Exome sequencing in the pediatric neuromuscular clinic leads to more frequent diagnosis of both neuromuscular and neurodevelopmental conditions.

INTRODUCTION/AIMS: Exome sequencing (ES) has proven to be a valuable diagnostic tool for neuromuscular disorders, which often pose a diagnostic challenge. The aims of this study were to investigate the clinical outcomes associated with utilization of ES in the pediatric neuromuscular clinic and to determine if specific phenotypic features or abnormal neurodiagnostic tests were predictive of a diagnostic result. METHODS: This was a retrospective medical record review of 76 pediatric neuromuscular clinic patients who underwent ES. Based upon clinical assessment prior to ES, patients were divided into two groups: affected by neuromuscular (n = 53) or non-neuromuscular (n = 23) syndromes. RESULTS: A diagnosis was made in 28/76 (36.8%), with 29 unique disorders identified. In the neuromuscular group, a neuromuscular condition was confirmed in 78% of those receiving a genetic diagnosis. Early age of symptom onset was associated with a significantly higher diagnostic yield. The most common reason neuromuscular diagnoses were not detected on prior testing was due to causative genes not being present on disease-specific panels. Changes to medical care were made in 57% of individuals receiving a diagnosis on ES. DISCUSSION: These data further support ES as a powerful diagnostic tool in the pediatric neuromuscular clinic and highlight the advantages of ES over gene panels, including the ability to identify diagnoses regardless of etiology, identify genes newly associated with disease, and identify multiple confounding diagnoses. Rapid and accurate diagnosis by ES can not only end the patient's diagnostic odyssey, but often impacts patients' medical management and genetic counseling of families.

Intron mutations and early transcription termination in Duchenne and Becker muscular dystrophy.

DMD pathogenic variants for Duchenne and Becker muscular dystrophy are detectable with high sensitivity by standard clinical exome analyses of genomic DNA. However, up to 7% of DMD mutations are deep intronic and analysis of muscle-derived RNA is an important diagnostic step for patients who have negative genomic testing but abnormal dystrophin expression in muscle. In this study, muscle biopsies were evaluated from 19 patients with clinical features of a dystrophinopathy, but negative clinical DMD mutation analysis. Reverse transcription-polymerase chain reaction or high-throughput RNA sequencing methods identified 19 mutations with one of three pathogenic pseudoexon types: deep intronic point mutations, deletions or insertions, and translocations. In association with point mutations creating intronic splice acceptor sites, we observed the first examples of DMD pseudo 3'-terminal exon mutations causing high efficiency transcription termination within introns. This connection between splicing and premature transcription termination is reminiscent of U1 snRNP-mediating telescripting in sustaining RNA polymerase II elongation across large genes, such as DMD. We propose a novel classification of three distinct types of mutations identifiable by muscle RNA analysis, each of which differ in potential treatment approaches. Recognition and appropriate characterization may lead to therapies directed toward full-length dystrophin expression for some patients.

The Genotypic and Phenotypic Spectrum of BICD2 Variants in Spinal Muscular Atrophy.

The bicaudal D cargo adaptor 2 (BICD2) gene encodes a conserved cargo adaptor protein required for dynein-mediated transport. Inherited and de novo variants in BICD2 cause SMALED2 (spinal muscular atrophy lower extremity dominant 2), and a subset have recently been reported to cause severe, often lethal disease. However, a true genotype-phenotype correlation for BICD2 has not been performed, and cases described to date are scattered among at least 14 publications. In this review, we identify the characteristics of disease-causing variants in BICD2 that distinguish them from benign variation and perform genotype-phenotype correlations for 99 BICD2 variant carriers from 35 families. ANN NEUROL 2020;87:487-496.

Time is muscle: A recommendation for early treatment for preterm infants with spinal muscular atrophy.

Implementation of newborn screening for spinal muscular atrophy (SMA) in 33 US states and increased genetic carrier screening have led to an increase in early, presymptomatic diagnosis of SMA. Early treatment is critically important and is recommended for presymptomatic infants with two to four copies of survival motor neuron 2. Currently, no specific treatment recommendations exist for preterm infants with SMA. The US Food and Drug Administration does not recommend using onasemnogene abeparvovec-xioi in preterm infants. Some insurance companies interpret "preterm" to be less than 40 weeks gestational age (GA) instead of the commonly accepted 37 weeks GA, which can be a barrier to treatment access. Given the risk of rapid decline in some infants, we recommend treatment of preterm infants when they reach 37 weeks GA, based on the definitions of term GA from the World Health Organization and Centers for Disease Control and Prevention, assuming all other treatment criteria are met.

