Читать книгу Genomic and Epigenomic Biomarkers of Toxicology and Disease - Группа авторов - Страница 23

MicroRNAs (miRNAs) are short, non-coding RNAs whose primary known function is to regulate the transcription and translation of messenger RNA. Targeting occurs through sequence specificity; however, this interaction is not limited to a single gene. Rather, a particular miRNA can regulate hundreds of genes. In addition, a transcript may be targeted by multiple miRNAs. As a result, it is estimated that between 30% and 80% of all transcribed genes in mammals may be regulated by miRNAs, depending on the tissue or cell type (Clark et al. 2014; Gu and Kay 2010; Lu and Clark 2012). Thus, alterations in miRNA amounts, even if minute, could feasibly have a large impact on transcriptional profiles. The consequence is important in terms of biological effect, as well as for the identification of biomarkers of health impact.

The need for specific and sensitive biomarkers spans research and development, regulatory, safety, and clinical sectors. Biomarkers can indicate exposure, biological effect, and sensitivity. All three areas have important purposes and applications. Those that measure biological effect more directly link to biological function, and thus to the putative adverse health effects that a perturbation may mediate. A biomarker may be a measurable alteration—chemical, biochemical, physiological, behavioral, or of some other kind—within an organism (World Health Organization and International Programme on Chemical Safety 1993). miRNAs have the potential to be good biomarkers of biological effect because they are well defined, chemically uniform, restricted to a manageable number, and stable (not readily degraded). They also get released into extracellular matrices, where they are accessible and measurable. In these biofluids such as blood, urine, and sputum, miRNAs serve as unique biomarkers for a minimally invasive prediction of toxicant exposure. The altered biological pathways that is consequent upon (and due to) miRNA changes can therefore reflect the mechanisms of toxicant-related diseases. Measurements of these miRNAs in biofluids can therefore serve as biomarkers of effect.

Indeed, changes in intracellular miRNA expression due to toxicant exposure have been well documented in multiple tissues and model organisms (Yu and Cho 2015) and are regulated by both genetic and epigenetic mechanisms. DNA hypo- and hypermethylation and histone modifications are involved in the regulation of the expression of miRNA promoters (Tomasetti et al. 2019). As a high proportion of miRNAs are embedded in CpG islands susceptible to methylation, miRNA genes are methylated more frequently than protein-coding genes (Kozomara et al. 2019; Morales et al. 2017). There is also evidence that some miRNAs can interact with other miRNAs, in a form of self-regulation (Hill and Tran 2021). In addition, mechanisms for the alteration of intracellular miRNA expression through exposure to environmental carcinogens have been proposed that involve the modulation or blocking of normal miRNA processing and maturation (Izzotti and Pulliero 2014). DNA damage can alter miRNA expression via p53-dependent mechanisms. p53 interaction with the Drosha/DGCR8 processing complex modulates the processing of pri-miRNAs to pre-miRNAs. In addition, Dicer, another processing unit that produces the final mature miRNA, is a direct transcriptional target for p53. Alternatively, the binding of electrophilic metabolites to nucleophilic sites of miRNA precursors can form miRNA adducts that are not able to access the catalytic pockets of Dicer, therefore arresting miRNA maturation. Further, toxicant metabolites binding to Dicer itself in the proximity of miRNA catalytic sites can block the maturation of miRNA precursors (Izzotti and Pulliero 2014).

The interest in miRNAs as biomarkers of exposure to environmental toxicants and resultant biological effects is bolstered by their stable detection in extracellular biofluids, where they can be non-invasively sampled. There are a number of mechanisms by which miRNAs are released from the cell into these matrices (Condrat et al. 2020) and, importantly, can be linked to tissue or cell type specificity (or both), as well as to mechanisms of biological perturbation that may relate to stress response, toxicity, and disease. In this chapter we review the promise of these putative biomarkers, together with the technical challenges that lie ahead if we want to establish them in the practice of toxicology and regulatory sciences.