Overexpression of human thymidine kinase mRNA without corresponding enzymatic activity in patients with chronic lymphatic leukemia

Self-assembly of DNA-gold nanoaggregate for visual detection of thymidine kinase 1 (TK1) mRNA via lateral flow assay

Nucleic acid lateral flow assay (NALFA) with gold nanoparticles (AuNPs) as colorimetric probes have been extensively adopted for point-of-care testing (POCT). However, the sensitivity of NALFA still needs to be improved. Herein, DNA-gold nanoaggregate (DNA-AuNA) was assembled as a signal amplification probe of NALFA for sensitive detection of tumor marker TK1 mRNA. Four functional oligonucleotides with complementary pairs were assembled to form DNA-AuNA that coupled more AuNPs to improve sensitivity. Thus, the limit of detection (LOD) was 0.36 pM, which is lower than that of conventional AuNPs-based NALFA. Moreover, the bioassay showed good reproducibility, stability, and specificity for detecting TK1 mRNA. The detection of TK1 mRNA in human serum was also satisfactory. Therefore, DNA-AuNA-based NALFA provides a sensitive method for portable detection of TK1 mRNA.

Evaluation of gene expression in peripheral blood cells as a potential biomarker for enzootic bovine leukosis

Expression of six selective genes in peripheral blood cells was evaluated as diagnostic biomarkers for enzootic bovine leukosis (EBL) by using 10 EBL and 15 clinically healthy cattle. The clinically healthy cattle generally showed lower gene expression levels. Although wide variations of gene expression were found in some clinical cases of EBL, 4 and 5 among 10 EBL cattle showed higher expression of interleukin 2 receptor gene (IL2R) and Wilms' tumor gene (WT1), respectively. Expression of IL2R in peripheral blood cells in EBL cattle was statistically increased; however, the lower sensitivity and higher variation in the gene expressions among clinical cases of EBL would be problems as diagnostic biomarkers.

The role of thymidine kinase in cancer diseases

Thymidine kinase 1 (TK 1-fetal) is a cell cycle-dependent marker that increases dramatically during the S-phase of the cell cycle. In this review, the authors discuss serum levels of thymidine kinase in a variety of neoplasias. Determination of thymidine kinase helps to monitor the follow-up of solid tumours and haematological malignancies as well as indicating the efficacy of adjuvant and palliative chemotherapy. Elevated levels of thymidine kinase must always be interpreted together with a detailed knowledge of the patient's condition because nonspecific elevations of serum levels (inflammatory and autoimmune diseases) must be excluded.

Calf form bovine leukosis with lameness in a Holstein heifer

A 12-month-old Holstein heifer with anorexia, lameness, and enlargement of peripheral lymph nodes was suspected of having bovine leukosis. Although lymphocytosis was not observed, cytology of fine needle aspirate from a superficial cervical node, and increased serum lactate dehydrogenase and thymidine kinase activities, strongly suggested lymphosarcoma. Increased numbers of mononuclear cells as well as mitotic cells were observed in synovial fluid collected from swollen joints. Pathological examination confirmed B-cell calf form bovine leukosis and joint swelling related to neoplastic cell infiltration. Both interleukin-2 receptor and thymidine kinase 1 genes were highly expressed in cells from superficial cervical lymph node aspirate.

Overexpression of interleukin 2 receptor, thymidine kinase and immunoglobulin-associated alpha-1 messenger RNA in a clinical case of enzootic bovine leukosis

A 49-month-old Holstein cow with anorexia, tachypnea, enlarged peripheral lymph nodes, and difficulty standing up was suspected of bovine leukosis. Hematological examination revealed lymphocytosis with the presence of neoplastic cells. Increased total lactate dehydrogenase (LDH) activity, isozymes of LDH-2 and LDH-3 activities and thymidine kinase activity were observed. Cytological findings of fine needle aspiration of subiliac lymph nodes indicated lymphosarcoma. Histopathology and antibody analysis confirmed the diagnosis of enzootic bovine leukosis, a B-cell bovine lymphoma caused by bovine leukemia virus. Gene expressions known as biomarkers of hematopoietic neoplasia in human were also examined in the present case. Increased messenger RNA expression of interleukin 2 receptor, thymidine kinase, and immunoglobulin-associated alpha-1 was observed in the case animal.

