Targeting DNA Damage Repair in Cancer Treatment Flashcards
What is the most effective treatment and those cured, how many does it cure?
Surgery
49%
How many people does chemotherapy cure?
11%
How many people does radiotherapy cure?
40%
When someone is ‘cured’ in cancer, what does this actually mean?
they have had 5-10 years of remission
What is chemotherapy?
The treatment of disease using chemical substances, especially the treatment of cancer by cytotoxic and other drugs
What is targeted therapy?
Inhibition of growth signalling
What is radiotherapy?
The treatment of disease using x-rays and similar forms of radiation
What do chemotherapy and radiotherapy both cause?
DNA damage
Why can DNA repair pathways be targeted for treatment?
In tumour progression, most tumours lose one or more repair pathway, increasing mutagenicity and heterogeneity
This means you can target the cells remaining repair pathway to cause apoptosis
How do you treat thyroid cancer?
Radioiodine
How do you treat bone metastases?
Strontium-89
What is the typical treatment time for radiotherapy?
5 days a week for 4-7 weeks
What is the typical dose of radiotherapy?
2 Gray
What is the issue with high energy radiation?
It can damage tissue around it
What is the issue with low energy radiation?
Spreads more than high energy
Give an example of an alkylating agent
Temozolomide (TMZ)
What is temozolomide used to treat?
Glioblastoma
What is temozomide used in combination with?
Radiotherapy
What does temozolomide effect?
MGMT levels
Give an example of an antimetabolite drug
Fluorouracil
What does fluorouracil treat?
Colon Rectal Breast Stomach Pancreatic
How does fluorouracil work?
interferes with metabolism of nucleotides
What are the side effects of anti-metabolites?
Nausea
Vomiting
Myelosuppression
What does cisplatin, carboplatin and oxaliplatin do?
Cause nucleotide damage and cross link damage
What are the side effects of cisplatin, carboplatin and oxaliplatin?
Nausea
Vomiting
Neurotoxicity
Neutropenia
Name a topoisomerase poison
Camptothecin
What does camptothecin do?
causes DNA breaks by interfering with action of topoisomerases
What do PARP inhibitors do?
Cause DNA breaks
What are the side effects of PARP inhibitors?
Nausea
Vomiting
Anaemia
Name a microtubule poison
Paclitaxel
What is the function of paclitaxel?
Inhibitor of microtubule assembly during cell division
What are the side effects of paclitaxel?
Nausea
Vomiting
Anaemia
How are alkylating agents able to treat glioblastoma?
They are small molecules which can pass through the blood brain barrier
What are cisplatin, carboplatin and oxaliplatin used for?
Breast and ovarian cancer
What is TMZ hydrolysed to?
Methyl triazino imidazole carboxamide (MTIC)
How fast is TMZ hydrolysed?
in 1 hour at peak plasma concentration
What does TMZ make when it alkylates DNA?
O6 methyl-guanine
What effect does O6 methylyguanine have?
It cannot pair with cytosine
What is O6 methylyguanine methyltransferase (MGMT)?
A transferase and receptor
How is MGMT targeted for degradation?
By methylation
What is the function of MGMT?
Helps to protect DNA against alkylating agents
In terms of treatment, what is the issue with MGMT?
Can cause resistance to TMZ
Issue with TMZ treatment?
Tumours with high MGMT levels were not as successful with treatment
How did the issue of TMZ tried to be overcome?
Using an MGMT inhibitor e.g. O6-benzylguanine pseudosubstrate
What was the issue with using an MGMT inhibitor?
Only marginal clinical benefit
Side effects: myelosuppression
What is myelosuppression?
Decreased bone marrow activity
What are defects in mismatch repair (MMR) associated with?
Tolerance to TMZ, platinum agents and some nucleoside analogues
How does 5-FU work in terms of 5-UMP?
5-FU is converted to an active metabolite F-UMP -> incorporated into RNA -> inhibits RNA processing -> Inhibit cell growth
What can 5-FU be converted to?
5-fluoroxyuridine monophosphate (F-UMP)
5-5-fluoro-2’-deoxyuridine-5’-O-monophosphate (F-dUMP)
How does 5-FU work in terms of 5-dUMP?
5-FU is converted to 5-dUMP -> inhibits thymidylate synthase -> depletes thymidine triphosphate (one of the 4 nucleotide triphosphates used for DNA synthesis)
How does replicative stress cause a DNA break?
If you run out of the building blocks for DNA, you form a replication fork -> DNA break
What is the function of topoisomerases?
To unwind DNA to control supercoiling of DNA during DNA replication
What two molecules inhibit topoisomerases in the body?
Camptothecin (topoisomerase I)
Etoposide (topoisomerase II)
What is the etoposide drug called?
Etopophos
What is etopophos used to treat?
Small cell lung cancer
Ovarian cancer
Testicular cancer
Lymphoma
Side effects of etopophos
Alopecia
Anaemia
Sepsis
What is less toxic, cisplatin or oxaliplatin?
