OncomiRs: the discovery and progress of microRNAs in cancers

0 Address: Department of Clinical Oncology, Queen Elizabeth Hospital , Room 1305, 13/F, Block R, 30 Gascoigne Road, Kowloon, Hong Kong SAR , PR China microRNAs (miRNAs) are evolutionarily conserved, endogenous, small, noncoding RNA molecules of about 22 nucleotides in length that function as posttranscriptional gene regulators. They are deemed to play a crucial role in the initiation and progression of human cancer, and those with a role in cancer are designated as oncogenic miRNAs (oncomiRs). For example, miR-15 and miR-16 induce apoptosis by targeting Bcl2. miRNAs from the miR-17-92 cluster modulate tumor formation and function as oncogenes by influencing the translation of E2F1 mRNA. miR-21 modulates gemcitabine-induced apoptosis by phosphatase and tensin homolog deleted on chromosome 10dependent activation of PI 3-kinase signaling. miR-34a acts as a suppressor of neuroblastoma tumorigenesis by targeting the mRNA encoding E2F3 and reducing E2F3 protein levels. The chromosomal translocations associating with human tumors disrupt the repression of High mobility group A2 by let-7 miRNA. In addition, the oncomiRs expression profiling of human malignancies has also identified a number of diagnostic and prognostic cancer signatures. This article introduces the roles of oncomiRs in neoplasm development, progression, diagnosis, prognostication, as well as their mechanism of actions on target mRNAs and the functional outcomes of their actions on mRNAs. The paper ends with a brief perspective to the future of oncomiRs. - Introduction microRNAs (miRNAs) are evolutionarily conserved, endogenous, small, noncoding RNA molecules of about 22 nucleotides in length that function as posttranscriptional gene regulators [1]. They are encoded in the genome and are generally transcribed by RNA polymerase II. miRNAs work via RNA-induced silencing complexes, targeting them to messenger RNAs where they either repress translation or direct destructive cleavage [2]. Recent evidences have shown that miRNAs have diverse functions, including the regulation of cellular development, differentiation, proliferation and apoptosis. The first miRNA was described in 1993, in which the C. elegans heterochronic gene lin-4 encoded small RNAs with antisense complementarity to lin-14 [3]. It is estimated that vertebrate genomes encode up to 1,000 unique miRNAs [4], which are predicted to regulate expression of at least 30% of genes [5]. Though more than 530 miRNAs have been identified in human, much remains to be understood about their precise cellular function and role in the development of diseases. Recent evidences indicate that miRNAs can function as tumor suppressors and oncogenes, and the miRNAs with a role in cancer are designated as oncogenic miRNAs (oncomiRs). This article introduces the roles of oncomiRs in neoplasm development, progression, diagnosis, prognostication, as well as their mechanism of actions on target mRNAs and the functional outcomes of their actions on mRNAs. microRNAs experiment miRNA microarray is a high-throughput approach to study the expression of miRNAs in cultured cells or tissues. Unlike the traditional cDNA microarray expression profiling, RNA samples used for miRNA microarray hybridization need to be enriched for small RNAs. Usually, the first step of a miRNA microarray experiment is the isolation of total RNA and the enrichment or direct isolation of small RNA. The miRNAs are then labeled and cleaned-up, following with miRNA hybridization to arrays spotted with miRNA probes. After washes and scanning, the differential miRNAs are identified. Subsequent validation is recommended using Northern blot, quantitative reverse transcription-polymerase chain reaction (qRT-PCR), or other analytical methods. The recent development of highly efficient labeling method and novel microarray probe design enable direct label as low as 120 ng of total RNA using Cy3 or Cy5, without fractionation or amplification, to produce precise and accurate measurements that span a linear dynamic range from 0.2 amol to 2 fmol of input miRNA. The assay is also applicable for formalin-fixed paraffin-embedded samples [6]. microRNAs and cancers It has been reported that miRNAs play a crucial role in the initiation and progression of human cancer. Deregulation of oncomiRs is associated with genetic or epigenetic alterations, including deletion, amplification, point mutation and aberrant DNA methylation [7]. Their expression profiling of human malignancies has identified signatures involving in cancer development, progression, diagnosis and prognosis (Table 1). The role of microRNAs in tumorigenesis Calin et al. first made the connection between microRNAs and cancer by showing that miR-15 and miR-16 are located at chromosome 13q14, a region deleted in more than half of B-cell chronic lymphocytic leukemia (CLL). Detailed deletion and expression analysis showed that miR-15 and miR-16 are located within a 30 kb region of loss in CLL, and that both genes were deleted or downregLung cancer, colon cancer Pancreatic cancer Pancreatic cancer ulated in approximately 68% of CLL cases [8]. They further showed that miR genes were frequently located in cancer-associated genomic regions or in fragile sites. The full complement of miRNAs in a genome might be extensively involved in cancers [9]. Bottoni et al. found that miR-15a and miR-16-1 were expressed at lower levels in pituitary adenomas as compared to normal pituitary tissue. Their expression was inversely correlated with tumor diameter and with arginyl-tRNA synthetase expression, but was directly correlated with p43 secretion, suggesting that these miRNAs might influence tumor growth [10]. Cimmino et al. then demonstrated that miR-15a and miR16-1 expressions were inversely correlated to Bcl2 expression in CLL and that both miRNAs negatively regulated Bcl2 at a posttranscriptional level. Bcl2 repression by these miRNAs induced apoptosis in a leukemic cell line model. Therefore miR-15 and miR-16 were natural antisense Bcl2 interactors that could be used for therapy of Bcl2-overexpressing tumors [11]. Recently, Garzon et al. showed that all-trans retinoic acid (ATRA) downregulation of Bcl2 and Ras was correlated with the activation of miR-15a/miR-161 [12]. Amplification and overexpression of the miR-17-92, which comprised 7 miRNAs and resided in intron 3 of the C13orf25 gene at 13q31.3, has been reported with pointers to functional involvement in the development of lymphoma and lung cancer. He et al. compared B-cell lymphoma samples and cell lines to normal tissues, and found that the levels of the primary or mature miRNAs derived from the miR-17-92 locus were often substantially increased in cancer cells. Their studies indicated that miRNAs could modulate tumor formation and function as oncogenes, implicating the miR-17-92 cluster as a potential human oncomicroRNAs (oncomiRs) [13]. O'Donnell et al. demonstrated that c-Myc activated expression of a set of 6 miRNAs (...truncated)