Pseudoexons of the DMD Gene

30 Years Since the Proposal of Exon Skipping Therapy for Duchenne Muscular Dystrophy and the Future of Pseudoexon Skipping

Thirty years ago, in 1995, I proposed a fundamental treatment for Duchenne Muscular Dystrophy (DMD) using antisense oligonucleotides (ASOs) to induce exon skipping and restore dystrophin expression. DMD is a progressive and fatal muscular dystrophy, and the establishment of an effective therapy has been a pressing demand among patients worldwide. Exon-skipping therapy utilizing ASOs has garnered significant attention as one of the most promising treatments for DMD, stimulating global research and development efforts in ASO technology. Two decades later, in 2016, one ASO was conditionally approved by the U.S. FDA as the first DMD treatment. This review summarizes the current status and challenges of ASO-based exon-skipping therapies for DMD and explores the prospects of pseudoexon skipping using ASOs, which holds the potential for achieving a complete cure for DMD.

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.

Two novel deep intronic variants cause Duchenne muscular dystrophy by splice-altering mechanism

Duchenne muscular dystrophy (DMD) is a genetic disorder characterized by progressive muscle degeneration and weakness, due to mutations in the DMD gene, which encodes the dystrophin protein. While mutations within the coding regions of DMD have been extensively studied, recent focus has shifted to deep intronic variants for their potential impact on disease severity. Here, we characterize two deep intronic variants, c.8669-19_8669-24del and c.6439-1016_6439-3376del, in unrelated DMD patients. These variants were identified using targeted long-read sequencing on patients' DNA. RNA sequencing/reverse transcription polymerase chain reaction on RNA extracted from muscle biopsies revealed the presence of a pseudoexon or retention of part of the intron in the transcript, resulting in the introduction of premature termination codons. This study enhances our understanding of pseudoexon activation mechanisms in DMD and underscores the diverse genetic abnormalities contributing to the disease's complexity.

The Role of MicroRNA in the Pathogenesis of Duchenne Muscular Dystrophy

Duchenne muscular dystrophy (DMD) is an X-linked progressive disorder associated with muscle wasting and degeneration. The disease is caused by mutations in the gene that encodes dystrophin, a protein that links the cytoskeleton with cell membrane proteins. The current treatment methods aim to relieve the symptoms of the disease or partially rescue muscle functionality. However, they are insufficient to suppress disease progression. In recent years, studies have uncovered an important role for non-coding RNAs (ncRNAs) in regulating the progression of numerous diseases. ncRNAs, such as micro-RNAs (miRNAs), bind to their target messenger RNAs (mRNAs) to suppress translation. Understanding the mechanisms involving dysregulated miRNAs can improve diagnosis and suggest novel treatment methods for patients with DMD. This review presents the available evidence on the role of altered expression of miRNAs in the pathogenesis of DMD. We discuss the involvement of these molecules in the processes associated with muscle physiology and DMD-associated cardiomyopathy.

Therapeutic strategies for aberrant splicing in cancer and genetic disorders

Accurate pre-mRNA splicing is essential for proper protein translation; however, aberrant splicing is commonly observed in the context of cancer and genetic disorders. Notably, in genetic diseases, these splicing abnormalities often play a pivotal role. Substantial challenges persist in accurately identifying and classifying disease-induced aberrant splicing, as well as in development of targeted therapeutic strategies. In this review, we examine prevalent forms of aberrant splicing and explore potential therapeutic approaches aimed at addressing these splicing-related diseases. This summary contributes to a deeper understanding of the complexities about aberrant splicing and provide a foundation for the development of effective therapeutic interventions in the field of genetic disorders and cancer.

