RNA therapy: rich history, various applications and unlimited future prospects

Abstract

RNA therapy refers to the treatment or prevention of diseases using RNA-based molecules. The recent advent of a series of effective messenger RNA-based vaccines in response to the COVID-19 pandemic has reignited research interest in RNA therapy. Based on the accumulated results of long-term research in the field of RNA therapy spanning several decades, therapeutic agents for various diseases are being rapidly developed. These therapeutics tend to target diseases that cannot be treated with other conventional drug groups, and several clinical studies are underway for a variety of RNA-based therapeutics against various incurable diseases. This review describes the history of several important discoveries in RNA biology and their impact on key developments in RNA therapy as well as the advantages of RNA therapy. In addition, it describes the action mechanisms and examples of drugs approved for RNA therapy. Finally, this review discusses methods for RNA drug delivery to target organs and cells. Given that RNA therapy is expected to advance and play an integral role in the development of novel therapeutic agents for human diseases in the future, this review is designed to offer an updated reference point for researchers in this field.

Fig. 1. The historical timeline of important discoveries in RNA biology and key developments in RNA therapy.

See text for details.

Fig. 2. Diverse cellular molecules can be targeted by RNA therapy.

RNA-based drugs can target various steps involved in the expression of both protein-coding and noncoding genes. Splicing can be modulated by antisense oligonucleotides (ASOs), and mature messenger RNAs (mRNAs) can be targeted by ASOs or small interfering RNAs (siRNAs). In addition, noncoding RNAs (ncRNAs), including small ncRNAs and long ncRNAs (lncRNAs), can be suppressed by ASOs or siRNAs. Protein function can be modulated by aptamer binding. Finally, exogenous mRNAs can be used to introduce specific proteins into cells to replenish a deficient enzyme or act as antigens to elicit a targeted immune response.

Fig. 3. Antisense oligonucleotide-based RNA drugs.

a Primary mechanism of action for antisense oligonucleotide (ASO)-based RNA drugs in human cells. ASOs can modulate splicing or induce RNase H-mediated degradation of the target mRNA. b Mipomersen, a drug used in the treatment of homozygous familial hypercholesterolemia, induces the degradation of apolipoprotein B-100 (ApoB-100) mRNA. c Nusinersen binds to the intronic splicing silencer element in the intron of survival motor neuron 2 (SMN2) pre-mRNA, which blocks the exon 7 skipping associated with SMN type diseases. Consequently, SMN2 mRNA is spliced like SMN1 mRNA, resulting in the production of stable SMN proteins. d In some patients with Duchenne muscular dystrophy, a mutation at exon 51 of dystrophin (DMD) mRNA induces premature termination of translation, preventing the production of functional protein. Eteplirsen binds to the exonic splicing enhancer in this exon and induces exon 51 skipping. This results in the production of smaller amounts of fully functional proteins.

Fig. 4. Small interfering RNA (siRNA) drugs.

a Mechanism of action of siRNA drugs in mammalian cells. siRNAs interact with the Argonaute (AGO) protein to produce a binding complex that recognizes its target mRNA and then induces its sequence-specific cleavage in a process called RNA interference (RNAi). b Patisiran binds to the 3ʹ untranslated region (UTR) of transthyretin (TTR) mRNA. Because mutation occurs within the coding region of this gene, patisiran suppresses the expression of both wild-type and mutant TTR mRNAs. c Givosiran suppresses the expression of aminolevulinic acid (ALA) synthase 1 (ALAS1) mRNA. This results in a reduction in ALAS1 protein, which is required for the production of ALA and porphobilinogen (PBG). Notably, givosiran contains the N-acetylgalactosamine (GalNAc) conjugate within the 3ʹ end of its passenger strand. d Inclisiran induces the cleavage of the mRNA encoding proprotein convertase subtilisin/kexin type 9 (PCSK9), a promising target for the therapeutic management of patients with high cholesterol. Reductions in PCSK9 protein result in reduced endocytosis of low-density lipoprotein (LDL) receptors.

Fig. 5. RNA aptamer drugs.

a The mechanism of action of aptamer-based drugs. Aptamers bind to their target protein, thus modulating their function. b The mechanism of action of the RNA aptamer pegaptanib, which binds to vascular endothelial growth factor (VEGF), thereby inhibiting its interaction with the VEGF receptor and inhibiting cell proliferation.

Fig. 6. Therapeutic applications of messenger RNA (mRNA).

a The mechanism of action of mRNA-based drugs. Exogenous mRNAs introduced into cells undergo translation to proteins and facilitate protein function. These mRNA constructs include a 5ʹ cap analog to facilitate their recognition by translation initiation factors, which is the first step in translation. In addition, mRNA sequences can be modified to allow evasion of the immune system, allowing them to exert their therapeutic effect for a longer period. b The use of mRNAs for enzyme replacement therapy. mRNAs are introduced into cells where the corresponding proteins are not produced due to mutation. c The use of mRNAs as vaccines. The introduced mRNAs produce proteins that may be recognized by the immune system as antigens.

Fig. 7. Delivery methods and administration routes for RNA-based drugs.

a The three major delivery methods for RNA-based therapeutics in mammalian cells. Naked RNAs can be recognized by receptors that are broadly expressed across various cell types, or they can be conjugated with a compound recognized by a specific receptor. In both cases, the RNAs are introduced into the cells through receptor-mediated endocytosis. Longer RNAs are usually encapsulated inside lipid nanoparticles and endocytosed into the cells. Finally, they are released into the cytoplasm, where they exert their therapeutic effect. b Diverse administration routes for approved RNA-based therapeutics. RNA drugs introduced via the intravitreal or intrathecal injection require less consideration for specific delivery, as these organs are relatively severed from circulation.