Alternative titles; symbols
HGNC Approved Gene Symbol: TLK2
Cytogenetic location: 17q23.2 Genomic coordinates (GRCh38) : 17:62,470,914-62,615,481 (from NCBI)
| Location | Phenotype |
Phenotype MIM number |
Inheritance |
Phenotype mapping key |
|---|---|---|---|---|
| 17q23.2 | Intellectual developmental disorder, autosomal dominant 57 | 618050 | Autosomal dominant | 3 |
The Tousled-like kinases, first described in Arabidopsis, are nuclear serine/threonine kinases that are potentially involved in the regulation of chromatin assembly (Sillje et al., 1999).
By screening an expression library for autophosphorylation activity, followed by screening a testis cDNA library, Yamakawa et al. (1997) cloned TLK2, which they called PKU-alpha. The deduced 717-amino acid protein has a 5-domain structure, including an N-terminal nuclear localization signal and a single catalytic domain near the C terminus. TLK2 shares 86% sequence identity with TLK1 (608438) overall, and 94% identity in the catalytic region. Northern blot analysis detected a 3.5-kb transcript expressed primarily in fetal kidney, liver, heart, and skeletal muscle, and in placenta. Immunofluorescence analysis of transfected COS-1 cells localized TLK2 to the nucleus. By PCR amplification of sequences similar to Arabidopsis TSL, followed by screening several cDNA libraries, Sillje et al. (1999) cloned TLK2. The deduced 749-amino acid protein has a calculated molecular mass of 85.3 kD. By RNase protection analysis of RNA isolated from various adult mouse organs, they found that Tlk2 is expressed at a high level in testis, and at lower levels in heart, brain, liver, and other organs. Western blot analysis detected endogenous HeLa cell TLK1 and TLK2 that migrated together at an apparent molecular mass of about 85 kD. Both proteins were also localized to the nucleus and were excluded from nucleoli.
Using myelin basic protein (MBP; 159430), casein (see 115450), and histone H1 (see 142709) as model substrates, Sillje et al. (1999) confirmed kinase activity in TLK2. MBP was readily phosphorylated, and histone was poorly phosphorylated. TLK2 also autophosphorylated on both serine and threonine, and phosphorylation was required for catalytic activity. Activity was also dependent upon asp-590 within the catalytic domain. The authors further found that both TLK1 and TLK2 displayed maximal activity during S phase of the cell cycle. Whereas protein levels were virtually constant throughout the cell cycle, both TLKs appeared to be regulated by cell-cycle-dependent phosphorylation. Inhibition of DNA replication caused a rapid loss of TLK activity, indicating that TLK function is tightly linked to ongoing DNA replication. With use of several human cell lines, Groth et al. (2003) determined that both TLK1 and TLK2 were novel targets of the DNA damage checkpoint. Both TLKs were rapidly inactivated upon exposure to ionizing radiation, and the inactivation was directly mediated by the S-phase DNA damage checkpoint.
Mortuza et al. (2018) reported that the TLK2 polypeptide is a predominantly nuclear protein composed of an N-terminal region harboring a nuclear localization signal, a middle region of helices predicted to contain 3 coiled coils (CC), and a C-terminal kinase domain. The CC domains and C-terminal kinase domain are highly conserved from plants to mammals. TLK2 can be activated with or without an external triggering kinase, and autophosphorylation follows a unimolecular process and occurs in the context of the TLK2 dimer. Mapping of TLK2 phosphosites showed that loops between the predicted CC domains and C-terminal kinase domain contain majority of the phosphosites. Deletion analysis showed that the CC1 domains of TLK2 mediate dimerization and are essential for activation through ordered autophosphorylation that in turn promotes higher order oligomers. Mortuza et al. (2018) hypothesized that the initial phosphorylations in one of the kinase domains in the dimer likely triggers a cascade of conformational changes and makes new sites available, and the new sites would be subsequently targeted by the second catalytic domain, leading to the full activation of the enzyme. The authors demonstrated that several de novo missense predicted loss-of-function mutations identified in patients with intellectual developmental disorder (Lelieveld et al., 2016) impaired TLK2 activity in vitro and showed a more than 50% decrease in autophosphorylation compared with the wildtype enzyme.
