Molecular insight into mitochondrial DNA depletion syndrome in two patients with novel mutations in the deoxyguanosine kinase and thymidine kinase 2 genes

Thymidine kinase 2 (TK2) and deoxyguanosine kinase (dGK) are the two key enzymes in mitochondrial DNA (mtDNA) precursor synthesis. Deficiencies in TK2 or dGK activity, due to genetic alteration, have been shown to cause tissue-specific depletion of mtDNA. In the case of TK2 deficiency, affected individuals suffer severe myopathy and, in the case of dGK deficiency, devastating liver or multi-systemic disease. Here, we report clinical and biochemical findings from two patients with mtDNA depletion syndrome. Patient A was a compound heterozygote carrying the previously reported T77M mutation and a novel mutation (R161K) in the TK2 gene. Patient B carried a novel mutation (L250S) in the dGK gene. The clinical symptoms of patient A included muscular weakness and exercise intolerance due to a severe mitochondrial myopathy associated with a 92% reduction in mtDNA. There was minimal involvement of other organs. Patient B suffered from rapidly progressive, early onset fatal liver failure associated with profoundly decreased mtDNA levels in liver and, to a lesser extent, in skeletal muscle. Site-directed mutagenesis was used to introduce the mutations detected in patients A and B into the TK2 and dGK cDNAs, respectively. We then characterized each of these recombinant enzymes. Catalytic activities of the three mutant enzymes were reduced to about 2-4% for TK2 and 0.5% for dGK as compared to the wild-type enzymes. Altered competition between dCyd and dThd was observed for the T77M mutant. The residual activities of the two mitochondrial enzymes correlated directly with disease development.     

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

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.     

Regulation of herpesvirus thymidine kinase activity in LM(TK) cells transformed by ultraviolet light-irradiated herpes simplex virus.

The content and composition of Semliki Forest virus polypeptides were not affected by a change of the host from BHK 21 cells to Aedes albopictus cells. Galactose and sialic acid were, however, lost from the viral oligosaccharides in the change; most of the cholesterol was also lost and the phospholipid composition was profoundly changed. The morphology of the virions was not changed.     

Analysis of spontaneous mutations in a chromosomally located hsv-1 thymidine kinase (TK) gene in a human cell line

We have developed a system for studying spontaneous mutations at a chromosomally located single-copy HSV-1 thymidine kinase (TK) gene in the human 143TK ⁻ cell line. The neo gene, which confers resistance to the antibiotic G418, was placed next to theTK gene for the purpose of screening out gross chromosomal alterations.TK ⁻ mutations were selected using the anti-TK nucleotide analogs trifluorothymidine, acyclovir, and DHPG 9-(1,3 dihydroxy-2-propoxymethyl)-guanine either separately, or in combination to eliminate leaky mutations. Analysis of theTK ⁻ mutations by Southern blotting revealed that the majority had undetectable alterations of less than 50 base pairs. The results using the methylationsensitive enzymes HpaII,AvaI, andSmaI suggest that the inactivation of the TK gene was not due to extensive methylation, although specific methylation of a limited number of MspI sites cannot be ruled out. Reversion studies, however, showed that of 16 mutants analyzed, about half had a very high reversion frequency (approximately 10⁻² This suggests that inactivation of theTK gene may have occurred by a variety of mutational events.     

Enhancing the sensitivity of the thymidine kinase assay by using DNA repair-deficient human TK6 cells

The OECD guidelines define the bioassays of identifying mutagenic chemicals, including the thymidine kinase (TK) assay, which specifically detects the mutations that inactivate the TK gene in the human TK6 lymphoid line. However, the sensitivity of this assay is limited because it detects mutations occurring only in the TK gene but not any other genes. Moreover, the limited sensitivity of the conventional TK assay is caused by the usage of DNA repair-proficient wild-type cells, which are capable of accurately repairing DNA damage induced by chemicals. Mutagenic chemicals produce a variety of DNA lesions, including base lesions, sugar damage, crosslinks, and strand breaks. Base damage causes point mutations and is repaired by the base excision repair (BER) and nucleotide excision repair (NER) pathways. To increase the sensitivity of TK assay, we simultaneously disrupted two genes encoding XRCC1, an important BER factor, and XPA, which is essential for NER, generating XRCC1 ^−/−/XPA ^−/− cells from TK6 cells. We measured the mutation frequency induced by four typical mutagenic agents, methyl methane sulfonate (MMS), cis-diamminedichloro-platinum(II) (cisplatin, CDDP), mitomycin-C (MMC), and cyclophosphamide (CP) by the conventional TK assay using wild-type TK6 cells and also by the TK assay using XRCC1 ^−/−/XPA ^−/− cells. The usage of XRCC1 ^−/−/XPA ^−/− cells increased the sensitivity of detecting the mutagenicity by 8.6 times for MMC, 8.5 times for CDDP, and 2.6 times for MMS in comparison with the conventional TK assay. In conclusion, the usage of XRCC1 ^−/−/XPA ^−/− cells will significantly improve TK assay.     