Targeted endoradiotherapy using nucleotides

Increased cellular proliferation is an integral part of the cancer phenotype. Hence, the sustained and continued demand on supply of DNA building blocks during the DNA replication presents a potential target for therapeutic intervention. For this propose, the α and Auger electron emitting nucleotides analogs are attractive for targeted endoradiotherapy, given that DNA of malignant cells is selectively addressed. This review summarizes development and preclinical and clinical studies of endoradiotherapeutic acting nucleoside analogs with a special focus on thymidine analogs.

Tumor 3'-deoxy-3'-(18)F-fluorothymidine ((18)F-FLT) uptake by PET correlates with thymidine kinase 1 expression: static and kinetic analysis of (18)F-FLT PET studies in lung tumors

We report the first, to our knowledge, findings describing the relationships between both static and dynamic analysis parameters of 3'-deoxy-3'-(18)F-fluorothymidine ((18)F-FLT) PET and the expression of the proliferation marker Ki-67, and the protein expression and enzymatic activity of thymidine kinase-1 (TK1) in surgically resected lung lesions.

METHODS

Static and dynamic analyses (4 rate constants and 2 compartments) of (18)F-FLT PET images were performed in a cohort of 25 prospectively accrued, clinically suspected lung cancer patients before surgical resection (1 lesion was found to be benign after surgery). The maximal and overall averaged expression of Ki-67 and TK1 were determined by semiquantitative analysis of immunohistochemical staining. TK1 enzymatic activity was determined by in vitro assay of extracts prepared from flash-frozen samples of the same tumors.

RESULTS

Static (18)F-FLT uptake (partial-volume-corrected maximum-pixel standardized uptake value from 60- to 90-min summed dynamic data) was significantly correlated with the overall (ρ = 0.57, P = 0.006) and maximal (ρ = 0.69, P < 0.001) immunohistochemical expressions of Ki-67 and TK1 (overall expression: ρ = 0.65, P = 0.001; maximal expression: ρ = 0.68, P < 0.001) but not with TK1 enzymatic activity (ρ = 0.34, P = 0.146). TK1 activity was significantly correlated with TK1 protein expression only when immunohistochemistry was scored for maximal expression (ρ = 0.52, P = 0.029). Dynamic analysis of (18)F-FLT PET revealed correlations between the flux constant (K(FLT)) and both overall (ρ = 0.53, P = 0.014) and maximal (ρ = 0.50, P = 0.020) TK1 protein expression. K(FLT) was also associated with both overall (ρ = 0.59, P = 0.005) and maximal (ρ = 0.63, P = 0.002) Ki-67 expression. We observed no significant correlations between TK1 enzyme activity and K(FLT). In addition, no significant relationships were found between TK1 expression, TK1 activity, or Ki-67 expression and any of the compartmental rate constants.

CONCLUSION

The absence of observable correlations of the imaging parameters with TK1 activity suggests that (18)F-FLT uptake and retention within cells may be complicated by a variety of still undetermined factors in addition to TK1 enzymatic activity.

Expression of deoxynucleoside kinases and 5'-nucleotidases in mouse tissues: implications for mitochondrial toxicity

Anti-HIV nucleoside therapy can result in mitochondrial toxicity affecting muscles, peripheral nerves, pancreas and adipose tissue. The cytosolic deoxycytidine kinase (dCK; EC 2.7.1.74) and thymidine kinase (TK1; EC 2.7.1.21), the mitochondrial thymidine kinase (TK2) and deoxyguanosine kinase (dGK; EC 2.7.1.113) as well as 5'-deoxynucleotidases (5'-dNT; EC 3.1.3.5) are enzymes that control rate-limiting steps in formation of intracellular and intra-mitochondrial nucleotides. The mRNA levels and activities of these enzymes were determined in mouse tissues, using real-time PCR and selective enzyme assays. The expression of mRNA for all these enzymes and the mitochondrial deoxynucleotide carrier was detected in all tissues with a 5-10-fold variation. TK1 activities were only clearly detected in spleen and testis, while TK2, dGK and dCK activities were found in all tissues. dGK activities were higher than any other dNK in all tissues, except spleen and testis. In skeletal muscle dGK activity was 5-fold lower, TK2 and dCK levels were 10-fold lower as compared with other tissues. The variation in 5'-dNT activities was about eight-fold with the highest levels in brain and lowest in brown fat. Thus, the salvage of deoxynucleosides in muscles is 5-10-fold lower as compared to other non-proliferating tissues and 100-fold lower compared to spleen. These results may help to explain tissue specific toxicity observed with nucleoside analogs used in HIV treatment as well as symptoms in inherited mitochondrial TK2 deficiencies.

