The novel contiguous gene syndrome of myotubular myopathy (MTM1), male hypogenitalism and deletion in Xq28:report of the first familial case

Symptomatic heterozygous X-Linked myotubular myopathy female patient with a large deletion at Xq28 and decrease expression of normal allele

X-linked myotubular myopathy (XLMTM; OMIM 310400) is a centronuclear congenital muscular disorder of X-linked recessive inheritance. Although female carriers are typically asymptomatic, affected heterozygous females have been described. Here, we describe the case of a sporadic female patient with suspicion of centronuclear myopathy and a heterozygous large deletion at Xq28 encompassing the MAMLD1, MTM1, MTMR1, CD99L2, and HMGB3 genes. The deletion was first detected using a custom next generation sequencing (NGS)-based multigene panel and finally characterized by comparative genomic hybridization array and multiplex ligation probe assay techniques. In this patient we have confirmed, by MTM1 mRNA quantification, a MTM1 gene expression less than the expected 50 percent in patient muscle. The significant 20% reduction in MTM1 mRNA expression in muscle, precludes low level of the normal myotubularin protein as the cause of the phenotype in this heterozygous female. We have also found that BIN1 expression in patient muscle biopsy was significantly increased, and postulate that BIN1 expression will be increased in XLMTM patient muscle as an attempt to maintain muscle function.

Affected female carriers of MTM1 mutations display a wide spectrum of clinical and pathological involvement: delineating diagnostic clues

X-linked myotubular myopathy (XLMTM), a severe congenital myopathy, is caused by mutations in the MTM1 gene located on the X chromosome. A majority of affected males die in the early postnatal period, whereas female carriers are believed to be usually asymptomatic. Nevertheless, several affected females have been reported. To assess the phenotypic and pathological spectra of carrier females and to delineate diagnostic clues, we characterized 17 new unrelated affected females and performed a detailed comparison with previously reported cases at the clinical, muscle imaging, histological, ultrastructural and molecular levels. Taken together, the analysis of this large cohort of 43 cases highlights a wide spectrum of clinical severity ranging from severe neonatal and generalized weakness, similar to XLMTM male, to milder adult forms. Several females show a decline in respiratory function. Asymmetric weakness is a noteworthy frequent specific feature potentially correlated to an increased prevalence of highly skewed X inactivation. Asymmetry of growth was also noted. Other diagnostic clues include facial weakness, ptosis and ophthalmoplegia, skeletal and joint abnormalities, and histopathological signs that are hallmarks of centronuclear myopathy such as centralized nuclei and necklace fibers. The histopathological findings also demonstrate a general disorganization of muscle structure in addition to these specific hallmarks. Thus, MTM1 mutations in carrier females define a specific myopathy, which may be independent of the presence of an XLMTM male in the family. As several of the reported affected females carry large heterozygous MTM1 deletions not detectable by Sanger sequencing, and as milder phenotypes present as adult-onset limb-girdle myopathy, the prevalence of this myopathy is likely to be greatly underestimated. This report should aid diagnosis and thus the clinical management and genetic counseling of MTM1 carrier females. Furthermore, the clinical and pathological history of this cohort may be useful for therapeutic projects in males with XLMTM, as it illustrates the spectrum of possible evolution of the disease in patients surviving long term.

A rare case of centronuclear myopathy with DNM2 mutation: genotype-phenotype correlation

Centronuclear myopathy (CNM) is a group of rare genetic muscle disorders characterized by muscle fibers with centrally located nuclei. The most common forms of CNM have been attributed to X-linked recessive mutations in the MTM1 gene; autosomal-dominant mutations in the DNM2 gene-encoding dynamin-2, the BIN1 gene; and autosomal-recessive mutations in BIN1, RYR1, and TTN genes. Dominant CNM due to DNM2 mutations usually follows a mild clinical course with the onset in adolescence. Currently, around 35 mutations of the DNM2 gene have been identified in CNM; however, the underlying molecular mechanism of DNM2 mutation in the pathology of CNM remains elusive, and the standard clinical characteristics have not yet been defined. Here, we describe the case of a 17-year-old female who presented with proximal muscle weakness along with congenital anomalous pulmonary venous connection (which has not been described in previous cases of CNM), scoliosis, and lung disease without a significant family history. Her creatine kinase level was normal. Histology, special stains, and electron microscope findings on her skeletal muscle biopsy showed CNM with the characteristic features of a DNM2 mutation, which was later confirmed by next-generation sequencing. This case expands the known clinical and pathological findings of CNM with DNM2 gene mutation.

