Widespread microRNA repression by Myc contributes to tumorigenesis. Nat Genet. 2008;40:43-50. [PMID: 18066065 DOI: 10.1038/ng.2007.30]

Biological principles of microRNA-mediated regulation: shared themes amid diversity

Regulation of gene activity by microRNAs is critical to myriad aspects of eukaryotic development and physiology. Amidst an extensive regulatory web that is predicted to involve thousands of transcripts, emergent themes are now beginning to illustrate how microRNAs have been incorporated into diverse settings. These include potent inhibition of individual key targets, fine-tuning of target activity, the coordinated regulation of target batteries, and the reversibility of some aspects of microRNA-mediated repression. Such themes may reflect some of the inherent advantages of exploiting microRNA control in biological circuits, and provide insight into the consequences of microRNA dysfunction in disease.

Regulating the regulators: mechanisms controlling the maturation of microRNAs

MicroRNAs (miRNAs) are small noncoding RNAs that control diverse cellular and developmental events through repression of large sets of target mRNAs. Regulated transcription of the genes encoding miRNAs by RNA polymerase II promotes specific expression patterns of individual miRNAs. However, recent studies have established that substantial regulation of mature miRNA accumulation also occurs after transcription. Here, we review the mechanisms of such post-transcriptional regulation, with a particular focus on examples where molecular mechanisms or physiological principles are beginning to emerge. Elucidating these mechanisms will increase our understanding of gene regulation and provide new insights into causes of miRNA misexpression in diseases such as cancer.

MicroRNAs: key players in the immune system, differentiation, tumorigenesis and cell death

Micro (mi)RNAs are small, highly conserved noncoding RNAs that control gene expression post-transcriptionally either via the degradation of target mRNAs or the inhibition of protein translation. Each miRNA is believed to regulate the expression of multiple mRNA targets, and many miRNAs have been linked to the initiation and progression of human cancer. miRNAs control various activities of the immune system and different stages of hematopoietic development, and their misexpression is the cause of various blood malignancies. Certain miRNAs have oncogenic activities, whereas others have the potential to act as tumor suppressors. Because they control fundamental processes such as differentiation, cell growth and cell death, the study of the role of miRNAs in human neoplasms holds great promise for novel forms of therapy. Here, we summarize the role of miRNAs and their targets in contributing to human cancers and their function as regulators of apoptotic pathways and the immune system.

c-Myb oncoprotein is an essential target of the dleu2 tumor suppressor microRNA cluster

The dleu2 tumor suppressor locus encodes two microRNAs, miR-15a and miR-16, which are thought to play an important role in B-cell neoplasms. However, relatively little is known about proteins that regulate or are regulated by this microRNA cluster. Here we demonstrate that the Pax5 oncoprotein downregulates the dleu2 gene and at the same time boosts expression of its own heterodimeric partner c-Myb. Interestingly, c-Myb upregulation occurs primarily at a post-transcriptional level, suggesting that it might be a target for microRNAs such as miR-15a/16. Indeed, miR-15a/16 have predicted binding sites in the c-Myb 3'-UTR and through them diminish protein output in luciferase sensor assays. Moreover, forced overexpression of miR-15a/16 reduces endogenous c-Myb levels and compromises Pax5 function. Conversely, restoration of c-Myb levels partly alleviates tumors suppressive effects of miR-15a/16, suggesting that c-Myb is a key downstream target of this microRNA cluster.

NF-kappaB-YY1-miR-29 regulatory circuitry in skeletal myogenesis and rhabdomyosarcoma

Studies support the importance of microRNAs in physiological and pathological processes. Here we describe the regulation and function of miR-29 in myogenesis and rhabdomyosarcoma (RMS). Results demonstrate that in myoblasts, miR-29 is repressed by NF-kappaB acting through YY1 and the Polycomb group. During myogenesis, NF-kappaB and YY1 downregulation causes derepression of miR-29, which in turn accelerates differentiation by targeting its repressor YY1. However, in RMS cells and primary tumors that possess impaired differentiation, miR-29 is epigenetically silenced by an activated NF-kappaB-YY1 pathway. Reconstitution of miR-29 in RMS in mice inhibits tumor growth and stimulates differentiation, suggesting that miR-29 acts as a tumor suppressor through its promyogenic function. Together, these results identify a NF-kappaB-YY1-miR-29 regulatory circuit whose disruption may contribute to RMS.

