Our work focuses on the DNA-damage response (DDR): the set of events that optimises cell survival by detecting DNA damage, signalling its presence and mediating its repair. The importance of the DDR is underscored by defects in it being associated with various pathologies, including neurodegenerative disease, immunodeficiency, premature ageing, infertility and cancer.

By working with both yeast and human cells, we are identifying new DDR factors, defining the functions of known DDR components, assessing how the DDR is affected by chromatin structure, and learning how DDR events are regulated. For example, we recently discovered that – like the yeast Sae2 protein – the human CtIP protein functionally interacts with the MRN complex to promote processing (resection) of DNA double-strand breaks (DSBs) to generate single-stranded DNA that activates the DDR kinase ATR and triggers DNA repair by homologous recombination. Moreover, we found that CtIP and Sae2 share a highly conserved cyclin-dependent kinase (CDK) phosphorylation site, and discovered that its phosphorylation facilitates resection, thus promoting ATR signalling and HR in S/G2 but not G0/G1. Furthermore, we recently discovered that the yeast Sgs1 helicase and its human counterpart BLM (mutated in cancer-predisposed Bloom syndrome patients) promote resection, HR and ATR signalling by a pathway that is distinct from that mediated by MRN and Exonuclease I. Another highlight over the past year has been our demonstration that phosphorylation-dependent interactions between MRN and the DDR-mediator protein MDC1 are required for the association of MRN with chromatin flanking DSB sites. Our current work is geared towards further defining the mechanisms and biological functions of these and other DDR events.

Fig 1: Common DNA-damage recruitment and activation mechanisms for ATM, DNA-PKcs and ATR. These proteins are recruited and activated at sites of damage through interactions with their partner proteins – NBS1 that is part of the MRE11-RAD50-NBS1 (MRN) complex, Ku80 and ATRIP – to bring about DDR signalling or repair, as indicated. Model derived from Falck J, Coates J, Jackson SP (2005). Nature 434:605-611.

Fig 2: Cell-cycle coordination of DSB signalling and repair. In G1, cells carry out little DSB resection, leading to activation of ATM-dependent signalling and DSB repair by non-homologous end-joining (NHEJ). In S and G2 cells, ATM signalling also occurs but in these circumstances, CtIP – in conjunction with the MRE11-RAD50-NBS1 (MRN) complex – promotes DSB processing to generate single-stranded DNA that triggers ATR activation and leads to repair by homologous recombination (HR). Our recent work has shown that CDK phosphorylation of CtIP and its yeast homologue, Sae2, promotes ssDNA formation and HR.

Selected publications:

• Sartori AA, Lukas C, Coates J, Mistrik M, Fu S, Bartek J, Baer R, Lukas J and Jackson SP (2007) Human CtIP promotes DNA end resection. Nature 450:509-14

• Chapman JR and Jackson SP (2008) Phospho-dependent interactions between NBS1 and MDC1 mediate chromatin retention of the MRN complex at sites of DNA damage. EMBO Reports 9:795-801

• Huertas P, Cortes-Ledesma F, Sartori AA, Aguilera A and Jackson SP (2008) CDK targets Sae2 to control DNA-end resection and homologous recombination. Nature 455:689-692

• Gravel S, Chapman JR, Magill C and Jackson SP (2008) DNA helicases Sgs1 and BLM promote DNA double-strand break resection. Genes and Development 22:2767-2772

Fig 3: MDC1-dependent retention of the MRN complex on γH2AX-coupled chromatin spanning DSBs. The Casein Kinase 2 (CK2) phosphorylated SDTD repeat-motifs of MDC1 are bound by the FHA-domain of NBS1, allowing the MRN complex to be recruited to damaged chromatin when MDC1 binds ATM-phosphorylated histone H2AX (γH2AX). The resulting co-localisation of MDC1 and NBS1 can be visualised along tracts of laser-induced DNA DSBs (lower left panels), yet this co-localisation is compromised in human cells expressing an MDC1 mutant lacking the SDTD motifs, resulting in an impaired NBS1 recruitment (lower right panels).