Novel findings associated with MTM1 suggest a higher number of female symptomatic carriers

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504 myopathic patients have been screened for MTM1 variants by NGS and CGH array approaches.

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Seven novel XLMTM patients and the fifth case of a large Xq28 deletion have been identified.

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The identification of two sporadic manifesting female carriers suggests that their number may be underestimated.

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Large NGS panels, including the MTM1 gene, are useful tools to identify sporadic female XLMTM patients.

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The identification of MTM1 variants, also as incidental findings, complicates genetic counseling.

Mutations in the MTM1 gene cause X-linked myotubular myopathy (XLMTM), characterized by neonatal hypotonia and respiratory failure, and are responsible for a premature mortality in affected males. Female carriers are usually asymptomatic but they may present with muscular weakness because of a hypothesized skewed pattern of X-chromosome inactivation.

By combining next generation sequencing (NGS) and CGH array approaches, we have investigated the role of MTM1 variants in a large cohort of undiagnosed patients with a wide spectrum of myopathies. Seven novel XLMTM patients have been identified, including two girls with an unremarkable family history for myotubular myopathy.

Moreover, we have detected and finely mapped a large deletion causing a myotubular myopathy with abnormal genital development.

Our data confirm that the severe neonatal onset of the disease in male infants is sufficient to address the direct molecular testing toward the MTM1 gene and, above all, suggest that the number of undiagnosed symptomatic female carriers is probably underestimated.

Human MAMLD1 Gene Variations Seem Not Sufficient to Explain a 46,XY DSD Phenotype

MAMLD1 is thought to cause disordered sex development in 46,XY patients. But its role is controversial because some MAMLD1 variants are also detected in normal individuals, several MAMLD1 mutations have wild-type activity in functional tests, and the male Mamld1-knockout mouse has normal genitalia and reproduction. Our aim was to search for MAMLD1 variations in 108 46,XY patients with disordered sex development, and to test them functionally. We detected MAMDL1 variations and compared SNP frequencies in controls and patients. We tested MAMLD1 transcriptional activity on promoters involved in sex development and assessed the effect of MAMLD1 on androgen production. MAMLD1 expression in normal steroid-producing tissues and mutant MAMLD1 protein expression were also assessed. Nine MAMLD1 mutations (7 novel) were characterized. In vitro, most MAMLD1 variants acted similarly to wild type. Only the L210X mutation showed loss of function in all tests. We detected no effect of wild-type or MAMLD1 variants on CYP17A1 enzyme activity in our cell experiments, and Western blots revealed no significant differences for MAMLD1 protein expression. MAMLD1 was expressed in human adult testes and adrenals. In conclusion, our data support the notion that MAMLD1 sequence variations may not suffice to explain the phenotype in carriers and that MAMLD1 may also have a role in adult life.

Screening of MAMLD1 mutations in 70 children with 46,XY DSD: identification and functional analysis of two new mutations

More than 50% of children with severe 46,XY disorders of sex development (DSD) do not have a definitive etiological diagnosis. Besides gonadal dysgenesis, defects in androgen biosynthesis, and abnormalities in androgen sensitivity, the Mastermind-like domain containing 1 (MAMLD1) gene, which was identified as critical for the development of male genitalia, may be implicated. The present study investigated whether MAMLD1 is implicated in cases of severe 46,XY DSD and whether routine sequencing of MAMLD1 should be performed in these patients.

Seventy children with severe non-syndromic 46,XY DSD of unknown etiology were studied. One hundred and fifty healthy individuals were included as controls. Direct sequencing of the MAMLD1, AR, SRD5A2 and NR5A1 genes was performed. The transactivation function of the variant MAMLD1 proteins was quantified by the luciferase method.

Two new mutations were identified: p.S143X (c.428C>A) in a patient with scrotal hypospadias with microphallus and p.P384L (c.1151C>T) in a patient with penile hypospadias with microphallus. The in vitro functional study confirmed no residual transactivating function of the p.S143X mutant and a significantly reduced transactivation function of the p.P384L protein (p = 0.0032). The p.P359S, p.N662S and p.H347Q variants are also reported with particularly high frequency of the p.359T- p.662G haplotype in the DSD patients.

Severe undervirilization in XY newborns can reveal mutations of MAMLD1. MAMLD1 should be routinely sequenced in these patients with otherwise normal AR, SRD5A2 and NR5A1genes.

