In the present work we studied the DNA-damaging effect of solar UV-radiation at different biological endpoints after exposure. Our results demonstrate the usefulness of the comet-assay and the micronucleus (MN)-assay in this research field.

1. Using comet-assay we could differentiate between UVA- and UVB-irradiation induced DNA-damage and repair in cultured human HaCaT keratinocytes. Our results are in agreement with literature data. Furthermore, we have found that the alkaline comet-assay is not able to show the effect of PUVA (8-MOP+UVA) on DNA.
2. We could demonstrate by means of neutral comet-assay the formation of DNA-double strand breaks during DNA-repair following PUVA-induced DNA-damage.
3. A flow cytometry method we modified and applied made it possible to measure MN-induction with differentiation between nuclei in G0/G1- and G2/M-phase at the same time in cultured normal human fibroblasts (FB) ad keratinocytes (KC). We observed positive correlation between MN-induction and proportion of G2/M-phase nuclei in the sample.
4. We were the first to report that UVA is able to induce chromosomal damage in a dose dependent manner in human FB.
5. We have found that UVB-irradiation also induces chromosomal damage in a dose dependent manner in human melanocytes (MC) and FB, but the extent of the DNA-damage is different in the two cell types regarding the same radiation doses.
6. By means of comet-assay we have found that upon 4 h and 8 h of exposure to very low concentrations of formaldehyde (FA) significant DNA-protein crosslink-formation is present in normal human FB and in normal human KC.
7. We were the first to describe DNA-damage and repair kinetics induced by UV-irradiation in cultured normal human KC in comparison with FB using comet-assay. The DNA-damage and repair following UV-irradiation were dependent on the wavelength of UV and the repair kinetics was similar in the two cell types.
8. We have found that FA at a low concentration level interfered with DNA-resynthesis/ligation step of nucleotide excision repair following UVC- and UVB-irradiation. FA had no effect on the DNA-repair after UVA-irradiation.
9. We have found that the delay in DNA-repair resulted in an increase of chromosomal damage. FA at a concentration not inducing MN caused significant increase of UVC-induced chromosomal damage.