Dystrophia Myotonica Protein Kinase Gene Research Articles

Pluripotent Stem Cells in Disease Modeling and Drug Discovery for Myotonic Dystrophy Type 1.

Myotonic dystrophy type 1 (DM1) is a progressive multisystemic disease caused by the expansion of a CTG repeat tract within the 3' untranslated region (3' UTR) of the dystrophia myotonica protein kinase gene (DMPK). Although DM1 is considered to be the most frequent myopathy of genetic origin in adults, DM1 patients exhibit a vast diversity of symptoms, affecting many different organs. Up until now, different in vitro models from patients' derived cells have largely contributed to the current understanding of DM1. Most of those studies have focused on muscle physiopathology. However, regarding the multisystemic aspect of DM1, there is still a crucial need for relevant cellular models to cover the whole complexity of the disease and open up options for new therapeutic approaches. This review discusses how human pluripotent stem cell-based models significantly contributed to DM1 mechanism decoding, and how they provided new therapeutic strategies that led to actual phase III clinical trials.

Delay of EGF-Stimulated EGFR Degradation in Myotonic Dystrophy Type 1 (DM1).

Myotonic dystrophy type 1 (DM1) is an autosomal dominant disease caused by a CTG repeat expansion in the 3′ untranslated region of the dystrophia myotonica protein kinase gene. AKT dephosphorylation and autophagy are associated with DM1. Autophagy has been widely studied in DM1, although the endocytic pathway has not. AKT has a critical role in endocytosis, and its phosphorylation is mediated by the activation of tyrosine kinase receptors, such as epidermal growth factor receptor (EGFR). EGF-activated EGFR triggers the internalization and degradation of ligand–receptor complexes that serve as a PI3K/AKT signaling platform. Here, we used primary fibroblasts from healthy subjects and DM1 patients. DM1-derived fibroblasts showed increased autophagy flux, with enlarged endosomes and lysosomes. Thereafter, cells were stimulated with a high concentration of EGF to promote EGFR internalization and degradation. Interestingly, EGF binding to EGFR was reduced in DM1 cells and EGFR internalization was also slowed during the early steps of endocytosis. However, EGF-activated EGFR enhanced AKT and ERK1/2 phosphorylation levels in the DM1-derived fibroblasts. Therefore, there was a delay in EGF-stimulated EGFR endocytosis in DM1 cells; this alteration might be due to the decrease in the binding of EGF to EGFR, and not to a decrease in AKT phosphorylation.

Predicting the CTG Repeat Size from a Single Spirometry Test Performed at Any Time during the Disease Course of Myotonic Dystrophy Type 1.

Objective In myotonic dystrophy type 1 (DM1), the CTG repeat size in the dystrophia myotonica protein kinase gene has been shown to correlate with disease severity and is a potential predictive marker for respiratory decline. However, genetic testing can be challenging in some clinical situations. We developed a simple formula for estimating the CTG repeat size using a single spirometry test in patients with DM1. Methods In this single-center retrospective study, we reviewed 50 consecutive patients with genetically confirmed DM1 whose follow-up visits were at our hospital. The patients were randomly assigned to training and test analysis subsets. By applying a linear mixed model to the longitudinal spirometry results of the training set, we calculated the fixed effects on the annual respiratory decline. Subsequently, we derived a prediction formula to calculate the repeat size that incorporated %vital capacity (%VC) and the patient's age at the time of the spirometry evaluation; the results were validated by the test set. Results A total of 157 spirometry tests were recorded. The fixed effects on the annual %VC decline were =-0.90. The derived formula [repeat size=-16.8×(age+%VC/0.90)+2663] had a moderate predictive performance with a mean coefficient of determination R2 of 0.41. Conclusion The CTG repeat size in patients with DM1 can be potentially predicted using a simple formula based on a single spirometry test conducted at any time over the disease course. It can be useful as a supportive tool for advance care planning when genetic testing is not available.