Oxaliplatin
As well as causing DNA crosslinks, what other effects can platinated drugs have?
inhibit thymidylate synthase
How do platinated drugs increase the immune response?
Interact with TLRs of dendritic cells and stimulate IFN-gamma
What is oxaliplatin often used in combination with?
5-FU
How does resistance to platinated drugs occur?
through decrease uptake or mutations in DNA damage response
Give an example of a radiomimetic?
Bleomycin
How do radiomimetics induce DNA damage?
Double strand breaks
What cancer are radiomimetics used in?
Testicular
Ovarian
Hodgkins lymphoma
Non-hodgkins lymphoma
What are the side effects of radiomimetics?
Pulmonary fibrosis (scaring of the lungs caused by lipid peroxidation)
What are the repair pathways for radiomimetics?
Homologous recombination
Non-homologous end joining
What are the repair pathways for platinated drugs?
nucleotide excision repair
interstrand crosslink repair
homologous recombination
What are the repair pathways for topoisomerase inhibitors?
Homologous recombination
Non-homologous end joining
What is PARP?
Poly ADP-ribose polymerase
Which PARPs are linked to DNA damage response?
1, 2 and 3
What do PARP synthesise?
Poly ADP ribose (PAR)
What is the different between ADP ribose and ATP?
Instead of having a phosphate group, there is a ribose group on the end
What is the constituent for PARP?
NAD+
What is the function of PARP1?
It is recruited to sites of DNA damage and it PARylates proteins and itself
How does base excision repair/ single strand repair occur?
A Poly ADP-ribose recruits repair factors -> gap is filled by PARP and DNA ligase
What is the rational for using PARP inhibitors?
Might sensitise cancer cells to conventional treatments
Inhibiting PARP-mediated repair of lesions created by standard chemotherapy and radiotherapy might increase the potency of these agents
How many cases of breast cancer are inherited?
5-10%
Of those who have inherited breast cancer, how many are due to mutations of BRCA1 and 2?
40%
What are BRCA1 and 2 for?
To repair damage via homologous recombination
What does BRCA1 determine in homologous recombination?
Determines if a lesion is repaired
What does BRCA2 determine in homologous recombination?
They load RAD51
What is the process of homologous recombination?
There is a double strand break in sister chromatids -> DNA ends resected to generate a single strand DNA -> BRCA1 promotes the initiation of homologous recombination -> BRCA2 loads RAD51 onto DNA -> DNA synthesis and branch migration -> ‘missing’ information replaced -> DNA ligation
What is synthetic lethality?
Combination of deficiencies in the expression of two or more genes leads to cell death, whereas, a deficiency in only one does not
How do PARP inhibitors induce synthetic lethality with BRCA1 and 2 gene mutations?
PARP inhibitors normally cause repair to go through the homologous recombination pathway since it blocks base excision repair
BRCA1 and 2 mutations means that homologous recombination cannot occur
Add the two together means there is not a repair pathway available
What is the issue with PARP inhibitors?
It increases ssbreaks, since HR is not available, it can cause end joining and this causes translocation (promotes tumourigenesis) and cell death
The potency correlates with trapping the PARP1 enzyme in the DNA
How can you detect someone may be response to PARP inhibitors?
Good response to platinum drugs also means a good response to PARP inhibitors
Genetic screenings for mutations in key genes, e.g. BRCA 1 and 2
DNA sequencing for genomic scars which indicate defects in homologous recombination
Identify defects in proteins involved in homologous recombination
What cancers has signature 3 been found in?
Breast
Ovarian
Pancreatic
What is signature 3 associated with?
Failure of DNA double-strand break-repair by homologous recombination
What mutational features are associated with signature 3?
Elevated numbers of large insertions and deletions with overlapping microhomology at breakpoint junctions
What DNA damage marker can you stain for?
H2X
What are the two benefits of using biomarkers?
Makes sure the person had the best drug
Improves the performance of the drug in drug trails - this is cost efficient
What are the mechanisms of PARP inhibitor resistance?
Genetic reversion
Genetic suppression
Drug efflux
What is genetic reversion?
Means they did have the mutation but the treatment causes their mutation to be reversed so the therapy isn’t successful
How can BRCA mutations undergo genetic reversion?
They are commonly caused by frameshift mutations which induce a stop codon
It can undergo another frameshift mutation so this stop codon is removed
What gene causes end joining?
53BP1
Which dominates in normal cells for the repair of DNA breaks, BRCA1 and 53BP1?
BRCA1
If they develop mutations in both BRCA1 and 53BP1 what happens?
They develop a secondary mutation which allows the homologous recombination route to be restored
How does drug efflux cause resistance to PARP inhibitors and Platinum drugs?
Mutations in the cellular pumps mean it can pump the drugs out before they are able to have an effect