MicroRNA signature and regulatory functions in the endometrium during normal and disease states

During the menstrual cycle, human endometrium undergoes extensive cyclic morphologic and biochemical modifications in preparation for embryo implantation. These processes are highly regulated by ovarian steroids and various locally expressed gene products and involve inflammatory reaction, apoptosis, cell proliferation, angiogenesis, differentiation (tissue formation), and tissue remodeling. MicroRNAs (miRNAs) have emerged as key regulators of gene expression, and their altered and/or aberrant expression has been associated with establishment and progression of various disorders, including tumorigenesis. This review highlights the endometrial expression of miRNAs and their potential regulatory functions under normal and pathologic conditions such as endometriosis, dysfunctional uterine bleeding, and endometrial cancer. Given the key regulatory function of miRNAs on gene expression stability, understanding the underlying mechanisms of how endometrial miRNAs are regulated and identifying their specific target genes and their functions might lead to the development of preventive and therapeutic strategies by regulating specific target genes associated with such reproductive disorders.

An analysis of human microRNA and disease associations

It has been reported that increasingly microRNAs are associated with diseases. However, the patterns among the microRNA-disease associations remain largely unclear. In this study, in order to dissect the patterns of microRNA-disease associations, we performed a comprehensive analysis to the human microRNA-disease association data, which is manually collected from publications. We built a human microRNA associated disease network. Interestingly, microRNAs tend to show similar or different dysfunctional evidences for the similar or different disease clusters, respectively. A negative correlation between the tissue-specificity of a microRNA and the number of diseases it associated was uncovered. Furthermore, we observed an association between microRNA conservation and disease. Finally, we uncovered that microRNAs associated with the same disease tend to emerge as predefined microRNA groups. These findings can not only provide help in understanding the associations between microRNAs and human diseases but also suggest a new way to identify novel disease-associated microRNAs.

MicroRNAs: new regulators of immune cell development and function

Decades of research went into understanding immune cell development and function without awareness that consideration of a key element, microRNA (miRNA), was lacking. The discovery of miRNAs as regulators of developmental events in model organisms suggested to many investigators that miRNA might be involved in the immune system. In the past few years, widespread examination of this possibility has produced notable results. Results have shown that miRNAs affect mammalian immune cell differentiation, the outcome of immune responses to infection and the development of diseases of immunological origin. Some miRNAs repress expression of target proteins with well established functions in hematopoiesis. Here we bring together much of this work, which has so far only scratched the surface of this very fertile field of investigation, and show how the results illuminate many historic questions about hematopoiesis and immune function.

A quantitative PCR method to detect blood microRNAs associated with tumorigenesis in transgenic mice

MicroRNA (miRNA) dysregulation frequently occurs in cancer. Analysis of whole blood miRNA in tumor models has not been widely reported, but could potentially lead to novel assays for early detection and monitoring of cancer. To determine whether miRNAs associated with malignancy could be detected in the peripheral blood, we used real-time reverse transcriptase-PCR to determine miRNA profiles in whole blood obtained from transgenic mice with c-MYC-induced lymphoma, hepatocellular carcinoma and osteosarcoma. The PCR-based assays used in our studies require only 10 nanograms of total RNA, allowing serial mini-profiles (20 - 30 miRNAs) to be carried out on individual animals over time. Blood miRNAs were measured from mice at different stages of MYC-induced lymphomagenesis and regression. Unsupervised hierarchical clustering of the data identified specific miRNA expression profiles that correlated with tumor type and stage. The miRNAs found to be altered in the blood of mice with tumors frequently reverted to normal levels upon tumor regression. Our results suggest that specific changes in blood miRNA can be detected during tumorigenesis and tumor regression.

The FLYWCH transcription factors FLH-1, FLH-2, and FLH-3 repress embryonic expression of microRNA genes in C. elegans

MicroRNAs (miRNAs) are small noncoding RNAs that regulate gene expression post-transcriptionally via antisense base-pairing. Although miRNAs are involved in a variety of important biological functions, little is known about their transcriptional regulation. Using yeast one-hybrid assays, we identified transcription factors with a FLYWCH Zn-finger DNA-binding domain that bind to the promoters of several Caenorhabditis elegans miRNA genes. The products of the flh-1 and flh-2 genes function redundantly to repress embryonic expression of lin-4, mir-48, and mir-241, miRNA genes that are normally expressed only post-embryonically. Although single mutations in either flh-1 or flh-2 genes result in a viable phenotype, double mutation of flh-1 and flh-2 results in early larval lethality and an enhanced derepression of their target miRNAs in embryos. Double mutations in flh-2 and a third FLYWCH Zn-finger-containing transcription factor, flh-3, also result in enhanced precocious expression of target miRNAs. Mutations of lin-4 or mir-48&mir-241 do not rescue the lethal flh-1; flh-2 double-mutant phenotype, suggesting that the inviability is not solely the result of precocious expression of these miRNAs. Therefore, the FLH-1 and FLH-2 proteins likely play a more general role in regulating gene expression in embryos.

