Mitochondrial thymidine kinase and the enzymatic network regulating thymidine triphosphate pools in cultured human cells.

In non-proliferating cells mitochondrial (mt) thymidine kinase (TK2) salvages thymidine derived from the extracellular milieu for the synthesis of mt dTTP. TK2 is a synthetic enzyme in a network of cytosolic and mt proteins with either synthetic or catabolic functions regulating the dTTP pool. In proliferating cultured cells the canonical cytosolic ribonucleotide reductase (R1-R2) is the prominent synthetic enzyme that by de novo synthesis provides most of dTTP for mt DNA replication. In non-proliferating cells p53R2 substitutes for R2. Catabolic enzymes safeguard the size of the dTTP pool: thymidine phosphorylase by degradation of thymidine and deoxyribonucleotidases by degradation of dTMP. Genetic deficiencies in three of the participants in the network, TK2, p53R2, or thymidine phosphorylase, result in severe mt DNA pathologies. Here we demonstrate the interdependence of the different enzymes of the network. We quantify changes in the size and turnover of the dTTP pool after inhibition of TK2 by RNA interference, of p53R2 with hydroxyurea, and of thymidine phosphorylase with 5-bromouracil. In proliferating cells the de novo pathway dominates, supporting large cytosolic and mt dTTP pools, whereas TK2 is dispensable, even in cells lacking the cytosolic thymidine kinase. In non-proliferating cells the small dTTP pools depend on the activities of both R1-p53R2 and TK2. The activity of TK2 is curbed by thymidine phosphorylase, which degrades thymidine in the cytoplasm, thus limiting the availability of thymidine for phosphorylation by TK2 in mitochondria. The dTTP pool shows an exquisite sensitivity to variations of thymidine concentrations at the nanomolar level.

Real-time multiplex PCR assay to quantify hepatitis C virus RNA in peripheral blood mononuclear cells.

Ultrasensitive methods to measure very low levels of hepatitis C virus (HCV) RNA in biological samples may have diagnostic and prognostic significance and be useful to evaluate the response to antiviral treatment. A sensitive assay to quantify HCV RNA in peripheral blood mononuclear cells (PBMCs) was developed and validated using the iCycler iQ Detection System (Bio-Rad) coupled with TaqMan chemistry. HCV was co-amplified with the endogenous control glyceraldehyde-3-phosphate dehydrogenase in a multiplex reaction. Calculated PCR amplification efficiencies for both target and control genes were used in a mathematical model for relative quantitation of HCV RNA. A linear relationship between input RNA and C(T) values over 6 log dilutions was observed for both HCV- and GAPDH-specific products (R(2) > or = 0.99). As few as 1.5 IU/reaction could be detected, with high accuracy (CV < or= 3.94%) and reproducibility (CV < or = 2.20%). Quantitation of HCV RNA levels ranging from 10(3) to 10(7) IU/ml as measured in 47 plasma samples was highly correlated with values obtained by the COBAS Amplicor HCV Monitor test, v2.0 (Roche) (R(2) = 0.977). In conclusion, this assay provides an excellent tool to determine accurately HCV kinetics in PBMCs during antiviral therapy and to assess the long-term significance of different patterns of response to treatment.

Human mitochondrial 5'-deoxyribonucleotidase. Overproduction in cultured cells and functional aspects.

Deoxynucleoside triphosphates (dNTPs) used for mitochondrial DNA replication are mainly formed by phosphorylation of deoxynucleosides imported into mitochondria from the cytosol. We earlier obtained evidence for a mitochondrial 5'-nucleotidase (dNT2) with a pronounced specificity for dUMP and dTMP and suggested that the enzyme protects mitochondrial DNA replication from excess dTTP. In humans, accumulation of dTTP causes a mitochondrial genetic disease. We now establish that dNT2 in vivo indeed is located in mitochondria. The native enzyme shows the same substrate specificity and affinity for inhibitors as the recombinant dNT2. We constructed ponasterone-inducible cell lines overproducing dNT2 with and without the green fluorescent protein (GFP) linked to its C terminus. The fusion protein occurred in mitochondria mostly in an inactive truncated form, with only a short C-terminal fragment of dNT2 linked to GFP. No truncation occurred when dNT2 and GFP were not linked. The cell mitochondria then contained a large excess of active dNT2 with or without the mitochondrial presequence. After removal of ponasterone overproduced dNT2 disappeared only slowly from the cells, whereas dNT2-mRNA was lost rapidly. Overproduction of dNT2 did not lead to an increased excretion of pyrimidine deoxyribonucleosides, in contrast to overproduction of the corresponding cytosolic deoxynucleotidase, suggesting that the mitochondrial enzyme does not affect overall cellular deoxynucleotide turnover.