Polymorphisms of MAMLD1 gene in hypospadias

PURPOSE

Mastermind-like domain containing 1 (MAMLD1) is a causative gene for the fetal development of male external genitalia. Almost 10% of patients with both severe and non-severe hypospadias exhibit mutations of MAMLD1. The aim of this work was to determine whether polymorphisms of MAMLD1 are a genetic risk factor for hypospadias.

MATERIAL AND METHODS

This study included 150 hypospadias with a range of severities and 150 controls. Direct sequencing of the MAMLD1 coding exons and their flanking splice sites was performed. In silico secondary and tertiary structure prediction and accessibility of changed amino acids were evaluated using JPred, Netsurf and PHYRE software. Functional studies of the transactivation of haplotypes on Hes3 promoter were performed in vitro using cDNAs of missense variants of MAMLD1.

RESULTS

The p.P286S polymorphism was identified in 17/150 patients and 12/150 controls (11.3% vs. 8.0%, p = 0.32). The p.N589S polymorphism was identified in 22/150 patients and 12/150 controls (14.6% vs. 8.0%, p = 0.068). The double polymorphism (S-S haplotype) was present in 16/150 patients and 6/150 controls (10.6% vs. 4.0%, p = 0.044, OR = 2.87, CI from 1.09 to 7.55). The association of polymorphisms consistently revealed a modification in the structure prediction or amino acid accessibility in all three in silico models. The P286S, N589S and P286S + N589S proteins did not exhibit reduced transactivating activity on Hes3 promoter.

CONCLUSION

Polymorphisms of MAMLD1 gene are frequent in patients with hypospadias. Although no change in transactivation was noted on Hes3 promoter, the in silico studies and the significantly increased incidence of the S-S haplotype in hypospadiac patients raise the hypothesis of a particular susceptibility conferred by these variants.

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

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

Is hypospadias a genetic, endocrine or environmental disease, or still an unexplained malformation?

Hypospadias is one of the most frequent genital malformations in the male newborn and results from an abnormal penile and urethral development. This process requires a correct genetic programme, time- and space-adapted cellular differentiation, complex tissue interactions, and hormonal mediation through enzymatic activities and hormonal transduction signals. Any disturbance in these regulations may induce a defect in the virilization of the external genitalia and hypospadias. This malformation thus appears to be at the crossroads of various mechanisms implicating genetic and environmental factors. The genes of penile development (HOX, FGF, Shh) and testicular determination (WT1, SRY) and those regulating the synthesis [luteinizing hormone (LH) receptor] and action of androgen (5alpha reductase, androgen receptor) can cause hypospadias if altered. Several chromosomal abnormalities and malformative syndromes include hypospadias, from anterior to penoscrotal forms. More recently, CXorf6 and ATF3 have been reported to be involved. Besides these genomic and hormonal factors, multiple substances found in the environment can also potentially interfere with male genital development because of their similarity to hormones. The proportion of hypospadias cases for which an aetiology is detected varies with the authors but it nevertheless remains low, especially for less severe cases. An interaction between genetic background and environment is likely.

MAMLD1 (CXorf6) is a New Gene for Hypospadias

MAMLD1 (mastermind-like domain containing 1), previously known as CXorf6 (chromosome X open reading frame 6), has been shown to be a causative gene for hypospadias. This is primarily based on the identification of nonsense mutations (E124X, Q197X, and R653X), which undergo nonsense mediated mRNA decay, in patients with penoscrotal hypospadias. Subsequent molecular studies have shown that the mouse homolog is transiently expressed in fetal Sertoli and Leydig cells around the critical period for sex development, and that transient knockdown of Mamld1 results in significantly reduced testosterone production in murine Leydig tumor cells. These findings suggest that the MAMLD1 mutations cause hypospadias primarily because of compromised testosterone production around the critical period for sex development.

Genome-wide analysis for micro-aberrations in familial exstrophy of the bladder using array-based comparative genomic hybridization

OBJECTIVE

Exstrophy of the bladder (EB) is part of the bladder-exstrophy-epispadias complex (BEEC). Because familial occurrence of BEEC is rare, exogenous factors are thought to play a major role in the etiology of most BEEC cases. We aimed to investigate a possible genetic basis of BEEC in a consanguineous kindred of Moroccan origin with three members showing the same phenotypic expression of BEEC.

