Ki-67 is necessary during DNA replication for forks protection and genome stability

Abstract

Background: The proliferation antigen Ki-67 has been widely used in clinical settings for cancer staging for many years but investigations on its biological functions have lagged. Recently, Ki-67 was shown to regulate both the composition of the chromosome periphery and chromosome behaviour in mitosis as well as to play a role in heterochromatin organisation and gene transcription. However, a role for Ki-67 in regulating cell cycle progression has never been reported. The progress towards understanding Ki-67 function have been limited by the tools available to deplete the protein coupled to its abundance and fluctuation during the cell cycle. Results: Here we have used an auxin-inducible degron tag (AID) to achieve a rapid and homogeneous degradation of Ki-67 in HCT116 cells. This system, coupled with APEX2 proteomics and phospho-proteomics approaches, allowed us to show for the first time that Ki-67 plays a role during DNA replication. In its absence, DNA replication is severely delayed, the replication machinery is unloaded, causing DNA damage that is not sensed by the canonical pathways and dependant on HUWE1 ligase. This leads to replication and sister chromatids cohesion defects, but it also triggers an interferon response mediated by the cGAS/STING pathway in all the cell lines tested. Conclusions: We have unveiled a new function of Ki-67 in DNA replication and genome maintenance that is independent of its previously known role in mitosis and gene regulation.