Molecular cloning and structural analysis of murine thymidine kinase genomic and cDNA sequences

Molecular analysis of chemically-induced mutations in mammalian cell assays

Two mammalian cell mutation assays, the HPRT/V79 assay and the TK/mouse lymphoma assay, were compared for their ability to respond to the genotoxic chemicals ethyl methanesulfonate (EMS) and mitomycin C (MMC). Whereas EMS induced a high mutant frequency at both loci, MMC produced few mutants at the hprt locus, but induced a large number of mutants at the tk locus. Southern blotting analysis showed that this difference was due to the type of genetic damage induced by the two chemicals. Intragenic changes ranging from point mutations to loss of the entire gene were recovered as viable mutants at both the hprt and tk loci. Thus, EMS which causes mainly intragenic mutations induced similar mutant frequencies at both loci. The large multilocus deletions induced by MMC, in which the damage was assumed in many cases to extend into a gene essential for growth since most TK mutants were slow-growing, could not be recovered at the hprt locus. Whereas both loci will detect intergenic mutations, mutants carrying large-scale damage are recoverable only at the heterozygous tk locus. At the hemizygous hprt locus no homologous chromosome exists to provide the function of essential genes if these are lost along with hprt in multilocus deletions. Most human cancers develop as a highly complex process involving both gene and multilocus mutations in oncogenes and tumour suppressor genes. Thus the TK/mouse lymphoma assay is a more appropriate in vitro test for the detection of chemicals capable of causing the types of DNA lesions important in human cancer.

Production and characterisation of a novel chicken IgY antibody raised against C-terminal peptide from human thymidine kinase 1

Egg yolk is a good source of highly specific antibodies against mammalian antigens because of the phylogenetic distance between birds and mammals. Chicken egg yolk immunoglobulins (IgY) were generated to a synthetic 31-amino acid peptide from the C-terminal of human HeLa thymidine kinase 1 (TK1) enzyme. The anti-TK1 IgY antibody was purified using affinity chromatography against the 31-amino acid peptide. The purified antibody inhibited the catalytic activity of the TK1 enzyme in the CEM TK1(+) cells and recognized the 25-kDa subunit and tetrameric form of TK1, which has a pI value of 8.3. No immunoreaction was observed in CEM TK1(-) cells. Western blot of the serum TK1 (S-TK1) also showed that only a single band was found in the serum of patients with malignancies. No band was seen in healthy serum. Furthermore, dot blots and enhanced chemiluminescence (ECL) detection of S-TK1 performed on sera of preoperative patients with gastric cancer (GC) (n=31) and healthy controls (n=62) showed that the levels of S-TK1 in the sera of cancer patients were significantly different (P<0.01). Using ECL dot blots, 0.1 pg of TK1 in 3 microl sera could be detected. Immunohistostaining of tissues in the 11 advanced-stage cancer patients (four breast carcinomas, three hepatocarcinomas and four thyroid carcinomas) indicated that a strong staining of TK1 enzyme was found in the cytoplasm of malignant cells. No staining or weak staining was seen in normal tissues. We suggest that screening for TK1 using anti-TK1 IgY may be potentially useful for serological and immunohistochemical detection of TK1 as an early prognosis and for monitoring patients undergoing treatment.

Mice deficient for cytosolic thymidine kinase gene develop fatal kidney disease

The thymidine kinase (Tk) gene codes for a cytosolic protein involved in the pyrimidine nucleotide salvage pathway. A functional Tk gene is not necessary for cells in culture, and a naturally occurring Tk deficient phenotype has not been described in humans or animal models. In order to determine the biological significance of the Tk gene, we created Tk(-/-) knockout (KO) mice through homologous recombination in mouse embryonic stem cells. Tk KO mice have shortened life spans compared with their wild-type or Tk heterozygous (HET) siblings. All Tk KO mice develop sclerosis of kidney glomeruli and die before one year of age of kidney failure. Among other changes in KO animals, the most consistent is a switch from exclusively mucous secretion to predominantly serous secretion in the sublingual salivary gland. HET parents can produce KO mice at a frequency approaching Mendelian inheritance. Other observations in KO animals include an elevated level of serum thymidine, a significant decrease in the cloning efficiency of splenic lymphocytes, an increase in the frequency of hypoxanthine guanine phosphoribosyl transferase gene mutant lymphocytes, and histological alteration in the lymphoid structure of the spleen. In addition, KO animals sporadically exhibit inflammation of the arteries, which taken together with the lymphocyte and spleen abnormalities, suggest an abnormal immune system. Alterations in Tk KO mice indicate that the pyrimidine nucleotide salvage pathway is indispensable in vivo.

Specific recognition of cytosolic thymidine kinase in the human lung tumor by monoclonal antibodies raised against recombinant human thymidine kinase

Anti-TK monoclonal antibodies (mAbs) were raised against recombinant human cytosolic thymidine kinase (rhTK) and characterized by Western immunoblotting, enzyme-linked immunosorbent assay (ELISA) and immunostaining of tumor cells. Twenty-three clones of TK mAbs were characterized to recognize specifically not only rhTK produced by Escherichia coli but also TK subunit of 25 kDa in human lung cancer. The anti-TK mAbs reacted specifically with cytosolic TK but not with mitochondrial TK. Only one clone of the mAbs inhibited the catalytic activity of TK. By solid phase sandwich enzyme immunoassay using these mAbs, we could quantitate the cytosolic TK content in tissues. Immunohistochemical staining analysis using one of the TK mAbs showed that human lung adenocarcinoma and squamous cell carcinoma exhibited much higher staining intensity than stromal cells. These mAbs are useful for biochemical studies on the regulation of human TK in proliferating cells such as tumor cells and for diagnosis of highly proliferating tumors.

