Genome Replication and Cancer
Problems in determining where and when DNA replication begins, lead to genomic instability which leads to genetic mutations which lead to selection of cells that can continue to proliferate under conditions where normal cell growth is arrested - CANCER.
Cancer cells exhibit three characteristics that are linked directly to genome duplication: genomic instability, uncontrolled proliferation, and resistance to apoptosis. Genomic stability depends on the number of active replication origins per genome. Low origin densities produce fewer replication forks that must travel greater distances, thus increasing the probability that forks will stall or suffer damage. Conversely, high origin densities increase the possibility of DNA damage during the initiation process, and the possibility that too many forks will trigger a DNA damage response.
Uncontrolled proliferation results from increased sensitivity of cancer cells to mitogenic stimuli and decreased sensitivity to contact inhibition. Cancer cells continue to proliferate under nutritional conditions in which normal cells complete cell division and then go into a quiescent state termed ‘G0-phase’. In addition, cancer cells continue to proliferate when they come into contact with other cells whereas normal cells stop proliferating when cell to cell contacts are made.
Normal cells have multiple checkpoints that can arrest further initiation events during S-phase, stabilize replication forks until they are repaired, and prevent the onset of mitosis until DNA replication is completed. If the problem cannot be corrected, then DNA damage response pathways trigger apoptosis (programmed cell death). Moreover, normal cells have multiple pathways that restrict genome duplication to once and only once each time a cell divides. These pathways involve phosphorylation of several of the 14 proteins that comprise the pre-replication complex, as well as ubiquitination by specific ubiquitin ligases followed by proteolysis mediated by the 26S proteasome.
Cancer cells lack several of these controls. It makes them dangerous in that mutations are more easily generated and restrictions on genome duplication more easily ignored, but it may also be their Achilles heel. The tumor suppressor genes Retinoblastoma (Rb) and TP53 are the cell proliferation control genes whose activities are most frequently altered in cancers (1). Rb regulates expression of a large number of genes that are required for S-phase. Cell cycle exit depends upon turning off these genes. TP53 is a transcription factor that is induced by DNA damage, hypoxia or oncogene activation. TP53 regulates expression of genes that either arrest cell proliferation or trigger apoptosis.
More recently, we discovered that cancer cells depend primarily, if not exclusively, on a single regulatory pathway to prevent DNA re-replication from occurring during mitotic cell division cycles. That pathway is geminin, a protein unique to multicellular organisms that is expressed only during S, G2 and early M-phases of the cell cycle, and that specifically inhibits loading of the replicative DNA helicase (termed the MCM helicase) onto chromatin. The result was that suppression of geminin expression in cancer cells induced DNA re-replication, and DNA re-replication triggered the ATR-Chk1-Cdc25 DNA damage response pathway, which soon induced apoptosis. In other words, suppression of geminin in cultured mammalian cells selectively killed cells derived from several different malignant human cancers but not normal human cells (2). This discovery opens the way for new therapeutic approaches to arresting or destroying a wide variety of cancers.