DDT 4 - Mutagens Flashcards
Toxicants
Any substance (poison) that causes harm to a living organism
- Distributed - Metabolized - Interact with cellular macromolecules - result in toxic endpoint - excretion and repair process can mitigate toxin effects
Toxicology
Study of toxicants (chemical/physical)
Include areas of:
- analytical toxicology (identifying toxicants and metabolites)
- toxicity testing (living systems to estimate toxicity effects, simple cell culture techniques)
- toxicologic pathology (investigates subcellular/cellular/tissue/organ changes due to toxicants)
- structure activity studies (identifies relationships between structure and toxic effects to predict toxicity)
- statistics and epidemiology (determine significance and risk associated with toxicant exposure within human populations)
Sir Percivall Pott (1776)
Reported chimney sweeps had a higher incidence of scrotal cancer
Attributed it to increase topical exposure to soot and tar
K. Yamagiwa, K. J. Itchikawa (1915-18)
Demonstrated multiple applications of coal tar to rabbit skin produced carcinoma
First demonstration that chemical could produce cancers in animals
Established link between epidemiology studies and animal carcinogenicity
Mutagens
Chemical/physical phenomenon (radiation) that can cause changes to the composition of DNA
Increase the rate of mutations in DNA compared to spontaneous mutation rate
Mutations can lead to development of cancer
Many mutagens are also carcinogens
Physical Mutagens
EM radiation
– UV light
(ionizing radiation, dimerization of pyrimidine, hydration of cytosine, indirectly damages DNA by production of reactive oxygen species)
– X-rays / Gamma rays
(shorter wavelength, travels through cell without collision, if collides with DNA, strand break)
Particle Radiation
– Alpha/Beta particles (upon impact with nuclear DNA, attract/repel charged areas of DNA, breaking DNA)
Chemical Mutagens
Testing for Potential Chemical Mutagens/Carcinogens
Ames Test (UC 1970s)
Montiors chemicals ability to bring about reverse mutation in Salmonella typhimurium strains that have defects in their histidine synthesis pathway
Strains do not grow in absence of histidine
Cells can mutate back to wild type and grow in absence of histidine
If no histidine, bacteria will not grow and will not form colonies
But cells can mutate back to wild type - reactivation of enzyme responsible of histidine synthesis in bacteria
Salmonella bacteria will be able to grow in absence of histidine in growth media
Ames Test
- Chemical (suspected carcinogen) and suspension of cells containing His- (strain which cannot synthesize histidine)
- Incubated with rat liver enzymes to produce activated electrophilic species (simulate metabolic activation for these suspected carcinogens in the body)
- Bacterial test culture plated onto agar plate with *no histidine
- Presence of colonies indicates some cells have reverted to wild type (positive result)
- Positive result in Ames test shows compound is a conformed mutagen
- Different strains can indicate particular types of mutations (base pair, frame shift)
- For validity, +/- controls and varying concentration of chemical are used in test
Carcinogenesis
Process through which cancer develops
- Chemical carcinogenesis = study of chemical carcinogens and mode of action
Chemical Carcinogens
Change composition of DNA, cause cancer
Tumor Suppressor Genes
Negative regulators of cell growth
Inactivated in cancer cells
Proto-oncogenes
Found in normal cells
involved in positive regulation of cell growth
mutated in cancer cells
Pre-neoplastic region
High probability of progressing into a malignant tumor
Carcinogenesis: Initiation to Progression
= Conversion
transformation of preneoplastic cell into one that expresses malignant phenotype
exposure to UV light can accelerate
= Progression
expression of malignant phenotype
cells acquire aggressive characteristics (genomic instability, uncontrolled growth, metastasis)
Direct Carcinogens
Intrinsically reactive
Form adducts with DNA without metabolic modification
e.g. nitrogen mustards, methyl nitrosurea
Procarcinogens
Require metabolic activation before damaging DNA by covalent adduct formation or via reactive oxygen species
Procarcinogen -> Proximate Carcinogen -> Ultimate carcinogen (metabolite that forms adducts with DNA)
e.g. Benzo(a)pyrene, Aflatoxin, Dimethylnitrosamine
How do mutagens and carcinogens cause damage to DNA
– Covalent adduct formation
formation of bulky aromatic-type adducts
alkylation (genrally small adduct)
dimerization
deamination
other reactions
– Oxidative damage
Bulky aromatic Adducts
- First chemically identified carcinogens
- Polycyclic aromatic hydrocarbons
- Very common environmental contaminants
- Composed of variable numbers of fused benzene rings
- Formed from incomplete combustion of fossil fuels / veg matter
- Chemically inert, require metabolic activation via Cytochrome P450
Aflatoxin B1
- Targets N7 on guanine when forming adduct
- Toxin produced by some Aspergillus species
- Found in soil and areas with decaying vegetation (e.g. not washing veg properly can expose them to Aflatoxin B1)
- Can contaminate food sources
- Exposure to high levels can cauase cirrhoses and *liver cancer
- Chronic exposure also associated with increased risk to liver cancer
Procarcinogen –> Aflatoxin B1 epoxide (ultimate carcignogen)
Alkylating agents
- Attach *small alkyl groups to nucleophilic sites to form DNA adducts
- Dimethylnitrosamine / nitrosamines
(industrial byproducts, present in tobacco smoke, cured meats) - Procarcinogens that produce reactive diazonium and carbonium ions
- Target DNA base sites
(Adenine N1,3,7 // Cytosine N3 // Guanine N3,O6,N7 // Thymine O2,N3,O4) - Can lead to loss of base due to instability
(forms Abasic site)
Deaminating agents
- Nitrous acid (HNO2) converts
= cytosine to uracil (amino group on cytosine converted to carbonyl)
= adenine to hypoxanthine (amino group on adenine converted to carbonyl, forms amide)
= guanine to xanthine - H-bonding potential/pattern of modified base is altered, resulting in mismatch of bases
- Hypoxanthine can pair with cytosine causing mismatch
DNA repair mechanisms
- Excision repair
- Mismatch repair
- Single strand break pair
- Direct reversal repair
Base excision repair
Nucleotide Excision repair of DNA damage