Thymidine utilization abnormality in proliferating lymphocytes and hepatocytes of the woodchuck

Effective incorporation of tritiated thymidine ([(3)H]TdR) into proliferating lymphocytes is important because [(3)H]TdR is a standard label to study proliferate T-cell responses. We analyzed the thymidine utilization of woodchuck peripheral blood lymphocytes (PBL) since the [(3)H]TdR incorporation assay was not applicable to measure proliferative immune responses in the woodchuck, a current major virus/host model for human hepatitis B virus infection. Incorporation of [(3)H]TdR into DNA as well as the activity of the salvage pathway enzyme thymidine kinase (TK) of proliferating woodchuck PBL was low compared to human lymphocytes. Furthermore, [(3)H]TdR incorporation of proliferating woodchuck PBL remained residual regardless of the use of methotrexate, an inhibitor of the competitive deoxythymidine monophosphate de novo synthesis pathway. Using a human probe, specific for the proliferation-associated TK1, we proved the genomic presence and transcription of TK1 sequences in various species. TK1 sequences were detected in the genome of human, mouse, woodchuck, and chicken specimens. In contrast to proliferating human PBL and 3T3 mouse fibroblasts, no TK1 transcript was found in proliferating woodchuck PBL and hepatic cells. Transfection experiments with vectors containing the murine or human TK1 and selection assays demonstrated the ability of woodchuck cells to transcribe TK1 and to express functional TK1 proteins. Our study characterizes the unique failure of sufficient [(3)H]TdR incorporation into proliferating woodchuck cells and demonstrates tritiated adenine and serine as alternative labels to monitor PBL proliferation in the woodchuck.

Tk+/- mouse model for detecting in vivo mutation in an endogenous, autosomal gene

Tk+/- transgenic mice were created using an embryonic stem cell line in which one allele of the endogenous thymidine kinase (Tk) gene was inactivated by targeted homologous recombination. Breeding Tk+/- parents produced viable Tk-/- knockout (KO) mice. Splenic lymphocytes from KO mice were used in reconstruction experiments for determining the conditions necessary for recovering Tk somatic cell mutants from Tk+/- mice. The cloning efficiency of KO lymphocytes was not affected by the toxic thymidine analogues 5-bromo-2'-deoxyuridine (BrdUrd) or trifluorothymidine (TFT), or by BrdUrd in the presence of lymphocytes from Tk+/- animals; however, it was easier to identify clones resistant to BrdUrd than to TFT when Tk+/- cells were present. Tk+/- mice were treated with vehicle or 100 mg/kg of N-ethyl-N-nitrosourea (ENU), and after 4 months, the frequency of Tk mutant lymphocytes was measured by resistance to BrdUrd. The frequency of Tk mutants was 22+/-5.9x10-6 in control animals and 80+/-31x10-6 in treated mice. In comparison, the frequency of Hprt mutant lymphocytes, as measured by resistance to 6-thioguanine, was 2.0+/-1.2x10-6 in control animals and 84+/-28x10-6 in the ENU-treated mice. Analysis of BrdUrd-resistant lymphocyte clones derived from the ENU-treated animals revealed point mutations in the non-targeted Tk allele. These results indicate that the selection of BrdUrd-resistant lymphocytes from Tk+/- mice may be used for assessing in vivo mutation in an endogenous, autosomal gene.

Regulation of the expression of enzymes involved in the replication of DNA in chemically-induced granulocytic differentiation of HL-60 leukemia cells

The expression of seven enzymes involved in the biosynthesis of DNA was measured in HL-60 promyelocytic leukemia cells treated with dimethylsulfoxide (DMSO) or all-trans retinoic acid (RA) to gain information on their role in the termination of proliferation in cells undergoing granulocytic differentiation. The steady-state levels of the mRNAs for topoisomerase I, topoisomerase II. DNA polymerase-alpha, thymidylate synthase, thymidine kinase and hypoxanthine-guanine phosphoribosyltransferase progressively declined from day 3 to day 7 of exposure to the polar solvent or the retinoid suggesting that the expression of these enzymes is coordinately regulated. In contrast, a pronounced difference between the two inducers of differentiation occurred in the expression of the mRNA of the M2 subunit of ribonucleotide reductase, with DMSO causing virtually complete inhibition of the expression of the M2 subunit of the enzyme from day 5 through day 7, with no change in the steady-state levels of the mRNA being produced by retinoic acid. Measurement of the enzymatic activities of two of these catalysts, thymidylate synthase and thymidine kinase, in cells exposed to the two inducers of maturation corroborated the findings at the level of the mRNAs, with corresponding decreases in the activity of these enzymes. The findings collectively demonstrate that the down-regulation of the expression of a relatively wide variety of enzymes involved in DNA replication occurs as late events in the granulocytic differentiation of HL-60 cells, ensuring that cellular replication cannot occur in terminally differentiated cells.

Purinergic receptors mediate cell proliferation and enhanced recovery from renal ischemia by adenosine triphosphate

Kidney dysfunction after ischemia can be improved by either limiting the initial injury or by enhancing the subsequent proliferative repair process. Adenosine triphosphate (ATP) favorably affects kidney function when it is given shortly after ischemia. We tested whether ATP promotes the proliferative repair response. Rats were subjected to occlusion of the left renal artery for 40 minutes and received an infusion of ATP, 12.5 micromol intravenously over 30 minutes, beginning at reperfusion. Control animals received saline solution or the hydroxyl radical scavenger dimethylthiourea (DMTU). Despite comparable functional protection by DMTU and ATP, only ATP specifically increased DNA synthesis (renal incorporation of tritiated thymidine) to an extent greater than that produced by ischemia alone. In other animals, ribonucleic acid was extracted from kidneys for Northern analysis. Expression of the proto-oncogenes c-fos and c-jun was enhanced in ATP-treated animals as compared with controls. Expression of a histone protein gene (H2b) and thymidine kinase was increased by ischemia but was not additionally affected by ATP. In vitro studies of primary cultures of renal proximal tubule epithelial cells confirmed the ability of ATP to stimulate cellular proliferation as a consequence of stimulation of purinergic P2 receptors, possibly of the P2x subclass. In summary, ATP given after ischemia increased new DNA synthesis and augmented expression of genes critical to cellular proliferation. These beneficial effects were not merely a consequence of limiting initial cellular damage, and they suggest a novel mechanism of action for ATP and other purinergic receptor agonists in renal ischemia.

