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PDA > 14 Genetic Toxicology > Flashcards

14 Genetic Toxicology Flashcards

1
Q

Mutagenicity, carcinogenicity & teratogenicity

A

Mutations
•Dominant mutation – may immediately cause disease •Recessive mutation – may be silent

Mutation in Somatic cell - Potential for: Carcinogenicity Teratogenesis

Mutation in Germ cell - Inherited genetic change: Potential for reproductive toxicity (teratogenic, fertility), carcinogenicity or other genetic disorder

BUT
•Teratogenicity can be caused by non-genetic mechanisms •Better correlation between mutagenicity & carcinogenicity

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2
Q

How do drugs or metabolite cause DNA damage?

A
  1. The drug covalently binds to DNA
  2. The drug chemically modifies DNA bases eg alkylate
  3. The drug is incorporated instead of DNA base
  4. Drug binds non-covalently to DNA (eg intercalators)
  5. Strand breaks
  6. Inter and intrastrand crosslinks
  7. Crosslink DNA to protein
  8. Drug interferes with DNA replication mechanisms
  9. Potentially - Drug interferes with DNA repair mechanisms
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3
Q

DNA lesions

A

Intracalators
•Bind between strands
•Sometimes sequence specific

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4
Q

A covalently modified base

A

•dG paired with modified dA after covalent binding of a benzopyrene to adenine N6

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5
Q

An intercalating molecule

A
  • A Bis naphthalimide drug LU-79553 (topoisomerase II inhibitor)
  • Each napthalamide unit stack with purines
  • Linker is in major groove
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6
Q

Esperamicin : An interchelating & strand cleaving molecule

A
  • Methoxyacryl –anthranilate moiety intercalates between GG
  • Trisaccharide moiety anchors drug in minor groove
  • Enediyne moiety generates free radicals to cleave strand
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7
Q

DNA repair

A
  • Repair mechanisms
  • Direct repair – eg demethylation (alkyltransferases)
  • MMR mismatch repair – mismatch, not damaged bases
  • NER nucleotide excision repair of bulky adducts – removal of oligonucleotide + resynthesize
  • BER base excision repair – removal of nucleotide to replace damaged base
  • HR: homologous recombination – to repair double strand breaks
  • NHEJ non-homologous end joining – to repair double strand breaks
  • Errors:
  • Repair enzymes may not determine which is the correct base •Excision repair polymerase – lower fidelity than in replication •HR and NHEJ – most error prone
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8
Q

DNA damage repair

- Mis-match repair (MMR)

A
  • repair mismatched bases.
  • damage recognition by a specific protein that binds to the mismatch,
  • cutting DNA at a distance from mismatch, excision past mismatch, resynthesis and ligation
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9
Q

DNA damage repair

- Nucleotide excision repair (NER)

A
  • removes bulky lesions from DNA.

* Removal of oligonucleotide, re-synthesis,and ligation

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10
Q

DNA damage repair

- Base excision repair (BER)

A
  • removal of damaged base.

* gap filled by DNA polymerase, followed by ligation to parent DNA

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11
Q

DNA damage repair

- Homologous recombination (HR)

A
  • repairs double strand breaks.

* single strand tail formed which invades an undamaged homologous chromatid and uses as template to repair

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12
Q

DNA damage repair

- non-homologous end-joining (NHEJ)

A
  • repairs double strand breaks. Break ends are directly ligated
  • No template so more error prone
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13
Q

Consequences

A 
1. Mutations within a single gene*
– Or within a restricted number of genes
2. Chromosomal aberrations
– Structural change in the chromosome
3. Genomic mutations
– Change in number of chromosomes - “aneuploidy”
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14
Q
  1. Mutations within a single gene (Initiation)
A

• Base pair substitution (eg A→T, G →A)
– Incorporation of base analogs
– Chemical modification of base
• O6-alkylguanine can pair with thymine
• N7alkyl guanine is labile – loss of base
– Spontaneous base changes
• may be important with drugs which interfere with DNA repair
– Silent (eg CCC(Pro)→CCA(Pro), missense (eg CCC→ GCC(Ala) or nonsense (→ TGA, TGG; stop codons)
– no effect, intermediate effect or drastic effect on protein function

