miR-15 and miR-16 induce apoptosis by targeting BCL2

Murine models for chronic lymphocytic leukaemia

CLL (chronic lymphocytic leukaemia) is characterized by the clonal outgrowth of B-lymphocytes with the distinctive phenotype: CD19(hi)CD5(+)CD23(+)IgM(lo). These malignant B-cells accumulate in the PB (peripheral blood) and lymphoid organs, and are generally arrested at the G(0)/G(1)-phase of cell cycle and display a resistance to apoptosis. To date, most of the CLL research has been carried out using PB samples obtained from patients with established CLL, which have proved instrumental in characterizing the disease. However, while CLL cells appear to have a defect in apoptosis in vivo, they rapidly undergo apoptosis ex vivo, suggesting that CLL cells are dependent on microenvironmental signals to enhance cell survival. One approach used to define the cellular and molecular events that govern CLL has been the development of murine models that replicate the human disease. As well as providing a deeper understanding of the potential triggers for CLL, these models provide preclinical in vivo systems to test novel therapies. The focus of the present review will be to highlight the recent advances in the development of mouse models for CLL.

Computational analysis of biological functions and pathways collectively targeted by co-expressed microRNAs in cancer

Background

Multiple recent studies have found aberrant expression profiles of microRNAome in human cancers. While several target genes have been experimentally identified for some microRNAs in various tumors, the global pattern of cellular functions and pathways affected by co-expressed microRNAs in cancer remains elusive. The goal of this study was to develop a computational approach to global analysis of the major biological processes and signaling pathways that are most likely to be affected collectively by co-expressed microRNAs in cancer cells.

Results

We report results of computational analysis of five datasets of aberrantly expressed microRNAs in five human cancers published by the authors (pancreatic cancer) and others (breast cancer, colon cancer, lung cancer and lymphoma). Using the combinatorial target prediction algorithm miRgate and a two-step data reduction procedure we have determined Gene Ontology categories as well as biological functions, disease categories, toxicological categories and signaling pathways that are: targeted by multiple microRNAs; statistically significantly enriched with target genes; and known to be affected in specific cancers.

Conclusion

Our global analysis of predicted miRNA targets suggests that co-expressed miRNAs collectively provide systemic compensatory response to the abnormal phenotypic changes in cancer cells by targeting a broad range of functional categories and signaling pathways known to be affected in a particular cancer. Such systems biology based approach provides new avenues for biological interpretation of miRNA profiling data and generation of experimentally testable hypotheses regarding collective regulatory functions of miRNA in cancer.

The role of microRNA and other non-coding RNA in the pathogenesis of chronic lymphocytic leukemia

New findings support the view that chronic lymphocytic leukemia (CLL) is a genetic disease in which the main alterations occur in a new class of genes named microRNAs (miRNAs). Cases with good prognostic features typically are characterized by miRNA down-regulation of genes miR-15a and miR-16-1, located at 13q14.3. Both microRNAs negatively regulate BCL2 at a post-transcriptional level. On the other hand, in CLL cases that use unmutated immunoglobulin heavy-chain variable-region genes (IgV(H)) or have high-level expression of the 70-kD zeta-associated protein (ZAP-70) have high levels of TCL1 due to low-level expression of miR-29 and miR-181, which directly target this oncogene. Conceivably, these miRNAs might be used to target BCL2 or TCL1 for therapy of this disease.

Prognostic markers in chronic lymphocytic leukaemia

Rai and Binet staging of chronic lymphocytic leukaemia (CLL) is being superseded by new prognostic markers. The mutational status of the immunoglobulin variable region heavy-chain genes segregates the disease into more benign and more malignant versions, and has been confirmed as an important prognostic marker in prospective clinical trials. A search for surrogate markers for this difficult-to-perform assay has led to flow cytometric assays for CD38 and ZAP-70 expression, although in both cases there are problems with standardization and interpretation of the assays. A separate pathway of research has revealed two chromosomal aberrations--deletions of 11q and 17p--as important prognostic markers. Fluorescent in-situ hybridization has made their detection readily available. These five markers are in different stages of evaluation, but some of them are ready to be used for risk-adapted therapy in clinical trials. Other assays are in earlier stages of assessment.

MicroRNA expression in lymphoma

MicroRNAs are a recently discovered class of short (approximately 22 nucleotide) naturally occurring RNA molecules that negatively regulate gene expression post-transcriptionally. There has been an explosion of interest in the microRNA field as these molecules have been found to play key roles in a wide range of biological processes and to be aberrantly expressed in many types of cancer, including haematological malignancies. Cancer-associated microRNAs can act as both tumour suppressor molecules (e.g., miR-15a and miR-16-1) and have oncogenic properties (e.g., miR-155 and miR-17-92 cluster). In this review the authors discuss the rapidly accumulating evidence for the central role that microRNAs play in both haematopoiesis and haematological malignancy, in particular focusing on their role in lymphoma.

High-resolution oligonucleotide array-CGH pinpoints genes involved in cryptic losses in chronic lymphocytic leukemia

Although recurrent chromosomal alterations occur in chronic lymphocytic leukemia (CLL), relatively few affected tumor suppressors and oncogenes have been implicated. To improve genetic characterization of CLL, we performed high-resolution gene copy number analysis of 20 CLL patients using oligonucleotide array comparative genomic hybridization (aCGH). The most recurrent losses were observed in 13q and 11q with variable sizes. The 11q losses varied between 7.44 Mb and 41.72 Mb in size and targeted ATM among others. Lost regions in 13q were generally smaller, spanning from 0.79 Mb to 29.33 Mb. The minimal common region (158 kb) in 13q14.3, which was also homozygously deleted in some cases, harbored five genes: TRIM13, KCNRG, DLEU2, DLEU1, and FAM10A4. Additionally, two micro-RNA genes (MIRN15A and MIRN16-1) locate to the region. New cryptic losses were detected in 1q23.2-->q23.3, 3p21.31, 16pter-->p13.3, 17p13.3-->p13.2, 17q25.3-->qter, and 22q11.22. In conclusion, our oligonucleotide aCGH study revealed novel aberrations and provided detailed genomic profiles of the altered regions.