Truncating variants in UBAP1 associated with childhood-onset nonsyndromic hereditary spastic paraplegia.

Hereditary spastic paraplegia (HSP) is a group of disorders with predominant symptoms of lower-extremity weakness and spasticity. Despite the delineation of numerous genetic causes of HSP, a significant portion of individuals with HSP remain molecularly undiagnosed. Through exome sequencing, we identified five unrelated families with childhood-onset nonsyndromic HSP, all presenting with progressive spastic gait, leg clonus, and toe walking starting from 7 to 8 years old. A recurrent two-base pair deletion (c.426_427delGA, p.K143Sfs*15) in the UBAP1 gene was found in four families, and a similar variant (c.475_476delTT, p.F159*) was detected in a fifth family. The variant was confirmed to be de novo in two families and inherited from an affected parent in two other families. RNA studies performed in lymphocytes from one patient with the de novo c.426_427delGA variant demonstrated escape of nonsense-mediated decay of the UBAP1 mutant transcript, suggesting the generation of a truncated protein. Both variants identified in this study are predicted to result in truncated proteins losing the capacity of binding to ubiquitinated proteins, hence appearing to exhibit a dominant-negative effect on the normal function of the endosome-specific endosomal sorting complexes required for the transport-I complex.

Homozygous variants in AMPD2 and COL11A1 lead to a complex phenotype of pontocerebellar hypoplasia type 9 and Stickler syndrome type 2.

Pontocerebellar hypoplasia type 9 (PCH9) is an autosomal recessive neurodevelopmental disorder caused by pathogenic variants in the AMPD2 gene. We evaluated the son of a consanguineous couple who presented with profound hypotonia and global developmental delay. Other features included sensorineural hearing loss, asymmetric astigmatism, and high myopia. Clinical whole-exome sequence analysis identified a homozygous missense variant in AMPD2 (NM_001257360.1:c.2201C > T, p.[Pro734Leu]) that has not been previously reported. Given the strong phenotypic overlap with PCH9, including the identification of the typical "Figure 8" appearance of the brainstem on neuroimaging, we suspect this variant was causative of the neurodevelopmental disability in this individual. An additional homozygous nonsense variant in COL11A1 (NM_001854.4:c.1168G > T, p.[Glu390Ter]) was identified. Variants in this alternatively spliced region of COL11A1 have been identified to cause an autosomal recessive form of Stickler syndrome type 2 characterized by sensorineural hearing loss and eye abnormalities, but without musculoskeletal abnormalities. The COL11A1 variant likely also contributed to the individual's phenotype, suggesting two potentially relevant genetic findings. This challenging case highlights the importance of detailed phenotypic characterization when interpreting whole exome data.

Update in Duchenne and Becker muscular dystrophy.

PURPOSE OF REVIEW: The purpose of this review is to highlight updates in the standard of care recommendations for DMD, and to describe approaches to and recent advances in genetic therapies for DMD. RECENT FINDINGS: Treatment of DMD patients with the corticosteroids prednisone or deflazacort remains the standard of care, and recent data shows that early treatment (as young as 5 months) with a weekend dosing regimen results in measurable improvement in motor outcomes. A mutation-specific therapy directed at restoring an open reading frame by skipping exon 51 is FDA-approved, and therapies directed at other exons are in trials. Gene replacement therapy shows significant promise in animal models, and trials are underway. Genome editing has received significant attention because of results in animal models, but challenges to implementation in humans remain. SUMMARY: The mainstay of treatment remains meeting well defined standards of care that have been shown to influence morbidity and mortality. These include use of systemic steroids, early nocturnal ventilatory support, appropriate cardiac care and prophylaxis, and wherever appropriate, scoliosis surgery. Early and accurate molecular diagnosis, along with appropriate and multidisciplinary care, provides the best opportunity for maximum benefit of both current standard and upcoming novel therapies for boys with DMD. Among the most promising of these is AAV-based gene replacement therapy, which is currently in clinical trials.

Variants in MED12L, encoding a subunit of the mediator kinase module, are responsible for intellectual disability associated with transcriptional defect.