A common microRNA signature in mouse models of retinal degeneration

Perturbed microRNA (miR) expression is a feature of, and may play a fundamental role in, certain disease states such as different forms of cancer. Retinitis pigmentosa (RP) a group of inherited retinal degenerations is characterised by a progressive loss of photoreceptor cells and consequent visual handicap. We have previously reported an altered pan-retinal expression of miR-96, -183, -1 and -133 in a P347S-Rhodopsin transgenic mouse model of RP. As many different mutations in Rhodopsin and other genes such as RDS/Peripherin can lead to RP, it was of interest to explore whether the characterized retinal miR expression signature was observed in three other mouse models of RP linked to rhodopsin and rds/peripherin. Therefore, pan-retinal expression of miR-96, -182, -183, -1, -133 and -142 was analysed using quantitative real-time RT-PCR. A common signature of altered miR expression was found; expression of miR-96, -182 and -183 decreased by 14.1-53.2%, while expression of miR-1, -133 and -142 was up-regulated by 186.1-538.5%. Significantly, the detected pan-retinal miR signature was mirrored by similar miR expression profiles in FACS-isolated rod photoreceptors from these mice. In an attempt to understand the function of these miRs, corresponding target genes were predicted using computational means. Many 'enriched' targets (with binding sites for at least two of the above miRs) were found to be regulatory molecules and members of intracellular signalling circuits. However, further studies are required to highlight which of the large number of in silico predicted targets are actually controlled by these miRs.

Expression profile of microRNAs in c-Myc induced mouse mammary tumors

c-Myc is a transcription factor overexpression of which induces mammary cancer in transgenic mice. To explore whether certain microRNAs (mirRNA) mediate c-Myc induced mammary carcinogenesis, we studied mirRNA expression profile in mammary tumors developed from MMTV-c-myc transgenic mice, and found 50 and 59 mirRNAs showing increased and decreased expression, respectively, compared with lactating mammary glands of wild type mice. Twenty-four of these mirRNAs could be grouped into eight clusters because they had the same chromosomal localizations and might be processed from the same primary RNA transcripts. The increased expression of mir-20a, mir-20b, and mir-9 as well as decreased expression of mir-222 were verified by RT-PCR, real-time RT-PCR, and cDNA sequencing. Moreover, we fortuitously identified a novel non-coding RNA, the level of which was decreased in proliferating mammary glands of MMTV-c-myc mice was further decreased to undetectable level in the mammary tumors. Sequencing of this novel RNA revealed that it was transcribed from a region of mouse chromosome 19 that harbored the metastasis associated lung adenocarcinoma transcript-1 (Malat-1), a non-protein-coding gene. These results suggest that certain mirRNAs and the chromosome 19 derived non-coding RNAs may mediate c-myc induced mammary carcinogenesis.

Role of microRNAs in vascular diseases, inflammation, and angiogenesis

The integrity of the endothelial monolayer is fundamental for the homoeostasis of the vascular system. Functional endothelial cells are also required for the growth of new blood vessels during neovascularization. Although multiple growth factors have been shown to regulate angiogenesis and vascular development, little is known about the complex upstream regulation of gene expression and translation. MicroRNAs (miRNAs) are an emerging class of highly conserved, non-coding small RNAs that regulate gene expression on the post-transcriptional level by inhibiting the translation of protein from mRNA or by promoting the degradation of mRNA. More than 500 human miRNAs have been identified so far, and increasing evidence indicates that miRNAs have distinct expression profiles and play crucial roles in various physiological and pathological processes such as cardiogenesis, haematopoietic lineage differentiation, and oncogenesis. Meanwhile, a few specific miRNAs that regulate endothelial cell functions and angiogenesis have been described. Let7-f, miR-27b, and mir-130a were identified as pro-angiogenic miRNAs. In contrast, miR-221 and miR-222 inhibit endothelial cell migration, proliferation, and angiogenesis in vitro by targeting the stem cell factor receptor c-kit and indirectly regulating endothelial nitric oxide synthase expression. Moreover, some miRNAs are involved in tumour angiogenesis such as the miR-17-92 cluster and miR-378. Early studies also indicate the contribution of specific miRNAs (e.g. miR-155, miR-21, and miR-126) to vascular inflammation and diseases. Thus, the identification of miRNAs and their respective targets may offer new therapeutic strategies to treat vascular diseases such as atherosclerosis, to improve neovascularization after ischaemia, or to prevent tumour progression.