Homozygous mutations in C1QBP as cause of progressive external ophthalmoplegia (PEO) and mitochondrial myopathy with multiple mtDNA deletions

Biallelic mutations in the C1QBP gene have been associated with mitochondrial cardiomyopathy and combined respiratory‐chain deficiencies, with variable onset (including intrauterine or neonatal forms), phenotypes, and severity. We studied two unrelated adult patients from consanguineous families, presenting with progressive external ophthalmoplegia (PEO), mitochondrial myopathy, and without any heart involvement. Muscle biopsies from both patients showed typical mitochondrial alterations and the presence of multiple mitochondrial DNA deletions, whereas biochemical defects of the respiratory chain were present only in one subject. Using next‐generation sequencing approaches, we identified homozygous mutations in C1QBP. Immunoblot analyses in patients' muscle samples revealed a strong reduction in the amount of the C1QBP protein and varied impairment of respiratory chain complexes, correlating with disease severity. Despite the original study indicated C1QBP mutations as causative for mitochondrial cardiomyopathy, our data indicate that mutations in C1QBP have to be considered in subjects with PEO phenotype or primary mitochondrial myopathy and without cardiomyopathy.     

Substrate specificity and molecular modelling of the feline herpesvirus-1 thymidine kinase

Feline herpesvirus-1 (FHV-1) causes a severe upper respiratory and ocular disease in cats. An effective antiviral compound is required for treating FHV-1 infections. The virus-encoded thymidine kinase (TK) is the molecular basis for selective activation of commonly used antiviral nucleoside analogue drugs, e.g. acyclovir (ACV), penciclovir (PCV) and ganciclovir (GCV). The substrate specificity of a recombinant FHV-1 TK, expressed in Escherichia coli, was studied. FHV-1 TK efficiently phosphorylated its natural substrate deoxythymidine. However, it exhibited relatively lower affinity for the guanosine analogue substrates. PCV was most efficiently phosphorylated, followed by GCV, with approximately twofold reduction in the phosphorylation rate. The lowest phosphorylation rate was recorded for ACV. To correlate these biochemical data with structural features of the FHV-1 TK, a three-dimensional (3D) model of this enzyme was constructed based on sequence homology with two other herpesviral TKs, encoded by equine herpesvirus-4 (EHV-4) and herpes simplex-1 (HSV-1). Mutational analysis of the amino acids forming the FHV-1 TK active site identified two residues (Y29 and F144) as being critical for the differential ability of this enzyme to phosphorylate nucleoside analogues. A double substitution of Y29H/F144Y resulted in a threefold increase in the ACV phosphorylation rate.     

Serological specificity of thymidine kinase activity in herpes simplex virus-transformed L cells.

Thymidine kinase (TK) activities of L cells transformed by herpes simplex virus type 1 (HSV-1) or type 2 (HSV-2) were neutralized type specifically by antisera from rabbits immunized with these viruses. When compared with activities of lytically infected cells the specificity of neutralization was similar. The data strongly suggest the conclusion that the TK activity in HSV-transformed L cells is virus coded.     

A novel mutation in the mitochondrial tRNA(Ser(AGY)) gene associated with mitochondrial myopathy, encephalopathy, and complex I deficiency

Purpose

To identify molecular defects in a girl with clinical features of MELAS (mitochondrial encephalomyopathy and lactic acidosis) and MERRF (ragged‐red fibres) syndromes.

Methods

The enzyme complex activities of the mitochondrial respiratory chain were assayed. Temporal temperature gradient gel electrophoresis was used to scan the entire mitochondrial genome for unknown mitochondrial DNA (mtDNA) alterations, which were then identified by direct DNA sequencing.

Results

A novel heteroplasmic mtDNA mutation, G12207A, in the tRNA^Ser(AGY) gene was identified in the patient who had a history of developmental delay, feeding difficulty, lesions within her basal ganglia, cerebral atrophy, proximal muscle weakness, increased blood lactate, liver dysfunction, and fatty infiltration of her muscle. Muscle biopsy revealed ragged red fibres and pleomorphic mitochondria. Study of skeletal muscle mitochondria revealed complex I deficiency associated with mitochondrial proliferation. Real time quantitative PCR analysis showed elevated mtDNA content, 2.5 times higher than normal. The tRNA^Ser(AGY) mutation was found in heteroplasmic state (92%) in the patient''s skeletal muscle. It was not present in her unaffected mother''s blood or in 200 healthy controls. This mutation occurs at the first nucleotide of the 5′ end of tRNA, which is involved in the formation of the stem region of the amino acid acceptor arm. Mutation at this position may affect processing of the precursor RNA, the stability and amino acid charging efficiency of the tRNA, and overall efficiency of protein translation.