PURPOSE: Mediator is a multiprotein complex that allows the transfer of genetic information from DNA binding proteins to the RNA polymerase II during transcription initiation. MED12L is a subunit of the kinase module, which is one of the four subcomplexes of the mediator complex. Other subunits of the kinase module have been already implicated in intellectual disability, namely MED12, MED13L, MED13, and CDK19. METHODS: We describe an international cohort of seven affected individuals harboring variants involving MED12L identified by array CGH, exome or genome sequencing. RESULTS: All affected individuals presented with intellectual disability and/or developmental delay, including speech impairment. Other features included autism spectrum disorder, aggressive behavior, corpus callosum abnormality, and mild facial morphological features. Three individuals had a MED12L deletion or duplication. The other four individuals harbored single-nucleotide variants (one nonsense, one frameshift, and two splicing variants). Functional analysis confirmed a moderate and significant alteration of RNA synthesis in two individuals. CONCLUSION: Overall data suggest that MED12L haploinsufficiency is responsible for intellectual disability and transcriptional defect. Our findings confirm that the integrity of this kinase module is a critical factor for neurological development.

Correction: Variants in MED12L, encoding a subunit of the Mediator kinase module, are responsible for intellectual disability associated with transcriptional defect.

In the Acknowledgements section of the paper the authors neglected to mention that the study was supported by a grant from the National Human Genome Research Institute (NHGRI) UM1HG007301 (S.H., M.L.T.). In addition, the award of MD was associated with the authors Michelle L. Thompson and Susan Hiatt instead of PhD. The PDF and HTML versions of the Article have been modified accordingly.

Diagnostic Utility of Whole Exome Sequencing in the Neuromuscular Clinic.

Next-generation sequencing is a powerful diagnostic tool, yet it has proven inadequate to establish a diagnosis in all cases of congenital hypotonia or childhood onset weakness. We sought to describe the impact of whole exome sequencing (WES), which has only recently become widely available clinically, on molecular diagnosis in the Nationwide Children's Hospital Neuromuscular clinics. We reviewed records of all patients in our clinic with pediatric onset of symptoms who had WES done since 2013. Patients were included if clinical suspicion was high for a neuromuscular disease. Clinical WES was performed in 30 families, representing 31 patients, all of whom were seen for hypotonia, weakness, or gait disturbance. Probands had between 2 and 12 genetic diagnostic tests prior to obtaining WES. A genetic diagnosis was established in 11 families (37%), and in 12 patients (39%), with mutations in 10 different genes. Five of these genes have only been associated with disease since 2013, and were not previously represented on clinically available disease gene panels. Our results confirm the utility of WES in the clinical setting, particularly for genetically heterogeneous syndromes. The availability of WES can provide an end to the diagnostic odyssey for parents and allow for expansion of phenotypes.

Low-level expression of EPG5 leads to an attenuated Vici syndrome phenotype.

Vici syndrome is a multisystem disorder characterized by agenesis of the corpus callosum, oculocutaneous hypopigmentation, cataracts, cardiomyopathy, combined immunodeficiency, failure to thrive, profound developmental delay, and acquired microcephaly. Most individuals are severely affected and have a markedly reduced life span. Here we describe an 8-year-old boy with a history of developmental delay, agenesis of the corpus callosum, failure to thrive, myopathy, and well-controlled epilepsy. He was initially diagnosed with a mitochondrial disorder, based in part upon nonspecific muscle biopsy findings, but mitochondrial DNA mutation analysis revealed no mutations. Whole exome sequencing revealed compound heterozygosity for two EPG5 variants, inherited in trans. One was a known pathogenic mutation in exon 13 (c.2461C > T, p.Arg821X). The second was reported as a variant of unknown significance found within intron 16, six nucleotides before the exon 17 splice acceptor site (c.3099-6C > G). Reverse transcription-polymerase chain reaction of the EPG5 mRNA showed skipping of exon 17-which maintains an open reading frame-in 77% of the transcript, along with 23% expression of wild-type mRNA suggesting that intronic mutations may affect splicing of the EPG5 gene and result in symptoms. However, the expression of 23% wild-type mRNA may result in a significantly attenuated Vici syndrome phenotype.

Clinical decision making around commercial use of gene and genetic therapies for spinal muscular atrophy.