miRNA genetic alterations in human cancers

MicroRNAs (miRNAs) are endogenous, non-coding, small RNAs, which negatively regulate gene expression in a sequence-specific manner via translational repression and/or mRNA degradation. Their discovery revealed a new and exciting aspect of post-transcriptional gene regulation that is universally involved in cellular homeostasis. Importantly, the advent of miRNAs added another level of complication in the already complex regulatory networks of the cell, undermining that RNA molecules in general, should be considered gene regulators of equal importance with proteins. Recently, the scientific community drew attention to the miRNA field for an additional reason: an increasing line of evidence indicated that miRNA genes are tightly connected with the process of tumorigenesis. Indeed, several miRNAs have already been demonstrated to behave as oncogenes or tumor suppressor genes in many types of cancer. Even though the underlying mechanisms by which miRNAs can destabilize the normal cellular processes, promoting cell transformation and tumor progression, are not well understood, genetic and epigenetic alterations most probably play a critical role. Significant technologic advances facilitated the profiling of the miRNA expression patterns in normal and cancer tissues and discovered an unexpected greater reliability of miRNA expression signatures in classifying cancer types than the respective signatures of protein-coding genes. Along with this extraordinary diagnostic potential, miRNAs have also proved their prognostic value in predicting clinical behaviors of cancer patients. The aim of this review is to describe miRNA expression and how its deregulation is involved in the pathophysiology of human cancers.

Role of miRNA in carcinogenesis and biomarker selection: a methodological view

miRNAs, their involvement in cancer development and their potential to be robust biomarkers of diagnosis, staging, prognosis and response to therapy are reviewed. In small RNA animal biogenesis, miRNA genes in the nucleus are transcribed to generate long primary transcripts (pri-miRNAs), which are first cropped by RNase-III-type enzyme Drosha to release hairpin intermediates (pre-miRNAs) in the nucleus. Pre-miRNA is then exported to the cytoplasm by exportin-5. Following arrival in the cytoplasm, pre-miRNAs are subjected to the second processing step (dicing) to release the mature miRNA duplex, which is then separated: one strand becomes the mature miRNA and the other is degraded. These tiny miRNAs induce messenger degradation, translational repression or both. However, there is no evidence to demonstrate that these two mechanisms exist in the regulation of the same gene. Since a miRNA can target numerous mRNAs, often in combination with other miRNAs, these miRNAs operate a highly complex regulatory network. The specific function in most mammalian miRNAs is unknown. However, data suggest that miRNA genes, approximately 1% of all human genes, regulate protein production for 20-30% or more of all genes. miRNA expression profiles are effective for classifying solid and hematologic human cancers, and have shown great promise for early cancer detection. This is of great importance for effective treatment before the cells metastasize; therefore, tumors can be surgically resected. Computer-based prediction approaches of miRNAs and their targets, and biological validation techniques for ascertaining these predictions, currently play a central role in the discovery of miRNAs and in elucidating their function. Guidelines have been established for the identification and annotation of new miRNAs to distinguish them from other RNAs, especially siRNAs. These guidelines take into account factors such as transcript structure, conservation and processing, and a centralized, searchable database of all possible miRNA sequence information and annotation for humans and of more than 38 other species. Two approaches are used to characterize miRNAs: studying expression of known miRNAs by hybridization-based techniques (e.g., northern blots, RNase protection, primer extension, real-time, quantitative PCR and microarrays) or discovery of novel miRNAs molecules by cloning and sequencing. Owing to their adaptability and high throughput, microarrays may prove to be the preferred platform for whole-genome miRNA expression analysis.

Gene expression trees in lymphoid development

BACKGROUND

The regulatory processes that govern cell proliferation and differentiation are central to developmental biology. Particularly well studied in this respect is the lymphoid system due to its importance for basic biology and for clinical applications. Gene expression measured in lymphoid cells in several distinguishable developmental stages helps in the elucidation of underlying molecular processes, which change gradually over time and lock cells in either the B cell, T cell or Natural Killer cell lineages. Large-scale analysis of these gene expression trees requires computational support for tasks ranging from visualization, querying, and finding clusters of similar genes, to answering detailed questions about the functional roles of individual genes.

RESULTS

We present the first statistical framework designed to analyze gene expression data as it is collected in the course of lymphoid development through clusters of co-expressed genes and additional heterogeneous data. We introduce dependence trees for continuous variates, which model the inherent dependencies during the differentiation process naturally as gene expression trees. Several trees are combined in a mixture model to allow inference of potentially overlapping clusters of co-expressed genes. Additionally, we predict microRNA targets.

CONCLUSION

Computational results for several data sets from the lymphoid system demonstrate the relevance of our framework. We recover well-known biological facts and identify promising novel regulatory elements of genes and their functional assignments. The implementation of our method (licensed under the GPL) is available at http://algorithmics.molgen.mpg.de/Supplements/ExpLym/.

Chromosomal rearrangements and microRNAs: a new cancer link with clinical implications

There is widespread aberrant expression of mature and/or precursor microRNAs in cancer cells, as microRNAs are deregulated consequent to chromosomal alterations and other genomic abnormalities. The identification of such abnormalities has a clear diagnostic and prognostic significance, and there are ever increasing examples of links between certain human cancers and modifications at microRNA loci.

The let-7 microRNA represses cell proliferation pathways in human cells

MicroRNAs play important roles in animal development, cell differentiation, and metabolism and have been implicated in human cancer. The let-7 microRNA controls the timing of cell cycle exit and terminal differentiation in Caenorhabditis elegans and is poorly expressed or deleted in human lung tumors. Here, we show that let-7 is highly expressed in normal lung tissue, and that inhibiting let-7 function leads to increased cell division in A549 lung cancer cells. Overexpression of let-7 in cancer cell lines alters cell cycle progression and reduces cell division, providing evidence that let-7 functions as a tumor suppressor in lung cells. let-7 was previously shown to regulate the expression of the RAS lung cancer oncogenes, and our work now shows that multiple genes involved in cell cycle and cell division functions are also directly or indirectly repressed by let-7. This work reveals the let-7 microRNA to be a master regulator of cell proliferation pathways.

Oblimersen for the treatment of patients with chronic lymphocytic leukemia

Among adults in Western countries, chronic lymphocytic leukemia (CLL) is the most prevalent form of leukemia. CLL primarily affects the elderly and may be associated with multiple comorbidities. A cure has not been identified, and new treatment options are needed. Expression of Bcl-2 protein is associated with the pathogenesis of CLL and chemotherapy resistance. Oblimersen, a Bcl-2 antisense phosphorothioate oligonucleotide, is being evaluated in patients with CLL and other cancers; trials through Phase III have been completed. In the setting of relapsed/refractory CLL, single-agent oblimersen demonstrates modest activity, whereas the addition of oblimersen to fludarabine/cyclophosphamide significantly improves the rate of complete and nodular partial responses; moreover, these responses are durable and associated with clinical benefit. Oblimersen is more efficacious in relapsed rather than refractory patients. The side effect profile of oblimersen, alone or in combination with standard chemotherapy, is favorable compared with currently available chemotherapies. In the first cycle, an infusion reaction with or without tumor lysis syndrome is uncommon, and transient thrombocytopenia is observed. Catheter-related complications are associated with the need for continuous intravenous infusion of oblimersen over several days; other routes of administration are under clinical investigation. Oblimersen is a promising therapeutic approach for patients with relapsed CLL and should be further evaluated in the front-line setting.

