Small Interfering RNAs: Heralding a New Era in Gene Therapy

Last decades have witnessed a tremendous expansion in knowledge and availability of the genome sequence, which was of great importance for advancements in the field of gene therapy. This led to improved strategies based on use of nucleic acids with sequences complementary to specific target genes in treatment of many diseases. Especially, advancements have been achieved in discovery and use of diverse RNA molecules other than messenger RNAs (mRNAs), transfer RNAs (tRNAs), or ribosomal RNAs (rRNAs). Such RNA molecules, known as non-coding RNAs (ncRNAs), serve diverse biological roles some of which are still elusive (Gesteland 2006). Generally, the ncRNA molecule is functional even when it does not encode for a protein. Recent evidence provided by many projects including the Encode project (The Encyclopedia Of DNA Elements) suggests that larger part of the genomes of mammals and other complex organisms is transcribed into ncRNAs. These ncRNAs are transcribed from both exon and intron DNA regions, and include small interfering RNAs (siRNAs), micro RNAs (miRNAs) and small nucleolar RNAs (snoRNAs), while many of such molecules remain yet to be discovered. A vast amount of evidence demonstrates that ncRNAs play essential roles in cellular physiology. Some biological processes known to be regulated by ncRNAs include transcriptional regulation of genes, gene silencing, messenger RNA stability and translation, development, proliferation, haematopoiesis, apoptosis, protein translocation and chromosome replication (Bühler 2007, Mattick 2006, Lee 1993). There is no doubt that RNA regulatory networks are critical for determining our most complex traits, and they play an important role in disease pathogenesis as well. The specific disease phenotypes might indeed result from deficiency of one or more specific ncRNA instead from protein structural defects, as is usually expected. A challenge for the future might thus be to map the whole cells/organisms complement of ncRNAs and to understand their biological role. Up to now, the use of ncRNAs as a research tool has greatly improved gene therapy approaches for various diseases (Gallaso 2010), but also substantially improved drug discovery and target validation. In this book chapter, we will therefore focus