MYC stimulates EZH2 expression by repression of its negative regulator miR-26a

The MYC oncogene, which is commonly mutated/amplified in tumors, represents an important regulator of cell growth because of its ability to induce both proliferation and apoptosis. Recent evidence links MYC to altered miRNA expression, thereby suggesting that MYC-regulated miRNAs might contribute to tumorigenesis. To further analyze the impact of MYC-regulated miRNAs, we investigated a murine lymphoma model harboring the MYC transgene in a Tet-off system to control its expression. Microarray-based miRNA expression profiling revealed both known and novel MYC targets. Among the miRNAs repressed by MYC, we identified the potential tumor suppressor miR-26a, which possessed the ability to attenuate proliferation in MYC-dependent cells. Interestingly, miR-26a was also found to be deregulated in primary human Burkitt lymphoma samples, thereby probably being of clinical relevance. Although today only few miRNA targets have been identified in human disease, we could show that ectopic expression of miR-26a influenced cell cycle progression by targeting the bona fide oncogene EZH2, a Polycomb protein and global regulator of gene expression yet unknown to be regulated by miRNAs. Thus, in addition to directly targeting protein-coding genes, MYC modulates genes important to oncogenesis via deregulation of miRNAs, thereby vitally contributing to MYC-induced lymphomagenesis.

Functional integration of microRNAs into oncogenic and tumor suppressor pathways

A large body of evidence has documented abnormal microRNA (miRNA) expression patterns in diverse human malignancies. Given that miRNA expression is tightly regulated during development and cellular differentiation, aberrant miRNA expression in cancer cells is likely to be in part a consequence of the loss of normal cellular identity that accompanies malignant transformation. Nevertheless, it is now clear that miRNAs function as critical effectors of several canonical oncogenic and tumor suppressor pathways, including those controlled by Myc and p53. Gain- and loss-of-function of these factors in cancer cells contributes to miRNA dysregulation, directly influencing neoplastic phenotypes including cellular proliferation and apoptosis.

Transcription-independent functions of MYC: regulation of translation and DNA replication

MYC is a potent oncogene that drives unrestrained cell growth and proliferation. Shortly after its discovery as an oncogene, the MYC protein was recognized as a sequence-specific transcription factor. Since that time, MYC oncogene research has focused on the mechanism of MYC-induced transcription and on the identification of MYC transcriptional target genes. Recently, MYC was shown to control protein expression through mRNA translation and to directly regulate DNA replication, thus initiating exciting new areas of oncogene research.

let-7 regulates Dicer expression and constitutes a negative feedback loop

microRNAs (miRNA) are small, endogenously expressed non-coding RNAs that are sequentially processed by Drosha and Dicer from primary transcripts, by negatively regulating the expression of protein-coding genes through either translational repression or RNA degradation. Their expression patterns are developmentally regulated and/or tissue specific, while altered expressions of certain miRNAs are frequently observed in human cancers, though the underlying regulatory mechanism is largely unknown. Herein, we show that Dicer expression was inversely correlated with expression levels of mature let-7 in a panel of human cancer cell lines, showing association with cell growth and cell cycle phases. Overexpression of let-7 significantly reduced the expression of Dicer at both the protein and messenger RNA levels, whereas antisense-mediated reduction of let-7 expression conversely increased Dicer at both levels. A luciferase assay using a reporter carrying a putative target site in the 3' untranslated region of Dicer revealed that let-7 directly affects Dicer expression. Downregulation of Dicer resulted in a reduced expression of mature let-7. Furthermore, overexpression of let-7 decreased the levels of expression of other mature miRNAs, while knockdown of let-7 increased those levels. Taken together, these findings strongly suggest the possible existence of a novel regulatory loop, in which let-7 may play a role as a key miRNA for implementing the tightly regulated, equilibrated state of Dicer and various miRNAs.

