Ian David Hickson
Panum, Bygning: 18.1.32
2200 København N
The chromosomes in all living organisms are under constant attack from mechanical stress and reactive chemical species that can damage the structural integrity of the DNA. All cells, therefore, devote considerable energy to both preventing and repairing DNA structural abnormalities. Despite this, thousands of DNA lesions still arise daily in each human cell. The majority of these lesions pose no threat to cell viability, but their cumulative effect in a long-lived species influences lifespan and the incidence of age-related diseases such as cancer and neurodegeneration.
Our laboratory investigates the biochemical, molecular and cell biological functions of DNA repair factors in eukaryotic cells and how these factors act to maintain chromosome stability. In particular, we focus on those genes that, when defective, give rise to disorders in humans associated with the premature onset of aging and/or age-associated diseases such as cancer.
In recent years, a major focus has been on characterization of BLM, the protein defective in Bloom's syndrome, a disorder associated with an elevated incidence of cancers of all types. The BLM gene encodes a DNA helicase of the RecQ family, which has important roles in DNA replication and repair via the homologous recombination pathway. We combine analysis of the biochemical properties of BLM with both studies in human cells lacking BLM and studies of SGS1, the budding yeast ortholog of BLM.
Following our recent move from the University of Oxford to the Center for Healthy Aging in the ICMM, we have re-aligned many of our studies to focus on the aging process in humans. Fortunately, chromosome maintenance pathways, including those involving RecQ helicases, are known to impact directly on the rate of human aging. This can be seen most strikingly in human disorders associated with accelerated aging, such as Werner's syndrome (WS). In WS, loss of the RecQ helicase encoded by the WRN gene leads to early onset, multi-organ aging.
Current Project Areas
(i) Biochemical analysis of RecQ helicases
We use purified recombinant proteins and defined DNA substrates to investigate the functions of RecQ helicases and their partner proteins in DNA replication and homologous recombination repair. For example, we discovered and re-constituted biochemically a pathway for processing recombination intermediates that requires BLM.
This is a process called Holliday junction dissolution.
Model for the role of BLM in the dissolution of double Holliday junctions
ii) Factors required for the regulation of homologous recombination
We are using molecular genetics and biochemistry to characterize factors, such as FBH1, that regulate the efficiency of homologous recombination repair.
(iii) Cellular responses to DNA damage
We are analyzing the role of selected post-translational modifications of proteins in regulating the cellular response to DNA damage and other stresses occurring during DNA replication.
An example of a late anaphase cell with a long anaphase bridge stained with BLM (red). DNA is in blue.
(iv) Characterization of a novel class of anaphase bridge structure.
Through analyzing the localization of BLM, we identified a class of ultra-fine anaphase bridges that link sister chromatids at either centromeres or fragile site loci. This latter class (see photo) have foci for the FANCD2 protein at their termini. FANCD2 is a protein required for the so-called Fanconi anaemia pathway for DNA repair. We are investigating the source of these bridges, and how BLM helps to resolve them.
BLM acts alongside a SNF2 family protein called PICH, which we are analyzing biochemically and functionally through the use of PICH-depleted human cells and chicken DT-40 cell knock-out mutants. We are also investigating how phosphorylation of BLM and PICH in mitosis regulates their function.
(v) Use of yeast to study chromosome maintenance pathways
Because the pathways we analyze are highly conserved, we can use budding yeast as a model organism to conduct detailed genetic characterization of the BLM ortholog, Sgs1.
These studies also enable the use of techniques, such as 2-dimensional DNA replication gel analysis and genome-wide screens to be conducted. For example, we have created a system for site-specific blockade of DNA replication, and are analyzing proteins recruited to the site of blockade and the mechanism by which the blockade is overcome.