most severe and often fatal X-linked form of myotubular myopathy (XLMTM) is caused by mutations in the gene encoding the ubiquitous lipid phosphatase myotubularin, an enzyme specifically dephosphorylating phosphatidylinositol-3-phosphate and phosphatidylinositol-3,5-bisphosphate. Because XLMTM patients have a predominantly muscle-specific phenotype a number of pathogenic mechanisms have been proposed, including a direct effect of the accumulated lipid on the skeletal muscle calcium channel ryanodine receptor 1, a negative effect on the structure of intracellular organelles and defective autophagy. Animal models knocked out for MTM1 show severe reduction of ryanodine receptor 1 mediated calcium release but, since knocking out genes in animal models does not necessarily replicate the human phenotype, we considered it important to study directly the effect of MTM1 mutations on patient muscle cells. The results of the present study show that at the level of myotubes MTM1 mutations do not dramatically affect calcium homeostasis and calcium release mediated through the ryanodine receptor 1, though they do affect myotube size and nuclear content. On the other hand, mature muscles such as those obtained from patient muscle biopsies exhibit a significant decrease in expression of the ryanodine receptor 1, a decrease in muscle-specific microRNAs and a considerable up-regulation of histone deacetylase-4. We hypothesize that the latter events consequent to the primary genetic mutation, are the cause of the severe decrease in muscle strength that characterizes these patients.

Extensive morphological and immunohistochemical characterization in myotubular myopathy

The X-linked myotubular myopathy (XLMTM) also called X-linked centronuclear myopathy is a rare congenital myopathy due to mutations in the MTM 1 gene encoding myotubularin. The disease gives rise to a severe muscle weakness in males at birth. The main muscle morphological characteristics (significant number of small muscle fibers with centralized nuclei and type 1 fiber predominance) are usually documented, but the sequence of formation and maintenance of this particular morphological pattern has not been extensively characterized in humans. In this study, we perform a reevaluation of morphological changes in skeletal muscle biopsies in severe XLMTM. We correlate the pathologic features observed in the muscle biopsies of 15 newborns with MTM 1-mutations according to the "adjusted-age" at the time of muscle biopsy, focusing on sequential analysis in the early period of the life (from 34 weeks of gestation to 3 months of age). We found a similar morphological pattern throughout the period analyzed; the proportion of myofibers with central nuclei was high in all muscle biopsies, independently of the muscle type, the age of the newborns at time of biopsy and the specific MTM 1 mutation. We did not observe a period free of morphological abnormalities in human skeletal muscle as observed in myotubularin-deficient mouse models. In addition, this study demonstrated some features of delayed maturation of the muscle fibers without any increase in the number of satellite cells, associated with a marked disorganization of the muscle T-tubules and cytoskeletal network in the skeletal muscle fibers.

The CMT4B disease-causing proteins MTMR2 and MTMR13/SBF2 regulate AKT signalling

Charcot-Marie-Tooth disease type 4B is caused by mutations in the genes encoding either the lipid phosphatase myotubularin-related protein-2 (MTMR2) or its regulatory binding partner MTMR13/SBF2. Mtmr2 dephosphorylates PI-3-P and PI-3,5-P2 to form phosphatidylinositol and PI-5-P, respectively, while Mtmr13/Sbf2 is an enzymatically inactive member of the myotubularin protein family. We have found altered levels of the critical signalling protein AKT in mouse mutants for Mtmr2 and Mtmr13/Sbf2. Thus, we analysed the influence of Mtmr2 and Mtmr13/Sbf2 on signalling processes. We found that overexpression of Mtmr2 prevents the degradation of the epidermal growth factor receptor (EGFR) and leads to sustained Akt activation whereas Erk activation is not affected. Mtmr13/Sbf2 counteracts the blockage of EGFR degradation without affecting prolonged Akt activation. Our data indicate that Mtmr2 and Mtmr13/Sbf2 play critical roles in the sorting and modulation of cellular signalling which are likely to be disturbed in CMT4B.

Endplate structure and parameters of neuromuscular transmission in sporadic centronuclear myopathy associated with myasthenia

Centronuclear myopathy is a pathologically diagnosed congenital myopathy. The disease genes encode proteins with membrane modulating properties (MTM1, DNM2, and BIN1) or alter excitation-contraction coupling (RYR1). Some patients also have myasthenic symptoms but electrodiagnostic and endplate studies in these are limited. A sporadic patient had fatigable weakness and a decremental EMG response. Analysis of centronuclear myopathy disease- and candidate-genes identified no mutations. Quantitative endplate electron microscopy studies revealed simplified postsynaptic regions, endplate remodeling with normal nerve terminal size, normal synaptic vesicle density, and mild acetylcholine receptor deficiency. The amplitude of the miniature endplate potential was decreased to 60% of normal. Quantal release by nerve impulse was reduced to 40% of normal due to a decreased number of releasable quanta. The safety margin of neuromuscular transmission is compromised by decreased quantal release by nerve impulse and by a reduced postsynaptic response to the released quanta.