Mouse thymidine kinase stability in vivo and after in vitro translation

Using a combination of centrifugal elutriation and recultivation of synchronised cell populations we could show that murine thymidine kinase (TK) is rapidly degraded during mitosis in polyoma virus-transformed mouse fibroblasts, in parallel to the time-course for loss of cyclin A. Transformation is no prerequisite for the instability phenotype since artificial overexpression of TK under the control of a constitutive promoter in normal mouse fibroblasts also resulted in rapid turnover of TK during mitosis. The decay of TK protein could be partially mimicked in vitro with enzymatically active protein translated in a rabbit reticulocyte lysate: full length polypeptide was lost slightly more rapidly in the presence of G2/M cytosolic extracts than with G1/S preparations. In addition, an enzymatically active C-terminal truncation of 37 amino acids at Gln-196 was completely stable under the conditions tested, confining the instability domain between residues 196 to 233. These experiments also indicated the border for intact TK since translation products up to Tyr-189 or less were completely inactive. This was also confirmed by a mutant TK protein from mouse F9tk- teratocarcinoma cells which harboured a similar deletion.

Estrogen-induced cyclin D1 and D3 gene expressions during mouse uterine cell proliferation in vivo: differential induction mechanism of cyclin D1 and D3

D-type cyclins are involved in the regulation of the G1/S transition of the cell cycle in various cell types cultured in vitro. Little is, however, known about the expression pattern and functional role of D-type cyclins in physiological processes in vivo. In this report, we studied whether the expression of murine D-type cyclins correlates with the states of mouse uterine cell proliferation in vivo. Time-course changes in cyclin D1 and D3 mRNA levels in the uterine tissues of immature mice primed with 17 beta-estradiol (E2) were examined by Northern blot hybridization. c-fos and thymidine kinase (TK) mRNA levels were also examined as markers for the transition from G0 to G1 and the onset of S phase, respectively. Cyclin D1 and D3 mRNAs were induced 2.5-fold between c-fos and TK mRNA peaks. The E2-induced cyclin D1 and D3 gene expressions were blocked by antiestrogens tamoxifen and ICI 182,780. We also investigated the effects of cycloheximide (CHX), a protein synthesis inhibitor, on cyclin D1 and D3 gene expressions. When CHX was treated alone, cyclin D3, but not cyclin D1, mRNA was immediately superinduced. The E2-induced cyclin D3 gene expression was shifted by approximately 6 h when CHX was pretreated 1 hr before E2 administration. Interestingly, the 3H-thymidine incorporation experiment showed that the mouse uterine cell cycle progression also shifted by 6 hr with pretreatment of CHX. The overall results suggest that both cyclin D1 and D3 mRNAs are constitutively expressed in uterine tissues and induced by E2 at G1 phase of the mouse uterine cell cycle. However, the superinducibility and temporal shift of cyclin D3 by CHX suggest that there is a different regulatory mechanism underlying cyclin D1 and D3 gene expressions in the mouse uterine cell cycle progression.

Salvage pathway of pyrimidine synthesis: divergence of substrate specificity in two related species of teleostean fish

For nucleotide synthesis, cells use purine and pyrimidine nucleosides generated either through de novo synthesis or through utilization of salvage pathways. In the pyrimidine salvage pathway, thymidine is taken up by transport proteins and phosphorylated by the enzyme thymidine kinase to thymidine monophosphate. So far, all vertebrates analyzed are able to use radioactively labeled thymidine for the biosynthesis of nucleotides in brain tissue. However, when standard autoradiographic, immunohistochemical and biochemical procedures were applied for the detection of the incorporation of tritiated thymidine and the thymidine analogue 5-bromo-2'-deoxyuridine into DNA to two species of gymnotiform fish, a divergence in substrate specificity has been revealed. Although brain cells of the two species, Apteronotus leptorhynchus and Eigenmannia sp., can utilize 5-bromo-2'-deoxyuridine for pyrimidine synthesis, only Eigenmannia sp. is able to incorporate tritiated thymidine into DNA during the S phase of the cell cycle. We hypothesize that this inability to use thymidine for nucleotide synthesis is caused either by a defect in the transport system mediating the uptake of thymidine or by a deficiency in the thymidine kinase of A. leptorhynchus.

Differential expression of D-type G1 cyclins during mouse development and liver regeneration in vivo

D-type G1 cyclins are the primary cell cycle regulators of G1/S transition in eukaryotic cells, and are differentially expressed in a variety of cell lines in vitro. Little is known, however, about the expression patterns of D-type G1 cyclins in normal mouse in vivo. Thus, in the present study, tissue-specific expressions of cyclin D1 and D3 genes were examined in several tissues derived from adult male mice, and stage-specific expression of cyclin genes was studied in brain, liver, and kidney of developing mice from embryonic day 13 to postnatal day 11. Cell cycle-dependent expression of cyclins was also examined in regenerating livers following partial hepatectomy. Our results indicate that (1) cyclins D1 and D3 are expressed in a tissue-specific manner, with cyclin D1 being highly expressed in kidney and D3 in thymus; (2) cyclin D3 mRNA is abundantly expressed in young proliferating tissues and is gradually reduced during development, whereas cyclin D1 mRNA fluctuates during development; and (3) compensatory regeneration of liver induces cyclin D1 gene expression 12 hr after partial hepatectomy, and cyclin D3 gene expression from 36 to 42 hr (at the time of G1/S transition). In conclusion, this study indicates that cyclin D1 and D3 genes are differentially expressed in vivo in a tissue-specific, developmental stage-dependent, and cell cycle-dependent manner.

Existence of phosphorylated and dephosphorylated forms of cytosolic thymidine kinase (TK1)

In this study we examine whether different TK1 variants of pI 6.9 and 8.3 found by isoelectric focusing gel electrophoresis (IFE) reflect just a phenotype difference due to phosphorylation modifications or have a real phenotypic background. The phosphorylation degree of purified TK1 variants was analyzed by determining the changes in the pI values after treatment with alkaline phosphatase, using IFE. The genetic origin of the two TK1 variants was studied by determining their mol wt. by means of SDS-gelelectrophoresis. Furthermore, the subcellular distribution of the two TK1 variants was also studied. Alkaline phosphatase treatment changed the pI value of purified TK1 from 6.9 to 8.3. No change in the pI value was found when purified TK1 corresponding to pI 8.3 was treated in the same way. Similar results were obtained when treated a cytosolic fraction with alkaline phosphatase. Antibody raised against the C-terminal part of human TK1 only recognized the dephosphorylated TK1 variant corresponding to pI 8.3. There was no difference in the molecular weight between the two TK1 variants. Thus, we concluded that the TK1 variants corresponding to pI 6.9 and 8.3 are of the same genetic origin, but consist of phosphorylated and dephosphorylated forms.

