Molecular basis of myotonic dystrophy: expansion of a trinucleotide (CTG) repeat at the 3' end of a transcript encoding a protein kinase family member

Alternative splicing dysregulation across tissue and therapeutic approaches in a mouse model of myotonic dystrophy type 1

Myotonic dystrophy type 1 (DM1), the leading cause of adult-onset muscular dystrophy, is caused by a CTG repeat expansion. Expression of the repeat causes widespread alternative splicing (AS) defects and downstream pathogenesis, including significant skeletal muscle impacts. The HSA LR mouse model plays a significant role in therapeutic development. This mouse model features a transgene composed of approximately 220 interrupted CTG repeats, which results in skeletal muscle pathology that mirrors DM1. To better understand this model and the growing number of therapeutic approaches developed with it, we performed a meta-analysis of publicly available RNA sequencing data for AS changes across three widely examined skeletal muscles: quadriceps, gastrocnemius, and tibialis anterior. Our analysis demonstrated that transgene expression correlated with the extent of splicing dysregulation across these muscles from gastrocnemius (highest), quadriceps (medium), to tibialis anterior (lowest). We identified 95 splicing events consistently dysregulated across all examined datasets. Comparison of splicing rescue across seven therapeutic approaches showed a range of rescue across the 95 splicing events from the three muscle groups. This analysis contributes to our understanding of the HSA LR model and the growing number of therapeutic approaches currently in preclinical development for DM1.

RNA mis-splicing in children with congenital myotonic dystrophy is associated with physical function

OBJECTIVES

Dysregulated RNA alternative splicing is the hallmark of myotonic dystrophy type 1 (DM1). However, the association between RNA mis-splicing and physical function in children with the most severe form of disease, congenital myotonic dystrophy (CDM), is unknown.

METHODS

Eighty-two participants (42 adults with DM1 and 40 children with CDM) with muscle biopsies and measures of myotonia, motor function, and strength were combined from five observational studies. Data were normalized and correlated with an aggregate measure of alternative splicing dysregulation, [MBNL]inferred, in skeletal muscle biopsies. Multiple linear regression analysis was performed to predict [MBNL]inferred using clinical outcome measures alone. Similar analyses were performed to predict 12-month physical function using baseline metrics.

RESULTS

Myotonia (measured via vHOT) was significantly correlated with RNA mis-splicing in our cross-sectional population of all DM1 individuals; CDM participants alone displayed no myotonia despite a similar range of RNA mis-splicing. Measures of motor performance and muscle strength were significantly associated with [MBNL]inferred in our cohort of all DM1 individuals and when assessing children with CDM independently. Multiple linear regression analyses yielded two models capable of predicting [MBNL]inferred from select clinical outcome assessments alone in all subjects (adjusted R2 = 0.6723) or exclusively in children with CDM (adjusted R2 = 0.5875).

INTERPRETATION

Our findings establish significant correlations between skeletal muscle performance and a composite measure of alternative splicing dysregulation, [MBNL]inferred, in DM1. The strength of these correlations and the development of predictive models will assist in designing efficacious clinical trials for individuals with DM1, particularly CDM.

Altered drug metabolism and increased susceptibility to fatty liver disease in a mouse model of myotonic dystrophy

Myotonic Dystrophy type 1 (DM1), a highly prevalent form of muscular dystrophy, is caused by (CTG)n repeat expansion in the DMPK gene. Much of DM1 research has focused on the effects within the muscle and neurological tissues; however, DM1 patients also suffer from various metabolic and liver dysfunctions such as increased susceptibility to metabolic dysfunction-associated fatty liver disease (MAFLD) and heightened sensitivity to certain drugs. Here, we generated a liver-specific DM1 mouse model that reproduces molecular and pathological features of the disease, including susceptibility to MAFLD and reduced capacity to metabolize specific analgesics and muscle relaxants. Expression of CUG-expanded (CUG)exp repeat RNA within hepatocytes sequestered muscleblind-like proteins and triggered widespread gene expression and RNA processing defects. Mechanistically, we demonstrate that increased expression and alternative splicing of acetyl-CoA carboxylase 1 drives excessive lipid accumulation in DM1 livers, which is exacerbated by high-fat, high-sugar diets. Together, these findings reveal that (CUG)exp RNA toxicity disrupts normal hepatic functions, predisposing DM1 livers to injury, MAFLD, and drug clearance pathologies that may jeopardize the health of affected individuals and complicate their treatment.

Small Molecule Screening Identifies HSP90 as a Modifier of RNA Foci in Myotonic Dystrophy Type 1

Myotonic dystrophy type 1 (DM1) is a multisystemic disorder caused by a CTG triplet repeat expansion within the 3' untranslated region of the DMPK gene. Expression of the expanded allele generates RNA containing long tracts of CUG repeats (CUGexp RNA) that form hairpin structures and accumulate in nuclear RNA foci; however, the factors that control DMPK expression and the formation of CUGexp RNA foci remain largely unknown. We performed an unbiased small molecule screen in an immortalized human DM1 skeletal muscle myoblast cell line and identified HSP90 as a modifier of endogenous RNA foci. Small molecule inhibition of HSP90 leads to enhancement of RNA foci and upregulation of DMPK mRNA levels. Knockdown and overexpression of HSP90 in undifferentiated DM1 myoblasts validated the impact of HSP90 with upregulation and downregulation of DMPK mRNA, respectively. Furthermore, we identified p-STAT3 as a downstream mediator of HSP90 impacting levels of DMPK mRNA and RNA foci. Interestingly, differentiated cells exhibited an opposite effect of HSP90 inhibition displaying downregulation of DMPK mRNA through a mechanism independent of p-STAT3 involvement. This study has revealed a novel mediator for DMPK mRNA and foci regulation in DM1 cells with the potential to identify targets for future therapeutic intervention.

Calcium handling abnormalities increase arrhythmia susceptibility in DMSXL myotonic dystrophy type 1 mice

BACKGROUND

Myotonic dystrophy type 1 (DM1) is a multiorgan disorder with significant cardiac involvement. ECG abnormalities, including arrhythmias, occur in 80 % of DM1 patients and are the second-most common cause of death after respiratory complications; however, the mechanisms underlying the arrhythmogenesis remain unclear. The objective of this study was to investigate the basis of the electrophysiological abnormalities in DM1 using the DMSXL mouse model.

METHODS

ECG parameters were evaluated at baseline and post flecainide challenge. Calcium transient and action potential parameters were evaluated in Langendorff-perfused hearts using fluorescence optical mapping. Calcium transient/sparks were evaluated in ventricular myocytes via confocal microscopy. Protein and mRNA levels for calcium handling proteins were evaluated using western blot and RT-qPCR, respectively.

RESULTS

DMSXL mice showed arrhythmic events on ECG including premature ventricular contractions and sinus block. DMSXL mice showed increased calcium transient time to peak without any change to voltage parameters. Calcium alternans and both sustained and non-sustained ventricular tachyarrhythmias were readily inducible in DMSXL mice. The confocal experiments also showed calcium transient alternans and increased frequency of calcium sparks in DMSXL cardiomyocytes. These calcium abnormalities were correlated with increased RyR2 phosphorylation without changes to the other calcium handling proteins.

CONCLUSIONS

The DMSXL mouse model of DM1 exhibited enhanced arrhythmogenicity associated with abnormal intracellular calcium handling due to hyperphosphorylation of RyR2, pointing to RyR2 as a potential new therapeutic target in DM1 treatment.

Quantum Chemical Characterization of Rotamerism in Thio-Michael Additions for Targeted Covalent Inhibitors

Myotonic dystrophy type I (DM1) is the most common form of adult muscular dystrophy and is a severe condition with no treatment currently available. Recently, small-molecule ligands have been developed as targeted covalent inhibitors that have some selectivity for and covalently inhibit cyclin-dependent kinase 12 (CDK12). CDK12 is involved in the transcription of elongated RNA sections that results in the DM1 condition. The covalent bond is achieved after nucleophilic addition to a Michael acceptor warhead. Previous studies of the conformational preferences of thio-Michael additions have focused on characterizing the reaction profile based on the distance between the sulfur and β-carbon atoms. Rotamerism, however, has not been investigated extensively. Here, we use high-level quantum chemistry calculations, up to coupled cluster with single, double, and perturbative triple excitations [CCSD(T)], to characterize the nucleophilic addition of an archetypal nucleophile, methanethiolate, to various nitrogen-containing Michael acceptors which are representative of the small-molecule covalent inhibitors. By investigating the structural, energetic, and electronic properties of the resulting enolates, as well as their reaction profiles, we show that synclinal additions are generally energetically favored over other additions due to the greater magnitude of attractive noncovalent interactions permitted by the conformation. The calculated transition states associated with the addition process indicate that synclinal addition proceeds via lower energetic barriers than antiperiplanar addition and is the preferred reaction pathway.

