Steve Jackson - Overview

Our research focuses on the detection, signalling and repair of the most dangerous form of DNA damage: the DNA double-strand break (DSB). DSBs can directly inactivate key genes and are also highly recombinogenic, leading to chromosomal translocations and abnormalities, and ultimately to cancer development. DSBs can arise through normal metabolic processes and are produced during site-specific recombination (for example, V(D)J recombination in the immune system) and during DNA replication. However, exogenous agents such as gamma irradiation and some chemotherapeutic drugs also cause DSBs. Through characterising the cellular events triggered by DSBs, we aim to better understand how genomic stability is normally preserved, how defects in DNA repair pathways can cause cancer predisposition, and how these pathways can be manipulated to improve cancer therapy.

Because DSBs are potentially life threatening, eukaryotic organisms have developed highly conserved systems to repair them. The system that we are particularly interested in is non-homologous end joining (NHEJ), and recently, much has been learnt about this pathway. For example, we have analysed the structures and functions of DNA-dependent protein kinase (DNA-PK), which comprises two small Ku subunits and a large catalytic subunit, DNA-PKcs. This complex is involved in initial DNA damage detection and is then thought to interact with and phosphorylate downstream NHEJ effectors, such as the XRCC4/DNA ligase IV complex, which repairs the DNA.

We are also interested in other proteins that, like DNA-PKcs, are members of the phosphatidylinositol-3 kinase-like (PIKK) family of kinases. Notably, two other PIKKs – mammalian ATM (mutated in ataxia-telangiectasia, a cancer predisposition syndrome), and ATR, and their yeast homologues Tel1p and Mec1p – are involved in "checkpoint" signalling. Their activation by DNA damage leads to numerous responses, including cell cycle arrest that provides enough time for DNA repair before DNA replication or cell division occurs. We are investigating how the activity of these yeast and mammalian kinases is triggered by DNA damage and are looking for their interacting proteins and downstream substrates. For example, we have found that Mec1p phosphorylates yeast histone H2A in response to DNA damage and that this changes chromatin structure and is required for efficient NHEJ. We are extending these studies to the analogous human system and are investigating whether modulation of other chromatin components may affect ability of the cell to tolerate DNA damage.

Finally, we are studying how DNA repair and DNA-damage signalling proteins influence other cellular events. For example, we have found that Ku is localised to telomeres in both yeast and mammalian cells, and that it plays a key role in maintaining telomere length and stability. Another DSB repair component that functions at the telomere is the human Mre11/Rad50/Nbs1 complex (Mre11p/Rad50p/Xrs2p in yeast), defects in which cause cancer predisposition syndromes. This multi-functional complex, which may help to prepare the DNA ends for ligation or for the action of telomerase, is also involved in signalling DNA damage to the checkpoint machinery. As with Ku, we are using a combination of approaches to investigate how this complex fulfills its multiple functions.