RNA interference, or RNAi, is an RNA-dependent mechanism that silences genes. RNAi is important in the gene silencing of developmental genes and show great potential for numerous fields of research. RNAi is commonly used to understand the function of genes in development, disease progression, reproduction, and upkeep of a genome. First discovered in transgenic petunias in the early 1990’s, the silencing of a gene that encodes chalcone synthase, the enzyme that plays a major role in the pigment color of the flower petals, resulted in flowers that were both purple and white, showing areas that lacked any pigmentation. This post-transcriptional gene silencing (PTGS) was conducted by a molecule that was found in the fungal organism Neurospora as well in other plants. In 1998, a research group led by Dr. Andrew Fire discovered that by mixing sense and antisense strands from target mRNA to form double stranded RNA (dsRNA) and injecting them into C.elegans (a type of nematode) resulted in gene silencing, demonstrating that dsRNA was the jump starter to RNAi. Subsequently, Dr. Fire, along with Craig C. Mello, was awarded the Nobel Prize for Physiology or Medicine in 2006.
So how does RNAi work? The RNAi pathway starts with a double stranded RNA being cleaved into sections that contain about 22-25 base pairs, called small interfering RNAs (siRNAs). The siRNA guide strand becomes bound to a protein complex, which forms the RNA-induced silencing complex, or RISC. The siRNA is cleaved into 2 strands with the help of ATP (adenosine tri-phosphate, which is a form of usable energy in living organisms). At this point RISC is activated and can be bound to the mRNA that is at the target site. The target mRNA is cleaved and other proteins degrade this cleaved strand, which halts the production of the protein being targeted. The end result is gene silencing at sequence specific locations.
There are three approaches scientists can take in utilizing RNAi technology. The one described above is utilized most commonly in non-mammalian systems. In a mammalian system, RNAi can be activated by expression vectors originating from DNA that are designed to express the short hairpin RNA (shRNA). The introduction of siRNA molecules created from long strands of dsRNA can also initiate the process. Ultimately they all end up in RISC and the target mRNA is cleaved. Delivery of synthetic siRNAs can be done in vitro via transcription, by means of chemical synthesis, or can be expressed in a plasmid or a viral vector. Tuschl and colleagues in 2001 found that gene silencing could be induced in mammalian cells by synthetic siRNAs, which is now used as a method for studying gene function in mammalian systems. A study conducted by Soutschek et al (2004) demonstrated the by developing mice that are heterozygous for having a knockout mutation of apoB, a gene that is responsible for the development of LDLs (low-density lipoproteins), the mice have lower cholesterol as well as resistance to developing high cholesterol from a diet high in cholesterol. On December 4, 2008, it was announced that AiRNA Pharmaceuticals and Boston Biomedical have published a paper called “Asymmetric RNA (aiRNA) Duplexes Mediate RNA Interference in Mammalian Cells” that investigates the effectiveness of aiRNA, which is only 15 bp long as a target for a variety of genes. They found that aiRNA was more efficient than the siRNA typically used in current research. Thus, the potential for RNAi being a huge contributor to genetic, epigenetic, and infectious disease research is becoming more evident. In the next few years, the sky is the limit for the potential breakthroughs society may see from the research being conducted utilizing RNAi.
For more information, check out Ambion’s Information page or the RNAi database.
