Roles of microRNAs in cancers and development.

MicroRNAs (miRNAs) are small noncoding RNA molecules thought to play an important role in regulating gene expression. Although knowledge of the biological functions of most miRNAs is still limited, miRNAs are thought to regulate the gene expression in various diseases and embryo development. In this chapter, the roles of miRNAs in human cancers are first examined from the viewpoint of up- and downregulation. Oncogenic miRNAs are involved in the overexpression/upregulation of cancers, whereas suppressive miRNAs are involved in the underexpression/downregulation of cancers. Statistical analysis of the positional nucleotide occurrence features of miRNAs revealed differences between the positional nucleotide occurrences of oncogenic and suppressive miRNAs. A miRNA gene-silencing score was then defined on the basis of the higher and lower levels of the statistical significances of positional nucleotides. Since the miRNA scores were closely related to miRNA frequencies, a method using the scores and nucleotide frequencies to distinguish whether a new miRNA is oncogenic or suppressive is proposed. This chapter also describes the roles of miRNAs in development. As miRNAs can act as cis-regulatory elements in the early embryonic development of Drosophila melanogaster, it is proposed that they mediate signal transduction between genes.