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© PhD Students Symposium 2005. All rights reserved.
Prof. Dr. Ron Laskey
MRC Cancer Cell Unit, Hutchison-MRC Research Centre,
Cambridge, United Kingdom
Initiation of DNA replication is a crucial regulatory step in cell proliferation. It is tightly coupled to the cell cycle in such a way that DNA is replicated precisely once, and only once in each cell cycle. This is achieved by binding of MCM proteins to DNA, licensing one round of DNA replication in each cell cycle. MCM proteins are exceptionally promising markers for cancer diagnosis and screening. We have assessed their value for improving screening for cancer of the cervix, colon, lung and bladder, amongst other sites.
The binding of MCM proteins to DNA is regulated in several ways. Together these ensure that re-initiation of DNA replication does not occur within a single cell cycle. First, the sequential action of different cyclins and cyclin-dependent kinases ensures that no more MCM proteins can bind once S phase has started. Second, A small protein called geminin regulates the loading of MCM proteins by preventing further loading once replication has started. Geminin depletion causes over-replication in mammalian cell lines. We have asked if this can explain the endoreduplication of trophoblast cells in mammalian development, which results in a thousand-fold increase in their DNA content. We find that geminin is downregulated in trophoblast giant cells by continuous proteolysis. Furthermore geminin knock-out causes early arrest in mouse embryos at eight cells. All eight cells endoreduplicate, and express trophoblast markers, and none form inner cell mass. We conclude that proteolytic degradation of geminin drives the endoreduplication cycles in mammalian development.
As geminin is a negative regulator of DNA replication, it would be expected to be a good candidate for a role as a tumour suppressor gene. However, it does not show the pattern of mutations, or the expression patterns expected of a tumour suppressor. Instead its expression pattern is a strong independent predictor of outcome in breast cancer. High geminin expression correlates with poor outcome and short times to distant metastases.