AntimiR treatment corrects myotonic dystrophy primary cell defects across several CTG repeat expansions with a dual mechanism of action

This study evaluated therapeutic antimiRs in primary myoblasts from patients with myotonic dystrophy type 1 (DM1). DM1 results from unstable CTG repeat expansions in the DMPK gene, leading to variable clinical manifestations by depleting muscleblind-like splicing regulator protein MBNL1. AntimiRs targeting natural repressors miR-23b and miR-218 boost MBNL1 expression but must be optimized for a better pharmacological profile in humans. In untreated cells, miR-23b and miR-218 were up-regulated, which correlated with CTG repeat size, supporting that active MBNL1 protein repression synergizes with the sequestration by CUG expansions in DMPK. AntimiR treatment improved RNA toxicity readouts and corrected regulated exon inclusions and myoblast defects such as fusion index and myotube area across CTG expansions. Unexpectedly, the treatment also reduced DMPK transcripts and ribonuclear foci. A leading antimiR reversed 68% of dysregulated genes. This study highlights the potential of antimiRs to treat various DM1 forms across a range of repeat sizes and genetic backgrounds by mitigating MBNL1 sequestration and enhancing protein synthesis.

The heterodimer of 2-amino-1,8-naphthyridine and 3-aminoisoquinoline binds to the CTG/CTG triad via hydrogen bonding

Myotonic dystrophy type 1 (DM1) is caused by the aberrant expansion of CTG repeats within the DMPK gene. This study investigated the potential binding of "X-linker-Y" type molecules to the CTG/CTG motif present in CTG repeats, using heterocyclic units X and Y capable of forming complementary hydrogen bonds with nucleobases. Among the tested molecules, the heterodimer of 2-amino-1,8-naphthyridine (X) and 3-aminoisoquinoline (Y) showed significant binding to the CTG/CTG motif. NMR analysis suggested hydrogen-bonded interactions between 3-aminoisoquinoline and thymine.

Structural and Dynamical Properties of Nucleic Acid Hairpins Implicated in Trinucleotide Repeat Expansion Diseases

Dynamic mutations in some human genes containing trinucleotide repeats are associated with severe neurodegenerative and neuromuscular disorders-known as Trinucleotide (or Triplet) Repeat Expansion Diseases (TREDs)-which arise when the repeat number of triplets expands beyond a critical threshold. While the mechanisms causing the DNA triplet expansion are complex and remain largely unknown, it is now recognized that the expandable repeats lead to the formation of nucleotide configurations with atypical structural characteristics that play a crucial role in TREDs. These nonstandard nucleic acid forms include single-stranded hairpins, Z-DNA, triplex structures, G-quartets and slipped-stranded duplexes. Of these, hairpin structures are the most prolific and are associated with the largest number of TREDs and have therefore been the focus of recent single-molecule FRET experiments and molecular dynamics investigations. Here, we review the structural and dynamical properties of nucleic acid hairpins that have emerged from these studies and the implications for repeat expansion mechanisms. The focus will be on CAG, GAC, CTG and GTC hairpins and their stems, their atomistic structures, their stability, and the important role played by structural interrupts.

Rescue of Scn5a mis-splicing does not improve the structural and functional heart defects of a DM1 heart mouse model

Myotonic Dystrophy Type 1 (DM1) is an autosomal dominant multisystemic disorder for which cardiac features, including conduction delays and arrhythmias, are the second leading cause of disease mortality. DM1 is caused by expanded CTG repeats in the 3' untranslated region of the DMPK gene. Transcription of the expanded DMPK allele produces mRNAs containing long tracts of CUG repeats, which sequester the Muscleblind-Like family of RNA binding proteins, leading to their loss-of-function and the dysregulation of alternative splicing. A well-characterized mis-regulated splicing event in the DM1 heart is the increased inclusion of SCN5A exon 6A rather than the mutually exclusive exon 6B that normally predominates in adult heart. As previous work showed that forced inclusion of Scn5a exon 6A in mice recapitulates cardiac DM1 phenotypes, we tested whether rescue of Scn5a mis-splicing would improve the cardiac phenotypes in a DM1 heart mouse model. We generated mice lacking Scn5a exon 6A to force the expression of the adult SCN5A isoform including exon 6B and crossed these mice to our previously established CUG960 DM1 heart mouse model. We showed that correction Scn5a mis-splicing does not improve the DM1 heart conduction delays and structural changes induced by CUG repeat RNA expression. Interestingly, we found that in addition to Scn5a mis-splicing, Scn5a expression is reduced in heart tissues of CUG960 mice and DM1-affected individuals. These data indicate that Scn5a mis-splicing is not the sole driver of DM1 heart deficits and suggest a potential role for reduced Scn5a expression in DM1 cardiac disease.

Change in Visual Acuity of Patients With Fuchs Endothelial Corneal Dystrophy Over 1 Year

PURPOSE

To determine whether the clinical and paraclinical course of Fuchs endothelial corneal dystrophy (FECD) over 1 year is related to the extent of triplet repetition in the transcription factor 4 (TCF4) gene.

METHODS

A prospective study with a 1-year follow-up was conducted. A total of 104 patients (160 eyes) with FECD and an equivalent number of age- and sex-matched control subjects without FECD were included. At inclusion, the corneas were graded using the modified Krachmer grade (KG) and patients were genotyped for the number of trinucleotide repeats (TNRs) in the TCF4 gene by the short tandem repeat assay. Visual acuity, Scheimpflug tomographic features, and the Visual Function and Corneal Health Status using a visual disability instrument were measured on 2 visits at 1-year intervals.

RESULTS

KGs ranged from 1 to 6, and 46% of eyes had grades 1 to 4. 71% of the patients harbored TNR expansion (>40) versus 13% in control subjects ( P < 0.001). Severity at inclusion was higher in the presence of TNR expansion when considering eyes independently (mean grade ±SD, 4.08 ± 1.42) without TNR expansion and 4.66 ± 1.27 with TNR expansion ( P = 0.024). In 1 year, the ETDRS score significantly decreased by -2.97 (95% confidence interval -4.69 to -1.26, P = 0.001) and the ETDRS score with glare by -4.25 (95% confidence interval -6.22 to -2.27, P < 10 -5 ). There was no relationship between the rate of decline and TNR expansion or KG. Central corneal thickness and Visual Function and Corneal Health Status scores did not significantly vary.

CONCLUSIONS

It is possible to measure a subtle progression of FECD over a period as short as 1 year. We did not find a relationship between the presence of TNR expansion and the speed of deterioration over 1 year. This work should facilitate the design of future clinical trials on FECD.

Comprehensive four-year disease progression assessment of myotonic dystrophy type 1

Myotonic dystrophy type 1 (DM1) is a heterogeneous neuromuscular disorder characterized by progressive muscle weakness and myotonia. This study investigates the progression of muscular strength and function over a four-year period. Patients with DM1 were examined at baseline and four years later. The following metrics were assessed over time: muscle strength (Medical Research Council-sumscore), hand-grip strength (Martin-Vigorimeter), hand-grip relaxation time (myotonia), and limitations in activities of daily living and (DM1ActivC questionnaire). A total of 648 patients entered the registry. Recruitment and follow-up is ongoing. In our manuscript, we focus on, 187 patients who were followed for 4 years. A significant decline in MRC sum score was observed, with distal muscles showing more deterioration. Hand-grip strength decreased significantly, with notable differences between sex and phenotype classified by disease onset. Surprisingly, an improvement of myotonia was observed. Follow-up analysis revealed a significant interaction between myotonia and grip-strength over time. Thus, the improvement in myotonia is likely explained by decreased in grip strength. Finally, there was a significant reduction in DM1ActivC score, indicating decreased activity and social participation. This study demonstrated variability in disease progression depending on sex, phenotype and disease status. This research demonstrates a nuanced pattern of disease progression, highlighting the need to combine different outcome measures to fully understand the complexity of DM1.

