Small interfering RNA, or siRNA, is a type of double-stranded ribonucleic acid molecule. It is a component of a natural cellular process known as RNA interference (RNAi), which regulates gene expression. Scientists have learned to utilize this biological tool for research and therapeutic purposes, harnessing it to target and silence the genes responsible for various diseases.
The Mechanism of Gene Silencing
The process of gene silencing begins when a double-stranded siRNA molecule enters the cytoplasm, the substance filling a cell. Once inside, it is recognized by an enzyme called Dicer. This enzyme functions like molecular scissors, cutting the longer double-stranded RNA into smaller fragments, typically 21 to 23 nucleotides in length. These smaller pieces become the functional siRNA molecules.
The newly formed siRNA fragments are loaded into a large protein assembly known as the RNA-induced silencing complex, or RISC. Within this complex, the two strands of the siRNA are separated. One of these strands, the guide strand, remains bound to the RISC, while the other passenger strand is cleaved and discarded. The guide strand contains the specific sequence that will identify the target for silencing.
The activated RISC, carrying its guide strand, patrols the cytoplasm searching for messenger RNA (mRNA) molecules that have a sequence complementary to the siRNA guide strand. The mRNA molecules are the cellular messengers that carry genetic instructions from DNA in the nucleus to the ribosomes, where proteins are made.
When the activated RISC finds and binds to its matching mRNA target, a protein within the complex called Argonaute cleaves the mRNA. This action is similar to a recipe being shredded before it can be read. By cutting the mRNA, the RISC prevents the ribosome from translating the instructions and producing the corresponding protein. This degradation of the mRNA effectively “silences” the gene by stopping the production of a specific protein.
Natural vs. Synthetic siRNA
In many organisms, siRNA plays a role in the innate immune system, acting as a defense mechanism against invading viruses, especially those that use RNA as their genetic material. When a virus injects its double-stranded RNA into a host cell, the cell’s own machinery recognizes it as foreign. The Dicer enzyme then chops up this viral RNA into small siRNA fragments. These fragments are then loaded into the RISC complex, which uses them as a template to find and destroy more of the viral RNA, thereby halting the infection.
This natural process provides a defense by turning the virus’s own genetic material against it. The amplification of the silencing signal ensures an efficient and rapid response to the viral threat. This role highlights how cells have evolved to use RNA interference as a protective measure.
Scientists are able to design and create synthetic siRNA molecules in the laboratory. These lab-made siRNAs can be engineered with a specific nucleotide sequence to precisely match any gene of interest. This allows researchers to target and silence virtually any gene they wish to study, a technique often referred to as “gene knockdown.”
The ability to create custom siRNA has become a valuable tool in biological research. By silencing a particular gene, scientists can observe the resulting changes in cellular function or an organism’s development, which helps to determine the gene’s specific role. This targeted approach has accelerated the pace of genetic discovery.
Therapeutic Applications of siRNA
The principle behind siRNA therapies is to halt the production of harmful proteins that cause disease. A precisely designed siRNA molecule can be used to silence the gene responsible for a particular protein’s creation. This approach targets the root cause of the disease at the genetic level, rather than just managing symptoms.
Several FDA-approved siRNA drugs are now available. A prominent example is Patisiran, sold under the brand name Onpattro, which was first approved in 2018. Patisiran is used to treat a rare genetic disorder called hereditary transthyretin-mediated amyloidosis (hATTR), where a mutated gene leads to the production of a misfolded protein that accumulates in nerves and organs. The drug silences the gene that produces this faulty transthyretin protein, reducing its production by about 80-85%.
Clinical trials are actively exploring siRNA therapies for a wide range of conditions. For instance, Inclisiran targets the PCSK9 gene to lower high cholesterol, a major risk factor for cardiovascular disease. Other research areas include targeting genes involved in the growth of certain cancers, other rare genetic disorders, and combating viral infections.
As of early 2024, the FDA has approved several other siRNA agents, including Givosiran for acute hepatic porphyria and Lumasiran for primary hyperoxaluria type 1. Each of these drugs works by silencing a specific gene in the liver that is responsible for the disease. The targeted nature of these therapies offers an advantage over traditional drugs, which can have widespread, unintended effects on the body.
Delivery and Future of siRNA Technology
A challenge for siRNA therapies is ensuring the molecules reach their intended destination within the body. On their own, siRNA molecules are fragile and can be quickly degraded by enzymes in the bloodstream. Furthermore, their size and negative charge make it difficult for them to cross the protective membrane of target cells.
To overcome these barriers, the most common solution is to package the siRNA inside protective carriers, most notably lipid nanoparticles (LNPs). These LNPs are tiny spheres of fat that act as delivery vehicles, shielding the siRNA from degradation in the bloodstream. The composition of the LNP can be tailored to help it fuse with the membrane of specific cells, releasing the siRNA payload into the cytoplasm.
The future of siRNA technology is focused on refining and expanding these delivery methods. Current research aims to create more advanced delivery systems that can target specific organs or tissues beyond the liver, such as the brain, heart, or cancerous tumors. This involves engineering nanoparticles with specific molecules on their surface that bind only to the desired cell types.
Scientists are also working on chemically modifying the siRNA molecules themselves to make them more stable and less likely to be broken down. These modifications can extend the half-life of the drug in the body, potentially reducing the frequency of doses needed for treatment. As these delivery technologies improve, the range of diseases that can be treated with siRNA is expected to grow.