Genetic toxicology

Question 1

What is Genetic Toxicology?

Answer 1

The study of genetic damage that results in alterations to heritable information.

Question 2

What are the different types of DNA damage?

Answer 2

Chromosome breakage (clastogenesis)

Alteration of the DNA base sequence (mutagenesis)

Mis-segregation of chromosomes (aneugenesis)

Interchange of sections of chromosomes altering gene regulation (recombination)

Question 3

Difference between dominant mutation and recessive mutation?

Answer 3

Dominant mutation – may immediately cause disease - loss of function

Recessive mutation – may be silent - inactive a gene

Question 4

What happens if there is a mutation in a germ cell and a somatic cell?

Answer 4

If you have a mutation in somatic cell, there is potential for Carcinogenicity and Teratogenesis.

If there is a mutation in germ cell, this can bring about inherited genetic change. So there is potential for reproductive toxicity (teratogenic, fertility), carcinogenicity or other genetic disorder.

Question 5

What is Teratogenicity?

Answer 5

Direct harm to the unborn child.
Can be caused by non-genetic mechanisms.

Reproductive toxicity can include teratogenicity but can also affect the fertility of the adult.

Question 6

How do drugs cause DNA damage? (or any metabolite)

Answer 6

1. The drug covalently binds to DNA
2. OR The drug chemically modifies DNA bases eg alkylate
3. The drug molecule may be incorporated into the DNA strand instead of DNA base
4. Drug may also bind non-covalently to DNA (eg intercalators – which slides in-between bases and changes the shape and the structure of that section of DNA)
5. Strand breaks
6. Inter and intra strand crosslinks occurs
7. Some drug might cause a crosslink between DNA and protein
8. OR the drug might interfere with DNA replication mechanisms
9. Potentially - Drug interferes with DNA repair mechanisms

Question 7

What might happen if the drug interferes with DNA replication?

Answer 7

You might not get accurate replication of DNA.

This might lead to accumulation of more damage (this is the same result for interfering with DNA repair mechanisms, accumulation).

Question 8

Examples of Damaging agents?

Answer 8

X-rays, oxygen radicals, alkylating agents, spontaneous reactions

UV light, polycyclic aromatic hydrocarbons

X-rays, anti-tumour agents

Replication errors

Question 9

Examples of repair processes?

Answer 9

Base excision repair (BER) -

Nucleotide excisior repair (NER)

Recombinational repair

Mismatch repair

Question 10

Explain the action of each damaging agents? And name their repair mechanism?

Answer 10

X-rays, oxygen radicals etc, cause a single strand break, abasic site and modified base —- repair is BER

UV light and more produces Pyrimidine dimers (T-T) and other bulky adduct —- repair is NER

X-rays and anti-tumour causes interstrand crosslinks and double strand break —- repair is recombinational repair

Replication errors causes bases mismatch, insertion and deletion —- repair is mismatch repair

Question 11

What are the consequences of damaging agents?

Answer 11

A transient cell cycle arrest

Apoptosis due to the inhibition of transcription, replication or chromosome segregation

Cancer, ageing and inborn disease can occur due to mutations and chromosome aberration

Question 12

What does Biotransformation do?

Answer 12

It can make a drug a lot more reactive and may create an electrophile.

It makes the drug a lot more polar, so it will be more water soluble and more easily excreted.

Question 13

How many nucleophilic sites does each DNA base have?

Answer 13

Adenine – 4 sites

Guanine – 5

Cytosine – 3

Thymine – 3

Question 14

Explain Mis-match repair (MMR)?

Answer 14

repair mismatched bases; damage recognition by a specific protein that binds to the mismatch, cutting DNA at a distance from mismatch, excision past mismatch, resynthesise and ligation

Question 15

Explain Base excision repair (BER)?

Answer 15

removal of damaged base, gap filled by DNA polymerase, followed by ligation to parent DNA

Question 16

Explain Homologous recombination (HR)?

Answer 16

repairs double strand breaks, single strand tail formed which invades an undamaged homologous chromatid and uses as template to repair

Question 17

Explain Non-homologous end-joining (NHEJ)?

Answer 17

• repairs double strand break. Broken ends are directly ligated
• No template so more error prone

Question 18

Does repair remove all the results of DNA damage?

Answer 18

No, Repair may cause the consequences of DNA damage to be less harsh but it will not completely remove or prevent the consequences of the DNA damage.

Question 19

What are some consequences of DNA damage?