Enhancing Binding Affinity to CGG/CGG Triad: Optimizing Naphthyridine Carbamate Dimer Derivatives with Varied Linker Lengths

This study examines the binding properties of six naphthyridine carbamate dimer (NCD) derivatives with varying linker lengths to the CGG/CGG triad, a non-canonical DNA structure linked to repeat expansion disorders. By altering the linker length from 2 to 4 methylene groups, we found changes in thermal stability of the ligand-bound complexes while maintaining a consistent 2 : 1 binding stoichiometry. Among the derivatives, CC23 showed superior binding affinity compared to the parent molecule CC33 (NCD). Spectroscopic analyses revealed that linker length influences the conformational equilibrium of NCD derivatives. Thermal melting temperature measurements demonstrated CC23's enhanced thermal stability over CC33. These findings underscore the potential of optimized NCD derivatives, like CC23, as tools to modulate CGG repeat structures, offering insights for therapeutic strategies targeting repeat expansion disorders.

MBL-1/Muscleblind regulates neuronal differentiation and controls the splicing of a terminal selector in Caenorhabditis elegans

The muscleblind family of mRNA splicing regulators is conserved across species and regulates the development of muscles and the nervous system. However, how Muscleblind proteins regulate neuronal fate specification and neurite morphogenesis at the single-neuron level is not well understood. In this study, we found that the C. elegans Muscleblind/MBL-1 promotes axonal growth in the touch receptor neurons (TRNs) by regulating microtubule stability and polarity. Transcriptomic analysis identified dozens of MBL-1-controlled splicing events in genes related to neuronal differentiation or microtubule functions. Among the MBL-1 targets, the LIM-domain transcription factor mec-3 is the terminal selector for the TRN fate and induces the expression of many TRN terminal differentiation genes. MBL-1 promotes the splicing of the mec-3 long isoform, which is essential for TRN fate specification, and inhibits the short isoforms that have much weaker activities in activating downstream genes. MBL-1 promotes mec-3 splicing through three "YGCU(U/G)Y" motifs located in or downstream of the included exon, which is similar to the mechanisms used by mammalian Muscleblind and suggests a deeply conserved context-dependency of the splicing regulation. Interestingly, the expression of mbl-1 in the TRNs is dependent on the mec-3 long isoform, indicating a positive feedback loop between the splicing regulator and the terminal selector. Finally, through a forward genetic screen, we found that MBL-1 promotes neurite growth partly by inhibiting the DLK-1/p38 MAPK pathway. In summary, our study provides mechanistic understanding of the role of Muscleblind in regulating cell fate specification and neuronal morphogenesis.

Advances in human pluripotent stem cell reporter systems

The capability of human pluripotent stem cells (hPSCs) to self-renew and differentiate into any cell type has greatly contributed to the advancement of biomedicine. Reporter lines derived from hPSCs have played a crucial role in elucidating the mechanisms underlying human development and diseases by acting as an alternative reporter system that cannot be used in living humans. To bring hPSCs closer to clinical application in transplantation, scientists have generated reporter lines for isolating the desired cell populations, as well as improving graft quality and treatment outcomes. This review presents an overview of the applications of hPSC reporter lines and the important variables in designing a reporter system, including options for gene delivery and editing tools, design of reporter constructs, and selection of reporter genes. It also provides insights into the prospects of hPSC reporter lines and the challenges that must be overcome to maximize the potential of hPSC reporter lines.

Parental diagnostic delay and developmental outcomes in congenital and childhood-onset myotonic dystrophy type 1

AIM

To investigate the timing of type 1 myotonic dystrophy (DM1) diagnosis in parents of affected children and describe children's perinatal characteristics and developmental outcomes.

METHOD

This was a descriptive case series of children with congenital myotonic dystrophy (CDM) and childhood-onset myotonic dystrophy (ChDM). Parental timing of DM1 diagnosis and the perinatal, motor, and cognitive outcomes of paediatric patients were recorded.

RESULTS

A total of 139 children followed by 12 highly specialized tertiary care neuromuscular centres in Italy and one tertiary neuromuscular centre in the USA were included: 105 children with CDM and 34 children with ChDM (mean age 8 years 8 months and 12 years 2 months respectively; 49 males and 17 males respectively). Seventy (50%) parents were diagnosed with adult-onset DM1 after the affected child was diagnosed. Only 12 (17%) of the 69 parents known to be affected had prenatal testing. Of the 105 children with CDM, 98% had maternally inherited CDM, 36% were born preterm, 83% required a stay in the neonatal intensive care unit for more than 48 hours, 84% and 79% had ambulation and speech delay, and 84% had an IQ lower than 70. Of the 34 children with ChDM, 59% had paternally inherited ChDM, 91% were born at term, and 36% had an IQ lower than 70.

INTERPRETATION

Delay in diagnosing DM1 affects family planning. The prenatal and perinatal outcomes of the affected offspring emphasize the need for proactive counselling as parents may be reluctant to conduct prenatal testing.

Expression levels of core spliceosomal proteins modulate the MBNL-mediated spliceopathy in DM1

Myotonic dystrophy type 1 (DM1) is a heterogeneous multisystemic disease caused by a CTG repeat expansion in DMPK. Transcription of the expanded allele produces toxic CUG repeat RNA that sequesters the MBNL family of alternative splicing (AS) regulators into ribonuclear foci, leading to pathogenic mis-splicing. To identify genetic modifiers of toxic CUG RNA levels and the spliceopathy, we performed a genome-scale siRNA screen using an established HeLa DM1 repeat-selective screening platform. We unexpectedly identified core spliceosomal proteins as a new class of modifiers that rescue the spliceopathy in DM1. Modest knockdown of one of our top hits, SNRPD2, in DM1 fibroblasts and myoblasts, significantly reduces DMPK expression and partially rescues MBNL-regulated AS dysfunction. While the focus on the DM1 spliceopathy has centered around the MBNL proteins, our work reveals an unappreciated role for MBNL:spliceosomal protein stoichiometry in modulating the spliceopathy, revealing new biological and therapeutic avenues for DM1.

Intellectual Profile in Myotonic Dystrophy Type 1 and Its Association With Its Onset: A Systematic Review and Meta-Analysis

BACKGROUND

Myotonic dystrophy type 1 (DM1) is caused by mutations in the DMPK gene, and it is associated with cognitive deficits and intelligence below normative values. The objective of this systematic review and meta-analysis was to estimate the overall intelligence and proportion of intellectual development disorder (IDD) in the population with DM1 and its association with its onset.

METHODS

Systematic searches of Medline, Scopus, Web of Science, and Cochrane Library were performed from inception to January 2023. Studies that determined the full intelligence quotient (FIQ) or the IDD proportion in populations with DM1 were included. Meta-analyses of the FIQ and IDD and the FIQ mean difference and IDD prevalence ratios (PRs) by disease onset, inheritance, and genotype were conducted.

RESULTS

Forty-five studies were included in the meta-analyses, and all were performed in the DM1 population. The FIQ and IDD in DM1 were 77.90 (71.98, 83.81) and 0.44 (0.27, 0.60), respectively. Furthermore, DM1 onset was negatively associated with intelligence. Thus, the comparison "Congenital versus Adult" onsets resulted in an intelligence quotient of -41.61 (-47.81, -35.40) points and a PR of IDD of 9.49 (3.23, 27.89). Finally, maternal inheritance was also negatively associated, but the genotype did not have a statistically significant association.

CONCLUSIONS

The alterations in intelligence in DM1 are highly associated with the onset of the disease. However, the genotype did not explain these alterations well and there may be other genetic or epigenetic factors that should be considered.

Greater cortical thinning and microstructural integrity loss in myotonic dystrophy type 1 compared to myotonic dystrophy type 2

BACKGROUND

Myotonic dystrophy is a multisystem disorder characterized by widespread organic involvement including central nervous system symptoms. Although myotonic dystrophy disease types 1 (DM1) and 2 (DM2) cover a similar spectrum of symptoms, more pronounced clinical and brain alterations have been described in DM1. Here, we investigated brain volumetric and white matter alterations in both disease types and compared to healthy controls (HC).

METHODS

MRI scans were obtained from 29 DM1, 27 DM2, and 56 HC. We assessed macro- and microstructural brain changes by surface-based analysis of cortical thickness of anatomical images and tract-based spatial statistics of fractional anisotropy (FA) obtained by diffusion-weighted imaging, respectively. Global MRI measures were related to clinical and neuropsychological scores to evaluate their clinical relevance.

RESULTS

Cortical thickness was reduced in both patient groups compared to HC, showing similar patterns of regional distribution in DM1 and DM2 (occipital, temporal, frontal) but more pronounced cortical thinning for DM1. Similarly, FA values showed a widespread decrease in DM1 and DM2 compared to HC. Interestingly, FA was significantly lower in DM1 compared to DM2 within most parts of the brain.

