Protein- and mRNA-based phenotype-genotype correlations in DMD/BMD with point mutations and molecular basis for BMD with nonsense and frameshift mutations in the DMD gene

The Effect of Genotype Differences on Cardiac Involvement in Cases Diagnosed with Duchenne Muscular Dystrophy

AIM

 Duchenne muscular dystrophy (DMD) is the most frequently seen muscular disease in childhood. Cardiac involvement is extremely important in terms of morbidity and mortality in these patients. Different studies have shown that mutations occurring in various exons are cardioprotective or increase cardiac involvement in DMD cases. The aim of this study was to examine the effect of genotype differences on cardiac involvement in patients diagnosed with DMD with genetic analysis.

MATERIAL AND METHOD

 A retrospective analysis of DMD patients followed up in the Muscle Diseases Centre of Health Sciences University Izmir Tepecik Training and Research Hospital was done.

RESULTS

 Evaluation was made of 120 male DMD patients with a mean age of 9.66 ± 5.10 years. According to the genetic analysis results, 76.7% deletions, 15.8% mutations, and 7.5% duplications were determined. Of the mutations determined, 65.8% were between exons 44 and 54, 17.5% between exons 1 and 18, and 9.2% between exons 19 and 43, 5.8% were non-sense mutations, and 1.7% were on exons >54. In the cases determined with cardiac involvement, the mean age of onset was 11.87 ± 3.11 years. When ejection fraction (EF) <56% or fractional shortening (FS) <28% was accepted as systolic dysfunction cardiac effect, 12.5% of the cases were determined with cardiac involvement. Of the cases determined with cardiac effects, 86.7% were aged >10 years. Electrocardiography was evaluated as normal in 54.5%, sinus tachycardia in 24.2%, short PR in 15.2%, and right and left ventricle hypertrophy in 8.1%. No statistically significant difference was determined in mutation types and location according to the age of cardiac involvement. The left ventricle (LV) posterior wall thickness value determined in the exon 44-54 group was higher than in DMD cases with other mutations. Although not statistically significant, an important result was that the LV posterior wall and IVSed values were evaluated to be high.

CONCLUSION

 The current study results and findings in literature have not found a clear relationship between genotypes and cardiac involvement in DMD cases.

A novel deep intronic variant in the DMD gene causes Duchenne muscular dystrophy by pseudoexon activation encoding a nonsense codon

Dystrophinopathies are a group of neuromuscular disorders, inherited in an X-linked recessive manner, caused by pathogenic variants in the DMD gene. Copy number variation detection and next generation sequencing allow the detection of around 99 % of the pathogenic variants. However, some patients require mRNA studies from muscle biopsies to identify deep intronic pathogenic variants. Here, we report a child suspected of having Duchenne muscular dystrophy, with a muscle biopsy showing dystrophin deficiency, and negative molecular testing for deletions, duplications, and small variants. mRNA analysis from muscle biopsy revealed a pseudoexon activation that introduce a premature stop codon into the reading frame. gDNA sequencing allowed to identified a novel variant, c.832-186 T>G, which creates a cryptic donor splice site, recognizing the underlying mechanism causing the pseudoexon insertion. This case highlights the usefulness of the mRNA analysis from muscle biopsy when routine genetic testing is negative and clinical suspicion of dystrophinopathies remains the main clinical diagnosis suspicion.

Comprehensive analysis of 2097 patients with dystrophinopathy based on a database from 2011 to 2021

BACKGROUND

An increasing number of clinical trials for new therapeutic strategies are underway or being considered for dystrophinopathy. Having detailed data on the natural progression of this condition is crucial for assessing the effectiveness of new drugs. However, there's a lack of data regarding the long-term data on the natural course and how it's managed in China. In this study, we offer a comprehensive overview of clinical and molecular findings, as well as treatment outcomes in the Chinese population.

METHODS

Institutional data on all patients with dystrophinopathy from August 2011 to August 2021 were retrospectively reviewed. The data included geographic distribution, age at diagnosis, molecular findings, and treatment options, such as corticosteroids, cardiac interventions, and clinical outcomes.

RESULTS

In total, 2097 patients with dystrophinopathy, including 1703 cases of Duchenne muscular dystrophy (DMD), 311 cases of Becker muscular dystrophy (BMD), 46 cases of intermediate muscular dystrophy (IMD), and 37 cases categorized as "pending" (individuals with an undetermined phenotype), were registered in the Children's Hospital of Fudan University database for dystrophinopathy from August 2011 to August 2021. The spectrum of identified variants included exonic deletions (66.6%), exonic duplications (10.7%), nonsense variants (10.3%), splice-site variants (4.5%), small deletions (3.5%), small insertions/duplications (1.8%), and missense variants (0.9%). Four deep intronic variants and two inversion variants were identified. Regarding treatment, glucocorticoids were administered to 54.4% of DMD patients and 39.1% of IMD patients. The median age at loss of ambulation was 2.5 years later in DMD patients who received glucocorticoid treatment. Overall, one cardiac medicine at least was prescribed to 7.4% of DMD patients, 8.3% of IMD patients, and 2.6% of BMD patients. Additionally, ventilator support was required by four DMD patients. Eligibility for exon skipping therapy was found in 55.3% of DMD patients, with 12.9%, 10%, and 9.6% of these patients being eligible for skipping exons 51, 53, and 45, respectively.

CONCLUSIONS

This is one of the largest studies to have evaluated the natural history of dystrophinopathy in China, which is particularly conducive to the recruitment of eligible patients for clinical trials and the provision of real-world data to support drug development.

Investigation of genotype-phenotype and familial features of Turkish dystrophinopathy patients

Duchenne muscular dystrophy (DMD) and Becker muscular dystrophy (BMD) are X-linked recessive allelic muscle diseases caused by dystrophin gene mutations. Eight hundred thirty-seven patients admitted between 1997 and 2022 were included in the study. Two hundred twenty patients were analyzed by multiplex PCR (mPCR) alone. Five hundred ninety-five patients were investigated by multiplex ligation-dependent probe amplification (MLPA), and 54 patients were examined by sequencing. Deletion was detected in 60% (132/220) of the cases in the mPCR group only and in 58.3% (347/595) of the cases with MLPA analysis. The rates of deletion and duplication were 87.7% and 12.3%, respectively, in the MLPA analysis. Single exon deletions were the most common mutation type. The introns 43-55 (81.8%) and exons 2-21 (13.1%) regions were detected as hot spots in deletions. It was determined that 89% of the mutations were suitable for exon skipping therapy. The reading frame rule did not hold in 7.6% of D/BMD cases (17/224). We detected twenty-five pathogenic/likely pathogenic variants in sequencing, five of which were novel variants. Nonsense mutation was the most common small mutation (44%). 21% of DMD patients were familial. We detected germline mosaicism in four families (4.3%) in the large rearrangement group and one gonosomal mosaicism in a family with a nonsense mutation. This is the largest study examining genotype and phenotype data in Turkish D/BMD families investigated by MLPA analysis. The reading frame hypothesis is not valid in all cases. Sharing the genotype and phenotype characteristics of these cases in the literature will shed light on the molecular structure of DMD and guide gene therapy research. In genetic counseling, carrier screening in the family and possible gonadal mosaicism should be emphasized.

Characterization of Phenotypic Variability in Becker Muscular Dystrophy for Clinical Practice and Towards Trial Readiness: A Two-Years Follow up Study

Background

Becker muscular dystrophy (BMD) is a dystrophinopathy due to in-frame mutations in the dystrophin gene (DMD) which determines a reduction of dystrophin at muscle level. BMD has a wide spectrum of clinical variability with different degrees of disability. Studies of natural history are needed also in view of up-coming clinical trials.

