Ribozymes as Human Therapeutic Agents

The protein encoded by a particular gene normally corresponds to an RNA sequence considerably shorter than the sequence transcribed from that gene. This is due to the fact that genes typically contain several exons (expressed sequences) separated by a series of introns (intervening sequences). When the RNA is transcribed from a gene, the corresponding intron sequences are spliced out and the exons are ligated in a transesterification reaction. The newly spliced series of exons is then translated into the appropriate proteins. In the early 1980s Cech and his colleagues discovered that certain RNA splicing reactions are catalyzed by RNA. It was unequivocally demonstrated that certain intervening sequences (Group I) were inherently capable of catalyzing RNA splicing reactions to give rise to mature RNA. Cech termed such RNA molecules, possessing enzymatic activity, "ribozymes". These RNA enzymes have now been found in a wide variety of biological systems. By suitable chemical or molecular manipulation, ribozymes can be engineered either to bind specifically to external desired RNA sequence targets and cleave them, thereby inhibiting a gene function, or to ligate new pieces of RNA onto the target by trans splicing to create a new gene function. Therein lies their therapeutic potential. In less than 15 years since the initial discovery by Cech and Altman, the fundamental importance of catalytic RNA (ribozymes) in chemistry and biology has become apparent. The demonstration that RNA plays an active catalytic role in the production of proteins from DNA and is not merely a "passive" participant has caused a major paradigm shift in the role of RNA in chemistry and biology. Furthermore, the availability of "catalytic RNA" to carry out processes previously reserved only for protein enzymes has caused a rethinking of the role RNA may have played in evolution. Ribozymes provide a broad and enabling technology applicable to human disease diagnosis and therapy, agriculture, and animal health. Correspondingly, research interest in ribozymes has grown exponentially over the past several years. Over 500 articles were published through the end of 1993 on various aspects of ribozymes and their role in chemistry, biology, and medicine. The broad potential of ribozyme technology is due to the fact that a ribozyme will, in principle, selectively bind and cleave any target RNA. Thus, highly specific control of gene expression by ribozyme cleavage and consequent nuclease destructi