Effects of chronic exposure to ultraviolet B radiation on DNA repair in the dermis and epidermis of the hairless mouse

It has previously been shown that chronic exposure to low fluences of ultra violet B radiation reduced DNA repair capacity in mouse skin. In this study we now extend this to examine the concentration dependence and tissue depe ndence of this phenomenon. We found that (6-4) photoproducts were repaired considerably faster than cyclobutane dimers and that the kinetics for photo product removal were comparable in the dermis and epidermis. Chronic ultrav iolet B irradiation significantly reduced the initial rate and extent of DN A repair. After low daily doses of ultraviolet B (6-4) photoproduct repair was most affected and after high daily doses the repair of both cyclobutane and (6-4) dimers was reduced. Whereas cyclobutane dimer repair was most af fected in the dermis, reduced (6-4) photoproduct repair was observed in bot h tissues. The deleterious effects of chronic ultraviolet exposure were sus tained for a considerable time after the chronic treatment ended.