γH2AX in the S Phase after UV Irradiation Corresponds to DNA Replication and Does Not Report on the Extent of DNA Damage

ABSTRACT

Ultraviolet (UV) radiation is a major environmental mutagen. Exposure to UV leads to a sharp peak of γH2AX, the phosphorylated form of the histone variant H2AX, in the S phase within an asynchronous population of cells. γH2AX is often considered a definitive marker of DNA damage inside a cell. In this report, we show that γH2AX in the S-phase cells after UV irradiation reports neither on the extent of primary DNA damage in the form of cyclobutane pyrimidine dimers nor on the extent of its secondary manifestations in the form of DNA double-strand breaks or in the inhibition of global transcription. Instead, γH2AX in the S phase corresponds to the sites of active replication at the time of UV irradiation. This accumulation of γH2AX at replication sites slows down the replication. However, the cells do complete the replication of their genomes and arrest within the G₂ phase. Our study suggests that it is not DNA damage, but the response elicited, which peaks in the S phase upon UV irradiation.

KEYWORDS:

• γH2AX
• cell cycle
• replication
• S phase
• UV
• DNA damage
• DNA damage response (DDR)
• DNA repair
• DNA replication

SUPPLEMENTAL MATERIAL

Supplemental material is available online only.

ACKNOWLEDGMENTS

A.M. and S.D. thank Tamal Das for access to the UV cross-linker. We also thank the two anonymous reviewers for their constructive feedback that helped us convey our results more effectively.

A.M., S.D., and S.R. conceived and designed the experiments. S.D. performed and analyzed the experiments. S.D., S.R., and A.M. interpreted the data. S.D. and A.M. wrote the manuscript. A.M. monitored the project and associated grants.

We declare no conflicts of interest.

This project was funded by intramural funds at TIFR Hyderabad from the Department of Atomic Energy (DAE), and partially by a DST SERB Early Career Research Award (ECR/2016/000907) to A.M. S.R. was supported by a DST SERB National Post-Doctoral Fellowship (N-PDF) scheme (PDF/2016/002781).