Chemical reagents that form interstrand cross-links have been used for a long time in cancer therapy. They covalently link two strands of DNA, thereby blocking transcription. Cross-link repair enzymes, however, can restore the transcription processes, causing resistance to certain anti-cancer drugs. The mechanism of these cross-link repair processes has not yet been fully revealed. One of the obstacles in this study is the lack of sufficient amounts of well-defined, stable, cross-linked duplexes to study the pathways of cross-link repair enzymes. Our group has developed a cross-link strategy where a furan moiety is incorporated into oligodeoxynucleotides (ODNs). These furan-modified nucleic acids can form interstrand crosslinks upon selective furan oxidation with N-bromosuccinimide. We here report on the incorporation of the furan moiety at the 2 0 -position of a uridine through an amido or ureido linker. The resulting modified ODNs display an unprecedented selectivity for cross-linking toward a cytidine opposite the modified residue, forming one specific cross-linked duplex, which could be isolated in good yield. Furthermore, the structure of the formed cross-linked duplexes could be unambiguously characterized.
' INTRODUCTIONThe high specificity with which oligonucleotides recognize nucleic acids has led to considerable interest in the design of modified oligonucleotides. 1 These customized nucleic acid derivatives can be used for a range of therapeutic and analytical purposes, including the treatment of diseases (antisense, 2-4 siRNA 5-8 ), the regulation of gene expression (antigene, 9-11 decoy DNA 12,13 ), the investigation of RNA tertiary structures, 14-16 and the study of DNA damage and the resulting repair processes. [17][18][19][20][21] DNA is not indefinitely stable, and numerous sources of DNAdamaging agents of endogenous and exogenous origin contribute further to the instability of DNA. 22 DNA interstrand cross-links (ICLs) are among the most cytotoxic lesions known, the covalent bond between the two DNA strands prohibiting strand separation and thereby blocking vital aspects of DNA metabolism. ICLs are mostly formed by reaction of cellular DNA with bifunctional electrophilic compounds that are either formed endogenously (e.g., lipid peroxidation) or present through exogenous exposure. 23 Additionally, generation of an abasic site in the DNA upon depurination can also give rise to ICL formation. 20,24 The cytotoxic properties of such ICL-forming agents are exploited by cancer chemotherapeutics, where their toxicity ultimately leads to apoptosis of the malignant cells. 25 Development of resistance of the tumor cells to treatment with antitumor drugs can often be attributed to enhanced ICL repair. 23 These repair pathways find their evolutionary origin in the necessity to counteract the threat posed by endogenous and exogenous crosslinking compounds, and failure to repair these lesions can lead to inherited disorders such as Fanconi anemia. 23 Thus, although DNA repair is essential for a heal...