In reality, a technique of gene silencing that function as a switch for controlling the gene in the cell according to our will - often sounded as RNA interference (RNAi) - keeps our hope alive. RNAi based drugs are now racing towards the clinic. RNAi, a process for turning genes off, has sparked a flurry of biotech and pharma investments.
In recent years, wide range of Ribonucleic Acid (RNA) outplayed DNA from the field of development, disease and human health. The small regulatory RNAs (siRNA) are turning out to be a major player by involving in the most vital activities of human cell. In practice, the RNA which was long viewed as mere copyists of the gene encoded by the famous double helix had come into limelight. It looks like that DNA is just a passive archive of information identical to old page of telephone directory, whereas RNA is an active partner and dictate not only the phone number to be picked up but establishes the connection and the time of each call.
Small regulatory RNA is playing a major role related to the most vital activities in human cell by silencing the homologous gene based on sequence similarity. This RNA-RNA interference, which is referred as RNAi, is a readymade knock down system applied in silencing an innumerable number of genes. It protects the intriguity of DNA in tumour cells, cellular differentiation, development, chromatin organisation, rapid cell proliferation, tumour formation and many more. Its application in human based defect in different diseases is now in reality. RNAi genome drugs are running towards the clinic.
A ready source of gene info
The foundation of RNAi technology in animals, known as cosuppression (1997) was established ten years back. It occurs invariably as a shield to fight against diseases, when multiple copies of same genetic materials of infectious microorganisms enter our body. The name, RNAi was coined in one year later by a group of scientists while documenting a similar process in the nematode worm Caenorhabditis elegans (1998) by introducing small RNA. These small RNAs can be used as a guide for destroying the machinery of infectious organisms once they enter our body. Yet, doctors and clinical researchers are now talking about beginning human trials within the next one or two years. Part of the excitement also stems from the power of RNAi. It has a capacity to destroy any genetic material at any time point of our life in any specific type of cells that is thought to be appeared as gene knock out system.
RNAi is a readymade source of information for what genes do in real life. When a gene is active, its blue print sequences are read to produce messenger RNA, which contains information to manufacture proteins. By introducing a small bit of double stranded RNA that corresponds to the specific mRNA sequences, researchers can shut down the activity of genes in question.
One of the obvious targets of RNAs is HIV - a virus that has no cure and no vaccine so far. Several research groups target different HIV genes related to their replication in human cells and are reduced dramatically. Same technique is applicable for other virus born diseases (for example Hepatitis C, polio, influenza etc). Many scientists are currently targeting different tropical viruses and successfully shutting down the genes of Japanese encephalitis, Human papilloma viruses even in recent threats in south Asia, SARS.
RNAi works like a magic in cell culture but what about in live animals? Different research groups use gene therapy vectors particularly retroviral vectors and adeno-associated viral vectors for delivery of genetic materials that read small interfering RNA, says Mark Kay, Professor of Stanford University, a pioneer in this field. He reported that mice having Hepatitis C siRNA are protected against Hepatitis C virus.
The major advantages of double stranded RNA are that they are not cleaved or digested by the RNAse activity, which are profoundly found in the cell's cytosol. So this double stranded RNA might be the potential target for the host immune system to block viral replication. The Nobel Prize in medicine (2006) was given Dr Craig Mello and Andrew Fire when they introduced double stranded RNA derived from a nematode gene and found that same nematode gene reduces its expression.
The early applications used long dsRNA but this was not effective in most mammalian cells, as it triggered antiviral Interferon (IFN) response which usually leads to cell death. A conserved machinery in different organisms reported that it cleaved long dsRNA into duplex of 21 to 28 nucleotide small interfering RNA (siRNA) that guides sequence specific degradation of mRNAs.
siRNA-based therapeutics
Several ODN and ribozyme molecules are already in clinical trial. One antisense ODN- fomivirsen (Vitravene) has been approved by US FDA for the treatment of cytomagolovirus infection of the eye. However, siRNA and their functionality in human cells have recently entered in clinical trials in the shortest period of drug development history. There is no scientific reason why siRNA will not be proven as effective therapeutic elements. Several proof of principle experiments have demonstrated the therapeutic potential of siRNAs: siRNAs protected mice from fulminant hepatitis, reduced cholesterol,viral infection, sepsis, tumour growth and ocular neovascularization causing macular degeneration and the age dependent blind diseases.
Gene silencing -A great therapeutic promise
Gene silencing is golden egg at least for two US biotechnology companies. They aim to reap the rewards for bringing a method for silencing genes into the clinic, where they claim it could lead to treatments for everything from viral diseases to cancer. The RNAi technique could offer a safe and effective way of turning off a gene and many see its potentiality in biotechnology and pharma based industries. RNAi is used in major drug firms as a next generation of drug targets, after the small molecules and proteins. However, RNAi therapy still faces one significant hurdle - drug delivery. The technique works by introducing a short section of RNA into the body that can then interfere with and switch off the gene in question. But RNA is rapidly chewed up by the immune system. So it is hard to make sure it reaches its genetic target inside the cell. Sirna Therapeutics performed extensive chemical modifications to its drug molecules and has devised a delivery system using lipid nanoparticles. In effect, the lipid bilayers are protective casings that promote the RNA to cross the cellular membranes and blood-brain barrier. The company plans to test its system in clinical trail.
The drug delivery problem does not stop the RNAi's initial forays into the clinics and it had been aimed the diseases affecting the eye because RNA molecules can be delivered to the eye without coming into contact with the immune cell that would destroy them. The first treatment of clinical trial was made by Acuity Pharmaceuticals in Pennsylvania. Its treatment for macular degeneration, a common cause for adult blindness began safety trails in October 2004. The phase II efficacy trails claims that the drug is effective.
The major potential of RNAi drug underlies on the broad interests in terms of different diseases from infectious diseases to chronic cancer. In near future it is going to become a major shot and money spinning capacity for biotechnology based pharma companies.
The siRNA approach for gene silencing holds great therapeutic promise, as siRNA and other regulatory RNAs are naturally used by cells and are therefore non toxic and highly effective. One potential draw back is that for a long term use siRNA could theoretically out-compete the function of endogenous small regulatory RNAs in certain human cells.
(Utpal Bhadra & SNCVL Pushpavalli are with Functional Genomics and Gene Silencing Group, Centre for Cellular and Molecular Biology, Hyderabad, while Manika Pal-Bhadra is with Centre for Chemical Biology, Indian Institute of Chemical Technology, Hyderabad.)