RNA Interference-Based Therapeutics: Harnessing the Powers of Nature

The central dogma of biology describes the transfer of biological information from DNA through to protein (Crick, 1970). In the first phase, known as transcription, DNA is converted into a complementary sequence of messenger RNA (mRNA). This mRNA allows the genetic message to be communicated outside of the nucleus, to other areas of the cell, where it is then translated into protein by ribosomes. Most human diseases arise from increased function or dysfunction of proteins within the body. Since these proteins are generated from DNA via mRNA, modulation of this flow of genetic information can convey a therapeutic effect on the disease state. Mammalian cells possess the genetic instruction to make 50,000 to 100,000 different proteins but only 10-20% of these are found in any single cell. Therefore, a gene must contain instructions for the regulation of the production of protein in correct amounts and at the correct time for each cell type. Gene regulation is one of the most complex molecular processes known, involving up to 10% of the proteins that cells produce. In 1998, Andrew Fire and Craig Mello described RNAi as an endogenous gene expression pathway activated by double-stranded RNA (dsRNA) in the worm Caenorhabditis elegans. For this pioneering work, Fire and Mello were awarded the 2006 Nobel Prize in Physiology or Medicine. The discovery of the natural RNAi mechanism for sequence-specific gene silencing launched a new era in antisense technology. During the 1990s, a number of genesilencing phenomena that occurred at the posttranscriptional level were discovered in plants, fungi, animals and ciliates, introducing the concept of post-transcriptional gene silencing (PTGS) or RNA silencing (Baulcombe, 2000; Matzke et al., 2001). The most important technologies for gene suppression are: antisense oligonucleotides, aptamers, ribozymes and RNA interference (RNAi). The first report that gene expression could be modulated by the use of reverse complementary (antisense) oligonucleotides was made in 1978. Antisense molecules are synthetic segments of DNA or RNA, designed to mirror specific mRNA sequences and block protein production, these molecules are designed to inhibit translation of a target gene to protein via interaction with mRNA. Aptamers are single-strand DNA or RNA oligomers, which can bind to a given ligand with high affinity and specificity due to their particular 3-D structures and thereby antagonize the biologic function of the ligand. Recent developments demonstrate that aptamers are valuable tools for diagnostics, purification processes, target validation, drug discovery and therapeutics. Ribozymes are