Gene Therapy: the potential of small interfering RNA-based therapies

“DNA makes RNA and RNA makes protein” is the central dogma in molecular biology. First stated by Francis Crick in 1957 it summarises the genetic information flow in a cell. the genetic information encoded in the nuclear DNA is first transcribed to messenger RNA (mRNA), a temporary intermediary. mRNA is the template that the cell’s ‘factories’, ribosomes, use to transduce the genetic information into a protein. Gene therapy encompasses different techniques aimed at modifying the genetic information flow, either at the DNA level or at the RNA level, so that the expression of the target protein is increased, reduced or stopped (siRNA silencing).

Central dogma of molecular biology (Credit: Wikipedia Commons)

Small interfering RNAs (siRNA) are one of the three main elements that operate the RNAi pathway. The mechanism by which siRNAs silence genes is well studied. In general, long double-stranded RNA molecules (dsRNA), either exogenous or endogenous are cleaved by a specialized endo-ribonuclease called Dicer in the cytosol. This enzyme cuts the dsRNA into small fragments, the siRNAs. Then, the siRNAs are unwound into two single-strand RNAs, the passenger strand and the guide strand. The passenger strand is degraded while the guide strand is incorporated into other proteins and forms RNA-induced Silencing Complex (RISC). This strand will then scan different mRNA looking for its complementary mRNA. Unlike microRNAs (miRNAs — another RNAi element), siRNAs have complete complementarity to their target. Once the guide strand has recognized a complementary mRNA, it binds to it and induces mRNA cleavage. The cleaved mRNA is then recognized as abnormal and is fully degraded by the cell, thus silencing the gene that encoded that mRNA (2,3) .

siRNA mechanism (Credit: Wikipedia Commons)

It is not surprising that, not long after being described by Fire and Mello, many researchers saw the potential of the RNAi, and especially siRNAs, as therapeutics. In 2001, Elbashir and colleagues successfully showed for the first time siRNA-mediated gene silencing in mammalian cells (4). This milestone provided the scientific community with the foundations to develop further siRNA applications.

Thanks to the extraordinary sequence-specificity of the siRNA mechanism, siRNA-based drugs can overcome the main limitations of small drug molecules. Most common small molecule drugs are enzyme inhibitors or competitors, which limits their target pool to certain protein families. Another family of small molecules drug are antibody-based therapies, which are mainly limited to circulant or cell surface proteins. By contrast, siRNA can virtually be used to silence any gene, provided that we know its sequence and that a specific siRNA can be designed for it. siRNA-based therapies can be used to treat single-gene disorders and conditions that involve over-expression of one or more proteins. This, along with the better understanding of the underlying genetic basis of many diseases, has opened the door to treat disorders that were once thought to be untreatable. First siRNA clinical trials date from 2007 and over the last decade they have progressed to late stage clinical trials. In 2018, the FDA approved the first-ever siRNA therapy for treatment of a rare hereditary disease, the transthyretin-mediated amyloidosis (5).

The efficacy and safety of siRNA-based therapies is widely demonstrated, but this technology has yet to overcome a few barriers before it is finally established in the marketplace. The efficacy of these therapies lies in the silencing of the target gene or genes but for this it is necessary that the siRNA reaches its destination. Therefore, the success of these therapies in the next few years will depend to a great extent on the development of a delivery system that allows the safe and controlled delivery of siRNA molecules to the target cells, protecting them from degradation mechanisms and minimising off-target effects.

References

1. Fire A, Xu S, Montgomery MK, Kostas SA, Driver SE, Mello CC. Potent and specific genetic interference by double-stranded RNA in Caenorhabditis elegans. Nature. 1998 Feb;391:806.

2. Carthew RW, Sontheimer EJ. Origins and Mechanisms of miRNAs and siRNAs. Cell. 2009 Feb;136(4):642–655.

3. Wilson RC, Doudna JA. Molecular Mechanisms of RNA Interference. Annu Rev Biophys. 2013 May 6;42(1):217–39.

4. Elbashir SM, Harborth J, Lendeckel W, Yalcin A, Weber K, Tuschl T. Duplexes of 21-nucleotide RNAs mediate RNA interference in cultured mammalian cells. Nature. 2001 May;411(6836):494–8.

5. Rizk M, Tuzmen S. Update on the clinical utility of an RNA interference-based treatment: focus on Patisiran. Pharmacogenomics Pers Med. 2017 Nov;Volume 10:267–78.