Technology Insight: therapy for Duchenne muscular dystrophy—an opportunity for personalized medicine?

2022 HRS expert consensus statement on evaluation and management of arrhythmic risk in neuromuscular disorders

This international multidisciplinary document is intended to guide electrophysiologists, cardiologists, other clinicians, and health care professionals in caring for patients with arrhythmic complications of neuromuscular disorders (NMDs). The document presents an overview of arrhythmias in NMDs followed by detailed sections on specific disorders: Duchenne muscular dystrophy, Becker muscular dystrophy, and limb-girdle muscular dystrophy type 2; myotonic dystrophy type 1 and type 2; Emery-Dreifuss muscular dystrophy and limb-girdle muscular dystrophy type 1B; facioscapulohumeral muscular dystrophy; and mitochondrial myopathies, including Friedreich ataxia and Kearns-Sayre syndrome, with an emphasis on managing arrhythmic cardiac manifestations. End-of-life management of arrhythmias in patients with NMDs is also covered. The document sections were drafted by the writing committee members according to their area of expertise. The recommendations represent the consensus opinion of the expert writing group, graded by class of recommendation and level of evidence utilizing defined criteria. The recommendations were made available for public comment; the document underwent review by the Heart Rhythm Society Scientific and Clinical Documents Committee and external review and endorsement by the partner and collaborating societies. Changes were incorporated based on these reviews. By using a breadth of accumulated available evidence, the document is designed to provide practical and actionable clinical information and recommendations for the diagnosis and management of arrhythmias and thus improve the care of patients with NMDs.

Skin cells for use in an alternate diagnostic method for Duchenne muscular dystrophy

The importance of molecular diagnosis and identification of disease-associated variants for Duchenne muscular dystrophy (DMD) is evident in the age of gene-based therapies and personalised medicine. Detection of the causative DMD variant and determination of its effects on dystrophin expression is best achieved by analysis of RNA extracted from muscle biopsy material. However, this is not done routinely, as the procedure can be traumatic, especially to young children, and carries risk of complications related to the use of anaesthetic. As skin biopsies are safer and straightforward to perform than muscle biopsies, we investigated the utility of cultured human epidermal melanocytes and dermal fibroblasts as alternative tools for RNA-based diagnosis of DMD. Shallow skin biopsies from 5 boys with genetically confirmed diagnoses of DMD were used to culture fibroblasts and melanocytes. Biopsies were sampled, and tolerated without complications, using local anaesthetic cream. Dystrophin expression in the cultured cells was assessed using immunocytochemical staining, quantitative real-time PCR and cDNA sequencing methodologies. We observed differential expression of the full-length dystrophin muscle transcript, with significantly more robust expression in melanocytes, compared to that in fibroblasts. Our results suggest that cultured skin melanocytes may present an alternative tool for RNA-based genetic diagnosis of DMD.

Spectrum of Neuromuscular Disorders With HyperCKemia From a Tertiary Care Pediatric Neuromuscular Center

Elevated creatine kinase is a useful screening test in the diagnostic workup of patients with neuromuscular disorders. We did a retrospective study of children with hyperCKemia (>175 IU/L) who were followed in the neuromuscular program of a tertiary care pediatric center from 2005 to 2016. Patients with hyperCKemia were divided into 2 groups: myopathic and nonmyopathic. Within the myopathic group, there were 3 arbitrary subgroups based on creatine kinase values: A (creatine kinase >10 times normal), B (creatine kinase 5-10 times normal), and C (creatine kinase 1-5 times normal). The 3 major categories of myopathies across all the subgroups were muscular dystrophies (commonest) followed by metabolic myopathies and inflammatory myopathies. Among the nonmyopathic causes of hyperCKemia, spinal muscular atrophy was the commonest. Muscular dystrophies should be considered in children with hyperCKemia, muscle weakness, or calf hypertrophy, and metabolic myopathies to be considered in children with recurrent rhabdomyolysis.

Mutational spectrum of Duchenne muscular dystrophy in Spain: Study of 284 cases

INTRODUCTION

Duchenne muscular dystrophy (DMD) is a severe X-linked recessive neuromuscular disease that affects one in 3500 live-born males. The total absence of dystrophin observed in DMD patients is generally caused by mutations that disrupt the reading frame of the DMD gene, and about 80% of cases harbour deletions or duplications of one or more exons.