The complex landscape of DMD mutations: moving towards personalized medicine

Duchenne muscular dystrophy (DMD) is a severe genetic disorder characterized by progressive muscle degeneration, with respiratory and cardiac complications, caused by mutations in the DMD gene, encoding the protein dystrophin. Various DMD mutations result in different phenotypes and disease severity. Understanding genotype/phenotype correlations is essential to optimize clinical care, as mutation-specific therapies and innovative therapeutic approaches are becoming available. Disease modifier genes, trans-active variants influencing disease severity and phenotypic expressivity, may modulate the response to therapy, and become new therapeutic targets. Uncovering more disease modifier genes via extensive genomic mapping studies offers the potential to fine-tune prognostic assessments for individuals with DMD. This review provides insights into genotype/phenotype correlations and the influence of modifier genes in DMD.

Uncovering the true features of dystrophin gene rearrangement and improving the molecular diagnosis of Duchenne and Becker muscular dystrophies

Duchenne and Becker muscular dystrophies (DMD/BMD) are caused by complex mutations in the dystrophin gene (DMD). Currently, there is no integrative method for the precise detection of all potential DMD variants, a gap which we aimed to address using long-read sequencing. The captured long-read sequencing panel developed in this study was applied to 129 subjects, including 11 who had previously unsolved cases. The results showed that this method accurately detected DMD mutations, ranging from single-nucleotide variations to structural variations. Furthermore, our findings revealed that continuous exon duplication/deletion in the DMD/BMD cohort may be attributed to complex segmental rearrangements and that noncontiguous duplication/deletion is generally attributed to intragenic inversion or interchromosome translocation. Mutations in the deep introns were confirmed to produce a pseudoexon. Moreover, variations in female carriers were precisely identified. The integrated and precise DMD gene screening method proposed in this study could improve the molecular diagnosis of DMD/BMD.

Case report: a novel deep intronic splice-altering variant in DMD as a cause of Becker muscular dystrophy

We present the case of a male patient who was ultimately diagnosed with Becker muscular dystrophy (BMD; MIM# 300376) after the onset of muscle weakness in his teens progressively led to significant walking difficulties in his twenties. A genetic diagnosis was pursued but initial investigation revealed no aberrations in the dystrophin gene (DMD), although immunohistochemistry and Western blot analysis suggested the diagnosis of dystrophinopathy. Eventually, after more than 10 years, an RNA analysis captured abnormal splicing where 154 nucleotides from intron 43 were inserted between exon 43 and 44 resulting in a frameshift and a premature stop codon. Normal splicing of the DMD gene was also observed. Additionally, a novel variant c.6291-13537A>G in DMD was confirmed in the genomic DNA of the patient. The predicted function of the variant aligns with the mRNA results. To conclude, we here demonstrate that mRNA analysis can guide the diagnosis of non-coding genetic variants in DMD.

Retrospective analysis of persistent HyperCKemia with or without muscle weakness in a case series from Greece highlights vast DMD variant heterogeneity

BACKGROUND

Persistent hyperCKemia results from muscle dysfunction often attributed to genetic alterations of muscle-related genes, such as the dystrophin gene (DMD). Retrospective assessment of findings from DMD analysis, in association with persistent HyperCKemia, was conducted.

PATIENTS AND METHODS

Evaluation of medical records from 1354 unrelated cases referred during the period 1996-2021. Assessment of data concerning the detection of DMD gene rearrangements and nucleotide variants.

RESULTS

A total of 730 individuals (657 cases, 569 of Greek and 88 of Albanian origins) were identified, allowing an overall estimation of dystrophinopathy incidence at ~1:3800 live male births. The heterogeneous spectrum of 275 distinct DMD alterations comprised exon(s) deletions/duplications, nucleotide variants, and rare events, such as chromosome translocation {t(X;20)}, contiguous gene deletions, and a fused gene involving the DMD and the DOCK8 genes. Ethnic-specific findings include a common founder variant in exon 36 ('Hellenic' variant).

CONCLUSIONS

Some 50% of hyperCKemia cases were characterized as dystrophinopathies, highlighting that DMD variants may be considered the most common cause of hyperCKemia in Greece. Delineation of the broad genetic and clinical heterogeneity is fundamental for actionable public health decisions and theragnosis, as well as the establishment of guidelines addressing ethical considerations, especially related to the mild asymptomatic patient subgroup.

