DNA damage responses in the context of the cell division cycle

Abstract

During my PhD, I have investigated aspects of the DNA damage response (DDR) in the context of three different cellular scenarios: DNA damage signalling in response to double-strand breaks during mitosis, coordination of DNA replication with DNA damage responses by regulation of the GINS complex, and checkpoint activation by the prototypical checkpoint protein Rad9. Here, I show that mitotic cells treated with DNA break-inducing agents activate a ‘primary’ DDR, including ATM and DNA-PK-dependent H2AX phosphorylation and recruitment of MDC1 and the MRN complex to damage sites. However, downstream DDR events and induction of a DNA damage checkpoint are inhibited in mitosis, with full DDR activation only ensuing when damaged mitotic cells enter G1. In addition, I provide evidence that induction of a primary DDR in mitosis is biologically important for cell viability. The GINS complex is an evolutionarily conserved component of the DNA replication machinery and may represent an ideal candidate for transferring the DNA damage signal to the replication apparatus. Here, I show the identification of a consensus ‘SQ’ PIKK phosphorylation motif at the carboxyl end of the GINS complex subunit, Psf1. In Saccharomyces cerevisiae, switching the conserved serine to a glutamic acid is lethal, indicating that the site is crucial for the protein’s function. Moreover, in human cells, I identified UV-DDB, a heterodimeric complex involved in NER repair, as a binding partner that specifically interacts with the Psf1 C-terminus in vitro. Finally, I discuss my findings in characterizing functional interactions between Rad9 and Chk1 in S. cerevisiae. I show that specific consensus CDK sites within Rad9 N-terminus are essential to enable Chk1 phosphorylation and activation, and that MCPH1, a human homologue of Rad9, may share a conserved function in binding and activating Chk1, underscoring the evolutionarily conservation of checkpoint activation mechanisms.