Session 3 - Oncogenes and tumour suppressor genes Flashcards
What is a universal truth of cancer mutations?
Cancers are universally formed by variant mutations in proto-oncogenes and tumour suppressor genes, activating the former and de-activating the latter.
Give five possible types of mutation in cancer
- Deletion – Base pairs, Genes and Chromosomes
- Insertion – Base Pairs, Viral DNA
- Substitution – Base pairs
- Amplification – Gene, chromosome
- Translocation – Chromosome
Name three oncogenes which can become activated and cause carcinogenesis
Growth factor receptors, GTP-binding proteins, Tyrosine specific protein kinases, steroid-type growth-factor receptors, nuclear protiens and Growth Factors.
What is the RAS gene family and the MAPK pathway?
Ras activates the MAPK pathway, by binding to GTP and rendering it active. The MAPK pathway is a chain of proteins in the cell that communicates a signal from a receptor on the surface of the cell to the DNA in the nucleus of the cell.
What happens to MAPK if RAS is permanently activated via mutation?
It is activated by RAS permanently and growth signals are transmitted to the cell nucleus without rest.
What is insertional mutagenesis?
DNA viruses can incorporate a viral oncogene in the host DNA, with deleterious effects. Examples include HPV 16/18, which produce the E6 & E7 proteins – inhibiting P53 and PRb respectively.
Give three ways in which oncogenes can be permanently activated
Insertional mutagenesis
Chromosomal translocation
Chromosomal amplification
What is chromosomal translocation and give an example of how it cause cancer formation?
Chromosomal translocation, such as the 9:22 translocation found in chronic myeloid leukaemia. In this condition C-abl (9) is truncated onto bcr (22), with the effect of producing abnormal fusion proteins which have effects such as abnormal tyrosine kinase activity.
How does chromosomal amplification cause cancer?
Tumour genomic instability leads to amplification and may lead to over production of the normal protein.
Give an example of an upregulated oncogene
HER-2 amplified and overexpressed in 10-30% of breast cancer
How can HER-2 amplification be detected?
By FISH and immunohistochemistry
How can HER-2 amplified breast cancers be treated?
Herceptin
Give give wways in which tumour supressor genes encourage stability
- DNA repair
- Carcinogen detoxification
- Cell cycle control
- Apoptosis
- Senescence
What is genetic silencing?
Gene silencing can occur via promotor methylation in the target gene, which causes promoters to entirely miss said gene and stop it from producing translating itself into life. This is of particular significance in the case of P53.
Promoter methylation of a number of genes may be the molecular link between again and cancer. This is an epigenetic change which does not cause change in base sequence.
What is p53?
Tumour suppressor protein mutated in 50% o all cancers. It is widely expressed, and causes cell cycle G1 arrest in response to DNA damage. Can trigger both cell differentiation and apoptosis.
Outline the incidence of colorectal cancer
Colorectal Cancer is the third most common cancer in the UK, and is the second most common cause of cancer deaths
Give three ways in which CRC can develop
Sporadically
Hereditarily
Familialy
What is the most common way in which CRC develops?
Sporadically, in which individuals have no family history of the condition
What is the difference between familial CRC and hereditary CRC?
Familial CRC relationships reward a vague genetic propensity to develop the condition, whereas the genetic link in hereditary CRC syndromes is more explicit.
Outline Duke’s staging of CRC
- Duke A (stage I): cancer confined within the bowel. Localized disease
- Duke B (stage II): Invasion of the muscle layer, no lymph nodes involvement
- Duke C (stage III): Involvement of lymph nodes
- Duke D (stage IV): Distal metastasis
What is the survival difference between duke stage A and D?
Survival vastly differs across the different stages, with Duke stage A having a 90% chance of survival, compared to <5% at Duke stage D.
What is the normal development of colrectal adenocarcinoma?
Normal -> Hyperplasia -> Preneoplasia -> Neoplasia
Normal colon epithelium -> (initiation) -> Adenoma -> (promotion) -> Adenocarcinoma of Colon
What are the two subtypes of CRC
Adenomatous polyps and serrated polyps
Give three subtypes of adenomatous polyps
Villous adenomas
Tubular adenomas
Tubulovillous adenomas
Give three types of serrated polyps
Hyperplastic polyps
Sessile serrated adenoma
Traditional serrated adenomas
What is dysplasia?
Low and high grade dysplasia indicate how much a lesion is different from the tissue of origin, and how close it is to being fully cancerous.
Give three categories of genomic alteration in CRC
Loss of tumour supressors
Gain of oncogenic mutations
Genomic instability
Give three tumour supressor genes
APC (Adenomatous Polyposis Coli)
P53
TGFB-R (transforming growth factor receptor)
Give three types of oncogenic mutation
Kras
Braf
B-catenin
Give three types of genomic instability in CRC
Chromosomal instability
Microsatellite instability
CpG island methylator phenotpe
Which is the most common type of genomic instability?
