Nucleotide excision repair from site-specifically platinum-modified nucleosomes

Nucleotide excision repair is a major cellular defense mechanism against the toxic effects of the anticancer drug cisplatin and other platinum-based chemotherapeutic agents. In this study, mononucleosomes were prepared containing either a site-specific cis-diammineplatinum(II)-DNA intrastrand d(GpG) or a d(GpTpG) cross-link. The ability of the histone core to modulate the excision of these defined platinum adducts was investigated as a model for exploring the cellular response to platinum-DNA adducts in chromatin. Comparison of the extent of repair by mammalian cell extracts of free and nucleosomal DNA containing the same platinum-DNA adduct reveals that the nucleosome significantly inhibits nucleotide excision repair. With the GTG-Pt DNA substrate, the nucleosome inhibits excision to about 10% of the level observed with free DNA, whereas with the less efficient GG-Pt DNA substrate the nucleosome inhibited excision to about 30% of the level observed with free DNA. The effects of post-translational modification of histones on excision of platinum damage from nucleosomes were investigated by comparing native and recombinant nucleosomes containing the same intrastrand d(GpTpG) cross-link. Excision from native nucleosomal DNA is approximately 2-fold higher than the level observed with recombinant material. This result reveals that post-translational modification of histones can modulate nucleotide excision repair from damaged chromatin. The in vitro system established in this study will facilitate the investigation of platinum-DNA damage by DNA repair processes and help elucidate the role of specific post-translational modification in NER of platinum-DNA adducts at the physiologically relevant nucleosome level.