Objectives

From an initial cohort of 32 BMD adult subjects, we present a detailed phenotypic characterization of 28 patients, then providing a description of their clinical natural history over the course of 12 months for 18 and 24 months for 13 of them.

Methods

Each patient has been genetically characterized. Baseline, and 1-year and 2 years assessments included North Star Ambulatory Assessment (NSAA), timed function tests (time to climb and descend four stairs), 6-minute walk test (6MWT), Walton and Gardner-Medwin Scale and Medical Research Council (MRC) scale. Muscle magnetic resonance imaging (MRI) was acquired at baseline and in a subgroup of 9 patients after 24 months. Data on cardiac function (electrocardiogram, echocardiogram, and cardiac MRI) were also collected.

Results and conclusions

Among the clinical heterogeneity, a more severe involvement is often observed in patients with 45-X del, with a disease progression over two years. The 6MWT appears sensitive to detect modification from baseline during follow up while no variation was observed by MRC testing. Muscle MRI of the lower limbs correlates with clinical parameters.Our study further highlights how the phenotypic variability of BMD adult patients makes it difficult to describe an uniform course and substantiates the need to identify predictive parameters and biomarkers to stratify patients.

Advances in Dystrophinopathy Diagnosis and Therapy

Dystrophinopathies are x-linked muscular disorders which emerge from mutations in the Dystrophin gene, including Duchenne and Becker muscular dystrophy, and dilated cardiomyopathy. However, Duchenne muscular dystrophy interconnects with bone loss and osteoporosis, which are exacerbated by glucocorticoids therapy. Procedures for diagnosing dystrophinopathies include creatine kinase assay, haplotype analysis, Southern blot analysis, immunological analysis, multiplex PCR, multiplex ligation-dependent probe amplification, Sanger DNA sequencing, and next generation DNA sequencing. Pharmacological therapy for dystrophinopathies comprises glucocorticoids (prednisone, prednisolone, and deflazacort), vamorolone, and ataluren. However, angiotensin-converting enzyme (ACE) inhibitors, angiotensin receptor blockers (ARBs), and β-blockers are the first-line to prevent dilated cardiomyopathy in dystrophinopathy patients. Duchenne muscular dystrophy gene therapy strategies involve gene transfer, exon skipping, exon reframing, and CRISPR gene editing. Eteplirsen, an antisense-oligonucleotide drug for skipping exon 51 from the Dystrophin gene, is available on the market, which may help up to 14% of Duchenne muscular dystrophy patients. There are various FDA-approved exon skipping drugs including ExonDys-51 for exon 51, VyonDys-53 and Viltolarsen for exon 53 and AmonDys-45 for exon 45 skipping. Other antisense oligonucleotide drugs in the pipeline include casimersen for exon 45, suvodirsen for exon 51, and golodirsen for exon 53 skipping. Advances in the diagnosis and therapy of dystrophinopathies offer new perspectives for their early discovery and care.

Dystrophin Short Product, Dp71, Interacts with AQP4 and Kir4.1 Channels in the Mouse Cerebellar Glial Cells in Contrast to Dp427 at Inhibitory Postsynapses in the Purkinje Neurons

Dystrophin is the causative gene for Duchenne and Becker muscular dystrophy (DMD/BMD), and it produces full-length and short dystrophin, Dp427 and Dp71, respectively, in the brain. The existence of the different dystrophin molecular complexes has been known for a quarter century, so it is necessary to derive precise expression profiles of the molecular complexes in the brain to elucidate the mechanism of cognitive symptoms in DMD/BMD patients. In order to investigate the Dp71 expression profile in cerebellum, we employed Dp71-specific tag-insertion mice, which allowed for the specific detection of endogenous Dp71 in the immunohistochemical analysis and found its expressions in the glial cells, Bergmann glial (BG) cells, and astrocytes, whereas Dp427 was exclusively expressed in the inhibitory postsynapses within cerebellar Purkinje cells (PCs). Interestingly, we found different cell-type dependent dystrophin molecular complexes; i.e., glia-associated Dp71 was co-expressed with dystroglycan (DG) and dystrobrevinα, whereas synapse-associated Dp427 was co-expressed with DG and dystrobrevinβ. Furthermore, we investigated the molecular relationship of Dp71 to the AQP4 water channel and the Kir4.1 potassium channel, and found biochemical associations of Dp71 with AQP4 and Kir4.1 in both the cerebellum and cerebrum. Immunohistochemical and cytochemical investigations revealed partial co-localizations of Dp71 with AQP4 and Kir4.1 in the glial cells, indicating Dp71 interactions with the channels in the BG cells and astrocytes. Taken together, different cell-types, glial cells and Purkinje neurons, in the cerebellum express different dystrophin molecular complexes, which may contribute to pathological and physiological processes through the regulation of the water/ion channel and inhibitory postsynapses.

Dystrophin (DMD) Missense Variant in Cats with Becker-Type Muscular Dystrophy

Muscular dystrophy due to dystrophin deficiency in humans is phenotypically divided into a severe Duchenne and milder Becker type. Dystrophin deficiency has also been described in a few animal species, and few DMD gene variants have been identified in animals. Here, we characterize the clinical, histopathological, and molecular genetic aspects of a family of Maine Coon crossbred cats with clinically mild and slowly progressive muscular dystrophy. Two young adult male littermate cats exhibited abnormal gait and muscular hypertrophy with macroglossia. Serum creatine kinase activities were highly increased. Histopathologically, dystrophic skeletal muscle exhibited marked structural changes including atrophic, hypertrophic, and necrotic muscle fibers. Immunohistochemistry showed irregularly reduced expression of dystrophin but the staining of other muscle proteins such as β- and γ-sarcoglycans as well as desmin was also diminished. Whole genome sequencing of one affected cat and genotyping of the littermate found both to be hemizygous mutant at a single DMD missense variant (c.4186C>T). No other protein-changing variants in candidate genes for muscular dystrophy were detected. In addition, one clinically healthy male littermate was hemizygous wildtype, while the queen and one female littermate were clinically healthy, but heterozygous. The predicted amino acid exchange (p.His1396Tyr) resides in a conserved central rod spectrin domain of dystrophin. Various protein modeling programs did not predict major disruption of the dystrophin protein by this substitution, but the altered charge of the region may still affect protein function. This study represents the first genotype-to-phenotype correlation of Becker-type dystrophin deficiency in companion animals.

Gene therapy for selected neuromuscular and trinucleotide repeat disorders - An insight to subsume South Asia for multicenter clinical trials

Background

In this article, the authors discuss how they utilized the genetic mutation data in Sri Lankan Duchenne muscular dystrophy (DMD), Spinal muscular atrophy (SMA), Spinocerebellar ataxia (SCA) and Huntington's disease (HD) patients and compare the available literature from South Asian countries to identifying potential candidates for available gene therapy for DMD, SMA, SCA and HD patients.

Methods

Rare disease patients (n = 623) with the characteristic clinical findings suspected of HD, SCA, SMA and Muscular Dystrophy were genetically confirmed using Multiplex Ligation Dependent Probe Amplification (MLPA), and single plex PCR. A survey was conducted in the "Wiley database on Gene Therapy Trials Worldwide" to identify DMD, SMA, SCA, and HD gene therapy clinical trials performed worldwide up to April 2021. In order to identify candidates for gene therapy in other neighboring countries we compared our findings with available literature from India and Pakistan which has utilized the same molecular diagnostic protocol to our study.

