Genome Instability Flashcards
What is genome instability?
Refers to unscheduled alterations, either temporary or permanent, within the genome
How do these genome instability appear in cancer?
- All malignant tumour types have been shown to contain chromosomal aberrations
- The pattern of abnormalities varies greatly between malignancies, ranging from simple rearrangments to complex where both chromosome number and structure is affected
Does changes caused by genome instability affect the tumour?
- May be involved in initiating the tumour
- Changes accumulate as tumour progresses as tumour cells are more prone to rearrangements compared to normal cells
What are the 2 types of genomic instability?
- Chromosomal instability (CIN)
- Nucleotide level instability
What is chromosomal instability?
Gains/losses of whole chromosomes including inversions, deletions, duplications and translocations of large chromosomal segments
What is nucleotide level instability?
Mutations of single or small groups of nucleotides (not visible morphologically)
How can you identify complete loss/gain with chromosomal instability?
Create a chromosome spread where mitotic chromosomes are laid out and dyed to stain the DNA in order to be able to count each chromosome number
How can you identify translocation with chromosomal instability?
Use chromosome painting where each chromosome number is stained a different colour to allow any translocation can be observed
What is balanced translocation?
When two chromosomes have broken off and each part has attached to the other chromosome
What is unbalanced translocation?
When a part of a chromosome has detached and attached to another but no other part of a chromosome has attached to its original
How can you identify deletion/duplication/inversions with chromosomal instability?
- When chromosomes are stained, there are visible bands of DNA as the grouped DNA is very dense
- Any DNA that has been deleted/duplicated/inversions on one chromosome can be see when it is compared with the other
How can you identify nucleotide level instability?
- They are small insertions/deletions so can are not visible with a microscope
- PCR is used to detect these changes
- Single base changes sequencing can also be used as it will detect the single specific nucleotide that has been altered
What are the effects of point mutations in genome instability?
- Altered gene products (amino acids/proteins)
- Altered control of the gene product (on/off)
What are the effects of partial/complete deletion in genome instability?
- Loss of gene products
What are the effects of duplications in genome instability?
- Possible interference in balance protein expression (eg chordoma)
What are the effects of inversions/translocations in genome instability?
- Altered gene products
- Altered gene control
- Generation fusion proteins with deleterious activities
Is genome instability always bad?
- Can be neutral, eg point mutations can give silent mutations or natural variation which will have no effect, especially in non coding areas
- Can be positive, eg immune cells are able to differentiate in order to protect against any pathogen
What 4 processes cause genome instability?
- Loss of high-fidelity during DNA replication in S phase
- Errors during chromosome segregation in mitosis
- Uncoordinated cell cycle progression
- Error prone repair of sporadic DNA damage
How do the different steps of high-fidelity maintenance become unstable?
- Polymerase accuracy (point mutation)
- Mismatch repair (point mutation)
- Origin licensing (over/under replication)
- Maturation of okasaki fragments (retention RNA)
- Restart stalled replication forks (loss of genome)
- Re-chromatinisation (stall replication)
- Telomere maintenance (loss of sequences)
- Preservation epigenetic signatures (lack of accurate transcriptional information)
Which process during chromosome segregation in mitosis lead to chromosome instability if not completed properly?
- Chromosome condensation
- Sister chromatic cohesion
- Kinetochore assembly and attachment
- Centrosome duplication and attachment
- Spindle formation
- Chromatid segregation
- Cytokinesis
How do the incomplete processes of mitosis cause chromosome instability?
Through chromosome breakage and/or mis-segregation
Do these mitotic processes have a large effect on the cell?
No, as the faults are usually detected and eradicated by the cell cycle checkpoints
What are the 5 cell cycle checkpoints?
- G1/S checkpoint
- G2/M checkpoint
- Intra-S checkpoint
- Spindle checkpoint
- Post mitotic checkpoint
What does the G1/S checkpoint check?
- Sufficient energy levels
- Sufficient nucleotide levels
- Important protein complexes are in the correct place for replication
What does the G2/M checkpoint check?
- Replication is finished
- Chromosomes are in the right state for separation (condensed)
What does the Intra-S checkpoint check?
- No unresolved DNA damage present
- No stalled replication forks present
What does the Spindle checkpoint check?
- Spindles are attached correctly to chromosomes
What does the post mitotic checkpoint check?
- Chromosome separation is complete
What happens if these checkpoints fail?
- Point mutations may be fixed in the new cells
- Cells may divide with incomplete replicated DNA which can result in inappropriate chromosome fusions
- Cells may divide carrying over re-replicated regions of DNA which also results in inappropriate chromosome fusions
- Cells may divide with chromosomes that are not properly attached to the spindle which results in uneven distribution of chromosomes
What is chromothripsis?
- Thousands of clustered chromosomal events occur in one single event in a confined genomic region
- Cause is unknown but it is suspected that it is due to a catastrophic event
- This causes the genome to fall apart however the cell repair processes are unable to rearrange them accurately
