University of Dundee

Recognizing and repairing chromosome damage

DNA is highly chemically reactive; there are many agents that occur normally inside cells that react with DNA in a way that could change the sequence and/or structure of the genome, with potentially catastrophic consequences. In addition to their potential mutagenicity, DNA damage can block important processes such as DNA replication, which can potentially prevent cell proliferation. So it’s important that DNA damage is repaired rapidly to prevent mutations, rearrangements or changes in chromosome number from occurring.

We are interested in how cells detect, signal and repair DNA damage and how they deal with blocks to DNA replication. In the past years we have discovered a range of completely new proteins in mammalian cells that are instrumental for repair of DNA damage and broken replication forks. Some of these – such as SLX4 – are mutated in debilitating human diseases. We are interested in figuring out the modes of action of these proteins and their relevance to disease, and in discovering more new players in DNA repair. Many important chemotherapeutic agents act by inducing DNA damage and/or DNA replication stress and we are interested in finding ways of making these therapies more effective and in preventing resistance. Furthermore we are involved in identifying new anti-cancer drug targets in the DNA repair arena.

We are particularly interested in DNA inter–strand crosslinks (ICLs). These are formed when bifunctional agents covalently link the two strands in a double helix. ICLs are toxic lesions that prevent strand separation necessary for transcription and DNA replication. ICLs are caused by endogenous metabolites, and the major route for ICL repair appears to be initiated when DNA replisomes collide with ICLs. The repair of ICLs involves multiple DNA repair pathways, but how they are removed and DNA replication re-started is unclear.

Project 1: How does protein ubiquitylation promote ICL repair?

Defects in the repair of ICLs causes Fanconi anaemia (FA), a rare inherited chromosome instability syndrome accompanied by developmental and skeletal defects, bone marrow failure and predisposition to cancer. There are fifteen FANC proteins, and the central component of the FA pathway is FANCD2, which is mono–ubiquitylated at Lys561 in S–phase and in response to ICLs. This is catalysed by the eight–subunit FA core complex. The mono-ubiquitylation of FANCD2 is essential for the repair of DNA inter-strand crosslinks (ICLs) but despite much work in this area exactly how mono–ubiquitylation of FANCD2 promotes ICL repair at the molecular level is unknown. This projects aims at identifying the proteins recruited by mono–ubiquitylated FANCD2 that repair ICLs.

Project 2: Screening for new regulators of DNA repair

We have set up new assays to identify new DNA repair genes and this project involves using these assays to screen for new regulators of DNA repair. We have already identified some new candidates that need to be characterised in detail.

Project 3: A “ synthetic rescue” approach to treating diseases caused by defective DNA repair

Women with a germline mutation in the BRCA1 tumour suppressor gene have a very high chance of developing in breast or ovarian cancers. Cells from patients with BRCA1 mutations show defects in a mode of DNA repair referred to as homologous recombinbation, and as a result they show a high degree of genome stability and problems with proliferation. A number of groups found that deleting the 53BP1 DNA repair gene is able to reverse many aspects of the phenotype associated with BRCA1 mutations. In animal models 53BP1 deletion greatly reduced the incidence of tumours associated with BRCA1 mutations. The logic behind this story is that when BRCA1 is absent, 53BP1 initiates an inappropriate mode of DNA repair that BRCA1 would normally prevent. The project on offer aims to use a synthetic rescue screening strategy to find ways of rescuing the DNA repair defects seen in Fanconi anaemia and other debilitating diseases caused by DNA repair.