The uptake of 3?-deoxy-3?-[18F]fluorothymidine into L5178Y tumours in vivo is dependent on thymidine kinase 1 protein levels

PURPOSE

The aim of this study was to investigate the role of thymidine kinase 1 (TK1) protein in 3'-deoxy-3'-[18F]fluorothymidine ([18F]FLT) positron emission tomography (PET) studies.

METHODS

We investigated the in vivo kinetics of [18F]FLT in TK1+/- and TK1-/- L5178Y mouse lymphoma tumours that express different levels of TK1 protein.

RESULTS

[18F]FLT-derived radioactivity, measured by a dedicated small animal PET scanner, increased within the tumours over 60 min. The area under the normalised tumour time-activity curve were significantly higher for the TK1+/- compared with the -/- variant (0.89+/-0.02 vs 0.79+/-0.03 MBq ml(-1) min, P=0.043; n=5 for each tumour type). Ex vivo gamma counting of tissues excised at 60 min p.i. (n=8) also revealed significantly higher tumour [18F]FLT uptake for the TK1+/- variant (6.2+/-0.6 vs 4.6+/-0.4%ID g(-1), P=0.018). The observed differences between the cell lines with respect to [18F]FLT uptake were in keeping with a 48% higher TK1 protein in the TK1+/- tumours versus the -/- variant (P=0.043). On average, there were no differences in ATP levels between the two tumour variants (P=1.00). A positive correlation between [18F]FLT accumulation and TK1 protein levels (r=0.68, P=0.046) was seen. Normalisation of the data for ATP content further improved the correlation (r=0.86, P=0.003).

CONCLUSION

This study shows that in vivo [18F]FLT kinetics depend on TK1 protein expression. ATP may be important in realising this effect. Thus, [18F]FLT-PET has the potential to yield specific information on tumour proliferation in diagnostic imaging and therapy monitoring.

Effect of valine 106 on structure-function relation of cytosolic human thymidine kinase

Information on the regulation and structure-function relation of enzymes involved in DNA precursor synthesis is pivotal, as defects in several of these enzymes have been found to cause depletion or deletion of mitochondrial DNA resulting in severe diseases. Here, the effect of amino acid 106 on the enzymatic properties of the cell-cycle-regulated human cytosolic thymidine kinase 1 (TK1) is investigated. On the basis of the previously observed profound differences between recombinant TK1 with Val106 (V106WT) and Met106 (V106M) in catalytic activity and oligomerization pattern, we designed and characterized nine mutants of amino acid 106 differing in size, conformation and polarity. According to their oligomerization pattern and thymidine kinetics, the TK1 mutants can be divided into two groups. Group I (V106A, V106I and V106T) behaves like V106WT, in that pre-assay exposure to ATP induces reversible transition from a dimer with low catalytic activity to a tetramer with high catalytic activity. Group II (V106G, V106H, V106K, V106L and V106Q) behaves like V106M in that they are permanently high activity tetramers, irrespective of ATP exposure. We conclude that size and conformation of amino acid 106 are more important than polarity for the catalytic activity and oligomerization of TK1. The role of amino acid 106 and the sequence surrounding it for dimer-tetramer transition was confirmed by cloning the putative interface fragment of human TK1 and investigating its oligomerization pattern.