Acute leg pain and weakness in pregnancy: A new diagnosis of myotonic dystrophy

We present a unique case of a 44-year-old woman who presented at 29 weeks’ gestation with proximal limb pain and elevated creatine kinase. This occurred in the background of premature cataracts, atrial fibrillation and abnormal liver function. Clinical, pathological and neurodiagnostic findings supported a diagnosis of myotonic dystrophy, confirmed by genetic testing which revealed dystrophia myotonica protein kinase gene expansion. Muscle biopsy found both recent necrotising and chronic myopathic processes. Following delivery, the mother's myalgia resolved and creatine kinase quickly declined. The fetus was diagnosed with congenital myotonic dystrophy. We review the impact of myotonic dystrophy on pregnancy and discuss potential explanations for this patient's clinical course. This case emphasises the importance of considering myotonic dystrophy as a differential diagnosis in the right clinical context and the need for pre-pregnancy assessment and genetic counselling in women with known myotonic dystrophy.

Myotonic Dystrophies: A Genetic Overview.

Myotonic dystrophies (DM) are the most common muscular dystrophies in adults, which can affect other non-skeletal muscle organs such as the heart, brain and gastrointestinal system. There are two genetically distinct types of myotonic dystrophy: myotonic dystrophy type 1 (DM1) and myotonic dystrophy type 2 (DM2), both dominantly inherited with significant overlap in clinical manifestations. DM1 results from CTG repeat expansions in the 3′-untranslated region (3′UTR) of the DMPK (dystrophia myotonica protein kinase) gene on chromosome 19, while DM2 is caused by CCTG repeat expansions in intron 1 of the CNBP (cellular nucleic acid-binding protein) gene on chromosome 3. Recent advances in genetics and molecular biology, especially in the field of RNA biology, have allowed better understanding of the potential pathomechanisms involved in DM. In this review article, core clinical features and genetics of DM are presented followed by a discussion on the current postulated pathomechanisms and therapeutic approaches used in DM, including the ones currently in human clinical trial phase.

Congenital Myotonic Dystrophy with Combined Heterozygous ATP8B1/ABCB4 Mutation Leading to Progressive Cholestasis and Liver Failure

Myotonic dystrophy (MyoD) is an inherited genetic disorder caused by the expansion of a CTG trinucleotide repeat in the dystrophia myotonica protein kinase gene. It manifests as a multisystem disease affecting not only skeletal muscles, but also heart, lung, eye, gastrointestinal tract, central nervous system, and endocrine system. However, MyoD is rarely associated with a progressive liver disorder. We report a case of congenital MyoD with combined heterozygous ATP8B1/ABCB4 mutation who developed chronic, progressive low gamma-glutamyltransferase cholestatic liver disease at early infancy, and eventually underwent successful liver transplantation.

Hemangiomas of the tongue and the oral cavity in a myotonic dystrophy type 1 patient

Rationale:Myotonic dystrophy type 1 (DM1) is an autosomal dominant disease caused by a cytosine, guanine, thymine (CTG) trinucleotide repeat expansion in the non-coding region of dystrophia myotonica protein kinase gene, causing a multisystem involvement. To date, few studies have been performed to evaluate skin features in DM1 patients, but none reported on the possible association between the disease and tongue hemangiomas.Patients concerns:We report a case of a 63-year-old woman affected by DM1 and presenting, at the intraoral examination, several swelling and buish lesions occurring on buccal and palatal mucosa, and in the anterior two-thirds and margins of the tongue.Diagnosis:Multiple tongue hemangiomas in DM1 patient.Interventions:Color Doppler ultrasound revealed hypoechoic lesions with intermittent color picking suggestive of vascular lesion. Surgical excision was performed under general anesthesia. Histopathological examination was compatible with the diagnosis of cavernous hemangiomas.Outcomes:At 6 months follow-up, a part from the cosmetic deformity, patient's hemangiomas did not bleed, but caused functional problems with speaking, mastication, and deglutition, in addition to the same symptoms induced by DM1.Lessons:This case may add new details to better characterize the DM1 phenotype, suggesting that even tongue hemangiomas may be part of the DM1 multisystem involvement.

Therapeutic Genome Editing for Myotonic Dystrophy Type 1Using CRISPR/Cas9.

Myotonic dystrophy type 1 (DM1) is caused by a CTG nucleotide repeat expansion within the 3′ UTR of the Dystrophia Myotonica protein kinase gene. In this study, we explored therapeutic genome editing using CRISPR/Cas9 via targeted deletion of expanded CTG repeats and targeted insertion of polyadenylation signals in the 3′ UTR upstream of the CTG repeats to eliminate toxic RNA CUG repeats. We found paired SpCas9 or SaCas9 guide RNA induced deletion of expanded CTG repeats. However, this approach incurred frequent inversion in both the mutant and normal alleles. In contrast, the insertion of polyadenylation signals in the 3′ UTR upstream of the CTG repeats eliminated toxic RNA CUG repeats, which led to phenotype reversal in differentiated neural stem cells, forebrain neurons, cardiomyocytes, and skeletal muscle myofibers. We concluded that targeted insertion of polyadenylation signals in the 3′ UTR is a viable approach to develop therapeutic genome editing for DM1.