Spinal muscular atrophy is no longer a leading cause of inherited infant death in the United States. Since 2016, three genetic therapies have been approved for the treatment of spinal muscular atrophy. Each therapy has been well studied with robust data for both safety and efficacy. However, there are no head-to-head comparator studies to inform clinical decision making. Thus, treatment selection, timing, and combination therapy is largely up to clinician preference and insurance policies. As the natural history of spinal muscular atrophy continues to change, more data is needed to assist in evidence-based and cost-effective clinical decision making.

Parental Experiences with Newborn Screening and Gene Replacement Therapy for Spinal Muscular Atrophy.

BACKGROUND: Spinal muscular atrophy (SMA) is a genetic neurodegenerative disorder with onset predominantly in infants and children. In recent years, newborn screening and three treatments, including gene replacement therapy (Onasemnogene abeparvovec-xioi), have become available in the United States, aiding in the diagnosis and treatment of children with SMA. OBJECTIVE: To evaluate parents' experiences with newborn screening and gene replacement therapy and to explore best practices for positive newborn screen disclosure and counseling of families. METHODS: We conducted semi-structured interviews (n = 32) and online surveys (n = 79) of parents whose children were diagnosed with SMA (on newborn screening or symptomatically) and treated with gene replacement therapy. RESULTS: Gene replacement therapy was most parents' first treatment choice, although concerns regarding long term efficacy (65%) and safety (51%) were common. Information provided during the newborn screening disclosure was quite variable. Only 34% of parents reported the information provided was sufficient and expressed need for more information about treatment. Although many parents experienced denial of the diagnosis at initial disclosure, 94% were in favor of inclusion of SMA on newborn screening. Parents were almost universally anxious following diagnosis and over half remained anxious at the time of study participation with uncertainty of the future being a key concern. Many parents had difficulty processing information provided during their first clinic appointment due to its complexity and their emotional state at the time. CONCLUSIONS: Utilizing this data, we provide a recommendation for the information provided in newborn screening disclosure, propose adjustments to education and counseling during the first clinic visit, and bring awareness of parents' mental health difficulties.

Increase in Full-Length Dystrophin by Exon Skipping in Duchenne Muscular Dystrophy Patients with Single Exon Duplications: An Open-label Study.

Single exon duplications account for disease in a minority of Duchenne muscular dystrophy patients. Exon skipping in these patients has the potential to be highly therapeutic through restoration of full-length dystrophin expression. We conducted a 48-week open label study of casimersen and golodirsen in 3 subjects with an exon 45 or 53 duplication. Two subjects (aged 18 and 23 years) were non-ambulatory at baseline. Upper limb, pulmonary, and cardiac function appeared stable in the 2 subjects in whom they could be evaluated. Dystrophin expression increased from 0.94 % ±0.59% (mean±SD) of normal to 5.1% ±2.9% by western blot. Percent dystrophin positive fibers also rose from 14% ±17% at baseline to 50% ±42% . Our results provide initial evidence that the use of exon-skipping drugs may increase dystrophin levels in patients with single-exon duplications.

Long-Term Follow-Up Cares and Check Initiative: A Program to Advance Long-Term Follow-Up in Newborns Identified with a Disease through Newborn Screening.

In the United States and around the world, newborns are screened on a population basis for conditions benefiting from pre-symptomatic diagnosis and treatment. The number of screened conditions continues to expand as novel technologies for screening, diagnosing, treating, and managing disease are discovered. While screening all newborns facilitates early diagnosis and treatment, most screened conditions are treatable but not curable. Patients identified by newborn screening often require lifelong medical management and community support to achieve the best possible outcome. To advance the long-term follow-up of infants identified through newborn screening (NBS), the Long-Term Follow-up Cares and Check Initiative (LTFU-Cares and Check) designed, implemented, and evaluated a system of longitudinal data collection and annual reporting engaging parents, clinical providers, and state NBS programs. The LTFU-Cares and Check focused on newborns identified with spinal muscular atrophy (SMA) through NBS and the longitudinal health information prioritized by parents and families. Pediatric neurologists who care for newborns with SMA entered annual data, and data tracking and visualization tools were delivered to state NBS programs with a participating clinical center. In this publication, we report on the development, use of, and preliminary results from the LTFU-Cares and Check Initiative, which was designed as a comprehensive model of LTFU. We also propose next steps for achieving the goal of a national system of LTFU for individuals with identified conditions by meaningfully engaging public health agencies, clinicians, parents, families, and communities.

