Genome instability Flashcards
Define genome instability
- unscheduled alterations, either of a temporary or permanent nature within the genome
Explain genome instability in cancer
- all malignant tumour types have been shown to contain chromosomal aberrations
- the pattern of abnormalities varies between malignancies, from simple balanced rearrangements to complex abnormalities affecting both chromosome structure and number
- the changes may be involved in initiating the tumour, but also accumulate as the tumour progresses as tumour cells are more prone to rearrangements than normal cells
What are the two types of genome instability?
- chromosomal instability (CIN) - gains or losses of whole chromosomes as well as inversions, deletions, duplications and translocations of large chromosomal segments
- nucleotide level instability - mutations of single or small groups of nucleotides - not visible morphologically
CIN - how are they detected?
- loss and gain of chromosomes
- look at chromosomal spread - at mitotic stage
- stain with dye which stains DNA
Chromsome translocations
- chromosome painting - easy to see translocations
- can be balanced - swap pieces between ends of chromsomes
- non-balanced - piece transferred to another chromosome, but no transfer back
How are deletions/duplications/inversions visualised
- giemsa staining - banding patterns can be identified - if segments are lost/duplicated/inerted
How are nucleotide level changes identified?
- small insertions and deletions are detected through PCR - region of interest can be amplified
- single base changes can be sequenced
What are the outcomes of genome instability and cancer?
- point mutations - affect gene products and control
- deletion of whole/partial chromsomes - loss of gene products
- duplications - possible interference in balance protein expression
- inversions/translocations - alteration gene products, alteration gene control - generation of fusion proteins with deleterious activities
Is genome instability always bad?
- can be neutral eg point mutations can give silent mutations and natural variation (SNP) - have no effect
- can be positive - immune cells
What are the processes which cause 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 is high fidelity DNA replication maintained?
- polymerase accuracy - mainly point mutations
- mismatch repair - mainly point mutations
- origin licensing - once per s phase - over replication =potential amplification, DNA breakage and recombination. under replication - loss of genome on cell division, CIN most common
- maturation of okasaki - fragments - retention RNA and generation of nicks and gaps causing DNA breakage and recombination - mostly CIN
- restart stalled replication forks - potential loss of genome on cell division, DNA breakage and recombination
- re-chromatinisation - potential to stall replication leading to loss of genome on cell division, DNA breakage and recombination
- telomere maintenance - loss of sequences at chromosome ends, DNA breakage and recombination - high susceptibility for translocations
- preservation epigenetic signatures - lack of accurate transcriptional information
What errors occur during chromosome segregation in mitosis?
- chromosome condensation
- sister chromatid cohesion
- kinetochore assembly and attachment
- centromere duplication and attachment
- spindle formation
- chromatid segregation
- cytokinesis
What happens if there is uncoordinated cell cycle progression?
G1/2 checkpoint - insufficient energy levels, insuffifient nucleotide levels, important protein complexes
G2/M checkpoint - replication is not finished, chromosome not in right state for separation
Intra-S checkpoint - unresolved DNA damage present, stalled replication forks present
Spindle checkpoint - spindle not attached correctly to chromosomes
Post mitotic checkpoint - chromosome separation not complete
What are the consequences of control/checkpoint failure?
- point mutations may be fixed in the new cells
- cells may divide with incompletely replicated DNA (chromsomes may be pulled apart - leads to cells with broken chromosome ends = aneu or polyploid
- cells may divide carrying over re-replicated regions of DNA - strand invasion, inappropriate fusions
- cells ay divide with chromosomes not properly attached to the spindle - uneven dist, chromosome leading to aneu/polyploidy
Describe chromothripsis
- thousands of chromosomal events occur in a single event in a confined genomic region
- exact details not clear - could be due to aborted apoptosis , ionising radiation during mitosis = shattering of chromosome
- when cell repair processes try to put it back together, doesn’t get exactly right so rearranged
- recently suggested that it is due to breakage of a single chromosome which has become incorporated into a micronucleus
- counteracts the argument that cancer is always due to gradual acquisition of genomic rearrangements and somatic mutations over time
