A mammalian microRNA cluster controls DNA methylation and telomere recombination via Rbl2-dependent regulation of DNA methyltransferases

Emerging roles of microRNAs as molecular switches in the integrated circuit of the cancer cell

Transformation of normal cells into malignant tumors requires the acquisition of six hallmark traits, e.g., self-sufficiency in growth signals, insensitivity to antigrowth signals and self-renewal, evasion of apoptosis, limitless replication potential, angiogenesis, invasion, and metastasis, which are common to all cancers (Hanahan and Weinberg 2000). These new cellular traits evolve from defects in major regulatory microcircuits that are fundamental for normal homeostasis. The discovery of microRNAs (miRNAs) as a new class of small non-protein-coding RNAs that control gene expression post-transcriptionally by binding to various mRNA targets suggests that these tiny RNA molecules likely act as molecular switches in the extensive regulatory web that involves thousands of transcripts. Most importantly, accumulating evidence suggests that numerous microRNAs are aberrantly expressed in human cancers. In this review, we discuss the emergent roles of microRNAs as switches that function to turn on/off known cellular microcircuits. We outline recent compelling evidence that deregulated microRNA-mediated control of cellular microcircuits cooperates with other well-established regulatory mechanisms to confer the hallmark traits of the cancer cell. Furthermore, these exciting insights into aberrant microRNA control in cancer-associated circuits may be exploited for cancer therapies that will target deregulated miRNA switches.

A 'higher order' of telomere regulation: telomere heterochromatin and telomeric RNAs

Protection of chromosome ends from DNA repair and degradation activities is mediated by specialized protein complexes bound to telomere repeats. Recently, it has become apparent that epigenetic regulation of the telomric chromatin template critically impacts on telomere function and telomere-length homeostasis from yeast to man. Across all species, telomeric repeats as well as the adjacent subtelomeric regions carry features of repressive chromatin. Disruption of this silent chromatin environment results in loss of telomere-length control and increased telomere recombination. In turn, progressive telomere loss reduces chromatin compaction at telomeric and subtelomeric domains. The recent discoveries of telomere chromatin regulation during early mammalian development, as well as during nuclear reprogramming, further highlights a central role of telomere chromatin changes in ontogenesis. In addition, telomeres were recently shown to generate long, non-coding RNAs that remain associated to telomeric chromatin and will provide new insights into the regulation of telomere length and telomere chromatin. In this review, we will discuss the epigenetic regulation of telomeres across species, with special emphasis on mammalian telomeres. We will also discuss the links between epigenetic alterations at mammalian telomeres and telomere-associated diseases.

Noncoding RNAs and Cancer

The eukaryotic complexity involves the expression and regulation of genes via RNA-DNA, RNA-RNA, DNA-protein and RNA-protein interactions. Recently, the role of RNA molecules in the regulation of genes in higher organisms has become more evident, especially with the discovery that about 97% of the transcriptional output in higher organisms are represented as noncoding RNAs: rRNA, snoRNAs, tRNA, transposable elements, 5' and 3' untranslated regions, introns, intergenic regions and microRNAs. MicroRNAs function by negatively regulating gene expression via degradation or translational inhibition of their target mRNAs and thus participate in a wide variety of physiological and pathological cellular processes including: development, cell proliferation, differentiation, and apoptosis pathways. MicroRNA expression profiles in many types of cancers have been identified. Recent reports have revealed that the expression profiles of microRNAs change in various human cancers and appear to function as oncogenes or tumor suppressors. Abnormal microRNA expression has increasingly become a common feature of human cancers. In this review, we summarize the latest progress on the involvement of microRNAs in different types of cancer and their potential use as potential diagnostic and prognostic tumor biomarkers in the future.