A simple array platform for microRNA analysis and its application in mouse tissues

MicroRNAs (miRNAs) are a novel class of small noncoding RNAs that regulate gene expression at the post-transcriptional level and play a critical role in many important biological processes. Most miRNAs are conserved between humans and mice, which makes it possible to analyze their expressions with a set of selected array probes. Here, we report a simple array platform that can detect 553 nonredundant miRNAs encompassing the entire set of miRNAs for humans and mice. The platform features carefully selected and designed probes with optimized hybridization parameters. Potential cross-reaction between mature miRNAs and their precursors was investigated. The array platform was used to analyze miRNAs in the mouse central nervous system (CNS, spinal cord and brain), and two other non-CNS organs (liver and heart). Two types of miRNAs, differentially expressed organ/tissue-associated miRNAs and ubiquitously expressed miRNAs, were detected in the array analysis. In addition to the previously reported neuron-related miR-124a, liver-related miR-122a, and muscle-related miR-133a, we also detected new tissue-associated miRNAs (e.g., liver-associated miR-194). Interestingly, while the majority of pre-miRNAs were undetectable, miR690, miR709, and miR720 were clearly detected at both mature and precursor levels by the array analysis, indicating a limited cross-reaction between pre-miRNAs and their mature miRNAs. The reliability of this array technology was validated by comparing the results with independent Northern blot analyses and published data. A new approach of data normalization based on Northern blot analysis of one ubiquitously expressed miRNA is introduced and compared with traditional approaches. We expect this miRNA array platform to be useful for a wide variety of biological studies.

OncomiRs: the discovery and progress of microRNAs in cancers

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 10-dependent 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.

Kaposi's sarcoma-associated herpesvirus encodes an ortholog of miR-155

MicroRNAs (miRNAs) are small noncoding RNAs that posttranscriptionally regulate gene expression by binding to 3'-untranslated regions (3'UTRs) of target mRNAs. Kaposi's sarcoma-associated herpesvirus (KSHV), a virus linked to malignancies including primary effusion lymphoma (PEL), encodes 12 miRNA genes, but only a few regulatory targets are known. We found that KSHV-miR-K12-11 shares 100% seed sequence homology with hsa-miR-155, an miRNA frequently found to be up-regulated in lymphomas and critically important for B-cell development. Based on this seed sequence homology, we hypothesized that both miRNAs regulate a common set of target genes and, as a result, could have similar biological activities. Examination of five PEL lines showed that PELs do not express miR-155 but do express high levels of miR-K12-11. Bioinformatic tools predicted the transcriptional repressor BACH-1 to be targeted by both miRNAs, and ectopic expression of either miR-155 or miR-K12-11 inhibited a BACH-1 3'UTR-containing reporter. Furthermore, BACH-1 protein levels are low in cells expressing either miRNA. Gene expression profiling of miRNA-expressing stable cell lines revealed 66 genes that were commonly down-regulated. For select genes, miRNA targeting was confirmed by reporter assays. Thus, based on our in silico predictions, reporter assays, and expression profiling data, miR-K12-11 and miR-155 regulate a common set of cellular targets. Given the role of miR-155 during B-cell maturation, we speculate that miR-K12-11 may contribute to the distinct developmental phenotype of PEL cells, which are blocked in a late stage of B-cell development. Together, these findings indicate that KSHV miR-K12-11 is an ortholog of miR-155.

Prediction and preliminary validation of oncogene regulation by miRNAs

Background

MicroRNAs (miRNAs) are one of the most abundant groups of regulatory genes in multicellular organisms, playing important roles in many fundamental cellular processes. More than four hundred miRNAs have been identified in humans and the deregulation of miRNA expression has been also shown in many cancers. Despite the postulated involvement of miRNAs in tumourigenesis, there are only a few examples where an oncogene or a tumour suppressor has been identified as a miRNA target.

Results

Here, we present an in silico analysis of potential miRNA- oncogene interactions. Moreover, we have tested the validity of two possible interactions of miRNAs with genes related to cancer. We present evidence for the down-regulation of c-MYC, one of the most potent and frequently deregulated oncogenes, by let-7 miRNA, via the predicted binding site in the 3'UTR, and verify the suppression of BCL-2 by miR16.

Conclusion

In this work both bioinformatic and experimental approaches for the prediction and validation of possible targets for miRNAs have been used. A list of putative targets for different oncomirs, validation of which would be of special interest, is proposed and two such interactions have been experimentally validated.

MicroRNAs in disease and potential therapeutic applications

MicroRNAs (miRNAs) are 21-24 nucleotide (nt) duplex RNAs that are created from precursor transcripts by subsequent processing steps mediated by members of the RNAseIII family, Drosha and Dicer. One of the two strands is incorporated into the active sites of the Argonaute family of proteins, where it serves as a guide for Watson-Crick base pairing with complementary sequences in target messenger RNAs (mRNAs). In mammals, the majority of miRNAs guide the RNA-induced silencing complex (RISC) to the 3' untranslated regions (UTRs) of mRNA targets, with the consequence that translation of the target mRNAs is inhibited. The importance of miRNAs in normal cellular development and metabolism is only now being realized. miRNA deficiencies or excesses have been correlated with a number of clinically important diseases ranging from myocardial infarction to cancers. The loss or gain of miRNA function can be caused by a single point mutation in either the miRNA or its target or by epigenetic silencing of primary miRNA transcription units. This review summarizes miRNA biogenesis and biology, explores the potential roles miRNAs can play in a variety of diseases, and suggests some therapeutic applications for restoring or inhibiting miRNA function.

