Development of a proxy-reported scale to assess motor function in infants and young children with early-onset neuromuscular disorders.

AIM: To develop a novel proxy-reported scale of motor function in infants and young children with early-onset neuromuscular disorders (NMD), entitled the Proxy Motor Outcome Measure (PMOM). DESIGN: A mixed method design was employed, applying both qualitative and quantitative research. METHODS: A framework technique using sensitivity analyses guided the development of the most appropriate and relevant subset of items, modelled after 30 neuromuscular disease instruments/scales. The PMOM was designed based on semi-structured interviews with 16 proxies; a focus group of 11 experts in neuromuscular diseases and scale development, 10 of whom also gave quantitative data using a two-round Delphi method survey; and cognitive interviews with five proxies. These processes were conducted between January 2014-March 2019. RESULTS: Nine themes and 32 subthemes were derived from the semi-structured interviews. Five domains and three subdomains of potential items were identified by the focus group. An initial version of the PMOM scale was created with 121 items. Using the two-round Delphi method, 43 items met agreement on pre-defined requirements. The second version of the PMOM scale included these 43 and two additional items based on expert feedback. Proxies gave 114 suggestions on cognitive interviews, 99 of which were successfully addressed by the research team. The final version of the PMOM scale included 43 items. CONCLUSION: We developed a preliminary proxy-reported instrument, the PMOM, to evaluate motor function in infants and young children with early-onset NMD. IMPACT: Proxies hold a wealth of knowledge on their child's motor function during early development, which may complement clinic-based motor function testing. However, there is no validated measure of motor function that incorporates the observation of proxies of infants and young children with NMD. Future work will be focused on assessing the reliability, validity and responsiveness of the PMOM scale and implementing this tool in clinical studies.

Intracellular calcium leak as a therapeutic target for RYR1-related myopathies.

RYR1 encodes the type 1 ryanodine receptor, an intracellular calcium release channel (RyR1) on the skeletal muscle sarcoplasmic reticulum (SR). Pathogenic RYR1 variations can destabilize RyR1 leading to calcium leak causing oxidative overload and myopathy. However, the effect of RyR1 leak has not been established in individuals with RYR1-related myopathies (RYR1-RM), a broad spectrum of rare neuromuscular disorders. We sought to determine whether RYR1-RM affected individuals exhibit pathologic, leaky RyR1 and whether variant location in the channel structure can predict pathogenicity. Skeletal muscle biopsies were obtained from 17 individuals with RYR1-RM. Mutant RyR1 from these individuals exhibited pathologic SR calcium leak and increased activity of calcium-activated proteases. The increased calcium leak and protease activity were normalized by ex-vivo treatment with S107, a RyR stabilizing Rycal molecule. Using the cryo-EM structure of RyR1 and a new dataset of > 2200 suspected RYR1-RM affected individuals we developed a method for assigning pathogenicity probabilities to RYR1 variants based on 3D co-localization of known pathogenic variants. This study provides the rationale for a clinical trial testing Rycals in RYR1-RM affected individuals and introduces a predictive tool for investigating the pathogenicity of RYR1 variants of uncertain significance.

Randomized controlled trial of N-acetylcysteine therapy for RYR1-related myopathies.

OBJECTIVE: To investigate the efficacy of N-acetylcysteine (NAC) for decreasing elevated oxidative stress and increasing physical endurance in individuals with ryanodine receptor 1-related myopathies (RYR1-RM). METHODS: In this 6-month natural history assessment (n = 37) followed by a randomized, double-blinded, placebo-controlled trial, 33 eligible participants were block-randomized (1:1) to receive NAC (n = 16) or placebo (n = 17), orally for 6 months (adult dose 2,700 mg/d; pediatric dose 30 mg/kg/d). The primary endpoint was urine 15-F2t isoprostane concentration and the clinically meaningful co-primary endpoint was 6-minute walk test (6MWT) distance. RESULTS: When compared to the general population, participants had elevated baseline 15-F2t isoprostane concentrations and most had a decreased 6MWT distance (mean ± SD 3.2 ± 1.5 vs 1.1 ± 1.7 ng/mg creatinine and 468 ± 134 vs 600 ± 58 m, respectively, both p < 0.001). 15-F2t isoprostane concentration and 6MWT distance did not change over the 6-month natural history assessment (p = 0.98 and p = 0.61, respectively). NAC treatment did not improve 15-F2t isoprostane concentration (least squares means difference 0.1 [95% confidence interval [CI] -1.4 to 1.6] ng/mg creatinine, p = 0.88) or 6MWT distance (least squares means difference 24 [95% CI -5.5 to 53.4] m, p = 0.11). NAC was safe and well-tolerated at the doses administered in this study. CONCLUSION: In ambulatory RYR1-RM-affected individuals, we observed stable disease course, and corroborated preclinical reports of elevated oxidative stress and decreased physical endurance. NAC treatment did not decrease elevated oxidative stress, as measured by 15-F2t isoprostane. CLASSIFICATION OF EVIDENCE: This study provides Class I evidence that, for people with RYR1-RM, treatment with oral NAC does not decrease oxidative stress as measured by 15-F2t isoprostane. CLINICALTRIALSGOV IDENTIFIER: NCT02362425.