Results

From the overall cohort of 623 rare disease patients with the characteristic clinical findings suspected of HD, SCA, SMA and Muscular Dystrophy, n = 343 (55%) [Muscular Dystrophy- 65%; (DMD-139, Becker Muscular Dystrophy -BMD-11), SCA type 1-3-53% (SCA1-61,SCA2- 23, SCA3- 39), HD- 52% (45) and SMA- 34% (22)] patients were positive for molecular diagnostics by MLPA and single plex PCR. A total of 147 patients in Sri Lanka amenable to available gene therapy; [DMD-83, SMA-15 and HD-49] were identified. A comparison of Sri Lankan finding with available literature from India and Pakistan identified a total of 1257 patients [DMD-1076, SMA- 57, and HD-124] from these three South Asian Countries as amenable for existing gene therapy trials. DMD, SMA, and HD gene therapy clinical trials (113 studies) performed worldwide up to April 2021 were concentrated mostly (99%) in High Income Countries (HIC) and Upper Middle-Income Countries (UMIC). However, studies on the potential use of anti-sense oligonucleotides (ASO) for treatment of SCAs have yet to reach clinical trials.

Conclusion

Most genetic therapies for neurodegenerative and neuromuscular disorders have been evaluated for efficacy primarily in Western populations. No multicenter gene therapy clinical trial sites for DMD, SMA and HD in the South Asian region, leading to lack of knowledge on the safety and efficacy of such personalized therapies in other populations, including South Asians. By fostering collaboration between researchers, clinicians, patient advocacy groups, government and industry in gene therapy initiatives for the inherited-diseases community in the developing world would link the Global North and Global South and breathe life into the motto "Together we can make a difference".

Tissue- and cell-specific whole-transcriptome meta-analysis from brain and retina reveals differential expression of dystrophin complexes and new dystrophin spliced isoforms

The large DMD gene encodes a group of dystrophin proteins in brain and retina, produced from multiple promoters and alternative splicing events. Dystrophins are core components of different scaffolding complexes in distinct cell types. Their absence may thus alter several cellular pathways, which might explain the heterogeneous genotype-phenotype relationships underlying central comorbidities in Duchenne muscular dystrophy (DMD). However, the cell-specific expression of dystrophins and associated proteins (DAPs) is still largely unknown. The present study provides a first RNA-Seq-based reference showing tissue- and cell-specific differential expression of dystrophins, splice variants and DAPs in mouse brain and retina. We report that a cell type may express several dystrophin complexes, perhaps due to expression in separate cell subdomains and/or subpopulations, some of which with differential expression at different maturation stages. We also identified new splicing events in addition to the common exon-skipping events. These include a new exon within intron 51 (E51b) in frame with the flanking exons in retina, as well as inclusions of intronic sequences with stop codons leading to the presence of transcripts with elongated exons 40 and/or 41 (E40e, E41e) in both retina and brain. PCR validations revealed that the new exons may affect several dystrophins. Moreover, immunoblot experiments using a combination of specific antibodies and dystrophin-deficient mice unveiled that the transcripts with stop codons are translated into truncated proteins lacking their C-terminus, which we called N-Dp427 and N-Dp260. This study thus uncovers a range of new findings underlying the complex neurobiology of DMD.

Generation of dystrophin short product-specific tag-insertion mouse: distinct Dp71 glycoprotein complexes at inhibitory postsynapse and glia limitans

Duchenne muscular dystrophy (DMD), the most severe form of dystrophinopathies, is a fatal X-linked recessive neuromuscular disorder characterized by progressive muscle degeneration and various extents of intellectual disabilities. Physiological and pathological roles of the responsible gene, dystrophin, in the brain remain elusive due to the presence of multiple dystrophin products, mainly full-length dystrophin, Dp427, and the short product, Dp71. In this study, we generated a Dp71-specific hemagglutinin (HA) peptide tag-insertion mice to enable specific detection of intrinsic Dp71 expression by anti-HA-tag antibodies. Immunohistochemical detections in the transgenic mice demonstrated Dp71 expression not only at the blood-brain barrier, where astrocytic endfeet surround the microvessels, but also at the inhibitory postsynapse of hippocampal dentate granule neurons. Interestingly, hippocampal cornu ammonis (CA)1 pyramidal neurons were negative for Dp71, although Dp427 detected by anti-dystrophin antibody was clearly present at the inhibitory postsynapse, suggesting cell-type dependent dystrophin expressions. Precise examination using the primary hippocampal culture validated exclusive localization of Dp71 at the inhibitory postsynaptic compartment but not at the excitatory synapse in neurons. We further performed interactome analysis and found that Dp71 formed distinct molecular complexes, i.e. synapse-associated Dp71 interacted with dystroglycan (Dg) and dystrobrevinβ (Dtnb), whereas glia-associated Dp71 did with Dg and dystrobrevinα (Dtna). Thus, our data indicate that Dp71 and its binding partners are relevant to the inhibitory postsynaptic function of hippocampal granule neurons and the novel Dp71-transgenic mouse provides a valuable tool to understand precise physiological expressions and functions of Dp71 and its interaction proteins in vivo and in vitro.

Analysis of Pathogenic Pseudoexons Reveals Novel Mechanisms Driving Cryptic Splicing

Understanding pre-mRNA splicing is crucial to accurately diagnosing and treating genetic diseases. However, mutations that alter splicing can exert highly diverse effects. Of all the known types of splicing mutations, perhaps the rarest and most difficult to predict are those that activate pseudoexons, sometimes also called cryptic exons. Unlike other splicing mutations that either destroy or redirect existing splice events, pseudoexon mutations appear to create entirely new exons within introns. Since exon definition in vertebrates requires coordinated arrangements of numerous RNA motifs, one might expect that pseudoexons would only arise when rearrangements of intronic DNA create novel exons by chance. Surprisingly, although such mutations do occur, a far more common cause of pseudoexons is deep-intronic single nucleotide variants, raising the question of why these latent exon-like tracts near the mutation sites have not already been purged from the genome by the evolutionary advantage of more efficient splicing. Possible answers may lie in deep intronic splicing processes such as recursive splicing or poison exon splicing. Because these processes utilize intronic motifs that benignly engage with the spliceosome, the regions involved may be more susceptible to exonization than other intronic regions would be. We speculated that a comprehensive study of reported pseudoexons might detect alignments with known deep intronic splice sites and could also permit the characterisation of novel pseudoexon categories. In this report, we present and analyse a catalogue of over 400 published pseudoexon splice events. In addition to confirming prior observations of the most common pseudoexon mutation types, the size of this catalogue also enabled us to suggest new categories for some of the rarer types of pseudoexon mutation. By comparing our catalogue against published datasets of non-canonical splice events, we also found that 15.7% of pseudoexons exhibit some splicing activity at one or both of their splice sites in non-mutant cells. Importantly, this included seven examples of experimentally confirmed recursive splice sites, confirming for the first time a long-suspected link between these two splicing phenomena. These findings have the potential to improve the fidelity of genetic diagnostics and reveal new targets for splice-modulating therapies.

