A new era in the study of individual differences in drug metabolism and pharmacokinetics.

The study of the epigenetic regulation of drug-metabolizing enzymes that might affect drug response and adverse drug reactions is important for understanding individual differences in drug response, drug metabolism and pharmacokinetics. Epigenetic modifications include transcriptional (DNA methylation and histone modification) and post-transcriptional (microRNA (miRNA) and ubiquitination) regulation, affecting phenotypic changes. Because of their dynamic nature, epigenetic factors provide a link between the genome and the phenotypic outcomes, and fill the gap between genome and proteins. Recently, among these epigenetic factors, miRNAs have been studied most extensively. MiRNAs are a large family of non-coding RNAs that are evolutionarily conserved, endogenous, and 21–23 nucleotides in length. MiRNAs regulate gene expression by targeting messenger RNAs (mRNAs) by binding to complementary regions of transcripts to repress their translation or mRNA degradation. MiRNAs are encoded by the genome, and more than 2,000 human miRNAs have been identified so far. In silico prediction estimates that 360% of human mRNAs could be targets of miRNAs. Similar to mRNA, miRNAs are expressed in a tissueor cell-specific manner. MiRNAs can potentially regulate every aspect of cellular processes such as differentiation, proliferation, apoptosis and necrosis as well as a wide range of physiological processes. Although miRNA research into the metabolism of xenobiotics/ endobiotics is still in its infancy, understanding of miRNAs is progressing rapidly. Recently, many reports concerning miRNAs related to cancer have been published; however, the miRNA research into the metabolism of xenobiotics and endobiotics has only recently been instigated. The expression of drugand xenobiotic-metabolizing enzymes and nuclear receptors and their regulation by miRNAs could be important factors for the outcomes of drug response and adverse drug reactions. The different profiles of the expression of P450 isoenzymes determine the amount of reactive intermediates formed and the resulting adverse drug response. The mechanisms of the transcriptional regulation of P450 and P450-related nuclear receptors have been considerably clarified, but the post-transcriptional regulation largely remains to be elucidated. Recently, various P450s and nuclear receptors have been found to be posttranscriptionally regulated by miRNAs as summarized in Table 1. For example, we reported that miR-148a regulates pregnane X receptor (PXR) but not CYP3A4. Notably, the miR-148a-dependent regulation of PXR affected the induction of endogenous CYP3A4 in human LS180 cells. Correlation analysis using human liver samples showed that the CYP3A4 protein level showed a significantly positive correlation with the CYP3A4 mRNA level. The PXR protein level was also correlated with CYP3A4 protein level. However, the PXR protein level was not correlated with the PXR mRNA level, indicating the post-transcriptional regulation. The transcriptional efficiency of PXR was inversely correlated with the miR-148a levels. Although more precise and quantitative investigations are needed, miR-148a-dependent PXR regulation would have great impact on the large inter-individual variability of CYP3A4 expression in human livers. Collectively, the regulation of nuclear receptors by miRNAs results in changes in the expression of a variety of target genes, constructing complex networks. Further studies will clarify the significance of miRNAs in the control of pharmacokinetics and the mechanism of interand intra-individual differences in drug responses and adverse drug reactions. SNPs (single nucleotide polymorphisms) occur in an average of 1/1000 bases in the human genome, and are present not only in the mRNA but also in mature miRNA sequences. SNPs in miRNAs could modify various biological processes by influencing the processing or target selection of miRNAs. Only 310% of human precursor (pre)-miRNAs have documented SNPs and <1% of miRNAs have SNPs in the functional seed sequence region. A SNP [rs7372209] in the primary (pri)-miR26a gene and a SNP [rs1834306] in the pri-miR-100 gene were significantly associated with the tumor response or time to progression in 61 metastatic colorectal cancer patients treated with 5-fluorouracil and CPT-11, suggesting a relationship between the clinical outcome of drug therapy and SNPs in the miRNA-biogenesis machinery, but the molecular mechanisms by which these polymorphisms act have not yet been clarified. Although seed sequence variations in miRNAs Table 1. MiRNAs targeting xenobiotic-related receptors and drug-metabolizing enzymes