Thymidine Kinase 2 Deficiency-Induced mtDNA Depletion in Mouse Liver Leads to Defect β-Oxidation

Xiaoshan Zhou,Sophie Curbo,Sindra Isetun,Kristina Kannisto,Ulrika Von Döbeln,Mats Gåfvels,Kjell Hultenby,Anna Karlsson,David C Samuels

doi:10.1371/journal.pone.0058843

Xiaoshan Zhou, Sophie Curbo + Show 7 more

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https://doi.org/10.1371/journal.pone.0058843

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Abstract

Thymidine kinase 2 (TK2) deficiency in humans causes mitochondrial DNA (mtDNA) depletion syndrome. To study the molecular mechanisms underlying the disease and search for treatment options, we previously generated and described a TK2 deficient mouse strain (TK2−/−) that progressively loses its mtDNA. The TK2−/− mouse model displays symptoms similar to humans harboring TK2 deficient infantile fatal encephalomyopathy. Here, we have studied the TK2−/− mouse model to clarify the pathological role of progressive mtDNA depletion in liver for the severe outcome of TK2 deficiency. We observed that a gradual depletion of mtDNA in the liver of the TK2−/− mice was accompanied by increasingly hypertrophic mitochondria and accumulation of fat vesicles in the liver cells. The levels of cholesterol and nonesterified fatty acids were elevated and there was accumulation of long chain acylcarnitines in plasma of the TK2−/− mice. In mice with hepatic mtDNA levels below 20%, the blood sugar and the ketone levels dropped. These mice also exhibited reduced mitochondrial β-oxidation due to decreased transport of long chain acylcarnitines into the mitochondria. The gradual loss of mtDNA in the liver of the TK2−/− mice causes impaired mitochondrial function that leads to defect β-oxidation and, as a result, insufficient production of ketone bodies and glucose. This study provides insight into the mechanism of encephalomyopathy caused by TK2 deficiency-induced mtDNA depletion that may be used to explore novel therapeutic strategies.

Highlights

Mitochondrial DNA depletion syndrome (MDS) is a group of rare and severe autosomal recessive disorders that are characterized by quantitative reduction of mtDNA in affected tissues [1]
To determine if the increased volume was due to hypertrophy or increased number of mitochondria the number of mitochondria profiles were estimated (2189 for thymidine kinase 2 (TK2)+/+ and 2128 for TK22/2) and correlated to the total areas of the cytoplasm giving a mean number of mitochondria/mm2 cytoplasm of 0.5260.02 for TK2+/+ and 0.5460.08 for TK22/2
In this study liver and plasma from both symptom-free (7 days old) and symptomatic (12 and 14 days old) TK22/2 mice were used to clarify how hepatic mitochondria are involved in the development of the encephalopathic phenotype of the TK22/2 mice

Summary

Introduction

Mitochondrial DNA (mtDNA) depletion syndrome (MDS) is a group of rare and severe autosomal recessive disorders that are characterized by quantitative reduction of mtDNA in affected tissues [1]. The different clinical symptoms have been associated with different mutations in nuclear genes encoding proteins involved in the mtDNA metabolism [4]. Several mutations in the thymidine kinase 2 (TK2), deoxyguanosine kinase (DGUOK), polymerase gamma (POLG) and MpV17 mitochondrial inner membrane protein (MPV17) genes have been linked to inheritable MDS in humans [5,6,7,8]. Humans carrying TK2 deficiency primarily present with severe myopathy and display neurological phenotypes whereas DGUOK, POLG and MPV17 deficiencies have been associated with liver failure and encephalomyopathy [5,6]. There are reports of patients harbouring TK2 deficiency that have liver and brain involvement [9,10,11,12,13]. Patients with a certain TK2 mutation (R172W/R172W) have been shown to harbor TK2-

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