The DNA damage spectrum produced by simulated sunlight

The mutagenic effects of ultraviolet and solar irradiation are thought to b e due to the formation of DNA photoproducts, most notably cyclobutane pyrim idine dimers (CPDs) and pyrimidine (6-4) pyrimidone photoproducts ((6-4)PPs ). Experimental systems for determining the levels and sequence dependence of photoproduct formation in DNA have often used high doses of short-wave ( UVC) irradiation. We have reassessed this issue by using DNA sequencing tec hnologies and different doses of UVC as well as more physiologically releva nt doses of solar irradiation emitted from a solar UV simulator. It has bee n questioned whether hot alkali treatment can detect (6-4)PPs at all sequen ce positions. With high UVC doses, the sequence distribution of (6-4)PPs wa s virtually identical when hot alkali or UV damage endonuclease (UVDE) were used for detection, which appears to validate both methods. The (6-4)PPs f orm at 5'-TpC and 5'CpC sequences but very low levels are seen at all other dipyrimidines including 5'-TpT. Contrary to expectation, we find that (6-4 ) photoproducts form at almost undetectable levels under conditions of irra diation for up to five hours with the solar UV simulator. The same treatmen t products high levels of CPDs. In addition, DNA glycosylases, which recogn ize oxidized and ring-opened bases, did not produce significant cleavage of sunlight-irradiated DNA. From these data, we conclude that cyclobutane pyr imidine dimers are at least 20 to 40 times more frequent than any other DNA photoproduct when DNA or cells are irradiated with simulated sunlight. (C) 2000 Academic Press.