PATIENTS AND METHODS

The three affected males (two cousins and their maternal uncle) all presenting with nonsyndromic classic EB, were born in Morocco or The Netherlands. One Moroccan patient had an open bladder surface for 22 years due to late surgical reconstruction, avoided upright posture and developed severe lumbar scoliosis. Because three maternally related males from a consanguineous family living in different environments are affected, a genetic basis for EB is likely in this family. To screen for chromosomal aberrations and micro-aberrations in the two patients who were still alive, conventional karyotyping and array-based comparative genomic hybridization (array-CGH) were used on DNA-microarrays with a genome-wide average resolution of <0.5 Mb.

RESULTS

Chromosome banding revealed normal karyotypes. By array-CGH, one of 8000 clones was aberrant in both affected cousins. This clone from 1p21.1 contained the AMY1B, AMY2B, AMY2A genes coding for salivary and pancreatic amylases.

CONCLUSION

The aberration detected by array-CGH in both affected cousins is a known copy-number variant and most likely unrelated to the EB-phenotype. Nevertheless, in this family the nonsyndromic EB could be a monogenic disorder inherited in an autosomal-recessive or X-linked fashion.

CXorf6 is a causative gene for hypospadias

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

Characterization of MTM1 mutations in 31 Japanese families with myotubular myopathy, including a patient carrying 240kb deletion in Xq28 without male hypogenitalism

X-linked myotubular myopathy is a congenital muscle disorder due to MTM1 mutation, and is characterized clinically by generalized muscle weakness and hypotonia at birth usually resulting in early death. We newly identified 26 unrelated Japanese patients with MTM1 mutations by genomic DNA and transcript analysis, including 12 novel mutations. Among 31 patients, including our previously reported five patients, the c.1261-10A>G splice site mutation was the most frequent mutation. Three mutations, one missense and two splice site, were associated with milder phenotype. Of particular interest, one boy had a 240 kb deletion in Xq28 encompassing CXorf6 (formerly F18), MTM1 and MTMR1 but was not accompanied by hypogenitalism. CXorf6, which have been implicated in male sexual development, was not entirely deleted in this boy, resulting in the fusion with the MTMR1 gene. A chimeric fusion transcript was detected in patient's muscle by RT-PCR, suggesting this fusion gene product avoids the phenotype. This deletion led us to refine the critical region of CXorf6 for the development of male genitalia.

Seven new cases of familial isolated bladder exstrophy and epispadias complex (BEEC) and review of the literature

Clustering of the bladder exstrophy and epispadias complex (BEEC) has been described in 29 families. To explore the genetic contribution we located new families through the German and Austrian Bladder Exstrophy Support Group and the Association for the Bladder Exstrophy Community (predominantly US and Canada). We report six new families with two occurrences of BEEC, one family where the proband is the product of a consanguineous union, and four discordant twin pairs. In conjunction with the published clinical and epidemiological reports this collection suggests that there is a significant genetic predisposition for susceptibility to the BEEC. It also highlights the importance of self-referral groups for recruiting families for multicenter collaborative research efforts to identify susceptibility loci.

Genotype-phenotype correlations in X-linked myotubular myopathy

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

A distinctive phenotype associated with an interstitial deletion 6q14 contained within a de novo pericentric inversion 6 (p11.2q15)

This report describes a nearly 25-year-old female with an interstitial deletion of band 14 in the long arm of one chromosome 6 (6q14). The deletion is contained within a de novo pericentric inversion with breakpoints in 6p11.2 and 6q15 (Karyotype 46,XX, del(6)(q13q15),inv(6)(p11.2q15). The distal breakpoint of the deletion and the pericentric inversion at 6q15 are the same, but the proximal breakpoints differ. Since cells with other chromosomal findings were not detected in cultured lymphocytes and fibroblasts, chromosome mosaicism seems unlikely. Thus, it is assumed that the inversion and the deletion originated from the same event. The development of a distinctive phenotype in the patient was observed over a period of 22 years. It includes characteristic dysmorphic facial features such as ocular hypertelorism, flat nasal bridge, prominent zygomatic bones, and a depressed glabella. A striking, non-progressive deficit of motor control is manifest in an inability to use her hands properly and a broad-based slow-motion-like gait. Although severely deficient in abstract mental abilities and speech development, she is well adapted to family life and to a school for retarded individuals. Normal height and head circumference, and reduced sensitivity to pain are noteworthy. Presumably the deletion caused the phenotype and the distinct behavioral pattern. This patient probably represents a novel chromosomal phenotype that results from aggregate haploinsufficiency of gene loci in the deleted region.

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).