CONCLUSION

Comparisons between DM1 and DM2 indicate a more pronounced cortical thinning of grey matter and a widespread reduction in microstructural integrity of white matter in DM1. Future studies are required to unravel the underlying and separating mechanisms for the disease courses of the two types and their neuropsychological symptoms.

Effectiveness and safety of mexiletine versus placebo in patients with myotonia: a systematic review and meta-analysis

BACKGROUND

The rare nature of dystrophic and non-dystrophic myotonia has limited the available evidence on the efficacy of mexiletine as a potential treatment. To address this gap, we conducted a systematic review and meta-analysis to evaluate the effectiveness and safety of mexiletine for both dystrophic and non-dystrophic myotonic patients.

METHODS

The search was conducted on various electronic databases up to March 2023, for randomized clinical trials (RCTs) comparing mexiletine versus placebo in myotonic patients. A risk of bias assessment was carried out, and relevant data was extracted manually into an online sheet. RevMan software (version 5.4) was employed for analysis.

RESULTS

A total of five studies, comprising 186 patients, were included in the meta-analysis. Our findings showed that mexiletine was significantly more effective than placebo in improving stiffness score (SMD =  - 1.19, 95% CI [- 1.53, - 0.85]), as well as in reducing hand grip myotonia (MD =  - 1.36 s, 95% CI [- 1.83, - 0.89]). Mexiletine also significantly improved SF-36 Physical and Mental Component Score in patients with non-dystrophic myotonia only. Regarding safety, mexiletine did not significantly alter ECG parameters but was associated with greater gastrointestinal symptoms (GIT) compared to placebo (RR 3.7, 95% CI [1.79, 7.64]). Other adverse events showed no significant differences.

CONCLUSION

The results support that mexiletine is effective and safe in myotonic patients; however, it is associated with a higher risk of GIT symptoms. Due to the scarcity of published RCTs and the prevalence of GIT symptoms, we recommend further well-designed RCTs testing various drug combinations to reduce GIT symptoms.

Myotonic dystrophy type 1 testing, 2024 revision: A technical standard of the American College of Medical Genetics and Genomics (ACMG)

Myotonic dystrophy type 1 (DM1) is a form of muscular dystrophy causing progressive muscle loss and weakness. Although clinical features can manifest at any age, it is the most common form of muscular dystrophy with onset in adulthood. DM1 is an autosomal dominant condition, resulting from an unstable CTG expansion in the 3'-untranslated region of the myotonic dystrophy protein kinase (DMPK) gene. The age of onset and the severity of the phenotype are roughly correlated with the size of the CTG expansion. Multiple methodologies can be used to diagnose affected individuals with DM1, including polymerase chain reaction, Southern blot, and triplet repeat-primed polymerase chain reaction. Recently, triplet repeat interruptions have been described, which may affect clinical outcomes of a fully-variable allele in DMPK. This document supersedes the Technical Standards and Guidelines for Myotonic Dystrophy originally published in 2009 and reaffirmed in 2015. It is designed for genetic testing professionals who are already familiar with the disease and the methods of analysis.

RNA mis-splicing in children with myotonic dystrophy is associated with physical function

Objectives

Dysregulated RNA alternative splicing is the hallmark of myotonic dystrophy type 1 (DM1). However, the association between RNA mis-splicing and physical function in children with the most severe form of disease, congenital myotonic dystrophy (CDM), is unknown.

Methods

82 participants (42 DM1 adults & 40 CDM children) with muscle biopsies and measures of myotonia, motor function, and strength were combined from five observational studies. Data were normalized and correlated with an aggregate measure of alternative splicing dysregulation, [MBNL] inferred in skeletal muscle biopsies. Multiple linear regression analysis was performed to predict [MBNL] inferred using clinical outcome measures alone. Similar analyses were performed to predict 12-month physical function using baseline metrics.

Results

Myotonia (measured via vHOT) was significantly correlated with RNA mis-splicing in our cross-sectional population of all DM1 individuals; CDM participants alone displayed no myotonia despite a similar range of RNA mis-splicing. Measures of motor performance and muscle strength were significantly associated with [MBNL] inferred in our cohort of all DM1 individuals and when assessing CDM children independently. Multiple linear regression analyses yielded two models capable of predicting [MBNL] inferred from select clinical outcome assessments alone in all subjects (adjusted R 2 = 0.6723) or exclusively in CDM children (adjusted R 2 = 0.5875).

Interpretation

Our findings establish significant correlations between skeletal muscle performance and a composite measure of alternative splicing dysregulation, [MBNL] inferred, in DM1. The strength of these correlations and the development of the predictive models will assist in designing efficacious clinical trials for individuals with DM1, particularly CDM.

Natural history of cardiac involvement in myotonic dystrophy type 1 - Emphasis on the need for lifelong follow-up

BACKGROUND

Cardiac involvement represents a major cause of morbidity and mortality in patients with myotonic dystrophy type 1 (DM1) and prevention of sudden cardiac death (SCD) is a central part of patient care. We investigated the natural history of cardiac involvement in patients with DM1 to provide an evidence-based foundation for adjustment of follow-up protocols.

METHODS

Patients with genetically confirmed DM1 were identified. Data on patient characteristics, performed investigations (12 lead ECG, Holter monitoring and echocardiography), and clinical outcomes were retrospectively collected from electronic health records.

RESULTS

We included 195 patients (52% men) with a mean age at baseline evaluation of 41 years (range 14-79). The overall prevalence of cardiac involvement increased from 42% to 66% after a median follow-up of 10.5 years. There was a male predominance for cardiac involvement at end of follow-up (74 vs. 44%, p < 0.001). The most common types of cardiac involvement were conduction abnormalities (48%), arrhythmias (35%), and left ventricular systolic dysfunction (21%). Only 17% of patients reported cardiac symptoms. The standard 12‑lead ECG was the most sensitive diagnostic modality and documented cardiac involvement in 24% at baseline and in 49% at latest follow-up. However, addition of Holter monitoring and echocardiography significantly increased the diagnostic yield with 18 and 13% points at baseline and latest follow-up, respectively. Despite surveillance 35 patients (18%) died during follow-up; seven due to SCD.

CONCLUSIONS

In patients with DM1 cardiac involvement was highly prevalent and developed during follow-up. These findings justify lifelong follow-up with ECG, Holter, and echocardiography.

CLINICAL PERSPECTIVE

What is new? What are the clinical implications?

Different neuropsychological and brain volumetric profiles in a pair of identical twins with myotonic dystrophy type 1 indicate a non-genetic modulation of clinical phenotype

We report on genetic and environmental modulation of social cognition abilities and brain volume correlates in two monozygotic twins (Twin1 and Twin2) with genetically confirmed myotonic dystrophy-type1 who grew up in different environmental settings. They both underwent neuropsychological assessment (i.e., Intelligent Quotient [IQ], theory of mind, emotion recognition tests), and MRI scanning to evaluate regional brain volumetrics compared to 10 gender and sex-matched healthy controls. Against a normal IQ level in both patients, Twin1 was more impaired in emotional processing and Twin2 in cognitive aspects of social cognition. Both patients showed grey matter (GM) atrophy in Brodmann Areas 23/31 (BA23/31) and BA7 bilaterally, while Twin2 showed additional GM loss in right BA46. Both patients showed a similar pattern of white matter atrophy involving the thalamus, basal ganglia, and uncinate fasciculus. White matter atrophy appeared to be mostly driven by genetics, while grey matter volumes appeared associated with different impairments in social cognition and possibly modulated by environment.

Next generation sequencing panel as an effective approach to genetic testing in patients with a highly variable phenotype of neuromuscular disorders

Neuromuscular disorders (NMDs) include a wide range of diseases affecting the peripheral nervous system. The genetic diagnoses are increasingly obtained with using the next generation sequencing (NGS). We applied the custom-design targeted NGS panel including 89 genes, together with genotyping and multiplex ligation-dependent probe amplification (MLPA) to identify a genetic spectrum of NMDs in 52 Polish patients. As a result, the genetic diagnosis was determined by NGS panel in 29 patients so its diagnostic utility is estimated at 55.8%. The most pathogenic variants were found in CLCN1, followed by CAPN3, SCN4A, and SGCA genes. Genotyping of myotonic dystrophy type 1 and 2 (DM1 and DM2) as a secondary approach has been performed. The co-occurrence of CAPN3 and CNBP mutations in one patient as well as DYSF and CNBP mutations in another suggests possibly more complex inheritance as well as expression of a phenotype. In 7 individuals with single nucleotide variant found in NGS testing, the MLPA of the CAPN3 gene was performed detecting the deletion encompassing exons 2-8 in the CAPN3 gene in one patient, confirming recessive limb-girdle muscular dystrophy type 1 (LGMDR1). Thirty patients obtained a genetic diagnosis (57.7%) after using NGS testing, genotyping and MLPA analysis. The study allowed for the identification of 27 known and 4 novel pathogenic/likely pathogenic variants and variants of uncertain significance (VUS) associated with NMDs.In conclusion, the diagnostic approach with diverse molecular techniques enables to broaden the mutational spectrum and maximizes the diagnostic yield. Furthermore, the co-occurrence of DM2 and LGMD has been detected in 2 individuals.