Blockade of invasion and metastasis of breast cancer cells via targeting CXCR4 with an artificial microRNA

miRNAs have been shown to function as regulatory molecules and to play an important role in cancer progression. Very little is currently known about the increasing invasion and metastasis of breast cancer due to the loss of expressive levels of certain miRNAs in breast tumor cells. In order to determine whether the CXCR4/SDF-1 pathway is regulated by expression of miRNAs, we designed and synthesized pre-miRNA against CXCR4. This double-stranded miRNA gene was ligated with a miR-155-based Block-iT Pol II miR RNAi Expression Vector (Invitrogen). Expression levels of CXCR4 in CXCR4-miRNA-transfected breast tumor cells had significantly declined. These cells exhibited reduced migration and invasion in vitro. Furthermore, they formed fewer lung metastases in vivo compared to ctrl-miRNA-transfected cells. These data support the conclusion that miRNA against CXCR4 can serve as an alterative means of therapy to lower CXCR4 expression and to block the invasion and metastasis of breast cancer cells.

mir-29 regulates Mcl-1 protein expression and apoptosis

Cellular expression of Mcl-1, an anti-apoptotic Bcl-2 family member, is tightly regulated. Recently, Bcl-2 expression was shown to be regulated by microRNAs, small endogenous RNA molecules that regulate protein expression through sequence-specific interaction with messenger RNA. By analogy, we reasoned that Mcl-1 expression may also be regulated by microRNAs. We chose human immortalized, but non-malignant, H69 cholangiocyte and malignant KMCH cholangiocarcinoma cell lines for these studies, because Mcl-1 is dysregulated in cells with the malignant phenotype. By in silico analysis, we identified a putative target site in the Mcl-1 mRNA for the mir-29 family, and found that mir-29b was highly expressed in cholangiocytes. Interestingly, mir-29b was downregulated in malignant cells, consistent with Mcl-1 protein upregulation. Enforced mir-29b expression reduced Mcl-1 protein expression in KMCH cells. This effect was direct, as mir-29b negatively regulated the expression of an Mcl-1 3' untranslated region (UTR)-based reporter construct. Enforced mir-29b expression reduced Mcl-1 cellular protein levels and sensitized the cancer cells to tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) cytotoxicity. Transfection of non-malignant cells (that express high levels of mir-29) with a locked-nucleic acid antagonist of mir-29b increased Mcl-1 levels and reduced TRAIL-mediated apoptosis. Thus mir-29 is an endogenous regulator of Mcl-1 protein expression, and thereby, apoptosis.

[MicroRNAs and intestinal pathophysiology]

MicroRNAs (miRNAs) represent an abundant class of endogenously expressed small RNAs, which is believed to control the expression of proteins through specific interaction with their mRNAs. MiRNAs are non-coding RNAs of 18 to 24 nucleotides that negatively regulate target mRNAs by binding to their 3'-untranslated regions (UTR). Most eukaryotic cells utilize miRNA to regulate vital functions such as cell differentiation, proliferation or apopotosis. The diversity of miRNAs and of their mRNA targets strongly indicate that they play a key role in the regulation of protein expression. To date, more than 500 different miRNAs have been identified in animals and plants. There are at least 326 miRNAs in the human genome, comprising 1-4% of all expressed human genes, which makes miRNAs one of the largest classes of gene regulators. A single miRNA can bind to and regulate many different mRNA targets and, conversely, several different miRNAs can bind to and cooperatively control a single mRNA target. The correlation between the expression of miRNAs and their effects on tumorigenesis and on the proliferation of cancer cells is beginning to gain experimental evidences. Recent studies showed that abnormal expression of miRNAs represents a common feature of cancer cells and that they can function as tumor suppressor genes or as oncogenes. Therefore, this diversity of action for miRNAs on several target genes could be one of the common mechanisms involved in the deregulation of protein expression observed during intestinal disorders. In this review, the emergent functions of miRNAs in colorectal cancer and their potential role in the intestinal inflammatory process are discussed.

MicroRNAs as therapeutic targets in human diseases

MicroRNAs (miRNAs) are important endogenous regulators of gene expression. The specific regulation at both the transcription and the translation level (inhibition or mRNA degradation) opens an avenue to use these small RNA molecules as potential targets for the development of novel drugs as well as for the diagnosis of several human diseases. Important information about the role of a miRNA in disease can be deduced by mimicking or inhibiting its activity and examining its impact on the phenotype/behaviour of the cell or organism. Modulating the activity of a miRNA is expected to lead to improvement in disease symptoms and this implies that the target miRNA plays an important role in the disease. It is also now possible to develop miRNA-based therapeutic products that can either increase or decrease the levels of proteins in pathophysiological conditions such as cancer, cardiovascular diseases, viral diseases, metabolic disorders and programmed cell death. The commercial potential of miRNA and related drugs is expected to exponentially increase within the next few years, yet there are several areas in miRNA biology and delivery that need to be extensively investigated.

Micro RNA 145 targets the insulin receptor substrate-1 and inhibits the growth of colon cancer cells

The insulin receptor substrate-1 (IRS-1), a docking protein for both the type 1 insulin-like growth factor receptor (IGF-IR) and the insulin receptor, is known to send a mitogenic, anti-apoptotic, and anti-differentiation signal. Several micro RNAs (miRs) are suggested by the data base as possible candidates for targeting IRS-1. We show here that one of the miRs predicted by the data base, miR145, whether transfected as a synthetic oligonucleotide or expressed from a plasmid, causes down-regulation of IRS-1 in human colon cancer cells. IRS-1 mRNA is not decreased by miR145, while it is down-regulated by an siRNA targeting IRS-1. Targeting of the IRS-1 3'-untranslated region (UTR) by miR145 was confirmed using a reporter gene (luciferase) expressing the miR145 binding sites of the IRS-1 3'-UTR. In agreement with the role of IRS-1 in cell proliferation, we show that treatment of human colon cancer cells with miR145 causes growth arrest comparable to the use of an siRNA against IRS-1. Taken together, these results identify miR145 as a micro RNA that down-regulates the IRS-1 protein, and inhibits the growth of human cancer cells.

