Mutagenesis and DNA damage tolerance
Cellular DNA requires continuous maintenance to counter spontaneous degradation and exogenous damage. All forms of life possess robust repair mechanisms that can excise damaged bases since these will interfere with essential cellular processes, especially transcription and replication. DNA damage encountered during replication poses a particular problem. The replicative polymerases are highly accurate and one reason for this is that they do not tolerate damaged bases in the DNA template. A damaged template leads them to stall and thus blocks progression of the replication fork.
Attempts at excision repair of DNA damage at the replication fork will result the formation of a double strand break that requires homologous recombination to repair and allow restart of the fork. To avoid this the replication machinery bypasses the damage, deferring definitive repair of the lesion until later, when a stable duplex is re-established. Cells have has two basic strategies for bypassing, or tolerating, DNA damage. First, they can find an alternative template that the the stalled replicative polymerases can 'borrow' to get around the lesion and restart replication, a form of recombination known as template switching. Alternatively, the cell can switch polymerases replacing the replicative polymerases with a combination of specialised translesion polymerases that are able to synthesise directly across the lesion. This process is known as translesion synthesis.
There is now good evidence that the activity of the specialised translesion polymerases account for the majority of mutagenesis in cells either because of their ability to read non- or mis-instructional bases or because of their intrinsically reduced ability to incorporate the correct nucleotide.
Error-prone lesion bypass makes some sort of sense in a single celled organism. In a changing environment being a mutator can be beneficial, at least to the population as a whole. However, similar behaviour in the somatic cells of multicellular organisms can result in loss of the normal control of cell growth and division and the development of a tumour. It was therefore rather a surprise when, in the late 1990s, it became clear that vertebrates possessed homologs of many of the bacterial and yeast translesion polymerases. Further, these polymerases are not only present, we and others have shown that they play a critical role in the vertebrate DNA damage response.
Our work in this area has focussed on the mechanisms by which the translesion polymerases are controlled and how translesion synthesis interacts with recombinational modes of DNA damage bypass. A particularly important finding for us was the identification of specific genetic requirements for translesion synthesis at the replication fork and behind the replication fork.
It was this observation that prompted us to start thinking about the effects the timing of lesion bypass have on histone management during replication and on the maintenance of epigenetic memory (see Replication associated epigenetic instability).