How does DNA replication initiate in animal cells?
Karim Labib
University of Dundee, Dundee, UK
Chromosome replication is one of the most complex processes in a eukaryotic cell, producing a single and near-perfect copy of the genome in every cell cycle. The initiation of chromosome replication is very highly regulated, to ensure that the chromosomes are copied just once. Mutations in initiation factors are associated with human disease syndromes, and there is considerable evidence that initiation is deregulated in human cancer, making the initiation step an interesting target for future therapies. Much of our understanding of how chromosome replication initiates has come from pioneering work with the budding yeast, Saccharomyces cerevisiae, for which the initiation, elongation and termination of DNA replication have been reconstituted in vitro with a large set of purified proteins. The elongation step has also been reconstituted in vitro with human orthologues of the factors required in budding yeast, illustrating the high evolutionary conservation of this process in eukaryotes, but until now the initiation of chromosome replication remains poorly characterised in animal cells and other eukaryotes. Using early embryos of the nematode Caenorhabditis elegans as a model system, we have found that a protein called DNSN-1 is required for initiation of chromosome duplication. Yeasts lack orthologues of DNSN-1, but the human equivalent – called DONSON – is mutated in a human microcephalic dwarfism syndrome. Moreover, the genes encoding two key initiation factors from budding yeast can be completely deleted in C. elegans without impairing viability. These findings point to a surprising degree of diversification in the mechanism by which eukaryotic DNA replication initiates, compared to the current model that is largely derived from budding yeast.