TLK2

TLK2 is a nuclear serine/threonine protein kinase that couples its catalytic activity to chromatin regulation, DNA damage responses, and cell-fate decisions (PMID:9427565, PMID:39727191, PMID:38712247). Its recruitment to chromatin is gated by autoregulation: TLK2 hyper-autophosphorylates its N-terminus through homo- and hetero-dimerization with TLK1, masking a non-canonical PIP box that mediates PCNA-dependent recruitment to DNA damage sites (PMID:39727191, PMID:38712247). Functionally, TLK2 organizes higher-order chromatin by phosphorylating DYNLL1 (LC8), which enhances DYNLL1–CTCF binding and promotes CTCF–cohesin loop/hub formation at the KLF4 locus during cancer stemness transitions (PMID:41120304), and it associates in proximity with the chromatin remodelers CHD7, CHD8, BRD4, and NACC1 (PMID:33323470). TLK2 abundance is controlled by ubiquitin-mediated turnover through the circadian E3 ligase SCFFBXL3 via CRY1/2, a recruitment that depends on TLK2 kinase activity (PMID:30655559). Through these activities TLK2 enforces genome stability, since its overexpression impairs Chk1/Chk2 G2-M checkpoint signaling, delays repair, and drives chromosomal instability (PMID:27489360), while loss-of-function and kinase-dead missense variants relax chromatin and increase DNA damage susceptibility, acting through haploinsufficiency to cause a neurodevelopmental syndrome (PMID:29861108, PMID:33323470). In neurons, TLK2 is regulated by activity-driven nucleocytoplasmic shuttling (PMID:42023051), and calcium overload activates TLK2 via multimerization to form a complex with LC8 and myosin IIA that ruptures the nuclear envelope in a degenerative death pathway (PMID:40210858).

1. 1997 Medium

   Established TLK2's basic identity, answering whether the gene encodes a functional kinase and where it acts by identifying catalytic activity and a functional nuclear localization signal.

   Evidence cDNA cloning with kinase-activity expression screening and GST-fusion nuclear localization assay in COS-1 cells

   PMID:9427565

   Open questions at the time

   • No substrates identified
   • No physiological pathway context
   • NLS function shown only for an isolated GST fusion
2. 2016 Medium

   Linked TLK2 dosage to genome maintenance by showing that its overexpression disrupts checkpoint signaling, framing TLK2 as a driver of chromosomal instability in cancer.

   Evidence Gain- and loss-of-function in breast cancer lines with G2-M checkpoint, Chk1/2 phosphorylation, repair kinetics, and DNA-damage sensitivity readouts

   PMID:27489360

   Open questions at the time

   • Direct kinase substrate in the checkpoint pathway not identified
   • Mechanism connecting TLK2 to Chk1/2 unresolved
   • Single cancer-cell context
3. 2018 Low

   Defined the genetic mechanism of TLK2-associated disease, showing that loss-of-function variants cause a neurodevelopmental syndrome through haploinsufficiency.

   Evidence Functional assessment of patient-derived cell lines carrying TLK2 mutations

   PMID:29861108

   Open questions at the time

   • Limited methodological detail in the source
   • Molecular pathway linking haploinsufficiency to neurodevelopmental phenotype not defined
   • Single study
4. 2018 Low

   Placed TLK2 upstream of an oncogenic signaling axis by showing it activates SRC to drive glioblastoma migration and invasion.

   Evidence Overexpression/knockdown with migration/invasion assays and SRC inhibition by saracatinib

   PMID:30207834

   Open questions at the time

   • Pharmacological epistasis only — no direct biochemical link to SRC
   • No demonstrated phosphorylation target
   • Single inhibitor used for pathway placement
5. 2019 Medium

   Identified how TLK2 protein levels are controlled, showing it is a kinase-activity-dependent substrate of the circadian SCFFBXL3+CRY1/2 ubiquitin ligase.