Short tandem repeat expansions in cortical layer-specific genes implicate in phenotypic severity and adaptability of autism spectrum disorder

AIM

Short tandem repeats (STRs) are repetitive DNA sequences and highly mutable in various human disorders. While the involvement of STRs in various genetic disorders has been extensively studied, their role in autism spectrum disorder (ASD) remains largely unexplored. In this study, we aimed to investigate genetic association of STR expansions with ASD using whole genome sequencing (WGS) and identify risk loci associated with ASD phenotypes.

METHODS

We analyzed WGS data of 634 ASD families and performed genome-wide evaluation for 12,929 STR loci. We found rare STR expansions that exceeded normal repeat lengths in autism cases compared to unaffected controls. By integrating single cell RNA and ATAC sequencing datasets of human postmortem brains, we prioritized STR loci in genes specifically expressed in cortical development stages. A deep learning method was used to predict functionality of ASD-associated STR loci.

RESULTS

In ASD cases, rare STR expansions predominantly occurred in early cortical layer-specific genes involved in neurodevelopment, highlighting the cellular specificity of STR-associated genes in ASD risk. Leveraging deep learning prediction models, we demonstrated that these STR expansions disrupted the regulatory activity of enhancers and promoters, suggesting a potential mechanism through which they contribute to ASD pathogenesis. We found that individuals with ASD-associated STR expansions exhibited more severe ASD phenotypes and diminished adaptability compared to non-carriers.

CONCLUSION

Short tandem repeat expansions in cortical layer-specific genes are associated with ASD and could potentially be a risk genetic factor for ASD. Our study is the first to show evidence of STR expansion associated with ASD in an under-investigated population.

Immortalized human myotonic dystrophy type 1 muscle cell lines to address patient heterogeneity

Historically, cellular models have been used as a tool to study myotonic dystrophy type 1 (DM1) and the validation of therapies in said pathology. However, there is a need for in vitro models that represent the clinical heterogeneity observed in patients with DM1 that is lacking in classical models. In this study, we immortalized three DM1 muscle lines derived from patients with different DM1 subtypes and clinical backgrounds and characterized them at the genetic, epigenetic, and molecular levels. All three cell lines display DM1 hallmarks, such as the accumulation of RNA foci, MBNL1 sequestration, splicing alterations, and reduced fusion. In addition, alterations in early myogenic markers, myotube diameter and CTCF1 DNA methylation were also found in DM1 cells. Notably, the new lines show a high level of heterogeneity in both the size of the CTG expansion and the aforementioned molecular alterations. Importantly, these immortalized cells also responded to previously tested therapeutics. Altogether, our results show that these three human DM1 cellular models are suitable to study the pathophysiological heterogeneity of DM1 and to test future therapeutic options.

Somatic and intergenerational G4C2 hexanucleotide repeat instability in a human C9orf72 knock-in mouse model

Expansion of a G4C2 repeat in the C9orf72 gene is associated with familial Amyotrophic Lateral Sclerosis (ALS) and Frontotemporal Dementia (FTD). To investigate the underlying mechanisms of repeat instability, which occurs both somatically and intergenerationally, we created a novel mouse model of familial ALS/FTD that harbors 96 copies of G4C2 repeats at a humanized C9orf72 locus. In mouse embryonic stem cells, we observed two modes of repeat expansion. First, we noted minor increases in repeat length per expansion event, which was dependent on a mismatch repair pathway protein Msh2. Second, we found major increases in repeat length per event when a DNA double- or single-strand break (DSB/SSB) was artificially introduced proximal to the repeats, and which was dependent on the homology-directed repair (HDR) pathway. In mice, the first mode primarily drove somatic repeat expansion. Major changes in repeat length, including expansion, were observed when SSB was introduced in one-cell embryos, or intergenerationally without DSB/SSB introduction if G4C2 repeats exceeded 400 copies, although spontaneous HDR-mediated expansion has yet to be identified. These findings provide a novel strategy to model repeat expansion in a non-human genome and offer insights into the mechanism behind C9orf72 G4C2 repeat instability.

Genetic forms of tauopathies: inherited causes and implications of Alzheimer's disease-like TAU pathology in primary and secondary tauopathies

Tauopathies are a heterogeneous group of neurologic diseases characterized by pathological axodendritic distribution, ectopic expression, and/or phosphorylation and aggregation of the microtubule-associated protein TAU, encoded by the gene MAPT. Neuronal dysfunction, dementia, and neurodegeneration are common features of these often detrimental diseases. A neurodegenerative disease is considered a primary tauopathy when MAPT mutations/haplotypes are its primary cause and/or TAU is the main pathological feature. In case TAU pathology is observed but superimposed by another pathological hallmark, the condition is classified as a secondary tauopathy. In some tauopathies (e.g. MAPT-associated frontotemporal dementia (FTD), progressive supranuclear palsy (PSP), corticobasal degeneration (CBD), and Alzheimer's disease (AD)) TAU is recognized as a significant pathogenic driver of the disease. In many secondary tauopathies, including Parkinson's disease (PD) and Huntington's disease (HD), TAU is suggested to contribute to the development of dementia, but in others (e.g. Niemann-Pick disease (NPC)) TAU may only be a bystander. The genetic and pathological mechanisms underlying TAU pathology are often not fully understood. In this review, the genetic predispositions and variants associated with both primary and secondary tauopathies are examined in detail, assessing evidence for the role of TAU in these conditions. We highlight less common genetic forms of tauopathies to increase awareness for these disorders and the involvement of TAU in their pathology. This approach not only contributes to a deeper understanding of these conditions but may also lay the groundwork for potential TAU-based therapeutic interventions for various tauopathies.

Neurocognitive disorder in Myotonic dystrophy type 1

Cognitive deficits and abnormal cognitive aging have been associated with Myotonic dystrophy type 1 (DM1), but the knowledge of the extent and progression of decline is limited. The aim of this study was to examine the prevalence of signs of neurocognitive disorder (mild cognitive impairment and dementia) in adult patients with DM1. A total of 128 patients with childhood, juvenile, adult, and late onset DM1 underwent a screening using the Montreal Cognitive Assessment (MoCA). Demographic and clinical information was collected. The results revealed that signs of neurocognitive disorder were relatively rare among the participants. However, 23.8 % of patients with late onset DM1 (aged over 60 years) scored below MoCA cut-off (=23), and this group also scored significantly worse compared to patients with adult onset. Age at examination were negatively correlated with MoCA scores, although it only explained a small portion of the variation in test results. Other demographic and clinical factors showed no association with MoCA scores. In conclusion, our findings indicate a low prevalence of signs of neurocognitive disorder in adult patients with DM1, suggesting that cognitive deficits rarely progress to severe disorders over time. However, the performance of patients with late onset DM1 suggests that this phenotype warrants further exploration in future studies, including longitudinal and larger sample analyses.

