TRMU deficiency: A broad clinical spectrum responsive to cysteine supplementation.

TRMU is a nuclear gene crucial for mitochondrial DNA translation by encoding tRNA 5-methylaminomethyl-2-thiouridylate methyltransferase, which thiolates mitochondrial tRNA. Biallelic pathogenic variants in TRMU are associated with transient infantile liver failure. Other less common presentations such as Leigh syndrome, myopathy, and cardiomyopathy have been reported. Recent studies suggested that provision of exogenous L-cysteine or N-acetylcysteine may ameliorate the effects of disease-causing variants and improve the natural history of the disease. Here, we report six infants with biallelic TRMU variants, including four previously unpublished patients, all treated with exogenous cysteine. We highlight the first report of an affected patient undergoing orthotopic liver transplantation, the long-term effects of cysteine supplementation, and the ability of the initial presentation to mimic multiple inborn errors of metabolism. We propose that TRMU deficiency should be suspected in all children presenting with persistent lactic acidosis and hypoglycemia, and that combined N-acetylcysteine and L-cysteine supplementation should be considered prior to molecular diagnosis, as this is a low-risk approach that may increase survival and mitigate the severity of the disease course.