An overview of microRNAs

Abstract

The discovery of the first microRNA (miRNA) over 20 years ago has ushered in a new era in molecular biology. There are now over 2000 miRNAs that have been discovered in humans and it is believed that they collectively regulate one third of the genes in the genome. miRNAs have been linked to many human diseases and are being pursued as clinical diagnostics and as therapeutic targets. This review presents an overview of the miRNA pathway, including biogenesis routes, biological roles, and clinical approaches.

Keywords: Argonaute; Dicer; Drosha; MicroRNA; RNAi; miRNA.

Figure 1. The miRNA biogenesis pathway

The canonical miRNA biogenesis pathway is shown in the central part of the figure. miRNAs are transcribed by RNA polymerase II. The pri-miRNA transcript is cut by the enzyme Drosha yielding the precursor miRNA. After Dicer cleavage the mature miRNA is loaded into the effector complex RISC where it directs translational repression of mRNA targets. Two well characterized non-canonical biogenesis routes are shown in the semi-transparent graphic. mirtrons bypass the Drosha processing step; a splicing reaction produces the precursor. Subsequent biogenesis steps are the same. The second non-canonical route shown is specific for miR-451. This miRNA precursor is directly bound and cut by Argonaute 2 in a Dicer independent reaction. The remaining RNA is trimmed back to yield a mature miRNA.

Figure 2. Anatomy of a miRNA poly-cistron

The miRNA cluster containing miR-106b, miR-93, and miR-25 is shown in its genomic context. Three miRNAs are located within an intron of the protein coding gene MCM7. Boxes in the MCM7 gene model represent exons, and the arrow line represents introns. Sequence conservation is shown. Data were adapted from the UCSC genome browser.

Figure 3. CRISPR/Cas as a genome editing tool

The guide RNA directs the Cas9 endonuclease to target DNA. Modification of the guide RNA sequence allows any DNA sequence to be targeted. The only sequence requirement is the PAM motif NCC downstream of the target site, and a C at the 5′ end of the target site. The double strand break is repaired by non-homologous end joining, leading to a short deletion.