Polymerase Theta-Mediated DNA Repair Pathways

DNA polymerase theta (Pol θ) orchestrates a distinct, error-prone route of double-strand break (DSB) repair known as microhomology-mediated end joining (MMEJ) or theta-mediated end joining (TMEJ). Unlike classical non-homologous end joining (cNHEJ) and homologous recombination (HR), TMEJ exploits short regions of microhomology flanking DNA breaks, allowing Pol θ to align and extend minimally paired DNA ends. This pathway is particularly active when HR is compromised or cNHEJ factors are absent, serving as both a backup and a driver of mutagenesis. Pol θ is tightly regulated throughout the cell cycle and can be activated by phosphorylation events in mitosis. Its overexpression in many cancers underlies synthetic lethality with HR defects and offers opportunities for targeted therapy. Small-molecule inhibitors that trap Pol θ on DNA or block its polymerase activity have emerged, potentiating existing treatments such as PARP inhibitors and radiotherapy. Beyond repair, Pol θ influences genome evolution, immunoglobulin class switching, and off-target integration during gene editing, highlighting its significance in both normal physiology and disease.

Research from Nature Portfolio

Recent studies have revealed that in mitotic cells, Pol θ is directly phosphorylated by Polo-like kinase 1, triggering recruitment to broken DNA ends via interactions with TOPBP1. This mitotic activation of Pol θ ensures the joining of DSBs when cNHEJ and HR are disabled, thereby maintaining chromosomal integrity and explaining its synthetic lethality with HR-deficient backgrounds.

Another advance is the discovery of a potent allosteric inhibitor that selectively binds Pol θ in its closed conformation on DNA/DNA substrates. This compound traps the polymerase for extended periods, blocks MMEJ, and synergises with PARP inhibitors to kill HR-deficient cells. Structural and biochemical analyses have elucidated the mechanism by which DNA-bound Pol θ is immobilised, offering a blueprint for further drug development.

Seminal work has also demonstrated that small-molecule inhibition of Pol θ polymerase activity induces synthetic lethality in BRCA1/2-mutant tumour models and overcomes resistance associated with loss of 53BP1 or Shieldin complex components. By elevating single-stranded DNA intermediates and impairing alternative end-joining, these inhibitors restore sensitivity to PARP inhibitors and define a precision strategy for HR-deficient cancers.

Polymerase Theta-Mediated DNA Repair Pathways publication trend

The graph below shows the total number of articles in polymerase theta-mediated dna repair pathways across all publications each year (not limited to Nature Index journals).

Technical terms

Double-strand break (DSB): A form of DNA damage in which both strands of the DNA helix are severed.

Microhomology-mediated end joining (MMEJ): An alternative DSB repair mechanism that uses short homologous DNA sequences to align and join broken ends, mediated by Pol θ.

Synthetic lethality: A genetic or pharmacological interaction where simultaneous impairment of two genes or pathways causes cell death, whereas loss of either alone is compatible with viability.

Homologous recombination (HR): An error-free DSB repair pathway that uses an intact DNA template to guide accurate repair.

cGAS-STING pathway: A cytosolic DNA sensing mechanism that activates innate immune signalling in response to cytosolic DNA fragments.

Allosteric inhibitor: A molecule that binds a site distinct from the enzyme’s active site, inducing a conformational change that reduces catalytic activity.

References

1. Polθ is phosphorylated by PLK1 to repair double-strand breaks in mitosis. Nature (2023).
2. Discovery of a small-molecule inhibitor that traps Polθ on DNA and synergizes with PARP inhibitors. Nature Communications (2024).
3. Polθ inhibitors elicit BRCA-gene synthetic lethality and target PARP inhibitor resistance. Nature Communications (2021).
4. POLQ inhibition elicits an immune response in homologous recombination-deficient pancreatic adenocarcinoma via cGAS/STING signaling. Journal of Clinical Investigation (2023).
5. Small-molecule Polθ inhibitors provide safe and effective tumor radiosensitization in preclinical models. Clinical Cancer Research (2023).
6. Polymerase Θ is a key driver of genome evolution and of CRISPR/Cas9-mediated mutagenesis. Nature Communications (2015).
7. Inactivation of Pol θ and C-NHEJ eliminates off-target integration of exogenous DNA. Nature Communications (2017).