METHODS

We reviewed 284 cases of males with a genetic diagnosis of DMD between 2007 and 2014. These patients were selected from 8 Spanish reference hospitals representing most areas of Spain. Multiplex PCR, MLPA, and sequencing were performed to identify mutations.

RESULTS

Most of these DMD patients present large deletions (46.1%) or large duplications (19.7%) in the dystrophin gene. The remaining 34.2% correspond to point mutations, and half of these correspond to nonsense mutations. In this study we identified 23 new mutations in DMD: 7 large deletions and 16 point mutations.

CONCLUSIONS

The algorithm for genetic diagnosis applied by the participating centres is the most appropriate for genotyping patients with DMD. The genetic specificity of different therapies currently being developed emphasises the importance of identifying the mutation appearing in each patient; 38.7% of the cases in this series are eligible to participate in current clinical trials.

Assessment of heritable genetic effects using new genetic tools and sentinels in an era of personalized medicine

The challenge of estimating human health effects from damage to the germ line may be met in the genomic era. Understanding the genetic, as opposed to postconception developmental basis of birth defects is critical to their use in monitoring heritable genetic damage. The causes of common birth defects are analyzed here: mendelian genetic, multigenic, developmental, inherited, or combinational. Only a small fraction of these (noninherited, mendelian genetic) are likely to be informative relative to germ cell mutagenesis, and these won't be discernible against the general background of birth defects. Targeted genetic testing as part of personalized medicine could be integrated into a strategy for assessing germ cell alterations in populations. Thus, "sentinel mutations," as originally proposed by Mulvihill and Ceizel, need not be restricted to X-linked or dominant mutations or conditions visible at birth. Several new sentinels related to personalized medicine are proposed, based on health impact (likelihood of monitoring), frequency, and genetic target suitability (responsiveness to diverse mutational mechanisms). Candidates could include CYP genes (related to metabolism of xenobiotics) important in optimizing drug doses and avoiding adverse reactions. High frequency LDLR mutations (related to familial high cholesterol) predict myocardial infarction in approximately50% of individuals. The more common recessive genetic diseases (cystic fibrosis, phenylketonuria, and others) monitored in newborn screening programs could be informative given parental analysis. New opportunities for genetic analyses need to be coupled with epidemiological studies on environmental exposures. These could focus on adverse outcomes related to tobacco, the mostubiquitous and potent environmental mutagen.

Human neurotrophin receptor p75NTR defines differentiation-oriented skeletal muscle precursor cells: implications for muscle regeneration

Satellite cells are resident stem cells of adult skeletal muscle that have roles in tissue repair. Although several efforts have led to the functional characterization of distinct myogenic populations in animal models, the translation of these findings to humans has been limited. Here, we analyzed the expression and function of the neurotrophin receptor p75NTR in human skeletal muscle precursor cells. We combined histological investigations of muscle biopsies with molecular and cellular analyses of primary muscle precursor cells. p75NTR is expressed by most satellite cells in vivo and is a marker for regenerating fibers in inflamed and dystrophic muscle. p75NTR mRNA and protein are also detectable in primary myoblasts, and these levels increase transiently when cell differentiation is triggered. Transcriptome analyses of p75NTR high versus p75NTR low muscle cells showed that p75NTR is the prototype marker for a precursor cell population that has a broad transcriptional repertoire associated with muscle development and maturation. Several in vitro experiments, including receptor blockade and gene silencing in myoblasts, proved that p75NTR specifically regulates myogenesis and dystrophin expression. Taken together, the results indicate that p75NTR is a novel marker of human differentiation-prone muscle precursor cells that is involved in myogenesis in vivo and in vitro.

Diagnosis and management of Duchenne muscular dystrophy, part 1: diagnosis, and pharmacological and psychosocial management

Duchenne muscular dystrophy (DMD) is a severe, progressive disease that affects 1 in 3600-6000 live male births. Although guidelines are available for various aspects of DMD, comprehensive clinical care recommendations do not exist. The US Centers for Disease Control and Prevention selected 84 clinicians to develop care recommendations using the RAND Corporation-University of California Los Angeles Appropriateness Method. The DMD Care Considerations Working Group evaluated assessments and interventions used in the management of diagnostics, gastroenterology and nutrition, rehabilitation, and neuromuscular, psychosocial, cardiovascular, respiratory, orthopaedic, and surgical aspects of DMD. These recommendations, presented in two parts, are intended for the wide range of practitioners who care for individuals with DMD. They provide a framework for recognising the multisystem primary manifestations and secondary complications of DMD and for providing coordinated multidisciplinary care. In part 1 of this Review, we describe the methods used to generate the recommendations, and the overall perspective on care, pharmacological treatment, and psychosocial management.