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.

Pathological defects in congenital myopathies

Congenital myopathies are a molecularly, pathologically and clinically heterogenous group of disorders defined by hypotonia and muscle weakness, that usually present at birth or early childhood, in association with a characteristic morphological defect. The most common morphological defects are nemaline rods, cores of varying size, central nuclei, and type I fibre hypotrophy, with or without an additional abnormality. The defective genes responsible for many of the congenital myopathies are known, but there is considerable clinico-pathological overlap. In particular, defects in more than one gene are associated with the presence of the same pathological feature, while defects in the same gene can result in more than one pathological feature. Understanding the complexities of these spectra is paramount to the elucidation of pathogenesis, and to the development of therapies.

Centronuclear (myotubular) myopathy

Centronuclear myopathy (CNM) is an inherited neuromuscular disorder characterised by clinical features of a congenital myopathy and centrally placed nuclei on muscle biopsy.

The incidence of X-linked myotubular myopathy is estimated at 2/100000 male births but epidemiological data for other forms are not currently available.

The clinical picture is highly variable. The X-linked form usually gives rise to a severe phenotype in males presenting at birth with marked weakness and hypotonia, external ophthalmoplegia and respiratory failure. Signs of antenatal onset comprise reduced foetal movements, polyhydramnios and thinning of the ribs on chest radiographs; birth asphyxia may be the present. Affected infants are often macrosomic, with length above the 90^th centile and large head circumference. Testes are frequently undescended. Both autosomal-recessive (AR) and autosomal-dominant (AD) forms differ from the X-linked form regarding age at onset, severity, clinical characteristics and prognosis. In general, AD forms have a later onset and milder course than the X-linked form, and the AR form is intermediate in both respects.

Mutations in the myotubularin (MTM1) gene on chromosome Xq28 have been identified in the majority of patients with the X-linked recessive form, whilst AD and AR forms have been associated with mutations in the dynamin 2 (DNM2) gene on chromosome 19p13.2 and the amphiphysin 2 (BIN1) gene on chromosome 2q14, respectively. Single cases with features of CNM have been associated with mutations in the skeletal muscle ryanodine receptor (RYR1) and the hJUMPY (MTMR14) genes.

Diagnosis is based on typical histopathological findings on muscle biopsy in combination with suggestive clinical features; muscle magnetic resonance imaging may complement clinical assessment and inform genetic testing in cases with equivocal features. Genetic counselling should be offered to all patients and families in whom a diagnosis of CNM has been made.

The main differential diagnoses include congenital myotonic dystrophy and other conditions with severe neonatal hypotonia.

Management of CNM is mainly supportive, based on a multidisciplinary approach. Whereas the X-linked form due to MTM1 mutations is often fatal in infancy, dominant forms due to DNM2 mutations and some cases of the recessive BIN1-related form appear to be associated with an overall more favourable prognosis.

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.

A newborn with spinal muscular atrophy type 0 presenting with a clinicopathological picture suggestive of myotubular myopathy

We report a male term newborn with genetically confirmed spinal muscular atrophy type 0, presenting with arthrogryposis and severe generalized weakness and requiring ventilatory support. Muscle biopsy revealed fibers with central nuclei resembling myotubes and negative myotubularin immunohistochemical staining compared with a control muscle biopsy. The absence of myotubularin associated with survival motor neuron protein deficiency suggests that survival motor neuron protein may have a role in muscle fiber maturation and myotubularin expression. Studying the pathology of this rare and lethal neonatal form of spinal muscular atrophy may further our understanding of spinal muscular atrophy pathogenesis.

[Myotubular myopathy. Case report and review of the literature]

The first Hungarian report of a case of myotubular myopathy is presented here, which is a recessive congenital disorder linked to X chromosome. The patient presented at birth with severe hypotonia, weak spontaneous movements, arthrogryposis and respiratory insufficiency. The biopsy showed the appearance of myotubular myopathy. The diagnosis was further confirmed by genetic analysis revealing a novel frameshift mutation (1314-1315insT) of the myotubularin-coding MTM1 gene.

Myofiber size correlates with MTM1 mutation type and outcome in X-linked myotubular myopathy

We aimed to correlate pathologic findings with MTM1 mutation type in a series of molecularly defined XLMTM cases. Clinical data from 15 XLMTM patients and their corresponding 16 muscle biopsies were studied. All patients were infants (range: 6-217 days old) when initially biopsied. The proportion of myofibers with central nuclei did not correlate with clinical outcome, however, morphometric studies showed that survivors had larger myofiber diameters in infancy than those who died (10.4+/-3.9microm versus 8.9+/-3microm; p<0.001). As a corollary, patients with MTM1 missense mutations had larger myofiber diameters (11.1+/-4microm), than those with truncation/deletion mutations (8.6+/-2.7microm) (controls 11.7+/-2.5microm) (p<0.0001). These data indicate that differences in myofiber size correlate with MTM1 mutation type and patient outcome. Failure to attain and/or maintain myofiber size, along with fiber type perturbations and the misplacement of myofiber nuclei and other organelles, are important components of XLMTM muscle pathology.