Lin-28 interaction with the Let-7 precursor loop mediates regulated microRNA processing

A hallmark of mammalian embryonic development is the widespread induction of microRNA (miRNA) expression. Surprisingly, the transcription of many of these small, noncoding RNAs is unchanged through development; rather, a post-transcriptional regulatory event prevents accumulation of the mature miRNA species. Here, we present a biochemical framework for the regulated production of the Let-7 family of miRNAs. Embryonic cells contain a Drosha Inhibitor that prevents processing of the Let-7 primary transcript. This inhibitor specifically binds to conserved nucleotides in the loop region of the Let-7 precursor, and competitor RNAs that mimic the binding site restore Let-7 processing. We have identified the Drosha Inhibitor as the embryonic stem cell specific protein Lin-28. Lin-28 has been previously implicated in developmental regulatory pathways in Caenorhabditis elegans, and it promotes reprogramming of human somatic cells into pluripotent stem cells. Our findings outline a microRNA post-transcriptional regulatory network and establish a novel role for the miRNA precursor loop in the regulated production of mature Let-7.

let-7 microRNAs in development, stem cells and cancer

MicroRNAs (miRNAs) are small noncoding RNAs, approximately 22 nucleotides in length, that repress target messenger RNAs (mRNAs) through an antisense mechanism. The let-7 miRNA was originally discovered in the nematode Caenorhabditis elegans, where it regulates cell proliferation and differentiation, but subsequent work has shown that both its sequence and its function are highly conserved in mammals. Recent results have now linked decreased let-7 expression to increased tumorigenicity and poor patient prognosis. Moreover, during normal development, accumulation of let-7 can be prevented by LIN28, a promoter of pluripotency. Based on these findings, we propose that let-7 regulates 'stemness' by repressing self-renewal and promoting differentiation in both normal development and cancer. A more complete understanding of its function will thus provide insights into these processes and might yield diagnostic and therapeutic advances for cancer treatment.

Loss of miRNA biogenesis induces p19Arf-p53 signaling and senescence in primary cells

Dicer, an enzyme involved in microRNA (miRNA) maturation, is required for proper cell differentiation and embryogenesis in mammals. Recent evidence indicates that Dicer and miRNA may also regulate tumorigenesis. To better characterize the role of miRNA in primary cell growth, we generated Dicer-conditional mice. Ablation of Dicer and loss of mature miRNAs in embryonic fibroblasts up-regulated p19^Arf and p53 levels, inhibited cell proliferation, and induced a premature senescence phenotype that was also observed in vivo after Dicer ablation in the developing limb and in adult skin. Furthermore, deletion of the Ink4a/Arf or p53 locus could rescue fibroblasts from premature senescence induced by Dicer ablation. Although levels of Ras and Myc oncoproteins appeared unaltered, loss of Dicer resulted in increased DNA damage and p53 activity in these cells. These results reveal that loss of miRNA biogenesis activates a DNA damage checkpoint, up-regulates p19^Arf-p53 signaling, and induces senescence in primary cells.

Pluripotent human stem cell lines: what we can learn about cancer initiation

Although the cancer stem cell (CSC) hypothesis has become an attractive model to account for tumor recurrence, failure to define a cell of origin has created the need to explore alternative models for cancer initiation and maintenance. Recent studies have linked an embryonic stem cell (ESC)-like gene signature with poorly defined high-grade tumors. Here, we review advances in the ESC field with an emphasis on how human pluripotent stem cells (hPSCs) can be used to define early tumorigenic events, including potential miRNA and epigenetic targets, as well as proto-oncogene and tumor suppressor networks that might facilitate hierarchal transformation. These studies allow for investigation of cancer initiation in a manner that cannot be achieved using primary tumors, where only retrospective evaluation of CSC development is possible. By comparing transformed hPSCs with their normal counterparts, we hope to develop novel cell-specific therapies that selectively target CSCs.

