MicroRNA
Introduction
MicroRNAs (miRNAs) are a class of small non-coding RNA molecules that play a crucial role in the regulation of gene expression. They were first discovered in the early 1990s and have since been identified in a wide range of organisms, including plants, animals, and viruses. MiRNAs are involved in many biological processes, such as development, cell proliferation, differentiation, and apoptosis.
Biogenesis
The biogenesis of miRNAs is a complex process that involves multiple steps. It begins in the nucleus, where the miRNA genes are transcribed by RNA polymerase II into primary miRNAs (pri-miRNAs). These pri-miRNAs are then processed by the enzyme Drosha into precursor miRNAs (pre-miRNAs), which are approximately 70 nucleotides in length. The pre-miRNAs are then exported to the cytoplasm by the protein exportin-5. In the cytoplasm, the pre-miRNAs are further processed by the enzyme Dicer into mature miRNAs, which are approximately 22 nucleotides in length.
Function
MiRNAs function by binding to the 3' untranslated regions (3' UTRs) of target messenger RNAs (mRNAs) and inhibiting their translation into proteins. This is achieved through a process known as RNA interference (RNAi). The miRNA and its target mRNA form a complex with a group of proteins known as the RNA-induced silencing complex (RISC). Within the RISC, the miRNA guides the complex to the target mRNA, leading to its degradation or translational repression.
Role in Disease
Alterations in miRNA expression have been linked to a variety of diseases, including cancer, cardiovascular disease, and neurological disorders. In cancer, for example, certain miRNAs can act as oncogenes (cancer-promoting genes) or tumor suppressors (cancer-inhibiting genes). Oncogenic miRNAs are often overexpressed in cancer cells, while tumor suppressor miRNAs are typically downregulated. This dysregulation of miRNA expression can lead to the abnormal proliferation of cancer cells and the development of tumors.
Therapeutic Potential
Given their role in disease, miRNAs have been proposed as potential therapeutic targets. For example, strategies to inhibit oncogenic miRNAs or to restore the expression of tumor suppressor miRNAs could potentially be used to treat cancer. In addition, miRNAs could also be used as diagnostic or prognostic biomarkers, as changes in miRNA expression can often be detected in biological fluids such as blood or urine.