ecology practicals2
TRANSCRIPT
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Mutagenicity and carcinogenicity of environmental factors
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Next week
• A test in molecular biology
– (will be based on professor Goetz and
professor Sedláček lectures)
• Don´t be late, please!
• Possibility of presentations (after
negotiation with professor Mareš)
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Find Kočárek´s presentations in:
• http://www.medik.cz/medik/kocarek/
or
• http://camelot2.lf2.cuni.cz/turnovec/ublg/vyuka/
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Mutation• A change in the DNA molecule• Process which produces changes in the
DNA that may be inherited.
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Mutagenicity
• = mutagenic activity
• = genotoxicity (genotoxic activity)
• Mutagenic (genotoxic) compound = a compound that causes mutation
• Mutations in certain genes could lead to malignant transformation of the cell.
• Many mutagens are supposed to be carcinogenic.
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Carcinogens
• Karkinos (Greek) =
crayfish
• Compounds or other
factors that induce
transformation of a
normal cell into a tumor
one
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Classification of mutagens
• Chemical (different compounds)
– Alkylating agents
– Aromatic hydrocarbons (e.g. benzo(a)pyrene)
– Intercalating agents (e.g. fluorescent dyes)
– Artificial derivatives of DNA bases
• Physical (ionizing radiation, UV-radiation)
• Biological (viruses, transposable
elements)
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According to metabolic conversion we distinguish:
• Direct acting mutagens (active without
metabolic conversion)
• Promutagens (require metabolic
activation)
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Benzo(a)pyrene• Polycyclic aromatic hydrocarbon
• Carcinogenic activity confirmed
• A product of combustion processes (e.g.
tobacco smoking)
• A typical promutagenic agent
• Metabolic conversion (addition of epoxide
group and two OH-groups).
• A metabolic product binds to DNA → adduct
(a compound that results from addition).
• Presence of adduct can cause mutation.
Metabolic conversion
binds to DNA
BaP – guanin adduct
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Metabolic activation of mutagens
PROMUTAGENACTIVE
(ULTIMATE)
MUTAGEN
DNA ADDUCT (OR OTHER CHEMICAL MODIFICATION OF
DNA BASES)
MUTATION
enzymes
met
abo
lic
acti
vati
on
TUMOR
liver
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Genotoxicity screening tests
• On molecular level
• On gene level
• On chromosomal level
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Tests on molecular level
• Identification of adducts
• Unscheduled DNA synthesis – measure of
DNA repair using radioactively labeled
nucleotide.
• Comet assay
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Comet assay
• Single Cell Gel Electrophoresis (SCGE)
• Identification of small DNA fragments inside nuclei of affected cells
• The fragments result from mutagenic affect.
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Comet assay
• DNA fragments are released from nuclei using electrophoresis
• Isolated nuclei are mounted into electrophoretic gel – after electrophoresis are stained with fluorescent dye.
• If DNA fragments are present a „comet tail“ is present observed in the vicinity of the nuclei.
– +
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Comet assay
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Possible results of a comet assay
Normal nucleus without fragments
(DNA is not damaged – mutagenicity excluded)
Two nuclei with DNA damage
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Evaluation of a comet assay
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Comet assay – computer analysis
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Tests on gene level• Induction of in vitro mammalian cells resistant to
certain chemical compound – (e.g. 8-azaguanine, 6-thioguanin, ouabain)
• SOS/umu test – measurement of DNA repair in bacteria– The bacteria is transformed by plasmide with mutator
(repair) genes umuC and umuD fused with the gene for galactosidase. If the DNA is damaged by mutagen both umu mutator genes and gal gene are transcribed and galactosidase turns a specific substrate into a color product.
• Ames test
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Ames test
Bruce Ames (born 1928)
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Ames test
• Test system – auxotrophic strain of Salmonella typhimurium – survives only in medium with histidine (dies in normal medium without histidine)
• After treatment with mutagen some auxotrophic cells are turned into normal ones that synthesize histidine and survive in a normal medium.
• These cells are called revertants (due to reverse mutation).
