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A comparative study on the effect of MNU on human lymphocyte cultures in vitro evaluated by O6-mdG formation,
G Stephanou1, D Vlastos, D Vlachodimitropoulos
1Department of Biology, University of Patras, Greece.
Abstract:
N-Nitroso-compounds are a large group of chemicals present in a number of environmental sources and many of them are mutagens as well as carcinogens in experimental animals. Among the known N-nitroso-compounds, N-nitroso-N-methylurea (MNU) is a strong mutagen. In this study an effort has been made to compare the ability of MNU to methylate the O6-guanosine site in DNA and to induce micronuclei and sister chromatid exchanges in human lymphocyte cultures in vitro. To quantitate O6-methyldeoxyguanosine (O6-mdG) a highly sensitive immunoassay, immuno-slot-blot (ISB), has been used. For the evaluation of micro nuclei (MN) the cytokinesis block micronucleus method has been used. Different concentrations (75, 100, 125 micrograms/ml) were tested. At the highest concentration tested for the MN induction, 125 micrograms/ml, the occurrence of binucleates and micro nuclei is higher than twice in relation to control and a reduction in NDI is also observed. The same concentrations were used for the estimation of sister chromatid exchanges (SCEs) induction. The mean number of SCEs at 125 micrograms/ml is almost three times that of the control level. The concentrations tested for the quantitation of O6-mdG were 200, 300 and 400 micrograms/ml and this was done because for the test system we used and for the given experimental conditions the first indication of O6-mdG formation was at 200 micrograms/ml. Our results show that methylation of O6-guanosine increases with concentration and at 400 micrograms/ml the concentration of O6-mdG is 5.83 fmol/microgram DNA, while at the control level it is 2.40 fmol/microgram DNA. Since O6-mdG formation is observed in higher concentrations than those of MN and SCE induction it would be interpreted that O6-mdG levels are not correlated with the studied cytogenetic effects although one has to take into consideration the total promutagenic lesions in DNA, induced by MNU, as well as AGT repair activity.
Insights
N-nitroso-N-methylurea (MNU) is a potent mutagen. This study found MNU induces DNA methylation, micronuclei, and sister chromatid exchanges in human cells, but O6-methyldeoxyguanosine levels did not correlate with cytogenetic effects.
Area of Science:
- Environmental Health
- Toxicology
- Molecular Biology
Background:
- N-nitroso-compounds are environmental mutagens and animal carcinogens.
- N-nitroso-N-methylurea (MNU) is a known potent mutagen.
- Understanding MNU's genotoxicity in human cells is crucial.
Purpose of the Study:
- To compare MNU's ability to methylate O6-guanosine in DNA.
- To assess MNU's capacity to induce micronuclei (MN) and sister chromatid exchanges (SCEs) in human lymphocyte cultures.
- To investigate potential correlations between DNA methylation and cytogenetic damage.
Main Methods:
- Quantification of O6-methyldeoxyguanosine (O6-mdG) using immuno-slot-blot (ISB).
- Evaluation of micronuclei (MN) induction via the cytokinesis block micronucleus method.
- Assessment of sister chromatid exchanges (SCEs) induction.
- Testing MNU concentrations of 75-125 µg/ml for MN and SCEs, and 200-400 µg/ml for O6-mdG.
Main Results:
- MNU significantly increased micronuclei and sister chromatid exchanges in a dose-dependent manner.
- O6-methyldeoxyguanosine formation increased with MNU concentration, reaching 5.83 fmol/µg DNA at 400 µg/ml.
- O6-mdG levels were observed at higher concentrations than those inducing significant MN and SCEs.
Conclusions:
- MNU induces DNA methylation and cytogenetic damage in human lymphocytes in vitro.
- The study suggests O6-mdG levels may not directly correlate with the observed MN and SCE induction.
- Further research should consider total promutagenic lesions and DNA repair activity (e.g., AGT).