doi:10.1038/nindia.2013.156 Published online 25 November 2013
The ultimate goal of medical sciences is to treat human ailments by chemical or biological molecules. Scientists have been working for ages to identify the biological mechanisms of diseases and then to treat them using several approaches.
In the last centuries, the focus was mainly on small chemical compounds to target biological systems and fix medical problems. However success for diseases with complex etiology has been pretty less with this approach , .
Cancer is the most significant example of that. These complex medical problems have forced scientists to think in a non-traditional way, close to the biological approach. In the last decades, many biological therapeutic approaches have come forth. However, most of them are isolated in nature and were not tested in a holistic way .
Since the discovery of RNAi, it has been an important tool in the study of biological phenomena but its real clinical potential is yet to be studied, mainly its short term nature, efficient delivery and targeted effects . On the other hand, RNAi can be expressed by the promoter and cellular machinery which processes ShRNA expression. This expression relies on the enzyme complex RNA polymerase III (or Pol III).
Most of the cell-specific and tissue-specific promoters are type II promoters. So achieving tissue-specific expression was really challenging. Scientists from the Cold Spring Harbor Laboratory in New York have designed a new way to efficiently express shRNA under the type II promoters. They placed the ShRNA on a microRNA30 (miR30) backbone and could efficiently express it under promoter II. So by identifying any disease specific promoter (type II) and putting these ShRNA under them, one can control the expression of ShRNA in that particular cell and minimise toxicity. In this way, one can overcome the problem of the short term nature of ShRNA and its undesirable expression in non-targeted cells .
The classical two-hit hypothesis, which explains the early onset of cancer at multiple sites in the body, suggests that the tumor suppressor gene loses it function and oncogenes gain. Using ShRNA, one can target these oncogenes. By expressing a tumor suppressor protein, it is possible to express two different molecules under tumor specific promoters. ShRNA can be placed in intronic or 3'UTR region of the oncogene. Using this approach, one can express any number of genes and ShRNA under one promoter and/or can add more promoters, ShRNA and genes.
This flexibility can be extended to other disease mechanisms. For instance, in infectious diseases one can use a promoter from an infectious organism and downstream of that promoter, express ShRNA to inhibit the expression of vital genes of infectious diseases. In case of HIV, one can take the long terminal repeat (or LTR) promoter and then express ShRNA of any vital gene or other important conserved proteins. These 'molecular arsenals' have to be explored thoroughly for their translational potential by molecular biologists.
This suggested approach has the advantage of combining and manipulating gene therapy-based approaches with disease specific desired outcomes. It is critical to optimize ShRNA for specificity to limit the off-target effects with increase in efficiency as also select a promoter which is specific to the pathological state under study.
Efficient delivery of gene vector has always been a key question for gene therapy. Nanoparticle or adeno-associated virus-based targeted delivery approaches are being developed and have shown efficiency for delivery of gene vector into cells , . There have been various approaches for ShRNA design to minimize off-target effects. In principle, it is feasible to design an efficient molecular therapy which is regulated, specific and has minimum side effects and toxicity. However, the therapeutic utility of this novel approach needs to be clinically tested .
There is a consistent decline in the number of approved chemical drugs every year (perhaps due to the stringent regulation bodies). There are limitations in the chemical space also, which pharmaceutical companies know for decades now . In the light of these drawbacks, it is time to explore newer biological approaches with enhanced flexibility in design and optimization, than the conventional ones.