Carcinogenesis Flashcards
Classes of cancer genes
Oncogenes
Tumour suppressor genes
DNA repair genes
Apoptosis genes
Normal genes which become abnormal
Somatic or germ line mutations
What occurs in the DNA damage response pathway
If damage occurs surveillance proteins are needed to pass on information. Examples of this are ATM and p53
Accumulation of p53 is mediated by action of ATM and this results in arrest of the cell cycle in order to facilitate apoptosis or repair (increase in p21 and gadd45)
In cancer we see that this pathway fails
What are the types of DNA damage
Pyrimidine (adenine and guanine) dimers caused by UV
DNA cross links caused by carcinogens
Can be between the same strand (intrastrand) or between the 2 strands (inter strand)
Base oxidation, hydrolysis or damage caused by ROS, UV and high temp
Single strand breaks caused by ionising radiation
Double strand breaks caused by ionising radiation, ROS
Replication errors
DNA damaging factors can be classified into 2….
Exogenous (extracellular) such as UV, x-rays, natural isotopes, chemicals
Endogenous (intracellular) = oxygen, water, reactive metabolism intermediates
How does UV (exogenous) damage DNA
Causes dimerisation of thymine and CC dimers
Interferes with base pairing during DNA replication
6-4 photo products = formation of covalent bond. Less frequent but more mutagenic than above
Can cause DNA to distort, CC can miss-pair with AA, CC to TT mutation signature
How do alkylating agents cause DNA damage
Methylation of O6 in guanine is toxic and mutagenic if unrepaired.
Can miss pair with thymine
How may endogenous biotransformation reactions cause DNA damage
Metabolism of various compounds such as drugs and carcinogens may lead to produce active agents that are toxic, carcinogenic or teratogenic
What occurs when contaminated food is consumed?
CYP450 adds highly reactive epoxide group which if not disarmed may cause dna damage
Aflatoxin (endogenous)
Above is produced most commonly by Aspergillus fungus and spores may contaminate food and enter access into body which then might have a carcinogenic effect
ATM is deficient in which condition and characteristics of rare condition
Autosomal recessive disorder
Causes progressive neurodegeneration (unsteady walk and balance) and premature ageing
Individual has predisposition to cancer (lymphoma or leukaemia)
Statistics about p53 mutations
P53 gene is directly affected in more than 50% of human cancers
P53 pathway is altered in more than 85-90% of cancers
Examples of endogenous and exogenous mutations of p53
Exogenous:
- UV sunlight which causes dimerisation of CC to TT (skin cancer)
- aflatoxin B1 in diet may result in liver cancer as it causes change AGG to AGT in codon 249
- benzopyrene in tobacco smoke causes lung cancer due to changes in codons 157,248,273
Endogenous:
- most p53 mutations occur at CpG sites
- methylation of C at CpG sites
- spontaneous deamination of methylcytosine is not repairable and resulting base is thymine
What are the mechanisms of DNA repair
- direct reversal through transferring the alkyl group from the base to the suicide enzyme O-6 methylguanine-DNA methyltransferase (MGMT)
Cancers with reduced MGMT (gene) expression are responsive to treatment with alkylating agents
- base excision repair = DNA Glycosylases remove base. This leaves a gap called the AP site (apurinic/apyrimidinic site). AP endonuclease repairs the AP site (gene = APEX1)
How do AP sites occur spontaneously
Via ROS
Estimated that human genome sustains 10,000 oxidative hits per cell per day
How is nucleotide excision repair different to BER:
Many forms of DNA damage not recognised by DNA glycosylases so not repaired by BER.