X-linked myotubular myopathy: report of a case with novel mutation

Myotubular myopathy is a well-defined entity within the centronuclear myopathy subgroup of congenital myopathies. The authors present a patient with the most severe X-linked recessive type (XLMTM). A baby boy presented at birth with severe hypotonia, weak spontaneous movements, arthrogryposis, and respiratory insufficiency. Muscle biopsy showed features of myotubular myopathy. The diagnosis was confirmed and further specified by genetic analysis, revealing a novel frameshift mutation (1314-1315insT) of the myotubularin-coding MTM1 gene. This case underlines the importance of interdisciplinary analysis of congenital muscle diseases, including histomorphological and genetic investigations.

Inositol phosphates and phosphoinositides in health and disease

In the past two decades, considerable progress has been made toward understanding inositol phosphates and PI metabolism. However, there is still much to learn. The present challenge is to understand how inositol phosphates and PIs are compartmentalized, identify new targets of inositol phosphates and PIs, and elucidate the mechanisms underlying spatial and temporal regulation of the enzymes that metabolize inositol phosphates and PIs. Answers to these questions will help clarify the mechanisms of the diseases associated with these molecules and identify new possibilities for drug design.

X-linked myotubular and centronuclear myopathies

Recent work has significantly enhanced our understanding of the centronuclear myopathies and, in particular, myotubular myopathy. These myopathies share similar morphologic appearances with other diseases, namely the presence of hypotrophic myofibers with prominent internalized or centrally placed nuclei. Early workers suggested that this alteration represented an arrest in myofiber maturation, while other hypotheses implicated either failure in myofiber maturation or neurogenic causes. Despite similarities in morphology, distinct patterns of inheritance and some differences in clinical features have been recognized among cases. A severe form, known as X-linked myotubular myopathy (XLMTM), presents at or near birth. Affected males have profound global hypotonia and weakness, accompanied by respiratory difficulties that often require ventilation. Most of these patients die in infancy or early childhood, but some survive into later childhood or even adulthood. The responsible gene (MTM1) has been cloned; it encodes a phosphoinositide lipid phosphatase known as myotubularin that appears to be important in muscle maintenance. In autosomal recessive centronuclear myopathy (AR CNM), the onset of weakness typically occurs in infancy or early childhood. Some investigators have divided AR CNM into 3 subgroups: 1) an early-onset form with ophthalmoparesis, 2) an early-onset form without ophthalmoparesis, and 3) a late-onset form without ophthalmoparesis. Clinically, autosomal dominant CNM (AD CNM) is relatively mild and usually presents in adults with a diffuse weakness that is slowly progressive and may be accompanied by muscle hypertrophy. Overall, the autosomal disorders are not as clinically uniform as XLMTM, which has made their genetic characterization more difficult. Currently the responsible gene(s) remain unknown. This review will explore the historical evolution in understanding of these myopathies and give an update on their histopathologic features, genetics and pathogenesis.

Membrane association of myotubularin-related protein 2 is mediated by a pleckstrin homology-GRAM domain and a coiled-coil dimerization module

Mutations in the myotubularin (MTM)-related protein 2 (MTMR2) gene are responsible for the severe autosomal recessive neuropathy Charcot-Marie-Tooth disease type 4B1. MTMR2 belongs to the MTM family of dual-specific phosphatases that use phosphatidylinositol (PI) 3,5-bisphosphate [PI(3,5)P2] and PI 3-phosphate [PI(3)P] as their substrate. Because these substrates are localized in the membrane bilayer, membrane targeting of Mtmr2 is an important regulatory mechanism. In hypoosmotically stressed COS cells with increased levels of PI(3,5)P2, Mtmr2 is bound to the membrane of vacuoles formed under these conditions. Using several mutant forms of Mtmr2, we identified two domains that are necessary for membrane association: (i) A pleckstrin homology-GRAM domain; and (ii) a coiled-coil module. Protein-lipid overlay assays show that the pleckstrin homology-GRAM domain binds to PI(3,5)P2 and PI(5)P, a substrate and a product of the Mtmr2 enzyme, respectively. We also demonstrate that Mtmr2 forms a dimer and that the C-terminal coiled-coil is responsible for homodimerization, in addition to membrane association. Our data indicate that phosphoinositide-protein interactions, as well as protein-protein interactions, are necessary for the correct regulation of MTMR2.

Congenital myopathies at their molecular dawning

The introduction and application of molecular techniques have commenced to influence and alter the nosology of congenital myopathies. Long-known entities such as nemaline myopathies, core diseases, and desmin-related myopathies have now been found to be caused by unequivocal mutations. Several of these mutations and their genes have been identified by analyzing aggregates of proteins within muscle fibers as a morphological hallmark as in desminopathy and actinopathy, the latter a subtype among the nemaline myopathies. Immunohistochemistry has played a crucial role in recognizing this new group of protein aggregate myopathies within the spectrum of congenital myopathies. It is to be expected that other congenital myopathies marked by inclusion bodies may turn out to be such protein aggregate myopathies, depending on analysis of individual proteins within these protein aggregates and their association with putative gene mutations.

