NAD dyshomeostasis in RYR1-related myopathies.

BACKGROUND

Pathogenic variants in RYR1 cause a spectrum of rare congenital myopathies associated with intracellular calcium dysregulation. Glutathione redox imbalance has been reported in several Ryr1 disease model systems and clinical studies. NAD and NADP are essential cofactors in cellular metabolism and redox homeostasis. NAD deficiency has been associated with skeletal muscle bioenergetic deficits in mitochondrial myopathy and sarcopenia.

METHODS

Using a new colorimetric assay and large control dataset (n = 299), we assessed redox balance (glutathione, NAD, and NADP) in whole blood from 28 RYR1-RM affected individuals (NCT02362425). Analyses were expanded to human skeletal muscle (n = 4), primary myotube cultures (n = 5), and whole blood and skeletal muscle specimens from Ryr1 Y524S mice. The in vitro effects of nicotinamide riboside (NR) on cellular NAD content and mitochondrial respirometry were also tested.

RESULTS

At baseline, a majority of affected individuals exhibited systemic NAD deficiency (19/28 [68%] < 21 µM) and increased NADPH concentrations (22/26 [85%] > 1.6 µM). When compared to controls, decreased NAD/NADH and NADP/NADPH ratios were observed in 9/28 and 23/26 individuals, respectively. In patient-derived myotube cultures (n = 5), NR appeared to increase cellular NAD concentrations in a dose and time-dependent manner at 72-h only and favorably modified maximal respiration and ATP production. Average whole blood GSH/GSSG ratio was comparable between groups, and redox imbalance was not observed in Ryr1 Y524S specimens.

CONCLUSIONS

NAD and NADP dyshomeostasis was identified in a subset of RYR1-RM affected individuals. Further experiments are warranted to confirm if NAD repletion could be an attractive therapeutic approach given the favorable outcomes reported in other neuromuscular disorders.