   Evidence APMS substrate identification plus CRY1/2 overexpression and genetic deletion with TLK2 abundance measurements

   PMID:30655559

   Open questions at the time

   • Ubiquitination sites on TLK2 not mapped
   • Physiological consequence of circadian TLK2 oscillation not tested
   • Whether kinase activity drives autoturnover unexplored
6. 2020 Medium

   Connected disease variants to molecular dysfunction by showing missense mutations abolish kinase activity and that TLK2 sits in a chromatin-remodeler interactome controlling chromatin compaction.

   Evidence In vitro kinase assays of variants, BioID proximity interactome, comet assay, and chromatin accessibility analysis

   PMID:33323470

   Open questions at the time

   • Direct substrates among CHD7/CHD8/BRD4/NACC1 not established
   • Whether proximity partners are phosphorylated by TLK2 unknown
   • Causal chain from kinase loss to chromatin relaxation incomplete
7. 2024 High

   Resolved the mechanism gating TLK2 chromatin recruitment, showing autophosphorylation masks a PIP box required for PCNA-dependent recruitment to damage sites.

   Evidence In vitro autophosphorylation assays, PIP-box mutagenesis, PCNA Co-IP, and live-cell imaging of damage-site recruitment

   PMID:38712247 PMID:39727191

   Open questions at the time

   • Dynamics of de-repression at damage sites not defined
   • Downstream chromatin substrates at damage foci not identified
   • Relative roles of TLK1 vs TLK2 dimers unresolved
8. 2025 High

   Defined a direct catalytic output of TLK2 in genome architecture, showing it phosphorylates DYNLL1 to promote CTCF-cohesin loop formation governing cancer stemness.

   Evidence CRISPR screen with live-cell CTCF-cohesin reporters, DYNLL1 phosphorylation assays, DYNLL1–CTCF Co-IP, loop analysis at KLF4, and mouse models

   PMID:41120304

   Open questions at the time

   • DYNLL1 phosphosite(s) not detailed in this synthesis
   • Generality beyond the KLF4 locus unclear
   • Link between this loop function and DNA-damage role unexplored
9. 2025 High

   Uncovered a pathological neuronal function, showing calcium-driven TLK2 multimerization and activity form an LC8–myosin IIA complex that ruptures the nuclear envelope to drive neuronal death.

   Evidence RNAi, small-molecule inhibition, overexpression, live imaging of NE rupture, TLK2–LC8–myosin IIA Co-IP, and a mouse glaucoma RGC degeneration model

   PMID:40210858

   Open questions at the time

   • Direct kinase substrate driving NE rupture not identified
   • How calcium triggers multimerization mechanistically unknown
   • Relationship to TLK2's chromatin functions unresolved
10. 2026 Medium

    Established neuron-specific spatial regulation of TLK2, showing differentiation and synaptic activity drive its nuclear export and NLS-lacking splice variants generate a cytoplasmic pool.

    Evidence Splice-specific in situ hybridization in mouse brain, subcellular fractionation/immunostaining during neuroblastoma differentiation, and live imaging after synaptic stimulation

    PMID:42023051

    Open questions at the time

    • Cytoplasmic substrates of exported TLK2 not identified
    • Export machinery not defined
    • Functional consequence of activity-driven shuttling untested
11. 2023 Medium

    Implicated TLK2 in metabolic control, showing it interacts with ATF4 to promote ASNS expression and amino-acid synthesis via mTORC1 in gastric cancer.

    Evidence IP-MS, knockdown/overexpression, TLK2–ATF4 and mTORC1–ASNS Co-IP, ubiquitination assays, and TLK2 kinase inhibition

    PMID:37542132

    Open questions at the time

    • Whether TLK2 phosphorylates ATF4 not shown
    • Direct vs indirect mTORC1 regulation unresolved
    • Single cancer-cell context

• How TLK2's distinct activities — DNA-damage recruitment, CTCF-cohesin looping, circadian turnover, neuronal nuclear-envelope rupture, and metabolic signaling — are coordinated by a single kinase, and what its full substrate repertoire is, remains unresolved.
• A unifying substrate logic across contexts is undefined
• Tissue-specific regulation of which function dominates is unknown
• Structural basis of activity-dependent multimerization vs dimerization unresolved