Linking DNA methylation and histone modification: patterns and paradigms

Both DNA methylation and histone modification are involved in establishing patterns of gene repression during development. Certain forms of histone methylation cause local formation of heterochromatin, which is readily reversible, whereas DNA methylation leads to stable long-term repression. It has recently become apparent that DNA methylation and histone modification pathways can be dependent on one another, and that this crosstalk can be mediated by biochemical interactions between SET domain histone methyltransferases and DNA methyltransferases. Relationships between DNA methylation and histone modification have implications for understanding normal development as well as somatic cell reprogramming and tumorigenesis.

Cancer DNA methylation: molecular mechanisms and clinical implications

DNA methylation plays a crucial role in the regulation of gene expression and chromatin organization within normal eukaryotic cells. In cancer, however, global patterns of DNA methylation are altered with global hypomethylation of repeat-rich intergenic regions and hypermethylation of a subset of CpG-dense gene-associated regions (CpG islands). Extensive research has revealed the cellular machinery that catalyzes DNA methylation, as well as several large protein complexes that mediate the transcriptional repression of hypermethylated genes. However, research is only just beginning to uncover the molecular mechanisms underlying the origins of cancer-specific DNA methylation. Herein, we present several recent advances regarding these mechanisms and discuss the relationship between histone modifications (i.e., H3K4me2/3, H4K16Ac, H3K9me2/3, H3K27me3, H4K20me3), chromatin-modifying enzymes (G9a, EZH2, hMOF, SUV4-20H), and aberrant DNA methylation. Additionally, the role played by inflammation, DNA damage, and miRNAs in the etiology of aberrant DNA methylation is considered. Finally, we discuss the clinical implications of aberrant DNA methylation and the utility of methylated biomarkers in cancer diagnosis and management.

MicroRNAs: novel regulators in the hallmarks of human cancer

MicroRNAs (miRNAs) are small non-coding RNAs of 18-25 nucleotides in length that function as negative regulators. miRNAs post-transcriptionally regulate gene expression by either inhibiting mRNA translation or inducing mRNA degradation, and participate in a wide variety of physiological and pathological cellular processes. Recent reports have revealed that the deregulation of miRNAs correlates with various human cancers and is involved in the initiation and progression of human cancers. miRNAs can act as oncogenes or tumor suppressors to inhibit the expression of cancer-related target genes and to promote or suppress tumorigenesis in various tissues. Therefore, abnormal miRNA expression can be regarded as a common feature of human cancers, and the identification of miRNAs and their respective targets may provide potential diagnostic and prognostic tumor biomarkers and new therapeutic strategies to treat cancers. In the present review, we discuss the emerging roles of miRNAs in the hallmarks of human cancers.

Embryonic stem cell-specific microRNAs promote induced pluripotency

This report demonstrates that introduction of microRNAs (miRNAs) specific to embryonic stem cells enhances the production of mouse induced pluripotent stem (iPS) cells. The miRNAs miR-291-3p, miR-294 and miR-295 increase the efficiency of reprogramming by Oct4, Sox2 and Klf4, but not by these factors plus cMyc. cMyc binds the promoter of the miRNAs, suggesting that they are downstream effectors of cMyc during reprogramming. However, unlike cMyc, the miRNAs induce a homogeneous population of iPS cell colonies.

Epigenetics: heterochromatin meets RNAi

The term epigenetics refers to heritable changes not encoded by DNA. The organization of DNA into chromatin fibers affects gene expression in a heritable manner and is therefore one mechanism of epigenetic inheritance. Large parts of eukaryotic genomes consist of constitutively highly condensed heterochromatin, important for maintaining genome integrity but also for silencing of genes within. Small RNA, together with factors typically associated with RNA interference (RNAi) targets homologous DNA sequences and recruits factors that modify the chromatin, commonly resulting in formation of heterochromatin and silencing of target genes. The scope of this review is to provide an overview of the roles of small RNA and the RNAi components, Dicer, Argonaute and RNA dependent polymerases in epigenetic inheritance via heterochromatin formation, exemplified with pathways from unicellular eukaryotes, plants and animals.

