Changes in mitochondrial thymidine metabolism and mtDNA copy number during induced pluripotency

Somatic cell reprogramming into human induced pluripotent stem cells entails significant intracellular changes, including modifications in mitochondrial metabolism and a decrease in mitochondrial DNA copy number. However, the mechanisms underlying this decrease in mitochondrial DNA copy number during reprogramming remain unclear. Here we aimed to elucidate these underlying mechanisms. Through a meta-analysis of several RNA sequencing datasets, we identified genes responsible for the decrease in mitochondrial DNA. We investigated the functions of these identified genes and assessed their regulatory mechanisms. In particular, the expression of the thymidine kinase 2 gene (TK2), located in the mitochondria and required for mitochondrial DNA synthesis, is decreased in human pluripotent stem cells as compared with its expression in somatic cells. TK2 was significantly downregulated during reprogramming and markedly upregulated during differentiation. Collectively, this decrease in TK2 levels induces a decrease in mitochondrial DNA copy number and contributes to shaping the metabolic characteristics of human pluripotent stem cells. However, contrary to our expectations, treatment with a TK2 inhibitor impaired somatic cell reprogramming. These results suggest that decreased TK2 expression may result from metabolic conversion during somatic cell reprogramming.