Conclusion

This case underscores the importance of comprehensive mutational analysis of the entire mitochondrial genome when a mtDNA defect is strongly suggested.     

The natural history of infantile mitochondrial DNA depletion syndrome due to RRM2B deficiency

PurposeMitochondrial DNA (mtDNA) depletion syndrome (MDDS) encompasses a group of genetic disorders of mtDNA maintenance. Mutation of RRM2B is an uncommon cause of infantile-onset encephalomyopathic MDDS. Here we describe the natural history of this disease.MethodsMultinational series of new genetically confirmed cases from six pediatric centers.ResultsNine new cases of infantile-onset RRM2B deficiency, and 22 previously published cases comprised a total cohort of 31 patients. Infants presented at a mean of 1.95 months with truncal hypotonia, generalized weakness, and faltering growth. Seizures evolved in 39% at a mean of 3.1 months. Non-neurological manifestations included respiratory distress/failure (58%), renal tubulopathy (55%), sensorineural hearing loss (36%), gastrointestinal disturbance (32%), eye abnormalities (13%), and anemia (13%). Laboratory features included elevated lactate (blood, cerebrospinal fluid (CSF), urine, magnetic resonance (MR), spectroscopy), ragged-red and cytochrome c oxidase–deficient fibers, lipid myopathy, and multiple oxidative phosphorylation enzyme deficiencies in skeletal muscle. Eight new RRM2B variants were identified. Patients with biallelic truncating variants had the worst survival. Overall survival was 29% at 6 months and 16% at 1 year.ConclusionsInfantile-onset MDDS due to RRM2B deficiency is a severe disorder with characteristic clinical features and extremely poor prognosis. Presently management is supportive as there is no effective treatment. Novel treatments are urgently needed.     

Thymidine kinase (TK) activity in herpes simplex virus type 1 recombinants that carry insertions affecting regulation of the TK gene

Determinations of the possible importance of herpes simplex virus type 1 (HSV) thymidine kinase (TK) expression in the pathogenesis of viral latency depend in part on the use of defined mutants. In a recent study by A. E. Sears, B. Meignier, and B. Roizman (J. Virol. 55, 410-416 (1985], in which they utilized genetically engineered viral recombinants considered to be TK-, the role of HSV TK expression in latency was reported to be minimal. To further investigate this conclusion we intensively studied the TK phenotypes of their M316-2 and M316-10 HSV-1 mutants. TK activity was investigated by phosphorylation of thymidine, by arabinosylthymine (ara-T) inhibition and by virus plaque autoradiography. TK activity of the M316-2 and M316-10 HSV mutants was not detected in 5-min assays (as performed by Sears et al.), but in longer assays substantial activity was apparent. In contrast, in assays of control TK- viruses, activity was minimal or absent at all time points. In ara-T inhibition assays the M316-2 and M316-10 viruses were inhibited more than 10-fold, consistent with viruses of intermediate TK activity. By plaque autoradiography both of these viruses produced plaques which incorporated significant amounts of thymidine. Based on these results we conclude that the M316-2 and M316-10 viruses should likely be considered to express intermediate levels of TK activity. HSV latency results using these mutants may need to be interpreted with this in mind.     

Nonaka myopathy is caused by mutations in the UDP-N-acetylglucosamine-2-epimerase/N-acetylmannosamine kinase gene (GNE)

This is the first report on mutations of the UDP-N-acetylglucosamine-2-epimerase/N-acetylmannosamine kinase gene (GNE) in Nonaka myopathy or distal myopathy with rimmed vacuoles (OMIM 605820), an autosomal recessive neuromuscular disorder. Sequence and haplotype analyses of GNE in two siblings with Nonaka myopathy from a Japanese family revealed that both patients were compound heterozygotes for a C→T transition (A460V) in exon 8 and a G→C transition (V572L) in exon 10. Their parents and a normal elder brother were all carriers for one or the other of the mutations. GNE mutations are known to cause two other disorders: sialuria (OMIM #269921) and autosomal recessive inclusion body myopathy (IBM2, OMIM #600737). Mutations associated with sialuria are located in the epimerase domain, and those associated with IBM2 are in the epimerase or the kinase domain or both, whereas the mutations we observed in the Nonaka myopathy patients were located in the sugar kinase domain of the gene. Thus, Nonaka myopathy is the third disease known to be caused by GNE mutations. Received: October 12, 2001 / Accepted: November 9, 2001     