MicroRNA-34b and MicroRNA-34c are targets of p53 and cooperate in control of cell proliferation and adhesion-independent growth

MicroRNAs (miRNA) are a recently discovered class of noncoding RNAs that negatively regulate gene expression. Recent evidence indicates that miRNAs may play an important role in cancer. However, the mechanism of their deregulation in neoplastic transformation has only begun to be understood. To elucidate the role of tumor suppressor p53 in regulation of miRNAs, we have analyzed changes in miRNA microarray expression profile immediately after conditional inactivation of p53 in primary mouse ovarian surface epithelium cells. Among the most significantly affected miRNAs were miR-34b and miR-34c, which were down-regulated 12-fold according to quantitative reverse transcription-PCR analysis. Computational promoter analysis of the mir-34b/mir-34c locus identified the presence of evolutionarily conserved p53 binding sites approximately 3 kb upstream of the miRNA coding sequence. Consistent with evolutionary conservation, mir-34b/mir-34c were also down-regulated in p53-null human ovarian carcinoma cells. Furthermore, as expected from p53 binding to the mir-34b/c promoter, doxorubicin treatment of wild-type, but not p53-deficient, cells resulted in an increase of mir-34b/mir-34c expression. Importantly, miR-34b and miR-34c cooperate in suppressing proliferation and soft-agar colony formation of neoplastic epithelial ovarian cells, in agreement with the partially overlapping spectrum of their predicted targets. Taken together, these results show the existence of a novel mechanism by which p53 suppresses such critical components of neoplastic growth as cell proliferation and adhesion-independent colony formation.

Ultraconserved regions encoding ncRNAs are altered in human leukemias and carcinomas

Noncoding RNA (ncRNA) transcripts are thought to be involved in human tumorigenesis. We report that a large fraction of genomic ultraconserved regions (UCRs) encode a particular set of ncRNAs whose expression is altered in human cancers. Genome-wide profiling revealed that UCRs have distinct signatures in human leukemias and carcinomas. UCRs are frequently located at fragile sites and genomic regions involved in cancers. We identified certain UCRs whose expression may be regulated by microRNAs abnormally expressed in human chronic lymphocytic leukemia, and we proved that the inhibition of an overexpressed UCR induces apoptosis in colon cancer cells. Our findings argue that ncRNAs and interaction between noncoding genes are involved in tumorigenesis to a greater extent than previously thought.

Upregulation of bfl-1 is a potential mechanism of chemoresistance in B-cell chronic lymphocytic leukaemia

B-cell chronic lymphocytic leukaemia (B-CLL) is characterised by the progressive accumulation of monoclonal CD5(+) B cells. In a previous study, we have analysed the expression profile of apoptosis-regulating genes using a cDNA-based microarray and found overexpression of the antiapoptotic bcl-2 family member, bfl-1, in B-CLL cells with an apoptosis-resistant phenotype. In this study, bfl-1 mRNA levels have been determined by competitive PCR in an extended population of B-CLL patients to characterise its role in disease progression and development of chemoresistance. bfl-1 levels were significantly higher in patients with no response (NR) to last chemotherapy than in patients responding (partial response (PR)) to last chemotherapy (P<0.05) and in patients who had not required treatment (P<0.05). We found no correlation between bfl-1 mRNA levels and disease progression, IGHV mutational status or other clinical parameters. In addition, bfl-1 mRNA levels were inversely correlated with apoptotic response to in vitro fludarabine treatment of B-CLL cells. Specific downregulation of bfl-1 using siRNA induced apoptosis in resistant cells. Our data suggest that bfl-1 contributes to chemoresistance and might be a therapeutic target in B-CLL.

Regulatory mechanisms of microRNAs involvement in cancer

MicroRNAs (miRNAs) are 19-24 nucleotide noncoding RNAs that regulate the translation and degradation of target mRNAs and are extensively involved in human cancers. One unexpected conclusion of the profiling and functional studies in tumourigenesis is that some miRNAs behave in cancer cells in a dual mode, resembling the 'Dr Jekyll and Mr Hyde' story, which centers on a conception of humanity as dual in nature. The authors and others have found that onco-miRNAs and suppressor-miRNAs can represent two different looks of the same gene, behaving as oncogenes or tumour suppressors depending on tissue type and specific targets. In this review, the authors analyse the regulatory mechanisms of the main miRNA genes involved in human tumourigenesis.

mRNA/microRNA gene expression profile in microsatellite unstable colorectal cancer

Background

Colorectal cancer develops through two main genetic instability pathways characterized by distinct pathologic features and clinical outcome.

Results

We investigated colon cancer samples (23 characterized by microsatellite stability, MSS, and 16 by high microsatellite instability, MSI-H) for genome-wide expression of microRNA (miRNA) and mRNA. Based on combined miRNA and mRNA gene expression, a molecular signature consisting of twenty seven differentially expressed genes, inclusive of 8 miRNAs, could correctly distinguish MSI-H versus MSS colon cancer samples. Among the differentially expressed miRNAs, various members of the oncogenic miR-17-92 family were significantly up-regulated in MSS cancers. The majority of protein coding genes were also up-regulated in MSS cancers. Their functional classification revealed that they were most frequently associated with cell cycle, DNA replication, recombination, repair, gastrointestinal disease and immune response.

Conclusion

This is the first report that indicates the existence of differences in miRNA expression between MSS versus MSI-H colorectal cancers. In addition, the work suggests that the combination of mRNA/miRNA expression signatures may represent a general approach for improving bio-molecular classification of human cancer.

MicroRNAs in human cancer: from research to therapy

Numerous miRNAs are deregulated in human cancers, and experimental evidence indicates that they can play roles as oncogenes or tumor suppressor genes. Similarly to cancer genes that encode proteins, deregulation of miRNA-encoding genes is associated with genetic or epigenetic alterations, such as deletions, amplifications, point mutations and aberrant DNA methylation. The discovery that miRNAs interact with known oncogenes has established further links with molecular pathways implicated in malignant transformation. Finally, miRNAs can be used as diagnostic markers, and their potential as therapeutic molecules has moved miRNAs from the area of basic research to the field of cancer biotechnology.

BH3 profiling identifies three distinct classes of apoptotic blocks to predict response to ABT-737 and conventional chemotherapeutic agents

Cancer cells exhibit many abnormal phenotypes that induce apoptotic signaling via the intrinsic, or mitochondrial, pathway. That cancer cells nonetheless survive implies that they select for blocks in apoptosis. Identifying cancer-specific apoptotic blocks is necessary to rationally target them. Using a panel of 18 lymphoma cell lines, we show that a strategy we have developed, BH3 profiling, can identify apoptotic defects in cancer cells and separate them into three main classes based on position in the apoptotic pathway. BH3 profiling identifies cells that require BCL-2 for survival and predicts sensitivity to the BCL-2 antagonist ABT-737. BCL-2 dependence correlates with high levels of proapoptotic BIM sequestered by BCL-2. Strikingly, BH3 profiling can also predict sensitivity to conventional chemotherapeutic agents like etoposide, vincristine, and adriamycin.

