DNA replication licensing and human cell proliferation

The convergence point of growth regulatory pathways that control cell proli feration is the initiation of genome replication, the core of which is the assembly of prereplicative complexes resulting in chromatin being 'licensed ' for DNA replication in the subsequent S phase. We have analysed regulatio n of the pre-replicative complex proteins ORC, Cdc6, and MCM in cycling and nonproliferating quiescent, differentiated and replicative senescent human cells. Moreover, a human cell-free DNA replication system has been exploit ed to study the replicative capacity of nuclei and cytosolic extracts prepa red 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. Fu rthermore, analysis of MCM protein expression in self-renewing, stable and permanent human tissues shows that the three classes of tissue have develop ed very different growth control strategies with respect to replication lic ensing. Notably, in breast tissue we found striking differences between the proportion of mammary acinar cells that express MCM proteins and those lab elled with conventional proliferation markers, raising the intriguing possi bility that progenitor cells of some tissues are held in a prolonged G1 pha se or 'in-cycle arrest'. We conclude that biomarkers for replication-licens ed cells detect, in addition to actively proliferating cells, cells with gr owth potential, a concept that has major implications for developmental and cancer biology.