Mutations in sunlight-induced melanoma arise from cyclobutane pyrimidine dimers (CPD), DNA photoproducts that are typically created picoseconds after an ultraviolet (UV) photon is absorbed at thymine or cytosine. Here we show that in melanocytes, CPD are generated for >3 hours after exposure to UVA, a major component of the radiation in sunlight and in tanning beds. These “dark CPD” constitute the majority of CPD and include the cytosine-containing CPD that initiate UV-signature C→T mutations. Dark CPD arise when UV-induced reactive oxygen and nitrogen species combine to excite an electron in fragments of the pigment melanin. This creates a quantum triplet state that has the energy of a UV photon but that induces CPD by energy transfer to DNA in a radiation-independent manner. Melanin may thus be carcinogenic as well as protective against cancer. These findings also validate the long-standing suggestion that chemically-generated excited electronic states are relevant to mammalian biology.

Sunlight-induced melanomas contain UV-signature mutations, which are caused by cyclobutane pyrimidine dimers (CPD). These photoproducts are typically created picoseconds after a UV photon is absorbed at adjacent thymines or cytosines. However, using immunohistochemistry, mass spectrometry, and RNAi, we find that melanocytes generate CPD for >3 hours after exposure to UVA or UVB, wavelengths found in sunlight and in tanning beds; these “dark CPD” constitute the majority of CPD induced. Using pharmacologic inhibitors, single-photon counting, and specific energy acceptors, we elucidated the mechanism. The process begins when UV-induced superoxide and nitric oxide combine to form peroxynitrite, one of the few biological molecules capable of exciting an electron. Excitation creates a quantum triplet state in the skin pigment melanin that has the energy of a UV photon but induces CPD by transferring its energy to DNA in a radiation-independent manner. Melanin is evidently carcinogenic as well as protective. These findings may underlie the dependence of UV-induced and spontaneous skin cancers on melanin type. The results also validate the long-standing suggestion that chemical generation of excited electronic states - the source of bioluminescence in lower organisms - is important in mammalian biology. Citation Format: Sanjay Premi, Silvia Wallisch, Camila Mano, Adam Weiner, Antonella Bacchiocchi, Kazumasa Wakamatsu, Etelvino Bechara, Ruth Halaban, Thierry Douki, Douglas E. Brash. Excited electrons in melanin induce cyclobutane dimers in the dark. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr LB-104. doi:10.1158/1538-7445.AM2015-LB-104