Reliability and Validity of Self-Report Questionnaires as Indicators of Fatigue in RYR1-Related Disorders.

BACKGROUND: RYR1-related disorders (RYR1-RD), are a spectrum of genetic neuromuscular disorders. Affected individuals frequently experience fatigue yet appropriate tools to assess RYR1-RD-associated fatigue remain underdeveloped. OBJECTIVE: This study assessed the reliability and validity of two self-report questionnaires, the multidimensional fatigue inventory (MFI-20) and adult/pediatric functional assessment of chronic illness-fatigue (FACIT-F/Peds-FACIT-F) as potential fatigue measures in RYR1-RD affected individuals. METHODS: Participants (n = 37) were enrolled in an RYR1-RD combined natural history study and clinical trial. At baseline, participants completed fatigue questionnaires, six-minute walk test (6MWT), cardiopulmonary exercise test (CPET) and saliva collection for fatigue biomarker index (FBI) quantification. RESULTS: All questionnaires exhibited good test-retest reliability (n = 18, ICC > 0.80). MFI-20 (n = 37), and FACIT-F (n = 28) also showed good internal consistency (Cronbach's α> 0.80). All MFI-20 subscales, except mental fatigue, and FACIT-F demonstrated evidence of criterion validity when correlated against percent predicted 6MWT distance (MFI-20 n = 37; r = -0.34 to -0.47, all p < 0.05, mental fatigue, r = -0.16, p = 0.35; FACIT-F n = 28, r = 0.41, p = 0.03). This was not the case for percent predicted VO2 peak (all p > 0.05). FBI correlated with MFI-20 general fatigue dimension only (r = -0.35, p = 0.03). Comparison of standardized questionnaire scores revealed that RYR1-RD affected individuals experience significantly greater fatigue than the general population. CONCLUSIONS: MFI-20 and FACIT-F are valid and reliable tools for assessing RYR1-RD-associated fatigue, a symptom centrally implicated in this rare disorder.

Use of Fatigue Index as a Measure of Local Muscle Fatigability in Ryanodine Receptor Isoform-1-Related Myopathies.

Introduction: Individuals affected with ryanodine receptor isoform-1-related myopathies (RYR1-RM) commonly experience fatigability in the quadriceps, which may limit physical function and potentially diminish quality of life. Fatigability, in RYR1-RM, results from skeletal muscle injury secondary to dysfunction of the major skeletal muscle Ca++ channel. However, during fatigability testing, affected individuals did not always reach the point of local muscle fatigue as defined by a fatigue index (FATI) at 50% of peak torque. Surakka et al. compared three versions of FATI equations, which vary by the area under the force curve (AUC). By performing this comparison, they were able to determine the optimal equation in individuals with Multiple Sclerosis. Purpose: Using a similar comparison, we sought to identify the optimal FATI equation in the RYR1-RM population. Secondly, because local muscle fatigability might have an impact on independent living, this study also assessed change in local muscle fatigability over a 6-month time frame. Methods: Thirty participants were analyzed from the RYR1-RM natural history study and double-blind, placebo-controlled N-acetylcysteine (NAC) trial, NCT02362425. Twenty-seven had fatigability data, from isometric knee extension and flexion fatigability tests, available for the purpose of establishing a method for predicting FATI at 50% peak torque. For the natural history study, 30 participants were used to assess disease progression of local muscle fatigability achieved during the knee extension fatigability test, and 29 participants for the knee flexion fatigability test. Results: Surakka's equation 1, using the prediction approach, led to the smallest median error, the smallest square-root of uncorrected sum of squares, and the smallest average of the absolute value of the differences. No difference was observed in FATI at 50% peak torque between month 0 and month 6 for extension (p = 0.606) and flexion (p = 0.740). Conclusion: Surakka's equation 1, with the prediction approach, was found to be the most accurate for imputing values when fatigue was not reached during a sustained knee isometric fatigability test in RYR1-RM. Furthermore, when used to assess fatigability-based disease stability, local muscle fatigability, in this RYR1-RM population remained stable.