week 19 p1 Flashcards
What are the DNA repair mechanism for single stranded DNA
Base excision repair
Nucleotide excision repair
Mismatch repair
What are the DNA mechanism for double strand DNA
Non-homologous repair
Homologous recombination and homology directed repair
What is the first DNA damage
If proof reading fails in DNA replication
DNA replication cannot happen
Needs mismatch repair pathway
What is Mismatch repair pathway
- Corrects replication errors that have been scaped editing by polymerase
- Includes repair of insertion and deletion produced as a result of slippage during replication
How does Mismatch repair pathway occur
MSH2 and MSH6 are proteins for the complex MutS
This scans the DNA for errors
It recruits MLH1/PMS2 I are proteins for MutL to the site of the mismatch
Mismatch base is excised
MLH1 recruits DNA polymerase to repair the excised area with correct sequence
And nicks are repaired by DNA ligase
What happen if there is a defect in MMP in cancer
Heritable mutation in MLH1 , MAH2,MASH6 and PMS2 can all contribute to cancer syndrome called heredity non-polyposis colorectal cancer(HNPCC) or Lynch syndrome
* This predisposes to colorectal cancer and other too like breast cancer * MLH1 is silences in somatic cancer eg in colorectal * MMR is disrupted cancer have a high mutation rates * This is seen in repetitive regions of DNA called microsatellites which frequently undergo replication slippage
Why is MMR harder to treat
MMR defects do no respond to chemotherapy
treatment
hence there is a need to develop agents that (i) restore MMR proficiency or (ii) are hypersensitive in cells that are irreversibly MMR deficient.
he evidence that resistance to chemotherapy is associated with hMSH2 and/or hMLH1 deficiency
What is endogenous damage and other reactive oxygen species
- In single stranded damage
- ROS and hydroxyl radicals can by products of cellular respiration or inflammation
Or an be produced by the effect of ionising radiation
How can ROS and other endogenous damage occur
- They oxidate the bases of DnA mostly in guanine
- The product is 7,8-dihydro-8-oxo-2-deoxyguanosine (OG)
This mimic thymine and can be by passed
What is cytosine deamination
- Type of endogenous damage on single stranded DNA
- Cytosine and 5 methyl cytosine are prone to reaction
- Converts them to uracil which is recognised by the thymine
- Important for introducing mutation into the V variable region for antibody maturation
- Common in tumours
What is base excision repair
- Used to repair targets chemically altered bases induced by endogenous damage
- So it repair oxidative or deamination damage
- BER is has a specific glycolate flips out the damaged base
- The cut that is caused from the flip is further cut by phosphate backdown AP endonuclease and protected by PARP1
DNA polymerase the and ligase repair the nucleotide gap
Any disease association with BER
- MUYTH is a glycosylase that detects incorrect A bases inserted opposite of OG resembling Ts
- Mutation in MYH gene that encodes a DNA glycosylase which is responsible for removal of mismatch as shown previews
- During the pairing with 80 goxogenuanine can cause colorectal adenoma syndrome
A family case had a high cases of colorectal cancer
was found that carried a high amount of g;c-t;a transversion
• Family did not have mutation genes • Genes from BER pathway were sequenced and had A point mutation • This mutation reduced the ability of MUTHY to recognise OG OGG1 gene codes for the glycosylase responsible
How is Uv damaged single stranded DNA
• UV exposure is due to sunlight+ tanning+ arc welding
• UVA rays are the weaken but most penetrated
• Lower energy levels of IR so DSBs are not formed
• However it form bulky adducts form that links to thymine
This bends DNA preventing transcription
How can UV damage be repaired
- Using nucleotide excision
- NER can be initiated in two mechanism
- 1: Global genome NER: for helix distortion
damage that interferes with transcription- Both recognise a wide variety of mutation that distorts the DNA double helix
