Messenger RNA

Messenger RNA (mRNA) is a single-stranded ribonucleic acid transcribed from a DNA template, carrying in its codons the instructions the ribosome reads to assemble a polypeptide. In eukaryotes the primary transcript is processed before it can be used: a 7-methylguanosine CAP is added at the 5′ end, introns are excised and exons joined by the spliceosome, and the 3′ end is cleaved and polyadenylated. Alternative splicing means one gene can yield several distinct mRNAs and so several proteins. The mature molecule is modular — a 5′ untranslated region, an open reading frame, and a 3′ untranslated region carrying the binding sites for microRNAs and RNA-binding proteins that set stability and translation rate — and the CAP and poly(A) tail together license translation and protect the ends from exonucleases. The pattern is near-universal rather than absolute: replication-dependent histone mRNAs end in a conserved stem-loop instead of a poly(A) tail, and bacterial mRNA, lacking a nucleus, is translated while still being transcribed.

An mRNA is a transient intermediate by design. Deadenylation followed by decapping and exonucleolytic decay gives most transcripts a half-life of hours, surveillance pathways such as nonsense-mediated decay destroy those bearing premature stop codons, and translation itself is regulated at initiation. Most of this occurs in the cytoplasm, though mitochondria transcribe and translate their own mRNAs, and some transcripts have documented nuclear roles. Because the molecule is read and then destroyed without entering the genome, the mRNA itself is temporary, although the proteins it produces can cause lasting effects, including heritable genomic changes — the property that makes it attractive as a drug.

Synthetic mRNA can be designed to encode essentially any protein, with substitution of uridine by N1-methylpseudouridine and tuning of the untranslated regions used to raise translation and blunt innate immune sensing. Its size and polyanionic charge prevent it from crossing membranes unaided, so mRNA drugs are almost always formulated in a carrier — most commonly lipid nanoparticle built from an ionizable lipid such as ALC-0315 together with phospholipid, cholesterol, and PEGylated lipid. The ionizable lipid binds mRNA at low pH during assembly and is protonated again inside the endosome, promoting escape into the cytosol; endosomal entrapment remains the principal bottleneck limiting delivery, and ionizable lipid chemistry also drives dose-limiting toxicity and proinflammatory cytokine responses. Polymeric micelles, electroporation of unformulated RNA, and biologically assembled vectors are alternative routes under investigation. Clinically, mRNA is best established in mRNA-based COVID-19 vaccines, and is being extended to in situ engineering of T-lymphocytes for CAR T cell therapy, transfection of stromal populations such as fibroblasts and macrophages within the tumor microenvironment, regenerative applications in skin, and delivery of genome-editing components including CRISPR-associated protein 9 (Cas9) mRNA and CRISPR-Cas12a systems.