Small interfering RNA (siRNA) is a powerful tool in molecular biology. This short, synthetic, double-stranded RNA, typically 20 to 25 base pairs long, interferes with specific gene expression. This makes siRNA valuable for understanding gene function and holds therapeutic promise.
The Concept of Gene Silencing
Gene expression is the fundamental process where genetic information is used to synthesize a functional product, such as a protein. This process is highly regulated, ensuring cells produce the correct proteins at the right time and in appropriate amounts. When this balance is disrupted, like by overproduction or faulty proteins, it can lead to various diseases.
Gene silencing regulates gene expression within a cell, preventing or reducing a specific gene’s product. This mechanism occurs naturally as a defense against foreign genetic material, like viruses, and to regulate a cell’s own gene activity. For instance, a cell can silence viral genes to stop an infection. The ability to selectively turn off overactive or harmful genes holds significant therapeutic potential. Unlike gene knockout, which completely removes a gene, silencing typically reduces gene expression by at least 70% without full elimination.
How siRNA Silences Genes
siRNA silences genes through a precise molecular pathway called RNA interference (RNAi). When introduced into a cell, siRNA is processed by the enzyme Dicer, which cleaves longer double-stranded RNA molecules into short siRNA duplexes.
After Dicer processing, the siRNA duplex enters the RNA-induced Silencing Complex (RISC). Inside RISC, the “guide” strand remains bound, while the “passenger” strand is discarded. The Argonaute 2 (Ago2) protein, a core RISC component, cleaves the passenger strand.
The single-stranded guide siRNA then directs active RISC to target messenger RNA (mRNA) molecules with a complementary sequence. This binding is highly specific, often requiring full complementarity. Once bound, Ago2 within RISC cleaves and degrades the target mRNA, preventing its translation into a protein.
Therapeutic Applications of siRNA
siRNA’s ability to specifically silence genes has opened avenues for treating various diseases. For viral infections, siRNA can target and degrade viral RNA, inhibiting replication. This approach is explored for infections like hepatitis B virus (HBV) and respiratory syncytial virus (RSV).
For cancer, siRNA can silence oncogenes, which promote uncontrolled cell growth and division. Reducing their expression can inhibit tumor growth and progression, and potentially induce programmed cell death. siRNA is also investigated for genetic disorders caused by faulty or overactive genes. Patisiran, an FDA-approved siRNA therapeutic, targets the transthyretin (TTR) gene for hereditary transthyretin-mediated amyloidosis. Another example is Inclisiran, which targets PCSK9 to lower cholesterol.
Despite its promising therapeutic potential, a significant challenge for siRNA is its efficient and safe delivery to target cells. Systemic administration faces hurdles such as degradation by enzymes in the bloodstream, rapid clearance from the body, and difficulty crossing cell membranes to reach intracellular targets. To overcome these barriers, researchers are developing delivery systems, including viral vectors (e.g., adeno-associated viruses) and non-viral methods (e.g., lipid nanoparticles, polymer-based carriers). These systems encapsulate and protect siRNA, enhance its stability, and facilitate its entry into specific cells.