RNA-seq analysis, targeted long-read sequencing and in silico prediction to unravel pathogenic intronic events and complicated splicing abnormalities in dystrophinopathy

Dystrophinopathy is caused by alterations in DMD. Approximately 1% of patients remain genetically undiagnosed, because intronic variations are not detected by standard methods. Here, we combined laboratory and in silico analyses to identify disease-causing genomic variants in genetically undiagnosed patients and determine the regulatory mechanisms underlying abnormal DMD transcript generation. DMD transcripts from 20 genetically undiagnosed dystrophinopathy patients in whom no exon variants were identified, despite dystrophin deficiency on muscle biopsy, were analyzed by transcriptome sequencing. Genome sequencing captured intronic variants and their effects were interpreted using in silico tools. Targeted long-read sequencing was applied in cases with suspected structural genomic abnormalities. Abnormal DMD transcripts were detected in 19 of 20 cases; Exonization of intronic sequences in 15 cases, exon skipping in one case, aberrantly spliced and polyadenylated transcripts in two cases and transcription termination in one case. Intronic single nucleotide variants, chromosomal rearrangements and nucleotide repeat expansion were identified in DMD gene as pathogenic causes of transcript alteration. Our combined analysis approach successfully identified pathogenic events. Detection of diseasing-causing mechanisms in DMD transcripts could inform the therapeutic options for patients with dystrophinopathy.

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.

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.

Whole-genome sequencing reveals the artificial selection and local environmental adaptability of pigeons (Columba livia)

To meet human needs, domestic pigeons (Columba livia) with various phenotypes have been bred to provide genetic material for our research on artificial selection and local environmental adaptation. Seven pigeon breeds were resequenced and can be divided into commercial varieties (Euro-pigeon, Shiqi, Shen King, Taishen, and Silver King), ornamental varieties (High Fliers), and local varieties (Tarim pigeon). Phylogenetic analysis based on population resequencing showed that one group contained local breeds and ornamental pigeons from China, whereas all commercial varieties were clustered together. It is revealed that the traditional Chinese ornamental pigeon is a branch of Tarim pigeon. Runs of homozygosity (ROH) and linkage disequilibrium (LD) analyses revealed significant differences in the genetic diversity of the three types of pigeons. Genome sweep analysis revealed that the selected genes of commercial breeds were related to body size, reproduction, and plumage color. The genomic imprinting genes left by the ornamental pigeon breeds were mostly related to special human facial features and muscular dystrophy. The Tarim pigeon has evolved genes related to chemical ion transport, photoreceptors, oxidative stress, organ development, and olfaction in order to adapt to local environmental stress. This research provides a molecular basis for pigeon genetic resource evaluation and genetic improvement and suggests that the understanding of adaptive evolution should integrate the effects of various natural environmental characteristics.

Macroglossia in a pig diagnosed as Becker muscular dystrophy due to dystrophin pseudoexon insertion derived from intron 26

We report a case of Becker muscular dystrophy in a 6-month-old, mixed-breed, castrated male pig detected with macroglossia at a meat inspection center. The pig presented a severely enlarged tongue extending outside its mouth. The tongue was firm and pale with discolored muscles. Histologically, there was severe fibrosis, fatty replacement, and myofiber necrosis, degeneration, and regeneration. Immunofluorescence showed focal and severely weak labeling for dystrophin at the sarcolemma of myocytes in the tongue. Analysis of dystrophin mRNA showed a 62 base pair insertion between exons 26 and 27. The insertion was derived from intron 26. Based on these findings, we diagnosed the case as Becker muscular dystrophy—the first known muscular dystrophy case induced by pseudoexon insertion in animals.

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.