Role of the repeat expansion size in predicting age of onset and severity in RFC1 disease

RFC1 disease, caused by biallelic repeat expansion in RFC1, is clinically heterogeneous in terms of age of onset, disease progression and phenotype. We investigated the role of the repeat size in influencing clinical variables in RFC1 disease. We also assessed the presence and role of meiotic and somatic instability of the repeat. In this study, we identified 553 patients carrying biallelic RFC1 expansions and measured the repeat expansion size in 392 cases. Pearson's coefficient was calculated to assess the correlation between the repeat size and age at disease onset. A Cox model with robust cluster standard errors was adopted to describe the effect of repeat size on age at disease onset, on age at onset of each individual symptoms, and on disease progression. A quasi-Poisson regression model was used to analyse the relationship between phenotype and repeat size. We performed multivariate linear regression to assess the association of the repeat size with the degree of cerebellar atrophy. Meiotic stability was assessed by Southern blotting on first-degree relatives of 27 probands. Finally, somatic instability was investigated by optical genome mapping on cerebellar and frontal cortex and unaffected peripheral tissue from four post-mortem cases. A larger repeat size of both smaller and larger allele was associated with an earlier age at neurological onset [smaller allele hazard ratio (HR) = 2.06, P < 0.001; larger allele HR = 1.53, P < 0.001] and with a higher hazard of developing disabling symptoms, such as dysarthria or dysphagia (smaller allele HR = 3.40, P < 0.001; larger allele HR = 1.71, P = 0.002) or loss of independent walking (smaller allele HR = 2.78, P < 0.001; larger allele HR = 1.60; P < 0.001) earlier in disease course. Patients with more complex phenotypes carried larger expansions [smaller allele: complex neuropathy rate ratio (RR) = 1.30, P = 0.003; cerebellar ataxia, neuropathy and vestibular areflexia syndrome (CANVAS) RR = 1.34, P < 0.001; larger allele: complex neuropathy RR = 1.33, P = 0.008; CANVAS RR = 1.31, P = 0.009]. Furthermore, larger repeat expansions in the smaller allele were associated with more pronounced cerebellar vermis atrophy (lobules I-V β = -1.06, P < 0.001; lobules VI-VII β = -0.34, P = 0.005). The repeat did not show significant instability during vertical transmission and across different tissues and brain regions. RFC1 repeat size, particularly of the smaller allele, is one of the determinants of variability in RFC1 disease and represents a key prognostic factor to predict disease onset, phenotype and severity. Assessing the repeat size is warranted as part of the diagnostic test for RFC1 expansion.

RNAi-based drug design: considerations and future directions

More than 25 years after its discovery, the post-transcriptional gene regulation mechanism termed RNAi is now transforming pharmaceutical development, proved by the recent FDA approval of multiple small interfering RNA (siRNA) drugs that target the liver. Synthetic siRNAs that trigger RNAi have the potential to specifically silence virtually any therapeutic target with unprecedented potency and durability. Bringing this innovative class of medicines to patients, however, has been riddled with substantial challenges, with delivery issues at the forefront. Several classes of siRNA drug are under clinical evaluation, but their utility in treating extrahepatic diseases remains limited, demanding continued innovation. In this Review, we discuss principal considerations and future directions in the design of therapeutic siRNAs, with a particular emphasis on chemistry, the application of informatics, delivery strategies and the importance of careful target selection, which together influence therapeutic success.

Cognitive impairment, neuroimaging abnormalities, and their correlations in myotonic dystrophy: a comprehensive review

Myotonic dystrophy (DM) encompasses a spectrum of neuromuscular diseases characterized by myotonia, muscle weakness, and wasting. Recent research has led to the recognition of DM as a neurological disorder. Cognitive impairment is a central nervous system condition that has been observed in various forms of DM. Neuroimaging studies have increasingly linked DM to alterations in white matter (WM) integrity and highlighted the relationship between cognitive impairment and abnormalities in WM structure. This review aims to summarize investigations into cognitive impairment and brain abnormalities in individuals with DM and to elucidate the correlation between these factors and the potential underlying mechanisms contributing to these abnormalities.

Challenges and Considerations of Preclinical Development for iPSC-Based Myogenic Cell Therapy

Cell therapies derived from induced pluripotent stem cells (iPSCs) offer a promising avenue in the field of regenerative medicine due to iPSCs' expandability, immune compatibility, and pluripotent potential. An increasing number of preclinical and clinical trials have been carried out, exploring the application of iPSC-based therapies for challenging diseases, such as muscular dystrophies. The unique syncytial nature of skeletal muscle allows stem/progenitor cells to integrate, forming new myonuclei and restoring the expression of genes affected by myopathies. This characteristic makes genome-editing techniques especially attractive in these therapies. With genetic modification and iPSC lineage specification methodologies, immune-compatible healthy iPSC-derived muscle cells can be manufactured to reverse the progression of muscle diseases or facilitate tissue regeneration. Despite this exciting advancement, much of the development of iPSC-based therapies for muscle diseases and tissue regeneration is limited to academic settings, with no successful clinical translation reported. The unknown differentiation process in vivo, potential tumorigenicity, and epigenetic abnormality of transplanted cells are preventing their clinical application. In this review, we give an overview on preclinical development of iPSC-derived myogenic cell transplantation therapies including processes related to iPSC-derived myogenic cells such as differentiation, scaling-up, delivery, and cGMP compliance. And we discuss the potential challenges of each step of clinical translation. Additionally, preclinical model systems for testing myogenic cells intended for clinical applications are described.

RNA-Small-Molecule Interaction: Challenging the "Undruggable" Tag

RNA targeting, specifically with small molecules, is a relatively new and rapidly emerging avenue with the promise to expand the target space in the drug discovery field. From being "disregarded" as an "undruggable" messenger molecule to FDA approval of an RNA-targeting small-molecule drug Risdiplam, a radical change in perspective toward RNA has been observed in the past decade. RNAs serve important regulatory functions beyond canonical protein synthesis, and their dysregulation has been reported in many diseases. A deeper understanding of RNA biology reveals that RNA molecules can adopt a variety of structures, carrying defined binding pockets that can accommodate small-molecule drugs. Due to its functional diversity and structural complexity, RNA can be perceived as a prospective target for therapeutic intervention. This perspective highlights the proof of concept of RNA-small-molecule interactions, exemplified by targeting of various transcripts with functional modulators. The advent of RNA-oriented knowledge would help expedite drug discovery.

Neurobehavioral Phenotype of Children With Congenital Myotonic Dystrophy

BACKGROUND AND OBJECTIVES

To describe the neurobehavioral phenotype of congenital myotonic dystrophy. Congenital myotonic dystrophy (CDM) is the most severe form of myotonic dystrophy, characterized by symptom presentation at birth and later, cognitive impairment, autistic features, and disordered sleep.

METHODS

The neurobehavioral phenotype was assessed in this cross-sectional study by a neuropsychological battery consisting of the Wechsler Preschool and Primary Scale of Intelligence, Third Edition, Weschler Intelligence Scale for Children, Fourth Edition, Vineland Adaptive Behavior Scale, Second Edition (Vineland-II), Behavior Rating Inventory of Executive Function including preschool and teacher reports, Autism Spectrum Screening Questionnaire, Social Communication Scale, and Repetitive Behavior Scale-Revised. Sleep quality was evaluated with the Pediatric Sleep Questionnaire and Pediatric Daytime Sleepiness Scale.

RESULTS

Fifty-five children with CDM, ages 5 weeks to 14 years, were enrolled. The mean age and (CTG)n repeats (±SD) were 6.4 ± 3.8 years and 1,263 ± 432, respectively. The mean IQ was 64.1 ± 14.9 on the Weschler scales with 65.6% of participants falling in the extremely low range for IQ. Adaptive functioning was significantly low for 57.1% of participants (n = 20). Caregiver report of executive functioning indicated 23.1% (9/39) of participants had clinically elevated levels of dysfunction, though teacher report was discrepant and indicated 53.3% of participants with CDM fell in this range (8/15). Spearman correlations were strongly positive (p ≤ 0.05) for estimated full scale IQ, overall adaptive functioning and with daily living and socialization domain standard scores on the Vineland-II ranging from r = 0.719 to r = 0.849 for all ages. Aspects of executive function were directly related to features of autism and sleep quality. Social communication was inversely related to all aspects of daily functioning, except communication, and directly related to aspects of autism behavior.

DISCUSSION

Depressed IQ, adaptive skills, and executive functioning, poor sleep quality, and features of autism and altered social functioning individually describe different aspects of the neurobehavioral phenotype in CDM. These neurobehavioral and sleep measures could help quantitatively measure and assess the burden of cognitive impairment in CDM.

Small and large fiber neuropathy in adults with myotonic dystrophy type 1

Introduction

Myotonic dystrophy type 1 (DM1) is an inherited neuromuscular disorder that affects multiple organs. In this study, we investigated symptoms of pain and presence of small and large fiber neuropathy in the juvenile and adult form of DM1.

Method

Twenty genetically verified DM1 patients were included. Pain was assessed, and neurological examination and investigations of the peripheral nervous system by quantification of small nerve fibers in skin biopsy, quantitative sensory testing and nerve conduction studies were performed. Results from skin biopsies were compared to healthy controls.

