Mutant mitochondrial thymidine kinase in mitochondrial DNA depletion myopathy

The mitochondrial deoxyribonucleotide (dNTP) pool is separated from the cyt osolic pool because the mitochondria inner membrane is impermeable to charg ed molecules. The mitochondrial pool is maintained by either import of cyto solic dNTPs through dedicated transporters(1,2) or by salvaging deoxynuclea sides within the mitochondria; apparently, enzymes of the de novo dNTP synt hesis pathway are not present in the mitochondria. In non-replicating cells , where cytosolic dNTP synthesis is downregulated, mtDNA synthesis depends solely on the mitochondrial salvage pathway enzymes, the deoxyribonucleosid es kinases. Two of the four human deoxyribonucleoside kinases, deoxyguanosi ne kinase (dGK) and thymidine kinase-2 (TK2), are expressed in mitochondria (3-6). Human dGK efficiently phosphorylates deoxyguanosine and deoxyadenosi ne, whereas TK2 phosphorylates deoxythymidine, deoxycytidine and deoxyuridi ne. Here we identify two mutations in TK2, histidine 90 to asparagine and i soleucine 181 to asparagine, in four individuals who developed devastating myopathy and depletion of muscular mitochondrial DNA in infancy. In these i ndividuals, the activity of TK2 in muscle mitochondria is reduced to 14-45% of the mean value in healthy control individuals. Mutations in TK2 represe nt a new etiology for mitochondrial DNA depletion, underscoring the importa nce of the mitochondrial dNTP pool in the pathogenesis of mitochondrial dep letion.