In their pursuit to cure infectious as well as inherited diseases, scientists are testing several cutting-edge therapies. These include gene therapy, antisense therapy, and an approach called RNAi therapy. Although each of these approaches has potential, many experts consider RNAi to be the most promising so far.

As this video segment demonstrates, RNAi has already been successfully used to treat macular degeneration, an eye condition in which too many blood vessels grow underneath the retina, which leads to sight loss. Other potential disease targets include Huntington's disease, hepatitis, and breast cancer. RNAi can theoretically be used to treat any infection or disease that is caused by the overproduction of a normal protein, the production of an abnormal protein, or the production of a harmful foreign protein.

RNAi therapy is based on cells' natural response when they detect infection or genetic abnormality. Double-stranded RNA molecules in the cytoplasm signal abnormality in a cell, as RNA normally produced by the cell is single stranded. Double-stranded RNA molecules may come from viruses or may be part of the cell's own mechanism to inhibit the production of certain proteins. When the RNAi molecules detect either of these abnormal molecules, a protein complex, which scientists call "Dicer," cuts the double-stranded RNA into fragments. Next, a molecule called "RISC" binds to one fragment of the offending RNA and uses this fragment to detect single-stranded mRNA with the corresponding sequence. Whenever RISC encounters corresponding mRNA molecules, it cuts and degrades those molecules so that they can no longer be used to synthesize proteins. By inhibiting the production of the protein it codes for, RNAi can effectively block, or "silence," a specific gene's activity.

Besides its ability to selectively target genes, another advantage of the RNAi approach is that it is comparatively easy for scientists to make RNAi drugs. These drugs consist of small strands of RNA—each about 22 bases long—that can be synthesized by a machine. Further, because the RNAi mechanism passes from cell to cell, it would therefore maintain or even increase its effectiveness over time. That said, the power of the RNAi mechanism also carries potential risk. To use it effectively, scientists need to better understand how to inhibit the synthesis of a target protein without influencing the synthesis of any other proteins. The challenge, then, is to deliver the synthetic RNA molecules only to the cells that matter.

Print Background Essay