• Addition and deletion of bases
– Can occur during DNA repair or during replication (eg interchelating agent)
– Can cause a frameshift
– Indels – how can you distinguish?
– potentially disastrous!
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15
Q
  1. Chromosomal aberrations (Progression)
A

•Result from strand breaks
•Ionizing radiation can cause double strand break. •Certain drugs can also directly cause strand breaks –
•eg bleomycin (squamous cell carcinoma, non-hodgkins lymphoma,
testicular cancer
•Drug induced strand breaks (eg topoisomerase inhibitors) - mostly result from error in DNA synthesis using damaged template or repair of bulky adduct
•Break in DNA creates cohesive end •Fragments rejoin inappropriately
•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 •Effect – minor to major (gene fusion, loss of gene(s))

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16
Q
  1. Genomic changes (Progression)
A

• Change in number of chromosomes
Normally 2 copies of each chromosome
– Haploid – 1 copy
– Diploid – 2 copies (“2n”)
– Aneuploid – altered number of chromosomes
– Polyploid – extra copy of complete set of chromosomes (4n, tetraploid)

Causes
•Drugs affecting nuclear spindle microtubules, eg Paclitaxel, vincristine, vinblastine, colchicine
•In principle, any drug affecting
•Chromosome replication •Chromatid segregation

17
Q

Risk and benefit

A

Anti cancer drugs can increase the likelihood of (a different type of) cancer:
•Ovarian cancer (Carboplatin/paclitaxel) – increased risk of leukemia
•Breast cancer (Melphalan) – increased risk of leukemia

18
Q

Model of chemical carcinogenesis

A

•Carcinogens cause mutation within a single gene Primary carcinogen – can causes mutation; Secondary carcinogen –requires biotransformation to cause
mutation; Co-carcinogen – enhances ability of carcinogen to cause mutation
•Essentially irreversible
•“Initiated” cells have no clear morphological change
•D-R: No threshold dose, maximum may be unattainable

  • Tumour Promoters
  • Promote development of cancer after mutation •Don’t cause disease on own but cause increased incidence/decrease latency
  • Reversible – mutation not required –”epigenetic”
  • May promote proliferation of initiated cells
  • D-R: Conventional dose-response
  • Some compounds are both initiators and promoters
  • Clinically observable neoplasia
  • Genomic instability –chromosomal aberrations and genomic mutations
  • Develops into malignant form (invasive, undifferentiated, high mitotic index) and then becomes metastatic
19
Q

Genes affected during Initiation

A

Activation of oncogenes
•Mutation of 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

Inhibition of tumour suppressors
•Mutation to inactivate
•Decreased expression of the oncogene due to genetic changes in regulatory elements
•Deletion of the gene
•Translocation to separate from an active promoter

20
Q

What is a teratogen?

A
  • Teratogen: an agent (drug) which when administered to a woman (and sometimes to a man) causes structural or functional abnormalities in a foetus
  • May not be apparent until later life
21
Q

Mechanisms of teratogenesis

A
  • Mutations within a single gene
  • Chromosome aberrations
  • Genomic changes
  • Enzyme inhibition
  • Receptor antagonism
  • Altered cell-cell interactions
  • Altered metabolism, reduced energy supply
  • Alteration of cell migration
22
Q

When do drugs cause reproductive

toxicity?

A

• Before pregnancy!
– Etretinate
– Acne, & psoriasis
– Accumulates in fat and gradually released - slow elimination
– Toxic drugs likely to cause spontaneous abortion
• 1st trimester
– Organogenesis →most critical period
– Toxic drugs can cause structural abnormalities
• 2nd ,3rd trimesters – Growth
– Toxic drugs affect growth, maturation, integrity of structures • In men!
– Temozolomide
– Glioblastoma
– males recommended to avoid fathering a child for upto 6 months after treatment and consider sperm cryopreservation

23
Q

What is genetic toxicology

A

Genetic toxicology represents the study of genetic
damage, the agents that induce such damage,
the mechanisms that respond within cells and the potential consequences of the damage or the damage responses. Genotoxic agents are
abundant in the environment (often due to human activity) and the consequences of the
effects they have on humans include
carcinogenesis and teratogenesis.

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