Introduction

RNA molecules have become a hot topic of research. While I was taught about messenger RNA (mRNA), ribosomal RNA (rRNA) and transfer RNA (tRNA), many more varieties have come into the nomenclature after I graduated with my science degrees. Even more interesting, these RNAs do not code for a protein, but instead have a role in regulating gene expression. From long non-coding RNA (lncRNA) to short interfering RNA (siRNA), microRNA (miRNA) and small nucleolar RNA (snoRNA), these classes of RNAs affect protein translation, whether by hindering ribosomal binding, targeting mRNA for degradation or even modifying DNA (e.g., methylation). This post will cover the topic of microRNAs, explaining what they are, how researchers understand their function and role in metabolism, cancer and cardiovascular disease, and some of the challenges in miRNA research.

What are microRNAs? MicroRNAs (miRNAs) are short noncoding RNAs 19–25 nucleotides long that play a role in protein expression by regulating translation initiation and degrading mRNA. miRNAs are coded as genes in DNA and transcribed by RNA polymerase as a primary transcript (pri-miRNA) that is hundreds or thousands of nucleotides long. After processing with a double-stranded RNA-specific nuclease, a 70–100 nucleotide hairpin RNA precursor (pre-miRNA) is generated and transported from the nucleus into the cytoplasm. Once in the cytoplasm, the pre-miRNA is cleaved into an 18- to 24-nucleotide duplex by ribonuclease III (Dicer). This cleaved duplex associates with the RNA-induced silencing complex (RISC), and one strand of the miRNA duplex remains with RISC to become the mature miRNA.