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Background and purpose Pathogenic variants in the RYR1 gene have been associated with a variety of conditions, ranging from congenital myopathy to adult manifestations. Our aim was to characterize the p.Leu2286Val variant in 17 Basque patients, to accurately determine its correlation with clinical features and to explore the possible founder effect of the variant. Methods Families harbouring the p.Leu2286 RYR1 variant underwent a detailed clinical evaluation, including muscle magnetic resonance imaging, electromyography and muscle biopsy. Haplotypes were analysed in available patients and their relatives. Results Individuals carrying the p.Leu2286Val shared a common haplotype, suggesting a founder event in the Basque Country population. The most prevalent features were exertional myalgia, high creatine kinase (CK) levels, cramps and muscle hypertrophy. None of the patients carrying only the p.Leu2286Val showed progression to severe muscle weakness and muscle magnetic resonance imaging showed a heterogeneous muscle involvement. Muscle biopsy revealed non‐specific findings in two patients and features associated with central core disease in one patient carrying only the p.Leu2286Val and two patients harbouring an additional RYR1 variant. Three individuals carrying an in trans RYR1 variant presented with an earlier onset and more severe phenotype. Conclusion Here, it is shown that the dominantly inherited p.Leu2286Val RYR1 founder variant is associated with a milder phenotype of exercise intolerance, myalgia and hyperCKemia.

Background Natural history studies in neuromuscular disorders are vital to understand the disease evolution and to find sensitive outcome measures. We performed a longitudinal assessment of quantitative magnetic resonance imaging (MRI) and phosphorus magnetic resonance spectroscopy (31P MRS) outcome measures and evaluated their relationship with function in lower limb skeletal muscle of dysferlinopathy patients. Methods Quantitative MRI/31P MRS data were obtained at 3 T in two different sites in 54 patients and 12 controls, at baseline, and three annual follow‐up visits. Fat fraction (FF), contractile cross‐sectional area (cCSA), and muscle water T2 in both global leg and thigh segments and individual muscles and 31P MRS indices in the anterior leg compartment were assessed. Analysis included comparisons between patients and controls, assessments of annual changes using a linear mixed model, standardized response means (SRM), and correlations between MRI and 31P MRS markers and functional markers. Results Posterior muscles in thigh and leg showed the highest FF values. FF at baseline was highly heterogeneous across patients. In ambulant patients, median annual increases in global thigh and leg segment FF values were 4.1% and 3.0%, respectively (P < 0.001). After 3 years, global thigh and leg FF increases were 9.6% and 8.4%, respectively (P < 0.001). SRM values for global thigh FF were over 0.8 for all years. Vastus lateralis muscle showed the highest SRM values across all time points. cCSA decreased significantly after 3 years with median values of 11.0% and 12.8% in global thigh and global leg, respectively (P < 0.001). Water T2 values in ambulant patients were significantly increased, as compared with control values (P < 0.001). The highest water T2 values were found in the anterior part of thigh and leg. Almost all 31P MRS indices were significantly different in patients as compared with controls (P < 0.006), except for pHw, and remained, similar as to water T2, abnormal for the whole study duration. Global thigh water T2 at baseline was significantly correlated to the change in FF after 3 years (ρ = 0.52, P < 0.001). There was also a significant relationship between the change in functional score and change in FF after 3 years in ambulant patients (ρ = −0.55, P = 0.010). Conclusions This multi‐centre study has shown that quantitative MRI/31P MRS measurements in a heterogeneous group of dysferlinopathy patients can measure significant changes over the course of 3 years. These data can be used as reference values in view of future clinical trials in dysferlinopathy or comparisons with quantitative MRI/S data obtained in other limb‐girdle muscular dystrophy subtypes.

Limb–girdle muscular dystrophy R1 (LGMDR1) is characterized by progressive proximal muscle weakness due to mutations in the CAPN3 gene. Little is known about CAPN3’s function in muscle, but its loss results in aberrant sarcomere formation. Human muscle structure was analyzed in this study, with observations including integrin β1D isoform (ITGβ1D) mislocalization, a lack of Talin-1 (TLN1) in the sarcolemma and the irregular expression of focal adhesion kinase (FAK) in LGMDR1 muscles, suggesting a lack of integrin activation with an altered sarcolemma, extracellular matrix (ECM) assembly and signaling pathway deregulation, which may cause frailty in LGMDR1 muscle fibers. Additionally, altered nuclear morphology, centrosome distribution and microtubule organization have been found in muscle cells derived from LGMDR1 patients.

Myotonic dystrophy type I (DM1) is an autosomal dominant disease of which clinical manifestations resemble premature aging. We evaluated the contribution of telomere length in pathogenesis in 361 DM1 patients (12 with serial measurements) and 223 unaffected relative controls using qPCR assay. While no differences in baseline leukocyte relative telomere length (RTL) was noted, the data suggested an accelerated RTL attrition in DM1 (discovery cohort: T/S change/year = −0.013 in DM1 vs. −0.005 in controls, P = 0.04); similar trend was noted in validation cohort. Further investigations are needed to examine the role of TL in the pathophysiology of DM1.

