microRNAs in human diseases and viral infections

microRNAs (miRNAs) are a class of short noncoding RNAs derived from either cellular or viral transcripts that act posttranscriptionally to regulate messenger RNA stability and translation. In humans, several hundred different miRNA species have been identified to fine-tune gene expression. Among them, an increasing number has been shown to be involved in the development and progression of a variety of diseases such as cancer, heart failure, asthma, and even allergies. Besides, several pathogenic DNA viruses, such as adeno-, herpes-, and polyomaviruses, encode for own viral miRNAs. These miRNAs are involved in the modulation of viral pathogenicity and replication properties. Moreover, the hepatitis C virus (HCV) exploits a cellular miRNA to boost viral replication and mRNA translation. As it requires no more than partial target recognition for inhibitory effects to occur, it is perspicuous that in principle, a single miRNA can regulate the expression of several hundred mRNAs. Thus, the dysregulated expression or mutation of just one miRNA species might be sufficient to cause an onset of pathological conditions. Hence, miRNAs are very interesting and highly promising targets for novel therapeutic approaches, and their functional modulation may represent an effective strategy to fight disease progression and viral infection. In this chapter, we describe the biology and mechanisms of action of miRNAs with particular emphasis on pathologically relevant species and current strategies for therapeutic intervention.