Deoxycytidine and Deoxythymidine Treatment for Thymidine Kinase 2 Deficiency

Abstract

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. Interpretation Our studies demonstrate that deoxynucleoside substrate enhancement is a novel therapy, which may ameliorate TK2 deficiency in patients.

Emerging therapies for mitochondrial diseases

“…The consequent clinical phenotypes range from a severe infantile neuromuscular form to adult-onset chronic progressive external ophthalmoplegia. Promising results were obtained in a Tk2 knockin mouse model (Tk2 H126N/ H126N ) with oral administration of deoxycytidine and deoxythymidine monophosphates and subsequently deoxycytidine and deoxythymidine; both treatments increased mtDNA levels and mitochondrial respiratory chain enzyme activities, and prolonged the lifespan of the homozygous mutant mice (75, 76). …”

Therapies for mitochondrial diseases and current clinical trials

“…TK2 deficiency causes a depletion of mitochondrial nucleotides leading to impairment of mtDNA synthesis and a myopathic mtDNA depletion syndrome [83]. In Tk2 deficient mice, deoxythymidine and deoxycytidine were found to delayed disease onset, prolonged life span of Tk2-deficient mice, and restored mtDNA content as well as ETC complexes activities and levels [84]. Several individuals with TK2 deficiency have received nucleoside bypass therapy, mostly in Italy and Spain.…”

Deoxynucleoside Therapy for Thymidine Kinase 2–Deficient Myopathy

“…in vitro and in vivo, 16,18 we administered dC and dT to all but 1 patient (P11), who continues dTMP and dCMP treatment. The therapies exerted striking effects on survival in the early onset severe myopathy patients through amelioration of muscle weakness, which enabled reductions or discontinuation of mechanical ventilation and gastrostomy feeding as well as gaining ability to walk in the majority of these severe cases.…”

“…15,16 Doses were titrated up to 400mg/kg/day depending on tolerance. 15,16 Doses were titrated up to 400mg/kg/day depending on tolerance.…”

“…3,4,[9][10][11][12][13] The infantile onset form manifests rapidly progressive myopathy weakness with motor regression and respiratory insufficiency occasionally accompanied by central nervous system involvement and virtually uniform early fatality with postonset survival of 1 year. 16 In this situation, deoxynucleosides function as a substrate enhancement therapy. The less severe forms begin in childhood through adulthood and exhibit slower progression; however, bulbar, proximal limb, and respiratory weakness can be severe, causing inability to walk or breathe independently.…”