Carcinogenesis Flashcards
Classes of cancer genes
Activated protooncogenes -> oncogenes
Mutation of :
Tumour suppression genes, apoptosis genes, somatic or germline cells
What are the metabolic features of cancer cells?
Deregulation of cellular energetics due to the energy required for excessive proliferation of tumours via
aerobic respiration, mainly glycolysis and
increased fatty acid and glutamine synthesis which directly enter Krebs cycle
Avoids immune destruction
How do cancer cells avoid immune destruction?
Induce T cell exhaustion, restrict antigen recognition and inhibit immune response.
What causes damage to DNA in cells?
UV rays, ionising radiation, chemical exposure and metabolic reactions and repair errors
What is the most cytotoxic damage a cell can have?
Break in double stranded DNA
WHat is an endonuclease?
Cleaves phosphodiester bond withinn a polynucleotide chain
What is an exonuclease?
Cleaves nucleotides at the end of the chain by breaking phosphodiester bonds
How can metabolic reactions cause damage to DNA?
Toxic metabolites such as reactive oxygen species (from oxidative phosphorylation) and alkylating agents and aldehydes (from lipid peroxidation)
What are ways that metaboites can be harmful?
Medicinal drugs, carcinogens or endogenous intermediate metabolites. This is initiated by cytochrome P450 in the liver.
What is base excision repair?
Removes small base distortions from the genome which occurs throughout the cell cycle.
What is the response to DNA damage?
DNA repair, apoptosis, cell cycle checkpoint, transcriptional program activation
What are the DNA repair mechanisms?
Direct reversal: requires no DNA synthesis to correct error and uses enzymes to remove damaging parts of the genome
Mismatch repair: maintains cell fidelity and is strand specific
Nucleotide and base excision repair
Homologous recombination
What are the exogenous DNA damaging factors?
Extracellular from UV light, Ionising radiations such as X-rays which contains carcinogenic isotopes, chemicals and natural isotopes
How does tobacco smoke cause cancer?
Benzoapyrene is the carcinogen which inhibits the codon 157, 248 and 273 coding for the p53 tumour suppression protein leading to increased lung cancer risk.
What are the endogenous DNA damaging factors?
Oxygen, water ,
reactive metabolic intermediates such as reactive oxygen species and aldehyde and alkylating agents causing depurination
Normal replication errors eg DNA polymerase
What is the most common morphology of DNA damage?
Loss of purine bases via glycosidic bond hydrolysis, creating an abasic site in DNA absent in bases with no coding information so are highly mutagenic sites.
What is the effect of UV on DNA?
Pyramidine dimers form where there is crosslinking of DNA between cytosine and thymine bases which prevents replication or transcription beyond this point.
What is the effect of sunlight on skin?
50-100 DNA damage errors, most commonly pyramidine dimers and most mutagenic is the photoproducts covalent bonds
What are the photoproducts?
UV causes pyramidine dimers to form covalent bond between adjacent base carbon 4 and carbon 6 and create a thymine dimer which prevents DNA replication progressing from the point of the dimer.
What is the effect of ionising radiation on th DNA strand?
Single/Double strand breaks
What DNA damage is inherent in mitosis?
Replication error
What is the effect of carcinogens on DNA
DNA adduct forms where a section of DNA is bound to the chemical which prevents replication or
DNA crosslinking to form covalent bond intrastrand or interstrand and prevent replication
How do alkylating agents affect DNA?
Causes the addition of groups like methylation or alkyl group, which most commonly to the O6 position of guanine. This affects the GC-AT transition of thymine.
They require direct reversal to remove the DNA damage.
What is the most commonly mutated tumour suppressor gene?
P53, which can be caused by exogenous causes of p53 mutations such as:
Sunlight
Aflatoxin in diet
Tobacco smoke carcinogenic components
What is aflatoxin?
Aflatoxin B1 is a toxic metabolite from a fungus found on nuts. During metabolism of contaimined food, cytochrome p450 causes the addition of epxoide group which is highly reactive towards DNA.
This increases risk of liver cancer.
How can the toxic effects of aflatoxin be reduced by the body?
Cytochrome p450 can inhibit epoxide group through addition of glutathione group.
What is ATM deficiency?
Deficiency in ATM which is important in regulation of damaged double stranded DNA. It is an autosomal recessive condition that increases cancer predisposition with progressive neurodegeneration and premature aging
What are the exogenous causes of p53 mutation?
