Lecture 7 (AT) Flashcards
(26 cards)
We have now covered the main sources of DNA damage. How does mismatch repair differ from the rest?
It deals with chemically normal DNA, its just the incorrect bases have been inserted.
Name the 3 types of nucleotide decay (summary of lecture 5).
Depurination, depyrimidination and spontaneous deamination.
Name the 3 types of endogenous/exogenous agents that damage DNA (summary of lecture 5).
Oxidative damage, chemical exposure, radiation (UV and x-ray).
How does mismatch repair detect an incorrect base?
The insertion of the wrong base creates structural distortions.
In mammals, what cleaves the nick created by a base mismatch?
hMLH1 and hMSH2 complex.
MMR genes are often mutated in cancer cells. Individuals born with a mutated copy are more likely to develop what?
Hereditary nonpolyposis colon cancer (HNPCC).
How has hMLH1 been used to illustrate that the loss of repair function is an early event in the development of cancer?
Tissues were staining for nuclei possessing hMLH1, and for nuclei lacking hMLH1. The tumour had lost its hMLH1, but so had some of the surrounding normal tissue. Implies that the loss of hMLH1 predates the emergence of a tumour.
Certain genes are more prone to mutation when MMR isn’t functioning. Why is this?
Because these genes contain short sequence repeats. For example, they are found in TGF beta (membrane receptor).
How does the short sequence repeat link TGF beta and cancer?
It has a short sequence repeat that is prone to mismatches and therefore the loss of MMR. TGF beta functions in the G1 phase and responds to anti-growth signals –> if the receptor isn’t functionning the cells are more likely to proliferate. Hallmark of cancer.
Mutation signatures tells us about the type of mutation we have and gives us a clue about the type of mutagenic agent that drove the tumour. The signatures are classified according to the type of tumour. Give some examples.
Sig 1 occurs in all tumours and is caused by spontaneous deamination. Sig 4 is found in specific tumours and is caused by cigarette smoke.
What triggers a DSB?
- Reactive oxygen species/ ionising radiation.
- DNA topoisomerase inhibitors (chemotherapies).
- ‘Replication Stress’
Two mechanisms that deal with a DSB?
Homologous recombination and non-homologous endjoining (NHEJ).
Limitation with homologous recombination?
Since the sister chromatid is required it can only occur in S/G2; not G1 as the DNA has not yet been replicated.
Proteins involved are also linked with cancer, e.g. BRCA1 and BRCA2.
What are the limitations of NHEJ?
Repair dependent on finding a short region of homology between 2 ends of the break. Involves the loss of bases so it is not favoured. Occurs during G1.
How are surveillance and repair pathways integrated into the cell cycle?
Through signalling pathways. Collectively called the DNA damage response.
What are sensors?
Proteins that detect genotoxic stress.
What are transducers?
Respond to sensors and transmit the signal.
What are effectors?
These can be CC proteins, DNA repair proteins etc…
What is the key thing to note about all of these proteins involved in the DNA damage response?
All of these proteins can become mutated and involved in one tumour type or another.
Give an example of a protein that is involved in the DNA damage response but also suggested to play a role in tumours.
Chk2 = checkpoint kinase 2. Transducer molecule that functions to arrest the CC. Chk2 mutations are thought to play an early role in tumours.
p53 is another transducer molecule that can arrest the cell cycle. How?
It promotes the expression of cyclin-dependent kinase inhibitors (p21). Inhibitors block the expression of cyclin E/A (cdk2), preventing entry into S phase.
Transversion vs. transition?
Transversion –> interchanges of purine for pyrimidine base.
Transition –> exchange of two purines (A, G) , or the exchange of two pyrimidines (C, T).
It should be noted that polycyclic aromatic hydrocarbons do not mutate DNA. How do they work?
Instead DNA replication may convert the damage into a mutation, for example may result in a G to T transversion.
The G to T transversion has been studied in lung cancer and the p53 gene. What were the key findings?
In smokers, a higher percentage of the base changes in the p53 gene are G to T transversions, providing evidence to suggest that smoke (polycyclic aromatic hydrocarbons) are mutagenic agents.
