L-Cysteine and N-Acetylcysteine Supplementation Improves Clinical Outcome in a Patient With COXPD10.

MTO1 is a nuclear gene that encodes a mitochondrial protein essential for modifying mitochondrial transfer RNAs (tRNAs) and stabilizing codon-anticodon interactions to ensure accurate and efficient mitochondrial protein synthesis and oxidative phosphorylation. Mitochondrial tRNA translation optimization 1 (MTO1) plays an important role in the mitochondrial tRNA taurinomethylation modification by using the amino acid taurine, obtained from cysteine metabolism, at the wobble position U34 of the anticodon loop. Biallelic pathogenic variants in MTO1 cause combined oxidative phosphorylation deficiency 10 (COXPD10) (OMIM#614702). In the severe end of the spectrum, COXPD10 is characterized by infantile-onset hypertrophic cardiomyopathy and lactic acidosis with perinatal mortality when associated with nonsense and frameshift variants. The extra cardiac phenotypes include muscle hypotonia, feeding difficulties, psychomotor delay, optic atrophy, encephalopathy, and seizures. Currently, there is no targeted treatment for this condition aside from supportive care. Herein, we report a 22-month-old child, diagnosed early with a genotype predictive of severe COXPD10, who was initiated on treatment with L-cysteine and N-acetylcysteine (NAC) early in life and did not develop cardiac manifestations. This outcome suggests a potential benefit and improved clinical outcome with early disease-specific treatment initiation.