Detection of MNNG-induced DNA lesions in mammalian cells; validation of comet assay against DNA unwinding technique, alkaline elution of DNA and chromosomal aberrations.

Human cells (VH10 or Hep G2) and hamster cells V79 were exposed to different concentrations of N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) and the level of DNA lesions was evaluated by the DNA unwinding technique, alkaline elution of DNA and the comet assay. All three methods were able to detect the effects of MNNG but with a clear difference in sensitivity. At low concentrations of MNNG the most sensitive method appeared to be the comet assay. After the short-term treatment the comet assay was able to detect the lesions induced by MNNG at approx. 0.1 microgram/ml, alkaline elution of DNA at 1 microgram/ml and DNA unwinding at 1-2 micrograms/ml. MNNG treated VH10 cells, human lymphocytes and V79 cells were also tested cytogenetically, confirming that MNNG induced chromosomal aberrations at concentrations > 1 microgram/ml in VH10 cells (short-term treatment): > 0.2 microgram/ml in V79 cells (long-term treatment) and > 8 micrograms/ml in human lymphocytes (long-term treatment). In some experiments we tried to increase the level of MNNG-induced DNA breaks with help of DNA repair inhibitors cytosine arabinoside (Ara C) and hydroxyurea (HU) which were applied either after or during MNNG treatment. Our results showed that the level of MNNG-induced lesions was increased by simultaneous treatment of cells with MNNG and Ara C and HU. 2 x 10(-5) M Ara C and 2 x 10(-3) MHU were as effective as 10-times higher concentrations of inhibitors. Ara C and HU increased the level of MNNG-induced DNA breaks mainly in combination with lower concentrations of MNNG (< 2 micrograms/ml). Rejoining of DNA breaks was observed in human cells VH10 and Hep G2 as well as in Chinese hamster cells V79 damaged by both lower and higher MNNG-concentrations. All methods showed that MNNG-induced DNA breaks had been gradually rejoined.