Dystrophia myotonica type 1 presenting with dysarthria: A case report and literature review.

Dystrophia myotonica (DM) type 1 is an autosomal dominant disorder, caused by a trinucleotide CTG repeat expansion in the 3′ untranslated region of the dystrophia myotonica protein kinase (DMPK) gene (chromosome 19q13.3). The disorder affects different organ systems, including the skeletal muscles, ocular lens, lungs, heart and gastrointestinal tract, as well as the endocrine and central nervous systems. The skeletal muscles are most frequently involved, whereby the disorder manifests as myotonia, muscle weakness and amyotrophy. However, DM type 1 presenting with dysarthria is rare. The current study presents a case of a 28-year-old male with DM type 1 presenting with dysarthria and associated multifocal hyperintense lesions in the white matter. Although electromyogram measurements identified myotonic discharges in all extremities, a muscle biopsy failed to detect the characteristic pathological features of DM type 1. A lack of a positive family history for DM type 1 also obscured diagnosis. However, genetic analysis detected a single allele in the P12 segment of the DMPK gene that included a CTG expansion of 13 repeats and a three-base gradient fragment in the P134 segment that included a CTG expansion of >600 repeats. According to the characteristics of dysarthria, multifocal hyperintense lesions in the white matter, electromyogram measurement results and genetic testing results, a diagnosis of DM type 1 was confirmed.

Single-Tube Dodecaplex PCR Panel of Polymorphic Microsatellite Markers Closely Linked to the DMPK CTG Repeat for Preimplantation Genetic Diagnosis of Myotonic Dystrophy Type 1.

Preimplantation genetic diagnosis (PGD) of myotonic dystrophy type 1 (DM1) currently uses conventional PCR to detect nonexpanded dystrophia myotonica protein kinase (DMPK) alleles or triplet-primed PCR to detect the CTG-expanded alleles, coupled with analysis of linked microsatellite markers to increase diagnostic accuracy. We aimed to simplify the process of identification and selection of informative linked markers for application to DM1 PGD. An in silico search was performed to identify all markers within 1-1.5 Mb flanking the DMPK gene. Five previously known (D19S559, APOC2, D19S543, D19S112, and BV209569) and 7 novel (DM45050, DM45178, DM45209, DM45958, DM46513, DM46892, and DM47004.1) markers with potentially high heterozygosity values and polymorphism information content were selected and optimized in a single-tube multiplex PCR panel. Analysis of 184 DNA samples of Chinese and Caucasian individuals (91 from unrelated, anonymized cord blood of Chinese babies born at the National University Hospital, Singapore, and 93 Caucasian DNA samples from the Human Variation Panel HD100CAU) confirmed the high polymorphism indices of all markers (polymorphism information content >0.5), with observed heterozygosity values ranging from 0.62-0.93. All individuals were heterozygous for at least 6 markers, with 99.5% of individuals heterozygous for at least 2 markers on either side of the DMPK CTG repeat. The dodecaplex marker assay was successfully validated on 42 single cells and 12 whole genome amplified single cells. The DM1 multiplex PCR panel is suitable for use in DM1 PGD either as a standalone linkage-based assay or as a complement to DMPK CTG repeat expansion-mutation detection.

Targeting DMPK with Antisense Oligonucleotide Improves Muscle Strength in Myotonic Dystrophy Type 1 Mice.