RNA Targeting in Inherited Neuromuscular Disorders: Novel Therapeutic Strategies to Counteract Mis-Splicing

Neuromuscular disorders represent multifaceted abnormal conditions, with little or no cure, leading to patient deaths from complete muscle wasting and atrophy. Despite strong efforts in the past decades, development of effective treatments is still urgently needed. Advent of next-generation sequencing technologies has allowed identification of novel genes and mutations associated with neuromuscular pathologies, highlighting splicing defects as essential players. Deciphering the significance and relative contributions of defective RNA metabolism will be instrumental to address and counteract these malignancies. We review here recent progress on the role played by alternative splicing in ensuring functional neuromuscular junctions (NMJs), and its involvement in the pathogenesis of NMJ-related neuromuscular disorders, with particular emphasis on congenital myasthenic syndromes and muscular dystrophies. We will also discuss novel strategies based on oligonucleotides designed to bind their cognate sequences in the RNA or targeting intermediary of mRNA metabolism. These efforts resulted in several chemical classes of RNA molecules that have recently proven to be clinically effective, more potent and better tolerated than previous strategies.

Genetic Profile of the Dystrophin Gene Reveals New Mutations in Colombian Patients Affected with Muscular Dystrophinopathy

Background

Duchenne and Becker muscular dystrophies (DMD/BMD) are the most common human dystrophinopathies with recessive X-linked inheritance. Dystrophin gene deletions and duplications are the most common mutations, followed by point mutations. The aim of this study is to characterize the mutational profile of the dystrophin gene in Colombian patients with DMD/BMD.

Material and Methods

Mutational profiling was determined in 69 affected patients using Sanger sequencing, next-generation sequencing (NGS) and/or multiplex ligation dependent-probes amplification (MLPA). Genetic variants were classified according to molecular consequence and new variants were determined through database and literature analysis.

Results

Mutational profile in affected patients revealed that large deletions/duplications analyzed by MLPA accounted for 72.5% of all genetic variations. By using Sanger sequencing or NGS, we identified point mutations in 15.9% and small deletions in 11.6% of the patients. New mutations were found, most of them were point mutations or small deletions (10.1%).

Conclusion

Our results described the genetic profile of the dystrophin gene in Colombian patients with DMD and contribute to efforts to identify molecular variants in Latin American populations. For our population, 18.8% of cases could be treated with FDA or MDA approved molecular therapies based on specific mutations. These data contribute to the establishment of appropriate genetic counseling and potential treatment.

Dystroglycan regulates proper expression, submembranous localization and subsequent phosphorylation of Dp71 through physical interaction

Dystrophin-dystroglycan complex (DGC) plays important roles for structural integrity and cell signaling, and its defects cause progressive muscular degeneration and intellectual disability. Dystrophin short product, Dp71, is abundantly expressed in multiple tissues other than muscle and is suspected of contributing to cognitive functions; however, its molecular characteristics and relation to dystroglycan (DG) remain unknown. Here, we report that DG physically interacts with Dp71 in cultured cells. Intriguingly, DG expression positively and DG knockdown negatively affected the steady-state expression, submembranous localization and subsequent phosphorylation of Dp71. Mechanistically, two EF-hand regions along with a ZZ motif of Dp71 mediate its association with the transmembrane proximal region, amino acid residues 788-806, of DG cytoplasmic domain. Most importantly, the pathogenic point mutations of Dp71, C272Y in the ZZ motif or L170del in the second EF-hand region, impaired its binding to DG, submembranous localization and phosphorylation of Dp71, indicating the relevance of DG-dependent Dp71 regulatory mechanism to pathophysiological conditions. Since Dp140, another dystrophin product, was also regulated by DG in the same manner as Dp71, our results uncovered a tight molecular relation between DG and dystrophin, which has broad implications for understanding the DGC-related cellular physiology and pathophysiology.

A novel DMD intronic alteration: a potentially disease-causing variant of an intermediate muscular dystrophy phenotype

Pathogenic germline variants in DMD gene, which encodes the well-known cytoskeletal protein named dystrophin, are associated with a wide range of dystrophinopathies disorders, such as Duchenne muscular dystrophy (DMD, severe form), Becker muscular dystrophy (BMD, mild form) and intermediate muscular dystrophy (IMD). Muscle biopsy, immunohistochemistry, molecular (multiplex ligation-dependent probe amplification (MLPA)/next-generation sequencing (NGS) and Sanger methods) and in silico analyses were performed in order to identify alterations in DMD gene and protein in a patient with a clinical manifestation and with high creatine kinase levels. Herein, we described a previously unreported intronic variant in DMD and reduced dystrophin staining in the muscle biopsy. This novel DMD variant allele, c.9649+4A>T that was located in a splice donor site within intron 66. Sanger sequencing analysis from maternal DNA showed the presence of both variant c.9649+4A>T and wild-type (WT) DMD alleles. Different computational tools suggested that this nucleotide change might affect splicing through a WT donor site disruption, occurring in an evolutionarily conserved region. Indeed, we observed that this novel variant, could explain the reduced dystrophin protein levels and discontinuous sarcolemmal staining in muscle biopsy, which suggests that c.9649+4A>T allele may be re-classified as pathogenic in the future. Our data show that the c.9649+4A>T intronic sequence variant in the DMD gene may be associated with an IMD phenotype and our findings reinforce the importance of a more precise diagnosis combining muscle biopsy, molecular techniques and comprehensive in silico approaches in the clinical cases with negative results for conventional genetic analysis.

Small mutations in Duchenne/Becker muscular dystrophy in 164 unrelated Polish patients

In the 164 patients with Duchenne/Becker muscular dystrophy, we found 142 different small mutations including 51 novel mutations not listed in the LOVD, the UMD-DMD, the ClinVar, and the HGMD databases. Among all mutations, nonsense mutations occurred in 45.7%, frameshift mutations in 32.9%, and splicing mutations in 19.5%. Small mutations were distributed throughout the whole dystrophin gene. Splicing mutations were twice more common in BMD patients than in DMD patients. Eighty-two percent of mothers of the males affected with DMD/BMD were found to be carriers of small mutations.

From Mice to Humans: An Overview of the Potentials and Limitations of Current Transgenic Mouse Models of Major Muscular Dystrophies and Congenital Myopathies

Muscular dystrophies are a group of more than 160 different human neuromuscular disorders characterized by a progressive deterioration of muscle mass and strength. The causes, symptoms, age of onset, severity, and progression vary depending on the exact time point of diagnosis and the entity. Congenital myopathies are rare muscle diseases mostly present at birth that result from genetic defects. There are no known cures for congenital myopathies; however, recent advances in gene therapy are promising tools in providing treatment. This review gives an overview of the mouse models used to investigate the most common muscular dystrophies and congenital myopathies with emphasis on their potentials and limitations in respect to human applications.

Very Low Residual Dystrophin Quantity Is Associated with Milder Dystrophinopathy

Objective

This study was undertaken to determine whether a low residual quantity of dystrophin protein is associated with delayed clinical milestones in patients with DMD mutations.

Methods

We performed a retrospective multicentric cohort study by using molecular and clinical data from patients with DMD mutations registered in the Universal Mutation Database–DMD France database. Patients with intronic, splice site, or nonsense DMD mutations, with available muscle biopsy Western blot data, were included irrespective of whether they presented with severe Duchenne muscular dystrophy (DMD) or milder Becker muscular dystrophy (BMD). Patients were separated into 3 groups based on dystrophin protein levels. Clinical outcomes were ages at appearance of first symptoms; loss of ambulation; fall in vital capacity and left ventricular ejection fraction; interventions such as spinal fusion, tracheostomy, and noninvasive ventilation; and death.

Results

Of 3,880 patients with DMD mutations, 90 with mutations of interest were included. Forty‐two patients expressed no dystrophin (group A), and 31 of 42 (74%) developed DMD. Thirty‐four patients had dystrophin quantities < 5% (group B), and 21 of 34 (61%) developed BMD. Fourteen patients had dystrophin quantities ≥ 5% (group C), and all but 4 who lost ambulation beyond 24 years of age were ambulant. Dystrophin quantities of <5%, as low as <0.5%, were associated with milder phenotype for most of the evaluated clinical outcomes, including age at loss of ambulation (p < 0.001).

