TLK2

TLK2 is a nuclear serine/threonine kinase that functions in chromatin maintenance, DNA damage responses, and cell fate decisions including neuronal survival and cancer stemness. TLK2 dimerizes to autophosphorylate its N-terminus, masking a conserved PIP box that otherwise mediates PCNA-dependent recruitment to damaged chromatin, thereby providing a regulatory switch for its engagement at DNA lesions (PMID:39727191). It phosphorylates DYNLL1 to promote CTCF-cohesin chromatin hub formation and loop organization (PMID:41120304), interacts with chromatin regulators CHD7, CHD8, and BRD4 (PMID:33323470), and its protein levels are controlled by SCF^FBXL3+CRY-mediated ubiquitin-dependent degradation linking TLK2 turnover to the circadian clock (PMID:30655559). Heterozygous loss-of-function mutations cause a neurodevelopmental disorder through haploinsufficiency and impaired kinase activity (PMID:29861108, PMID:33323470), while calcium overload activates TLK2 to drive nuclear envelope disruption and neuronal death via a TLK2–LC8–myosin IIA complex (PMID:40210858).

1. 1997 Medium

   Establishing TLK2 as a nuclear kinase resolved the basic subcellular context for its function, showing that an N-terminal NLS directs the protein to the nucleus.

   Evidence GST-fusion NLS localization assays and transient transfection in COS-1 cells

   PMID:9427565

   Open questions at the time

   • No substrates or biological function identified
   • Kinase activity not yet demonstrated in cells
   • Single cell type examined
2. 2016 Medium

   TLK2 was placed in the DNA damage checkpoint pathway when overexpression was shown to impair Chk1/2-mediated G2-M checkpoint signaling, linking TLK2 dosage to chromosomal instability.

   Evidence TLK2 overexpression in breast cancer cell lines with checkpoint signaling and DNA repair assays

   PMID:27489360

   Open questions at the time

   • Direct phosphorylation targets in the checkpoint pathway not identified
   • Mechanism of Chk1/2 impairment not resolved
   • Overexpression system may not reflect physiological regulation
3. 2018 Medium

   Discovery that heterozygous TLK2 loss-of-function mutations cause a neurodevelopmental disorder established clinical relevance and demonstrated that half-normal TLK2 dosage is insufficient for normal brain development.

   Evidence Whole-exome/genome sequencing of affected individuals with functional validation in patient-derived cell lines

   PMID:29861108

   Open questions at the time

   • Specific neuronal substrates and processes disrupted by haploinsufficiency unknown
   • Animal model recapitulation not reported
   • Functional assays limited to two subjects
4. 2019 High

   Identification of TLK2 as a substrate of the SCF^FBXL3+CRY ubiquitin ligase revealed that TLK2 protein abundance is under circadian clock control, providing a post-translational regulatory layer.

   Evidence AP-MS identification of CRY1/2-TLK2 interaction; CRY overexpression decreased and CRY knockout increased TLK2 protein levels

   PMID:30655559

   Open questions at the time

   • Physiological circadian oscillation of TLK2 protein not demonstrated in vivo
   • Ubiquitination sites on TLK2 not mapped
   • Downstream consequences of circadian TLK2 regulation unclear
5. 2020 High

   Demonstration that disease-associated TLK2 missense variants abolish kinase activity and alter chromatin state linked the neurodevelopmental phenotype to impaired kinase function and chromatin maintenance, while BioID revealed an interactome including CHD7, CHD8, and BRD4.

   Evidence In vitro kinase assays with mutant TLK2, BioID proximity proteomics, comet assays in patient lymphoblastoid cells

   PMID:33323470

   Open questions at the time

   • Whether CHD7/CHD8/BRD4 are direct substrates or scaffolding partners is unresolved
   • Causal link between relaxed chromatin and neurodevelopmental phenotype not demonstrated
   • BioID proximity interactions not all validated by reciprocal methods
6. 2023 Medium

   Discovery that TLK2 interacts with ATF4 and promotes ASNS expression via mTORC1 extended TLK2 function to amino acid metabolism and metabolic signaling.

   Evidence IP-MS, co-immunoprecipitation of TLK2-ATF4, TLK2 knockdown/overexpression with pathway readouts

   PMID:37542132

   Open questions at the time

   • Whether TLK2 directly phosphorylates ATF4 is not established
   • Mechanism linking TLK2 to mTORC1 activation is indirect
   • Single-lab study without independent replication
7. 2025 High

   Resolution of TLK2's autoregulatory mechanism showed that dimerization-dependent N-terminal autophosphorylation masks a PIP box, creating a phospho-switch that gates PCNA-dependent recruitment to DNA damage sites.

   Evidence In vitro autophosphorylation assays, PIP box mutagenesis, live-cell imaging at laser-induced damage sites, co-IP with PCNA

   PMID:39727191

   Open questions at the time

   • Phosphatase(s) that reverse the PIP box masking unknown
   • In vivo significance of PCNA-dependent recruitment for repair outcomes not tested
   • Relative contributions of homo- vs hetero-dimerization with TLK1 unclear
8. 2025 High

   Identification of a TLK2–LC8–myosin IIA complex that mediates nuclear envelope rupture under calcium overload established a non-chromatin cytomechanical function for TLK2 in neuronal cell death, validated by TLK2 inhibition rescuing retinal ganglion cells in a glaucoma model.

   Evidence Co-IP of TLK2/LC8/myosin IIA, TLK2 RNAi and small-molecule inhibition, nuclear envelope phenotype assays, mouse glaucoma model

   PMID:40210858

   Open questions at the time

   • Direct phosphorylation target(s) in the LC8-myosin IIA complex not mapped
   • Whether this pathway operates in non-neuronal cell death is unknown
   • Mechanism linking calcium to TLK2 multimerization and activation unresolved
9. 2025 High

   Discovery that TLK2 phosphorylates DYNLL1 to promote CTCF-cohesin hub formation identified a direct kinase-substrate relationship controlling 3D chromatin architecture and cancer stemness.

   Evidence CRISPR screen, in vitro kinase assay of TLK2-DYNLL1, co-IP, chromatin conformation capture, mouse breast cancer model

   PMID:41120304

   Open questions at the time

   • Phosphorylation site(s) on DYNLL1 not fully characterized
   • Whether this mechanism operates at loci beyond KLF4 is unknown
   • Contribution of TLK1 to the same pathway not addressed
10. 2025 Medium

    Demonstration that TLK2 undergoes activity-dependent nuclear export during neuronal differentiation, combined with NLS-lacking splice variants, revealed a mechanism for compartment-specific TLK2 function in neurons.

    Evidence Splice-specific in situ hybridization, subcellular fractionation, live-cell imaging in differentiating rat neuroblastoma cells

    PMID:42023051

    Open questions at the time

    • Cytoplasmic substrates of TLK2 in neurons not identified
    • Nuclear export signal and transport receptor not mapped
    • Findings from a neuroblastoma cell line, not confirmed in primary neurons

• Key open questions include the full spectrum of TLK2 substrates in neurons, the structural basis of TLK2 dimerization and its regulation by calcium, and whether circadian TLK2 degradation modulates chromatin maintenance or DNA repair in vivo.
• No comprehensive substrate inventory exists
• Full-length TLK2 dimer structure not solved
• In vivo circadian oscillation of TLK2 and its functional consequences not demonstrated