Development of a novel mouse tk+/- embryonic stem cell line for use in mutagenicity studies

A tk+/- mouse embryonic stem (ES) cell line, designated 1G2, has been created in which one allele of the thymidine kinase (tk) gene was inactivated by targeted homologous recombination. This line is an analog of the mouse lymphoma tk+/- L5178Y cell line, which is used widely to assess the mutagenicity of chemical agents. Treatment of 1G2 cells with the alkylating agent N-ethyl-N-nitrosourea (ENU) resulted in a dose-related increase in trifluorothymidine-resistant colonies. Mutant frequencies of 152 and 296 per 10(6) cells were determined for 0.1 and 0.3 mg/ml doses of ENU, compared with a spontaneous mutant frequency of 15 per 10(6) cells. The data indicate that tk+/- 1G2 ES cells may be useful for the creation of a transgenic mouse model for assessing in vivo mutation using an endogenous autosomal gene.

Molecular analysis of the TK locus in L5178Y large and small colony mouse lymphoma cell mutants induced by hycanthone methanesulfonate

Mouse lymphoma cells of the L5178Y TK + /-3.7.2C line were exposed to varying concentrations of the anti-schistosomal drug hycanthone methanesulfonate. The trifluorothymidine (TFT)-resistant cells fell into two classes based on colony size. Southern blot analyses were performed using NcoI-digested DNA from a number of large and small mutant colonies from each treatment group. Two different restriction fragment banding patterns were identified in these analyses, those colonies that contained the 6.4 kb NcoI restriction fragment and those that did not. A total of 471 mutant colonies were analyzed and 84.5% (398) of these colonies did not exhibit the 6.4 kb fragment. There did not appear to be a hycanthone methanesulfonate dose response effect in the number of colonies that did not contain the 6.4 kb fragment among the treated groups. In addition, 82% (154 out of 188) of spontaneous mutants did not contain the 6.4 kb fragment. The results imply that greater than 80% of all spontaneous mutations found in the mouse lymphoma assay regardless of colony size do not contain the 6.4 kb fragment and each may be considered to be a large scale mutation. In addition, greater than 80% of the hycanthone induced mutations in the mouse lymphoma assay do not contain the 6.4 kb fragment and thus may be considered to be a large scale mutation.

Thymidine incorporation is less sensitive to lack of zinc in human than in rodent cells

The inhibition of thymidine incorporation by inadequate availability of zinc induced by adding a chelator to the culture medium was significantly less in human cell lines than in rodent cell lines. In contrast, zinc uptake into the human cells was inhibited by the chelator to a greater extent than with rodent cells. The possible implications of these observations for the dietary zinc requirements of humans and rodents are discussed.

Analysis of thymidine kinase gene expression in preimplantation mouse embryos

Thymidine kinase (TK) activity was examined during the development of preimplantation mouse embryos. TK activity was increased approximately 20-fold from day 2 embryos (2-cell) to day 5 embryos (late blastocyst). TK activity did not change along with the progression into S-phase of the first and the second cell cycles but increased sharply at S-phase of the third cell cycle. Analysis of TK mRNA with a reverse transcription-polymerase chain reaction (RT-PCR) method showed that the level of TK mRNA was low in ovulated eggs and 1-cell embryos and was hardly detectable in day 2 embryos (2-cell), but sharply increased in day 3 embryos (mixture of 5- to 8-cell and morula). The functional role of 5'-flanking sequence of TK gene was also investigated in preimplantation embryos after microinjection with the DNA construct of 5'-flanking sequence of TK (2.4 kb) linked to bacterial lacZ gene (TK2.5lacZ) into the pronucleus of 1-cell and subsequently by histochemical staining with X-gal. beta-Galactosidase activity was first detected in day 3 embryos (8-cell), and 30% of embryos were stained with X-gal in day 4 and day 5 embryos, respectively. These results show that an increase in TK activity occurred after 2-cell stage, and this increase was primarily due to the embryonic activation of TK gene expression. Also, it appears that the 5'-flanking sequence of TK may directly regulate the TK gene expression at the transcriptional level during preimplantation murine development.

Difference in allelic expression of genes probably associated with tumor progression in murine fibrosarcomas and cell lines

Allelic expression was examined by single-strand conformation polymorphism analysis in murine fibrosarcomas from inter-subspecific F1 mice between C57BL/6 and MSM. Ten genes encoding p53, mdm2, E-cadherin, 72 kD metalloproteinase and its inhibitor (Timp2), thymidine kinase and four glucose transporters (Gluts) were examined. These genes were chosen because of their probable association with tumor development and progression. In some of the tumors and cell lines, p53, E-cadherin and Glut3 genes showed remarkable differences in allelic expression, one allele being poorly expressed. The allele-specificity persisted in nine cell lines obtained by repeated transplantations from one tumor. These results suggest that expression of some genes is allele-specific in tumor cells and the pattern of specificity is stable. Such a decrease or a loss of expression in one of the alleles may be functionally equivalent to the loss of heterozygosity of the gene, and therefore this may confer malignant properties on tumor cells. It is also suggested that differential expression of two alleles is a common event in tumor cells.

Expression of c-Myc in glucocorticoid-treated fibroblastic cells

Glucocorticoids inhibit proliferation of L929 fibroblastic cells in culture. Inhibition of proliferation is reversible and is not associated with changes in the plating efficiency of the cells. Flow cytometric analysis indicates that glucocorticoid-treated cells exhibit a decrease in the percentage of cells with DNA content > 2 N. Thymidine kinase expression is inhibited as cells with 2 N DNA content accumulate. These observations indicate that glucocorticoids arrest proliferation of L929 cells in the G1 phase of the cell cycle. The abundance of c-Myc mRNA does not decrease in glucocorticoid-treated cells, and c-Myc protein content in dexamethasone-treated cells is approximately the same as that detected in mid-log phase cells. Nuclear run-on transcription of c-Myc is not inhibited by glucocorticoids. These observations indicate that glucocorticoid regulation of fibroblastic cell proliferation does not involve inhibition of c-Myc transcription. Although regulation of c-Myc expression is central to the mechanism whereby glucocorticoids regulate proliferation of lymphoid cells, it is clear that different mechanisms must be involved in glucocorticoid regulation of fibroblastic cell proliferation.

