MicroRNAs in cellular transformation and tumorigenesis

Abstract

MicroRNAs (miRNAs) are a novel class of small (approximately 23 nucleotides long), highly conserved, non-coding RNAs that function by broadly regulating gene expression. In animals, this regulation is achieved via interaction with target messenger RNAs (mRNAs), largely through their imperfect base pairing with the 3' untranslated regions (3' UTRs) of these target transcripts. Through this imperfect base pairing, miRNAs induce a repression of mRNA translation that is frequently coupled to enhanced turnover of the targeted transcript. This miRNA-mediated repression is highly related to that of RNA interference (RNAi), in which small non-coding RNAs exhibit perfect base pairing with target mRNA transcripts, leading to the endonucleolytic cleavage and degradation of these targeted mRNAs. Computational algorithms have been designed to predict putative miRNA binding sites within mRNAs. Using these predictions, it has been suggested that more than half of all mRNAs within mammals are under the control of miRNAs. Some of the earliest discovered miRNAs (characterized by genetic studies in the nematode Caenorhabditis elegans) were found to control the proliferation and differentiation of the cells in which they were expressed. As altered control of proliferation and differentiation frequently manifest in cancer in mammals, it was suggested that miRNAs might contribute to the development of tumorigenesis.

(cont.) In fact, several miRNAs are frequently deleted or amplified in human cancer and miRNA expression profiling studies have shown widespread reductions in steady-state miRNA levels in human cancers relative to normal tissue. These observations have implied a role for miRNAs in tumorigenesis. However, there is a paucity of functional studies demonstrating a role for miRNAs in oncogenic transformation. In the studies described below, we first provide strong evidence for the global loss of miRNAs in human cancers functionally enhancing cellular transformation and tumorigenesis. This enhanced transformation only occurred within tumor cells, suggesting that inhibiting miRNA biogenesis would not be sufficient to induce tumor formation. Moreover, we demonstrate that inhibition of miRNA processing in cancer must be incomplete, as Dicerl, a component of the miRNA processing pathway, suppresses tumorigenesis via haploinsufficiency. Finally, we examine the role of a specific miRNA family, the let-7 family, in the development of non-small cell lung cancer by showing that let-7 can suppress tumorigenesis via inhibition of its targets K-Ras and HMGA2. Taken together, these findings offer promise, not just for understanding the relationship of miRNAs and cancer, but for developing therapeutic agents against the disease.