The 6-4 photoproduct is the trigger of UV-induced replication blockage and ATR activation

Proc Natl Acad Sci U S A. 2020 Jun 9;117(23):12806-12816. doi: 10.1073/pnas.1917196117. Epub 2020 May 22.


The most prevalent human carcinogen is sunlight-associated ultraviolet (UV), a physiologic dose of which generates thousands of DNA lesions per cell, mostly of two types: cyclobutane pyrimidine dimers (CPDs) and 6-4 photoproducts (6-4PPs). It has not been possible, in living cells, to precisely characterize the respective contributions of these two lesion types to the signals that regulate cell cycle progression, DNA replication, and cell survival. Here we coupled multiparameter flow cytometry with lesion-specific photolyases that eliminate either CPDs or 6-4PPs and determined their respective contributions to DNA damage responses. Strikingly, only 6-4PP lesions activated the ATR-Chk1 DNA damage response pathway. Mechanistically, 6-4PPs, but not CPDs, impeded DNA replication across the genome as revealed by microfluidic-assisted replication track analysis. Furthermore, single-stranded DNA accumulated preferentially at 6-4PPs during DNA replication, indicating selective and prolonged replication blockage at 6-4PPs. These findings suggest that 6-4PPs, although eightfold fewer in number than CPDs, are the trigger for UV-induced DNA damage responses.

Keywords: 6-4PP; CPD; Chk1; DNA damage response; DNA replication.

Publication types

  • Research Support, N.I.H., Extramural
  • Research Support, Non-U.S. Gov't

MeSH terms

  • Animals
  • Ataxia Telangiectasia Mutated Proteins / metabolism*
  • Cells, Cultured
  • Checkpoint Kinase 1 / metabolism
  • DNA Damage*
  • DNA Repair
  • DNA Replication*
  • HCT116 Cells
  • Humans
  • Pyrimidine Dimers / genetics*
  • Ultraviolet Rays*


  • Pyrimidine Dimers
  • pyrimidine-pyrimidone dimer
  • ATR protein, human
  • Ataxia Telangiectasia Mutated Proteins
  • CHEK1 protein, human
  • Checkpoint Kinase 1