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New Modalities in Oncology: Ribozymes

For years, one of the assumptions of cell biology was that all enzymatic actions were performed by proteins. Although nucleic acids (DNA and RNA) could serve as blueprints for the transcription and translation that would produce the proper amino acid sequences to make up an enzymatic protein, it was the protein that did the actual work. 
 
However, in 1986, Zaug and Cech found that processing of RNA from the unicellular organism Tetrahymena thermophilia clearly depended upon an intron from its own RNA (1). This intron exhibited characteristics of an enzyme, and it was soon realized that the RNA was self-splicing. This was the first time a nonprotein enzyme was observed to perform a biochemical reaction. Because the enzyme was ribonucleotide based, it was termed a ribozyme. To some extent, all ribozymes fall under the category of antisense RNAs (reviewed in the previous issue), because they depend upon the binding of their nucleic acid sequence to complementary sequences in the target mRNA. Yet, whereas antisense approaches depend upon the activation of other enzymes (i.e., RNase H) to destroy the target RNA, ribozymes are capable of directly cleaving the target themselves. Ribozymes offer an additional advantage over antisense molecules because one molecule of ribozyme can cleave many mRNA molecules, thus amplifying its effect. As reviewed in the introduction to this series, these new targeted approaches allow therapy to be directed to specific genetic mutations characteristic of the malignant state.