L10- DNA repair and cancer Flashcards
(21 cards)
What happens if damaged DNA is not recognised or if repair mechanisms fail?
Mutations occur
What are the exogenous and endogenous sources of DNA damage?
Exogenous:
- ionising radiation
- alkylating agents
- mutagenic chemicals
- anti-cancer drugs
- free radicals
Endogenous:
- replication errors
- free radicals
Types of DNA damage?
- Deamination
- mismatches
- double stranded breaks
- single stranded breaks
- intercalating agent
a) What is DNA replication stress?
b) When does it occur?
a) Inefficient replication that leads to replication fork slowing, stalling and/or breakage
b) - when there is replication machinery defects e.g. polymerase
- when there is replication fork progression hindrance e.g. repetitive DNA
- when there is defects in response pathways
What is the normal function of DNA polymerase? What can happen if DNA polymerase doesn’t function as normal?
- the enzymes that build DNA in cells. During DNA replication (copying), most DNA polymerases undergo proofreading.
- If the polymerase detects that a wrong nucleotide has been added, it will remove and replace the nucleotide right away, before continuing with DNA synthesis
- if this isnt functioning normally then proofreading isnt functioning normally and hence bases that aren’t supposed to be in the strand will not be picked up and removed
- leads to mutations and can give rise to tumours
How can repetitive DNA cause replication stress/damage?
- tandem repeats are unstable regions of the genome where frequent insertions and deletions of nucleotides can take place, resulting in genome rearrangements.
- When DNA polymerase encounters a direct repeat, it can undergo a replication slippage
- leads to fork slippage
What is the difference between backward slippage and forward slippage?
Backward slippage: makes the strand longer
Forward slippage: makes the strand shorter
What is an example of a disorder that is due to repetitive DNA?
Explain
Huntington’s Disease (HD)
- CAG repeats
- backward slipping
- leads to neurone degeneration
- progressive, late onset disease
a) What is the normal DNA damage response pathway?
b) What are the types of final responses to DNA damage?
a) Signals –> Sensors –> transducers –> effectors
b) - If DNA damage levels are too high or persist: Senescence: in permanent cell cycle arrest
Apoptosis: cell death (normally ends here)
- if DNA damage levels are not too high:
- cell cycle checkpoints: temporary arrest to allow DNA repair
What are the cell cycle checkpoints, at what stage do they occur and what are their functions?
- G1 checkpoint: just before S phase to make sure DNA okay to replicate
- G2 checkpoint: just after G2 before mitosis, to check if all DNA is replicated and that all DNA damage is repaired
- MITOSIS CHECKPOINT: during mitosis, to make sure all chromosomes are properly attached to the mitotic spindle
What are the types of DNA repair mechanisms?
- Base-excision repair (BER)
- Nucleotide-excision repair (NER)
- Recombinational repair
- Mismatch repair
Outline the process of base-excision repair
- Enzymes called glycosylases will detect and remove damaged bases
- once the base has been removed the empty piece of DNA backbone is also removed
- the gap is filled with the right base by a DNA polymersase
- gap is sealed by a ligase
Outline the process of nucleotide excision repair
- detects and corrects types of damage that distort the DNA double helix e.g. detects bases that have been modified with bulky chemical groups
MOST COMMON: uv radiation produces a thymine dimer
- thymine dimer detected, the surrounding DNA is opened (using helicase) to form a bubble
- enzymes cut the damaged region out of bubble
- DNA polymerase replaces the cut out DNA and a ligase seals it
What type of DNA repair mechanism would be used by your body when it recognises DNA exposed to chemicals in cigarette smoke?
Nucleotide excision repair
Outline the process of mismatch DNA repair
- happens right after new DNA has been made
- remove and replace mis-paired bases
- mismatch is detected and is cut out, replaced with new nucleotides by DNA polymerase and then sealed by ligase
What is the basic process of DNA repair systems?
- Damage recognised
- Dna bit cut out
- correct nucleotides replaced
- DNA ligase seals backbone
Types of Double strand break repair
- Non-homologous end joining: the two broken ends simply glued back together.
- tends to produce a mutation as it sometimes involves the loss or addition of nucleotides - Homologous-directed pair
- information from the homologous chromosome that matches the damaged one (or from sister chromatid) is used to repair the break
- does not usually cause mutations
a) What does “multi-step cancer model” mean?
b) Seeing as DNA replication stress can lead to mutation, what does this mean in terms of cancer?
a) - several mutations are required for cancer to occur
- each mutation drives a wave of cellular multiplication associated with gradual increases in tumour size
- mutation accumulation
b) DNA replication stress can stimulate carcinogenesis
a) What is the difference between inter-tumour heterogeneity and intra-tumour heterogeneity?
b) Why is intra-tumour heterogeneity relevant clinically
a) 1. Inter- tumour heterogeneity: different between types of cancer
2. Intra-tumour: variability among cancer cells within a single tumour- different types of neoplastic cells in a single tumour
b) Because it is closely related to cancer progression, resistance to therapy and recurrences
It is observed that in some cases tumours seem to respond well to chemotherapy; growth slows down and the tumour reduces in size significantly. However after some time the tumour growth resumes. Describe two ways how this observation can be explained.
TUMOURS ARE HETEROGENEOUS:
- Differential sensitivity:
- Chemotherapy given and one cancer cell type in the tumour may be killed due to the treatment but another might be resistant and hence will grow and proliferate and tumour comes back - Chemotherapy induced mutagenesis:
- Chemotherapy given and reduces the tumour but the chemical can cause DNA mutation an make the cell line mutate and make it more aggressive or resistant
a) What is synthetic lethality?
b) How can it be used/ exploited to treat cancer?
–> examples
a) when a combination of deficiencies in the expression of two or more genes leads to cell death, where as a deficiency in only one of those genes does not
b) Only used as studies now at the moment but drugs could potentially target only the mutated cancer cells and block both genes so it results in the death of cancer cells
e. g. PARP inhibitors are being used in clinical trials