Comparison of common gene mutation tests in mammalian cells in culture: a position paper of the GUM Commission for the Development of Guidelines for Genotoxicity Testing

In gene mutation tests a decision concerning mutations is made on the basis of hereditary functional changes. In terms of the large amount of data available, the most suitable tests for routine testing in mammalian cells in culture are the tests for acquisition of 6-thioguanine resistance in Chinese hamster cells (V79 and CHO) and for acquisition of alpha,alpha,alpha-trifluorothymidine resistance in the mouse lymphoma line L5178Y TK+/- 3.7.2C. The molecular bases, peculiarities, advantages and disadvantages of these systems will be presented. Which system is to be preferred in any particular case depends among other things on the purpose of the study and the extent to which a technically competent performance of these comparatively exacting tests can be guaranteed.

Changes in cell cycle-associated gene expression in a model of impaired liver regeneration

Following partial hepatectomy (PH) there is compensatory regeneration of the remnant liver which eventually restores hepatic mass and function. The response to PH was studied in normal BALB/c and athymic nude mice, a model of impaired liver regeneration. Following PH, nude mice demonstrated diminished peak hepatic [3H]thymidine uptake and delayed liver mass restoration through 60 h post-PH. However, between 72-120 h there was no significant difference in mass restoration between the groups. The expression of genes associated with different stages of the cell cycle was evaluated in both models. In nude mice, there was an increase in peak expression of c-jun transcripts, while c-myc transcript expression was moderately attenuated. Thymidine kinase (TK) and cyclin-dependent kinase 1 (CDK1) mRNA expression was also diminished in athymic nude mice. The results suggest that while the defect in the regenerative response of the nude mouse after PH affects events in several phases of the cell cycle, mass restoration of the liver is only delayed and not attenuated.

Molecular analysis of mutations at the tk locus of L5178Y mouse-lymphoma cells induced by ethyl methanesulphonate and mitomycin C

Mutations at the tk locus of mouse-lymphoma L5178Y cells were induced by treatment with ethyl methanesulphonate (EMS), primarily a point mutagen and mitomycin C (MMC), a potent clastogen. Mutant colony size was distinctly bimodal with 35% of spontaneous mutants growing as small colonies and 65% large. The proportion of small colonies increased only slightly to 41% in EMS-treated cultures but to 64% after MMC treatment. Mutations were analysed by Southern and Northern blotting. Digestion of DNA with the restriction enzyme, Nco I, revealed that many mutants had lost a 6.3-kb fragment which constituted the loss of the entire tk gene. Almost all of the EMS-induced large-colony mutants analysed (9/10) retained the tk+ allele suggesting the presence of an intragenic mutation. Of the small-colony mutants, half (6/12) had lost the tk+ gene and presumably other genes affecting growth and half retained the tk+ allele suggesting point mutations in both the tk gene and other sites in the genome affecting growth. A very different spectrum of mutation was induced with MMC. Only 1/12 of the large-colony mutants were due to intragenic mutation, the remaining large-colony mutants having lost the tk+ allele while all the small-colony mutants had lost the tk+ gene presumably with the deletion extending to genes essential for normal growth. Northern blot analysis showed no changes in the size of tk transcript in any mutants. Alterations in the amount of tk mRNA were not detectable since all mutants produced an mRNA of similar size and amount, which may indicate the production of an abnormal mRNA from the tk- allele. Unlike cell-mutation assays that use hemizygous loci (such as hprt+/0) for detecting potential chemical carcinogens, the mouse-lymphoma tk+/- assay allows the recovery of both intragenic and intergenic mutations thus enabling the detection of both point mutagens such as EMS and potent clastogens like MMC.

Sequence analysis of tka(-)-1 and tkb(+)-1 alleles in L5178Y tk+/- mouse-lymphoma cells and spontaneous tk-/- mutants

The mouse-lymphoma mutagenesis assay detects forward mutations at the heterozygous thymidine kinase (tk-1) locus in L5178Y tk+/- 3.7.2C cells. This assay of genotoxicity is widely used to quantitate the mutagenic potential of a variety of chemical and physical agents. A NcoI heteromorphism between the tka- and tkb+ alleles allows the use of Southern blotting to broadly detect two major categories of mutations. These consist of deletions of the functional allele, characterized by absence of a 6.3-kb tk-hybridizing band, and apparent point mutations, indistinguishable from wild-type on blots. Rarely, Southern blots reveal a partial deletion of tkb. The variety of lesions recorded at the heterozygous tk-1 locus may be representative of events important in mammalian carcinogenesis and may include a greater range of mutagenic events than can be observed at hemizygous test loci. To further assess the ability of the mouse-lymphoma assay to detect a variety of mutations and to allow identification of point mutations, we have sequenced the entire tk-1 coding region from both tka- and tkb+ alleles of L5178Y 3.7.2C mouse-lymphoma cells. Sequences were obtained using PCR amplified double-stranded DNA templates prepared from cytoplasmic RNA from the heterozygous cell line. The two alleles were found to differ by a single TA to GC transversion, altering one amino acid in the deduced amino acid sequence. 4 spontaneous mutants were also sequenced and demonstrated a variety of mutations, including a 6-base pair in-frame deletion, a CG to GC transversion upstream of the start codon, a mutant apparently lacking expression of the tkb allele, and a mutant with apparent wild-type coding sequence for both tka- and tkb+ alleles. The diverse nature of the mutants isolated from L5178Y cells suggests that the mouse-lymphoma mutagenesis assay is capable of detecting a number of mutation types, enhancing the utility of the assay in studying the range of genetic lesions important in human disease. The lesions produced are readily analyzed using a combination of Southern blotting and sequence analysis.