Result

Seventeen patients reported chronic pain. Large and/or small fiber abnormalities were present in 50% of the patients. The intraepidermal nerve fiber density was significantly lower in the whole group of patients compared to healthy controls.

Conclusion

Small-fiber neuropathy might be an important cause of pain in DM1.

Analysis of splicing abnormalities in the white matter of myotonic dystrophy type 1 brain using RNA sequencing

Myotonic dystrophy type 1 (DM1) is a neuromuscular disorder caused by the genomic expansion of CTG repeats, in which RNA-binding proteins, such as muscleblind-like protein, are sequestered in the nucleus, and abnormal splicing is observed in various genes. Although abnormal splicing occurs in the brains of patients with DM1, its relation to central nervous system symptoms is unknown. Several imaging studies have indicated substantial white matter defects in patients with DM1. Here, we performed RNA sequencing and analysis of CTG repeat lengths in the frontal lobe of patients with DM1, separating the gray matter and white matter, to investigate splicing abnormalities in the DM1 brain, especially in the white matter. Several genes showed similar levels of splicing abnormalities in both gray and white matter, with an observable trend toward an increased number of repeats in the gray matter. These findings suggest that white matter defects in DM1 stem from aberrant RNA splicing in both gray and white matter. Notably, several of the genes displaying abnormal splicing are recognized as being dominantly expressed in astrocytes and oligodendrocytes, leading us to hypothesize that splicing defects in the white matter may be attributed to abnormal RNA splicing in glial cells.

High-Resolution NMR Structures of Intrastrand Hairpins Formed by CTG Trinucleotide Repeats

The CAG and CTG trinucleotide repeat expansions cause more than 10 human neurodegenerative diseases. Intrastrand hairpins formed by trinucleotide repeats contribute to repeat expansions, establishing them as potential drug targets. High-resolution structural determination of CAG and CTG hairpins poses as a long-standing goal to aid drug development, yet it has not been realized due to the intrinsic conformational flexibility of repetitive sequences. We herein investigate the solution structures of CTG hairpins using nuclear magnetic resonance (NMR) spectroscopy and found that four CTG repeats with a clamping G-C base pair was able to form a stable hairpin structure. We determine the first solution NMR structure of dG(CTG)4C hairpin and decipher a type I folding geometry of the TGCT tetraloop, wherein the two thymine residues form a T·T loop-closing base pair and the first three loop residues continuously stack. We further reveal that the CTG hairpin can be bound and stabilized by a small-molecule ligand, and the binding interferes with replication of a DNA template containing CTG repeats. Our determined high-resolution structures lay an important foundation for studying molecular interactions between native CTG hairpins and ligands, and benefit drug development for trinucleotide repeat expansion diseases.

Chronic endoplasmic reticulum stress in myotonic dystrophy type 2 promotes autoimmunity via mitochondrial DNA release

Myotonic dystrophy type 2 (DM2) is a tetranucleotide CCTG repeat expansion disease associated with an increased prevalence of autoimmunity. Here, we identified an elevated type I interferon (IFN) signature in peripheral blood mononuclear cells and primary fibroblasts of DM2 patients as a trigger of chronic immune stimulation. Although RNA-repeat accumulation was prevalent in the cytosol of DM2-patient fibroblasts, type-I IFN release did not depend on innate RNA immune sensors but rather the DNA sensor cGAS and the prevalence of mitochondrial DNA (mtDNA) in the cytoplasm. Sublethal mtDNA release was promoted by a chronic activation of the ATF6 branch of the unfolded protein response (UPR) in reaction to RNA-repeat accumulation and non-AUG translated tetrapeptide expansion proteins. ATF6-dependent mtDNA release and resulting cGAS/STING activation could also be recapitulated in human THP-1 monocytes exposed to chronic endoplasmic reticulum (ER) stress. Altogether, our study demonstrates a novel mechanism by which large repeat expansions cause chronic endoplasmic reticulum stress and associated mtDNA leakage. This mtDNA is, in turn, sensed by the cGAS/STING pathway and induces a type-I IFN response predisposing to autoimmunity. Elucidating this pathway reveals new potential therapeutic targets for autoimmune disorders associated with repeat expansion diseases.

CGG repeat expansion in LOC642361/NUTM2B-AS1 typically presents as oculopharyngodistal myopathy

CGG repeat expansions in LOC642361/NUTM2B-AS1 have recently been identified as a cause of oculopharyngeal myopathy with leukoencephalopathy. However, since only three patients from a single family were reported, it remains unknown whether their clinicopathological features are typical for CGG repeat expansions in LOC642361/NUTM2B-AS1. Here, using repeat-primed-polymerase chain reaction and long-read sequencing, we identify 12 individuals from 3 unrelated families with CGG repeat expansions in LOC642361/NUTM2B-AS1, typically presenting with oculopharyngodistal myopathy. The CGG repeat expansions range from 161 to 669 repeat units. Most of the patients present with ptosis, restricted eye movements, dysphagia, dysarthria, and diffuse limb muscle weakness. Only one patient shows T2-weighted hyperintensity in the cerebellar white matter surrounding the deep cerebellar nuclei on brain magnetic resonance imaging. Muscle biopsies from three patients show a myopathic pattern and rimmed vacuoles. Analyses of muscle biopsies suggest that CGG repeat expansions in LOC642361/NUTM2B-AS1 may deleteriously affect aggrephagic capacity, suggesting that RNA toxicity and mitochondrial dysfunction may contribute to pathogenesis. Our study thus expands the phenotypic spectrum for the CGG repeat expansion of LOC642361/NUTM2B-AS1 and indicates that this genetic variant typically manifests as oculopharyngodistal myopathy with chronic myopathic changes with rimmed vacuoles and filamentous intranuclear inclusions in muscle fibers.

RNA therapeutics for neurological disease

Neurological disorders are the group of diseases that primarily affect the center nervous system, which could lead to a significant negative impact on the ability of learning new skills, speaking, breathing, walking, judging, making decision, and other essential living skills. In the last decade, neurological disorders have significantly increased their impact to our community and become the one of leading causes of disability and death. The World Health Organization has identified neurological disorders including Alzheimer's disease and other dementia as the health crisis for the modern life. Tremendous ongoing research efforts focus on understanding of disease genetics, molecular mechanisms and developing therapeutic interventions. Because of the urgent need of the effective therapeutics and the recent advances in the toolkits and understanding for developing more drug-like RNA molecules, there is a growing interest for developing RNA therapeutics for neurological disorders. This article will discuss genetics and mechanisms of neurological disorders and how RNA-based molecules have been used to develop therapeutics for this group of diseases, challenges of RNA therapeutics and future perspectives on this rising therapeutic intervention tool.

A phenome-wide association study of tandem repeat variation in 168,554 individuals from the UK Biobank

Most genetic association studies focus on binary variants. To identify the effects of multi-allelic variation of tandem repeats (TRs) on human traits, we performed direct TR genotyping and phenome-wide association studies in 168,554 individuals from the UK Biobank, identifying 47 TRs showing causal associations with 73 traits. We replicated 23 of 31 (74%) of these causal associations in the All of Us cohort. While this set included several known repeat expansion disorders, novel associations we found were attributable to common polymorphic variation in TR length rather than rare expansions and include e.g. a coding polyhistidine motif in HRCT1 influencing risk of hypertension and a poly(CGC) in the 5'UTR of GNB2 influencing heart rate. Causal TRs were strongly enriched for associations with local gene expression and DNA methylation. Our study highlights the contribution of multi-allelic TRs to the "missing heritability" of the human genome.

Neuromuscular disorders in the omics era

Neuromuscular disorders encompass a spectrum of conditions characterized by primary lesions within the peripheral nervous system, which include the anterior horn cell, peripheral nerve, neuromuscular junction, and muscle. In pediatrics, most of these disorders are linked to genetic causes. Despite the considerable progress, the diagnosis of these disorders remains a challenging due to wide clinical presentation, disease heterogeneity and rarity. It is noteworthy that certain neuromuscular disorders, once deemed untreatable, can now be effectively managed through novel therapies. Biomarkers emerge as indispensable tools, serving as objective measures that not only refine diagnostic accuracy but also provide guidance for therapeutic decision-making and the ongoing monitoring of long-term outcomes. Herein a comprehensive review of biomarkers in neuromuscular disorders is provided. We highlight the role of omics-based technologies that further characterize neuromuscular pathophysiology as well as identify potential therapeutic targets to guide treatment strategies.

