Abstract
Why solid tumours show high chromosome instability is still poorly understood. Seminal work in the host lab has shown in the yeast model system that precocious CDK activation and reduced origin licensing in G1 cause S-phase extension and chromosome rearrangements in mitosis. Since most cancer cells have genetic or epigenetic alterations in one or more G1/S cell cycle regulators that can impede origin licensing, we analysed chromosome replication dynamics in fourteen human epithelial cancer cell lines using newly developed techniques, and compared it to normal human fibroblasts, mammary and retinal pigment epithelial cells. Our results show that all cancer cells spend longer time in S phase (10-29h) than untransformed cells (7-9h). Interestingly, most cancer cell lines displayed a lower global instant density of replication forks (GIFD), partly compensated for some cell lines by increased fork velocity (FV). We define replication potency (RP = GIFD x FV) as a new descriptor of cells’ capacity to synthesize DNA that integrates this compensation mechanism, and found that it was lower for cancer cell lines.The consequences of this longer S phase on the cell cycle and mitosis were assessed by 4D microscopy. We detected mitotic DNA synthesis (MiDAS) and chromosome segregation failures in cancer cells not treated with replication drugs, indicating constitutive chromosome instability (CIN). Importantly, the low GIFD and long S phase of pRb+ cancer cells was reversed by slightly extending G1 using low dose of the CDK4/6 inhibitor Palbociclib. Our data strongly suggest that S-phase extension due to lowered origin licensing in G1 is a common feature and perhaps the main trigger for the genomic instability in cancer cells.