DNA damage responses
Sunday 10 May 2009
DNA damage responses (DDR) encompass DNA repair and signal transduction pathways that effect cell cycle checkpoint arrest and/or apoptosis.
When the genomic DNA of eukaryotic cells becomes damaged by spontaneous processes, chemical mutagens, or sunlight exposure, the replication of damaged DNA triggers a cellular response called a post-replication checkpoint.
This response prevents cell cycle progression until post-replication repair processes are completed, and may control the activity of these DNA repair pathways.
In cell types that execute S phase before mitosis, such as fission yeast and human cells, the post-replication checkpoint makes time for repair by delaying the onset of mitosis.
In cell types where mitosis and S phase are concurrent, such as budding yeast, the postreplication checkpoint delays the progress of mitosis at metaphase.
The CHK1 gene is required to mediate the postreplication checkpoint and is conserved in yeast and humans. Fission yeast cells in which the chk1 gene has been disrupted progress normally through the cell cycle after exposure to UV radiation until they have carried damaged DNA through S-phase and the subsequent mitosis, at which point cells begin to die and exhibit gross chromosomal damage.
The BRCA1 tumor suppressor plays a role in the activation of human chk1, therefore the posreplication checkpoint may prevent the genetic changes that lead to cancer.
A number of genes required for the post-replication checkpoint encode proteins that recognize single-stranded DNA and the 5’ end of transitions between single-stranded DNA and double-stranded DNA.
These structure are known to be generated in two different ways during S phase.
The replication of DNA containing damaged bases leaves gaps in the newly-synthesized DNA strand, and the replication of nicked or gapped DNA creates double-strand breaks. Both structures are thought to activate the post-replication checkpoint.
Exposure of eukaryotic cells to ionizing radiation, of high doses of UV radiation or chemical mutagens, induces additional responses such as G1/S, intra-S, and G2/M checkpoints (see cell cycle checkpoints).
There is mounting evidence that these checkpoints do not ensure the completion of repair or occur under physiological conditions, therefore their functional significance remains unclear.
One possibility is that ionizing radiation and high doses of UV create DNA structures in G1 and G2 phase, such as double-strand breaks and post-replication gaps, that are created by the replication of damaged DNA under physiological conditions.
cell cycle checkpoints