MicroRNAs (miRNAs) are small 21-25 nucleotide-long non-coding RNAs that have emerged as key negative post-transcriptional regulators of gene expression [1, 2], Currently there are more than 700 mammalian miRNAs that can potentially target up to one-third of protein-coding human genes [1] involved in diverse physiological and pathological processes, including cancer [3, 4]. Indeed, aberrant levels of miRNAs have been reported in all major human malignancies [5,6]. In tumors, altered expression of miRNAs has been demonstrated to inhibit tumor suppressor genes or inappropriately activate oncogenes and has been associated with every aspect of tumor biology, including tumor progression, invasiveness, metastasis, and acquisition of resistance by malignantcells to chemotherapeutic agents [3, 4, 7, 8]. These observations lead to the suggestion that aberrant expression of miRNAs may contribute to tumorigenesis [9]. However, most of the tumor-miRNA-related studies are based on expression analysis of miRNAsin tumors in comparison with corresponding adjacent normal tissues [4-6]. The altered expression of any given miRNA in neoplastic cells is not sufficient to address conclusively the role of these changes in tumorigenesis [10]. Additionally, despite the established biological significance of miRNA dysregulation in neoplastic cells, there is a lack of knowledge on the role of miRNAs during early stages of tumor development, especially if variations in the expression of specific miRNAs are associated withdifferences in the susceptibility to tumorigenesis.
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