LONDON—English researchers found UVA rays may be more carcinogenic than previously thought (J Investigat Dermatol. Oct. 6, 2011). UVB readily induces cyclobutane pyrimidine dimers, mainly thymine dimers (TTs) and pyrimidine (6-4) pyrimidone photoproducts (6-4PPs) in DNA. These lesions result in “UVB signature mutations" found in skin cancers. Researcher investigated the induction of TTs and 6-4PPs in human skin in vivo by broadband UVA, and compared it with comparable erythemal doses of monochromatic UVB (300 nm).
In vitro and ex vivo studies showed the production of TTs, without 6-4PPs, by UVA. We show that UVA induces TTs, without 6-4PPs, in the epidermis of healthy volunteers in vivo, whereas UVB induced both photoproducts. UVB induced more TTs than UVA1 for the same level of erythema. The level of UVA-induced TTs increased with epidermal depth in contrast to a decrease that was seen with UVB. UVA- and UVB-induced TTs were repaired in epidermal cells at a similar rate. The mechanism by which UVA induces TTs is unknown, but a lack of intra-individual correlation between the subjects’ UVB and UVA minimal erythema doses implies UVA and UVB erythema occur by different mechanisms.