Chapter 5 Flashcards
Name 4 mechanisms causing DNA damage?
a) spontaneous reactions of DNA in the aqueous environment
b) due to metabolic byproducts such as ROS or NOS
c) action of environmental mutagens
d) error during DNA replication
Name different mechanisms resulting in genomic instability:
- Aquire DNA-damaging events: chemicals, radiation, virus
- Intrinsic cases of DNA damage: ROS, DNA-replication, spontaneous reactions in aqueous environment
- Inherited mutations in repair, growth and cell death genes
- Epigenetic events - acetylation, methylation
- Gene amplification
What is gene methylation?
When identified in cancer there is methylation on normally unmethylated DNA segments. It is the CpG islands within the DNA, and result in transcriptional silencing.
What is gene acetylation?
Normally histones are the core proteins in chromatin, and their acetylation status regulates gene expression. Deacetylated histones are generally coiled with DNA and are associated with silencing of genes. When the histones are acetylated, chromatin uncoil and genes can be expressed.
How can gene acetylation be a treatment target?
By administering an enzyme inhibitor of deacetylation, HDAC inhibitors, acetylation levels are increased allowing gene expression of tumour suppressor genes. Has been used in solid and haematological malignancy.s
Name two mechanisms for exogenous causes of genetic instability?
Ultraviolet and ionising radiation
What is translesion DNA synthesis?
When a lesion that block progression of the replication fork during DNA replication is repaired by a low fidelity and high flexibility DNA polymerase. This often allow for replicative bypass of the base damage contained within DNA - the use of these polymerases can contribute to high error rates during DNA replication and may lead to malignant transformation.
What is a mismatch repair?
A DNA repair mechanism used to repair DNA when an incorrect base or helical distortion (insertion-deletion loops) are made by mistake of the DNA polymerase.
What is base excision repair?
DNA base damage is a result of endogenous and exogenous factors, these will be repaired by base excision repair.
Mutation in which tumour suppressor gene may reduce the efficacy of base excision repair mechanisms?
p53
What is the MOA of a PARP-inhibitor?
PARP is an intermediate in base excision repair, when inhibited it causes accumulation of single-strand gaps and can cause collapse of the replication fork during DNA replication. In cells with defective homologous recombination, such as BRAC-2, PARP inhibitors may lead to accumulation to DSB breaks and cell death.
The concept of inhibiting compensatory pathways in tumour cells while sparing normal tissue is called synthetic lethality
How will bulky DNA adducts and inter strand cross-links be repaired?
Nucleoside excision repair
How are double strand breaks repaired?
By homologous recombination or non-homologous end-joining
When can homologous recombination occur in the cell cycle and why?
It can only happen during S and G2, because the mechanism require homology between the broken strand and the template, which is the new sister chromatid. which will only be present in these two phases.
Is homologous recombination prone to errorous DNA repair?
no- as it uses a correct template to pair the new DNA bases
What happenes to the cells ability to repair DNA by homologous repair if it is BRCA-2 deficient?
IT has a 10 -fold lower HR level,
Abnormalities in which genes have been associated with reduced HR ability, suggesting that tumour genesis is associated with altered HR function in spontaneous tumours?
BRCA-1/2, RAD-51/52, MRE-11
When will cells use non-homologous end-joining repair to fix double strand breaks?
When the DNA DSBs have damages ends, as this pathway does not need homology, it simply links the ends of the DNA break together.
What is a common consequence of non homologous end-joining repair?
It is highly error-prone, as it usually results in the loss or gain of nucleotides during modification of the damaged DNA to produce ligatable ends
When in the cell cycle can nonhomologous end-joining occur?
As it is not dependent on a template, it can occur throughout the cell-cycle.
When in the cell cycle can nonhomologous end-joining occur?
As it is not dependent on a template, it can occur throughout the cell-cycle.
Which mechanism is used for DNA-crosslink repair?
Fanconi proteins
What are the two general types of check-point in the cell-cycle?
The mitotic spindle assembly checkpoint - ensure the mitotic spindle is correct formed prior to division
DNA-integrity checkpoint - delay progression in response to DNA damage
Which 3 related kinases are important for initiating the cellular response to DS DNA breaks
ATM, DNA-PKs and ATR
Describe the events occurring after DS DNA break to the checkpoint mechanism?