Background and objectiveTK2 deficiency (TK2d) is a rare mitochondrial disorder that manifests predominantly as a progressive myopathy with a broad spectrum of severity and age of onset. The rate of progression is variable, and the prognosis is poor due to early and severe respiratory involvement. Early and accurate diagnosis is particularly important since a specific treatment is under development. This study aims to evaluate the diagnostic value of lower limb muscle MRI in adult patients with TK2d.MethodsWe studied a cohort of 45 genetically confirmed patients with mitochondrial myopathy (16 with mutations in TK2, 9 with mutations in other nuclear genes involved in mitochondrial DNA [mtDNA] synthesis or maintenance, 10 with single mtDNA deletions, and 10 with point mtDNA mutations) to analyze the imaging pattern of fat replacement in lower limb muscles. We compared the identified pattern in patients with TK2d with the MRI pattern of other non-mitochondrial genetic myopathies that share similar clinical characteristics.ResultsWe found a consistent lower limb muscle MRI pattern in patients with TK2d characterized by involvement of the gluteus maximus, gastrocnemius medialis, and sartorius muscles. The identified pattern in TK2 patients differs from the known radiological involvement of other resembling muscle dystrophies that share clinical features.ConclusionsBy analyzing the largest cohort of muscle MRI from patients with mitochondrial myopathies studied to date, we identified a characteristic and specific radiological pattern of muscle involvement in patients with TK2d that could be useful to speed up its diagnosis.

Myotonic dystrophy type 1 (DM1; MIM #160900) is an autosomal dominant disorder, clinically characterized by progressive muscular weakness and multisystem degeneration. The broad phenotypes observed in patients with DM1 resemble the appearance of an accelerated aging process. However, the molecular mechanisms underlying these phenotypes remain largely unknown. Transcriptomic analysis of fibroblasts derived from patients with DM1 and healthy individuals revealed a decrease in cell cycle activity, cell division, and DNA damage response in DM1, all of which related to the accumulation of cellular senescence. The data from transcriptome analyses were corroborated in human myoblasts and blood samples, as well as in mouse and Drosophila models of the disease. Serial passage studies in vitro confirmed the accelerated increase in senescence and the acquisition of a senescence-associated secretory phenotype in DM1 fibroblasts, whereas the DM1 Drosophila model showed reduced longevity and impaired locomotor activity. Moreover, functional studies highlighted the impact of BMI1 and downstream p16INK4A/RB and ARF/p53/p21CIP pathways in DM1-associated cellular phenotypes. Importantly, treatment with the senolytic compounds Quercetin, Dasatinib, or Navitoclax reversed the accelerated aging phenotypes in both DM1 fibroblasts in vitro and in Drosophila in vivo. Our results identify the accumulation of senescence as part of DM1 pathophysiology and, therefore, demonstrate the efficacy of senolytic compounds in the preclinical setting.

Objective Approximately half of patients with hereditary myopathies remain without a definitive genetic diagnosis after DNA next‐generation sequencing (NGS). Here, we implemented transcriptome analysis of muscle biopsies as a complementary diagnostic tool for patients with muscle disease but no definitive genetic diagnosis after exome sequencing. Methods In total, 70 undiagnosed cases with suspected genetic muscular dystrophies or congenital myopathies were included in the study. Muscle RNAseq comprised the analysis of aberrant splicing, aberrant expression, and monoallelic expression. In addition, existing NGS data or variant calling from RNAseq were reanalyzed, and genome sequencing was performed in selected cases. Four aberrant splicing open‐source tools were compared and assessed. Results RNAseq established a diagnosis in 10/70 patients (14.3%) by identifying aberrant transcripts produced by single nucleotide variants (7/10) or copy number variants (3/10). Reanalysis of NGS data allowed the diagnosis in 9/70 individuals (12.9%). Based on this cohort, FRASER was the tool that reported more splicing outlier events per sample while showing the highest accuracy (81.26%). Conclusions We demonstrate the utility of RNAseq in identifying causative variants in muscle diseases. Evaluation of four aberrant splicing tools allowed efficient identification of most pathogenic splicing events, obtaining a manageable number of candidate events for manual inspection, demonstrating feasibility for translation into a clinical setting. We also show how the integration of omic technologies reduces the turnaround time to identify causative variants.

Myotonic dystrophy type 1 (DM1 or Steinert's disease) and type 2 (DM2) are multisystem disorders of genetic origin. Progressive muscular weakness, atrophy and myotonia are the most prominent neuromuscular features of these diseases, while other clinical manifestations such as cardiomyopathy, insulin resistance and cataracts are also common. From a clinical perspective, most DM symptoms are interpreted as a result of an accelerated aging (cataracts, muscular weakness and atrophy, cognitive decline, metabolic dysfunction, etc.), including an increased risk of developing tumors. From this point of view, DM1 could be described as a progeroid syndrome since a notable age-dependent dysfunction of all systems occurs. The underlying molecular disorder in DM1 consists of the existence of a pathological (CTG) triplet expansion in the 3' untranslated region (UTR) of the Dystrophia Myotonica Protein Kinase (DMPK) gene, whereas (CCTG)n repeats in the first intron of the Cellular Nucleic acid Binding Protein/Zinc Finger Protein 9 (CNBP/ZNF9) gene cause DM2. The expansions are transcribed into (CUG)n and (CCUG)n-containing RNA, respectively, which form secondary structures and sequester RNA-binding proteins, such as the splicing factor muscleblind-like protein (MBNL), forming nuclear aggregates known as foci. Other splicing factors, such as CUGBP, are also disrupted, leading to a spliceopathy of a large number of downstream genes linked to the clinical features of these diseases. Skeletal muscle regeneration relies on muscle progenitor cells, known as satellite cells, which are activated after muscle damage, and which proliferate and differentiate to muscle cells, thus regenerating the damaged tissue. Satellite cell dysfunction seems to be a common feature of both age-dependent muscle degeneration (sarcopenia) and muscle wasting in DM and other muscle degenerative diseases. This review aims to describe the cellular, molecular and macrostructural processes involved in the muscular degeneration seen in DM patients, highlighting the similarities found with muscle aging.