CELL DIVISION AND CANCER
Cell division proceeds in a series of steps, the order of which is rigorously regulated. The most important step before one cell divides into two is the replication of its DNA. The retinoblastoma protein is critical for this because it controls the cell cycle clock, permitting DNA replication only when the cells are big enough (i.e. they’ve passed through the G1 growth phase) and when growth signals are telling the cell to divide (Fig. 1).
1. The retinoblastoma protein controls the cell cycle clock.
RB1 functions by regulating transcription of a specific group of genes. RB1 is really a sort of master regulator because it works by associating with a set of conventional transcription factors comprising the E2F family. RB1 binds to E2Fs, forming a complex that prevents transcription of a set of genes. When RB1 releases its hold on the E2Fs they become free to turn on this set. Some of these genes encode proteins required for DNA replication as well as regulators of cell cycle progression (e.g. MYC). The enzymes that actually make RNA from DNA (the RNA polymerases) are also controlled by the RB1-E2F complex. All of that means that not only does RB1 determine whether a cell can replicate its DNA and continue through the cell cycle but it also determines whether the cell can work at all – because the RNA polymerases in effect make all the components of a living cell.
An RB1 loss is a significant step towards cancer because it de-regulates the transcriptional power of the E2Fs. Abnormal duplication of DNA ensues and the stage is set for the accumulation of mutations. RB1 is a target of the cyclin-dependent kinases that control cell cycle progression. Under normal circumstances, these control the switch from transcriptional repression to transcriptional activation by phosphorylating RB1, which releases the E2Fs.
RB1 has an even more celebrated counterpart that emerged some years before the retinoblastoma gene and protein were isolated. TP53 (usually called p53) was first detected in a lysate of cells that had been infected with a DNA tumour virus and it was the first tumour suppressor gene to be cloned. What it did remain a mystery until the 1990s when two experiments gave very clear indications of its function. The first was the generation of a transgenic mouse by Allan Bradley’s group in Houston that had both P53 alleles knocked out – that is, the mice were unable to make any p53 at all. To general surprise, this didn’t seem to bother the mice at all. They developed and grew into normal adult animals and they could even breed quite happily, although the females were a bit less efficient at it than their wild-type counterparts. Fortunately, Bradley’s group persisted by continuing to observe the transgenic mice, and they were rewarded over the next six months as it gradually emerged that these mice were extremely prone to cancer. So much so that about three-quarters of the mice had developed neoplasms of one sort or another.
The second experiment that highlighted the role of p53 was carried out in Dundee by David Lane’s group who directed a mild burst of sunlight (UV irradiation) onto their own forearms and then measured the amount of p53 protein in the skin cells. The result was amazing: before irradiation p53 was almost undetectable (as it is in most normal cells) but after a short exposure to sunlight, the amount of p53 had dramatically increased.
Thus was p53 revealed. It isn’t necessary for normal cells to grow and divide; indeed it’s so unnecessary that normal cells hardly bother to make it. But subject a cell to any kind of stress that damages DNA – even mild UV radiation is very stressful in this respect – and p53 is made in large amounts. This explains why the knock-out mice were fine but it also explains why they were so susceptible to cancers: as we’ve noted, all animals are exposed to a wide range of DNA damaging assaults and p53 has evolved as part of a major mechanism for protecting cells against such assaults.