UV light, Aflatoxin B1 from diet and carcinogen benzoapyrene in tobacco smoke
What is a CPG site?
Area where cytosine nucleotide is followed by guanine nucleotide, separated by phosphate group. They regulate gene expression through silencing of corresponding gene
What are the endogenous p53 mutations?
Methylation of cytosine at the CpG site occurs. This is reversible however, because cytosine is vulnerable to deamination spontaneous deamination can occur to convert -> thymine which is irreversible. This causes G-C to A-T transition.
What happens when cytosine is deaminated?
Deamination of cytosine causes it to form-> thymine base.
How is alkylating damage repaired?
Direct reversal- Covalent transfer of alkyl group to a cytosine residue O-6 methylguanine DNA transferase and form an irreversible complex. This is a mechanism of suicide enzymes
What is an O-6 methylguanine DNA transferase?
DNA repair enzyme for the direct reversal of alkylation.
This is coded for by the human MGMT gene to maintain genetic stability.
What are the types of base excision repair?
Short patch repair of single base
Long patch repair of 2-10 nucleotides
How does base excision repair occur?
Specific DNA glycosylases remove the small and non-distorting base lesion which forms an abasic AP (apurinic/apyramidinic region).
The base which is removed is repaired by action of AP endonucleases which break the phosphodiester bond. If uracil is removed, uracil endocnuclease enzymes acts to insert the base.
DNA polymerase will synthesis new DNA to replace the empty space, and DNA lipase will seal the DNA strand.
Why do AP sites occur?
DNA damage by oxidation
Action of DNA glycosylases to repair base lesions
Why does nucleotide excision repair occur?
DNA damaged area cannot be recognised by DNA glycosylase, therefore the region of the damage contianing the base are removed.
What kind of DNA damage is repaired by nucleotide excision repair?
Large distorting mutations such as:
Crosslinks
Pyramidine dimers such as thymine diners from UV damage
6,4 photoproducts
Chemical adducts
Which proteins are involved in the nucleotide excision repair complex?
XPA, XPB, XPC, XPD, XPF, XPG, ERCC1 and RPA
How does nucleotide excision repair occur?
XPC is part of DNA damage sensor complex for recognition. XPA binds to the damage site. XPB and XPD separate the strands which are kept apart by RPA.
ERCC1, XPG and XPF are endonucleases that cleave DNA on either side of the damage for removal of the defective nucleotide. The gap is filled in by DNA polymerases that synthesises new functional DNA. It is then sealed by DNA ligases
NER: What recongises the DNA damage?
XPC
NER: What acts as a scaffold for recruitment of other proteins?
XPA which binds to the site of damage.
NER:What causes uncoiling?
Helicase enzymes XPB and XPD which separates the strands.
NER: What keeps the strands apart?
RPA
NER: What causes removal of nucleotide section?
Endonucleases ERCC1, XPG and XPF which cleave DNA on either side pf the damage
What is xeroderma pigmentosum?
Genetic mutation in the nucleotide excision repair gene which results in the extreme photosensitivity of the skin and eyes to UV which dramatically reduced life expectancy to around 30 years old. Results in blistering, sunburn, telangiectasia. cancers and ulcers of eyes that can metasise to the brain.
How is a double stranded DNA repaired?
Homologous recombination or non-homologous end-joining.
What is homologous recombination?
DNA repair of double stranded break. Resection occurs to expose 3’ ends. One of these 3’ tails invades a homologous complementary strand which can be used as a template for DNA polymerase to synthesise new DNA by forming a displacement (D) loop.
Eventually, the newly synthesised DNA will then disassociate from the D loop and rejoin its original DNA strand, with DNA polymerase filling in any remaining gaps and DNA ligase sealing it.
What is non-homologous end-joining?
Primary pathway when damage to Double stranded DNA occurs early in cell cycle such as G1 that there is no homologous template to use for DNA repair.
What is the process of non-homologous end joining?
Resection causes first stage to cleave the broken DNA ends. Ku protein binds and leaes the ends open, recruiting DNA-PKCS. These recruit and phosphorylate artemis which trims single stranded DNA overhangs.Ligase IV, XRCC4 and XL4 bind and repair the broken ends
What type of mutations are the predominant cause of cancer?
Somatic mutations
What are the types of point mutations?
Missense: different amino acid
Nonsense: early chain termination
Frameshift: different protein translation
Silent: does not affect amino acid produced
What is used as a comparison for gene mutations?
