Fialuridine and its metabolites inhibit DNA polymerase gamma at sites of multiple adjacent analog incorporation, decrease mtDNA abundance, and cause mitochondrial structural defects in cultured hepatoblasts

Sensitive Assay for Mitochondrial DNA Polymerase γ

Background: The mitochondrial DNA polymerase γ is the principal polymerase required for mitochondrial DNA replication. Primary or secondary deficiencies in the activity of DNA polymerase γ may lead to mitochondrial DNA depletion. We describe a sensitive and robust clinical assay for this enzyme.

Methods: The assay was performed on mitochondria isolated from skeletal muscle biopsies. High-molecular weight polynucleotide reaction products were captured on ion-exchange paper, examined qualitatively by autoradiography, and quantified by scintillation counting.

Results: Kinetic analysis of DNA polymerase γ by this method showed a Km for dTTP of 1.43 μmol/L and a Ki for azidothymidine triphosphate of 0.861 μmol/L. The assay was linear from 0.1 to 2 μg of mitochondrial protein. The detection limit was 30 units (30 fmol dTMP incorporated in 30 min). The linear dynamic range was three orders of magnitude; 30–30 000 units. Imprecision (CV) was 6.4% within day and 12% between days. Application of this assay to a mixed population of 38 patients referred for evaluation of mitochondrial disease revealed a distribution with a range of 0–2506 U/μg, reflecting extensive biologic variation among patients with neuromuscular disease.

Conclusion: This assay provides a useful adjunct to current laboratory methods for the evaluation of patients with suspected mitochondrial DNA depletion syndromes.

Mitochondrial DNA replication in human T lymphocytes is regulated primarily at the H-strand termination site

The most unique feature in the replication of mitochondrial DNA (mtDNA) is that most of the newly synthesized heavy strands (H-strands) terminate prematurely, resulting in the formation of displacement loop (D-loop) strands. Only the H-strand which proceeds past the termination site is a true nascent H-strand leading to the overall replication on a circular mtDNA molecule. The physiological significance of the D-loop formation has long been unclear. To examine the role of premature termination in mtDNA replication, we therefore developed a method for selectively measuring both the total amount of nascent H-strands and the amount of true nascent H-strands using ligation-mediated polymerase chain reaction, which, for the first time, enabled us to estimate the frequency of premature termination. The stimulation of cell proliferation with interleukin 2 and phytohemagglutinin in human peripheral T lymphocytes caused an increase in the net replication rate of mtDNA. In stimulated cells, in comparison to resting ones, the amount of true nascent H-strands increased approx. 2.6-fold while the total amount of nascent H-strands remained unchanged, indicating that premature termination decreased while the initiation of replication remained the same. Our findings thus demonstrate the first clear example that premature termination plays a primary role in the up-regulation of the net rate of mtDNA replication in human cells.

1-Methyl-4-phenylpyridinium ion (MPP+) selectively inhibits the replication of mitochondrial DNA

1-Methyl-4-phenyl-1,2,3,6-tetrahydropyridine is known to cause Parkinsonism in its neurotoxic form, 1-methyl-4-phenylpyridinium ion (MPP+). We have previously reported that MPP+ decreases the content of mitochondrial DNA (mtDNA) independently of the inhibition of complex I in human cells [Miyako, K., Kai, Y., Irie, T., Takeshige, K., and Kang, D. (1997) J. Biol. Chem. 272, 9605-9608]. Here we study the mechanism causing the decrease in mtDNA. MPP+ inhibits the incorporation of 5-bromo-2'-deoxyuridine into mtDNA but not into nuclear DNA, indicating that MPP+ inhibits the replication of mtDNA but not that of the nuclear genome. The replication of mtDNA is initiated by the synthesis of the heavy strand switched from the transcription of the light strand. MPP+ decreases the nascent heavy strands per mtDNA and increases the transcript of the ND6 gene, encoded on light strand, per mtDNA. The amount of mitochondrial transcription factor A is not decreased. These data suggest that the transcription is not inhibited and therefore the transition from transcription to replication of mtDNA is lowered in the MPP+-treated cells. Electron microscopy shows that the number of mitochondria is not decreased in the MPP+-treated cells, suggesting that MPP+ does not affect the overall biogenesis of mitochondria. Thus, MPP+ selectively inhibits the replication of mtDNA.

