Autosomal recessive mutations in the thymidine kinase 2 gene ( TK2 ) cause mitochondrial DNA depletion, multiple deletions, or both due to loss of TK2 enzyme activity and ensuing unbalanced deoxynucleotide triphosphate (dNTP) pools. To bypass Tk2 deficiency, we administered deoxycytidine and deoxythymidine monophosphates (dCMP+dTMP) to the Tk2 H126N ( Tk2 ^−/− ) knock‐in mouse model from postnatal day 4, when mutant mice are phenotypically normal, but biochemically affected. Assessment of 13‐day‐old Tk2 ^−/− mice treated with dCMP+dTMP 200 mg/kg/day each ( Tk2 ^{−/− 200dCMP/ dTMP} ) demonstrated that in mutant animals, the compounds raise dTTP concentrations, increase levels of mtDNA, ameliorate defects of mitochondrial respiratory chain enzymes, and significantly prolong their lifespan (34 days with treatment versus 13 days untreated). A second trial of dCMP+dTMP each at 400 mg/kg/day showed even greater phenotypic and biochemical improvements. In conclusion, dCMP/dTMP supplementation is the first effective pharmacologic treatment for Tk2 deficiency.

MS-derived oligodendrocytes conserve their ability to fully interact with axons and glial cells in vivo.

Importance Mendelian forms of complex I deficiency are usually associated with fatal infantile encephalomyopathy. Application of “MitoExome” sequencing (deep sequencing of the entire mitochondrial genome and the coding exons of >1000 nuclear genes encoding the mitochondrial proteome) allowed us to reveal an unusual clinical variant of complex I deficiency due to a novel homozygous mutation inACAD9. The patient had an infantile-onset but slowly progressive encephalomyopathy and responded favorably to riboflavin therapy. Observation A 13-year-old boy had exercise intolerance, weakness, and mild psychomotor delay. Muscle histochemistry showed mitochondrial proliferation, and biochemical analysis revealed severe complex I deficiency (15% of normal). The level of complex I holoprotein was reduced as determined by use of Western blot both in muscle (54%) and in fibroblasts (57%). Conclusions and Relevance The clinical presentation of complex I deficiency dueACAD9mutations spans from fatal infantile encephalocardiomyopathy to mild encephalomyopathy. Our data support the notion that ACAD9 functions as a complex I assembly protein. ACAD9 is a flavin adenine dinucleotide–containing flavoprotein, and treatment with riboflavin is advisable.