Chromsomal instability
What types of CRC are APC mutations found in?
Adenomatous polyps, but absent in serrated lesions
What are the two roles of B-catenin signalling?
B-catenin is associated with E-cadherin and essential for cadherin function. It is also involved in gene promotion when in excess.
How does B-catenin signalling become oncogenic?
Oncogenic B-catenin mutations or loss of APC (which breaks down b-C) promotes B-catenin stability, which can lead to translocation of excess b-catenin to the nucleus and the stimulation of cell proliferation, by promoting abberant expression of genes such as c-myc and cyclin D1.
What do mutations in p53 gene effect on the p53 protein?
DNA binding domain
In what types of tumour do p53 mutations occur?
Appears in high grade CRC adenomas, and appears to be necessary to promote extreme malignant transformation.
Give five roles of p53
Growth arrest Apoptosis Prevention of angiogenesis Translation DNA repair
Give two oncogenes
Ras and Braf
What kinds of genes do oncogenes stimulate?
survival, growth, proliferation and angiogenesis.
What do RAS mutations do?
• Mutations effect the GTP/GDP binding sites, which causes decreased GTPase activity and activation of RAS signalling
What do Braf mutations do?
- Ser/thr kinase of MAPK pathway
- Valine to glutamate change within the kinase permenantly activates
- Most common mutation in serrated polyps
What are two types of chromosomal instability?
Chromosomal instability is characterized by widespread imbalances in chromosome number (aneuploidy) and structure (translocation, deletions).
What does chromosomal instability result from?
Results from defects in chromosomal segregation, telomere instability and the DNA damage response. Loss of P53 contributes to CIN.
What is DNA methylation
Mechanism of epigenetic silencing of gene expression
What is CIMP?
CpG island methylator phenotype is the addition of a methyl group to the base cytosine
What does CIMP do?
Widespread methylation leads to compact hetrochromatic DNA which is inaccessible to transcripition factors. Genes which can be methylated in CRC are p53 and APC (P16 and P21 being less important examples).
What is microsatellite instability?
MSI (microsatellite instability) is a defect affecting microsatellites, which are repeating sequences of 2-5bp. These areas are prone to replication errors, and thus insert frameshift mutations which affect coding regions of DNA. Affected genes include TGFB-R and MSH3.
Give some CRC therapies for each Duke’s Stage
- Duke A (stage I): Surgery
- Duke B (stage II): Surgery and chemotherapy(?)
- Duke C (stage III): Surgery and chemotherapy
- Duke D (stage IV): Chemotherapy and surgery
- Rectal cancer: radiotherapy and surgery
Give three agents used in chemotherapy of CRC?
- Fluorouracil: inhibits DNA synthesis
- Oxaliplatin and irinotecan: DNA damaging agents (apoptosis)
- Leucovorin: ameliorates FU uptake and metabolism
What is chemotherapy in stage 2 and 3 of cancer used for?
adjuvant chemotherapy to prevent recurrence or distant metastasis or as neoadjuvant therapy to shrink the tumour mass before surgery.
Give a molecular therapy for patients with stage 4 metastatic CRC
EGFR blockade therapy based on monoclonal antibodies cetuximab and paninutumab
What happens to those given molecular therapies for CRC?
Initial responders invariably develops resistance after few months: in the vast majority of cases there is acquisition of mutations in Kras.
What is the purpose of CRC screening, and when is it given?
- Meant to identify early cancer lesions
- Offered to people aged 60 to 75
- Reduces CRC death rate by 13% (Nottingham trial)
- To be substitute by immunological FOBt
Give to hereditary CRC syndromes
FAP
Lynch syndrome
What is FAP?
- Autosomal dominant disease
- Affects 1 per 5000 in UK/ <1% of all CRCs
- Mutation in APC gene
- Linkage of FAP locus to 5q21 has led to identification of APC gene
- Hundreds to thousands of adenomas
- All patients develop carcinoma by the age of 40/50
What is the therapy for FAP?
• Therapy: colectomy around age 16. Endoscopic polypectomy can be consider in patients with attenuated disease
What is Lynch syndrome?
- Autosomal dominant disease
- Accounts for 1 to 3% of all CRCs
- Do not present with evident polyps
- Mutation in the MMR genes: MLH1, MSH2, MSH6 MSI
- 30 to 70% increase risk of CRC
- No associated LOH
- Age 40/50 or younger.
- Intensive surveillance (colonoscopy every 2 years starting at age 25).
