Each cell in our body – and our bodies contain trillions of cells – harbours all of the genetic information to make a human. However, different cells express different genes in the genome that specifically define muscle, skin, brain etc. How these expression patterns are maintained during the proliferation of cells needed for organ development is unknown. Every time a cell divides, it must first replicate its entire genome. This involves removing all proteins from the DNA, at least transiently, unwinding the DNA duplex, and copying each strand. Thus, the machinery involved in specific gene expression patterns is transiently displaced from DNA during replication and must be quickly re-established. Amongst the proteins that are displaced during DNA replication are an abundant class called histones. Histones bind very tightly to DNA in structures called nucleosomes and we know little about how nucleosomes are disrupted during DNA synthesis. Although there are relatively few different histones and histone ‘variants’, each of the four core histones (H2A, H2B, H3 and H4) have tails that are the sites of a multitude of covalent modifications including acetylation, methylation, phosphorylation ubiquitinylation, etc. These modifications can specifically recruit proteins involved in gene activation or repression. It is believed that the displacement of nucleosomes during replication must somehow be coupled to the re-assembly of nucleosomes from the same histones behind the replication fork to ensure the accurate re-establishment of gene expression patterns. We know very little about how parental nucleosomes are re-deposited behind the replication fork or how this is coordinated with the assembly of nucleosomes from newly synthesised histones.
In addition to providing a deep understanding of a fundamentally important cellular process, our work may have important implications for understanding the aetiology of human cancer. It has emerged in recent years that most and perhaps all cancers exhibit some form of ‘replicative stress’, loosely defined as perturbation of normal replication rates and accumulation of DNA damage during replication. It is unclear whether there is a single cause of replicative stress or whether replicative stress is an umbrella term for a variety of biochemical mistakes, which may be different in different cancers. Our work will help understand the biochemical mechanism by which chromosomes are replicated. This work will synergise with other work from my lab aimed at understanding replicative pathways which are misregulated in cancer.
The aim of our project is to understand how replication forks contend with nucleosomes and how gene expression patterns are re-established during and after DNA replication using a ‘bottom-up’ approach to reconstitute this process with purified proteins, based on a DNA replication system developed in our laboratory.
