Synthesis and characterisation of DNA adducts derived from the bracken toxin ptaquiloside.

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Bracken is a highly invasive plant found extensively in the UK and Europe with a well-documented toxicity in cattle and other animals. Bracken toxicity derives from the presence of ptaquiloside, a norsesquiterpenoid glucoside that is both mutagenic and carcinogenic. Ptaquiloside can be transmitted to humans following direct exposure from contaminated surface or ground water in bracken infested areas, or indirectly for example from contaminated dairy products. At slightly alkaline pH ptaquiloside decomposes to form a potent alkylating agent known as bracken dienone. The exposure of DNA to bracken dienone produces N3-adenine and N7-guanine base adducts. In contrast, the corresponding O6-alkylguanine adducts (O6-(pta)G adducts) have not been characterized in DNA, although the biological consequences of their formation are substantially more significant: O6-alkylguanine adducts are promutagenic and carcinogenic, typically coding for the misincorporation of thymine during DNA replication leading to GC to AT transition mutations.This proposal seeks the first characterization of O6-alkylguanine (specifically O6-(pta)guanine) adducts in DNA following exposure to ptaquiloside/bracken dienone that we will extract and purify from bracken. DNA containing O6-(pta)guanine adducts will be characterised using the O6-alkylguanine-binding protein Atl1 and by comparison with a synthesized O6-modified deoxyribonucleoside standard following enzymatic digestion of O6-(pta)guanine-containing DNA. Integral to this proposal, we will develop chemical syntheses of the naturally occurring enantiomer of pterosin B and novel ODNs containing O6-(pta)guanine adducts. The ability of the protein O6-methylguanine-DNA methyltransferase (MGMT) to repair O6-(pta)guanine-containing DNA and the substrate properties of this adduct for human DNA polymerase enzymes will be examined. The reaction of ptaquiloside or bracken dienone with water (decomposition) or the bases of DNA produces pterosin B and derivatives thereof. We aim to develop methods for the detection of such pterosin B-containing compounds. Our proposal will lead to a much more coherent understanding of the nature and risk to humans of exposure to bracken and its constituents.