MicroRNA: Utility as Biomarkers and Therapeutic Targets in Squamous Cell Carcinoma
Daniel W. Lambert, Hataitip Tasena and Paul M. Speight
Abstract In recent years there has been an explosion in understanding of the roles non-coding RNA play in the pathogenesis of malignancy, including in squamous cell carcinomas. The majority of this research effort has focussed on microRNA, a class of small RNA able to regulate the expression of protein coding targets which frequently show aberrant expression patterns in cancer. Owing to their ready detection in bodily fluids, including saliva, interest has grown in their utility as biomarkers for diagnosis, prognostication and monitoring treatment response. In addition, evidence is growing that they may represent viable therapeutic targets. This chapter will summarise the current knowledge of microRNA expression changes in head and neck squamous cell carcinomas and give an overview of the translational opportunities they currently offer.
MicroRNA: Small but Powerful Regulators of Gene Expression
MicroRNA (miRNA) are defined as short, * 22 nucleotide, RNA molecules which do not act as templates for the translation of a protein product (hence they are part of a large and diverse group of ‘non-coding’ RNA which also includes ribosomal RNA, transfer RNA and long non-coding RNA, amongst other RNA species). They are predominantly derived from sequential processing of RNA polymerase II-generated precursor transcripts by Drosha/Pasha and the RNase III endonuclease DICER (Fig. 9.1). Genes encoding miRNA are found throughout the genome, but
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Fig. 9.1 Biogenesis of microRNA. Schematic illustrating the biogenesis of mature miRNA. Source: https://commons.wikimedia.org/wiki/File:MiRNA_processing.svg are often found associated in clusters, and frequently associate with genomic loci which encode proteins. They may also be generated from intronic regions (so called ‘mirtrons’), transfer RNA (tRNA) and small nucleolar RNA (snoRNA) precursors, as well as long non-coding RNA (lncRNA) through a different mechanism (Fig. 9.1). At the time of writing 35,828 mature miRNA products have been annotated (www.mirbase.org), but it is likely the number of functionally active miRNA is considerably lower than this (around 1900 miRNA have been annotated with high confidence across all species analysed).
The *22 base pair mature miRNA associate with Argonaute (Ago) proteins which form part of the RNA-induced silencing complex (RISC) (Fig. 9.1). The guide strand is retained in the complex and forms Watson-Crick base pairs with complementary 7-8 nucleotide ‘seed sequences’ present predominantly in the 3' untranslated region (3'UTR) of target transcripts. This base pairing leads to reduced expression of the protein encoded by the target RNA either by reducing translation by competing with translational machinery or promoting degradation of the transcript by recruiting nucleases following deadenylation of 5' de-capping. Although the mechanisms dictating the method by which protein production is decreased are not fully understood, the ultimate consequence is repression of expression of the target gene(s) [reviewed in Krol et al. (2010)]. It is thought that over 60% of protein-encoding genes are subject to miRNA-mediated regulation, and a single miRNA may target several hundred transcripts. It is likely, therefore, that miRNA-mediated regulation of gene expression is highly dynamic, depending on contextual cues. In addition, the magnitude of suppression of protein expression by a single miRNA is often mild, leading to the hypothesis that miRNA ‘fine tune’ gene expression rather than acting as the dominant regulatory mechanism; however, evidence exists to suggest that the miRNA-mediated targeting of multiple components of a pathway responsible for a particular cellular response may collectively have a profound effect on cell behavior; indeed, frequently a single miRNA may target several members of the same pathway, leading to amplification of its effects.