Valine, not methionine, is amino acid 106 in human cytosolic thymidine kinase (TK1). Impact on oligomerization, stability, and kinetic properties

Cytosolic thymidine kinase (TK1) cDNA from human lymphocytes was cloned, expressed in Escherichia coli, purified, and characterized with respect to the ATP effect on thymidine affinity and oligomerization. Sequence analysis of this lymphocyte TK1 cDNA and 21 other cDNAs or genomic TK1 DNAs from healthy cells or leukemic or transformed cell lines revealed a valine at amino acid position 106. The TK1 sequence in NCBI GenBank(TM) has methionine at this position. The recombinant lymphocyte TK1(Val-106) (rLy-TK1(Val-106)) has the same enzymatic and oligomerization properties as endogenous human lymphocyte TK1 (Ly-TK1); ATP exposure induces an enzyme concentration-dependent reversible transition from a dimer to a tetramer with 20-30-fold higher thymidine affinity (K(m) about 15 and 0.5 microm, respectively). Substitution of Val-106 with methionine to give rLy-TK1(Met-106) results in a permanent tetramer with the high thymidine affinity (K(m) about 0.5 microm), even without ATP exposure. Furthermore, rLy-TK1(Met-106) is considerably less stable than rLy-TK1(Val-106) (t(12) at 15 degrees C is 41 and 392 min, respectively). Because valine with high probability is the naturally occurring amino acid at position 106 in human TK1 and because this position has high impact on the enzyme properties, the Val-106 form should be used in future investigations of recombinant human TK1.

Mammalian deoxyribonucleoside kinases

The mammalian deoxyribonucleoside kinases are deoxycytidine kinase, thymidine kinase 1 and 2 and deoxyguanosine kinase. These enzymes phosphorylate deoxyribonucleosides and thereby provide an alternative to de novo synthesis of DNA precursors. Their activities are essential for the activation of several chemotherapeutically important nucleoside analogues. In recent years, these enzymes have been thoroughly characterised with regard to structure, substrate specificity and patterns of expression. In this review, these results are reviewed and furthermore, the physiologic metabolic role of the anabolic enzymes is discussed in relation to catabolic pathways. The significance of this information for the development of therapeutic protocols and choice of animal model systems is discussed. Finally, alternative pathways for nucleoside analogue phosphorylation are surveyed, such as the phosphotransfer capacity of 5'-nucleotidase.

Human thymidine kinase 1. Regulation in normal and malignant cells

In mammalian cells, salvage pathway phosphorylation of thymidine is catalyzed by two thymidine kinases: the cell-cycle regulated cytoplasmic TK1 and the constitutively expressed mitochondrial TK2. Since TK1 is virtually absent in non-dividing cells, TK2 is probably the only thymidine kinase present in these cells. In cellular metabolism, TK1 and TK2 presumably serve to maintain sufficient dTTP for DNA replication and repair. TK1 purified from phytohemagglutinin-stimulated human lymphocytes is a dimer in the absence and a tetramer in the presence of ATP. In addition to the molecular weight transition, incubation with ATP at 4 degrees C or storage with ATP induces a reversible, enzyme concentration-dependent, kinetically slow transition from a low to a high affinity form of TK1, with Km values of 14 microM and 0.5 microM, respectively. This affinity difference implies that at cellular thymidine concentrations, the difference in catalytic activity between the two TK1 forms will be 3-5-fold. Calculations of cellular TK1 concentration suggested that the low affinity dimer form was dominant in G0/G1 cells and the high affinity tetramer form in S-phase cells. Hence, the transition may serve to fine-tune the cell-cycle regulation of thymidine kinase activity on the post-translational level. To study the ATP effect on the molecular level, an IPTG inducible T7 RNA polymerase-dependent expression system for the entire human TK1 polypeptide in E. coli was established. The recombinant TK1 has the same subunit mass and specific activity as the native enzyme. However, the recombinant TK1 solely displayed the kinetics of the high affinity form, with Km values of 0.3-0.4 microM regardless of pre-exposure to ATP, indicating that the ATP effect may be dependent on post-translational modifications absent in E. coli. Surprisingly, we did not observe any effect of ATP on TK1 purified from bone-marrow cells from a patient with acute monocytic leukemia (AMOL). Furthermore, the Km values of TK1 from these cells were 45 microM for the ATP-free enzyme and 65 microM for the ATP-incubated enzyme. With TK1 purified from HL-60 cells, we obtained the same pattern and kinetic values as for TK1 from lymphocytes. In the light of the results with the recombinant TK1, we presume that the lack of ATP effect and very high Km values observed for the AMOL TK1 may be due to changes in post-translational regulatory mechanisms in acute monocytic cells.