Objective—Thymidine kinase 2 (TK2), a critical enzyme in the mitochondrial pyrimidine
salvage pathway, is essential for mitochondrial DNA (mtDNA) maintenance. Mutations in the
nuclear gene TK2 cause TK2 deficiency, which manifests predominantly in children as myopathy
with mtDNA depletion. Molecular bypass therapy with the TK2 products, dCMP and dTMP,
prolongs the lifespan of Tk2-deficient (Tk2-/-) mice by 2-3 fold. Because we observed rapid
catabolism of the deoxynucleoside monophosphates to deoxythymidine (dT) and deoxycytidine
(dC), we hypothesized that: 1) deoxynucleosides might be the major active agents and 2)
inhibition of deoxycytidine deamination might enhance dTMP+dCMP therapy.
Methods—To test these hypotheses, we assessed two therapies in Tk2-/- mice: 1) dT+dC and 2)
co-administration of the deaminase inhibitor, tetrahydrouridine (THU), with dTMP+dCMP.
Results—We observed that dC+dT delayed disease onset, prolonged lifespan of Tk2-deficient
mice, and restored mtDNA copy number as well as respiratory chain enzyme activities and levels.
In contrast, dCMP+dTMP+THU therapy decreased lifespan of Tk2-/- animals compared to dCMP
+dTMP.
