The deoxyguanosine kinase gene is mutated in individuals with depleted hepatocerebral mitochondrial DNA
ABSTRACT Mitochondrial DNA (mtDNA)-depletion syndromes (MDS; OMIM 251880) are phenotypically heterogeneous, autosomal-recessive disorders characterized by tissue-specific reduction in mtDNA copy number. Affected individuals with the hepatocerebral form of MDS have early progressive liver failure and neurological abnormalities, hypoglycemia and increased lactate in body fluids. Affected tissues show both decreased activity of the mtDNA-encoded respiratory chain complexes (I, III, IV, V) and mtDNA depletion. We used homozygosity mapping in three kindreds of Druze origin to map the gene causing hepatocerebral MDS to a region of 6.1 cM on chromosome 2p13, between markers D2S291 and D2S2116. This interval encompasses the gene (DGUOK) encoding the mitochondrial deoxyguanosine kinase (dGK). We identified a single-nucleotide deletion (204delA) within the coding region of DGUOK that segregates with the disease in the three kindreds studied. Western-blot analysis did not detect dGK protein in the liver of affected individuals. The main supply of deoxyribonucleotides (dNTPs) for mtDNA synthesis comes from the salvage pathway initiated by dGK and thymidine kinase-2 (TK2). The association of mtDNA depletion with mutated DGUOK suggests that the salvage-pathway enzymes are involved in the maintenance of balanced mitochondrial dNTP pools.
- SourceAvailable from: Ewen William Sommerville
[Show abstract] [Hide abstract]
- "Both patients harboured recessive DGUOK mutations that had been previously reported in earlyonset mitochondrial disease patients with mtDNA depletion . DGUOK mutations remain associated in early-onset mitochondrial disease causing hepatocerebral mitochondrial depletion  and a very rare cause of late-onset PEO. "
ABSTRACT: Background: Progressive external ophthalmoplegia (PEO) is an eye movement disorder characterised by paresis of the extra ocular muscles and muscle restricted multiple mitochondrial DNA (mtDNA) deletions. Classification of patients is particularly difficult due to overlapping phenotypes and a poor genotype-phenotype relationship. Despite the identification of several nuclear encoded genes causing PEO, over half of patients with clinically confirmed PEO do not have a genetic diagnosis. Objective: To systematically review genotypic and phenotypic correlates of published cases of adult-onset PEO. Methods: Patients were identified from interrogation of articles from Scopus, Medline via PubMed, and Genetic Abstracts databases using electronic searches (1st January 1970 to 8th November 2013). Reference lists and UniProt entries were also manually checked for additional articles. Results: Twelve nuclear encoded genes were identified (TYMP, SLC25A4, POLG, C10ORF2, OPA1, POLG2, RRM2B, TK2, DGUOK, MPV17, MGME1, and DNA2) systematically from 583 patients. At the time of writing, mutations in SPG7 and AFG3L2 genes were reported to be associated with ophthalmoparesis and multiple mtDNA deletions in fourteen additional adult-onset PEO patients, bringing the total number of known genes to fourteen. Conclusions: Diagnostic yield is still critically dependent on the meticulous clinical and biochemical characterisation of patients. Understanding the intimate relationship between genotype and phenotype remains a fundamental challenge. The results of this systematic review provide guidance to both patients and clinician about future prognosis, and will serve, in future, to assess methods of disease prevention and evaluation of targeted therapeutic strategies.