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.

Nemaline and myotubular myopathies

Nemaline myopathy is caused by mutations in one of at least six different genes. The clinical picture also varies widely, in terms of the grade and the distribution of muscle weakness. In familial cases, autosomal-recessive inheritance is more common than autosomal-dominant inheritance, and in some patients the disorder is caused by new dominant mutations. Because of the genetic heterogeneity and the large size of one of the genes commonly involved, that is, nebulin, no routine molecular genetic testing is yet available. Thus, the diagnosis often still rests on clinical and histologic criteria. Prenatal diagnosis can only reliably be performed in families where the causative mutation(s) have been identified. No clear-cut prognostic indicators are known, and treatment decisions can only be taken in casu. In the long-term management of patients with nemaline myopathy, respiratory capacity requires regular monitoring for early detection of insidious hypoventilation.

PTEN and myotubularin: novel phosphoinositide phosphatases

Protein tyrosine phosphatases (PTPs) are a diverse group of enzymes that contain a highly conserved active site motif, Cys-x5-Arg (Cx5R). The PTP superfamily enzymes, which include tyrosine-specific, dual specificity, low-molecular-weight, and Cdc25 phosphatases, are key mediators of a wide variety of cellular processes, including growth, metabolism, differentiation, motility, and programmed cell death. The PTEN/MMAC1/TEP1 gene was originally identified as a candidate tumor suppressor gene located on human chromosome 10q23; it encodes a protein with sequence similarity to PTPs and tensin. Recent studies have demonstrated that PTEN plays an essential role in regulating signaling pathways involved in cell growth and apoptosis, and mutations in the PTEN gene are now known to cause tumorigenesis in a number of human tissues. In addition, germ line mutations in the PTEN gene also play a major role in the development of Cowden and Bannayan-Zonana syndromes, in which patients often suffer from increased risk of breast and thyroid cancers. Biochemical studies of the PTEN phosphatase have revealed a molecular mechanism by which tumorigenesis may be caused in individuals with PTEN mutations. Unlike most members of the PTP superfamily, PTEN utilizes the phosphoinositide second messenger, phosphatidylinositol 3,4,5-trisphosphate (PIP3), as its physiologic substrate. This inositol lipid is an important regulator of cell growth and survival signaling through the Ser/Thr protein kinases PDK1 and Akt. By specifically dephosphorylating the D3 position of PIP3, the PTEN tumor suppressor functions as a negative regulator of signaling processes downstream of this lipid second messenger. Mutations that impair PTEN function result in a marked increase in cellular levels of PIP3 and constitutive activation of Akt survival signaling pathways, leading to inhibition of apoptosis, hyperplasia, and tumor formation. Certain structural features of PTEN contribute to its specificity for PIP3, as well as its role(s) in regulating cellular proliferation and apoptosis. Recently, myotubularin, a second PTP superfamily enzyme associated with human disease, has also been shown to utilize a phosphoinositide as its physiologic substrate.

Synthesis and function of 3-phosphorylated inositol lipids

The 3-phosphorylated inositol lipids fulfill roles as second messengers by interacting with the lipid binding domains of a variety of cellular proteins. Such interactions can affect the subcellular localization and aggregation of target proteins, and through allosteric effects, their activity. Generation of 3-phosphoinositides has been documented to influence diverse cellular pathways and hence alter a spectrum of fundamental cellular activities. This review is focused on the 3-phosphoinositide lipids, the synthesis of which is acutely triggered by extracellular stimuli, the enzymes responsible for their synthesis and metabolism, and their cell biological roles. Much knowledge has recently been gained through structural insights into the lipid kinases, their interaction with inhibitors, and the way their 3-phosphoinositide products interact with protein targets. This field is now moving toward a genetic dissection of 3-phosphoinositide action in a variety of model organisms. Such approaches will reveal the true role of the 3-phosphoinositides at the organismal level in health and disease.

Normal innervation and differentiation of X-linked myotubular myopathy muscle cells in a nerve-muscle coculture system

To study the pathogenesis of X-linked recessive myotubular myopathy (XLMTM), we used a nerve-muscle coculture system which allows the reconstitution of functional motor units in vitro after coupling of human skeletal muscle cells with embryonic rat spinal cord explants. We used three skeletal muscle cell lines derived from subjects with known mutations in the MTM1 gene (two from embryonic tissues, associated with mutations predicted to give a severe phenotype, and one from a neonate still alive at 3 years 6 months and exhibiting a mild phenotype). We compared these three XLMTM muscle cell cultures with control cultures giving special attention to behaviour of living cocultures (formation of the myofibres, contractile activity, survival), expression of muscular markers (desmin, dystrophin, alpha-actinin, troponin-T, myosin heavy chain isoforms), and nerve-muscle interactions (expression and aggregation of the nicotinic acetylcholine receptors). We were unable to reproduce any 'myotubular' phenotype since XLMTM muscle cells behaved like normal cells with regard to all the investigated parameters. Our results suggest that XLMTM muscle might be intrinsically normal and emphasize the possible involvement of the myotubularin-deficient motor neurons in the development of the disease.