Spinal Muscular Atrophy Update in Best Practices: Recommendations for Diagnosis Considerations.

Background and Objectives: Spinal muscular atrophy (SMA) is an autosomal recessive progressive neurodegenerative primary motor neuron disorder caused by biallelic variants of the survival motor neuron 1 (SMN1) gene. The most recent SMA best practice recommendations were published in 2018 shortly after the approval of the first SMN-enhancing treatment. The availability of disease-modifying therapies for 5q SMA and implementation of SMA newborn screening (NBS) has led to urgency to update the SMA best practice recommendations for diagnosis and to reevaluate the current classification of SMA. In addition, the availability of disease-modifying therapies has opened the door to explore improved diagnosis of adult-onset SMA. Methods: A systematic literature review was conducted on SMA NBS. An SMA working group of American and European health care providers developed recommendations through a modified Delphi technique with serial surveys and virtual meeting feedback on SMA diagnosis to fill information gaps for topics with limited evidence. A community working group of an individual with SMA and caregivers provided insight and perspective on SMA diagnosis and support through a virtual meeting to guide recommendations. Results: The health care provider working group achieved consensus that SMA NBS is essential to include in the updated best practice for SMA diagnosis (100%). Recommendations for the following are described: characterizing NBS-identified infants before treatment; minimum recommendations for starting or offering SMA NBS in a state or country; recommendations for activities and services to be provided by an SMA specialty care center accepting SMA NBS referrals; and recommendations for partnership with individuals with SMA and caregivers to support NBS-identified infants and their caregivers. Limited data are available to advance efficient diagnosis of adult-onset SMA. Discussion: Updating best practice recommendations for SMA diagnosis to include SMA NBS implementation is essential to advancing care for individuals with SMA. In addition to testing, processes for the efficient management of positive newborn screen with access to knowledgeable and skilled health care providers and access to treatment options is critical to successful early diagnosis. Additional evidence is required to improve adult-onset SMA diagnosis.

Continued safety and long-term effectiveness of onasemnogene abeparvovec in Ohio.

5q spinal muscular atrophy (SMA) is an autosomal recessive neurodegenerative disease caused by absence of the SMN1 gene with three FDA approved genetic therapies which significantly improve outcomes. The AAV9 mediated gene replacement therapy, onasemnogene abeparvovec, has the greatest potential for side effects. Here we report the safety and outcomes from 46 children treated with onasemnogene abeparvovec in the state of Ohio between December 2018 and January 2023. In our cohort, onasemnogene abeparvovec treatment remained safe and no child experienced any significant adverse events, including thrombotic microangiopathy, liver failure or death. All children experienced benefit, although the benefit in those with 2 copies of SMN2 was variable. 79 % of the children treated when symptomatic had a SMN2 modifying therapy added on. With careful screening and post treatment monitoring, onasemnogene abeparvovec is safe and effective for children with SMA in the state of Ohio, but more work needs to be done to ensure optimal outcomes for all children with 2 copies of SMN2.

Heterozygous loss-of-function variants in SPTAN1 cause a novel early childhood onset distal myopathy with chronic neurogenic features.

Background: Neurogenetic disorders caused by pathogenic variants in four genes encoding non-erythrocytic spectrins ( SPTAN1, SPTBN1, SPTBN2, SPTBN4) range from peripheral and central nervous system involvement to complex syndromic presentations. Heterozygous pathogenic variants in SPTAN1 are exemplary for this diversity with phenotypes spanning almost the entire spectrum. Methods: Through international collaboration we identified 14 families with genetically unsolved distal weakness and unreported heterozygous SPTAN1 loss-of-function variants including frameshift, nonsense and splice-acceptor variants. Clinical data, electrophysiology, muscle CT or MRI and muscle biopsy findings were collected and standardized. SPTAN1 protein, mRNA expression analysis and cDNA sequencing was performed on muscle tissue from two patients. Results: All 20 patients presented with early childhood onset distal weakness. The severity varied both within families and between different families. Foot abnormalities ranged from hammer toes and pes cavus to distal arthrogryposis. Electrophysiology showed mixed myogenic and neurogenic features. Muscle MRI or CT in 10 patients showed fatty infiltration of the distal lower limb anterior compartment and/or selective involvement of the extensor hallucis longus muscle. Muscle biopsy revealed myopathic changes with mild dystrophic and chronic neurogenic changes in 7 patients. Finally, we provide proof for nonsense mediated decay in tissues derived from two patients. Conclusions: We provide evidence for the association of SPTAN1 loss-of-function variants with childhood onset distal myopathy in 14 families. This finding extends the phenotypic spectrum of SPTAN1 loss-of-function variants ranging from intellectual disability to distal weakness with a predominant myogenic cause. KEY MESSAGES: SPTAN1 loss-of-function variants, including frameshift, nonsense and splice site variants cause a novel childhood onset distal weakness syndrome with primarily skeletal muscle involvement. Hereditary motor neuropathies and distal myopathic disorders present a well-known diagnostic challenge as they demonstrate substantial clinical and genetic overlap. The emergence of SPTAN1 loss-of-function variants serves as a noteworthy example, highlighting a growing convergence in the spectrum of genotypes linked to both hereditary motor neuropathies and distal myopathies.