Mortuza et al. (2018) presented the crystal structure of the unphosphorylated (preactivated) kinase domain of human TLK2 in complex with ATP-gamma-S at 2.8-angstrom resolution. TLK2 contains an unusual ATP-binding motif, GxGxxS, instead of the canonical GxGxxG motif. Investigation of the docking of several small-molecule inhibitors of TLK activity indicated that the crystal structure may be useful for guiding the rationale design of novel inhibition strategies.
By FISH, Yamakawa et al. (1997) mapped the TLK2 gene to chromosome 17q23.
In 2 unrelated patients with autosomal dominant intellectual developmental disorder-57 (MRD57; 618050), Lelieveld et al. (2016) identified de novo heterozygous mutations in the TLK2 gene (608439.0001 and 608439.0002). The patients were part of a cohort of 820 parent-child trios with intellectual disability who underwent exome sequencing. Functional studies of the variants were not performed, but the mutations were predicted to result in a loss of function and haploinsufficiency. Subsequent addition of data from 4 large previously published family-based sequencing studies identified 3 additional patients with TLK2 mutations. The entire cohort contained 2,104 family trios. Functional studies of the variants and studies of patients cells were not performed.
In 38 patients and 2 of their mothers with MRD57, Reijnders et al. (2018) identified heterozygous mutations in the TLK2 gene (see, e.g., 608439.0001-608439.0005). Five of the patients had previously been reported by Lelieveld et al. (2016). The vast majority of the mutations reported by Reijnders et al. (2018) occurred de novo, but there were 2 families in which the proband inherited a mutation from an affected mothers. Mutation types included 4 frameshift variants, 10 nonsense variants, 12 splice-site variants, and 9 missense variants. One patient carried a de novo balanced translocation. The mutations occurred throughout the gene. None of the missense mutations were found in the ExAC database, and only 1 (R546W) was found in the gnomAD database at a very low frequency. The authors performed RNA analysis of cells derived from 3 patients, which demonstrated that 2 of the mutations were subject to nonsense-mediated mRNA decay (NMD), resulting in haploinsufficiency; the third mutation was a nonsense mutation that escaped NMD, but was predicted to result in a truncated protein and haploinsufficiency. The mutations were found by whole-exome or whole-genome sequencing, and the patients and data were collected from 26 different research institutions in 7 different countries by means of data sharing by collaborators and matchmaker databases. Genotype-phenotype analysis and comparison of computationally modeled faces showed that phenotypes of patients with loss-of-function variants overlapped with phenotypes of individuals with missense and C-terminal truncated mutations, suggesting that that haploinsufficiency of TLK2 is the most likely underlying disease mechanism, leading to a consistent neurodevelopmental phenotype.
In a 20-year-old Turkish man (patient 17), born of consanguineous parents, with autosomal dominant intellectual developmental disorder-57 (MRD57; 618050), Lelieveld et al. (2016) identified a de novo heterozygous G-to-T transversion (c.1720+1G-T, NM_006852.3) in the TLK2 gene, resulting in a splice site alteration. The mutation was found by exome sequencing.
Reijnders et al. (2018) noted that this mutation resulted in frameshift and premature termination (Ser517fsTer1). Analysis of patient cells showed the presence of an aberrant transcript consistent with the skipping of exon 18. The variant was demonstrated to result in nonsense-mediated mRNA decay, consistent with a loss of function and haploinsufficiency.
In a 7-year-old boy (patient 439) with autosomal dominant intellectual developmental disorder-57 (MRD57; 618050), Lelieveld et al. (2016) identified a de novo heterozygous c.2092C-T transition (c.2092C-T, NM_006852.3) in the TLK2 gene, resulting in an arg698-to-ter (R698X) substitution in the catalytic domain. The mutation was found by whole-exome sequencing.
Reijnders et al. (2018) noted that the R698X mutation occurred in the last exon of the gene. Analysis of patient cells by showed that the mutation escaped nonsense-mediated mRNA decay and may have produced a truncated protein.
