Alterations in microRNA expression patterns in liver diseases

In the past few years there has been growing interest for a type of short RNAs called microRNAs, which are involved in the regulation of gene expression mainly in a negative way. There are about 1000 known microRNA today. It has been demonstrated that expression level of microRNA may become altered from normal to diseased state, thus microRNAs could be employed as a reliable tool in the diagnosis of diseases. A liver-characteristic microRNA (miR-122) needed for functioning hepatocytes has been identified, which usually shows a decreased expression level upon liver injury. miR-122 has been suggested as a biomarker since it was downregulated in the liver tissue upon acetaminophen-induced toxicity and in turn elevated miR-122 level was detected in the plasma. Moreover, miR-122 level in the plasma was found to be more sensitive as compared with conventional assays based on the release of liver enzymes. Also, miR-122 expression tends to decrease as carcinogenesis progresses. In addition, miR-122 enhances the replication of hepatitis C virus and its level seems to influence the efficiency of interferon therapy. Nowadays, many microRNAs are known whose distinctive alterations in their specific patterns seem to characterize individual pathological processes. In this article, the major alterations in microRNA expression patterns in liver diseases such as drug- and alcohol-induced liver diseases, non-alcoholic fatty liver diseases, fibrosis, viral infections (hepatitis), cirrhosis and hepatocellular carcinoma are summarized.