Background splicing as a predictor of aberrant splicing in genetic disease

Mutations of splice sites, auxiliary splicing elements and the splicing machinery cause a wide range of genetic disease. Here we report that many of the complex effects of splicing mutations can be predicted from background splicing information, with emphasis on BRCA1, BRCA2 and DMD. Background splicing arises from very low level splicing between rarely used background splice sites and from low-level exon skipping between intron splice sites. We show how this information can be downloaded from the Snaptron database of spliced RNA, which we then compared with databases of human splice site mutations. We report that inactivating mutations of intron splice sites typically caused the non-mutated partner splice site to splice to a known background splice site in over 90% of cases and to the strongest background splice site in the large majority of cases. Consequently, background splicing information can usefully predict the effects of splice site mutations, which include cryptic splice activation and single or multiple exon skipping. In addition, de novo splice sites and splice sites involved in pseudoexon formation, recursive splicing and aberrant splicing in cancer show a 90% match to background splice sites, so establishing that the enhancement of background splicing causes a wide range of splicing aberrations. We also discuss how background splicing information can identify cryptic splice sites that might be usefully targeted by antisense oligonucleotides (ASOs) and how it might indicate possible multiple exon skipping side effects of ASOs designed to induce single exon skipping.

The identification of a novel splicing mutation in the DMD gene of a Chinese family

The proband is a five-year-old boy diagnosed with Duchenne muscular dystrophy (DMD) by clinical manifestations and laboratory examination, but clinical phenotype of his parents is normal. In the study, his mother had a second pregnancy, and they went to obstetrics for genetic counseling to make informed reproductive choices.

A spotter's guide to SNPtic exons: The common splice variants underlying some SNP-phenotype correlations

BACKGROUND

Cryptic exons are typically characterised as deleterious splicing aberrations caused by deep intronic mutations. However, low-level splicing of cryptic exons is sometimes observed in the absence of any pathogenic mutation. Five recent reports have described how low-level splicing of cryptic exons can be modulated by common single-nucleotide polymorphisms (SNPs), resulting in phenotypic differences amongst different genotypes.

METHODS

We sought to investigate whether additional 'SNPtic' exons may exist, and whether these could provide an explanatory mechanism for some of the genotype-phenotype correlations revealed by genome-wide association studies. We thoroughly searched the literature for reported cryptic exons, cross-referenced their genomic coordinates against the dbSNP database of common SNPs, then screened out SNPs with no reported phenotype associations.

RESULTS

This method discovered five probable SNPtic exons in the genes APC, FGB, GHRL, MYPBC3 and OTC. For four of these five exons, we observed that the phenotype associated with the SNP was compatible with the predicted splicing effect of the nucleotide change, whilst the fifth (in GHRL) likely had a more complex splice-switching effect.

CONCLUSION

Application of our search methods could augment the knowledge value of future cryptic exon reports and aid in generating better hypotheses for genome-wide association studies.

Integrated Genome and Transcriptome Sequencing to Solve a Neuromuscular Puzzle: Miyoshi Muscular Dystrophy and Early Onset Primary Dystonia in Siblings of the Same Family

BACKGROUND

Neuromuscular disorders (NMD), many of which are hereditary, affect muscular function. Due to advances in high-throughput sequencing technologies, the diagnosis of hereditary NMDs has dramatically improved in recent years.

METHODS AND RESULTS

In this study, we report an family with two siblings exhibiting two different NMD, Miyoshi muscular dystrophy (MMD) and early onset primary dystonia (EOPD). Whole exome sequencing (WES) identified a novel monoallelic frameshift deletion mutation (dysferlin: c.4404delC/p.I1469Sfs∗17) in the Dysferlin gene in the index patient who suffered from MMD. This deletion was inherited from his unaffected father and was carried by his younger sister with EOPD. However, immunostaining staining revealed an absence of dysferlin expression in the proband's muscle tissue and thus suggested the presence of the second underlying mutant allele in dysferlin. Using integrated RNA sequencing (RNA-seq) and whole genome sequencing (WGS) of muscle tissue, a novel deep intronic mutation in dysferlin (dysferlin: c.5341-415A > G) was discovered in the index patient. This mutation caused aberrant mRNA splicing and inclusion of an additional pseudoexon (PE) which we termed PE48.1. This PE was inherited from his unaffected mother. PE48.1 inclusion altered the Dysferlin sequence, causing premature termination of translation.