In a patient with autosomal dominant intellectual developmental disorder-57 (MRD57; 618050), Reijnders et al. (2018) identified a de novo heterozygous c.989C-A transversion (c.989C-A, NM_006852.3) in the TLK2 gene, resulting in a ser330-to-thr (S330X) substitution in a coiled-coil domain. Analysis of patient cells by showed that the mutation was subject to nonsense-mediated mRNA decay. The findings were consistent with a loss of function and haploinsufficiency.
In a mother and son with autosomal dominant intellectual developmental disorder-57 (MRD57; 618050), Reijnders et al. (2018) identified a heterozygous T-to-G transversion in intron 16 of the TLK2 gene (c.1460+2T-G, NM_006852.3), predicted to result in a splicing abnormality and a loss of function. Functional studies of the variant and studies of patient cells were not performed.
In a patient with autosomal dominant intellectual developmental disorder-57 (MRD57; 618050), Reijnders et al. (2018) identified a de novo heterozygous c.890G-A transition (c.890G-A, NM_006852.3) in the TLK2 gene, resulting in a gly297-to-asp (G297D) substitution. The variant was not found in the ExAC or gnomAD databases. Functional studies of the variant and studies of patient cells were not performed.
Groth, A., Lukas, J., Nigg, E. A., Sillje, H. H. W., Wernstedt, C., Bartek, J., Hansen, K. Human Tousled like kinases are targeted by an ATM- and Chk1-dependent DNA damage checkpoint. EMBO J. 22: 1676-1687, 2003. [PubMed: 12660173] [Full Text: https://doi.org/10.1093/emboj/cdg151]
Lelieveld, S. H., Reijnders, M. R. F., Pfundt, R., Yntema, H. G., Kamsteeg, E.-J., de Vries, P., de Vries, B. B. A., Willemsen, M. H., Kleefstra, T., Lohner, K., Vreeburg, M., Stevens, S. J. C., and 10 others. Meta-analysis of 2,104 trios provides support for 10 new genes for intellectual disability. Nature Neurosci. 19: 1194-1196, 2016. [PubMed: 27479843] [Full Text: https://doi.org/10.1038/nn.4352]
Mortuza, G. B., Hermida, D., Pedersen, A.-K., Segura-Bayona, S., Lopez-Mendez, B., Redondo, P., Ruther, P., Pozdnyakova, I., Garrote, A. M., Munoz, I. G., Villamor-Paya, M., Jauset, C., Olsen, J. V., Stracker, T. H., Montoya, G. Molecular basis of tousled-like kinase 2 activation. Nature Commun. 9: 2535, 2018. Note: Electronic Article. [PubMed: 29955062] [Full Text: https://doi.org/10.1038/s41467-018-04941-y]
Reijnders, M. R. F., Miller, K. A., Alvi, M., Goos, J. A. C., Lees, M. M., de Burca, A., Henderson, A., Kraus, A., Mikat, B., de Vries, B. B. A., Isidor, B., Kerr, B., and 58 others. De novo and inherited loss-of-function variants in TLK2: clinical and genotype-phenotype evaluation of a distinct neurodevelopmental disorder. Am. J. Hum. Genet. 102: 1195-1203, 2018. [PubMed: 29861108] [Full Text: https://doi.org/10.1016/j.ajhg.2018.04.014]
Sillje, H. H. W., Takahashi, K., Tanaka, K., Van Houwe, G., Nigg, E. A. Mammalian homologues of the plant Tousled gene code for cell-cycle-regulated kinases with maximal activities linked to ongoing DNA replication. EMBO J. 18: 5691-5702, 1999. [PubMed: 10523312] [Full Text: https://doi.org/10.1093/emboj/18.20.5691]
Yamakawa, A., Kameoka, Y., Hashimoto, K., Yoshitake, Y., Nishikawa, K., Tanihara, K., Date, T. cDNA cloning and chromosomal mapping of genes encoding novel protein kinases termed PKU-alpha and PKU-beta, which have nuclear localization signal. Gene 202: 193-201, 1997. [PubMed: 9427565] [Full Text: https://doi.org/10.1016/s0378-1119(97)00495-2]