Abstract
Mitochondrial inner membrane protein MPV17 is a protein of unknown function that is associated with mitochondrial DNA (mtDNA)-depletion syndrome (MDS). MPV17 loss-of-function has been reported to result in tissue-specific nucleotide pool imbalances, which can occur in states of perturbed folate-mediated one-carbon metabolism (FOCM), but MPV17 has not been directly linked to FOCM. FOCM is a metabolic network that provides one-carbon units for the de novo synthesis of purine and thymidylate nucleotides (e.g. dTMP) for both nuclear DNA (nuDNA) and mtDNA replication. In this study, we investigated the impact of reduced MPV17 expression on markers of impaired FOCM in HeLa cells. Depressed MPV17 expression reduced mitochondrial folate levels by 43% and increased uracil levels, a marker of impaired dTMP synthesis, in mtDNA by 3-fold. The capacity of mitochondrial de novo and salvage pathway dTMP biosynthesis was unchanged by the reduced MPV17 expression, but the elevated levels of uracil in mtDNA suggested that other sources of mitochondrial dTMP are compromised in MPV17-deficient cells. These results indicate that MPV17 provides a third dTMP source, potentially by serving as a transporter that transfers dTMP from the cytosol to mitochondria to sustain mtDNA synthesis. We propose that MPV17 loss-of-function and related hepatocerebral MDS are linked to impaired FOCM in mitochondria by providing insufficient access to cytosolic dTMP pools and by severely reducing mitochondrial folate pools.