2026-07-10 | Aberrant neuronal differentiation and splicing defects in Congenital Myotonic Dystrophy (DM1) iPSC models.

Myotonic Dystrophy type 1 (DM1) is an autosomal multisystem disorder manifested due to unstable CTG nucleotide repeat expansion within the 3'-untranslated region of the dystrophia myotonica protein kinase ( DMPK ) gene. Although progress towards understanding of molecular pathogenesis in muscle and heart has been made, the pathways that affect the brain in DM1 is fundamentally unknown. In addition, the congenital DM1 manifest even more complicated brain abnormalities. Despite the wealth of existing cellular and animal models, iPSCs based studies are being fostered as they replicate the human model more closely to the disease. In view of this context, we set out to characterize the differentiation potential of congenital DM1 patient derived iPSC lines towards neuronal cells. Using neurogenin2 (NGN2) induced direct reprogramming of iPSCs into neurons and chemically defined media-induced neural induction protocol, we find that congenital DM1 mutant iPSC derived neurons exhibited precocious differentiation, as evidenced by their expression of pan-neuronal markers TUJ1 and Map2, along with increased processes extension and neurite length. Moreover, unbiased RNA sequencing analyses and qPCR validation revealed precocious and enhanced expression of several neurogenic transcription factors including, Ascl1, NeuroG2, and NeuroD1. Furthermore, immunofluorescence imaging of MBNL1 and MBNL2, RNA-splicing factors, displayed enhanced nuclear aggregations, a hallmark of the DM1 disease, in the mutant lines. Moreover, investigation of RNA splicing events identified mis-splicing in many important genes/transcripts including RMST, ANK3 and MBD1 during the neural conversion of congenital DM1 lines. These studies reveal novel paradigms that may contribute to neurological pathogenesis in CDM1 patients. These studies also provide a strong foundation for future mechanistic investigation aimed at understanding CDM1 pathology and may open new avenues for the development of gene therapy approaches for individuals with DM1.

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2026-07-09 | Tissue-specific CTG•CAG expansion rate and disease severity are modified by DNA repair genes expression levels in myotonic dystrophy type 1 patients.

Myotonic dystrophy type 1 (DM1) is a multisystemic autosomal dominant disorder caused by the expansion of an unstable CTG•CAG repeat in the DMPK gene. This study examined whether differential expression of DNA repair genes in three different tissues from the same DM1 patient contributed to tissue-specific somatic instability of the repeat tract. RNA-Seq was used to quantify expression levels of eight DNA repair genes (MSH2, MSH3, MSH6, MLH1, MLH3, PMS2, LIG1, and FAN1). Results indicate that the expression levels varied significantly across tissues, with no inter-tissue correlations, suggesting independent regulation and patient-specific differences. Somatic expansion of the repeat tract in blood and muscle was effectively predicted by a complex ePALxAge interaction, while in muscle it was further influenced by MSH3 and PMS2 gene expression, confirming their role as tissue-specific genetic modifiers in DM1. Although only marginally significant, muscle expression of PMS2 and FAN1 appeared to affect age-at-onset: higher FAN1 expression was associated with reduced somatic expansion and later onset. Our findings also suggest a complex competitive balance between promoters and stabilizers of repeat instability, shaping muscle expansion dynamics and potentially modifying clinical onset. Overall, these results indicate that certain DNA repair genes exert stronger, tissue-dependent effects on somatic instability. We confirm that MSH3, PMS2, and FAN1 act as key modifiers not only of somatic expansion, especially in skeletal muscle, but also of DM1 severity. Although RT-qPCR data might be required to validate some of these results, these genes therefore represent promising therapeutic targets for modulating disease progression.

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2026-07-09 | Elimination of myotonia improves myopathy in a muscleblind-like knockout model of myotonic dystrophy.

A cardinal sign of myotonic dystrophy type 1 (DM1) is myotonia, slow muscle relaxation after voluntary contraction. Myotonia results from mis-regulated splicing of chloride channel 1 (ClC-1), leading to loss of channel function and runs of involuntary action potentials in muscle fibers. Preceding the onset of weakness, myotonia is often the first symptom of DM1, and thus this raises the possibility that muscle hyperexcitability contributes to the subsequent weakness and myopathy. Here, we show that genomic deletion of ClC-1 exon 7a (E7a), a cryptic exon abnormally regulated in DM1, completely rescues of ClC-1 function and yields permanent elimination of myotonia in the muscleblind-like 1 (Mbnl1) knockout mouse model of DM1. The restoration of normal excitability results in normalization of muscle force generation, correction of fiber-type distribution, and improvement of muscle histology. E7a deletion also partially corrects the muscle transcriptome, including changes of differential gene expression and alternative splicing. These results indicate that E7a inclusion is a lynchpin splice event that contributes to myotonic myopathy, and support myotonia reduction as a therapeutic objective in DM1.

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