[Show abstract] [Hide abstract]
- "Mitochondrial DNA depletion syndrome (MDS) is a frequent cause of severe childhood encephalomyopathy characterized molecularly by reduction of mtDNA copy number in tissues and insufficient synthesis of mitochondrial RC complexes (Hirano et al, 2001; Spinazzola & Zeviani, 2009). Mutations in eight nuclear genes have been identified as causes of infantile MDS (TK2, DGUOK, POLG, MPV17, RRM2B, SUCLA2, SUCLG1, and C10orf2) (Mandel et al, 2001; Saada et al, 2003; Naviaux & Nguyen, 2004; Elpeleg et al, 2005; Spinazzola et al, 2006; Bourdon et al, 2007; Ostergaard et al, 2007; Sarzi et al, 2007); 7 of the genes encode proteins involved in mtDNA replication or in the metabolism of deoxynucleotide triphosphate pools utilized as precursors for DNA replication (Copeland, 2012). Treatment for MDS, like most mitochondrial disorders, has been limited to supportive therapies; however, understanding the pathomechanism of MDS enables the design of treatment strategies targeting either the cause of the disease or the downstream metabolic defects. "
ABSTRACT: Autosomal recessive mutations in the thymidine kinase 2 gene (TK2) cause mitochondrial DNA depletion, multiple deletions, or both due to loss of TK2 enzyme activity and ensuing unbalanced deoxynucleotide triphosphate (dNTP) pools. To bypass Tk2 deficiency, we administered deoxycytidine and deoxythymidine monophosphates (dCMP+dTMP) to the Tk2 H126N (Tk2−/−) knock-in mouse model from postnatal day 4, when mutant mice are phenotypically normal, but biochemically affected. Assessment of 13-day-old Tk2−/− mice treated with dCMP+dTMP 200 mg/kg/day each (Tk2−/−200dCMP/dTMP) demonstrated that in mutant animals, the compounds raise dTTP concentrations, increase levels of mtDNA, ameliorate defects of mitochondrial respiratory chain enzymes, and significantly prolong their lifespan (34 days with treatment versus 13 days untreated). A second trial of dCMP+dTMP each at 400 mg/kg/day showed even greater phenotypic and biochemical improvements. In conclusion, dCMP/dTMP supplementation is the first effective pharmacologic treatment for Tk2 deficiency. Subject Categories Genetics, Gene Therapy & Genetic Disease; MetabolismEMBO Molecular Medicine 06/2014; 6(8). DOI:10.15252/emmm.201404092 · 8.25 Impact Factor
[Show abstract] [Hide abstract]
- "Balanced pools of deoxyribonucleoside triphosphates (dNTPs) are required for mitochondrial DNA replication, and alterations of this balance, caused by mutations in genes encoding proteins involved in the metabolism of deoxyribonucleoside triphosphates (RRM2B, DGUOK, TYMP and TK2) are relevant to human mitochondrial diseases (Nishino et al., 1999; Mandel et al., 2001; Saada et al., 2001; Bourdon et al., 2007). Autosomal recessive mutations in TYMP are responsible for mitochondrial neurogastrointestinal encephalopathy (MNGIE) (Nishino et al., 1999). "
ABSTRACT: Balanced pools of deoxyribonucleoside triphosphate precursors are required for DNA replication, and alterations of this balance are relevant to human mitochondrial diseases including mitochondrial neurogastrointestinal encephalopathy. In this disease, autosomal recessive TYMP mutations cause severe reductions of thymidine phosphorylase activity; marked elevations of the pyrimidine nucleosides thymidine and deoxyuridine in plasma and tissues, and somatic multiple deletions, depletion and site-specific point mutations of mitochondrial DNA. Thymidine phosphorylase and uridine phosphorylase double knockout mice recapitulated several features of these patients including thymidine phosphorylase activity deficiency, elevated thymidine and deoxyuridine in tissues, mitochondrial DNA depletion, respiratory chain defects and white matter changes. However, in contrast to patients with this disease, mutant mice showed mitochondrial alterations only in the brain. To test the hypothesis that elevated levels of nucleotides cause unbalanced deoxyribonucleoside triphosphate pools and, in turn, pathogenic mitochondrial DNA instability, we have stressed double knockout mice with exogenous thymidine and deoxyuridine, and assessed clinical, neuroradiological, histological, molecular, and biochemical consequences. Mutant mice treated with exogenous thymidine and deoxyuridine showed reduced survival, body weight, and muscle strength, relative to untreated animals. Moreover, in treated mutants, leukoencephalopathy, a hallmark of the disease, was enhanced and the small intestine showed a reduction of smooth muscle cells and increased fibrosis. Levels of mitochondrial DNA were depleted not only in the brain but also in the small intestine, and deoxyribonucleoside triphosphate imbalance was observed in the brain. The relative proportion, rather than the absolute amount of deoxyribonucleoside triphosphate, was critical for mitochondrial DNA maintenance. Thus, our results demonstrate that stress of exogenous pyrimidine nucleosides enhances the mitochondrial phenotype of our knockout mice. Our mouse studies provide insights into the pathogenic role of thymidine and deoxyuridine imbalance in mitochondrial neurogastrointestinal encephalopathy and an excellent model to study new therapeutic approaches.Brain 04/2014; 137(5). DOI:10.1093/brain/awu068 · 10.23 Impact Factor