Lecture 13 - Exploiting genome instability Flashcards
(14 cards)
Why is it important to understand genome stability?
- to understand the molecular causes of disease
- better healthcare diagnostics for patients
- develop new technologies - e.g. CRISPR
- To develop new treatments for disease - e.g. cancer
- to educate the public on lifestyle choices
- understand how our cells function & how we age
Ageing (living longer) is a problem for the populations, as types of cancer are changing. Cancers associated with old age is becoming more common.
Where are many major signalling pathways mutated?
In cancer
What are Cancer cell properties?
Features of cancer cells that make them susceptible to DNA damage
- altered metabolism (Warburg effect - increased glucose uptake)
- More ROS due to increased metabolism
- have reduced contact inhibition (tissue limits)
- resistant to normal apoptosis
- highly proliferative (lack of cell cycle checkpoints)
- have unstable genome
Have mutations in DDR genes (e.g. TP53, BRCA1) - 50% of cancers have mutated p53
Loss of contact inhibition (ability to stop growing when in contact with other cells) also leads to genome instability.
What is the effect of DDR deficiencies present in cancer cells?
This reduced ability to repair DNA damage & fast growth can be exploited
What are PARP inhibitors?
PARP - Poly ADP-Ribose Polymerase - involved in BER/ssDNA repair
PARP inhibitors are effective anti-cancer drugs - inhibit BER/ssDNA repair, leading to cells accumulating DNA lesions/ssDNA breaks.
Synthetic lethality - cancer cells may also lack BRCA1, making them defective in HDR, rely more heavily on BER.
Therefore adding a PARP inhibitor (Olaparib):
- means cells don’t have HDR or BER
- Alternative resolution of lesions not possible
- Impacts cancer cells more than normal cells
How do PARP inhibitors work?
Inhibiting repair pathway - cause cells to build up more lesion & ssDNA breaks. This creates SYNTHETIC LETHALITY - cancer cells may be deficient in BRA1, through mutations at this tumour (e.g. breast cancer). Proteins usually used in homology directed repair (HDR).
Homology directed repair used in G2, as this is where there is sister chromatids, as this after DNA replication.
This makes cancer cells more reliant on other repair pathways.
What are PARP inhibitors used to treat?
Used to treat ovarian cancer
- clinical treats are ongoing for a range of other cancers - HOWEVER, cancers can become resistant to PARPi - reversion mutations in DDR genes.
The PARP inhibitor market estimated to be ~$13 billion by 2031
Often used in combination with chemo/radiotherapy
Why use PARP inhibitors?
- sensitization to other DNA damaging agents
- these treatments cause DNA lesions/DNA breaks
- greater pressure on cancer cells in the presence of PARPi
Can end up selecting for resistant clones
Used in combination with radiotherapy to cause more damage, so that the cells become more stressed.
What is chemotherapy - Cisplatin?
Chemotherapy drug based on platinum
- is an alkylating agent
When inside the cell undergoes aquination:
- Water replaces the chlorine ions
- Becomes a highly reactive molecule
- Binds to DNA to form a lesion
Alkylating agent interacts with DNA backbone. When it enters cell & water, it sticks to DNA
Binds directly (crosslinks) to DNA
- Intrastrand (90% of events) - between guanines on the same DNA strand
- Interstrand - between guanines on different DNA strands
Both block DNA replication & transcription.
THIS TRIGGERS CELL CYCLE ARREST & THE INITIATION OF DNA REPAIR MECHANISMS
Describe how cells try to repair Cisplatin-DNA lesion
- causes DNA damage
- initiates a DDR
- normal cells are better able to repair these lesions
- cancer cells lack the necessary repair pathways
- affects fast growing cells
- more greatly damaged
Highly effective in treatment of testicular cancer
Describe Cisplatin
- Has cytotoxic effect (even to normal cells) - sometimes used together with DNA repair inhibitors (such as Gemcitabine)
- is metabolised into gemcitabine diphosphate (dFdCDP)
- interferes with nucleotide production
- this gets incorporated into newly synthesised DNA during repair
- causes more genomic stress to cancer cells vs normal cells
- cancer cells can become cisplatin resistant.
Inside cell you rarely find free nucleotides floating. The cell makes them as needed.
What is radiotherapy?
2 types:
- EBRT (external beam therapy)
- internal radiation therapy
- Uses X-rays, gamma rays, neutrons
- Proton therapy
- 50% of cancer patients receive radiotherapy
Radiation is focused on cancer cells - directly damages DNA - DSBs, SSBs, ROS
Can be better than chemotherapy as you can focus it in a specific area & produce localised damage.
What is Gamma Knife therapy?
- Stereotactic radiosurgery
- Precision beams of gamma radiation
- Very intense dose in a small area
- Used often for head/neck cancers
Again, normal cells are better able to repair damage caused through a functional DDR
How are forward genetic screens used in exploring genome instability?
- The ability to cause mutations enables the study of genes
- DNA repair is not 100% efficient
