Detection of DNA-psoralen photoadducts in mammalian skin

An immunofluorescence (IF) method for the detection of 8-methoxypsoralen (8-MOP) photoadducts to DNA has been developed to assess nuclear damage in keratinocytes and melanocytes after psoralen plus UVA (PUVA) treatment, both under in vitro and in vivo conditions. Cryostat sections of the albino and pigmented guinea pig and human skin were used for in vitro studies to establish minimal and maximal drug concentration and UVA dosimetry for the detection of DNA-8-MOP photoadducts. Limits of detection were as low as 10 ng/cm2 8-MOP and 1 J/cm2 UVA for skin sections and sodium bromide-split epidermal sheets. Guinea pigs treated with topical PUVA revealed positive IF stain in epidermal cell nuclei at a threshold dose of 100 micrograms/cm2 8-MOP and 13 J/cm2 UVA. Pretreatments of cryostat cuts with ethanol and alkali before IF test enhanced the sensitivity of detection in vivo about 10-fold and enabled us to follow the repair of DNA damage after treating normal guinea pig skin with a dose of 50 micrograms/cm2 8-MOP plus 6 J/cm2 UVA. The most interesting findings were as follows: A sensitive method to detect PUVA-induced nuclear damage in epidermal and dermal cells was developed. PUVA treatment induced nuclear DNA damage to melanocytes as well as to adjacent keratinocytes, and melanocytes appeared to be 10 times less vulnerable to photo-damage than keratinocytes. There was a greater propensity for the proliferative cells to be damaged by PUVA. PUVA induced nuclear damage up to 700 micron depth in the dermis. The usefulness of the IF test in detecting DNA damage in microgram and ng amounts in vivo and in following the repair of damaged DNA induced by PUVA.