MicroRNAs

MicroRNA (miRNA) is a small, regulatory RNA approximately 22 nucleotides long that does not code for protein. Instead, it regulates gene expression by inhibiting translation of mRNA into protein or causing mRNA degradation. This represents a form of post-transcriptional regulation distinct from transcription-level control.

Mature miRNA is produced through sequential enzymatic cleavage. The primary miRNA transcript forms a stem-loop structure. Drosha, an endonuclease in the nucleus, shortens this into pre-miRNA. After methylation and export to the cytoplasm, Dicer cleaves the pre-miRNA into a duplex, releasing a single mature miRNA strand.

The RNA-induced silencing complex (RISC) is a protein complex that binds mature miRNA and guides it to complementary sequences on target mRNA, typically in the 3-prime untranslated region. RISC binding triggers gene silencing through either mRNA cleavage and degradation or direct interference with the translation process.

Extensive or near-perfect complementary base-pairing between miRNA and target mRNA causes irreversible mRNA degradation. Limited base-pairing results in reversible translation inhibition, allowing stable mRNA to resume protein synthesis after miRNA repressors are eliminated. The extent of complementarity directly determines which silencing pathway occurs.

Let-7 miRNA normally inhibits oncogenes that promote cell growth, survival, and proliferation. Loss of let-7 removes this inhibition, allowing oncogenes to be expressed unchecked. This dysregulation promotes tumor formation and is observed in lung, liver, breast, prostate, and ovarian cancers.

Altered miRNA expression or function disrupts normal post-transcriptional regulation, leading to abnormal protein levels and cellular dysfunction. This dysregulation is correlated with potentially deadly diseases including cancer and heart disease, where miRNAs fail to properly silence disease-promoting genes or oncogenes.

MicroRNAs are transcribed from introns, which are non-coding regions within genes, or from intergenic regions, which are stretches of DNA located between genes. These genomic sources are processed through multiple enzymatic steps to generate the biologically active mature miRNA that regulates gene expression.