X-linked myotubular myopathy: clinical observations in ten additional cases

X-linked myotubular myopathy in a family of two infant siblings: A case report and review

X-linked myotubular myopathy (XLMTM) is a severe type of congenital skeletal muscle disorder usually presenting at birth requiring extensive resuscitation. While having phenotypic variability, its diagnosis carries a poor prognosis due to high rates of hospitalization and mortality by early infancy. Management of patients with XLMTM should therefore be guided by shared decision-making with parents, considering the severity and progression of the disease, quality of life, and demands on caregivers. We describe a family unit of two half-siblings presenting with the severe neonatal form of XLMTM, with varying prognosis and outcomes. Furthermore, a novel maternally-derived c.343-1G > A variant in intron-5 of the MTM1 gene was identified in this family. Hereby, we propose an algorithm for the management of XLMTM, outlining important considerations during the antenatal and postnatal follow-up period.

Genetic etiologies associated with infantile hydrocephalus in a Chinese infantile cohort

BACKGROUND

Infantile hydrocephalus (IHC) is commonly related to other central nervous system diseases, which may have adverse effects on prognosis. The causes of IHC are heterogeneous, and the genetic etiologies are not fully understood. This study aimed to analyze the genetic etiologies of an IHC cohort.

METHODS

The data for 110 IHC patients who had received exome sequencing at the Clinical Genetic Center of the Children's Hospital of Fudan University between 2016 and 2019 were reviewed and analyzed retrospectively. An exome-wide association analysis (EWAS) was performed within this cohort using IHC as the study phenotype.

RESULTS

Of the 110 IHC patients, a pathogenic or likely pathogenic variant was identified in 16 (15%) patients, spanning 13 genes. The genes were mainly associated with metabolic disorders, brain abnormalities, and genetic syndromes. IHC patients who had unclear clinical etiology were more likely to possess a genetic etiology. Based on previous studies and on our EWAS results, ZEB1, SBF2, and GNAI2 were over-represented among IHC patients and might affect the signaling pathways involved in IHC formation.

CONCLUSIONS

Our study showed heterogeneous genetic etiologies in an IHC cohort. It is essential to perform genetic testing on IHC patients who have unclear clinical etiology, and genes associated with metabolic disorders, brain abnormalities, and genetic syndromes should be noted. In addition, when aiming to discover IHC susceptibility genes, genes that might influence the signaling pathways involved in IHC formation should be prioritized.

Respiratory care in myotubular myopathy

X-linked myotubular myopathy is a neuromuscular condition caused by pathogenic variants in the MTM1 gene, which encodes for myotubularin, a phosphatidylinositol 3-phosphate phosphatase. Affected individuals typically require intensive medical intervention to survive, though there are some milder phenotypes. To date, respiratory management has been primarily supportive, optimising clearance of airway secretions, providing ventilatory support and prevention/early intervention of respiratory infections. Encouragingly, there has been significant progress in the development of novel therapeutic strategies such as gene therapy, enzyme replacement therapy and drugs that modulate downstream pathways. In this review, we discuss the common respiratory issues using four illustrative real-life cases, and summarise recent translational research, which offers hope to many patients and their families.

Structural Chromosomal Rearrangements Require Nucleotide-Level Resolution: Lessons from Next-Generation Sequencing in Prenatal Diagnosis

In this exciting era of "next-gen cytogenetics," integrating genomic sequencing into the prenatal diagnostic setting is possible within an actionable time frame and can provide precise delineation of balanced chromosomal rearrangements at the nucleotide level. Given the increased risk of congenital abnormalities in newborns with de novo balanced chromosomal rearrangements, comprehensive interpretation of breakpoints could substantially improve prediction of phenotypic outcomes and support perinatal medical care. Herein, we present and evaluate sequencing results of balanced chromosomal rearrangements in ten prenatal subjects with respect to the location of regulatory chromatin domains (topologically associated domains [TADs]). The genomic material from all subjects was interpreted to be "normal" by microarray analyses, and their rearrangements would not have been detected by cell-free DNA (cfDNA) screening. The findings of our systematic approach correlate with phenotypes of both pregnancies with untoward outcomes (5/10) and with healthy newborns (3/10). Two pregnancies, one with a chromosomal aberration predicted to be of unknown clinical significance and another one predicted to be likely benign, were terminated prior to phenotype-genotype correlation (2/10). We demonstrate that the clinical interpretation of structural rearrangements should not be limited to interruption, deletion, or duplication of specific genes and should also incorporate regulatory domains of the human genome with critical ramifications for the control of gene expression. As detailed in this study, our molecular approach to both detecting and interpreting the breakpoints of structural rearrangements yields unparalleled information in comparison to other commonly used first-tier diagnostic methods, such as non-invasive cfDNA screening and microarray analysis, to provide improved genetic counseling for phenotypic outcome in the prenatal setting.