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Ames test
His–
bacteria
Dies in a normal medium
Medium containing histidine
Normalmedium
His+
bacteriaReverse mutation
Mutagen
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Ames test
COMPOUND
CONFIRMED
MUTAGENIC
COMPOUND
0 Negative control
Positive control
A dish with a compound to
be tested
GENOTOXICITY
CONFIRMED
SPONTANEOUS
REVERTANTS
IS USED FOR
CONTROL OF
THE TEST
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Result of the Ames test
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Ames test – automatic evaluation of results
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Evaluation of mutagenic activity of 1-nitropyrene by means of the Ames test
• The graph describes results of the Ames test of various 1-nitropyrene doses.
• Describe the relationship between the dose and number of revertant colonies. Is it possible to confirm the mutagenic activity?
• Why we test the compound in various concentrations?
Nu
mb
er
of
rev
ert
an
t c
olo
nie
s
Dose of 1-nitropyrene (micrograms per dish)
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Result of the task
Number of colonies increases with the dose – mutagenicity confirmed.
Number of colonies decreases due to toxic effects of the tested compound
Nu
mb
er o
f re
vert
ant
colo
nie
s
Dose of 1-nitropyrene (micrograms per dish)
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Tests on chromosomal level• Micronucleus test
– Micronucleus – a small body in the vicinity of the nucleus (the body usually consists of an acentric chromosomal fragment that originated from deletion)
• Frequency of chromosomal aberrations, mainly breakages– Possible in laboratory animals, cultivated mammalian
cells and humans (e.g. persons exposed to radiation of other supposed mutagenic factors.
• SCE – sister chromatid exchanges – A special cultivation method that allows identification
of exchanges between sister chromatids
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Result of micronucleus test
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Tests on chromosomal level• Micronucleus test
– Micronucleus – a small body in the vicinity of the nucleus (the body usually consists of an acentric chromosomal fragment that originated from deletion)
• Frequency of chromosomal aberrations, mainly breakages– Possible in laboratory animals, cultivated mammalian
cells and humans (e.g. persons exposed to radiation of other supposed mutagenic factors.
• SCE – sister chromatid exchanges – A special cultivation method that allows identification
of exchanges between sister chromatids
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Possible results of chromosomal analysis
Percentage of cells with chromosomal aberrations
Result
Less then 2%Normal finding, spontaneous
level of aberrations
2 – 4 %A border result – mutagenic effect is neither confirmed
nor excluded
More than 4%Mutagenic effect confirmed
with high probability
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Tests on chromosomal level• Micronucleus test
– Micronucleus – a small body in the vicinity of the nucleus (the body usually consists of an acentric chromosomal fragment that originated from deletion)
• Frequency of chromosomal aberrations, mainly breakages– Possible in laboratory animals, cultivated mammalian
cells and humans (e.g. persons exposed to radiation of other supposed mutagenic factors.
• SCE – sister chromatid exchanges – A special cultivation method that allows identification
of exchanges between sister chromatids
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Sister Chromatid Exchanges
Normal chromosome A chromosome with SCE
The higher SCE frequency – the higher mutagenic activity.
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About the origin of a tumor cell
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Carcinogenesis (simplified scheme)
mutagen mutagen
apoptosis
mutation repaired
A cell with the first mutation
A cell with the second mutation
Dividing tumor cells
normal cell
normal cell
„Rubicon“
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Mutagenic X carcinogenic agent
• All mutagens are supposed to be carcinogenic but in some of them the carcinogenicity was not detected.
• Some carcinogenic agents are not mutagenic – they belong to the group of epigenetic carcinogens.
• Evaluation of carcinogenicity to humans is difficult and requires combination of various tests.
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Carcinogenic agents
• Chemical (mostly mutagenic compounds)
• Physical (radiation, asbestos)
• Biological (oncogenic viruses – e.g. HTLV
= human T-leukemia virus).
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Direct evaluation of carcinogenicity
• Laboratory tests
– In animals
– In vitro transformation of cultivated cells
• Epidemiologic studies of exposed human
population – mostly reliable
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IARC database
International Agency for Research of
Cancer
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IARC database (International Agency for Research of Cancer)
• Direct connection to „Monographs“:
http://monographs.iarc.fr/index.php
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IARC classification
• Group 1: Compounds (mixtures)
carcinogenic to humans
• Group 2A: Probable human carcinogens
• Group 2B: Possible human carcinogens
• Group 3: Carcinogenicity not confirmed
• Group 4: Compounds probably not
carcinogenic to humans
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See you after 3 weeks!