Zidovudine-induced mitochondrial disorder with massive liver steatosis, myopathy, lactic acidosis, and mitochondrial DNA depletion

Zidovudine is known to be responsible for a mitochondrial myopathy with ragged-red fibres and mitochondrial DNA depletion in muscle. Lactic acidosis alone or associated with hepatic abnormalities has also been reported. A single report mentioned the concomitant occurrence of muscular and hepatic disturbances and lactic acidosis in a patient receiving zidovudine, but muscle and liver tissues were not studied. A 57-year-old man with AIDS, who had been treated with zidovudine for 3 years, developed fatigue and weight loss. Serum creatine kinase and hepatic enzyme levels were high. Lactic acidosis was present. Liver biopsy showed diffuse macrovacuolar and microvacuolar steatosis. After withdrawal of zidovudine, creatine kinase, aspartate aminotransferase, and alanine aminotransferase levels normalised within 5 days, and lactacidaemia decreased. Acidosis persisted. The patient became confused and febrile and died 8 days after detection of high blood lactic acid. A muscle sample obtained at autopsy showed mitochondrial abnormalities with ragged-red fibres and lipid droplet accumulation. Southern blot analysis showed depletion of mitochondrial DNA, affecting skeletal muscle and liver tissue. No depletion was found in myocardium and kidney. This case emphasises that zidovudine treatment can induce mitochondrial multisystem disease, as revealed in our case by myopathy, liver steatosis and lactic acidosis.

Phosphorylation of anticancer nucleoside analogs by human mitochondrial deoxyguanosine kinase

The kinetic properties of recombinant human mitochondrial deoxyguanosine kinase (dGK, EC 2.7.1.113) for 2'-deoxyguanosine and the clinically important nucleoside analogs 2-chloro-2'-deoxyadenosine (CdA), 9-beta-D-arabinofuranosylguanine (araG) and 2',2',-difluorodeoxyguanosine (dFdG) were determined. The Michaelis-Menten kinetic parameters, comparing ATP and UTP as phosphate donors, demonstrated a marked increase in phosphorylation efficiency (VmaxKm) with UTP in comparison with ATP for both CdA and araG. The difluoro analog dFdG was an efficient substrate for recombinant dGK with an apparent Km of 16 microM with ATP as phosphate donor. We compared the kinetic properties of dGK with those of the related enzyme deoxycytidine kinase (dCK, EC 2.7.1.74). Although the purines 2'-deoxyguanosine (dGuo) and 2'-deoxyadenosine are substrates for both dGK and dCK, only CdA among the purine nucleoside analogs tested was an efficient substrate for both dCK and dGK. In competition with dGuo, the most efficient analog for phosphorylation by dGK was araG, as indicated by a lower Ki value than for CdA and dFdG. Of the purine analogs tested as substrates for dCK, only CdA could compete with 2'-deoxycytidine (dCyd). No inhibition of dCK-mediated dCyd phosphorylation was found by either araG or dFdG. In crude cell extract of HeLa and Capan 2 cells, the major CdA phosphorylation was contributed by dCK, while most araG phosphorylation was a result of dGK activity. Our study with pure recombinant enzymes confirms that dGK is mainly responsible for araG and dFdG phosphorylation, whereas dCK is the most important enzyme for activation of CdA and 2',2'-difluorodeoxycytidine (dFdC).

The identification and development of antiviral agents for the treatment of chronic hepatitis B virus infection

Hepatitis B virus (HBV) is the leading cause of chronic hepatitis throughout the world. Notwithstanding the availability of a safe and effective vaccine, the world prevalence of HBV has not declined significantly, thus resulting in the need for a selective antiviral agent. HBV is a small, partially double-stranded DNA virus which replicates through an RNA intermediate. Most efforts to develop anti-HBV agents have been targeted to the viral DNA polymerase which possesses reverse transcriptase activity. Currently, the most promising anti-HBV agents are nucleoside analogs which interfere with viral DNA replication. Although earlier nucleoside analogs such as vidarabine (ara-A) and fialuridine (FIAU) have displayed unacceptable toxicities, newer analogs such as lamivudine (3TC), bis-POM PMEA (GS-840), lobucavir, and BMS-200,475 have demonstrated clinical utility. In particular, the use of lamivudine has generated considerable interest in the development of other L-enantiomeric nucleoside analogs for use against HBV. Here, we provide an overview of HBV structure and replication strategy and discuss the use of cell culture systems, in vitro viral polymerase systems, and animal models to identify and evaluate anti-HBV agents. We also discuss the various classes of nucleoside analogs in terms of structure, mechanism of action, status in clinical development, ability to select for resistant HBV variants, and use in combination therapies. Finally, we present a discussion of novel antiviral approaches, including antisense and gene therapy, and address the various challenges to successful anti-HBV chemotherapeutic intervention.

