Dark cyclobutane pyrimidine dimers are formed in the epidermis of Fitzpatrick skin types I/II and VI in vivo after exposure to solar-simulated radiation.

INTRODUCTION: Unlike "light" cylobutane pyrimidine dimers (CPD) formed during ultraviolet radiation (UVR) exposure, dark CPD (dCPD) are formed afterwards. Studies have attributed this to delayed melanin sensitization. There are no data on the role of melanin in dCPD formation in human skin. METHODS AND RESULTS: Volunteers of Fitzpatrick skin types (FST I/II vs. VI) were exposed to erythemally equivalent doses of solar simulated radiation. CPD were assessed by semi-quantitative immunostaining in whole epidermis and in three epidermal zones, and quantitative HPLC-MS/MS (whole epidermis) at different times post-exposure up to 24 hr. A CPD peak that appeared at 1-2 hr post-exposure in whole epidermis measurements, in all skin types, demonstrated dCPD. However, both dCPD and light CPD were absent in the basal layer of FST VI with the greatest melanin concentration. Modelling the whole epidermis data showed no differences between the repair kinetics of FST I/II and VI. DISCUSSION: Melanin may be a sensitizer or "sunscreen" for dCPD depending on its location and concentration. Previous CPD repair studies in human skin have assumed peak CPD immediately after UVR exposure and so have overestimated total repair.