Cyclin G1 is a target of miR-122a, a microRNA frequently down-regulated in human hepatocellular carcinoma

We investigated the role of microRNAs (miRNAs) in the pathogenesis of human hepatocellular carcinoma (HCC). A genome-wide miRNA microarray was used to identify differentially expressed miRNAs in HCCs arisen on cirrhotic livers. Thirty-five miRNAs were identified. Several of these miRNAs were previously found deregulated in other human cancers, such as members of the let-7 family, mir-221, and mir-145. In addition, the hepato-specific miR-122a was found down-regulated in approximately 70% of HCCs and in all HCC-derived cell lines. Microarray data for let-7a, mir-221, and mir-122a were validated by Northern blot and real-time PCR analysis. Understanding the contribution of deregulated miRNAs to cancer requires the identification of gene targets. Here, we show that miR-122a can modulate cyclin G1 expression in HCC-derived cell lines and an inverse correlation between miR-122a and cyclin G1 expression exists in primary liver carcinomas. These results indicate that cyclin G1 is a target of miR-122a and expand our knowledge of the molecular alterations involved in HCC pathogenesis and of the role of miRNAs in human cancer.

p53-mediated activation of miRNA34 candidate tumor-suppressor genes

BACKGROUND

In response to varied cell stress signals, the p53 tumor-suppressor protein activates a multitude of genes encoding proteins with functions in cell-cycle control, DNA repair, senescence, and apoptosis. The role of p53 in transcription of other types of RNAs, such as microRNAs (miRNAs) is essentially unknown.

RESULTS

Using gene-expression analyses, reporter gene assays, and chromatin-immunoprecipitation approaches, we present definitive evidence that the abundance of the three-member miRNA34 family is directly regulated by p53 in cell lines and tissues. Using array-based approaches and algorithm predictions, we define genes likely to be directly regulated by miRNA34, with cell-cycle regulatory genes being the most prominent class. In addition, we provide functional evidence, obtained via antisense oligonucleotide transfection and the use of mouse embryonic stem cells with loss of miRNA34a function, that the BCL2 protein is regulated directly by miRNA34. Finally, we demonstrate that the expression of two miRNA34s is dramatically reduced in 6 of 14 (43%) non-small cell lung cancers (NSCLCs) and that the restoration of miRNA34 expression inhibits growth of NSCLC cells.

CONCLUSIONS

Taken together, the data suggest the miRNA34s might be key effectors of p53 tumor-suppressor function, and their inactivation might contribute to certain cancers.

MicroRNAs and haematology: small molecules, big function

MicroRNAs are a recently discovered class of small ( approximately 22nt) endogenously expressed translational-repressor RNAs that play key roles in many cellular pathways and whose aberrant expression appears to be a common feature of malignancy. MicroRNAs are expressed in specific haematological cell types and play important regulatory roles in early haematopoietic differentiation, erythropoiesis, granulocytosis, megakaryocytosis and lymphoid development. Additionally, there is an emerging body of research to suggest that microRNAs play an important role in the pathology of haematological malignancies. MicroRNAs have been found to act as both tumour suppressor molecules [e.g. MIRN15A (miR-15a), MIRN16-1 (miR-16-1)] in leukaemias and have oncogenic properties [e.g. MIRN155 (miR-155) and MIRN17-92 (miR-17-miR-92) cluster] in lymphomas. This review discusses the rapidly accumulating research that points to the major role microRNAs play in both haematopoiesis and haematological malignancy.

Downregulation of death-associated protein kinase 1 (DAPK1) in chronic lymphocytic leukemia

The heritability of B cell chronic lymphocytic leukemia (CLL) is relatively high; however, no predisposing mutation has been convincingly identified. We show that loss or reduced expression of death-associated protein kinase 1 (DAPK1) underlies cases of heritable predisposition to CLL and the majority of sporadic CLL. Epigenetic silencing of DAPK1 by promoter methylation occurs in almost all sporadic CLL cases. Furthermore, we defined a disease haplotype, which segregates with the CLL phenotype in a large family. DAPK1 expression of the CLL allele is downregulated by 75% in germline cells due to increased HOXB7 binding. In the blood cells from affected family members, promoter methylation results in additional loss of DAPK1 expression. Thus, reduced expression of DAPK1 can result from germline predisposition, as well as epigenetic or somatic events causing or contributing to the CLL phenotype.

Expression profiling in vivo demonstrates rapid changes in lung microRNA levels following lipopolysaccharide-induced inflammation but not in the anti-inflammatory action of glucocorticoids

BACKGROUND

At present, nothing is known of the role of miRNAs in the immune response in vivo despite the fact that inflammation is thought to underlie multiple acute and chronic diseases. In these circumstances, patients are commonly treated with corticosteroids such as dexamethasone.

RESULTS

To address this question, we have examined the differential expression of 104 miRNAs using real-time PCR during the innate immune response in mouse lung following exposure to aerosilised lipopolysaccharide (LPS). Following challenge, we observed rapid and transient increase in both the mean (4.3-fold) and individual levels of miRNA expression (46 miRNAs) which peaked at 3 hrs. Crucially, this increase was correlated with a reduction in the expression of tumour necrosis factor (TNF)-alpha, keratinocyte-derived chemokine (KC) and macrophage inflammatory protein (MIP)-2, suggesting a potential role for miRNAs in the regulation of inflammatory cytokine production. Examination of the individual miRNA expression profiles showed time dependent increases in miR-21, -25, -27b, -100, 140, -142-3p, -181c, 187, -194, -214, -223 and -224. Corticosteroid studies showed that pre-treatment with dexamethasone at concentrations that inhibited TNF-alpha production, had no effect either alone or upon the LPS-induced miRNA expression profile.

CONCLUSION

We have shown that the LPS-induced innate immune response is associated with widespread, rapid and transient increases in miRNA expression in the mouse lung and we speculate that these changes might be involved in the regulation of the inflammatory response. In contrast, the lack of effect of dexamethasone in either control or challenged animals implies that the actions of glucocorticoids per se are not mediated through changes in miRNAs expression and that LPS-induced increases in miRNA expression are not mediated via classical inflammatory transcription factors.

