Nitrogen Mustard Induces Formation of DNA–Histone Cross-Links in Nucleosome Core Particles

Nitrogen mustards have long been used in cancer chemotherapy, and their cytotoxicity has traditionally been attributed to the formation of DNA interstrand cross-links and DNA monoalkylation. Recent studies have shown that exposure to nitrogen mustards also induces the formation of DNA–protein cross-links (DPCs) via bridging between N7 of a deoxyguanosine residue in the DNA and the side chain of a Cys residue in the protein. However, the formation of nitrogen mustard-induced DNA–histone cross-links has never been observed. Herein, we demonstrate that treating reconstituted nucleosome core particles (NCPs) with the nitrogen mustard mechlorethamine results in the formation of DNA–histone cross-links in addition to DNA monoalkylation and interstrand cross-link formation. The yields of these three types of DNA lesions in the NCPs decreased in the following order: DNA monoalkylation ≫ DNA interstrand cross-links > DNA–histone cross-links. Mechanistic studies involving tailless histones and competitive inhibition by a polyamine demonstrated that Lys residues in the N- and C-terminal tails of the histones were the predominant sites involved in DNA–histone cross-link formation. Given that NCPs are the fundamental repeating units of chromatin in eukaryotes, our findings suggest that nitrogen mustard-induced formation of DNA–histone cross-links may occur in living cells and that DPC formation may contribute to the cytotoxicity of nitrogen mustards.