Identification and localization of a new human myotubularin-related protein gene, mtmr8, on 8p22-p23

Myotubularin and myotubularin-related proteins are dual-specificity phosphatases. Several myotubularin-related proteins have been identified in humans and mice. The members of the myotubularin protein family are highly conserved, from humans to yeast. Mutations in the human myotubularin gene (MTM1) lead to X-linked myotubular myopathy. Here we isolate and localize a novel putative myotubularin-related protein gene (MTMR8) on chromosome 8p22--p23,between the markers D8S550 and D8S265, by exon-trapping experiments and RT-PCR. Genomic sequencing revealed that the gene consists of 10 exons and spans approximately 43 kb. The corresponding cDNA is 7081 bp. The open reading frame predicts a protein of 549 amino acids and a calculated molecular mass of 63 kDa. Like myotubularin-related protein-5, MTMR8 has no dual-specificity phosphatase domain. It contains a double-helical motif similar to the SET interaction domain, which is thought to have a role in the control of cell proliferation.

Congenital myopathies

Most congenital myopathies have been defined on account of the morphological findings in enzyme histochemical preparations. In effect, the diagnosis of this group of diseases continues to be made on the histological pattern of muscle biopsies. However, progress has been made in elucidating the molecular genetic background of several of the congenital myopathies. In this updated review we address those congenital myopathies for which gene defects and mutant proteins have been found (central core disease, nemaline myopathies, desminopathy, actinopathy, certain vacuolar myopathies, and myotubular myopathy) and the other disease with central nuclei (centronuclear myopathy).

Progress in desmin-related myopathies

Desmin-related myopathies are sporadic and familial neuromuscular conditions of considerable clinical heterogeneity uniformly marked by the pathologic accretion of desmin, often in a filamentous fashion. A large variety of other proteins, some of them cytoskeletal, also accrue. Morphologically, two types may be distinguished, one characterized by inclusions such as cytoplasmic and spheroid bodies or desmin-dystrophin plaques and another marked by granulofilamentous material. The genetic spectrum of desmin-related myopathies is quite diverse in that missense mutations and deletions in the desmin gene and a missense mutation in the alpha-B crystallin gene have been detected and several genes on other chromosomes have been mapped; the encoded protein products of these genes, however, are unknown. Accumulation of desmin and other proteins appears to be due to impaired nonlysosomal proteolysis. Mutant desmin that appears to be hyperphosphorylated seems to act as a seed protein for filament aggregation, inducing formation of inclusions and granulofilamentous material in these conditions. This condition is part of the group of disorders known as "surplus protein myopathies."

Confirmation of prenatal diagnosis results of X-linked recessive myotubular myopathy by mutational screening, and description of three new mutations in the MTM1 gene

X-linked recessive myotubular myopathy (XLMTM; MTM1) is a severe neonatal disorder often causing perinatal death of the affected males. The responsible gene, designated MTM1, was localized to proximal Xq28 and recently isolated. The characterization of MTM1 allowed us to screen for causing mutations in three families, previously investigated by linkage analysis. Using exon amplification, single strand conformation polymorphism, and subsequent sequencing analysis, three new mutations and their mutational origin were characterized by analyzing 10 exons. An acceptor splice site and a frameshift mutation were correlated with the concurrent appearance of XLMTM in two families. A third intronic mutation was also analyzed by reverse transcription PCR and revealed a cryptic splice site mutation cosegregating with the presumed XLMTM haplotype in the third family. These results further support the implication of the MTM1 gene in XLMTM and allow efficient and reliable carrier and prenatal diagnosis in these families. Direct mutational diagnosis of families at risk in combination with haplotype analysis avoid the drawbacks using only linkage analysis, make genetic counselling far more reliable, and early clinical management of this disease more appropriate. Moreover, pedigree analyses provide first information on de novo mutation frequency in this newly identified human disease gene.

MTM1 mutations in X-linked myotubular myopathy

X-linked myotubular myopathy (XLMTM; MIM# 310400) is a severe congenital muscle disorder caused by mutations in the MTM1 gene. This gene encodes a dual-specificity phosphatase named myotubularin, defining a large gene family highly conserved through evolution (which includes the putative anti-phosphatase Sbf1/hMTMR5). We report 29 mutations in novel cases, including 16 mutations not described before. To date, 198 mutations have been identified in unrelated families, accounting for 133 different disease-associated mutations which are widespread throughout the gene. Most point mutations are truncating, but 26% (35/133) are missense mutations affecting residues conserved in the Drosophila ortholog and in the homologous MTMR1 gene. Three recurrent mutations affect 17% of the patients, and a total of 21 different mutations were found in several independent families. The frequency of female carriers appears higher than expected (only 17% are de novo mutations). While most truncating mutations cause the severe and early lethal phenotype, some missense mutations are associated with milder forms and prolonged survival (up to 54 years).