Sarcoglycanopathies: molecular pathogenesis and therapeutic prospects

Sarcoglycanopathies are a group of autosomal recessive muscle-wasting disorders caused by genetic defects in one of four cell membrane glycoproteins, alpha-, beta-, gamma- or delta-sarcoglycan. These four sarcoglycans form a subcomplex that is closely linked to the major dystrophin-associated protein complex, which is essential for membrane integrity during muscle contraction and provides a scaffold for important signalling molecules. Proper assembly, trafficking and targeting of the sarcoglycan complex is of vital importance, and mutations that severely perturb tetramer formation and localisation result in sarcoglycanopathy. Gene defects in one sarcoglycan cause the absence or reduced concentration of the other subunits. Most genetic defects generate mutated proteins that are degraded through the cell's quality control system; however, in many cases, conformational modifications do not affect the function of the protein, yet it is recognised as misfolded and prematurely degraded. Recent evidence shows that misfolded sarcoglycans could be rescued to the cell membrane by assisting their maturation along the ER secretory pathway. This review summarises the etiopathogenesis of sarcoglycanopathies and highlights the quality control machinery as a potential pharmacological target for therapy of these genetic disorders.

A homogeneous cell-based bicistronic fluorescence assay for high-throughput identification of drugs that perturb viral gene recoding and read-through of nonsense stop codons

Recoding mechanisms are programmed protein synthesis events used commonly by viruses but only very rarely in cells for cellular gene expression. For example, HIV-1 has an absolute reliance on frameshifting to produce the correct ratio of key proteins critical for infectivity. To exploit such recoding sites as therapeutic targets, a simple homogeneous assay capable of detecting small perturbations in these low-frequency (<5%) events is required. Current assays based on dual luciferase reporters use expensive substrates and are labor-intensive, both impediments for high-throughput screening. We have developed a cell-based bifluorophore assay able to measure accurately small recoding changes (<0.1%) with a high Z'-factor in 24- or 96-well formats that could be extended to 384 wells. In cases of nonsense mutations arising within coding regions of genes, the assay is suitable for assessing the potential of screened compounds to increase read-through at these nonprogrammed stop signals of variable termination efficiency.

Developmental biology: Transgenic primate offspring

Genetically engineered monkeys carrying a foreign gene that is passed on to their offspring provide a potentially valuable bridge between mouse models of disease and treatment for human disorders.

A novel custom high density-comparative genomic hybridization array detects common rearrangements as well as deep intronic mutations in dystrophinopathies

Background

The commonest pathogenic DMD changes are intragenic deletions/duplications which make up to 78% of all cases and point mutations (roughly 20%) detectable through direct sequencing. The remaining mutations (about 2%) are thought to be pure intronic rearrangements/mutations or 5'-3' UTR changes. In order to screen the huge DMD gene for all types of copy number variation mutations we designed a novel custom high density comparative genomic hybridisation array which contains the full genomic region of the DMD gene and spans from 100 kb upstream to 100 kb downstream of the 2.2 Mb DMD gene.

Results

We studied 12 DMD/BMD patients who either had no detectable mutations or carried previously identified quantitative pathogenic changes in the DMD gene. We validated the array on patients with previously known mutations as well as unaffected controls, we identified three novel pure intronic rearrangements and we defined all the mutation breakpoints both in the introns and in the 3' UTR region. We also detected a novel polymorphic intron 2 deletion/duplication variation. Despite the high resolution of this approach, RNA studies were required to confirm the functional significance of the intronic mutations identified by CGH. In addition, RNA analysis identified three intronic pathogenic variations affecting splicing which had not been detected by the CGH analysis.

Conclusion

This novel technology represents an effective high throughput tool to identify both common and rarer DMD rearrangements. RNA studies are required in order to validate the significance of the CGH array findings. The combination of these tools will fully cover the identification of causative DMD rearrangements in both coding and non-coding regions, particularly in patients in whom standard although extensive techniques are unable to detect a mutation.