DNA replication licensing and human cell proliferation
The convergence point of growth regulatory pathways that control cell proliferation is the initiation of genome replication, the core of which is the assembly of pre-replicative complexes resulting in chromatin being 'licensed' for DNA replication in the subsequent S phase. We have analysed regulation of the pre-replicative complex proteins ORC, Cdc6, and MCM in cycling and non-proliferating quiescent, differentiated and replicative senescent human cells. Moreover, a human cell-free DNA replication system has been exploited to study the replicative capacity of nuclei and cytosolic extracts prepared from these cells. These studies demonstrate that downregulation of the Cdc6 and MCM constituents of the replication initiation pathway is a common downstream mechanism for loss of proliferative capacity in human cells. Furthermore, analysis of MCM protein expression in self-renewing, stable and permanent human tissues shows that the three classes of tissue have developed very different growth control strategies with respect to replication licensing. Notably, in breast tissue we found striking differences between the proportion of mammary acinar cells that express MCM proteins and those labelled with conventional proliferation markers, raising the intriguing possibility that progenitor cells of some tissues are held in a prolonged G1 phase or 'in-cycle arrest'. We conclude that biomarkers for replication-licensed cells detect, in addition to actively proliferating cells, cells with growth potential, a concept that has major implications for developmental and cancer biology.