Myotonic dystrophy type 1 (DM1), a dominant hereditary muscular dystrophy, is caused by an abnormal expansion of a (CTG)n trinucleotide repeat in the 3′ UTR of the human dystrophia myotonica protein kinase (DMPK) gene. As a consequence, mutant transcripts containing expanded CUG repeats are retained in nuclear foci and alter the function of splicing regulatory factors members of the MBNL and CELF families, resulting in alternative splicing misregulation of specific transcripts in affected DM1 tissues. In the present study, we treated DMSXL mice systemically with a 2′-4′-constrained, ethyl-modified (ISIS 486178) antisense oligonucleotide (ASO) targeted to the 3′ UTR of the DMPK gene, which led to a 70% reduction in CUGexp RNA abundance and foci in different skeletal muscles and a 30% reduction in the heart. Furthermore, treatment with ISIS 486178 ASO improved body weight, muscle strength, and muscle histology, whereas no overt toxicity was detected. This is evidence that the reduction of CUGexp RNA improves muscle strength in DM1, suggesting that muscle weakness in DM1 patients may be improved following elimination of toxic RNAs.

Identification of Plant-derived Alkaloids with Therapeutic Potential for Myotonic Dystrophy Type I

Myotonic dystrophy type I (DM1) is a disabling neuromuscular disease with no causal treatment available. This disease is caused by expanded CTG trinucleotide repeats in the 3' UTR of the dystrophia myotonica protein kinase gene. On the RNA level, expanded (CUG)n repeats form hairpin structures that sequester splicing factors such as muscleblind-like 1 (MBNL1). Lack of available MBNL1 leads to misregulated alternative splicing of many target pre-mRNAs, leading to the multisystemic symptoms in DM1. Many studies aiming to identify small molecules that target the (CUG)n-MBNL1 complex focused on synthetic molecules. In an effort to identify new small molecules that liberate sequestered MBNL1 from (CUG)n RNA, we focused specifically on small molecules of natural origin. Natural products remain an important source for drugs and play a significant role in providing novel leads and pharmacophores for medicinal chemistry. In a new DM1 mechanism-based biochemical assay, we screened a collection of isolated natural compounds and a library of over 2100 extracts from plants and fungal strains. HPLC-based activity profiling in combination with spectroscopic methods were used to identify the active principles in the extracts. The bioactivity of the identified compounds was investigated in a human cell model and in a mouse model of DM1. We identified several alkaloids, including the β-carboline harmine and the isoquinoline berberine, that ameliorated certain aspects of the DM1 pathology in these models. Alkaloids as a compound class may have potential for drug discovery in other RNA-mediated diseases.

Myotonic Dystrophy Type 1 with Syringomyelia in a Young Patient

Myotonic Dystrophy Type 1 with Syringomyelia in a Young Patient

Congenital and childhood myotonic dystrophy: Current aspects of disease and future directions.

Myotonic dystrophy type 1 (DM1) is multisystem disease arising from mutant CTG expansion in the non-translating region of the dystrophia myotonica protein kinase gene. While DM1 is the most common adult muscular dystrophy, with a worldwide prevalence of one in eight thousand, age of onset varies from before birth to adulthood. There is a broad spectrum of clinical severity, ranging from mild to severe, which correlates with number of DNA repeats. Importantly, the early clinical manifestations and management in congenital and childhood DM1 differ from classic adult DM1. In neonates and children, DM1 predominantly affects muscle strength, cognition, respiratory, central nervous and gastrointestinal systems. Sleep disorders are often under recognised yet a significant morbidity. No effective disease modifying treatment is currently available and neonates and children with DM1 may experience severe physical and intellectual disability, which may be life limiting in the most severe forms. Management is currently supportive, incorporating regular surveillance and treatment of manifestations. Novel therapies, which target the gene and the pathogenic mechanism of abnormal splicing are emerging. Genetic counselling is critical in this autosomal dominant genetic disease with variable penetrance and potential maternal anticipation, as is assisting with family planning and undertaking cascade testing to instigate health surveillance in affected family members. This review incorporates discussion of the clinical manifestations and management of congenital and childhood DM1, with a particular focus on hypersomnolence and sleep disorders. In addition, the molecular genetics, mechanisms of disease pathogenesis and development of novel treatment strategies in DM1 will be summarised.

Altered nuclear structure in myotonic dystrophy type 1-derived fibroblasts.

