18 May 2005
RNA interference (RNAi) is a naturally occurring mechanism for silencing gene expression, initially discovered in plants but subsequently identified in other organisms including mammals. Gene expression - the production of proteins encoded by genes - requires an intermediate molecule, mRNA, to carry genetic information from the DNA to the cellular sites of protein synthesis. In RNA interference, small double-stranded RNA molecules called short interfering RNAs (siRNAs) are incorporated into a silencing complex (the RNA-induced silencing complex, or RISC) that directs degredation of homologous (complementary) mRNA sequences, preventing their translation into protein. The therapeutic potential of this process to silence the expression of harmful disease-associated genes has been the subject of intense research, and it has been repeatedly shown that chemically synthesized siRNAs can silence complementary mRNAs in cultured human cells. However, in vivo delivery of siRNAs remains a major obstacle to progress with this form of therapy; intravenous injection and viral vector mediated delivery have had very limited success, with generally poor tissue uptake.