MicroRNA-mediated control of cell fate in megakaryocyte-erythrocyte progenitors

Lineage specification is a critical issue in developmental and regenerative biology. We hypothesized that microRNAs (miRNAs) are important participants in those processes and used the poorly understood regulation of megakaryocyte-erythrocyte progenitors (MEPs) in hematopoiesis as a model system. We report here that miR-150 modulates lineage fate in MEPs. Using a novel methodology capable of profiling miRNA expression in small numbers of primary cells, we identify miR-150 as preferentially expressed in the megakaryocytic lineage. Through gain- and loss-of-function experiments, we demonstrate that miR-150 drives MEP differentiation toward megakaryocytes at the expense of erythroid cells in vitro and in vivo. Moreover, we identify the transcription factor MYB as a critical target of miR-150 in this regulation. These experiments show that miR-150 regulates MEP fate, and thus establish a role for miRNAs in lineage specification of mammalian multipotent cells.

MicroRNAs and prostate cancer

Prostate cancer (CaP) is the most frequently diagnosed malignant tumour and the second leading cause of cancer deaths in American men. One of the most troubling aspects of this disease is that, after androgen ablation therapy, androgen-dependent cancer cells inevitably progress to an androgen-independent status, for which no effective treatment has yet been developed. To date, the mechanisms that underlie the occurrence and progression of CaP remain largely unknown. Recent studies suggest that microRNAs (miRNAs) are involved in human tumourigenesis. Some aberrantly expressed miRNAs have been discovered in CaP cell lines, xenografts and clinical tissues and these CaP-related miRNAs may play critical roles in the pathogenesis of CaP. This review provides an overview of current findings about aberrantly expressed miRNAs in CaP. Although a number of CaP-related miRNAs were discovered, to date, only five are characterized for their functionalities: three as oncogenes and two as tumour suppressors. To understand the mechanisms of miRNA action as oncogenes or tumour suppressors, mRNA targets of miRNAs were characterized. Oncogenic miRNAs down-regulate the expression of apoptosis-related genes, and tumour suppressor miRNAs target the proliferation-related genes. Importantly, there is evidence that CaP-related miRNAs are regulated through androgen signalling and that this regulation may contribute to the development of androgen independence. Due to the oncogenic or tumour-suppressive properties of CaP-related miRNAs, they are highly likely to be of clinical use first as biomarkers but more importantly as therapeutic targets for prostate cancer treatment in the near future.

MicroRNA: basic mechanisms and transcriptional regulatory networks for cell fate determination

Characterization of regulatory mechanisms affecting microRNA (miRNA) expression and activity is providing novel clues for the identification of genes and complex regulatory circuits that determine cell and tissue specificity. Here, we review the molecular events leading to miRNA biogenesis and activity, focusing above all on endogenous and epigenetic transcriptional networks involving miRNA in early development, cellular lineage specification/differentiation of nervous, skeletal and cardiac muscle tissues and in haematopoiesis, as the de-regulation of such networks may be relevant to disease pathogenesis.

Breast cancer metastasis: a microRNA story

MicroRNAs (miRNAs) are small noncoding RNAs with regulatory functions, which play an important role in breast cancer. Several studies have shown that miRNAs can act either as tumor suppressors or as oncogenes, and that measurement of miRNA expression in malignancies may have diagnostic and prognostic implications. This article highlights a series of three recent studies that prove the involvement of miRNAs in breast cancer metastases. The first proves that miR-10b indirectly activates the pro-metastatic gene RHOC by suppressing HOXD10, thus leading to tumor invasion and metastasis. The second proves that miR-373 and miR-520c can also promote tumor invasion and metastasis, at least in part by regulating the gene CD44. The third identifies miR-335, miR-206, and miR-126 as suppressors of breast cancer metastasis. Loss of miR-335 leads to the activation of SOX4 and TNC (encoding tenascin C), which are responsible for the acquisition of metastatic properties. Altogether, these remarkable findings are important for our understanding of malignant transformation in the breast and may have implications for the management of patients with advanced breast cancer. The use of miRNAs as anticancer therapeutic agents is promising, and such fine molecular studies certainly help in bringing miRNAs closer to clinical practice.

The reprogramming language of pluripotency

In metazoans, lineage-specific transcription factors and epigenetic modifiers function to establish and maintain proper gene expression programs during development. Recent landmark studies in both mouse and human have defined a set of transcription factors whose ectopic expression by retroviral transduction is capable of reprogramming a somatic nucleus to the pluripotent state. The identification of factors that are sufficient for the induction of pluripotency suggests that rewiring transcriptional regulatory networks at the molecular level can be used to manipulate cell fate in vitro. These findings have broad implications for understanding development and disease and for the potential use of stem cells in therapeutic applications.