Limited dCTP availability accounts for mitochondrial DNA depletion in mitochondrial neurogastrointestinal encephalomyopathy (MNGIE)

• Published in: PLoS genetics Vol. 7; no. 3; p. e1002035
• Main Authors: González-Vioque, Emiliano, Torres-Torronteras, Javier, Andreu, Antoni L, Martí, Ramon
• Format: Journal Article
• Language: English
• Published: United States Public Library of Science 01.03.2011; Public Library of Science (PLoS)
• Subjects: Animals; Bars; Biology; Cell Culture Techniques; Cell cycle; Deoxycytosine Nucleotides - genetics; Deoxycytosine Nucleotides - metabolism; Deoxyribonucleic acid; DNA; DNA Replication; DNA, Mitochondrial - genetics; Encephalopathy; Experiments; Fibroblasts - cytology; Gene mutations; Genes; Genetic aspects; Genetics; Health aspects; Humans; Medicine; Metabolic disorders; Mice; Mitochondria; Mitochondria, Liver - metabolism; Mitochondrial DNA; Mitochondrial Encephalomyopathies - genetics; Mitochondrial Encephalomyopathies - metabolism; Physiological aspects; Ribonucleotide reductase; Risk factors; Thymine Nucleotides - genetics; Thymine Nucleotides - metabolism; Spain
• ISSN: 1553-7404; 1553-7390; 1553-7404
• DOI: 10.1371/journal.pgen.1002035

Mitochondrial neurogastrointestinal encephalomyopathy (MNGIE) is a severe human disease caused by mutations in TYMP, the gene encoding thymidine phosphorylase (TP). It belongs to a broader group of disorders characterized by a pronounced reduction in mitochondrial DNA (mtDNA) copy number in one or more tissues. In most cases, these disorders are caused by mutations in genes involved in deoxyribonucleoside triphosphate (dNTP) metabolism. It is generally accepted that imbalances in mitochondrial dNTP pools resulting from these mutations interfere with mtDNA replication. Nonetheless, the precise mechanistic details of this effect, in particular, how an excess of a given dNTP (e.g., imbalanced dTTP excess observed in TP deficiency) might lead to mtDNA depletion, remain largely unclear. Using an in organello replication experimental model with isolated murine liver mitochondria, we observed that overloads of dATP, dGTP, or dCTP did not reduce the mtDNA replication rate. In contrast, an excess of dTTP decreased mtDNA synthesis, but this effect was due to secondary dCTP depletion rather than to the dTTP excess in itself. This was confirmed in human cultured cells, demonstrating that our conclusions do not depend on the experimental model. Our results demonstrate that the mtDNA replication rate is unaffected by an excess of any of the 4 separate dNTPs and is limited by the availability of the dNTP present at the lowest concentration. Therefore, the availability of dNTP is the key factor that leads to mtDNA depletion rather than dNTP imbalances. These results provide the first test of the mechanism that accounts for mtDNA depletion in MNGIE and provide evidence that limited dNTP availability is the common cause of mtDNA depletion due to impaired anabolic or catabolic dNTP pathways. Thus, therapy approaches focusing on restoring the deficient substrates should be explored.