Antiviral activity and toxicity of fialuridine in the woodchuck model of hepatitis B virus infection

Woodchucks were used to study the antiviral activity and toxicity of fialuridine (FIAU; 1,-2'deoxy-2'fluoro-1-beta-D-arabinofuranosyl-5-iodo-uracil). In an initial experiment, groups of six chronic woodchuck hepatitis virus (WHV) carrier woodchucks received daily doses of FIAU by intraperitoneal injection for 4 weeks. At 0.3 mg/kg/d, the antiviral effect was equivocal, but at 1.5 mg/kg/d, FIAU had significant antiviral activity. No evidence of drug toxicity was observed during the 4-week period of treatment or during posttreatment follow-up. In a second experiment, groups of nine WHV carriers or uninfected woodchucks were given 1.5 mg/kg/d of FIAU orally for 12 weeks, and the results compared with placebo-treated controls. After 4 weeks, the serum WHV-DNA concentration in the FIAU-treated carrier group was two to three logs lower than that in the placebo-treated group. After 12 weeks of FIAU treatment, serum WHV DNA was not detectable by conventional dot-blot analysis, hepatic WHV-DNA replicative intermediates (RI) had decreased 100-fold, and hepatic expression of WHV core antigen was remarkably decreased. No evidence of toxicity was observed after 4 weeks, but, after 6 to 7 weeks, food intake decreased and, after 8 weeks, the mean body weights of woodchucks treated with FIAU were significantly lower than controls. Anorexia, weight loss, muscle wasting, and lethargy became progressively severe, and all FIAU-treated woodchucks died or were euthanized 78 to 111 days after treatment began. Hepatic insufficiency (hyperbilirubinemia, decreased serum fibrinogen, elevated prothrombin time), lactic acidosis, and hepatic steatosis were characteristic findings in the final stages of FIAU toxicity in woodchucks. The syndrome of delayed toxicity in woodchucks was similar to that observed previously in humans treated with FIAU, suggesting that the woodchuck should be valuable in future investigations of the molecular mechanisms of FIAU toxicity in vivo and for preclinical toxicological evaluation of other nucleoside analogs before use in patients.

Inhibitory effect of 2'-fluoro-5-methyl-beta-L-arabinofuranosyl-uracil on duck hepatitis B virus replication

The antiviral activity of 2'-fluoro-5-methyl-beta-L-arabinofuranosyluracil (L-FMAU), a novel L-nucleoside analog of thymidine known to be an inhibitor of hepatitis B virus (HBV) replication in hepatoma cells (2.2.1.5 cell line), was evaluated in the duck HBV (DHBV) model. Short-term oral administration (5 days) of L-FMAU (40 mg/kg of body weight/day) to experimentally infected ducklings induced a significant decrease in the level of viremia. This antiviral effect was sustained in animals when therapy was prolonged for 8 days. The histological study showed no evidence of liver toxicity in the L-FMAU-treated group. By contrast, microvesicular steatosis was found in the livers of dideoxycytidine-treated animals. L-FMAU administration in primary duck hepatocyte cultures infected with DHBV induced a dose-dependent inhibition of both virion release in culture supernatants and intracellular viral DNA synthesis, without clearance of viral covalently closed circular DNA. By using a cell-free system for the expression of an enzymatically active DHBV reverse transcriptase, it was shown that L-FMAU triphosphate exhibits an inhibitory effect on the incorporation of dAMP in the viral DNA primer. Thus, our data demonstrate that L-FMAU inhibits DHBV replication in vitro and in vivo. Long-term administration of L-FMAU for the eradication of viral infection in animal models of HBV infection should be evaluated.

Cloning of the cDNA and chromosome localization of the gene for human thymidine kinase 2

Human thymidine kinase 2 (TK2) is a deoxyribonucleoside kinase that phosphorylates thymidine, deoxycytidine, and deoxyuridine. The enzyme also phosphorylates anti-viral and anti-cancer nucleoside analogs. We have identified an expressed sequence tag cDNA that encoded a 27.5-kDa protein approximately 30% similar to the human deoxycytidine kinase and deoxyguanosine kinase. The protein was expressed in Escherichia coli and shown to have similar substrate specificity as reported for purified native human TK2. The recombinant TK2 was shown to phosphorylate the anti-cancer nucleoside analog 2',2'-difluorodeoxycytidine. Northern blot analysis showed two mRNA species at 2.4 and 4.0 kilobases predominantly expressed in liver, pancreas, muscle, and brain. We identified a sequence-tagged site designed from the 3' region of the TK2 cDNA. The sequence-tagged site has been mapped to 81-84 centimorgans from the top linkage group of chromosome 16, which corresponds to the 16q22 region. Our data show that deoxycytidine kinase, deoxyguanosine kinase, and TK2 belong to a family of closely related enzymes. At the time of this report all four of the known human deoxyribonucleoside kinases have been cloned. This provides the opportunity to characterize their individual contribution to therapeutic and toxic effects of nucleoside analogs.