- The lesions alone the adjacent nucleotides is excised out by endonucleases and DNA polymerase is used to fill the gap using the opposite strand as a template
Is there any issues associate with NER
- Xeroderma pigmentosum (XP) is an inherited disorder characterises by a defect in NER
- Affected Invidial are hypersensitive to the sun and have a 1000 fold increases for can skin cancer
- 7 XP genes products (XPA-XPG) have been identified out of 25 proteins involved in NER
How is double standard breaks caused
- IR
- Some chemicals eg radiomimetic
DNA replication error
Why is IR damaging?\
• Electromatic waves with enough energy can liberate electrons form atoms
When it hits DNA the energy deposited is sufficient to break the bonds holding DNA together
How does IR induce a DSBs
• Linear transfer (LET) describes the way in which energy is released from radiation to damage DNA
Radiation travels as gamma ways
• Low LET radiation cause less damage
• Beta particles can cause thyroid cancer due to High LT- DNA damage
Another ways that IR can cause DSBs
- Rt- IR is used to treat cancer, creates DNA damage sufficient to cause cell death
- 2 mechanisms
- Direct damage: of IR causes DSBs and cell death
•
How does DNA replication induce DSbs
SSBs breaks during replication
How do you repair DSBs
- Non homologous end joining
- NHEJ is an active gene rearrangements that leads to formation of functional antibodies (VDJ recombination)
- Quick process
- Done in G1
- No sister chromatids needed
- Ku complexes Reponses and circle ends\
- DNA-PK phosphorylation artemia to tide up loo9se ends
- XRCC4 bridges the two ends, allowing DNA ligase to stick them back together
Why is NHEJ error prone
Rarely mutated in cancer
Another way to repair DSB is homology directed repair
• This depends on sister chromatids presences
Process of homology directed repair
- Happens in in the late S phase and G2 of the cell cycles
- Requires sister chromatids to copy
- Takes longer then N HEJ
- MRN cleans the ends
- DDR checkpoints are recruited to these ends included ATM ands ATR
- These phosphorylates H2Ax and activated downstream factors causing cancer cell arrest
- CtLP- is activated by ubiquitination (brca1, rnf168)
- RPA binds and protects ss DNA
RAD51 replaces RPA and catalyses the search for compelementary strands together with PALB2 and BRCA2
Once formed strand inversion occurs and new DNA synthesis of the 3’ end
Used to repair the other strand
Holliday junction resolves
DNA ligase seals the nicks
What is the difference between NHEJ and HR
NHEJ:
In G1
Has a gap that’s filled up bt IR
HDR:
IN G2
HAS A SISTER CHROMATID TO FILL TH GAP AND IR
How does in DSB repair in cancer happen
- DDR is the checkpoint that’s sense DSB and then arrests the cell cycle while they are repaired
- Inherited mutation occurs in ATM which predispose to cancer and cause neurogenerative disorder syndrome ataxia-telangiectasia
Confers resistances to chemo therapy and poor prognosis
Is there any other gene related in DDR
CHK2 and P53
Therapies for DSDB
PARP inhibitors
How does PARP inhibitors work
PARP1 and PARP2 are crucial in repairing SSB by BER
Their inhibition causes an increase in DSBs, which are normally repaired by HR.
In BRCA1/2 mutant cells, inhibition of PARP enzymes results in cell-cycle arrest and apoptosis of cancer cells through synthetic lethality.
Approved by FDA in BRCA1/2 mutant breast and ovarian cance
What does synthetic lethality mean
• Two genes are considered synthetic lethal when cell viability is conserved if either of them is inactive,
whereas the impairment of both results in cell death
In PARP inhibitors’
• PARP inhibitors (PARPi) demonstrate synthetic lethality in cells with impaired homologous recombination (HR)-mediated DNA repair function
• The genetic interaction between PARP and BRCA can be described as synthetic lethal.
What is OG in BER
- oxo-guanine or 8-oxo-G which is the version of guanine that is seen as a thymine T
- by the replication machinery.
- It is caused by reactive oxygen species which oxidate DNA bases, particularly guanine.