MicroRNA Expression Profiling in Prostate Cancer

MicroRNAs (miRNA) are small, endogenously expressed noncoding RNAs that negatively regulate expression of protein-coding genes at the translational level. Accumulating evidence, such as aberrant expression of miRNAs, suggests that they are involved in the development of cancer. They have been identified in various tumor types, showing that different sets of miRNAs are usually deregulated in different cancers. To identify the miRNA signature specific for prostate cancer, miRNA expression profiling of 6 prostate cancer cell lines, 9 prostate cancer xenografts samples, 4 benign prostatic hyperplasia (BPH), and 9 prostate carcinoma samples was carried out by using an oligonucleotide array hybridization method. Differential expression of 51 individual miRNAs between benign tumors and carcinoma tumors was detected, 37 of them showing down-regulation and 14 up-regulation in carcinoma samples, thus identifying those miRNAs that could be significant in prostate cancer development and/or growth. There was a significant trend (P=0.029) between the expression of miRNAs and miRNA locus copy number determined by array comparative genomic hybridization, indicating that genetic aberrations may target miRNAs. Hierarchical clustering of the tumor samples by their miRNA expression accurately separated the carcinomas from the BPH samples and also further classified the carcinoma tumors according to their androgen dependence (hormone naive versus hormone refractory), indicating the potential of miRNAs as a novel diagnostic and prognostic tool for prostate cancer.

MicroRNA and brain tumors: a cause and a cure?

Malignant brain tumors, including high-grade gliomas, are among the most lethal of all cancers. Despite considerable advances, including multi-modal treatments with surgery, radiotherapy, and chemotherapy, the overall prognosis remains dismal for patients diagnosed with these tumors. With the discovery of RNA interference (RNAi) for target-specific gene silencing via small interfering RNA (siRNA), a novel method to target malignant gliomas has been exposed, an endeavor that is aggressively being carried out in numerous laboratories. However, practical difficulties in tissue- or organ-specific targeting of therapeutic quantities of siRNA still preclude its applicability in a clinical setting. MicroRNA (miRNA), an endogenously expressed form of siRNA, not only presents an alternate method to induce RNAi in a given diseased tissue or organ, but also exposes a unique set of diagnostic markers that can be used to identify, and then differentiate between tumor grades. Thus, miRNA can be considered the cells' answer to siRNA. Discovered over a decade ago, miRNA is fast becoming recognized as crucial in regulating gene expression in cancers. Therein lies the therapeutic potential of miRNA, as it may now be possible to induce or inhibit RNAi in a given diseased cell population by controlling the cells' miRNA expression profile. This review outlines the potential of miRNA as a therapeutic strategy against high-grade gliomas, and also the technological hurdles that need to be addressed before this promising technique can be administered in a clinical setting.

MicroRNA-143 and -145 in colon cancer

MicroRNAs (miRNAs) are endogenous, small non-coding RNAs (20-22 nucleotides) that negatively regulate gene expression at the translational level by base pairing to the 3' untranslated region of target messenger RNAs. More than 400 miRNAs have been identified in humans and are evolutionally conserved from plants to animals. It has been revealed that miRNAs regulate various biological processes, such as development, cell differentiation, cell proliferation, and cell death. It is predicted that 30% of protein-encoding genes are regulated by miRNAs. Inappropriate expression of miRNAs has been found in cancer. Especially, the expression level of miRNAs that act like anti-oncogenes is frequently reduced in cancers because of chromosome aberrations. In addition, since the processing of miRNAs has been characterized to be enzymatic in nature, the expression levels of miRNAs are closely associated with the activity and levels of such enzymes. In this review, we discuss recent remarkable advances in miRNA biogenesis, bio-networking involving miRNAs, and their roles in carcinogenesis. Further, we discuss the expression of miRNA-143 and -145 in colon cancer and their roles in carcinogenesis. The available data suggest that miRNAs would be potentially useful as diagnostic and therapeutic tools.

miRNA profiling for diagnosis and prognosis of human cancer

MicroRNAs (miRNAs) are a recently discovered class of small (approximately 18-24 nt) nucleic acids that negatively regulate gene expression. This novel class of molecules modulates a wide array of growth and differentiation processes in human cancers. High throughput analyses, utilizing the solid phase, array platform, or liquid phase, bead-based hybridization have variously demonstrated that miRNA expression was commonly dysregulated in human cancer. miRNA expression profiling has shown promise in defining malignant status in retrospective studies. Considerable disagreement remains with respect to the miRNA signature for a specific cancer cell type, which appears to depend largely on the analytical platform. Nonetheless, various internally controlled studies have successfully identified the histotype of tumors of unknown origin according to miRNA expression profile. The evaluation of miRNAs expression may also be of prognostic value, as best exemplified by the correlation of let-7 and mir-155 levels with disease survival in nonsmall cell lung cancer.

Unique MicroRNA signature and clinical outcome of cancers

MicroRNAs (miRNAs) are endogenous small non-protein-coding RNAs (18-25 nucleotides) that conduct the post-transcriptional repression of hundreds of their target proteins. They regulate many kinds of basic cellular processes including proliferation, differentiation, stress response, and cell death. Recent evidence has shown that deregulation of miRNAs correlates with certain features of diverse cancers (such as tumorigenesis, differentiation status, and outcome of tumor patients), and indicates that miRNAs can act as oncogenes or tumor suppressors. Here, we summarize these recent studies and provide a new perspective on understanding the role of miRNAs in initiation and progression of cancers. We also discuss the future applications of miRNAs in diagnostic and prognostic implications of cancers.

The role of microRNAs in cancer: no small matter

MicroRNAs are a recently discovered class of small, evolutionarily conserved, RNA molecules that negatively regulate gene expression at the post-transcriptional level. Mature microRNAs of approximately 20-22 nucleotides are formed from longer primary transcripts by two sequential processing steps mediated by a nuclear (Drosha) and a cytoplasmic (Dicer) RNAse III endonuclease. In the context of a protein complex, the RNA-induced silencing complex (RISC), microRNAs base-pair with target messenger RNA sequences causing translational repression and/or messenger RNA degradation. MicroRNAs have been implicated in the control of many fundamental cellular and physiological processes such as tissue development, cellular differentiation and proliferation, metabolic and signalling pathways, apoptosis and stem cell maintenance. Mounting evidence indicates that microRNAs also play a significant role in cellular transformation and carcinogenesis acting either as oncogenes or tumour suppressors. This review briefly introduces microRNAs in a historical perspective and focuses on the biogenesis of microRNAs, their mode of action, mammalian microRNA functions with emphasis on their involvement in disease - particularly cancer - and their potential therapeutic use.

