Constant Vigilance for Constant Attack
The integrity of our genomes is under constant attack from various sources such as environmental agents (ex. UV radiation, tobacco smoke), or endogenous sources such as oxidative damage from byproducts of metabolism (ex. free radicals), and errors made during the process of DNA replication. The cell employs overlapping signalling networks and DNA repair pathways to handle the barrage of genomic insults encountered on a daily basis. In response to a genomic insult, the cell responds in one of three possible ways:
1) Repair the fault
2) Ignore it and risk mutation
3) Altruistic suicide to prevent propagation of the genomic insult
DNA repair mechanisms are closely tied into the regulatory "checkpoints" of the cell cycle. Cell cycle checkpoint mechanisms ensure genomic integrity before permitting DNA replication and cell division to occur. Failures in these checkpoints can lead to an accumulation of damage, which in turn leads to mutations. Breaks in the DNA backbone are detected by cell cycle checkpoint proteins which sit at the top of complex signalling cascades that recruit "repair proteins" to the damage site. The kinase ATM is one such 'checkpoint' protein in humans which detects dsDNA breaks, and responds by activating other proteins to initiate one of two possible repair responses:
1) Homologous recombination (HR)
2) Non-homologous end-joining (NHEJ)
HR is activated in response to ssDNA breaks, while NHEJ is activated in addition to HR in response to dsDNA breaks. When double-strand breaks (DSBs) are generated, ring-shaped Ku proteins localize onto broken ends to act as "clamps" to allow ligases to re-ligate the broken strand together again.
If a single strand of the double-helix breaks, HR is triggered. When a cell is exposed to ionizing radiation, RAD proteins clump at damage sites in the cell nucleus. RAD51 "picks up" the broken end and searches for a complementary base-sequence. Once a suitable template is located by RAD51, DNA polymerases use the identified complimentary sequence to re-synthesize the damaged strands.
DNA Repair and Cancer
Cigarette smoke forms DNA adducts, tiny chemical bolt-ons to the double-helix, which can damage the way our cells operate if not removed. DNA adducts disrupt the DNA repair machinery by perturbing enzyme function, causing alterations in the DNA sequence. DNA polymerases can make mistakes causing mutations or breaks in the DNA backbone. This genome instability can lead to de-regulated activation of different genes, and drastic alterations in cell physiology.
Counter-intuitively, disrupting DNA repair machinery can be a useful anti-cancer strategy. The drug, cisplatin, used in cancer chemotherapy, interferes with normal cellular processes by causing DNA lesions. These lesions form crosslinks between DNA bases that can interfere with several cellular processes. The DNA repair or surveillance system kicks in and cells are killed off. While the drug helps get rid of cancer cells, normal cells suffer too, which is why patients receiving chemotherapy lose all their hair.
Problems arise when cancer cells find a way around the drug. For instance, cancer cells try to make polymerases that can replicate over cisplatin so they can survive drug treatment, because the DNA repair checks are evaded, and cells aren't repaired or removed. Structural biologists seek to understand how such enzymes can recognise the cisplatin lesion. What is the active site chemistry of the interaction? Answers to these questions will direct design of other chemotherapeutics that this polymerase cannot replicate over.
LINKS
http://www.nature.com/scitable/topicpage/dna-damage-repair-mechanisms-for-maintaining-dna-344
http://www.dna-repair.nl/start/index.php
1) Repair the fault
2) Ignore it and risk mutation
3) Altruistic suicide to prevent propagation of the genomic insult
DNA repair mechanisms are closely tied into the regulatory "checkpoints" of the cell cycle. Cell cycle checkpoint mechanisms ensure genomic integrity before permitting DNA replication and cell division to occur. Failures in these checkpoints can lead to an accumulation of damage, which in turn leads to mutations. Breaks in the DNA backbone are detected by cell cycle checkpoint proteins which sit at the top of complex signalling cascades that recruit "repair proteins" to the damage site. The kinase ATM is one such 'checkpoint' protein in humans which detects dsDNA breaks, and responds by activating other proteins to initiate one of two possible repair responses:
1) Homologous recombination (HR)
2) Non-homologous end-joining (NHEJ)
HR is activated in response to ssDNA breaks, while NHEJ is activated in addition to HR in response to dsDNA breaks. When double-strand breaks (DSBs) are generated, ring-shaped Ku proteins localize onto broken ends to act as "clamps" to allow ligases to re-ligate the broken strand together again.
If a single strand of the double-helix breaks, HR is triggered. When a cell is exposed to ionizing radiation, RAD proteins clump at damage sites in the cell nucleus. RAD51 "picks up" the broken end and searches for a complementary base-sequence. Once a suitable template is located by RAD51, DNA polymerases use the identified complimentary sequence to re-synthesize the damaged strands.
DNA Repair and Cancer
Cigarette smoke forms DNA adducts, tiny chemical bolt-ons to the double-helix, which can damage the way our cells operate if not removed. DNA adducts disrupt the DNA repair machinery by perturbing enzyme function, causing alterations in the DNA sequence. DNA polymerases can make mistakes causing mutations or breaks in the DNA backbone. This genome instability can lead to de-regulated activation of different genes, and drastic alterations in cell physiology.
Counter-intuitively, disrupting DNA repair machinery can be a useful anti-cancer strategy. The drug, cisplatin, used in cancer chemotherapy, interferes with normal cellular processes by causing DNA lesions. These lesions form crosslinks between DNA bases that can interfere with several cellular processes. The DNA repair or surveillance system kicks in and cells are killed off. While the drug helps get rid of cancer cells, normal cells suffer too, which is why patients receiving chemotherapy lose all their hair.
Problems arise when cancer cells find a way around the drug. For instance, cancer cells try to make polymerases that can replicate over cisplatin so they can survive drug treatment, because the DNA repair checks are evaded, and cells aren't repaired or removed. Structural biologists seek to understand how such enzymes can recognise the cisplatin lesion. What is the active site chemistry of the interaction? Answers to these questions will direct design of other chemotherapeutics that this polymerase cannot replicate over.
LINKS
http://www.nature.com/scitable/topicpage/dna-damage-repair-mechanisms-for-maintaining-dna-344
http://www.dna-repair.nl/start/index.php