Answer 19

1. Mutations within a single gene
   or within a restricted number of genes

Both in the coding and non-coding regions; mutation outside coding region can affect gene expression

2. Chromosomal aberrations

structural change in the chromosome

3. Genomic mutations

change in number of chromosomes

Question 20

Explain base pair substitution?

Answer 20

Incorporation of base analogs or a chemical modification of base

Spontaneous base changes (this may be important with drugs which interfere with DNA repair).

These may be silent, missense or nonsense. It may have no effect, intermediate effect or drastic effect on the protein function.

Question 21

Explain addition or deletion of bases?

Answer 21

Can occur during DNA repair or during replication

Can cause frameshift – indels

Could potentially be devasting

Question 22

What are the different ways in which strand can breaks?

Answer 22

Ionizing radiation can cause double strand break.

Certain drugs can also directly cause strand breaks, eg bleomycin

Drug induced strand breaks (eg topoisomerase inhibitors) mostly result from error in DNA synthesis where a damaged template is used or there is a repair of an bulky adduct.

Question 23

Consequences of strand breakage?

Answer 23

Loss of part of chromosome

Amplification of part of chromosome

Inversion of part of chromosome

Insertion of part of one chromosome into another

Translocation of part of one chromosome to another

Formation of ring structures, chromosomes with two centromeres

Range of effects; minor to major (gene fusion, loss of gene(s))

Question 24

What is Genomic changes (progression)?

Answer 24

Changes in a number of chromosomes.

This can lead to:
- Aneuploid – altered number of chromosomes

• Polyploid – extra copy of complete set of chromosomes (4n, tetraploid)

Question 25

Cause of Genomic changes?

Answer 25

Any drug affecting chromosome replication and/or chromatid segregation can result in some of these changes.

Especially drugs affecting nuclear spindle microtubules

Question 26

Explain Initiation in Chemical Carcinogenesis?

Initial damage

Answer 26

Carcinogens cause mutation within a single gene:

• Primary carcinogen can cause mutations; but Secondary carcinogen requires biotransformation to cause mutation;

Co-carcinogen enhances the ability of carcinogen to cause mutation

Question 27

Explain promotion in Chemical Carcinogenesis?

Expansion of the damage

Answer 27

Tumour Promoters will promote development of cancer after mutation

• They don’t cause disease on own but cause increased incidence/decrease latency
• They are reversible; mutation not required ”epigenetic”
• May promote prolifer’n of initiated cells
• Dose response: Conventional dose-response

Some compounds are both initiators and promoters.

Question 28

Explain progression in Chemical Carcinogenesis?

Answer 28

At this point you’ve reached clinically observable neoplasia

• Genomic instability is present with chromosomal aberrations and genomic mutations
• this allows the cells to develop into malignant forms
• malignant cells are highly invasive, undifferentiated, high mitotic index, and then can potentially become metastatic

Question 29

What are the two categories of genes affected during the initiation stage?

Answer 29

Activation of oncogenes or Inhibition of tumour suppressors.

Question 30

What causes the Activation of oncogenes?

Answer 30

• The mutation of the oncogene itself
• Increased expression of the oncogene due to genetic changes in regulatory elements
• Amplification of gene
• Translocation to fuse with a highly expressed gene

Question 31

What causes Inhibition of tumour suppressors?

Answer 31

• There may be mutation to inactivate these - resulting in decreased expression of the oncogene due to genetic changes in regulatory elements
• OR Deletion of the gene
• OR Translocation to separate from an active promoter

Question 32

Explain The Hallmarks of Cancer?

Answer 32

Self-sufficiency of growth signals leads to insensitivity to anti-growth signals.

This leads to tissue invasion and metastasis, limitless replicative potential and sustained angiogenesis.

Then cell invade apoptosis.

Then cycle starts again

Question 33

What is Teratogen?

Answer 33

An agent (drug) which when administered to a woman causes structural or functional abnormalities in a foetus.

Question 34

What is Absence of evidence and Evidence of absence?

Answer 34

Absence of evidence = there is no proof that something does something or exist as it.

Evidence of absence = is where you have proven that something does or doesn’t do something.

Question 35

What are the mechanisms of Teratogenesis?

Answer 35

• You can have: Mutations within a single gene, Chromosome aberrations or Genomic changes.
• Enzyme inhibition
• OR Receptor antagonism
• OR Altered cell-cell interactions
• OR Altered metabolism, reduced energy supply
• OR Alteration of cell migration

All these will lead to teratogenic effects.

Question 36

What is a known Teratogen?

Answer 36

Valporate

It is dose-dependent so higher dose = higher risk