Interpretation

Very low residual dystrophin protein quantity can cause a shift in disease phenotype from DMD toward BMD. ANN NEUROL 2021;89:280–292

Clinical correlations and long-term follow-up in 100 patients with sarcoglycanopathies

BACKGROUND AND PURPOSE

To describe a large series of patients with α, β, and γ sarcoglycanopathies (LGMD-R3, R4, and R5) and study phenotypic correlations and disease progression.

METHODS

A multicentric retrospective study in four centers in the Paris area collecting neuromuscular, respiratory, cardiac, histologic, and genetic data. The primary outcome of progression was age of loss of ambulation (LoA); disease severity was established according to LoA before or after 18 years of age. Time-to-event analysis was performed.

RESULTS

One hundred patients (54 γ-SG; 41 α-SG; 5 β-SG) from 80 families were included. The γ-SG patients had earlier disease onset than α-SG patients (5.5 vs. 8 years; p = 0.022) and β-SG patients (24.4 years). Axial muscle weakness and joint contractures were frequent and exercise intolerance was observed. At mean follow-up of 22.9 years, 65.3% of patients were wheelchair-bound (66.7% α-SG, 67.3% γ-SG, 40% β-SG). Dilated cardiomyopathy occurred in all sarcoglycanopathy subtypes, especially in γ-SG patients (p = 0.01). Thirty patients were ventilated and six died. Absent sarcoglycan protein expression on muscle biopsy and younger age at onset were associated with earlier time to LoA (p = 0.021 and p = 0.002). Age at onset was an independent predictor of both severity and time to LoA (p = 0.0004 and p = 0.009). The α-SG patients showed genetic heterogeneity, whereas >90% of γ-SG patients carried the homozygous c.525delT frameshift variant. Five new mutations were identified.

CONCLUSIONS

This large multicentric series delineates the clinical spectrum of patients with sarcoglycanopathies. Age at disease onset is an independent predictor of severity of disease and LoA, and should be taken into account in future clinical trials.

Pseudoexons of the DMD Gene

The DMD gene is the largest in the human genome, with a total intron content exceeding 2.2Mb. In the decades since DMD was discovered there have been numerous reported cases of pseudoexons (PEs) arising in the mature DMD transcripts of some individuals, either as the result of mutations or as low-frequency errors of the spliceosome. In this review, I collate from the literature 58 examples of DMD PEs and examine the diversity and commonalities of their features. In particular, I note the high frequency of PEs that arise from deep intronic SNVs and discuss a possible link between PEs induced by distal mutations and the regulation of recursive splicing.

Practical approach to the genetic diagnosis of unsolved dystrophinopathies: a stepwise strategy in the genomic era

OBJECTIVE

To investigate the diagnostic value of implementing a stepwise genetic testing strategy (SGTS) in genetically unsolved cases with dystrophinopathies.

METHODS

After routine genetic testing in 872 male patients with highly suspected dystrophinopathies, we identified 715 patients with a pathogenic DMD variant. Of the 157 patients who had no pathogenic DMD variants and underwent a muscle biopsy, 142 patients were confirmed to have other myopathies, and 15 suspected dystrophinopathies remained genetically undiagnosed. These 15 patients underwent a more comprehensive evaluation as part of the SGTS pipeline, which included the stepwise analysis of dystrophin mRNA, short-read whole-gene DMD sequencing, long-read whole-gene DMD sequencing and in silico bioinformatic analyses.

RESULTS

SGTS successfully yielded a molecular diagnosis of dystrophinopathy in 11 of the 15 genetically unsolved cases. We identified 8 intronic and 2 complex structural variants (SVs) leading to aberrant splicing in 10 of 11 patients, of which 9 variants were novel. In one case, a molecular defect was detected on mRNA and protein level only. Aberrant splicing mechanisms included 6 pseudoexon inclusions and 4 alterations of splice sites and splicing regulatory elements. We showed for the first time the exonisation of a MER48 element as a novel pathogenic mechanism in dystrophinopathies.

CONCLUSION

Our study highlights the high diagnostic utility of implementing a SGTS pipeline in dystrophinopathies with intronic variants and complex SVs.

The position of nonsense mutations can predict the phenotype severity: A survey on the DMD gene

A nonsense mutation adds a premature stop signal that hinders any further translation of a protein-coding gene, usually resulting in a null allele. To investigate the possible exceptions, we used the DMD gene as an ideal model. First, because dystrophin absence causes Duchenne muscular dystrophy (DMD), while its reduction causes Becker muscular dystrophy (BMD). Second, the DMD gene is X-linked and there is no second allele that can interfere in males. Third, databases are accumulating reports on many mutations and phenotypic data. Finally, because DMD mutations may have important therapeutic implications. For our study, we analyzed large databases (LOVD, HGMD and ClinVar) and literature and revised critically all data, together with data from our internal patients. We totally collected 2593 patients. Positioning these mutations along the dystrophin transcript, we observed a nonrandom distribution of BMD-associated mutations within selected exons and concluded that the position can be predictive of the phenotype. Nonsense mutations always cause DMD when occurring at any point in fifty-one exons. In the remaining exons, we found milder BMD cases due to early 5’ nonsense mutations, if reinitiation can occur, or due to late 3’ nonsense when the shortened product retains functionality. In the central part of the gene, all mutations in some in-frame exons, such as in exons 25, 31, 37 and 38 cause BMD, while mutations in exons 30, 32, 34 and 36 cause DMD. This may have important implication in predicting the natural history and the efficacy of therapeutic use of drug-stimulated translational readthrough of premature termination codons, also considering the action of internal natural rescuers. More in general, our survey confirm that a nonsense mutation should be not necessarily classified as a null allele and this should be considered in genetic counselling.

Exon skipping induced by nonsense/frameshift mutations in DMD gene results in Becker muscular dystrophy

Duchenne muscular dystrophy (DMD) is caused by a nonsense or frameshift mutation in the DMD gene, while its milder form, Becker muscular dystrophy (BMD) is caused by an in-frame deletion/duplication or a missense mutation. Interestingly, however, some patients with a nonsense mutation exhibit BMD phenotype, which is mostly attributed to the skipping of the exon containing the nonsense mutation, resulting in in-frame deletion. This study aims to find BMD cases with nonsense/frameshift mutations in DMD and to investigate the exon skipping rate of those nonsense/frameshift mutations. We searched for BMD cases with nonsense/frameshift mutations in DMD in the Japanese Registry of Muscular Dystrophy. For each DMD mutation identified, we constructed minigene plasmids containing one exon with/without a mutation and its flanking intronic sequence. We then introduced them into HeLa cells and measured the skipping rate of transcripts of the minigene by RT-qPCR. We found 363 cases with a nonsense/frameshift mutation in DMD gene from a total of 1497 dystrophinopathy cases in the registry. Among them, 14 had BMD phenotype. Exon skipping rates were well correlated with presence or absence of dystrophin, suggesting that 5% exon skipping rate is critical for the presence of dystrophin in the sarcolemma, leading to milder phenotypes. Accurate quantification of the skipping rate is important in understanding the exact functions of the nonsense/frameshift mutations in DMD and for interpreting the phenotypes of the BMD patients.

