Hybridization of antisense reagents to RNA.

Despite the simplicity of the concept, almost every step in an antisense experiment poses difficulties. Finding a site that is accessible to intermolecular hybridization with complementary nucleic acids is a major problem and determines the success or failure of an antisense experiment. A major determinant of accessibility appears to be the intramolecular folding in mRNAs that renders much of the molecule inaccessible. However, owing to our poor understanding of RNA folding and the mechanisms of heteroduplex formation, theoretical methods have limited use in finding accessible sites. Such methods are unable to address two major considerations when designing an antisense reagent, i.e., which is the most accessible sequence in the target and what length of the reagent works best in terms of activity and specificity. Empirical approaches appear more successful. Of notable interest, and reviewed here, are 'global' methods based on DNA arrays and on mapping of transcripts with RNase H.