Catalytic DNA: a novel tool for gene suppression.

RNA, as an intermediate in the production of every gene encoded protein and the genetic material of many pathogenic viruses, presents an attractive target for both biological and therapeutic manipulation. Despite its extensive involvement in living systems, its chemical diversity based on four units is relatively low compared with protein. This provides the opportunity for a generic approach to targeting with specificity based on primary structure rather than complex higher order structures. This form of recognition occurs naturally in complementary nucleic acids, due to an ability to bind their single stranded target through Watson-Crick interactions. The most established nucleic acid based approach to gene suppression at the RNA level is through antisense oligodeoxynucleotides (ODNs). These compounds form heteroduplex with target RNA which are thought to either block its function or mediate its destruction by activation of RNase H. Alternatively, RNA can be targeted by catalytic RNA such as the hammerhead ribozyme. Ribozymes have the advantage of being equipped with their own RNA cleavage apparatus and are therefore independent of host nuclear protein activity. At present, the utility of ribozyme oligonucleotides is restricted by the relative difficulty synthesising active molecules with sufficient resistance to nuclease degradation. Recently the power of in vitro selection has been used to evolve catalytic DNA sequences with RNA cleavage specificity and activity rivalling the very best ribozymes, while maintaining the more robust chemistry of an ODN. These deoxyribozymes or DNAzymes have tremendous potential as gene suppression agents for both target validation and therapeutic applications. A number of studies evaluating the biological activity of these compounds have shown promising results. However, as with other oligonucleotide based strategies, future exploitation of this approach may depend on accessory technology to assist with the accessibility of a target which is folded by its own secondary structure and hidden within the intracellular compartment.