Prenatal diagnosis of congenital myopathies and muscular dystrophies

Congenital myopathies and muscular dystrophies constitute a genetically and phenotypically heterogeneous group of rare inherited diseases characterized by muscle weakness and atrophy, motor delay and respiratory insufficiency. To date, curative care is not available for these diseases, which may severely affect both life-span and quality of life. We discuss prenatal diagnosis and genetic counseling for families at risk, as well as diagnostic possibilities in sporadic cases.

Validity of a Neurological Scoring System for Canine X-Linked Myotubular Myopathy

A simple clinical neurological test was developed to evaluate response to gene therapy in a preclinical canine model of X-linked myotubular myopathy (XLMTM). This devastating congenital myopathy is caused by mutation in the myotubularin (MTM1) gene. Clinical signs include muscle weakness, early respiratory failure, and ventilator dependence. A spontaneously occurring canine model has a similar clinical picture and histological abnormalities on muscle biopsy compared with patients. We developed a neuromuscular assessment score, graded on a scale from 10 (normal) to 1 (unable to maintain sternal recumbency). We hypothesize that this neurological assessment score correlates with genotype and established measures of disease severity and is reliable when performed by an independent observer. At 17 weeks of age, there was strong correlation between neurological assessment scores and established methods of severity testing. The neurological severity score correctly differentiated between XLMTM and wild-type dogs with good interobserver reliability, on the basis of strong agreement between neurological scores assigned by independent observers. Together, these data indicate that the neurological scoring system developed for this canine congenital neuromuscular disorder is reliable and valid. This scoring system may be helpful in evaluating response to therapy in preclinical testing in this disease model, such as response to gene therapy.

Pathogenic mechanisms in centronuclear myopathies

Centronuclear myopathies (CNMs) are a genetically heterogeneous group of inherited neuromuscular disorders characterized by clinical features of a congenital myopathy and abundant central nuclei as the most prominent histopathological feature. The most common forms of congenital myopathies with central nuclei have been attributed to X-linked recessive mutations in the MTM1 gene encoding myotubularin (“X-linked myotubular myopathy”), autosomal-dominant mutations in the DNM2 gene encoding dynamin-2 and the BIN1 gene encoding amphiphysin-2 (also named bridging integrator-1, BIN1, or SH3P9), and autosomal-recessive mutations in BIN1, the RYR1 gene encoding the skeletal muscle ryanodine receptor, and the TTN gene encoding titin. Models to study and rescue the affected cellular pathways are now available in yeast, C. elegans, drosophila, zebrafish, mouse, and dog. Defects in membrane trafficking have emerged as a key pathogenic mechanisms, with aberrant T-tubule formation, abnormalities of triadic assembly, and disturbance of the excitation–contraction machinery the main downstream effects studied to date. Abnormal autophagy has recently been recognized as another important collateral of defective membrane trafficking in different genetic forms of CNM, suggesting an intriguing link to primary disorders of defective autophagy with overlapping histopathological features. The following review will provide an overview of clinical, histopathological, and genetic aspects of the CNMs in the context of the key pathogenic mechanism, outline unresolved questions, and indicate promising future lines of enquiry.

Two Cases of X-Linked Myotubular Myopathy with Novel MTM1 Mutations

Background

Myotubular myopathy (MTM) is a congenital myopathy characterized by centrally placed nuclei in muscle fibers. Mutations in the myotubularin 1 gene (MTM1) have been identified in the most of the patients with the X-linked recessive form.

Case Report

This report describes two male infants with X-linked MTM (XLMTM). Both patients presented with generalized hypotonia and respiratory difficulties since birth. We did not perform a muscle biopsy in either patient, but their conditions were diagnosed by genetic testing of MTM1. One splicing mutation, c.63+1G>C, and a frame-shift mutation, c.473delA (p. Lys158SerfxX28), were identified. Neither mutation has been reported previously.

Conclusions

Genetic testing for MTM1 is helpful for the differential diagnosis of floppy male infants. We suggest that advanced molecular genetic testing may permit a correct diagnosis while avoiding invasive procedures.

The Integration of Children Dependent on Medical Technology Into Public Schools

Advances in medicine have increased the survival rates of children with complex medical conditions, including those who are dependent on technology such as ventilators and tracheostomies. The process of integrating children dependent on medical technology into public schools requires the collaboration of a multidisciplinary team to ensure that both their educational and health care needs are met. As coordinator of the school health program, the school nurse plays a vital role throughout the integration process and during the time the student is enrolled in the school. This article presents a case study that demonstrates the steps in the integration process, the roles of the team members, and implications for school nursing practice.

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.

