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The DNA damage-dependent intra-S phase checkpoint is regulated by parallel pathways

Jacob Falck1, John H J Petrini, Bret R Williams

  • 1Institute of Cancer Biology, Danish Cancer Society, Strandboulevarden 49, DK-2100 Copenhagen, Denmark.

Nature Genetics
|February 19, 2002
PubMed

Insights

Ionizing radiation triggers DNA damage checkpoints via ATM kinase. Blocking Nbs1-Mre11 and Chk2-Cdc25A-Cdk2 pathways together completely halts DNA replication inhibition, revealing parallel DNA repair pathways.

Area of Science:

  • Cellular biology
  • Molecular biology
  • Genetics

Background:

  • Mammalian cells activate ATM kinase in response to ionizing radiation to preserve genetic integrity.
  • This activation initiates S-phase checkpoint pathways to delay DNA replication.
  • Defects in ATM, Nbs1, Chk2, Mre11, or Cdc25A-Cdk2 lead to radio-resistant DNA synthesis (RDS).

Purpose of the Study:

  • To investigate whether Nbs1, Chk2, and associated proteins function in a common signaling cascade.
  • To elucidate the cooperative mechanisms of DNA damage-dependent S-phase checkpoint pathways.

Main Methods:

  • Experimental blockade of Nbs1-Mre11 function in human cells.
  • Interference with Chk2-triggered events and the Chk2-Cdc25A-Cdk2 cascade.
  • Assessment of DNA synthesis inhibition and radio-resistant DNA synthesis (RDS) phenotype.
  • Analysis of Cdk2-dependent loading of Cdc45 onto replication origins.

Main Results:

  • Blocking Nbs1-Mre11 or Chk2-Cdc25A-Cdk2 pathways individually resulted in a partial RDS phenotype.
  • Concomitant interference with both pathways completely abolished DNA synthesis inhibition by ionizing radiation, causing complete RDS.
  • Irradiation prevented Cdk2-dependent Cdc45 loading in normal and Nbs1/Mre11-defective cells, but not in ATM-defective cells.

Conclusions:

  • ATM kinase phosphorylates Nbs1 and Chk2, initiating two parallel branches of the DNA damage-dependent S-phase checkpoint.
  • These parallel pathways cooperate to inhibit distinct steps in DNA replication following ionizing radiation exposure.
  • Understanding these pathways is crucial for comprehending cellular responses to DNA damage.

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