RNA Interference (RNAi)
A New Method for Decreasing the Expression of Specific Gene Products
Mar 15, 2008
The Nobel Prize in Physiology or Medicine was awarded in 2006 to Andrew Z. Fire and Craig C. Mello of the United States for their discovery of a new technique for silencing the expression of genes. They identified a cellular process that they named ‘RNA interference’. Small, specific double-stranded RNA molecules interact in a complex with mRNAs to which they are homologous and cause their degradation leading to a decrease in the expression of the corresponding protein.
Information Flow Inside Cells.
The biochemical instructions for constructing an organism reside in its genes. The information of the genes is chemically maintained in the form of DNA (deoxyribonucleic acid). When a cell needs a particular protein to be synthesized, the gene for this protein is activated. In the process known as transcription, an RNA (ribonucleic acid) copy of the information contained within the gene is made and transferred to the protein synthetic apparatus of the cell. Once the RNA arrives at the synthetic machinery, the information it carries is used to synthesize the needed protein in a process known as translation. This information flow from DNA to RNA to protein has often been referred to as ‘The Central Dogma of Molecular Biology’.
Altering Gene Expression
The modern era of molecular biology has been highlighted by the development of methods that can alter the expression of target genes in intact cells and organisms. Using ‘transgenic’ techniques, it is possible to generate mice or other organisms that express a specific gene, normal or mutated, in either a specific tissue or throughout the organism for the purpose of studying its effects on cellular biology. A more recent development allows for the targeted inactivation of a specific gene in mice (what are referred to as ‘knock-outs’) so that the effects of its absence can be determined.
Immediate Methods for Turning Off Gene Expression
Scientists routinely study cells that are maintained in dishes in tissue culture. For many experiments, it is a much simpler task to study the biology of a specific biochemical pathway in ‘cultured cells’ than it is to study an intact organism. It is often desirable to turn down or turn off the expression of a specific protein when studying its role. Antisense oligonucleotides have been widely used in the recent past to transiently reduce the expression of a specific protein in a given cell with some success. RNA interference (RNAi) lets a scientist turn down the expression of a gene relatively quickly and measure the effects caused by this reduction.
What is the Mechanism of RNA interference
One of the places where this form of gene regulation occurs naturally is in cellular defenses against the infection by specific classes of viruses. Cells have developed a mechanism that responds to the presence of double-stranded RNA, which is often associated with infection by these specific viruses. When the system is activated specific enzyme complexes are able to degrade the RNA represented by the double stranded sequence, silencing the expression of the gene product. Learning to co-opt this mechanism and utilize it to study the function of specific genes has provided a powerful new tool for scientific research.
Practical Applications of RNAi
Turning off the expression of a particular gene and protein can be done by synthesizing a double-stranded RNA (dsRNA) that matches the sequence of the gene of interest. Once introduced into the cell, this synthetic dsRNA activates the silencing apparatus and the RNA molecules that correspond to the targeted gene sequence are degraded. Beyond its usefulness in laboratory research, companies have emerged that are trying to utilize this technology to combat illnesses. For example, if a disorder is caused by a virus, one that does not normally activate this cellular defense mechanism, one could generate an activating interfering RNA that matches the virus and induce the cells to degrade the viral RNA, limiting the illness.
Future Directions of RNAi Therapy
The capacity of RNAi to effect changes in gene expression has been proven in laboratories over and over again. One of the challenges of using this therapy for medical needs will be the development of methods that will allow for the introduction of interfering RNAs into various cells and tissues of the body. Access to the blood stream and its cells is easy; the same can not be said for most other cells and tissues. Delivery systems that allow this technology to be applied to many tissues will be a great advantage in its future use.
