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Myotonic dystrophy type 1 (DM1) is caused by a CTG repeat expansion in DMPK that leads to global dysregulation of RNA splicing and subsequent multisystemic symptomatic presentation. The gradual decline in physical function of DM1 individuals necessitates large participant numbers or extended trial durations when using motor function tests to demonstrate significant benefit of disease-modifying interventions. In this setting a biomarker capable of measuring the association between RNA mis-splicing and muscle weakness with prognostic utility would be useful for evaluating early indications of functional response or selection of appropriate participants at trial enrollment. We evaluated a 22-event composite measure of RNA mis-splicing called the Splice Index (SI) as a potential biomarker of muscle weakness in DM1. A total of 46 adults with DM1 provided baseline tibialis anterior muscle biopsies along with measurements of muscle strength, functional mobility, and myotonia, the hallmark symptom of DM1. A subset of 34 participants provided 3-month biopsies and measurements to assess longitudinal stability of the SI. The SI was derived using muscle RNA and a streamlined, easy to deploy, amplicon sequencing methodology to derive percent exon inclusion (PSI) values with high resolution. Individuals were then scored on a linear scale based on the average of a normalized PSI for all 22 splicing events. The SI demonstrated significant associations with ankle dorsiflexion (ADF) (Pearson r = -0.697) and other indicators of muscle function, including 10-meter run/fast walk and hand grip strength. These correlations were primarily driven by strong individual associations of CACNA1S and CLCN1 with all outcome measures. Mis-splicing of these two events is linked to both myotonia and muscle weakness in DM1 models. In a timepoint mismatch analysis we observed a significantly stronger correlation of baseline SI to 3-month ADF (r = -0.78) indicating that RNA mis-splicing as captured by the SI may be predictive of future performance. Linear regression modeling revealed that the combination of baseline ADF and SI was highly predictive of strength at 3 months in our cohort (adjusted R2 = 0.83). Importantly, while the SI exhibited reliability in cross-sectional comparisons between baseline and 3-months, asymmetric variation in SI dynamics within mild and moderately affected participant subgroups may affect this reliability. Overall, the SI is a reliable biomarker that accurately captures associations with physical strength and functional mobility in DM1 adults. Early analyses also suggest its potential value to predict future function. The SI can sensitively detect changes in mis-splicing over 3-months, which may be more dynamic than expected given the slow clinical progression of DM1. Collectively, our validation underscores this biomarker's utility for assessment of disease severity/progression and potential stratification of individuals for trial enrollment. [ABSTRACT FROM AUTHOR]

Limb girdle muscular dystrophy type R7 (LGMDR7) is caused by loss-of-function mutations in the Telethonin (TCAP) gene which occur with increased prevalence in Brazil and Southeast Asia. The deficiency or absence of TCAP protein results in progressive limb muscle weakness and loss of ambulation by the fourth decade of life. Cardiomyopathy frequently manifests in LGMDR7, consistent with allelic autosomal dominant mutations of TCAP which cause dilated and hypertrophic forms of cardiomyopathy. There are currently no disease modifying therapies for LGMDR7, however this disease is a strong candidate for gene therapy as the entire transgene can be inserted into a single capsid. Here we develop and test a novel gene replacement therapy in a Tcap mutant mouse model of LGMDR7 using a myotropic (AAVMYO2) vector. Tcap mutant mice express a frameshifting 8 bp duplication mutation within exon 1 (Tcapdup/dup) which results in a loss of Tcap RNA and protein expression. We find these mice show histopathology characterized by dystrophic damage, centrally nucleated myofibers, and myofiber size variability, and eventually develop kyphosis phenotypes consistent with muscle disease progression. In a 4-week treatment study we find that AAVMYO2 therapy displays improved safety versus legacy AAV8 vector through a greater than 1,000-fold de-targeting of liver tissue. Muscle and heart tissue are effectively transduced by all vectors. However, AAVMYO2 gene replacement therapy shows a more efficient expression of Tcap protein resulting in 75% expression of Tcap in muscle versus the 30% restoration observed for legacy AAV8 vector. Qualitative histopathology shows Tcap mutant muscles more closely resemble wild type after treatment, showing an improvement in phenotype. Quantification of histopathology, longer-term treatment regimens, and impacts to heart tissue are being examined in ongoing analyses. Together our data show progress towards a first LGMDR7 therapy, can inform allelic mutation groups that impact muscle or heart, and can impact muscle diseases more broadly through further development of myotropic gene therapy vectors with improved safety. [ABSTRACT FROM AUTHOR]