胸苷激酶1（TK1）在骨肉瘤中的表达及其临床病理意义

目的　探讨骨肉瘤肿瘤细胞胸苷激酶1（TK1）的表达状况及骨肉瘤患者血清TK1的水平,分析其临床病理意义。 方法 采用免疫组化MaxVision法检测40例骨肉瘤和20例骨巨细胞瘤组织标本中TK1的表达;点印迹免疫酶化学发光法检测20例骨肉瘤患者和20例骨外伤患者血清TK1的水平。 结果　骨肉瘤肿瘤细胞TK1阳性信号位于胞浆及胞核,高级别骨肉瘤阳性率显著高于低级别骨肉瘤（P<0.05）;骨肉瘤肿瘤细胞TK1阳性率亦显著高于骨巨细胞瘤（P<0.05）;骨肉瘤患者血清TK1的水平(2.95±1.33)pmol/L（0.97~7.25 pmol/L）高于外伤除外肿瘤患者(0.89±0.51) pmol/L (0.32~2.58 pmol/L),两者差异具有显著性统计学意义（P<0.05）。 结论　TK1在骨肉瘤与骨的良性病变的鉴别诊断中具有一定的意义。此外,TK1在骨肉瘤患者血清中可被检测到较高的水平,是一项检测骨肉瘤增殖活性较灵敏的指标。     

Thymidine kinase (TK) induction after infection of TK-deficient rabbit cell mutants with bovine herpesvirus type 1 (BHV-1): Isolation of TK BHV-1 mutants

Cytosol thymidine kinase (TK) activity is enhanced at 6 hr after bovine embryo tracheal (EBTr) and rabbit skin fibroblast (RAB-9) cells are infected with the Los Angeles and Cooper strains of bovine herpesvirus type 1 (BHV-1). To learn whether this enhancement resulted from the induction of a virus-specific TK activity, biochemical and genetic studies were carried out. The biochemical experiments demonstrated that: (i) the BHV-1-induced TK activity had a relative disc PAGE mobility (Rm) characteristic of other herpesvirus-encoded TKs and distinctly different from the Rm value of the cytosol TK of host cells; and (ii) the BHV-1-induced TK was significantly more sensitive to competitive inhibition by arabinosylthymine (araT) than the cytosol TKs of EBTr and RAB-9 cells. The genetic experiments entailed the isolation of a bromodeoxyuridine (BrdUrd)-resistant rabbit cell line [RAB(BU)] deficient in cytosol TK activity and of BrdUrd- and araT-resistant BHV-1 mutants. RAB(BU) cells acquired TK activity after they were infected by wild-type, TK+ BHV-1, but not drug-resistant BHV-1 mutants. The experiments strongly suggest that wild-type BHV-1 induces a virus-specific TK activity.     

Intrauterine cardiomyopathy and cardiac mitochondrial proliferation in mitochondrial trifunctional protein (TFP) deficiency

Because of a switch in energy-producing substrate utilization from glucose in the fetal period to fatty acids postnatally, intrauterine morbidity of fatty acid oxidation defects has widely been denied. We report the intrauterine development of severe cardiomyopathy in a child with mitochondrial trifunctional protein deficiency after 27 weeks of gestation. The child was born at 31 weeks of gestation and died on day 3 of life. Severe cardiac mitochondrial proliferation was observed. Molecular analysis of both TFP genes was performed and confirmed a homozygous mutation in the TFP α-subunit introducing a stop codon at amino acid position 256 (g.871C>T, p.R256X). Despite severe intrauterine decompensation in our patient, no HELLP-syndrome or acute fatty liver of pregnancy was observed in the mother. In the pathogenesis of maternal HELLP-syndrome, toxic effects of accumulating long-chain hydroxy-acyl-CoAs or long-chain hydroxy-acylcarnitines are suspected. In our patient, acylcarnitine analysis on day 2 of life during severest metabolic decompensation did not reveal massive accumulation of long-chain hydroxy-acylcarnitines in blood, suggesting other pathogenic factors than toxic effects. The most important pathogenic mechanism for the development of intrauterine cardiomyopathy appears to be significant cardiac energy deficiency. In conclusion, our report implicates that fatty acid oxidation does play a significant role during intrauterine development with special regard to the heart. Severe cardiac mitochondrial proliferation in TFP deficiency suggests pathophysiologically relevant energy deficiency in this condition.     

Restriction enzyme analysis of the mitochondrial genome in mitochondrial myopathy.

The mitochondrial myopathies are a heterogeneous group of disorders some of which may be caused by mutations in the mitochondrial genome. Mitochondrial DNA from 10 patients with mitochondrial myopathy and their mothers was analysed using five restriction enzymes and 11 mitochondrial probes in bacteriophage M13. No abnormalities were found in seven out of the 10 patients. Polymorphisms which have not previously been reported were detected in three patients and two of their mothers. These results exclude the presence of deletions or insertions of greater than 60 bp in the region of the mitochondrial genome examined. Any causative mitochondrial DNA mutations in these disorders are therefore likely to be point mutations or small structural rearrangements.