Homology of a 130-kb region enclosing the alpha-globin gene cluster, the alpha-locus controlling region, and two non-globin genes in human and mouse

The human alpha-globin gene cluster (30 kb) is embedded in a GC-rich isochore very close to the telomere of Chromosome (Chr) 16p. The alpha-Locus Controlling Region (alpha-LCR) is located upstream of the adult alpha-globin genes and has been shown to be essential for their expression. In this study we have been looking for expressed genes in the region upstream of the alpha-globin cluster to understand the role of the LCR-like element in the expression and replication timing of flanking gene clusters. We show that the upstream alpha-globin region is conserved over a 75-kb range and includes at least two oppositely transcribed non-globin genes, here referred to as Mid1 and Dist1. Complementary DNA sequences of 250 bp and 2.5 kb from Mid1 (coordinate -68) and Dist1 (coordinate -90 to -99), respectively, were isolated from human and mouse. The deduced partial amino acid sequences of these cDNAs are 81% and 95% identical for the Mid1 and Dist1 gene respectively. We have cloned a mouse cosmid "contig" which includes Dist1, Mid1, and the entire murine alpha-globin cluster. The murine homolog of the alpha-LCR was mapped upstream of the mouse globin genes at approximately the same position as in the human locus. Our results indicate that, in mouse and human, the alpha-globin loci and their flanking sequences are homologous over a range of at least 130 kb. The structural homology of this region in both mammals suggests also a functional one and indicates the mouse as a potential model for studying the role of the alpha-LCR controlling element in the regulation of expression and replication timing of the flanking gene clusters.

In vitro mammalian mutagenesis as a model for genetic lesions in human cancer

Recently in vitro assays of mutagenesis have been criticized as being poorly predictive of long-term in vivo rodent assays of carcinogenicity. Questions have also been raised concerning the relevance of rodent assays to human risk. In vitro assays using mammalian cells can detect most types of genetic lesions thought to be important in human malignant disease. Molecular and cytogenetic analyses of mutations induced by a variety of genotoxic compounds at the heterozygous thymidine kinase locus in mouse lymphoma cells indicate that this in vitro assay does indeed register the range of genetic lesions recently found in a wide variety of human tumors. The types and complexity of the induced lesions are reflected in mutant colony phenotype in a compound-specific fashion. These studies point to the use of appropriate in vitro mammalian mutagenesis assays as new model systems for dissecting the genetic lesions important in human carcinogenesis, and as a means of determining the potential for compounds to induce such lesions.

Characterization of the murine thymidine kinase-encoding gene and analysis of transcription start point heterogeneity

We have determined the molecular organization and transcription start points (tsp) for the murine gene (TK) encoding thymidine kinase. The exon/intron structure and sequences present at the splice junctions of the mammalian TK genes have been highly conserved; however, the promoter sequences of these genes have diverged widely. Both the human and Chinese hamster TK promoter regions contain CCAAT and TATA consensus motifs, whereas the mouse promoter has neither element. This difference between species is reflected in that, unlike the hamster and human TK genes, transcription initiates from numerous specific tsp within a 100-bp region in the mouse TK gene. The complex pattern of tsp seen in the endogenous gene was not maintained in transfected cell lines containing TK promoter::beta-globin (HBB) fusions. Transcription from the murine TK:HBB fusion genes initiated from a small number of tsp that were clustered downstream from the ATG in hybrids containing TK coding sequences, and in the HBB 5' UTR in hybrids that did not. Few or no specific tsp were detected from the upstream sites used in the endogenous mouse TK gene.

Thymidine kinase-deficient mutants of Aedes albopictus mosquito cells

Aedes albopictus mosquito cells resistant to the thymidine analog 5-bromodeoxyuridine (BrdU) were obtained using a single-step selection procedure. The resistant cells were characterized with respect to growth in the presence of BrdU, incorporation of [3H]uridine and [3H]thymidine, and thymidine kinase activity in crude extracts. The LC50 for TK-6 cells was 95 micrograms BrdU/ml, and for TK-8 cells was 45 micrograms/ml. Both clones incorporated [3H]uridine at levels corresponding to those in wild-type cells. TK-6 and TK-8 cells did not incorporate [3H]thymidine into acid-precipitable material, nor did they contain measurable thymidine kinase activity. Thymidine kinase activity in crude extracts from wild-type cells had a Km of 2 microM and a Vmax of 10 pmol.min-1.mg-1 protein.

Transcription initiation and temporal expression of thymidine kinase mRNA in Chinese hamster cells

The induction of thymidine kinase mRNA has proved to be a valuable model for understanding regulatory events at the G1/S boundary of the cell cycle (1, 2, 3). As an initial step toward characterizing the regulation of this gene in Chinese hamster cells, I have mapped the transcription start sites for TK mRNA in CHEF/18 cells. Two closely spaced sites of transcription initiation were detected downstream of a nonconsensus TATAA element in the promoter region. Using primer extension analyses, I demonstrated that the transcription initiation sites remained constant while the absolute levels of TK mRNA varied during the cell cycle in synchronized populations of Chinese hamster cells.

Mapping and molecular characterization of a functional thymidine kinase from Amsacta moorei entomopoxvirus

A thymidine kinase (TK) gene from the entomopoxvirus of Amsacta moorei (AmEPV) has been identified, mapped, cloned, and sequenced. The AmEPV TK was shown to be biologically functional as cloning of the gene into a TK-derivative of the orthopoxvirus vaccinia creates a TK+ virus. The gene has been localized to a 1.5-kb EcoRI-Q DNA fragment which maps to the far left end of the viral genome. Sequence analysis reveals an open reading frame (ORF) of 182 amino acids potentially encoding a polypeptide of 21.2 kDa. Amino acid homology comparisons indicate that the gene is most closely related to the TKs of a variety of poxviruses (approximately 45%) and less so to the TKs of vertebrates (approximately 40%). The TK from African swine fever virus (ASF) showed the least homology (31.4%) to the AmEPV TK gene, suggesting that these two viruses are not closely related although ASF shares some biological features of poxviruses, and both ASF and AmEPV can replicate within arthropod hosts.