No association between residual forefoot adduction and the position of the navicular in clubfeet treated by posterior release

Medial displacement of the navicular has been considered a major explanation for residual forefoot adduction (FFA) in congenital clubfoot and also a frequent reason for dissatisfaction after limited surgery. In this study, it was hypothesized that there would be an association between the degree of medial displacement of the navicular and residual FFA in clubfeet. The position of the navicular was retrospectively measured by ultrasonography in 49 clubfeet in 35 children at ages 3 to 6 years and correlated to residual FFA measured on footprints and radiographs (talo-first metatarsal angle). In the 49 clubfeet, the navicular was significantly more medially displaced toward the medial malleolus than in the 21 contralateral normal feet (P < 0.001). However, there was no correlation between the degree of medial displacement of the navicular and the degree of FFA measured on footprints (P = 0.690) or on radiographs (P = 0.390). Thus, there were clubfeet with straight forefoot and a medially displaced navicular, that is, "spurious correction," and clubfeet with FFA and the navicular in correct position in relation to the head of the talus. Both patient satisfaction and foot score declined with larger FFA. The results support the view that ultrasonography is a helpful tool for assessing the position of the navicular. The critical issue for analysis is whether the FFA is due to malalignment in the talonavicular joint or more distally.

A double mutation in a patient with X-linked myotubular myopathy

In this report a double mutation was identified in a patient with X-linked myotubular myopathy. The mutations present in the patient were a C-->T substitution of nucleotide 163, which led to an Arg 55 stop codon (nonsense mutation), and an "A" insertion at nucleotide 440, which caused a shift of the reading frame and a premature stop at codon 153 (frameshift mutation). The nonsense mutation was heterozygously present in the mother but not identified in the father or in normal controls. The frameshift mutation was not identified in either parent or normal controls (de novo mutation). These mutations are predicted to truncate the myotubularin protein.

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.

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.

Medical complications in long-term survivors with X-linked myotubular myopathy

OBJECTIVES

X-linked myotubular myopathy (MTM1) is a rare developmental disorder of skeletal muscle characterized by the presence of central nuclei in biopsy specimens from affected male subjects. Until recently, the disorder was usually fatal within the first year of life. This study was undertaken to determine the outcome in long-term survivors (>1 year of age) with MTM1.

METHODS

Clinical data were obtained on 55 male subjects from 49 independent North American families for which a mutation was identified in the X-linked myotubularin gene by direct genomic sequencing. Medical records were reviewed and families were interviewed to ascertain features at birth, length of survival, developmental milestones, and medical complications.

RESULTS

Seventy-four percent (26 of 35) of the affected male subjects over the age of 1 year are living (range, 1 to 27 years); 80% remain completely or partially ventilator-dependent. In the absence of significant hypoxia, cognitive development is normal, and the muscle disorder appears nonprogressive. Several patients have had other medical problems not previously reported to be associated with MTM1. These include pyloric stenosis (4 male subjects from 3 families), spherocytosis (2 patients), gallstones (4 patients), kidney stones or nephrocalcinosis (2 patients), a vitamin K responsive bleeding diathesis (2 patients), and height >/=90% for age (40% of the patients). Six patients have had biochemical evidence of liver dysfunction, and 2 patients died after significant liver hemorrhage.

CONCLUSIONS

These data suggest that the prognosis for X-linked MTM may not be as poor as previously reported. However, at least some long-term survivors appear at risk for medical complications involving other organ systems, and patients should be carefully monitored for these potentially life-threatening complications. The pleiotropic symptoms demonstrated in these patients strongly suggest that the function of the MTM1 protein is not limited to developing muscle cells.

Comparative mapping on the mouse X chromosome defines a myotubular myopathy equivalent region

The gene for X-linked myotubular myopathy (MTM1) has been localized to a 300-kb critical region in human Xq28 between IDS and GABRA3. As part of an effort to clone this gene, we developed a YAC contig on the mouse X Chromosome (Chr) which includes loci homologous to those within the human MTM1 critical region. The murine contig consists of 18 YACs and spans 2.5-3.0 Mb. We have aligned the human and murine physical maps by isolating conserved mouse genomic fragments, including CpG islands and trapped exons. We believe that the simultaneous isolation of genes from both mouse and human and continued comparative mapping will prove helpful in the eventual identification of MTM1 and other genes in the region.

XLMR genes: update 1996

A current list of all known forms of X-linked mental retardation (XLMR) and a slightly revised classification are presented. The number of known disorders has not increased because 6 disorders have been combined based on new molecular data or on clinical grounds and only 6 newly described XLMR disorders have been reported. Of the current 105 XLMR disorders, 34 have been mapped, and 18 disorders and 1 nonspecific XLMR (FRAXE) have been cloned. The number of families with nonspecific XLMR with a LOD score of > or = 2.0 has more than doubled, with 42 (including FRAXE) now being known. a summary of the localization of presumed nonspecific mental retardation (MR) genes from well-studied X-chromosomal translocations and deletions is also included. Only 10-12 nonoverlapping loci are required to explain all localizations of nonspecific MR from both approaches. These new trends mark the beginning of a significantly improved understanding of the role of genes on the X chromosome in producing MR. Continued close collaboration between clinical and molecular investigators will be required to complete the process.