Importance of muscle biopsy to establish pathogenicity of DMD missense and splice variants

A precise genetic diagnosis of a dystrophinopathy has far-reaching implications for affected boys and their families. We present three boys with DMD single nucleotide variants associated with Becker muscular dystrophy presenting with myalgia, reduced exercise capacity, neurodevelopmental symptoms and elevated creatine kinase. The DMD variants were difficult to classify: AIII:1 a synonymous variant in exon 13 c.1602G>A, p.Lys534Lys; BIII:1 an essential splice-site variant in intron 33 c.4674+1G>A, and CII:1 a missense mutation within the cysteine-rich domain, exon 66 c.9619T>C, p.Cys3207Arg. Complementary DNA (cDNA) analysis using muscle-derived mRNA established splice-altering effects of variants for AIII:1 and BIII:1, and normal splicing in CII:1. Western blot analysis demonstrated mildly to moderately reduced dystrophin levels (17.6 - 36.1% the levels of controls), supporting dystrophinopathy as a probable diagnosis. These three cases highlight the diagnostic utility of muscle biopsy for mRNA studies and western blot to investigate DMD variants of uncertain pathogenicity, by exploring effects on splicing and dystrophin protein levels.

X-chromosome inactivation pattern of amniocytes predicts the risk of dystrophinopathy in fetal carriers of DMD mutations

OBJECTIVE

To predict the risk of dystrophinopathy in fetal carriers of dystrophin gene (DMD) mutations.

METHODS

Twenty-three pregnant women, with a total of 25 female fetuses carrying DMD mutations, were recruited. Among them, 13 pregnant women who participated in this study were only used to analyse the incidence of induced abortion after fetuses were diagnosed as dystrophinopathy carriers. Eleven fetal carriers from 10 pregnant women were tested to analyse X-chromosome inactivation (XCI) using amniocytes to assess the risk of dystrophinopathy. Follow-ups were conducted on all cases.

RESULTS

Approximately one-third of fetuses were aborted before assessing the risk of dystrophinopathy. XCI analysis of amniocytes showed that 10 fetuses had random XCI patterns, and one fetus exhibited a highly skewed XCI pattern (100:0) with primary expression of the maternal X chromosome that carried the mutant allele. These 11 fetal carriers were born, and follow-up showed that the girl who showed the skewed XCI pattern as a fetus was diagnosed with Duchenne muscular dystrophy (DMD) at the age of four. The others did not present with dystrophinopathy-associated symptoms.

CONCLUSIONS

XCI was significantly implicated in symptomatic female carriers of dystrophinopathies, and XCI pattern analysis of amniocytes may be useful in predicting the risk of dystrophinopathy in fetal carriers.

Cardiac Dysfunction in Duchenne Muscular Dystrophy Is Less Frequent in Patients With Mutations in the Dystrophin Dp116 Coding Region Than in Other Regions

Supplemental Digital Content is available in the text.

Background:

Duchenne muscular dystrophy (DMD), the most common inherited muscular disease in childhood, is caused by dystrophin deficiency because of mutations in the DMD gene. Although DMD is characterized by fatal progressive muscle wasting, cardiomyopathy is the most important nonmuscle symptom threatening the life of patients with DMD. The relationship between cardiac involvement and dystrophin isoforms has not been analyzed.

Methods and Results:

The results of 1109 echocardiograms obtained from 181 Japanese DMD patients with confirmed mutations in the DMD gene were retrospectively analyzed. Patients showed an age-related decline in left ventricular ejection fraction. Patients were divided by patterns of dystrophin isoform deficiency into 5 groups. The cardiac dysfunction-free survival was significantly higher in the group with mutations in the Dp116 coding region than the others, whereas no significant differences in the other 3 groups. At age 25 years, the cardiac dysfunction-free rate was 0.6 in the Dp116 group, but only 0.1 in others. PCR amplification of Dp116 transcript in human cardiac muscle indicated promoter activation.

Conclusions:

Left ventricular ejection fraction in DMD declined stepwise with age. Cardiac dysfunction was less frequent in Dp116-deficient than other patients with DMD. Dp116 transcript was identified in human cardiac muscle for the first time. These results indicate that Dp116 is associated with cardiac involvement in DMD.

Multiple Exon Skipping in the Duchenne Muscular Dystrophy Hot Spots: Prospects and Challenges

Duchenne muscular dystrophy (DMD), a fatal X-linked recessive disorder, is caused mostly by frame-disrupting, out-of-frame deletions in the dystrophin (DMD) gene. Antisense oligonucleotide-mediated exon skipping is a promising therapy for DMD. Exon skipping aims to convert out-of-frame mRNA to in-frame mRNA and induce the production of internally-deleted dystrophin as seen in the less severe Becker muscular dystrophy. Currently, multiple exon skipping has gained special interest as a new therapeutic modality for this approach. Previous retrospective database studies represented a potential therapeutic application of multiple exon skipping. Since then, public DMD databases have become more useful with an increase in patient registration and advances in molecular diagnosis. Here, we provide an update on DMD genotype-phenotype associations using a global DMD database and further provide the rationale for multiple exon skipping development, particularly for exons 45–55 skipping and an emerging therapeutic concept, exons 3–9 skipping. Importantly, this review highlights the potential of multiple exon skipping for enabling the production of functionally-corrected dystrophin and for treating symptomatic patients not only with out-of-frame deletions but also those with in-frame deletions. We will also discuss prospects and challenges in multiple exon skipping therapy, referring to recent progress in antisense chemistry and design, as well as disease models.

DMD genotype correlations from the Duchenne Registry: Endogenous exon skipping is a factor in prolonged ambulation for individuals with a defined mutation subtype

Antisense oligonucleotide (AON)-mediated exon skipping is an emerging therapeutic for individuals with Duchenne muscular dystrophy (DMD). Skipping of exons adjacent to common exon deletions in DMD using AONs can produce in-frame transcripts and functional protein. Targeted skipping of DMD exons 8, 44, 45, 50, 51, 52, 53, and 55 is predicted to benefit 47% of affected individuals. We observed a correlation between mutation subgroups and age at loss of ambulation in the Duchenne Registry, a large database of phenotypic and genetic data for DMD (N = 765). Males amenable to exon 44 (N = 74) and exon 8 skipping (N = 18) showed prolonged ambulation compared to other exon skip groups and nonsense mutations (P = 0.035 and P < 0.01, respectively). In particular, exon 45 deletions were associated with prolonged age at loss of ambulation relative to the rest of the exon 44 skip amenable cohort and other DMD mutations. Exon 3-7 deletions also showed prolonged ambulation relative to all other exon 8 skippable mutations. Cultured myotubes from DMD patients with deletions of exons 3-7 or exon 45 showed higher endogenous skipping than other mutations, providing a potential biological rationale for our observations. These results highlight the utility of aggregating phenotypic and genotypic data for rare pediatric diseases to reveal progression differences, identify potentially confounding factors, and probe molecular mechanisms that may affect disease severity.

Left bundle branch block in Duchenne muscular dystrophy: Prevalence, genetic relationship and prognosis

Background

Duchenne muscular dystrophy (DMD) is an inherited myogenic disorder due to mutations in the dystrophin gene on chromosome Xp21.1. We designed this study to determine the prevalence of left bundle branch block (LBBB), whether there is a relationship between LBBB and genetic pattern, and to assess predictive factors for acute cardiac events and mortality in adult DMD patients.

Methods

We reviewed the charts of DMD followed at the Home Mechanical Ventilation Unit of the Raymond Poincare University Hospital.