CONCLUSION

Using integrated genome and transcriptome sequencing, we discovered hereditary MMD and EOPD affecting two siblings of same family. Our results added further weight to the combined use of RNA-seq and WGS as an important method for detection of deep intronic gene mutations, and suggest that integrated sequencing assays are an effective strategy for the diagnosis of hereditary NMDs.

Mutations in cis that affect mRNA synthesis, processing and translation

Genetic mutations that cause hereditary diseases usually affect the composition of the transcribed mRNA and its encoded protein, leading to instability of the mRNA and/or the protein. Sometimes, however, such mutations affect the synthesis, the processing or the translation of the mRNA, with similar disastrous effects. We here present an overview of mRNA synthesis, its posttranscriptional modification and its translation into protein. We then indicate which elements in these processes are known to be affected by pathogenic mutations, but we restrict our review to mutations in cis, in the DNA of the gene that encodes the affected protein. These mutations can be in enhancer or promoter regions of the gene, which act as binding sites for transcription factors involved in pre-mRNA synthesis. We also describe mutations in polyadenylation sequences and in splice site regions, exonic and intronic, involved in intron removal. Finally, we include mutations in the Kozak sequence in mRNA, which is involved in protein synthesis. We provide examples of genetic diseases caused by mutations in these DNA regions and refer to databases to help identify these regions. The over-all knowledge of mRNA synthesis, processing and translation is essential for improvement of the diagnosis of patients with genetic diseases.

Novel Intronic Mutations Introduce Pseudoexons in DMD That Cause Muscular Dystrophy in Patients

Background: Duchenne muscular dystrophy (DMD) and Becker muscular dystrophy (BMD) are two subtypes of muscular dystrophy diseases caused by pathogenic mutations in the DMD gene. Until now, more than 4,600 disease-causing mutations in DMD have been reported. However, only 33 mutations were deep intronic, cases with this type of mutations were limited. Methods: In this study, we used a combination of complementary DNA (cDNA) and target DNA sequencing analysis in addition to conventional whole-exome sequencing (WES). Results: Three novel hemizygous mutations IVS11 + 17811C > G (c.1331 + 17811C > G), IVS21 + 3252A > G (c.2803 + 3252A > G) and IVS40 + 362A > G (c.5739 + 362A > G) were identified in DMD patients, while a reported hemizygous mutation IVS62-285A > G (c.9225-285A > G) was found in the BMD patient. These DMD mutations lead to pseudoexon insertions, causing the generation of truncated and dysfunctional dystrophin. Conclusion: This study defines three novel and one reported intronic mutations, which can result in DMD/BMD. We also emphasize the need to combine WES and cDNA-based methods to detect the variant in the very large DMD gene in which the mutational spectrum is complex.

Splicing Characteristics of Dystrophin Pseudoexons and Identification of a Novel Pathogenic Intronic Variant in the DMD Gene

Pseudoexon (PE) inclusion has been implicated in various dystrophinopathies; however, its splicing characteristics have not been fully investigated. This study aims to analyze the splicing characteristics of dystrophin PEs and compare them with those of dystrophin canonical exons (CEs). Forty-two reported dystrophin PEs were divided into a splice site (ss) group and a splicing regulatory element (SRE) group. Five dystrophin PEs with characteristics of poison exons were identified and categorized as the possible poison exon group. The comparative analysis of each essential splicing signal among different groups of dystrophin PEs and dystrophin CEs revealed that the possible poison exon group had a stronger 3′ ss compared to any other group. As for auxiliary SREs, different groups of dystrophin PEs were found to have a smaller density of diverse types of exonic splicing enhancers and a higher density of several types of exonic splicing silencers compared to dystrophin CEs. In addition, the possible poison exon group had a smaller density of 3′ ss intronic splicing silencers compared to dystrophin CEs. To our knowledge, our findings indicate for the first time that poison exons might exist in DMD (the dystrophin gene) and present with different splicing characteristics than other dystrophin PEs and CEs.