NCAM, vimentin and neonatal myosin heavy chain expression in human muscle diseases

The intermediate filament protein vimentin, the neonatal isoform of the myosin heavy chain gene (MHCn), and the neural cell adhesion molecule (NCAM) are developmentally and/or neurally regulated molecules that reappear transiently after the induction of necrosis, or denervation. Immunostaining using antibodies against these molecules helps to identify regenerating and/or denervated muscle fibres even if they are not recognized by conventional staining procedures. This study examined the expression of vimentin, MHCn, and NCAM using immunohistochemistry in 82 biopsy specimens from muscular dystrophies, inflammatory myopathies, and neurogenic atrophies. Anti-vimentin labelled significantly more fibres than anti-MHCn staining in the inflammatory myopathies (P<0.03) but not in the muscular dystrophies (P=0.58) and neurogenic atrophies (P=0. 58). The fraction of NCAM+ fibres was always more elevated than vimentin+ or MHCn+ fibres. In the necrotizing myopathies, most NCAM+ fibres were regenerating ones (co-expressing vimentin). In neurogenic atrophies, half the NCAM+ fibres were regenerating and half of them were NCAM+/vimentin- and thus were considered to be denervated. Taken together, anti-vimentin staining detects a broader spectrum of regenerating fibres than anti-MHCn, at least in the inflammatory myopathies. The number of anti-NCAM labelled fibres in the necrotizing myopathies is similar, but not identical, to the number of regenerating fibres. Co-staining with anti-vimentin (or anti-MHCn) and anti-NCAM identifies a subset of fibres that is considered to be denervated.

Identification of novel mutations in the MTM1 gene causing severe and mild forms of X-linked myotubular myopathy

X-linked myotubular myopathy (XLMTM) is a congenital muscular disease characterized by severe hypotonia and generalized muscle weakness, leading in most cases to early postnatal death. The gene responsible for the disease, MTM1, encodes a dual specificity phosphatase, named myotubularin, which is highly conserved throughout evolution. To date, 139 MTM1 mutations in independent patients have been reported, corresponding to 93 different mutations. In this report we describe the identification of 21 mutations (14 novel) in XLMTM patients. Seventeen mutations are associated with a severe phenotype in males, with death occurring mainly before the first year of life. However, four mutations-three missense (R241C, I225T, and novel mutation P179S) and one single-amino acid deletion (G294del)-were found in patients with a much milder phenotype. These patients, while having a severe hypotonia at birth, are still alive at the age of 4, 7, 13, and 15 years, respectively, and display mild to moderate muscle weakness.

Germline mosaicism in X-linked myotubular myopathy

X-linked myotubular myopathy (XLMTM; OMIM310400) is a congenital muscle disorder characterized by severe hypotonia and respiratory insufficiency. The disorder was mapped to Xq28 by linkage studies and the MTM1 gene was isolated by positional cloning. The gene product is a 603 amino acid protein named myotubularin. A small domain in its sequence shows high homology to a consensus active site of tyrosine phosphatases, a diverse class of proteins involved in signal transduction, control of cell growth, and differentiation. In this report, two brothers affected with XLMTM are shown to have a point mutation (G1187A) in exon 11 of the MTM1 gene. Surprisingly, their mother does not have this mutation in her lymphocytes. Therefore, she likely has a germline mosaicism. As this is the third report of germline mosaicism in XLMTM, the finding has important implications for genetic counseling.

Characterization of 34 novel and six known MTM1 gene mutations in 47 unrelated X-linked myotubular myopathy patients

X-linked myotubular myopathy (XLMTM) is a congenital muscle disorder mainly affecting newborn males. Neonatal muscle weakness and hypotonia usually leads to a rapid demise. The responsible gene, MTM1, was isolated in 1996, and mutational data derived from 90 patients have been published. We report on our findings in a further 53 patients, using genomic DNA and mRNA screening protocols. Thirty-four novel mutations were identified in 37 cases, and six known mutations found in 10 other patients. The 34 new mutations include five large deletions, eight nonsense, six frameshift, five missense, and eight splice-site mutations, whereas two intronic variants causing partial exon skipping represent the first report on such a mechanism in MTM1. Two deletions, one involving exon 1, and the second exon 15, are the first defects to be identified in these exons. The heterogeneity of the mutations, their mutational origins, and the varied ethnic backgrounds of the patients, indicate that the majority of XLMTM families are affected by unique MTM1 mutations.

Identification of kinesin-like molecules in myogenic cells

Numerous organelles are repositioned during myogenic differentiation and are maintained in an asymmetric distribution throughout the life span of a myotube. It is likely that members of the kinesin superfamily may be responsible for some or all of these microtubule-dependent movements. Consequently, we have attempted to identify kinesin-like molecules expressed throughout myogenesis. Using a standard PCR-based strategy, we cloned two kinesin-like molecules from a rat myogenic cell line, L6. Sequence analysis of the first of these, KIF3C, defines it as a novel member of the KIF3 subfamily of kinesin-like proteins. KIF3C is expressed throughout myogenesis as well as in numerous rat tissues. Like other members of the KIF3 subfamily, KIF3C has an N-terminal motor domain. The second molecule identified is a rat homolog of murine KIF1B, a putative mitochondrial transporter. KIF1B is also expressed ubiquitously both in myogenic cells at all stages and in a variety of rat tissues.

