carcinogens Flashcards
what are carcinogens?
chemical or physical sources that initiate or promote carcinogenesis
what is a DNA damage?
abnormal chemical structure in DNA which causes changes
what is a genotoxin?
a physical or chemical agent that can induce DNA damages
what is a mutation?
an alteration of the nucleotide sequence in the genome
what is a mutagen?
a physical or chemical agent that can induce mutations
what types of genotoxins are there? What do they cause?
Mutages (DNA adducts and mutations)
clastogens (chromosome breaks)
Aneugens (mitotic spindle)
what are the key characteristics of carcinogens?
- is genotoxic
- alters DNA repair
- causes genomic instability
- induces oxidative stres
- induces chronic inflammation
- is immunosuppressive
- models receptor mediated effects
-causes immortalization
-alters cell proliferation/ cell death/ nutrient supply
what is the difference between genotixic carcinogens and non-genotoxic carcinogens?
genotoxic: initiate tumor formation by DNA damage and DNA mutagenesis. Is not concentration dependent. DNA damage is reversible but mutation is not. mutagenic
non-genotoxic: promote tumor formation by pathways other than DNA damage –> cell proliferation, cytotoxicity, hormonal effects
is concentration dependent, reversible and non-mutagenic –> provides growth advantage
Give examples for genotoxins and non-genotoxins
genotoxin: Aflatoxin B1, Formaldehyde, Nitrosamine, Vinyl Chloride
non-genotoxic: Asbestos, Dioxins, Estradiol, Phenobarbital
what are common types of DNA damage?
8-oxo-2’-deoxyguanosine: oxidative lesion in duplex DNA
ß-N-glycosidic bond between base and deoxyribose is hydrolyzed
Single Strand break: : directly or indirectly (BER)
double strand breaks
O6-MG: alkylation of oxygen antom of guanine –> repaired by suicide enzyme MGMT
spontaneous hydrolysis reaction of cytosine into Uracil
What is MGMT and how does it work?
is a suicide enzyme that is synthesized if DNA damage is detected: Transfers CH3 group of DNA to its self and is then degraded
how can genotoxic carcinogens be activated? what are direct and indirect genotoxins?
many genotoxins are not genotoxic by themselves but can form genotoxic metabolites through cellular metabolism: eg cytochrome P450 enzymes
direct genotoxins: active without metabolic activation
indirect: carcinogenic after metabolic activation
Procarcinogens are concerted by phase I enzymes. They then either form inactive metabolites which are secreted or ultimate carcinogens which are active and can lead to DNA damage
is DNA damage random?
No, each carcinogen has its specific pattern how it reacts with the DNA bases
how can DNA adducts be detected?
32P Postlabeling
Immunoassay
CG/MS and LC/MS
what is the relation between DNA damage, mutation and cell division?
cell division is required to fix a DNA damage
once a DNA damage is fixed in a mutation, it cannot be repaired
which mutations are more likely: transition mutation or transversions of bases?
transition more likely than transversion
what can lead to specific mutational patterns?
chemical exposures and DNA repair defects
why is tumor formation a rare event even though a lot of DNA damages occur per day per cell?
-efficient DNA repair systems
-many mutations are silent (introns, third base of codon)
-possibility of neutral mutation
-most gnees are not involved in cancer
is DNA damage sufficient for carcinogenesis?
no, but it is a prerequisite
what are the effects of genotoxic and non-genotoxic mechanisms?
genotoxic: DNA adducts; chromosome fusion, deletions, breackage, mis- segregation, non-disjunction
non-genotoxic: inflammation, immunosuppression, ROS, receptor activation, epigenetic silencing –> altered signal transduction
what is assesed in safety testing of drugs?
carcinogetic properties are evaluated in mutagenicity and carcinogenicity studies –> most time and cost consuming
how are carcinogens regulated? What is paracelsus principle?
paracelsus principle: any poison can be non-toxic if the dose is below an appropiate threshold
–> but no threshold at any exposure level for genotoxic carcinogens
–> drug development stopped if genotoxicity Tests are positive, but non-genotoxic carcinogens may be used
how does carcinogenicity testing work?
2-years rodent bioassay:
-rats
-male and female
-400 to 600 animals per study
-one control and three dosis groups
how does genotoxicity testing work?
several in vivo and in vitro tests
assessing genotoxicity via detection of mutations, chromosome abnormalities or DNA breaks
–>bacteria: Ames Test
–>rodent/human cells: MLA/HPRT, Micronucleus test, chromosomal aberrations
–> in vivo test as follow up
how does the Ames test work?
salmonella or e coli which are auxotrophic for an essential amino acid
incubation of tester strains with a potential genotoxin
induced mutations lead to increase in number of revertants –> are able to grow on minimal essential amino acid medium
how does the mammalian mutagenicity tesing (MLA TK test) work?
in vitro gene mutation assay:
nucleoside analogues are incorporated into DNA in TK-positive cells –> cytotoxic
genotoxin induced mutaions in TK lead to resistance –> survival
how does the micronucleus test work? What does it test for?
it tests for: clastogenic and aneugenic chemicals
cytokinesis blocking using cytochalsain B (inhibits actin polymerization) –> binucleate cells –> staining using DNA specific dyes/flourescent labelled DNA probes
How can chromosome aberrations be tested?
Analysis of structural abnormalities:
exposure to test substance –> cells are treated with a metaphase-arresting substance –> chromosome preparation (hypotonic treatment, fixation, staining)
how can single strand or double strand breaks be detected? what is the name of the assay and how does it work?
Comet assay:
-exposed single cells are seeded into an aqueous gel matrix
-direct current electric filed is established
-DNA migration towards anode
-DNA fragments induced by DNA strand breaks migrate faster than whole chromosomes
-
What are the limitations of the comet assay?
-very sensitive
-high concentration of test substance often give positive results because necrosis or apoptosis can induce DNA damage
What is MOE? what does it determine?
Margin of exposure:
MOE=TD10/estimated human exposure
with TD10= dose to give tumors to 10 % rats or mice
What is the ALARA principle?
carcinogen exposure As Low As Reasonably Achievable
At which MOE is the existing carcinogenic risk to be low?
> 10.000