The ?-subunit of the skeletal muscle sodium channel is encoded proximal to Tk-1 on mouse Chromosome 11

Recent evidence suggests that the human neuromuscular disorders, hyperkalemic periodic paralysis and paramyotonia congenita, are both caused by genetic defects in the alpha-subunit of the adult skeletal muscle sodium channel, which maps near the growth hormone cluster (GH) on Chromosome (Chr) 17q. In view of the extensive homology between this human chromosome and mouse Chr 11, we typed an interspecies backcross to determine whether the murine homolog (Scn4a) of this sodium channel gene mapped within the conserved chromosomal segment. The cytosolic thymidine kinase gene, Tk-1, was also positioned on the genetic map of Chr 11. Both Scn4a and Tk-1 showed clear linkage to mouse Chr 11 loci previously typed in this backcross, yielding the map order: TrJ-(Re, Hox-2, Krt-1)-Scn4a-Tk-1. No mouse mutant that could be considered a model of either hyperkalemic periodic paralysis or paramyotonia congenita has been mapped to the appropriate region of mouse Chr 11. These data incorporate an additional locus into the already considerable degree of homology observed for these human and mouse chromosomes. These data are also consistent with the view that the conserved segment region may extend to the telomere on mouse Chr 11 and on human 17q.

Deoxynucleoside phosphorylating enzymes in monkey and human tissues show great similarities, while mouse deoxycytidine kinase has a different substrate specificity

Three key enzymes in the anabolic phosphorylation of deoxyribonucleosides and deoxyribonucleoside analogs were purified i.e. cytoplasmic thymidine kinase (TK1), mitochondrial thymidine kinase (TK2) and cytoplasmic deoxycytidine kinase (dCK) from human, mouse and monkey liver and spleen. Their subunit structure and substrate specificities were compared. Extensive purification of TK1 and dCK from mouse spleen and TK2 from mouse and monkey livers revealed major polypeptide bands of 25, 30 and 28 kD, respectively, on sodium dodecyl sulphate-polyacrylamide gel electrophoresis which are very similar to the subunit molecular weights of the corresponding human enzymes. Affinity purified polyclonal antibodies against human dCK also cross-reacted with 30 kD bands in extracts from both mouse and monkey spleen. Thus, the molecular weights of the subunits of these three enzymes appeared to be very similar in all three species. TK1 and TK2 from these different sources appeared to have similar substrate specificities against several deoxyribonucleoside analogs. However, mouse dCK differed significantly from monkey and human dCK in its capacity to phosphorylate dAdo and 2',3'-dideoxycytidine (ddCyd) with a Vmax approximately 10-fold lower than that of the two latter enzymes. The Km and Vmax values for dCyd and arabinocytosine appeared to be very similar with the enzymes from all three species. The fact that mouse dCK shows low activity with dAdo and ddCyd explains differences reported previously in the metabolism of dAdo and ddCyd in mouse compared to that in human lymphocytes. These results argue against the use of mice as model systems for human deoxynucleoside metabolism.

Purification and characterization of thymidine kinase from regenerating rat liver

Thymidine kinase (EC 2.7.1.21) from regenerating rat liver has been purified 70,000-fold to apparent homogeneity by affinity chromatography. Molecular weight of the native enzyme was found to be about 54,000, as determined by gel filtration. Electrophoresis in polyacrylamide gels containing sodium dodecyl sulfate yielded a single band with a molecular weight of 26,000, suggesting that thymidine kinase is a dimer of very similar or identical subunits. The Michaelis constant for thymidine is 2.2 microM. ATP acts as a sigmoidal substrate with a 'Km' of 0.2 mM. Reaction kinetics and product inhibition studies reveal the enzymatic mechanism to be sequential.

Isolation and molecular characterization of the swinepox virus thymidine kinase gene

Swinepox virus (SPV), the only member of the Suipoxvirus genus, shows little antigenic relatedness or DNA homology to members of the other poxvirus genera. A SPV thymidine kinase (TK) gene was detected and mapped to the left end of the HindIII G fragment using degenerate oligonucleotide probes. Cloning and sequencing of a 1.8-kb HindIII-BamHI fragment containing the SPV TK gene revealed an open reading frame (ORF) of 181 amino acids yielding a predicted polypeptide of Mr 20.6 kDa with significant homology to both poxvirus and vertebrate thymidine kinases. Comparison with other TK protein sequences showed that the SPV thymidine kinase was closely related to the TK genes of avipoxviruses (52.0%) and vertebrates (57.1-59.7%). The TK gene from African swine fever virus (ASF) showed little homology (30.5%) to the SPV TK gene suggesting that these two viruses are not closely related though they share many biochemical features and infect a single, common mammalian host (swine). The SPV TK gene, like that of other poxviruses, is transcribed early, and when cloned into a TK- strain of vaccinia converted the virus to a TK+ phenotype. BUdRR mutants of SPV contained frameshift, deletion, and missense mutations in the TK ORF.

Localization of the mouse thymidine kinase gene to the distal portion of chromosome 11

We report the cytogenetic mapping of the thymidine kinase (tk-1) gene in the mouse using two complementary and independent analyses: (1) investigation of chromosome aberrations associated with tk-1 gene inactivation in the L5178Y TK+/- -3.7.2C cell line, and (2) fluorescence in situ molecular hybridization of cloned tk-1 cDNA probes to mitotic chromosomes of this cell line. The consensus location from both analyses is 11E1-E2. Consideration of the mouse tk-1 gene localization, along with evidence that the homologous human TK1 gene is located distally on the large arm of chromosome 17, appears to extend the region of homology between MMU11 and HSA17 to the distal end of both chromosomes.

A strain-specific modifier on mouse chromosome 4 controls the methylation of independent transgene loci

A transgene, pHRD, is highly methylated in 12 independent mouse lines when in a C57BL/6 strain background, but becomes progressively less methylated when bred into a DBA/2 background. Transgenes inherited from the mother are generally more methylated; however, this parental effect disappears following continued breeding into the nonmethylating strain. Mapping experiments using BXD recombinant inbred mice as well as other inbred strains indicate that a single strain-specific modifier (Ssm-1) linked to, but distinct from, Fv-1 is responsible for the strain effect. In addition to the methylated and unmethylated transgenic phenotypes, certain mice exhibit a partial methylation pattern that is a consequence of an unusual cellular mosaicism. The pHRD transgene, containing target sequences for the V(D)J recombinase, undergoes site-specific recombination only in lymphoid tissues. This V-J joining is restricted primarily to unmethylated transgene copies.