Myotonic Dystrophy Type 1 (DM1): Clinical Characteristics and Disease Progression in a Large Cohort

BACKGROUND

DM1 is a multisystem disorder caused by expansion of a CTG triplet repeat in the 3' non-coding region of DMPK. Neuropsychological consequences and sleep abnormalities are important associations in DM1.

OBJECTIVE

To describe the clinical phenotype, disease progression and characterize the sleep alterations and cognitive abnormalities in a sub-set of patients.

MATERIALS AND METHODS

A retrospective study on 120 genetically confirmed DM1 cases. Findings in neuropsychological assessment and multiple sleep questionnaires were compared with 14 age and sex matched healthy individuals. All 120 patients were contacted through letters/telephonic consultation/hospital visits to record their latest physical and functional disabilities.

RESULTS

The mean age at symptom onset was 23.1 ± 11.4 years, M: F = 3.8:1, mean duration of illness = 14.3 ± 9.5 years. Clinically 54.2% had adult onset form, juvenile = 27.5%, infantile = 10.8%, late adult onset = 7.5%. Paternal transmission occurred more frequently. The predominant initial symptoms were myotonia (37.5%), hand weakness (21.7%), lower limb weakness (23.3%) and bulbar (10%). Twenty patients completed sleep questionnaires (SQ). Abnormal scores were noted in Epworth sleepiness scale (55%); Pittsburgh sleep quality index (45%); Berlin SQ (30%); Rapid eye movement sleep Behaviour Disorder SQ (15%); Restless leg syndrome rating scale (10%). Neuropsychological assessment of 20 patients revealed frontal executive dysfunction, attention impairment and visuospatial dysfunction. Frontal lobe was most affected (72%) followed by parietal (16%) and temporal lobe (12%).

CONCLUSIONS

The current study provides a comprehensive account of the clinical characteristics in Indian patients with DM1. Hypersomnolence was most commonly seen. Excessive daytime sleepiness and Sleep disordered breathing were the most common sleep related abnormality. Cognitive impairment comprised predominantly of frontal lobe dysfunction.

Recent Advances in Pyrazole-based Protein Kinase Inhibitors as Emerging Therapeutic Targets

BACKGROUND

Pyrazole-scaffold protein kinase inhibitors (PKIs) have emerged as promising therapeutic agents for the treatment of various diseases, such as cancer, inflammatory disorders, and neurological diseases. This review article provides an overview of the pharmacological properties of pyrazole-scaffold PKIs, including their mechanism of action, selectivity, potency, and toxicity. The article also summarizes the recent developments in the design and synthesis of pyrazole-scaffold PKIs, highlighting the structural features and modifications that contribute to their pharmacological activity. In addition, the article discusses the preclinical and clinical studies of pyrazole-scaffold PKIs, including their efficacy, safety, and pharmacokinetic properties.

METHODS

A comprehensive search has been conducted on several online patent databases, including the United States Patent and Trademark Office (USPTO), the European Patent Office (EPO), and the World Intellectual Property Organization (WIPO). The search was conducted using pyrazole as the keyword. The search was limited to patents filed between 2015 and 2022. Patents were included if they involved articles in the fields of protein kinase inhibitors, and included literature on some pyrazoles and their pharmacological activities.

RESULTS

Data were extracted from each included patent on the following variables: patent title, patent number, inventors, assignee, filing date, publication date, patent type, and field of invention. Data were extracted from each patent using a standardized form to ensure consistency and accuracy.

CONCLUSION

The design and pharmacological evaluation of organic compounds containing pyrazole structure as biologically active substances have been done, and the key structures from the pharmacological data obtained as protein kinase inhibitors have been addressed in detail. The review concludes with a discussion on the current challenges and future directions for the development of pyrazole-scaffold PKIs as therapeutic agents. Overall, this review article provides a comprehensive summary of the pharmacological properties of pyrazole-scaffold PKIs, which will be of interest to researchers and clinicians in the field of drug discovery and development.

Inherited myotonias

The inherited myotonias are a complex group of diseases caused by variations in genes that encode or modulate the expression of ion channels that regulate muscle excitability. These variations alter muscle membrane excitability allowing mild depolarization, causing myotonic discharges. There are two groups of inherited myotonia, the dystrophic and the nondystrophic myotonias (NDM). Patients with NDM have a pure muscle phenotype with variations in channel genes expressed in muscle. The dystrophic myotonias are caused by genes that alter splicing leading to more systemic effects with myotonia being one of a number of systemic symptoms. This chapter therefore focuses on the key aspects of the NDMs. The NDMs manifest with varying clinical phenotypes, which change from infancy to adulthood. The pathogenicity of different variants can be determined using heterologous expression systems to understand the alteration in channel properties and predict the likelihood of causing disease. Myotonia itself can be managed by lifestyle modifications. A number of randomized controlled trials demonstrate efficacy of mexiletine and lamotrigine in treating myotonia, but there is an evidence that specific variants may be more or less well-treated by the different agents because of how they alter the channel kinetics. More work is needed to develop more targeted genetic treatments.

Expert Insights from a Delphi-driven Neurologists' Panel: Real-world Mexiletine use in Patients with Myotonic Disorders in Italy

Background

Myotonic disorders, such as non-dystrophic myotonias (NDMs) and myotonic dystrophies (DMs) are characterized by a delay in muscle relaxation after a contraction stimulus. There is general consensus that protocols to treat myotonia need to be implemented.

Objective

Mexiletine is the only pharmacological agent approved for the symptomatic treatment of myotonia in adult patients with NDM and is considered to be the first-line treatment for DMs; however, its production in Italy was halted in 2022 making its availability to patients problematic.

Methods

A panel of 8 Italian neurologists took part in a two-round Delphi panel between June and October 2022, analyzing the current use of mexiletine in Italian clinical practice.

Results

The panelists assist 1126 patients (69% DM type1, 18% NDM and 13% DM type2). Adult NDM patients receive, on average, 400-600 mg of mexiletine hydrochloride (HCl) while adult DM patients receive 100-600 mg, per day in the long-term. The severity of symptoms is considered the main reason to start mexiletine treatment for both NDM and DM patients. Mexiletine is reckoned to have a clinical impact for both NDM and DM patients, but currently drug access is problematic.

Conclusions

Mexiletine treatment is recognized to have a role in the reduction of the symptomatic burden for NDM and DM patients. Patient management could be improved by facilitating access to therapy and developing new drug formulations.

Peptide-conjugated antimiRs improve myotonic dystrophy type 1 phenotypes by promoting endogenous MBNL1 expression

Myotonic dystrophy type 1 (DM1) is a rare neuromuscular disease caused by a CTG repeat expansion in the DMPK gene that generates toxic RNA with a myriad of downstream alterations in RNA metabolism. A key consequence is the sequestration of alternative splicing regulatory proteins MBNL1/2 by expanded transcripts in the affected tissues. MBNL1/2 depletion interferes with a developmental alternative splicing switch that causes the expression of fetal isoforms in adults. Boosting the endogenous expression of MBNL proteins by inhibiting the natural translational repressors miR-23b and miR-218 has previously been shown to be a promising therapeutic approach. We designed antimiRs against both miRNAs with a phosphorodiamidate morpholino oligonucleotide (PMO) chemistry conjugated to cell-penetrating peptides (CPPs) to improve delivery to affected tissues. In DM1 cells, CPP-PMOs significantly increased MBNL1 levels. In some candidates, this was achieved using concentrations less than two orders of magnitude below the median toxic concentration, with up to 5.38-fold better therapeutic window than previous antagomiRs. In HSALR mice, intravenous injections of CPP-PMOs improve molecular, histopathological, and functional phenotypes, without signs of toxicity. Our findings place CPP-PMOs as promising antimiR candidates to overcome the treatment delivery challenge in DM1 therapy.

Sleep in pediatric neuromuscular disorders

Sleep disordered breathing (SDB) is prevalent among children with neuromuscular disorders (NMD). The combination of respiratory muscle weakness, altered drive, and chest wall distortion due to scoliosis make sleep a stressful state in this population. Symptomatology can range from absent to snoring, nocturnal awakenings, morning headaches, and excessive daytime sleepiness. Sequelae of untreated SDB includes cardiovascular effects, metabolic derangements, and neurocognitive concerns which can be compounded by those innate to the NMD. The clinician should have a low threshold for obtaining polysomnography and recognize the nuances of individual disorders due to disproportionately impacted muscle groups such as hypoventilation in ambulating patients from diaphragm weakness. Non-invasive or invasive ventilation are the mainstay of treatment. In this review we explore the diagnosis and treatment of SDB in children with various NMD.