microRNAs (miRNAs) are a new class of non-protein-coding, endogenous, small RNAs.
They are important regulatory molecules in animals and plants. miRNA regulates gene
expression by translational repression, mRNA cleavage, and mRNA decay initiated by
miRNA-guided rapid deadenylation. Recent studies show that some miRNAs regulate cell
proliferation and apoptosis processes that are important in cancer formation. By using
multiple molecular techniques, which include Northern blot analysis, real-time PCR,
miRNA microarray, up- or down-expression of specific miRNAs, it was found that several
miRNAs were directly involved in human cancers, including lung, breast, brain, liver,
colon cancer, and leukemia. In addition, some miRNAs may function as oncogenes or
tumor suppressors. More than 50% of miRNA genes are located in cancer-associated genomic
regions or in fragile sites, suggesting that miRNAs may play a more important role
in the pathogenesis of a limited range of human cancers than previously thought. Overexpressed
miRNAs in cancers, such as mir-17-92, may function as oncogenes and promote cancer
development by negatively regulating tumor suppressor genes and/or genes that control
cell differentiation or apoptosis. Underexpressed miRNAs in cancers, such as let-7,
function as tumor suppressor genes and may inhibit cancers by regulating oncogenes
and/or genes that control cell differentiation or apoptosis. miRNA expression profiles
may become useful biomarkers for cancer diagnostics. In addition, miRNA therapy could
be a powerful tool for cancer prevention and therapeutics.