The role of immunocytochemistry in congenital myopathies

Immunocytochemistry is playing an increasingly important role in the field of congenital myopathies. It is not yet the diagnostic tool that it is for the muscular dystrophies, but nevertheless it provides useful information on the nature of the structural defects that define each congenital myopathy, and on the additional secondary changes that accompany them. Immunocytochemistry may have a role in identifying primary protein defects but this may be dependent on the effect of a mutation on protein expression. Mutations affecting function, rather than expression, of a protein may not be detected by immunocytochemistry. Studies of candidate proteins, such as nebulin in nemaline myopathy, and the ryanodine receptor in central core disease, are, however, in progress, and as more defective genes are identified, and antibodies become available, immunocytochemistry will have an increasingly important role. Myofibrillar components are frequently affected in congenital myopathies and immunocytochemical localisation of their isoforms can reveal the nature of the structural abnormalities, such as rods, cores, and a variety of inclusions. Developmentally regulated proteins such as myosin heavy chains and intermediate filaments are also relevant to the understanding of the maturational process, in particular in myotubular/centronuclear myopathies, and also to the plasticity of muscle fibre types. Immunocytochemistry will continue to play an active role in studies of congenital myopathies and provide insight into their pathogenesis.

Myotubular myopathy: morphological, immunohistochemical and clinical variation

Myotubular myopathy frequently presents in male infants with severe generalised muscular hypotonia and weakness associated with ventilatory insufficiency, and is diagnosed on biopsy by the presence of many fibres with central nuclei and mitochondrial aggregation. In a 6-year period, we have investigated five unrelated patients with clinical and pathological features suggesting an X-linked myotubular myopathy, including one female patient. In one male infant, a biopsy of vastus lateralis showed less than 2% centrally-nucleated fibres, while biceps brachii showed up to 15% centrally-nucleated fibres. Immunohistochemical expression of the neural cell adhesion molecule (CD56) was more intense in the biceps muscle than in vastus lateralis, while expression of desmin and vimentin was similar. Morphometric evaluation of tissue from each of the patients revealed a wide spread of values for the number of centrally-nucleated fibres per microscopic field, and variation in the extent of immunohistochemical expression of NCAM, utrophin, laminin alpha 5 chain, vimentin and HLA1 antigen. These variations in the manifestations of myotubular myopathy have not been previously described, and will need to be correlated with the increasing knowledge of the mutations in the MTM1 gene coding for myotubularin.

Centronuclear myopathy. Histopathological aspects in ten patients with childhood onset

Centronuclear myopathy is a rare congenital myopathy. According to the period of onset of signs and symptoms and the degree of muscular involvement three clinical forms are distinguished: severe neonatal; childhood onset; and adult onset. We describe herein the muscle biopsy findings of ten patients with the childhood onset form of the disease including three cases with ultrastructural study. The biopsies disclosed increased nuclear centralization that varied from 25 to 90% of the fibers, type I predominance, great variability in fiber diameters, involvement in the internal fiber's architecture, and focal areas of myofilament disorganization. The main histopathologic differential diagnoses included type I fiber predominance, congenital fiber type disproportion, and myotonic dystrophy. The histologic abnormalities in centronuclear myopathy may be due to an arrest of maturation on the fetal myotubular stage. The cause of this arrest remains elusive.

Sequential muscle biopsy changes in a case of congenital myopathy

Muscle biopsies at age 7 months in a set of dizygotic male twins born floppy showed typical features of congenital fiber-type disproportion (CFTD). One of the twins died at age 1 year due to respiratory complications. The second one subsequently developed facial diplegia and external ophthalmoplegia. He never walked, remained wheelchair bound, and required continuous ventilatory support. He underwent repeat biopsies at ages 2 and 4, which showed many atrophic type 1 muscle fibers containing central nuclei and severe type 2 fiber deficiency compatible with centronuclear myopathy (CNM). Two-dimensional gel electrophoresis of muscle showed decreases of type II myosin light chains 2 and 3, suggestive of histochemical type I fiber deficiency. The progressive nature of morphological changes in one of our patients cannot be explained by maturational arrest. Repeat biopsies in cases of CFTD with rapid clinical deterioration may very well show CNM.