Myotonic dystrophy type 1 (DM1) is a multisystem genetic disorder caused by a triplet nucleotide repeat expansion in the 3' untranslated region of the Dystrophia Myotonica-Protein Kinase (DMPK) gene. DMPK gene transcripts containing CUG expanded repeats accumulate in nuclear foci and ultimately cause altered splicing/gene expression of numerous secondary genes. The study of primary cell cultures derived from patients with DM1 has allowed the identification and further characterization of molecular mechanisms underlying the pathology in the natural context of the disease. In this study we show for the first time impaired nuclear structure in fibroblasts of DM1 patients. DM1-derived fibroblasts exhibited altered localization of the nuclear envelope (NE) proteins emerin and lamins A/C and B1 with concomitant increased size and altered shape of nuclei. Abnormal NE organization is more common in DM1 fibroblasts containing abundant nuclear foci, implying expression of the expanded RNA as determinant of nuclear defects. That transient expression of the DMPK 3' UTR containing 960 CTG but not with the 3' UTR lacking CTG repeats is sufficient to generate NE disruption in normal fibroblasts confirms the direct impact of mutant RNA on NE architecture. We also evidence nucleoli distortion in DM1 fibroblasts by immunostaining of the nucleolar protein fibrillarin, implying a broader effect of the mutant RNA on nuclear structure. In summary, these findings reveal that NE disruption, a hallmark of laminopathy disorders, is a novel characteristic of DM1.

Transcriptionally correlated subcellular dynamics of MBNL1 during lens development and their implication for the molecular pathology of myotonic dystrophy type 1

DM1 (myotonic dystrophy type 1) is caused by elongation of a CTG repeat in the DMPK (dystrophia myotonica-protein kinase) gene. mRNA transcripts containing these CUGexp (CUG expansion) repeats form accumulations, or foci, in the nucleus of the cell. The pathogenesis of DM1 is proposed to result from inappropriate patterns of alternative splicing caused by sequestration of the developmentally regulated alternative splicing factor MBNL1 (muscleblind-like 1) by these foci. Since eye lens cataract is a common feature of DM1 we have examined the distribution and dynamics of MBNL1 in lens epithelial cell lines derived from patients with DM1. The results of the present study demonstrate that only a small proportion of nuclear MBNL1 accumulates in CUGexp pre-mRNA foci. MBNL1 is, however, highly mobile and changes localization in response to altered transcription and splicing activity. Moreover, immunolocalization studies in lens sections suggest that a change in MBNL1 distribution is important during lens growth and differentiation. Although these data suggest that the loss of MBNL1 function due to accumulation in foci is an unlikely explanation for DM1 symptoms in the lens, they do demonstrate a strong relationship between the subcellular MBNL1 localization and pathways of cellular differentiation, providing an insight into the sensitivity of the lens to changes in MBNL1 distribution.

Two is better than one: A case of homozygous myotonic dystrophy type 1

Myotonic dystrophy type 1 is an autosomal dominant condition caused by a trinucleotide CTG repeat expansion in the 3' untranslated region of the dystrophia myotonica protein kinase gene. The phenotypic features of myopathic facies, generalized weakness, and myotonia are thought to be dependent on repeat number, with larger expansions generally leading to earlier and/or more severe disease. The vast majority of individuals are heterozygous for an expanded allele and an allele in the normal range. In this clinical report, we describe two brothers with congenital myotonic dystrophy type 1. The younger of the two siblings is one of only 13 homozygous patients ever reported in the literature. He carries two expanded alleles: one with 1,170 repeats and the other with >100 repeats. We present his clinical picture in relation to his more severely affected heterozygous brother as well as other published homozygous cases. Finally, we discuss the inconsistency between repeat size and symptomatic expression as it applies to the current proposed mechanisms of myotonic dystrophy type 1 pathogenicity.

Genetics correlates with lung function and nocturnal ventilation in myotonic dystrophy

