microRNAs are small naturally occurring non-coding RNAs that are 20 to 25 nucleotides in length. microRNAs regulate expression of target genes through sequence-specific hybridization to the 3′ untranslated region (UTR) of messenger RNAs and either block translation or direct degradation of their target messenger RNAs. This novel class of small non-coding RNAs was first discovered in the model organism C. elegans in 1993 and since then microRNAs have been identified in almost every species, including humans. They are highly conserved and research has shown that microRNAs function as important regulators of gene expression and play a role in multiple cellular processes.
microRNA genes are expressed in the cell nucleus by RNA polymerase II as a long double-stranded precursor called the primary microRNA, or pri-microRNA. The microRNA portion of the pri-microRNA transcript forms a hairpin which signals for double-stranded RNA-specific nuclease cleavage by an enzyme called Drosha. Drosha processes the pri-microRNA to release the precursor microRNA, or pre-microRNA. The pre-microRNA is exported into the cytoplasm by the nuclear export protein Exportin 5. In the cytoplasm, the pre-microRNA is cleaved by the enzyme Dicer into a 20 to 25 nucleotide-long double-stranded RNA, which is then separated into two single strands. The single-stranded mature microRNA assembles into a protein-RNA complex called the RNA-Induced Silencing Complex (RISC). Through the RISC, the microRNA targets messenger RNAs by direct base pairing. The 5′ region of microRNA, known as the “seed” region (nucleotides 1 through 8), is the most critical for targeting and function. The microRNA target sites, located in the 3’ UTR of messenger RNAs, are often imperfectly matched to the rest of the microRNA sequence. Since microRNAs do not require perfect complementarity for target recognition, a single microRNA is able to regulate multiple messenger RNAs. While microRNAs exert subtle effects on each individual messenger RNA target, the combined effect is significant and produces measurable phenotypic results.
microRNAs play an integral role in numerous biological processes, including the immune response, cell-cycle control, metabolism, viral replication, stem cell differentiation and human development. Most microRNAs are conserved across multiple species, indicating the evolutionary importance of these molecules as modulators of critical biological pathways. Indeed, microRNA expression or function is significantly altered in many disease states, including cancer, inflammatory diseases, metabolic diseases, and fibrosis. Targeting pathways of human disease with microRNA-based drugs represents a novel and potentially powerful therapeutic approach.