Remodeling of the cytoskeletal lattice in denervated skeletal muscle

The effect of denervation-induced atrophy on the cytoskeletal lattice in rat fast- and slow-twitch skeletal muscle has been investigated. Immunochemical analyses and immunofluorescence microscopy experiments employing monospecific antibodies to dystrophin, desmin, and alpha-tubulin were carried out on intact and denervated muscles. The relative cellular content of dystrophin and desmin were reduced in the soleus muscle (slow-twitch), while significant increases were shown in the gastrocnemius muscle (fast-twitch). In both muscles, alpha-tubulin levels increased up to 12-fold as a function of time compared to control values. Immunofluorescence microscopy revealed a distinct rearrangement of the microtubule network toward a predominantly longitudinal alignment, which was accompanied by an increase in the density of the fluorescence. It is concluded that the relative increase of the three structural proteins in the fast-twitch gastrocnemius muscle may be related to the apparent resistance of this muscle type to denervation-induced atrophy. The increased alpha-tubulin content in denervated slow- and fast-twitch muscles could be indicative of an adaptive mechanism designed to maintain the integrity of the muscle fiber in view of eventual regenerative activities.

Patterns of abnormal protein expression in target formations and unstructured cores

Streaming of Z-disks and focal myofibrillar degeneration occur in target formations (TF) and unstructured cores (UC). Similar myofibrillar alterations are also part of the spectrum of ultrastructural reactions that can occur in the myopathies associated with myofibrillar degeneration and abnormal foci of desmin positivity. In the latter disorders, there is ectopic overexpression of dystrophin, neural cell adhesion molecule (NCAM), gelsolin, beta-amyloid precursor protein (beta APP) epitopes, alpha 1-antichymotrypsin (alpha 1-ACT), and many abnormal fiber regions are also strongly congophilic. Therefore, we searched for similar abnormalities in TF and UC. The UC and the center of TF show increased immunoreactivity for actin, alpha-actinin, gelsolin, dystrophin, beta APP epitopes, alpha 1-ACT, beta 2-microglobulin, desmin, and NCAM, but minimal or no congophilia. The periphery of the TF reacts strongly for nebulin but not for actin. The observed immunocytochemical alterations in TF and UC may represent a stereotyped cellular response associated with myofibrillar degeneration due to any cause. However, the three-dimensional profile of the TF and UC as well as their fiber-type specificity distinguish them from lesions that have similar immunocytochemical profiles in other myopathies.

X-linked myotubular myopathy: refinement of the critical gene region

X-linked recessive myotubular myopathy (XLMTM) is a severe neonatal neuro-muscular disease characterized by muscle weakness, hypotonia, and respiratory problems. The locus for the XLMTM gene (MTM1) has previously been mapped to Xq28 between the markers DXS304 and DXS497 by linkage analyses and by determining the breakpoints of deletion patients. We report linkage analysis data or 20 XLMTM families who were tested using the DNA markers DXS1113, DXS304, DXS455, DXS1684, DXS305 and DXS52 and present two families showing recombination between MTM1 and either DXS304, DXS334 or DXS305. We found each of the families to be informative for at least three markers. Based on these findings we excluded 30 women from being carriers, the carrier status of 17 obligate carrier mothers could be confirmed and eight mothers and sisters were identified as to be at high risk of carrying a MTM1 mutation. By combining recently published data with the results of our recombinant families, we suggest that the MTM1 locus maps between DXS334 and DXS497 narrowing the region of interest from 600 kb to an estimated < 500 kb interval. This additional refinement in the localization of MTM1 means a further step towards the isolation of the gene in the near future, and allows more reliable and efficient carrier detection and prenatal diagnosis.

Congenital myopathies

The congenital myopathies (CM) are a group of non or little progressive neuromuscular conditions, often hereditary, delineated by morphological techniques, ie, enzyme histochemistry and electron microscopy. The catalogue of CM entailing well known "classic" conditions as central core disease, nemaline myopathy, and centronuclear myopathy has continuously been expanded, now comprising some 40 conditions. Nosologic advances have occurred with immunohistochemical techniques that show generalized or focal protein abnormalities within muscle fibers of certain CM, but at much slower pace as to localization of CM genes. So far, only those for central core disease, nemaline myopathy, and myotubular myopathy have been reported. Epidemiological rarity and nosographic controversy of CM have contributed to this lack of molecular genetic progress in CM.

A gene mutated in X-linked myotubular myopathy defines a new putative tyrosine phosphatase family conserved in yeast

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.

Severe neonatal nemaline myopathy with delayed maturation of muscle

A Japanese infant with a severe neonatal form of nemaline myopathy showed features of muscle immaturity as well as rods in the biopsied quadriceps femoris muscle, and involvement of diaphragm was first confirmed at autopsy. The biopsied muscle showed numerous rods in 80% of muscle fibers, and an increased number of type 2C fibers (23.2%). Electron microscopic findings were characterized by the presence of many small, immature round fibers with central nuclei and sparse myofibrils, and an increased number of satellite cells in close association with the small muscle fibers, as well as numerous rod structures. These microscopic and electron microscopic findings are interpreted as immature muscle fibers. Maturational delay or arrest implies deprivation of a development promoting factor such as a neuronal signal. Severe involvement with numerous rods was demonstrated in both diaphragm and intercostal muscles at the time of the postmortem examination, compatible with the patient's respiratory failure.