Results

A total of 121 patients, aged from 18 to 41 years have been included in our study. Median vital capacity (VC) was 12% [7; 19.5] of predicted. Almost all patients were on home mechanical ventilation (95%). LBBB was present in 15 patients (13%); among them, 10 disclosed exonic deletions. After a median follow up of 6 years, 21 patients (17%) experienced acute heart failure (AHF), 7 patients (6%) supraventricular arrhythmia, 3 patients (2.4%) ventricular tachycardia, 4 patients (3%) significant electrical disturbances. LBBB was significantly associated with cardiac events (OR = 12.7; 95%CI [3.78–42.7]; p <0.0001) and mortality (OR = 4.4; 95%CI [1.44–13.7]; p 0.009). Presence of residual dystrophin protein was not associated with significant less cardiac events. Age and LVEF were also predictive factors for cardiac events and mortality.

Conclusion

LBBB is relatively frequent in DMD and is a major predictive factor for cardiac events and mortality. Presence of residual dystrophin protein was not associated with a lower incidence of cardiac events.

Low-level dystrophin expression attenuating the dystrophinopathy phenotype

The reading frame rule suggests that Duchenne muscular dystrophy (DMD) results from DMD mutations causing an out-of-frame transcript, whereas the milder Becker muscular dystrophy results from mutations causing an in-frame transcript. However, predicted nonsense mutations may instead result in altered splicing and an in-frame transcript. Here we report a 10-year-old boy with a predicted nonsense mutation in exon 42 who had a 6-minute walk time of 157% of that of age matched DMD controls, characterized as intermediate muscular dystrophy. RNA sequencing analysis from a muscle biopsy revealed only 6.0-9.8% of DMD transcripts were in-frame, excluding exon 42, and immunoblot demonstrated only 3.2% dystrophin protein expression. Another potential genetic modifier noted was homozygosity for the protective IAAM LTBP4 haplotype. This case suggests that very low levels of DMD exon skipping and dystrophin protein expression may result in amelioration of skeletal muscle weakness, a finding relevant to current dystrophin-restoring therapies.

Comprehensive analysis for genetic diagnosis of Dystrophinopathies in Japan

BACKGROUND

Duchenne muscular dystrophy (DMD) is the most common disease in children caused by mutations in the DMD gene, and DMD and Becker muscular dystrophy (BMD) are collectively called dystrophinopathies. Dystrophinopathies show a complex mutation spectrum. The importance of mutation databases, with clinical phenotypes and protein studies of patients, is increasingly recognized as a reference for genetic diagnosis and for the development of gene therapy.

METHODS

We used the data from the Japanese Registry of Muscular Dystrophy (Remudy) compiled during from July 2009 to March 2017, and reviewed 1497 patients with dystrophinopathies.

RESULTS

The spectrum of identified mutations contained exon deletions (61%), exon duplications (13%), nonsense mutations (13%), small deletions (5%), small insertions (3%), splice-site mutations (4%), and missense mutations (1%). Exon deletions were found most frequently in the central hot spot region between exons 45-52 (42%), and most duplications were detected in the proximal hot spot region between exons 3-25 (47%). In the 371 patients harboring a small mutation, 194 mutations were reported and 187 mutations were unreported.

CONCLUSIONS

We report the largest dystrophinopathies mutation dataset in Japan from a national patient registry, "Remudy". This dataset provides a useful reference to support the genetic diagnosis and treatment of dystrophinopathy.

RNA splicing in human disease and in the clinic

Defects at the level of the pre-mRNA splicing process represent a major cause of human disease. Approximately 15-50% of all human disease mutations have been shown to alter functioning of basic and auxiliary splicing elements. These elements are required to ensure proper processing of pre-mRNA splicing molecules, with their disruption leading to misprocessing of the pre-mRNA molecule and disease. The splicing process is a complex process, with much still to be uncovered before we are able to accurately predict whether a reported genomic sequence variant (GV) represents a splicing-associated disease mutation or a harmless polymorphism. Furthermore, even when a mutation is correctly identified as affecting the splicing process, there still remains the difficulty of providing an exact evaluation of the potential impact on disease onset, severity and duration. In this review, we provide a brief overview of splicing diagnostic methodologies, from in silico bioinformatics approaches to wet lab in vitro and in vivo systems to evaluate splicing efficiencies. In particular, we provide an overview of how the latest developments in high-throughput sequencing can be applied to the clinic, and are already changing clinical approaches.

Clinical profiles and prognosis of acute heart failure in adult patients with dystrophinopathies on home mechanical ventilation

Aims

Duchenne muscular dystrophy (DMD) is characterized by respiratory and heart involvements. In the context of permanently wheelchair bound and on mechanical ventilation (MV) patients, the clinical presentation of acute heart failure (AHF) syndrome may be atypical. We sought to describe clinical and genetic profiles and to determine prognosis of DMD and Becker muscular dystrophy (BMD) patients on home MV (HMV), hospitalized for AHF.

Methods and results

We included genetically proven DMD and BMD patients on HMV admitted for AHF. A total of 13 patients (11 DMD and 2 BMD) fulfilled the inclusion criteria. Median age was 34.0 [interquartile range (IQR) 26.0; 40.0] years. Median pulmonary vital capacity was 9.0% (6.0; 15.0) of predicted value. Long‐term invasive ventilation was performed in 69% of patients. All the 11 DMD patients carried out‐of‐frame DMD gene mutations. At admission, dyspnoea was present in 46%, lipothymia in 23%, and abdominal discomfort in 38.4% of patients. A total of 53.8% of patients showed anasarca. Cardiogenic shock presentation was found in six patients (46%). Ejection fraction was severely altered [median 25% (IQR 20; 30)]. Intra‐hospital mortality rate was 30%, reaching 53.8 % after 1 year. Previous episodes of AHF ≥ 2 were associated with intra‐hospital mortality (P = 0.025). In patients with cardiogenic shock, intra‐hospital mortality rate was 66.6%, reaching 83.3% after 1 year.

Conclusions

In adult DMD and BMD patients with severe ejection fraction alteration and on HMV, admitted for AHF, right cardiac signs are frequent. The intra‐hospital and 1 year mortality rate was high and was associated with previous episodes of AHF ≥ 2.

Cryptic splice activation but not exon skipping is observed in minigene assays of dystrophin c.9361+1G>A mutation identified by NGS

Next-generation sequencing (NGS) discloses nucleotide changes in the genome. Mutations at splicing regulatory elements are expected to cause splicing errors, such as exon skipping, cryptic splice site activation, partial exon loss or intron retention. In dystrophinopathy patients, prediction of splicing outcomes is essential to determine the phenotype: either severe Duchenne or mild Becker muscular dystrophy, based on the reading frame rule. In a Vietnamese patient, NGS identified a c.9361+1G>A mutation in the dystrophin gene and an additional DNA variation of A>G at +117 bases in intron 64. To ascertain the consequences of these DNA changes on dystrophin splicing, minigene constructs were prepared inserting dystrophin exon 64 plus various lengths of intron 64. Exon 64 skipping was observed in the minigene construct with 160 nucleotide (nt) of intron 64 sequence with both c.9361+1A and +117G. In contrast, minigene constructs with larger flanking intronic domains resulted in cryptic splice site activation rather than exon skipping. Meanwhile, the cryptic splice site activation was induced even in +117G when intron 64 was elongated to 272 nt and longer. It was expected that cryptic splice site activation is an in vivo splicing outcome.