Wild-type allele
What is the two hit hypothesis?
Tumour suppressor genes require both alleles to be inactivated via mutations for phenotypic change such as cancer to occur
What is first hit?
The first mutation of the tumour suppression gene, which is typically Genetically inherited if onsent occurs earlier in life
What is second hit?
Acquired mutation of the second allele due to
Allele loss via deletion or
Hypermethylation of the CPG site
Allele loss
Point mutation
What are the hereditary genes?
Rb1: Retinoblastoma
BRCA1/BRAC2
APC
MSH2 or MLH1
What is Retinoblastoma cancer?
Malignant tumour of retina cells due to mutation of Rb gene.
Germline has bilateral tumour and has earlier onset due to first hit being inherited
Sporadic is non hereditary and results in unilateral tumour
What is BRCA1/2?
Hereditary germline mutation of DNA strand repair gene, activated by ATM/R phosphorylation. Leads to ovarian and breast cancer.
What is colorectal cancer?
Characterised by
1) Formation of adenoma via K-RAS mutation
2) Formation of carcinoma via p53 and DCC mutation
What is the progression of colorectal cancer?
Sessile (fixed) polyp- benign pre-cancerous growth which is flat
Pedunculated polyp with a mushroomlike structrure and a stalk
Both of these lie flat against the mucous membrane
Adenocarcinoma
How does clonal expansion of damaged DNA occur?
DNA damage occurs in dividing cell which forms mutations that accumulate and are repetitively selected for and is subclinical for up to 2 decades.
What is the most common cause of colorectal cancer?
1) Sporadic non-hereditary
2) familial colorectal cancer
What are the hereditary forms of colorectal cancer?
Familial Adenomatous Polyposis
Hereditary non-polyposis Colon cancer
What is familial adenomatous polyposis (FAP)?
Autosomal dominant inheritance caused by a mutation of the gatekeeper protein APC tumour which downregulates beta catenin. It is associated with the hyperproliferation of the bowel tissue, resulting in the formation of many pre-malignant sessile and pedunculated polyps.
What is hereditary non-polyposis colon cancer? (HNPCC)
AKA Lynch syndrome which is a type of colorectal cancer occurring predominately in the right colon. This is a microsattelite instability caused by mutation in the caretaker protein DNA mismatch repair genes hMSH2 and hMHSH3. There is no polyposis and it has an early onset before age 50 due to inherited first hit mutation.
What are DNA mismatch repair genes?
Responsible for the correction of incorrect nucleotide pairings and replication errors that can lead to cancer. The mismatch repair genes are MSHM2 and MSHl1
How does APC gene work?
Gatekeeper protein which causes the ubiquitination and degradation of beta-catenin along with AXIN. This prevents the activation of Wrt genes for control of cell polarity and cell migration.
What are gatekeeper genes?
Inhibit cell growth and promote cell apoptosis. Cancer occurs if 2nd allele is mutated and causes initiation of tumour formation
What are caretaker proteins?
Promote cell growth and proliferation and when mutated, cause tumour progression when gatekeeper proteins are already inhibited.
What is the cause of sporadic colorectal cancer?
Chromosomal instability due to Gatekeeper and caretaker protein mutations
What are the molecular subtypes of colorectal cancer?
->Chromosomal instability which is most common
->Microsatellite instability is the prescence of repetitive sequences of DNA
What are the features of chromosomal instability?
Changes in alleles or whole chromosome itself.
Loss of heterozygosity with aneuploidy (abnormal number) or polyploidy (more than 2 chromsome sets) due to activation of oncogenes. Location of tumour is in left-sided colon, highly differentiated with little lymphocytic infiltration and worse prognosis when adjusted for stage
What is CIMP?
Chromsomal instability pathway with hypermethylation of CPG islands, causing gene silencing.
What is microsatellite instability?
increase in DNA sequence repetition. Occurs due to epigenetic silencing of MLH1. Chromosomes are diploid. Tumour is predominately right sided, poorly differentiated and there is lymphocytic infiltration but better prognosis when adjusted for cancer stage. HNPCC is a type of this.
What is CMS1?
Immune mediated colorectal cancer
What is CMS2?
Canonical sequence of colorectal cancer via adenoocarcinoma formation
What is CMS3?
Colorectal cancer due to metabolic dysregulation
What is CMS4?
Mesenchymal colorectal cancer with stromal infiltration and angiogenesis. Has the worst survival
