Carcinogenesis

Question 1

Classes of cancer genes

Answer 1

Oncogenes
Tumour suppressor genes
DNA repair genes
Apoptosis genes
Normal genes which become abnormal
Somatic or germ line mutations

Question 2

What occurs in the DNA damage response pathway

Answer 2

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

Question 3

What are the types of DNA damage

Answer 3

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

Question 4

DNA damaging factors can be classified into 2….

Answer 4

Exogenous (extracellular) such as UV, x-rays, natural isotopes, chemicals

Endogenous (intracellular) = oxygen, water, reactive metabolism intermediates

Question 5

How does UV (exogenous) damage DNA

Answer 5

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

Question 6

How do alkylating agents cause DNA damage

Answer 6

Methylation of O6 in guanine is toxic and mutagenic if unrepaired.
Can miss pair with thymine

Question 7

How may endogenous biotransformation reactions cause DNA damage

Answer 7

Metabolism of various compounds such as drugs and carcinogens may lead to produce active agents that are toxic, carcinogenic or teratogenic

Question 8

What occurs when contaminated food is consumed?

Answer 8

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

Question 9

ATM is deficient in which condition and characteristics of rare condition

Answer 9

Autosomal recessive disorder
Causes progressive neurodegeneration (unsteady walk and balance) and premature ageing
Individual has predisposition to cancer (lymphoma or leukaemia)

Question 10

Statistics about p53 mutations

Answer 10

P53 gene is directly affected in more than 50% of human cancers

P53 pathway is altered in more than 85-90% of cancers

Question 11

Examples of endogenous and exogenous mutations of p53

Answer 11

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

Question 12

What are the mechanisms of DNA repair

Answer 12

• 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)

Question 13

How do AP sites occur spontaneously

Answer 13

Via ROS
Estimated that human genome sustains 10,000 oxidative hits per cell per day

Question 14

How is nucleotide excision repair different to BER:

Answer 14

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

Question 15

Pathway of nucleotide excision repair

Answer 15

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

Question 16

What is xeroderma pigmentosum

Answer 16

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

Question 17

How is double strand breaker repaired

Answer 17

DSB’s arise spontaneously and due to ionising radiation
Repaired via non-homologous end-joining NHEJ and homologous recombination HR

Question 18

How may tumour suppressor genes be implicated in cancer

Answer 18

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)

Question 19

Examples of hereditary cancer genes

Answer 19

RB1 = retinoblastoma
BRCA1 BRCA2 = hereditary breast and ovarian cancers syndrome
APC = familial adenomatous polyposis colorectal cancer
MSH2, MLH1 in hereditary non polyposis colorectal cancer

Question 20

Difference between sporadic and inherited form of retinoblastoma

Answer 20

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

Question 21

Sequence of familial colon cancer

Answer 21

Sessile polyp (precursor lesion has possibility of becoming malignant)
Pedunculated polyp
Adenocarcinoma

Question 22

Types of colorectal cancer

Answer 22

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

Question 23

Types of inherited colorectal cancer

Answer 23

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

Question 24

Route of cancer progression sporadic (chromosomal instability) vs MSI (micro satellite sequence expansion)

Answer 24

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

Question 25

How does clonal expansion occur

Answer 25

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

Question 26

What is the nature of the mutation of APC gene that brings about FAP: and the role of APC protein

Answer 26

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

Question 27

What is MSI and how is it different to chromosomal instability CIN

Answer 27

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)

Question 28

What is lynch syndrome:

Answer 28

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

Question 29

How to differentiate between CIN and MSI

Answer 29

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