ATM is phosphorylated, and together with DNA-PK activation a protein complex creating a platform where other proteins in involved in signalling and repair can attach. ATM + other proteins instigate the G1-S checkpoint by stabilising p53. The p53 accumulation result in increase expression cyclin D/E kinase inhibitor p21 as p53 mainly works as a transcription factor.
p21 hypophosphorylates Rb protein, resulting in cell cycle arrest
Name the main effects of ATM/ATR/DNA-PK activation?
- inhibit cell cycle progression by inhibition cyclin-CKD expression
- Instigate DNA repair through stimulation of gene expression
- Cell death
What is the purpose of S phase checkpoint?
to check the quality of DNA replication
Xeroderma pigmentosum; affected pathway and cancer predisposition
Nucleotide excision repair, UV-induced skin cancer
Ataxia telangiectasia: affected pathway and cancer predisposition
DNA DSB response, ATM protein affected, lymphoma
Nijmegen breakage syndrome; affected pathway and cancer predisposition
DNA DSB response - NBS1 defect, lymphoma
BRCA 1/2; affected pathway and cancer predisposition
Homologous recombination, breast
Werner syndrome:
affected pathway and cancer predisposition
Homologous recombination, various cancers
Fanconi anemia: affected pathway and cancer predisposition
DNA crosslink repair, FANC-D2, leukemia
Li Freumeni
DNA DSB response, p53, many
Which enzyme is essential for continued cell division in cancer cells?
temlomerase reverse transcriptase (TERT)
What is the effect of reactivated telomerase transcriptase in cancer cells?
It adds telomeric sequences which compensate for gradual erosion of chromosome ends and enable cells to undergo divisions while maintaining telomere length and chromosome stability
By which mechanism does UV light cause genomic instability?
Mutations in P53
How is genetic instability induced by radiation?
- Induce mutations in genes involved in control of DNA synthesis or DNA repair
- Induction of chromosome instability
- persisting aberrant production of oxygen radicals that can harm DNA
Which tumours in vet med has been associated with chronic inflammation?
FISS
Feline ocular post traumatic sarcoma
Which extrinsic factors have been evaluated for causing neoplasia in vet med?
UV- evidence consistent for tumour development - SCC
Trauma or chronic inflammation - FISS
Magnetic fields- no clear evidence
Radiation - evidence for secondary tumours after RT - mesenchymal most common
Surgery and implants - struggle to have evidence for cause and effect relationship, hard to differentiate from infection and ostemyelitis
Asbestos - mesothelioma
Virus - FeLV, FIV, papilloma, feline sarcoma virus
Which tumours have an increased risk after castration/Spay?
Castration; Cardiac, OSA, prostatic epithelial tumours, TCC, LSA
Tumours increased risk after spay: Cardiac tumour, OSA, HSA; MCT LSA
Which methods can be used to study specific DNA lesions caused by radiation?
- Velocity sedimentation
- Filter elution
- Assay for chromosomal change
- DNA electrophoresis
Which techniques have been used fr quantification of single or double strand breaks after RI?
FISH
Premature chromosome condentsation
What are telomeres?
Network of proteins that bind the chromosomal ends and protect the from being recognised as DNA damage
What happened of the cell have loss of teleomere DAN or disruption of protein function at the temolmere?
Will be equivalent to a DSB signal and induce cessation of cell division or apoptosis
By which mechanisms does alternative lengthening of telomeres (ALT) work?
They elongate the protein complexes through recombination, exchange or copying the existing telomere
Which factors are recruited in the case of telomere dysfunction?
Activation of ATM/ATR completes which phosphorylate Chk1 and Chk2, resulting in p53 and p21 dependent cell cycle arrest and apoptosis
What is shelterin?
A complex binding telomeres ensuring telomerase axes, replication and integrity, t-loop assembly and stability .
Give 3 examples of methods investigated for telomerase as treatment target
Antiosense oligonucleotides (GRN1563L)
Telomerase promotoer driven suicide therapy
Telomerase immunotherapy
Why is telomerase directed therapy cancer specific, and for which patient group may it be more difficult to use?
Because dividing cells, such as cancer cells have shorter telomeres, should be able too preserve slowly dividing cells
Pediatric cancer have longer telomeres, may be less effective