Drug discovery using the regulation of gene expression

The expression of a disease-relevant protein is controlled by a transcriptional program specifically regulated at all stages of normal development and during the adult life. Thus, regulation of gene expression as an approach to drug discovery is conceptually appealing because it provides a rational basis for molecular strategies aimed at modulating gene expression in given cell types and/or at a given time. Indeed, numerous pharmacologic agents have been identified that can either restore or suppress disease-relevant protein expression. In this review, the author critically examines new strategies and methodologies that are being used and developed to identify and validate new therapeutic targets by taking advantage of our knowledge on mechanisms regulating their expression at the transcriptional and post-transcriptional levels. The author also examines the impact of genome-wide approaches and methods aimed at controlling epigenetic mechanisms of gene regulation and concludes by extrapolating on future trends.

Dmp1 and tumor suppression

Dmp1 (cyclin D binding myb-like protein 1; also called Dmtf1) is a transcription factor that was isolated in a yeast two-hybrid screen through its binding property to cyclin D2. Although it was initially predicted to be involved in the cyclin D-Rb pathway, overexpression of Dmp1 in primary cells induces cell cycle arrest in an Arf, p53-dependent fashion. Dmp1 is a unique Arf regulator, the promoter of which is activated by oncogenic Ras-Raf signaling. Dmp1 expression is repressed by physiological mitogenic stimuli as well as by overexpressed E2F proteins; thus, it is a novel marker of cells that have exited from the cell cycle. Spontaneous and oncogene-induced tumor formation is accelerated in both Dmp1(+/-) and Dmp1(-/-) mice; the Dmp1(+/-) tumors often retain and express the wild-type allele; thus, Dmp1 is haplo-insufficient for tumor suppression. Tumors from Dmp1(+/-) and Dmp1(-/-) mice often retain wild-type Arf and p53, suggesting that Dmp1 is a physiological regulator of the Arf-p53 pathway. The human DMP1 (hDMP1) gene is located on chromosome 7q21, the locus of which is often deleted in myeloid leukemia and also in some types of solid tumors. Post-translational modification of Dmp1 and its role in human malignancy remain to be investigated.

The 13q and 11q B-cell chronic lymphocytic leukaemia-associated regions derive from a common ancestral region in the zebrafish

Loss of the long arm of chromosomes 11 and 13 is the most consistent cytogenetic abnormalities for patients with B-cell chronic lymphocytic leukaemia (B-CLL). They suggest the presence of as yet unidentified tumour suppressor genes within well-defined minimal-deleted regions (MinDRs). We have identified 38 orthologues of the human genes in MinDRs in zebrafish cDNA and syntenic regions for the human deletions in the zebrafish genome. One region on chromosome 9 in the zebrafish genome is of potential interest. Within chromosome 9, five genes and two microRNAs were identified with shared synteny to the MinDRs in B-CLL (two genes to human chromosome 11, three to human chromosome 13 and two chromosome 13 microRNAs). The critical region on zebrafish chromosome 9 maps to the MinDR for both human chromosomes, suggesting a common ancestry for B-CLL tumour suppressor genes. Target-selected mutagenesis to identify zebrafish mutants with knock-outs of genes in this region will allow analysis of their in vivo potential for lymphoproliferation and may define causative genes for B-CLL within human chromosomes 11q and 13q. Our study provides an explanation for involvement of both 11q and 13q in B-CLL and the potential to develop animal models for this common lymphoproliferative disorder.

Transforming Growth Factor-β and microRNA:mRNA Regulatory Networks in Epithelial Plasticity

Noncoding microRNAs act as posttranscriptional repressors of gene function and are often deregulated in cancers and other diseases. Here we review recent findings on microRNA roles in tumorigenesis and report a microRNA profiling screen in transforming growth factor-beta1 (TGF-beta)-induced epithelial-mesenchymal transition (EMT) in human keratinocytes, a model of epithelial cell plasticity underlying epidermal injury and skin carcinogenesis. We describe a novel EMT-specific microRNA signature that includes induction of miR-21, a candidate oncogenic microRNA associated with carcinogenesis. By integrating the microRNA screen results with target prediction algorithms and gene expression profiling data, we outline a framework for TGF-beta-directed microRNA:messenger RNA (mRNA) regulatory circuitry and discuss its biological relevance for tumor progression.

Retroviral insertional activation of the Fli-3 locus in erythroleukemias encoding a cluster of microRNAs that convert Epo-induced differentiation to proliferation

MicroRNAs (miRNAs) are a newly discovered class of posttranscriptional regulatory noncoding small RNAs. Recent evidence has shown that miRNA misexpression correlates with progression of various human cancers. Friend erythroleukemia has been used as an excellent system for the identification and characterization of oncogenes and tumor suppressor genes involved in neoplastic transformation. Using this model, we have isolated a novel integration site designated Fli-3, from a Friend murine leukemia virus (F-MuLV)-induced erythroleukemia. The Fli-3 transcription unit is a murine homologue of the human gene C13orf25 that includes a region encoding the mir-17-92 miRNA cluster. C13orf25 is the target gene of 13q31 chromosomal amplification in human B-cell lymphomas and other malignancies. The erythroleukemias that have acquired either insertional activation or amplification of Fli-3 express higher levels of the primary or mature miRNAs derived from mir-17-92. The ectopic expression of Fli-3 in an erythroblastic cell line switches erythropoietin (Epo)-induced differentiation to Epo-induced proliferation through activation of the Ras and PI3K pathways. Such a response is associated with alteration in the expression of several regulatory factors, such as Spi-1 and p27 (Kip1). These findings highlight the potential of the Fli-3 encoding mir-17-92 in the development of erythroleukemia and its important role in hematopoiesis.

MicroRNAs and regenerative medicine

MicroRNAs, identified only relatively recently, are regulators of gene expression with potential medical benefits. The combination of microRNAs and regenerative medicine is an emerging interdisciplinary medical field that can yield exciting new possibilities for clinical medicine. In this paper, we review the prospects of microRNAs as future therapies in regenerative medicine. Recently, researchers have demonstrated the crucial roles of microRNAs, not only in the differentiation and proliferation of stem cells, which have a key function in the regeneration and transplantation of organs, but also in oncogenesis. Several lines of indirect evidence show that the initiation and maintenance of cancer stem cells might also be under the control of microRNAs. Further, microRNAs have been indicated to be involved in diverse biological processes, suggesting the potential role of these molecules in the treatment of diseases.

MicroRNAs in search of a target

As the number of known microRNAs (miRNAs) increases, and their importance in physiology and disease becomes apparent, the identification of their regulatory targets is a requisite for a full characterization of their biological functions. Computational methods based on sequence homology and phylogenetic conservation have spearheaded this effort in the last 3 years, but they may not be sufficient. Experimental studies are now needed to extend and validate the computational predictions and further our understanding of target recognition by miRNAs.