L3 - Early molecular events in development of cancer Flashcards
What is the fidelity of DNA repair?
Extremely high but each cell experiences thousands of DNA lesions per day
What are 3 causes of a mutation being allowed through repair?
- repair is not 100% effective and gets less precise with age
- Can be compromised by mutation
- Can be overwhelmed by number of damaged sites (massive carcinogen exposure)
What is needed other than a mutation to cause cancer?
Programmed cell death must also be disabled by mutation
How many errors does DNA polymerase 1 make and what does proofreading change this to?
1 in 10^5
Proofreading to 1 in 10^7
What does the Kaplin Mayer plot show?
Survival of a population of mice over 12 months.
WT survives 100%
Double mutation in proofreading subunit ends at 35%
How does the mismatch repair system know what the right sequence is?
Does it during replication and uses the okazaki fragments to detect the correct sequence
How does the mismatch repair system work?
- hMSH2 and hMLH1 scans new DNA for nicks
2. Remove strand with error then resynthesised
What cancer has mutations in the mismatch repair system?
Hereditary nonpolyposis colon cancer
What is the mutation of mismatch repair in the TGFb receptor and what does it cause?
2 more As added to sequence
Truncated receptor protein
How was it shown that the loss of MLH1 is early in the development of cancer?
Stained for MLH1 in endometrial tissue.
Showed cells that were not in the tumour had no MLH1 too.
How can pyrimidines damaged by UV light sometimes be repaired wrongly?
If they are not repaired in time the cell uses ‘error prone’ DNA polymerase to replicate past the lesion.
This can sometimes incorporate the wrong base fixing the mutation
Of 15000 cancers of the p53 gene in lung tumours ….. how many were caused by G-T transitions?
15% of total
In mutations of p53 gene in lung cancers how did smoking change the amount of G-T transitions? What does this mean?
From 21% in non smokers to 33% in smokers
Type of mutation provides clues about mutagenic agent
What can you take from the locations of common mutations in cancers?
Clues about the functional sites of the protein
What does melanin do?
Forms a cap on the top of skin cells protecting again UV damage
What are the 4 main pathways for correcting DNA damage?
- Mismatch repair
- Nucleotide excision repair
- Base excision repair
- Double strand break repair
What repair system is used for UV damage repair?
Nucletide excision repair (NER)
How does the NER system work?
- Detects distortion
- Excision upstream and downstream from lesion by endonuclease
- DNA synthesis by specialist polymerase
- Ligation
About what age does cancer percentage shoot up and what is the difference with people with a mutation in NER system
Around 40 and hits near 100% at about 90.
With mutation tumours start at 0 and hit 100% at 25
What is Xeroderma pigmentosum?
Patients with mutations in NER who have extreme sensitivity to UV light that results in skin cancer
What does base excision repair (BER) usually deal with?
Spontaneous deamination
How does BER work?
- Detection and removal by DNA glycosylase
- Removal of sugar and phosphate by endonuclease and phosphodiesterase
- DNA synthesis by polymerase beta
- ligation
What can cause a double strand break?
Ionising radiation or ROS
DNA topisomerase inhibitors (chemo)
Replicative stress
How does double strand break repair work?
Homology based repair uses the other chromatid as template
- Resections upstream and downstream by exonuclease
- Melting of helix on normal chromatid
- Invasion in normal chromatid and synthesis
- Rejoining of severed strands
When can a cell go through sister chromatid double strand break repair?
During G1 and S/G2
How many times do human cells undergo chromatid exchanges per round of replication?
about 10
What are breast cancer susceptibility genes that are involved in double strand break repair?
BRCA1 and BRCA2
How does non-homologous end joining (NHEJ) work?
Enzymes detect and bind to the free end of the DNA and catalyze ligation.
Can lead to large loss of sequence.