NER involves removal of oligonucleotide fragment. Works on bulky lesions such as chemical cross links, pyrimidine dimers, 6-4 photo products and large chemical adducts
Pathway of nucleotide excision repair
XPC = DNA damage sensor
XPA binds to dimer and recruits other proteins to form repair complex
XPB and XPD are helicases that separate the strands and RPA keeps them apart
XPG and XPF are endonucleases that cut DNA on either side of damage
Cut fragment is removed. And gap is filled in by DNA polymerases and sealed by DNA ligaments
What is xeroderma pigmentosum
Rare genetic disorder in which NER is defective. Results in extreme sensitivity to sunlight and risk of skin cancer is x1000 higher by the age of 8
How is double strand breaker repaired
DSB’s arise spontaneously and due to ionising radiation
Repaired via non-homologous end-joining NHEJ and homologous recombination HR
How may tumour suppressor genes be implicated in cancer
TSG’s encode for repressors of cell proliferation. Abnormal inactivation leads to loss of function
1st hit = most likely a point mutation can be somatic or germ line
2nd hit = point mutation, allele deletion, Hypermethylation of CpG site would silence gene
(Knudsons two hit hypothesis)
Examples of hereditary cancer genes
RB1 = retinoblastoma
BRCA1 BRCA2 = hereditary breast and ovarian cancers syndrome
APC = familial adenomatous polyposis colorectal cancer
MSH2, MLH1 in hereditary non polyposis colorectal cancer
Difference between sporadic and inherited form of retinoblastoma
Inherited = 1st hit normally present from birth and 2nd occurs during life. Cancer is early onset and present bilaterally
Sporadic form both hits occur during life and is late onset and unilateral
Sequence of familial colon cancer
Sessile polyp (precursor lesion has possibility of becoming malignant)
Pedunculated polyp
Adenocarcinoma
Types of colorectal cancer
Sporadic colorectal cancer 73%
- 85% CIN chromosome instability in which the tsgs and oncogenes are affected
-15% CIMP micro satellite = defect in mismatch repair hMLH1 silencing
Types of inherited colorectal cancer
FAP vs HNPCC (more common)
FAP = polyposis, autosomal dominant inheritance, cytogenitically visible deletion and APC gene affected. (Hypothesised that it is mainly due to CIN)
HNPCC = lynch syndrome, no polyposis, early onset and affects right colon mostly, increased risk of further colorectal cancers. Mismatch repair genes hMSH2 and hMLH1 affected. (Due to a mutation in DNA mismatch repair genes) (mostly MSI)
In FAP it is a gatekeeper gene which if becomes mutated (2nd allele) cancer is initiated eg APC. In caretaker genes eg MSH2 and MLH1 in the case of HNPCC there are errors in replication and when DNA repair is faulty mutations are more likely to occur and in mutated sequences even more mutations are likely to occur leading to concept known as mutator phenotype
Route of cancer progression sporadic (chromosomal instability) vs MSI (micro satellite sequence expansion)
For cancers caused by chromosomal instability they progress more normally and slowly whilst FAP (gatekeeper) and HNPCC (caretaker) are likely to worsen more quickly
Very likely that CIN brings about MSI
How does clonal expansion occur
DNA damage that undergoes repair and may become mutated. More mutations occur in mutator genes which allows for repetitive selection of mutants and mutators. This whole process takes about 20 yrs and is clinically detected when the malignant phenotype is selected
What is the nature of the mutation of APC gene that brings about FAP: and the role of APC protein
It is a germ line mutation that is sufficient to allow hyperproliferation in the bowel to form lots of polyps. Each one has the potential to become malignant.
APC protein is a negative regulator of beta catenin protein which binds to Transcription factors normally allowing gene expression. To regulate beta catenins APC and other complex proteins are bound to beta catenin inactivating it and not allowing it to bind to TF. In FAP there is a mutation in APC gene which does not allow for production of APC protein
What is MSI and how is it different to chromosomal instability CIN
The presence of alternate sized repetitive DNA sequences that are not present in the corresponding germ line DNA
Chromosomal instability = high rate of loss and gain of whole chromosomes. Whole or parts of the chromosome may become duplicated or deleted and this would lead to an unequal distribution upon mitosis (aneuploidy)
What is lynch syndrome:
Inherited disorder in which the individual has higher predisposition to some forms of cancer. This is due to their DNA mismatch repair genes being mutated and not being expressed
Mainly increases their risk for types of colorectal cancer, and other GI cancers
How to differentiate between CIN and MSI
CIN = widespread of heterozygosity. MSI = widespread micro satellite instability
CIN = aneuploidy or polyploidy MSI = diploidy
CIN are predominantly left sided cancer in colon whilst MSI are right sided
CIN = highly differentiated histological features whilst it is poor in MSI
CIN = little lymphocytic infiltration whilst MSI has
CIN is rarely mucinous whilst MSI is often mucinous
CIN has worse prognosis after adjustment for stage
