Abstract
Mitochondrial DNA depletion syndromes are a genetically heterogeneous group of often severe diseases, characterized by reduced cellular mitochondrial DNA content. Investigation of potential therapeutic strategies for mitochondrial DNA depletion syndromes will be dependent on good model systems. We have previously suggested that myotubes may be the optimal model system for such studies. Here we firstly validate this technique in a diverse range of cells of patients with mitochondrial DNA depletion syndromes, showing contrasting effects in cell lines from genetically and phenotypically differing patients. Secondly, we developed a putative therapeutic approach using variable combinations of deoxynucleoside monophosphates in different types of mitochondrial DNA depletion syndromes, showing near normalization of mitochondrial DNA content in many cases. Furthermore, we used nucleoside reverse transcriptase inhibitors to precisely titrate mtDNA depletion in vitro. In this manner we can unmask a physiological defect in mitochondrial depletion syndrome cell lines which is also ameliorated by deoxynucleoside monophosphate supplementation. Finally, we have extended this model to study fibroblasts after myogenic transdifferentiation by MyoD transfection, which similar to primary myotubes also showed deoxynucleoside monophosphate responsive mitochondrial DNA depletion in vitro, thus providing a more convenient method for deriving future models of mitochondrial DNA depletion. Our results suggest that using different combinations of deoxynucleoside monophosphates depending on the primary gene defect and molecular mechanism may be a possible therapeutic approach for many patients with mitochondrial DNA depletion syndromes and is worthy of further clinical investigation.