DNA Strand Breaks, Platinum Compounds

Question 1

Name the DNA repair pathways

Answer 1

Recombination (homologous recombination) is the only error free method. It uses a copy of DNA to make accurate repairs
Other repair methods:
Non-homologous End Joining (NHEJ)
Bread-induced Replication (BIR) are error prone, leading ro mutations and genetic errors

Question 2

What is the MOA of platinum anti-cancer drugs?

Answer 2

The drug enters via passive diffusion or active transport
Inside the cell the drug undergoes aquation (chloride ligands replaced by water) - making them more reactive
The drug binds covalently to DNA, primarilky to gaunine bases
Forms:
Intra-strand crosslinks
Inter-strand crosslinks
DNA-protein crosslinks
DNA damage response
Cross links disort the DNA helix block DNA replication and transcription and trigger cell cycle arrest
if damage is too severe to repair - leads to apoptosis

Question 3

What is the mechanisms of platinum drug resistane?

Answer 3

Reduced drug uptake
Inactivation in the cytoplasm
Increased drug efflux

Question 4

If the nucleoside excision repair is overactivated what can is lead to?

Answer 4

Platinum resistance by removing DNA damage before it can trigger apoptosis

Question 5

Name some other repair mechanisms

Answer 5

Loss of DNA Mismatch repair (MMR) - makes cell less sensitive to DNA damage
Bypass by polymerases β and η - enzymes that let DNA replication continue despite addcuts
Suppression of apoptosis - even if the damage is detected - the cell avoids death

Question 6

How can we overcome platinum resistance?

Answer 6

Increase drug delivery to the tumours - use liposomal platinum formulation
Combine with targeted therapies
- Bevacizumab (a VEGF inhibitor)
Use alternative platinum drugs - drugs like oxaliplatin can bypass resistance
Add resistance exploiting agents

Question 7

Explain homologous repair mechanism

Answer 7

Double-strand break (DSB) - both strands of DNA are damaged due to radiation
Processing the break - the DNA ends are trimmed back, leaving ssDNA regions around the break
Search for matching sequence - Rad51 helped replication protein A (RPA) binds to ssDNA - forms a searching filament that looks for a matching DNA sequence
Repair and resolution - once a match is found, DNA synthesis uses the intact strand as a template to fill in damaged area. After branch migration and proper alignment, the DNA strands are resolved and fully repaired

Question 8

What is the mode of action of Avastin (Bevacizumab)?

Answer 8

Monoclonal antibody
that binds specifcally to VEGF
Blocks VEGF from interacting with its receptors on endothelial cells
Prevents blood vessel growth towards the tumour
Leads to:
- stravation of tumour
- reduced materials
- improved effectivesnness of chemotherapy

Question 9

What does activated platinum prefer to bind to?

Answer 9

Sulfur-rich molecules such as GSH and metallothioneins