Cancers are Genetic Diseases; Mutations Flashcards
What is the difference between mutation and epimutation?
Mutation; changes in DNA sequence
Epimutation; DNA/protein modifications passed on to daughter cells (e.g. methylation of histone proteins)
How many mutations are required to initiate cancer?
- At least two; e.g. a RAS-like oncogene (cytoplasmic; signalling protein) and a MYC-like oncogene (nuclear; transcription factor)
- Once initiated by two, tumour progression is accelerated by tumour promoters and further mutations (5 or 6 in total to ‘kick off’ process
What is the mutator phenotype hypothesis?
- Cancer cells contain thousands of mutations; 200 times more than we’d expect
(Spontaneous mutation rate of human cells is approximately 1.4 × 10^−10 per base pair per cell generation) - > 100 genes contribute to DNA repair (protecting against accumulation of mutations, initiating apoptosis if irreparable)
- Mutations in any of those could predispose to a higher mutation rate
- Vicious circle of mutations (in DNA repair genes) allowing further mutation = tumour progresses
»> Cell less able to protect itself, positive feedback loop
»> Results in chromosome instability
What causes oncogenic changes in gene expression?
- Mutations in proto-oncogenes and tumour suppressor genes; change how they behave (normally implicated in controlling cell division, differentiation and death - causes of cancer if malfunctioned)
- Increase/decrease in activity e.g. changing AA sequence influences ligand-binding domain/active site mutation, promoters, UTRs
- Lack of response to proper regulation
Why are p53 mutations important in cancer?
- TP53 gene (encodes p53) is the most mutated in human cancers; > 70% cancers have mutant p53
- Over 29,000 mutations have been identified in the TP53 gene
What are ‘hotspot residues’ WRT TP53?
- 30% of TP53 mutation falls within 6 hotspot residues; residues that are more susceptible to infection.
e. g. nucleotides exposed/liable to modification by mutagens - R175 (Arginine 175)
- G245
- R248
- R249
- R273
- R282
What are the types of mutation that occur to the TP53 gene to produce an oncogenic p53?
- Loss of function mutation; p53 loses tumour suppressor activity
- Gain of function mutation; gains oncogene activity e.g. antagonises unmutated p53 function by aberrant binding to DNA or proteins (transcription factor; tumour suppressing/antagonising normal functions instead)
What does INK4 deletion entail? Why is it detrimental?
- Whole gene deletion
- Normally inhibits cyclin dependent kinases (e.g. Cyclin D-CDK4s; required for cell cycle to proceed as they phosphorylate RB1)
- INK4 is a tumour suppressor
»> Frequently deleted in cancers
»> Removing the safety net/blockage of Cyclin D-CDK4; which will now readily phosphorylate RB1 and allow cell cycle to continue regardless
What does MDM2 amplification entail, and how often does it occur?
- Errors in DNA replication and repair lead to the copying of MDM2 gene multiple times; loads of MDM2s are produced
- MDM2 is amplified in 7% of all cancers
What is the impact of a mutation resulting in MDM2 amplification?
- MDM2 is an oncoprotein; an ubiquitin ligase
- Normally results in ubiquitylation of the tumour suppressor p53, marking it for degradation
- Amplification of MDM2 would result in a big decrease of p53 levels as a result; more MDM2 to ubiquitylate p53, more degradation of vital tumour suppressor
What is the Philadelphia chromosome, and what kind of mutation is it an example of?
- Chromosomal rearrangement
- BCR is a serine threonine kinase and GTPase
- ABL is a signalling TK
»> Reciprocal translocation of chromosomes 9 and 22 occurs, fusing BCR and ABL
(genes for BCR and ABL are normally found in different chromosomes; Philadelphia chromosome is half BCR, half ABL) - Fusion has TK activity, but is NO longer regulated
What cancers is the Philadelphia chromosome mutation often found in, and what therapies are availible?
- Frequently associated w/CML (Chronic Myeloid Leukaemia)
- Imatinib (Gleevec), Nilotinib (Tasigna) and Dasatinib (Sprycel) are TKIs that target BCR/ABL
»> ATP analogues, so protein no longer has phosphate source
What is EGFRvIII (EGFR variant III), and what mutation is it an example of?
- Variant of EGFR; but lacks most of its extracellular domain (ligand-binding domain)
- Constitutively active as a result; continues to phosphorylate downstream proteins despite no ligand-docking (RAS-RAF-MEK-ERK, PI3K-)
- Mutation (deletion) of 801 base pairs (267 AAs); a partial deletion
Which cancer is EGFRvIII prevalent in?
- Present in 20-60% glioblastoma multiforme
What is RAS, and what does a point mutation of RAS result in? Where does the mutation occur?
- GTPase regulating pathways (RAF-MEK-ERK) for proliferation and survival
- Mutations affecting codons 12, 13, or 61 leads constitutively active RAS protein; active downstream pathways as a result
- GTP binding favoured over GDP binding (active conformation of RAS protein favoured over inactive)
How often do RAS mutations occur?
- 20% of cancers
- 99% of RAS mutations are at the codons 12, 13, 61
What is the significance of Pharmacogenetics/Pharmacogenomics?
- Effect of genes/genomes on response to treatment; looking at individual gene sequences
- Variation between patients; drug targets, metabolising enzymes (pro-drug activation/active drug removal)
- Will become more prevalent as testing becomes cheaper
> Could inform prognosis, diagnosis, treatment choice
Describe tamoxifen metabolism as an example of future pharmacogenetics application.
- TAM is a pro-drug; metabolised to active form (4-hydroxytamoxifen) by CYP2D6 (CYP450 superfamily)
- 7% of population have inherited a gene that encodes a less active CYP2D6 protein
»> Poor metabolisers require increased dose of TAM to compensate for lower levels of active 4-OH metabolites
> Recommendation for DOUBLE DOSE for poor metabolites - Genetic testing (of normal DNA) would identify poor metabolisers prior to starting treatment
(but not yet recommended in the UK)
What are examples of how genetic information can influence lifestyle and health management choices?
- SNP (Single Nucleotide Polymorphisms) in genes including MCR1, CDK10 and IIRF4 are associated with melanoma risk; extra care in the sun
- Chromosome 15q24 susceptibility locus associated w/lung cancer risk in smokers (23% vs. 14%)
Do genetic tests costing upwards of 100GBP have any clinical value, or simply prey on people’s vulnerabilities and insecurities?
- Not useful in direct-to-consumer form ATM
- Too much variation between different tests/expense
Why must genetic information be treated with extra care compared to other sensitive patient information?
Genetic exceptionalism:
- Predictive of genetic predispositions (share half DNA w/parents, quarter w/siblings)
- Impact on family members
- Significance may not be apparent at time of collection
- May have cultural significance
> Constant throughout life (genetic information is)
Can be obtained from any cell, even if separate from body (illegal w/o consent)
Of additional value to scientific research
Potential for abuse (insurance companies, employer etc.)
What do the genotypes of tumor cells tell us?
- Have a profound influence on treatment response
- Tumour genomes can be sequenced to personalise treatment; but currently expensive, thus not done in UK
What are ‘immunohistochemical methods’ and what do they do? Give examples.
- Looking at expression of specific target proteins using monoclonal antibodies
E.g. for:
> HER2 for Herceptin
EGFR for erlotinib cetuximab (TKI, mAB)
What are the different mutations that can cause cancer? Give examples for each.
- Chromosome rearrangement (BCR/ABL; big in size)
- Amplification (MDM2)
- Deletion (INK4)
- Partial deletion (EGFR)
- Point mutations (RAS, P53; v. small)
