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A CHK1-mediated phosphorylation switch suppresses human Topoisomerase 1-associated genomic instability

Ananda Guha Majumdar1,2, Nitish Chauhan1,2, Pooja Gupta1,2

  • 1Bio-Organic Division, Bhabha Atomic Research Centre, Mumbai, India.

The EMBO Journal
|May 13, 2026
PubMed

Insights

Checkpoint kinase 1 (CHK1) phosphorylates Topoisomerase 1 (TOP1) at Serine-320, limiting genotoxic TOP1-DNA complexes during normal cell metabolism and maintaining genomic stability.

Area of Science:

  • Molecular Biology
  • Genetics
  • Cell Biology

Background:

  • Topoisomerase 1 (TOP1) resolves DNA supercoils, but its intermediates (TOP1-DNA covalent complexes, TOP1ccs) are genotoxic when stabilized.
  • Mechanisms resolving chemotherapy-induced TOP1ccs are known, but physiological prevention of TOP1cc accumulation remains unclear.

Purpose of the Study:

  • To identify regulatory pathways limiting steady-state TOP1cc levels under physiological conditions.
  • To investigate the role of CHK1-mediated TOP1 phosphorylation in genomic stability.

Main Methods:

  • Phosphorylation site mapping of TOP1.
  • Analysis of TOP1cc levels in wild-type and mutant cells.
  • Assessment of DNA damage, replication fork progression, and R-loop formation.
  • Studies on proteasomal and autophagic degradation pathways.

Main Results:

  • CHK1 phosphorylates TOP1 at Serine-320, regulating its religation activity and limiting steady-state TOP1cc levels.
  • TOP1cc stabilization can evade proteasomal/autophagic clearance but is targeted by CtIP, SPRTN, and p97.
  • TOP1 S320 phosphorylation defects cause replication/transcription-associated DNA breaks, R-loop stabilization, genomic instability, and sensitivity to TOP1 poisons.

Conclusions:

  • CHK1 directly regulates human TOP1cc dynamics via S320 phosphorylation, crucial for genomic integrity.
  • This pathway is essential for preventing TOP1cc accumulation during unperturbed metabolism.
  • Findings have implications for understanding genomic stability and developing combinatorial chemotherapy strategies.

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