The primary structure of the thymidine kinase gene of fish lymphocystis disease virus

The DNA nucleotide sequence of the thymidine kinase (TK) gene of fish lymphocystis disease virus (FLDV) which has been localized between the coordinates 0.678 to 0.688 of the viral genome was determined. The analysis of the DNA nucleotide sequence located between the recognition sites of HindIII (0.669 map unit; nucleotide position 1) and AccI (nucleotide position 2032) revealed the presence of an open reading frame of 954 bp on the lower strand of this region between nucleotide positions 1868 (ATG) and 915 (TAA). It encodes for a protein of 318 amino acid residues. The evolutionary relationships of the TK gene of FLDV to the other known TK genes was investigated using the method of progressive sequence alignment. These analyses revealed a high degree of diversity between the protein sequence of FLDV TK gene and the amino acid composition of other TKs tested. However, significant conservations were detected at several regions of amino acid residues of the FLDV TK protein when compared to the amino acid sequence of TKs of African swine fever virus, fowlpox virus, shope fibroma virus, and vaccinia virus and to the amino acid sequences of the cellular cytoplasmic TK of chicken, mouse, and man.

Nucleotide sequence of the Escherichia coli thymidine kinase gene provides evidence for conservation of functional domains and quaternary structure

Using lambda bacteriophage clones from the Kohara Escherichia coli library spanning minutes 25.5 to 28.5 on the E. coli chromosome (strain W3110), two overlapping DNA fragments were identified which were able to confer thymidine kinase (TK) enzyme activity to a TK- strain of E. coli (KY895). This genetic complementation assay was used in concert with subcloning procedures to identify the minimal region (a 900 bp EcoRI-SalI fragment) which contained the E. coli thymidine kinase gene (tdk). The nucleotide sequence of the EcoRI-SalI fragment and a small portion of the adjoining downstream fragment was determined. Computer analysis of the derived sequence indicated the presence of a rightward-reading open reading frame of 615 bp which was capable of encoding a 205-amino-acid polypeptide with a predicted Mr of 23458 daltons. The in vivo transcriptional activity of this locus was confirmed by Northern blot hybridization analysis of RNA isolated from E. coli JM101 or KY895 which detected a 650-nucleotide RNA transcribed from this region. This places the tdk gene at approximately minute 27.35 on the E. coli W3110 chromosome, about 15 kb downstream from the narG locus and approximately 25 kb upstream of the trp operon. Although the predicted Mr of the E. coli TK protein was 23.5 kDa, gel-filtration analyses suggested that, like eukaryotic thymidine kinases, the active form of this enzyme is a multimeric complex.(ABSTRACT TRUNCATED AT 250 WORDS)

Expression and characterization of the thymidine kinase gene of African swine fever virus

The thymidine kinase (TK) gene of African swine fever virus (ASFV) was located within the viral genome by using two degenerate oligonucleotide probes derived from sequences of the vaccinia virus and cellular TK genes. The TK gene was mapped within a 0.72-kbp BglII-XhoI fragment (0.242 to 0.246 map units) derived from a 23.9-kbp SalI-B fragment of the ASFV genome. Identification of this region as the ASFV TK gene was confirmed by expression of TK in Escherichia coli and by the synthesis of active TK in a cell-free system programmed with RNA synthesized in vitro. The sequenced gene for TK includes an open reading frame of 588 nucleotides encoding a protein of 196 amino acids. The deduced amino acid sequence shows 32.4% identity with the TK of vaccinia virus.

Linkage of acid alpha-glucosidase (Gaa) and thymidine kinase (Tk-1) to esterase-3 (Es-3) on mouse chromosome 11

Inheritance in recombinant inbred (RI) strains of restriction fragment length variants (RFLVs) detected by probes specific for Gaa and Tk-1 showed tight linkage of both to Es-3 on mouse Chromosome (Chr) 11. This result extends the region of homology between mouse Chr 11 and human chr 17q.

Suppression of programmed death and G1 arrest in B-cell hybridomas by interleukin-6 is not accompanied by altered expression of immediate early response genes

The murine B-cell hybridoma B9 requires interleukin-6 (IL-6) for its survival and proliferation in vitro. We show here that withdrawal of IL-6 from B9 cultures results in programmed death, concomitant with arrest of the cells in the G1 phase of the cell cycle. Unlike several other systems that undergo programmed cell death, no induction of transcripts corresponding to the testosterone-repressed message-2 or transglutaminase genes is observed during this process. Upon readdition of IL-6 to G1-arrested B9 cells, viability is maintained and entry into S phase occurs after a lag period of 10 to 12 hr. Northern blot analysis showed that the immediate-early mRNAs normally induced shortly after growth factor stimulation in quiescent fibroblasts (c-fos, c-jun, Egr-1, c-myc, JE, and KC), and other growth-related genes (2F1, c-Ha-ras, and p53), are either not induced or remain unchanged during G1 to S phase progression. A correlation was found, however, between the temporal pattern of expression of several G1/S phase genes (dihydrofolate reductase, thymidine kinase, transferrin receptor, and histone H3) and DNA synthesis. These results demonstrate that IL-6-induced viability and growth of hybridoma (and, presumably, plasmacytoma) cells is mediated via novel signal transduction pathways.

Sequence and evolutionary relationships of African swine fever virus thymidine kinase

The thymidine kinase gene of African swine fever virus was mapped in a 1.4-kb EcoRI-PstI fragment located in the left half of the Eco RI K fragment of African swine fever virus DNA by using degenerate oligonucleotide probes derived from regions of the thymidine kinase sequence conserved in several poxviruses, man, mouse, and chicken. The nucleotide sequence of this region revealed an open reading frame of 196 codons, whose translated amino acid sequence showed significant similarity to the thymidine kinases of vaccinia virus, variola virus, monkeypox virus, shope fibroma virus, fowlpox virus, capripox virus, man, mouse, and chicken. The similarity scores obtained after comparison of known thymidine kinase sequences indicated that the African swine fever virus thymidine kinase is more distantly related than the poxvirus thymidine kinases to their cellular homologs. The evolutionary implications of these findings are discussed.