A genome-wide association analysis of loss of ambulation in dystrophinopathy patients suggests multiple candidate modifiers of disease severity.

The major determinant of disease severity in Duchenne muscular dystrophy (DMD) or milder Becker muscular dystrophy (BMD) is whether the dystrophin gene (DMD) mutation truncates the mRNA reading frame or allows expression of a partially functional protein. However, even in the complete absence of dystrophin, variability in disease severity is observed, and candidate gene studies have implicated several genes as modifiers. Here we present the largest genome-wide search to date for loci influencing severity in N = 419 DMD patients. Availability of subjects for such studies is quite limited, leading to modest sample sizes, which present a challenge for GWAS design. We have therefore taken special steps to minimize heterogeneity within our dataset at the DMD locus itself, taking a novel approach to mutation classification to effectively exclude the possibility of residual dystrophin expression, and utilized statistical methods that are well adapted to smaller sample sizes, including the use of a novel linear regression-like residual for time to ambulatory loss and the application of evidential statistics for the GWAS approach. Finally, we applied an unbiased in silico pipeline, utilizing functional genomic datasets to explore the potential impact of the best supported SNPs. In all, we obtained eight SNPs (out of 1,385,356 total) with posterior probability of trait-marker association (PPLD) ≥ 0.4, representing six distinct loci. Our analysis prioritized likely non-coding SNP regulatory effects on six genes (ETAA1, PARD6G, GALNTL6, MAN1A1, ADAMTS19, and NCALD), each with plausibility as a DMD modifier. These results support both recurrent and potentially new pathways for intervention in the dystrophinopathies.

Phenotypic Spectrum of Dystrophinopathy Due to Duchenne Muscular Dystrophy Exon 2 Duplications.

BACKGROUND AND OBJECTIVES: To describe the phenotypic spectrum of dystrophinopathy in a large cohort of individuals with DMD exon 2 duplications (Dup2), who may be particularly amenable to therapies directed at restoring expression of either full-length dystrophin or nearly full-length dystrophin through utilization of the DMD exon 5 internal ribosome entry site (IRES). METHODS: In this retrospective observational study, we analyzed data from large genotype-phenotype databases (the United Dystrophinopathy Project [UDP] and the Italian DMD network) and classified participants into Duchenne muscular dystrophy (DMD), intermediate muscular dystrophy (IMD), or Becker muscular dystrophy (BMD) phenotypes. Log-rank tests for time-to-event variables were used to compare age at loss of ambulation (LOA) in participants with Dup2 vs controls without Dup2 in the UDP database and for comparisons between steroid-treated vs steroid-naive participants with Dup2. RESULTS: Among 66 participants with Dup2 (UDP = 40, Italy = 26), 61% were classified as DMD, 9% as IMD, and 30% as BMD. Median age at last observation was 15.4 years (interquartile range 8.79-26.0) and 75% had been on corticosteroids for at least 6 months. Age at LOA differed significantly between participants with Dup2 DMD and historical controls without Dup2 DMD (p < 0.001). Valid spirometry was limited but suggested a delay in the typical age-related decline in forced vital capacity and 24 of 55 participants with adequate cardiac data had cardiomyopathy. DISCUSSION: Some patients with Dup2 display a milder disease course than controls without Dup2 DMD, and prolonged ambulation with corticosteroids suggests the potential of IRES activation as a molecular mechanism. As Dup2-targeted therapies reach clinical applications, this information is critical to aid in the interpretation of the efficacy of new treatments.