Disease relevance of TK2

• Here we identify two mutations in TK2, histidine 90 to asparagine and isoleucine 181 to asparagine, in four individuals who developed devastating myopathy and depletion of muscular mitochondrial DNA in infancy [1].
• Mitochondrial myopathy of childhood associated with mitochondrial DNA depletion and a homozygous mutation (T77M) in the TK2 gene [2].
• CONCLUSION: The clinical spectrum of TK2 mutations is not limited to severe infantile myopathy with motor regression and early death but includes spinal muscular atrophy type 3-like presentation, rigid spine syndrome, and subacute myopathy without motor regression and with longer survival [3].
• The protein was expressed in Escherichia coli and shown to have similar substrate specificity as reported for purified native human TK2 [4].
• TK2 activity occurred in peripheral blood lymphocytes of normal individuals, patients with chronic lymphocytic leukemia, or solid lymphoid tissue, exhibiting either nonneoplastic histological findings or those of diffuse well-differentiated lymphocytic lymphoma [5].

High impact information on TK2

• The main supply of deoxyribonucleotides (dNTPs) for mtDNA synthesis comes from the salvage pathway initiated by dGK and thymidine kinase-2 (TK2) [6].
• Two of the four human deoxyribonucleoside kinases, deoxyguanosine kinase (dGK) and thymidine kinase-2 (TK2), are expressed in mitochondria [1].
• Human dGK efficiently phosphorylates deoxyguanosine and deoxyadenosine, whereas TK2 phosphorylates deoxythymidine, deoxycytidine and deoxyuridine [1].
• In these individuals, the activity of TK2 in muscle mitochondria is reduced to 14-45% of the mean value in healthy control individuals [1].
• RESULTS: Total TK (TK1 plus TK2) levels in tumors were significantly (P < .001) elevated in patients who subsequently had recurrence compared with levels in those who did not [7].

Chemical compound and disease context of TK2

• RECENT FINDINGS: The most recently described mitochondrial myopathies are due to defects in nuclear DNA, including coenzyme Q10 deficiency and mutations in genes controlling mitochondrial DNA abundance and structure, such as POLG, TK2, and MPV17 [8].
• In this study, hTK2 was cloned and expressed at high levels in Escherichia coli as a fusion protein with maltose-binding protein [9].

Biological context of TK2

• Patient A was a compound heterozygote carrying the previously reported T77M mutation and a novel mutation (R161K) in the TK2 gene [10].
• We identified a sequence-tagged site designed from the 3' region of the TK2 cDNA [4].
• The consequences of the two point mutations of TK2 and the role of TK2 in mt disorders are discussed [11].
• The pathogenicity of these mutations was confirmed by reduced TK2 activity in muscle (28% to 37% of controls) [12].
• This report extends the phenotypic expression of primary TK2 deficiency and suggests that factors other than TK2 may modify expression of the clinical phenotype in patients with MDS syndrome [13].

Anatomical context of TK2

• There were no detectable difference in the molecular size of photolabeled TK2 isolated from human spleen, brain or placenta, monkey liver, beef heart and beef heart mitochondria [14].
• The biological activity of a representative compound, 2'-O-decanoyl-BVaraU, was also investigated in normal human fibroblasts and was found to impair mitochondrial function due to TK2 inhibition [15].
• The substrate specificity of TK in macrophages strongly suggests that mitochondrial TK2 was the enzyme phosphorylating thymidine and AZT in these cells, whereas it was cytosolic TK1 in stimulated lymphocytes [16].
• The lack of catabolism together with phosphorylation by TK2 clarifies how AZT can inhibit human immunodeficiency virus in macrophages [16].
• To determine whether kinase (TK) isozyme status adds clinically useful information in adult non-Hodgkin's lymphoma (NHL), we have analyzed peripheral blood plasma and lymphocytes of 44 patients with NHL for either TK1 or TK2 isozyme activity [17].

Associations of TK2 with chemical compounds

• Sequencing of the TK2 gene showed a homozygous C-->T transition at nucleotide 228 in exon 5, which changes a threonine to a methionine at position 77 (T77M) [2].
• The H121N mutant enzyme had normal K(m) values for thymidine (dThd) and deoxycytidine (dCyd), 6 and 11 microm, respectively, but 2- and 3-fold lower V(max) values as compared with wild type TK2 and markedly increased K(m) values for ATP, leading to decreased enzyme efficiency [11].
• Thymidine kinase 2 (TK2) is a mitochondrial (mt) pyrimidine deoxynucleoside salvage enzyme involved in mtDNA precursor synthesis [11].
• The recombinant TK2 was shown to phosphorylate the anti-cancer nucleoside analog 2',2'-difluorodeoxycytidine [4].
• Cleavage of labeled TK2 with cyanogen bromide showed that dTTP was incorporated into a single 3-kDa peptide [14].

Regulatory relationships of TK2

• 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 [18].

Other interactions of TK2

• The activities of TK1 and TK2 had returned to approximate control levels 24 h after irradiation [19].
• Our data show that deoxycytidine kinase, deoxyguanosine kinase, and TK2 belong to a family of closely related enzymes [4].
• The localization of four genes (CHEL3, TK2, TRG1, and MMP9) reported to map to chromosome 16 could not be confirmed, and for three of these localizations to other human chromosomes are reported [20].
• The two thymidine (dThd) kinases in human cells, the cytosolic, S-phase-specific TK1 and the mitochondrial, constitutively expressed TK2 were purified to homogeneity as judged from sodium dodecyl sulfate-gel electrophoresis [21].

Analytical, diagnostic and therapeutic context of TK2

• Site-directed mutagenesis was used to introduce the mutations detected in patients A and B into the TK2 and dGK cDNAs, respectively [10].
• TK2 activity was below control levels during the first 4 h after irradiation but rose 3-fold at 8 and 16 h after treatment [19].
• The fact that TK2 and not TK1 phosphorylates AZT in macrophages should have important implications for combination chemotherapy [16].
• RT-PCR analysis revealed that in H9-ddC cells the expression of both TK1 and TK2 mRNA was reduced to 27.1% and 79.4%, respectively [22].
• Further, high resolution isoelectric focusing confirmed the existence of two isoenzymes at pl values of 5.1 and 5.3, representing mitochondrial (TK2) and cytoplasmic (TK1) isoenzymes [23].

References