Nucleic acid enzymes as a novel generation of anti-gene agents

Recent molecular and cellular studies have highlighted the important role of some gene products in the cause and/or perpetuation of human pathological conditions including cancer and autoimmune diseases. The identification of such gene products has led to the development of new candidate therapies. The discovery of catalytic nucleic acid enzymes has provided researchers with a potentially important tool to block the expression of abnormal genes, provided that their sequences are known. The cleavage specificity of these compounds is determined by their hybridizing antisense arms, which anneal with the target mRNA in a complementary fashion. Nucleic acid enzymes can be delivered to cells either endogenously as gene encoding RNA enzymes (ribozymes) or exogenously as in vitro made agents. Given the progress reported during the last years, a wide range of molecular designs and chemical modifications can be introduced into these compounds, in particular the hammerhead type ribozyme. Here, we review the design, stability and the therapeutic application of these agents with the goals of illustrating relevant gene targets and signal pathways for molecular medicine. Relevant in vivo problems of the technology, mRNA repair by group I intron ribozymes and gene regulation by endogenous RNA will also be discussed.