DNA Cleavers and Repair Flashcards
What are examples of DNA strand breakers?
Bleomycin and the Phleomycins
DNA double strand repair
Platinum drugs for chemotherapy.
What are bleomycins?
A mixture of cytotoxic glycopeptide antibiotics isolated from a strain of Streptomyces Verticillus that is freely soluble in water.
They are related glycopeptides that differ in terminal amine substituent of the common structural unit.
They are used in combination therapy against lymphomas, squamous cell carcinomas and germ like tumours.
They bind transition metals and oxygen in the presence of electron reductant and can catalyse the formation of single stranded and double stranded DNA.
What part of bleomycins is essential for dsDNA cleavage?
The linker between the metal and the bithiazole DNA binding domain and the flexibility of the bithiazole moiety itself.
What are common outcomes of Bleomycin treatment?
They are complex, cell line and genotype dependent.
Extended cell cycle arrest, apoptosis and mitotic cell death.
What is the mechanism of action of bleomycin induced double stranded DNA cleavage?
They initiate single stranded DNA cleavage at pyrimidines 3’ to guanine in a sequence specific method.
Secondary site of cleavage depends on the residue which si 3’ to the primary cleavage site.
It requires bleomycin reactivation and bleomycin reorganisation after or during the first cleavage of DNA stand.
The linker and the flexibility of the bithiazole tail bound by partial intercalation. Rotation around the bond between the two thiazole rings in the tail of belomycin with other motions make the peroxide of the activated drug available for interaction with the second DNA strand.
How is double stranded DNA repaired?
Recombination can be initiated by a double strand break which may be caused by an endonuclease or DNA damaging agent.
DNA is processed at the site of the break to yield regions of single stranded DNA.
Rad51 and replication protein A coat the single stranded DNA to form a filament which searches for homologous sequences.
The break is repaired by DNA synthesis using intact strands as templates, then the repaired recombination products are released after branch migration and resolution.
What is recombination?
An error free method to ensure the repair of DNA double strand.
How is tumour resistance formed when platinum enter cells?
It enters using transporters or passive diffusion. The loss of CTR1 results in less platinum entering the cells and drug resistance.
When it is inside the cells cisplatin is activated by addition of water molecules to form a chemically reactive aqua species facilitated by relatively low chloride concentrations found inside cells.
Activated aqua species reacts with species containing high sulphur levels, the species include tripeptide glutathione or metallothionein’s.
In platinum resistant cancer cells the levels of glutathione and metallothionein levels are high which means activated platinum is mopped up in the cytoplasm before DNA binding can occur causing resistance.
Active export of platinum from cells through the copper exporters ATP7A and ATP7B and the GS-X pump contribute to platinum drug resistance.
Once activated the aqua platinum species enters the nucleus and covalently binds to the nitrogen on position 7 of guanine.
Major covalent bis-adduct formed has adjacent guanines on the same strand of DNA and the minor adduct involves binding to guanines on opposite DNA stands.
The DNA adducts are removed by nucleotide excision repair. This can happen in tumours and lead to platinum resistance.
Resistance can occur through increased tolerance to platinum DNA adducts, through DNA mismatch pair, bypassing of adducts by polymerase beta and nu, or through down regulation of apoptotic pathways.
What are strategies to circumvent cisplatin and carboplatin resistance?
Resistance can be tackled by:
Increasing levels of platinum reaching tumours resulting in greater killing and combining existing platinum drugs with molecularity targeted drugs.
Using novel platinum drugs which can circumvent cisplatin mediated mechanisms.
Using other drugs which exploit particular cisplatin mediated resistance mechanisms.
How do angiogenesis inhibitors work?
By blocking the growth of new blood vessels to tumours.
It is a monoclonal antibody binding to the vascular endothelial growth factor which tumour cells release to stimulate blood vessel growth.
Binding to the drug and growth factor prevents vascular endothelial growth factor from interacting with its receptors and on the endothelial cells that line the blood vessels.
Activation of these receptors normally leads to endothelial cell growth and the formation of new blood vessels to the tumour, preventing this growth inhibits metastasis and may improve sensitivity to cytotoxic drugs.
What is the structure of Bleomycins?
They have a metal binding domain where nitrogen atoms coordinate the metal. There is a linker region and a bithiazole tail.
There is a disaccharide and the tail is positively charged.
