What are the different RNA platforms?
Synthesized mRNA can be introduced into cells where it is translated into an innate or acquired protein by the existing cellular machinery.
mRNAs expressing an antigen have been experimentally introduced into cells in an attempt to vaccinate against viral diseases, like HIV and flu, as well as against cancers, including melanoma. This tactic has been successfully applied in COVID-19 vaccines, in which mRNA is packaged in lipid nanoparticles (LNPs).
Protein replacement therapies
This gene therapy corrects the adverse effects of a missing or defective protein. This can be done by transfecting cells with DNA, upon which it is inserted into the genome and stably expressed, or by adding mRNA to cells, after which a functional protein is transiently expressed.
Companies are developing mRNA replacement therapies for metabolic conditions, cystic fibrosis, heart diseases, and immunomodulators for oncology treatments.
It has also been used, as part of a CRISPR-Cas9 treatment, to knock out the mutant transthyretin gene present in patients with transthyretin (ATTR) amyloidosis.
Small interfering RNAs are short (roughly 20 nucleotides) double-stranded oligonucleotides with sequences complementary to the mRNA of an expressed gene. They bind to that mRNA, forming a double-strand RNA molecule that is then targeted by the cell for degradation. This process is known as gene silencing and is a normal part of post-transcriptional regulation of gene expression.
siRNA has been used to treat HIV and cancer. Three siRNA drugs have been approved by the FDA, including Onpattro, Giviaari, and Oxlumo, while at least seven are in phase 3 clinical trials.
miRNAs are similar in function to siRNA but are typically shorter chains of nucleotides designed to bind more promiscuously to mRNA; whereas siRNA binds specifically to the mRNA of one gene, miRNA can interact with multiple mRNAs.
These are short (13–30 nucleotides), synthetic single-strand oligonucleotides that, like siRNA, bind to a specific cellular mRNA, depending on their sequence. This binding alters gene expression through enzyme-mediated degradation of the mRNA or, alternatively, through a process known as alternative exon splicing. ASOs are extensively modified with synthetic nucleosides, rendering them stable and removing the need for a delivery vehicle.
Spinal muscular atrophy is caused by incorrect mRNA splicing of the SMN2 gene in which one of the exons in the pre-mRNA is skipped and not included in the final mRNA. This means an incomplete, and dysfunctional, protein is translated. Spinraza (nusinersen) is an FDA-approved RNA drug that binds to the pre-mRNA and allows it to be processed correctly.
Other FDA-approved examples include milasen, used in a single patient to treat Batten disease; fomivirsen for the treatment of cytomegalovirus retinitis; several exon-skipping oligos for the treatment of Duchenne muscular dystrophy (e.g., eteplirsen, golodirsen); mipomersen for familial hypercholesterolemia; and inotersen for hereditary transthyretin-mediated amyloidosis.
Single guide RNA molecules act to steer enzymes that target DNA in CRISPR gene editing systems. They are typically more than 100 nucleotides in length and consist of two sequences fused to make one sgRNA: crispr or seeking RNA is 17–20 nucleotides long and binds to the target DNA sequence to be edited; tracrRNA or scaffold RNA binds to the DNA editing enzyme.
Aptamers are short (15–45 nucleotides) single-stranded oligonucleotides that have high-affinity binding to cellular targets, such as proteins and viruses. The hope is that they could be used therapeutically to inactivate viruses, for example by binding to the spike protein of the SARS-CoV-2 virus. Pegaptanib (27 nucleotides) is the only FDA-approved RNA aptamer drug and is used to treat macular degeneration.
Other cutting-edge RNA-based therapies
A number of experimental RNA therapies exist that are not yet therapeutically relevant, including self-amplifying RNA (saRNA), circular RNA (cirRNA), and tRNA. There are approximately 80 different types of synthetic and natural RNAs, suggesting there is still plenty of room for experimentation and development.