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Methods of Methods of genotoxicologygenotoxicology
No 520 Development
Primary event in DNA ► DNA breaks, alkylation or other alteration of bases, „cross links“
▼
Reparation, defect in reparation, abnormal
reparation, abnormal joining of broken ends of
chromosomes, unrepaired DNA damage
▼
Chromosomal aberration
Method of mutagenic studyIn vitro:
Ames test – on bacteria – auxotrophic strains of Salmonella typhimurium
In vivo:
Micronucleus test – micronucleus = chromosomal fragment in cytoplasma of erythrocytes or binuclear cells, lymfocytes
Cytogenetic analysis of bone marrow cells of experimental animals or of human peripheral lymphocytes - chromosomal breaks or rearrangements
Comet assay – single strand DNA breaks
Bacteria : Salmonella typhimurium
- auxotrophic strains (bacterial cells are enable to
synthetize histidine because of mutation)
Mutagens cause reverse mutations and bacterial cells
thereby get the property to grow on the medium without
histidine and form colonies
Auxotrophy is the inability of an organism to synthesize
a particular organic compound required for its growth
Ames test
Scoring: counting of a number of colonies by computer (image analysis).
We detects mutagenic effect by Increased number of colonies (revertants) of sample in comparison with number of colonies in controls
Direct mutagens – original molecule has mutagenic
qualities
Indirect mutagens - need metabolic activation = are active only after enzymatic metabolisation
– S9 fraction of liver homogenate from rats treated by a mixture of mutagens – contains enzymes able to metabolize tested compound
Micronucleus testMicronucleus test
Micronuclei is either the whole chromosome or a chromosomal fragment as a small body outside nucleus
Principle: detection of the frequency of micronuclei: a) in erythrocytes of bone marrow of laboratory animals b) in peripheral lymphocytes of persons
Micronucleus testMicronucleus test
Mutagenes induce changes of chromosomes, breaks.
Chromosomal fragment (or whole chromosome) is not
attached to mitotic spindle, stays in cytoplasma after
formation of daughter nuclei.
A mutagenic effect of tested compound is presented by
increased number of micronuclei in 1000 of cells in
comparison with control.
MiMiccronuronuccleus testleus test
Detection of centromere by FISH
Micronucleus in polychromatophile Micronucleus in polychromatophile erythrocyteserythrocytes
http://faculty.ksu.edu.sa/al-saleh/Pages/LabAlbum.aspx
Micronuclei in peripheral lymphocytesMicronuclei in peripheral lymphocytes
Comet assay - single gel electrophoresis
Method testing mutagenic effects by the DNA strand break analysis
Method based on the electrophoresis
Quantitative analysis of DNA breaks
Advantage: it detects actual state of DNA damage, just before reparation.
Method of comet assayMethod of comet assay
Laboratory animals are exposed by the tested compound
Collection of tissue samples (bone marrow, liver, blood, intestine…)
Isolation of cells (trypsinisation)
Lysis of cells
Releasing of DNA double strands in alkali solution
Electrophoresis of DNA on microscopic slide covered with agarose
Staining (by fluorescent dye - ethidiumbromide)
Detection of DNA fragments as „tail of comet“ in fluorescent microscope
Intensity of fluorescence in tail compared with fluorescence in head of comet is quantitatively analyzed by computer program
Comet assay
Analysis of chromosomal aberrations in
human peripheral lymphocytes :
In vivo: in persons treated by mutagens
(professional exposure, or exposure by accident…) –
cultivation of peripheral lymphocytes – cytogenetic
method, staining and scoring of a number and types of
chromosomal aberrations
Scoring of 100-200 of cells per sample
Normal level (not increased) till 2% of cells with
chromosomal aberrations
post-radiation chromosomal aberrations
difragment
dicentric
Chromatid breaks
Chromatid exchange – after chemicals
Chromosomal changes (exchanges) after chemicals
– Cell cultivation (T lymphocytes) in the presence of
bromdeoxyuridine (5-bromo-2-deoxyuridine, BrdU ) during 2
cell cycles
– In the 1st mitosis - both chromatids are dark
– In the 2nd mitosis - one chromatid is dark, second one pale
pale chromatid = both DNA strands substituted with BrdU → delayed DNA spiralization → pale colour
Sister chromatid exchanges
1.
2.
+ BrdU for 2 cycles of division
BrdU = thymidine analogue
First mitosis:
BrdU is introduced to one DNA strand (new strand) of both chromatids during replication→ dark colour
Second mitosis:
One chromatid is dark, second (with both strands substituted by BrdU) is pale - delayed spiralization => pale staining
Observation of sister chromatid exchanges = method of testing of mutagenic effects
Different staining of chromatids in second mitosis with sister chromatid exchanges-marked by arrows
Sister chromatid exchanges – increased level after mutagene action
Metods of mutagenicity testing serve to prediction of carcinogenic effects (mutagenic test are quick in
comparison with long-term tests of carcinogenicity
Each mutagen = potential carcinogen!
but not all carcinogens are mutagenic (non-genotoxic carcinogens – support proliferation or have other effects, but do not induct mutations)
Experimental organismsExperimental organisms
Models in vivo: Drosophila melanogaster Mus musculus Rattus norvegicus Xenopus laevis Ceanorhabditis elegans
Models in vitro: Tissue cultures Cell lines (He-La cells) Primary cultures
Tobacco mosaic virus
Bacillus subtilis
Saccharomyces cerevisiae
Zea maysArabidopsis thaliana
Mus musculus, Rattus norvegicus
70% homology of human and mice genome
Mutation of same gene
Drosophila melanogasterXenopus laevisCaenorhabditis elegansSchizosaccharomyces pombe
Thanks for attention