Progressive loss of mitochondrial DNA in thymidine kinase 2-deficient mice

Abstract

Deficient enzymatic activity of the mitochondrial deoxyribonucleoside kinases deoxyguanosine kinase (DGUOK) or thymidine kinase 2 (TK2) cause mitochondrial DNA (mtDNA)-depletion syndromes in humans. Here we report the generation of a Tk2-deficient mouse strain and show that the mice develop essentially normally for the first week but from then on exhibit growth retardation and die within 2-4 weeks of life. Several organs including skeletal muscle, heart, liver and spleen showed progressive loss of mtDNA without increased mtDNA mutations or structural alterations. There were no major histological changes in skeletal muscle, but heart muscle showed disorganized and damaged muscle fibers. Electron microscopy showed mitochondria with distorted cristae. The Tk2-deficient mice exhibited pronounced hypothermia and showed loss of hypodermal fat and abnormal brown adipose tissue. We conclude that Tk2 has a major role in supplying deoxyribonucleotides for mtDNA replication and that other pathways of deoxyribonucleotide synthesis cannot compensate for loss of this enzyme.

Inhibition of oxidative metabolism leads to p53 genetic inactivation and transformation in neural stem cells

“…To determine whether these changes could occur also in another model of mitochondrial dysfunction, we used genetically modified NPCs, where oxidative phosphorylation is decreased due to loss of thymidine kinase 2 (TK2), a key component of the salvage pathway for nucleotide biosynthesis within mitochondria (24)(25)(26). TK2 knockout (KO) animals are ataxic and die by postnatal day 15 due to defects in multiple tissues, including brain (24)(25)(26).…”

“…TK2 knockout (KO) animals are ataxic and die by postnatal day 15 due to defects in multiple tissues, including brain (24)(25)(26). TK2 deficiency in postmitotic cells results in decreased mtDNA synthesis, in turn leading to diminished expression of mtDNA-encoded ETC components and impaired ETC (24)(25)(26). We isolated NPCs from the SVZ of WT and KO mice (preparation WT1 and KO1; Dataset S1).…”

Zidovudine Induces Downregulation of Mitochondrial Deoxynucleoside Kinases: Implications for Mitochondrial Toxicity of Antiviral Nucleoside Analogs

“…Recent studies using TK1-and dCK-knockout mouse models indicate that cytosolic TK1 and dCK regulate hematopoiesis by linking salvage deoxynucleotide synthesis and replication stress (3,4). The mitochondrial enzymes TK2 and dGK are vital for mitochondrial function, because deficiency in either causes severe mitochondrial DNA (mtDNA) depletion syndrome (MDS) in human patients and in TK2-knockout mouse models (5)(6)(7)(8). TK2 has also been shown to play an important role in providing dTTP for nuclear DNA repair and thus genomic stability in quiescent cells (9).…”

Identification of a de novo thymidylate biosynthesis pathway in mammalian mitochondria

“…Depletion of de novo dTMP synthesis results in deoxyuridine misincorporation into nuclear DNA leading to genome instability (2), whereas depletion of mitochondrial dTTP pools, due to mitochondrial deoxyribonucleoside kinases, deoxyguanosine kinase, or thymidine kinase 2 (TK2) mutations are associated with mtDNA-depletion syndromes and severe mitochondrial dysfunction in humans (3). Elevations in dTTP pools, as observed in mitochondrial neurogastrointestinal encephalomyopathy (4), an autosomal recessive disease resulting from decreased levels of cytoplasmic thymidine phosphorylase, results in mtDNA depletion (5), deletions (6), and site-specific point mutations (6).…”