Reassessment of Genomic Sequence Variation to Harmonize Interpretation for Personalized Medicine

Accurate interpretation of DNA sequence variation is a prerequisite for implementing personalized medicine. Discrepancies in interpretation between testing laboratories impede the effective use of genetic test results in clinical medicine. To better understand the underpinnings of these discrepancies, we quantified differences in variant classification internally over time and those between our diagnostic laboratory and other laboratories and resources. We assessed the factors that contribute to these discrepancies and those that facilitate their resolution. Our process resolved 72% of nearly 300 discrepancies between pairs of laboratories to within a one-step classification difference and identified key sources of data that facilitate changes in variant interpretation. The identification and harmonization of variant discrepancies will maximize the clinical use of genetic information; these processes will be fostered by the accumulation of additional population data as well as the sharing of data between diagnostic laboratories.

Phenotypic contrasts of Duchenne Muscular Dystrophy in women: Two case reports

We discussed two cases of symptomatic female carriers to Duchenne Muscular Dystrophy. The first case is a 20 year-old girl with classical phenotypic manifestation of the disease, similar to the condition in boys. The case 2 is a 62 year-old woman with progressive muscular weakness. The disease is much less common in woman than men so both cases described here are considered rare forms of the disease, with several clinical implications. In both cases, a progressive muscle weakness, impairment in walking and sleeping was observed, in addition to obstructive sleep apnea syndrome and alveolar hypoventilation, that required noninvasive ventilatory support.

Somatic mosaicism due to a reversion variant causing hemi-atrophy: a novel variant of dystrophinopathy

We describe a case of hemi-atrophy in a young adult male, with a positive family history of three maternal uncles with Duchenne muscular dystrophy (DMD). The patient showed progressive weakness localized to the left side, an abnormal electromyography, and creatine kinase levels >3000 IU/l. Muscle biopsy showed both dystrophin-positive and -negative myofibers. An out-of-frame duplication variant in DMD, that is, c.(93+1_94-1)_(649+1_650-1)dup(p.?) resulting in duplication of exons 3-7 was inherited, but the muscle biopsy showed dystrophin mRNA with and without the duplication. Dystrophin quantification using mass spectrometry showed 25% normal dystrophin protein levels in the muscle biopsy from the stronger right side. Sex chromosome aneuploidy was ruled out. We conclude that the patient inherited the duplication variant, but early in development an inner cell mass underwent a somatic recombination event removing the duplication and restoring dystrophin expression. To our knowledge, this is the first report of a reversion leading to somatic mosaicism in DMD.

Laminin α2 Deficiency-Related Muscular Dystrophy Mimicking Emery-Dreifuss and Collagen VI related Diseases

Background: Laminin α2 deficient congenital muscular dystrophy, caused by mutations in the LAMA2 gene, is characterized by early muscle weakness associated with abnormal white matter signal on cerebral MRI.

Objective: To report on 4 patients with LAMA2 gene mutations whose original clinical features complicated the diagnosis strategy.

Methods: Clinical, electrophysiological, muscle imaging and histopathological data were retrospectively collected from all patients. DNA samples were analysed by next-generation sequencing or direct gene sequencing. Laminin α2 was analysed by western-blot and immunohistochemistry.

Results: The four patients achieved independent walking. All had proximal muscle weakness with scapular winging and prominent joint contractures without peripheral neuropathy. During follow-up, two patients suffered from refractory epilepsy associated with brain leukoencephalopathy in one, polymicrogyria and lissencephaly without white matter changes in the other. In two patients, the distribution of fatty infiltration resembles that of collagen-VI related myopathies. Dilated cardiomyopathy contstartabstractwith conduction defects, suggestive of Emery-Dreifuss myopathy, emerged in two of them within the 4th decade. Molecular diagnosis remained elusive for many years. Finally, targeted capture-DNA sequencing unveiled the involvement of the LAMA2 gene in two families, and led us to further identify LAMA2 mutations in the remaining family using Sanger sequencing.

Conclusions: This report extends the clinical and radiological features of partial Laminin α2 deficiency since patients showed atypical manifestations including dilated cardiomyopathy with conduction defects in 2, epilepsy in 2, one of whom also had sole cortical brain abnormalities. Importantly, clinical findings and muscle imaging initially pointed to collagen-VI related disorders and Emery-Dreifuss muscular dystrophy.

DMD Mutations in 576 Dystrophinopathy Families: A Step Forward in Genotype-Phenotype Correlations

Recent advances in molecular therapies for Duchenne muscular dystrophy (DMD) require precise genetic diagnosis because most therapeutic strategies are mutation-specific. To understand more about the genotype-phenotype correlations of the DMD gene we performed a comprehensive analysis of the DMD mutational spectrum in a large series of families. Here we provide the clinical, pathological and genetic features of 576 dystrophinopathy patients. DMD gene analysis was performed using the MLPA technique and whole gene sequencing in blood DNA and muscle cDNA. The impact of the DNA variants on mRNA splicing and protein functionality was evaluated by in silico analysis using computational algorithms. DMD mutations were detected in 576 unrelated dystrophinopathy families by combining the analysis of exonic copies and the analysis of small mutations. We found that 471 of these mutations were large intragenic rearrangements. Of these, 406 (70.5%) were exonic deletions, 64 (11.1%) were exonic duplications, and one was a deletion/duplication complex rearrangement (0.2%). Small mutations were identified in 105 cases (18.2%), most being nonsense/frameshift types (75.2%). Mutations in splice sites, however, were relatively frequent (20%). In total, 276 mutations were identified, 85 of which have not been previously described. The diagnostic algorithm used proved to be accurate for the molecular diagnosis of dystrophinopathies. The reading frame rule was fulfilled in 90.4% of DMD patients and in 82.4% of Becker muscular dystrophy patients (BMD), with significant differences between the mutation types. We found that 58% of DMD patients would be included in single exon-exon skipping trials, 63% from strategies directed against multiexon-skipping exons 45 to 55, and 14% from PTC therapy. A detailed analysis of missense mutations provided valuable information about their impact on the protein structure.

Dystrophin genotype-cardiac phenotype correlations in Duchenne and Becker muscular dystrophies using cardiac magnetic resonance imaging

Duchenne and Becker muscular dystrophies are caused by mutations in dystrophin. Cardiac manifestations vary broadly, making prognosis difficult. Current dystrophin genotype-cardiac phenotype correlations are limited. For skeletal muscle, the reading-frame rule suggests in-frame mutations tend to yield milder phenotypes. We performed dystrophin genotype-cardiac phenotype correlations using a protein-effect model and cardiac magnetic resonance imaging. A translational model was applied to patient-specific deletion, indel, and nonsense mutations to predict exons and protein domains present within truncated dystrophin protein. Patients were dichotomized into predicted present and predicted absent groups for exons and protein domains of interest. Development of myocardial fibrosis (represented by late gadolinium enhancement [LGE]) and depressed left ventricular ejection fraction (LVEF) were compared. Patients (n = 274) with predicted present cysteine-rich domain (CRD), C-terminal domain (CTD), and both the N-terminal actin-binding and cysteine-rich domains (ABD1 + CRD) had a decreased risk of LGE and trended toward greater freedom from LGE. Patients with predicted present CTD (exactly the same as those with in-frame mutations) and ABD1 + CRD trended toward decreased risk of and greater freedom from depressed LVEF. In conclusion, genotypes previously implicated in altering the dystrophinopathic cardiac phenotype were not significantly related to LGE and depressed LVEF. Patients with predicted present CRD, CTD/in-frame mutations, and ABD1 + CRD trended toward milder cardiac phenotypes, suggesting that the reading-frame rule may be applicable to the cardiac phenotype. Genotype-phenotype correlations may help predict the cardiac phenotype for dystrophinopathic patients and guide future therapies.