Dystrophia myotonica (DM) is the most frequent adult-onset muscular dystrophy. Type 1 is caused by the cytosine-thymine-guanine (CTG) repeat expansion in the DM protein kinase gene. Respiratory muscle weakness and altered central ventilatory control lead to hypercapnia and lung volume restriction. This study aims to review the respiratory involvement in DM patients and study its relation with genetics. Retrospective study of patients with DM referred for respiratory assessment was made. Noninvasive ventilation (NIV) was considered to daytime hypercapnia or symptoms of nocturnal hypoventilation. Forty-two consecutive patients (37.9 ± 13.6 years) were evaluated. Mean CTG length was 642.8 ± 439.2 repeats. In the first evaluation, mean forced vital capacity (FVC) was 74.4 ± 20.2 %, maximal expiratory pressure (MEP) 35 ± 16 %, maximal inspiratory pressure 52 ± 23 %, peak cough flow (PCF) 327.3 ± 97.7 L/min, arterial pressure of oxygen 79.7 ± 11.3 mmHg, arterial pressure of carbon dioxide 45.5 ± 6.2 mmHg, overnight minimal peripheral oxygen saturation (SpO2) 79.6 ± 11.6 %, and apnea-hypopnea index 13.9 ± 9.9. CTG length was found to be related with MEP (r = -0.67; p = 0.001) and SpO2 (r = -0.37; p = 0.039). NIV was started in 25 patients. Ventilated patients had lower FVC (2.19 to 3.21 L; p < 0.001) and PCF (285.3 to 388.5 L/min; p = 0.003) and more CTG repeats (826.6 to 388.5 repeats; p = 0.02). NIV compliance was poor in seven patients (28 %) and related with hypercapnia (r = 0.87; p = 0.002) and inspiratory positive airway pressure setting (r = 0.65; p = 0.009). Ventilation improved symptoms and nocturnal hypoventilation. Comparing the first and last evaluations, only PCF was significantly lower (275.0 to 310.8 L/min; p = 0.019). Ventilatory insufficiency is very common in patients with DM and CTG length may be useful to predict it. Prolonged NIV improves symptoms, nocturnal hypoventilation and maintains daily blood gases. Routine evaluation of PCF should not be forgotten and assisted coughing training provided.

MBNL1 gene variants as modifiers of disease severity in myotonic dystrophy type 1

Myotonic dystrophy type 1 (DM1) is a multisystemic autosomal dominant disorder characterized by a highly variable phenotype and caused by an unstable CTG repeat expansion in the 3' untranslated region of the dystrophia myotonica protein kinase (DMPK) gene. Longer CTG repeat expansions often correlate with an anticipated age at onset and CTG repeat number may account for 45-60 % of the variance in disease severity. In order to search for candidate genes that could act as modifiers of disease severity, we studied the association between Muscleblind-like protein-1 (MBNL1) gene polymorphisms and the DM1 phenotype. In a group of 301 patients diagnosed with DM1 based on clinical symptoms, diagnosis was confirmed by molecular analysis of the DMPK gene. Patients were divided into four subtypes. The first subtype corresponded to asymptomatic patients or those with a mild phenotype, the second included those with a classic phenotype, the third concerned childhood onset, and the fourth corresponded to the congenital form of DM1. Three SNPs located in the MBNL1 gene promoter, rs323622, rs17283597, and rs17433672, were studied. Case-control analysis revealed that allele frequencies for the latter two were significantly associated with DM1 (p = 0.037 and p = 0.020). Multivariate linear regression analysis using phenotype as the dependent variable demonstrated that the TT genotype of the third SNP, rs323622, was associated with a more severe phenotype (p = 0.0034) and accounted for 1.88 % of the variance in disease severity. We report the association of several genetic variants of the MBNL1 gene with DM1 or with the severity of the disease.

New function for the RNA helicase p68/DDX5 as a modifier of MBNL1 activity on expanded CUG repeats

Myotonic Dystrophy type I (DM1) is caused by an abnormal expansion of CTG triplets in the 3′ UTR of the dystrophia myotonica protein kinase (DMPK) gene, leading to the aggregation of the mutant transcript in nuclear RNA foci. The expanded mutant transcript promotes the sequestration of the MBNL1 splicing factor, resulting in the misregulation of a subset of alternative splicing events. In this study, we identify the DEAD-box RNA helicase p68 (DDX5) in complexes assembled onto in vitro-transcribed CUG repeats. We showed that p68 colocalized with RNA foci in cells expressing the 3′UTR of the DMPK gene containing expanded CTG repeats. We found that p68 increased MBNL1 binding onto pathological repeats and the stem–loop structure regulatory element within the cardiac Troponin T (TNNT2) pre-mRNA, splicing of which is misregulated in DM1. Mutations in the helicase core of p68 prevented both the stimulatory effect of the protein on MBNL1 binding and the colocalization of p68 with CUG repeats, suggesting that remodeling of RNA secondary structure by p68 facilitates MBNL1 binding. We also found that the competence of p68 for regulating TNNT2 exon 5 inclusion depended on the integrity of MBNL1 binding sites. We propose that p68 acts as a modifier of MBNL1 activity on splicing targets and pathogenic RNA.