Alteration in the choice of DNA repair pathway with increasing sequence selective DNA alkylation in the minor groove

Background: Many conventional DNA alkylating anticancer drugs form adducts in the major groove of DNA. These are known to be chiefly repaired by both nucleotide (NER) and base (BER) excision repair in eukaryotic cells. Much l ess is known about the repair pathways acting on sequence specific minor gr oove purine adducts, which result from a promising new class of anti-tumour agents. Results: Benzoic acid mustards (BAMs) tethering 1-3 pyrrole units (compound s 1, 2 and 3) show increasing DNA sequence selectivity for alkylation from BAM and 1, alkylating primarily at guanine-N7 in the major groove, to 3 whi ch is selective for alkylation in the minor groove at purine-N3 in the sequ ence 5'-TTTTGPu (Pu = guanine or adenine). This increasing sequence selecti vity is reflected in increased toxicity in human cells. In the yeast Saccha romyces cerevisiae, the repair of untargeted DNA adducts produced by BAM, 1 and 2 depends upon both the NER and BER pathways. In contrast, the repair of the sequence specific minor groove adducts of 3 does not involve known B ER or NER activities. In addition, neither recombination nor mismatch repai r are involved. Two disruptants from the RAD6 mutagenesis defective epistas is group (rad6 and rad18), however, showed increased sensitivity to 3. In p articular, the rad18 mutant was over three orders of magnitude more sensiti ve to 3 compared to its isogenic parent, and 3 was highly mutagenic in the absence of RAD18. Elimination of the sequence specific DNA adducts formed b y 3 was observed in the wild type strain, but these lesions persisted in th e rad18 mutant. Conclusions: We have demonstrated that the repair of DNA adducts produced b y the highly sequence specific minor groove alkylating agent 3 involves an error free adduct elimination pathway dependent on the Rad18 protein. This represents the first systematic analysis of the cellular pathways which mod ulate sensitivity to this new class of DNA sequence specific drugs, and ind icates that the enhanced cytotoxicity of certain sequence specific minor gr oove adducts in DNA is the result of evasion of the common excision repair pathways.