MicroRNA in the ovary and female reproductive tract

Posttranscriptional gene regulation plays a vital role in male and female germ cell function, but our understanding of this regulatory process in somatic cells and its effect on reproductive tissue development and function is not understood. In mammalian cells, microRNA (miRNA) are key posttranscriptional regulators and function by modulating translation or degradation of their target mRNA. Mature miRNA are synthesized through a multi-step process that concludes with the cleavage of stem-loop pre-miRNA by the RNase III enzyme, Dicer1. To determine the extent of miRNA regulation and establish a baseline, miRNA profiling has indicated the presence of large numbers of miRNA within reproductive tissues and cells. Moreover, several studies have indicated that miRNA expression in reproductive tissues varies in response to pituitary and gonadal hormones. To understand the role that miRNA-mediated posttranscriptional gene regulation plays in female reproduction, a global Dicer1 hypomorph mouse and several tissue-specific Dicer1 knockout mice have been studied. Interestingly, when Dicer1 expression is decreased in reproductive tissues or cells, the females are infertile. This review discusses all the work regarding miRNA regulation within the mammalian female reproductive system published to date.

The putative tumor suppressor microRNA-101 modulates the cancer epigenome by repressing the polycomb group protein EZH2

The Polycomb Repressive Complex 2 (PRC2) mediates epigenetic gene silencing by trimethylating histone H3 lysine 27 (H3K27me3) and is known to aberrantly silence tumor suppressor genes in cancer. EZH2, the catalytic subunit of PRC2, enhances tumorigenesis and is commonly overexpressed in several types of cancer. Our microRNA profiling of bladder transitional cell carcinoma (TCC) patient samples revealed that microRNA-101 (miR-101) is down-regulated in TCC, and we showed that miR-101 inhibits cell proliferation and colony formation in TCC cell lines. Furthermore, our results confirm that miR-101 directly represses EZH2 and stable EZH2 knockdowns in TCC cell lines create a similar growth suppressive phenotype. This suggests that abnormal down-regulation of miR-101 could lead to the overexpression of EZH2 frequently seen in cancer. We conclude that miR-101 may be a potent tumor suppressor by altering global chromatin structure through repression of EZH2.

Transcriptional inhibiton of Hoxd4 expression by miRNA-10a in human breast cancer cells

Background

Small noncoding RNAs (ncRNAs), including short interfering RNAs (siRNAs) and microRNAs (miRNAs), can silence genes at the transcriptional, post-transcriptional or translational level [1,2].

Results

Here, we show that microRNA-10a (miR-10a) targets a homologous DNA region in the promoter region of the hoxd4 gene and represses its expression at the transcriptional level. Mutational analysis of the miR-10a sequence revealed that the 3' end of the miRNA sequence is the most critical element for the silencing effect. MicroRNA-10a-induced transcriptional gene inhibition requires the presence of Dicer and Argonautes 1 and 3, and it is related to promoter associated noncoding RNAs. Bisulfite sequencing analysis showed that the reduced hoxd4 expression was accompanied by de novo DNA methylation at the hoxd4 promoter. We further demonstrated that trimethylation of histone 3 lysine 27 (H3K27me3) is involved in the miR-10a-induced hoxd4 transcriptional gene silence.

Conclusion

In conclusion, our results demonstrate that miR-10a can regulate human gene expression in a transcriptional manner, and indicate that endogenous small noncoding RNA-induced control of transcription may be a potential system for expressional regulation in human breast cancer cells.

MicroRNAs: key regulators of stem cells

The hallmark of a stem cell is its ability to self-renew and to produce numerous differentiated cells. This unique property is controlled by dynamic interplays between extrinsic signalling, epigenetic, transcriptional and post-transcriptional regulations. Recent research indicates that microRNAs (miRNAs) have an important role in regulating stem cell self-renewal and differentiation by repressing the translation of selected mRNAs in stem cells and differentiating daughter cells. Such a role has been shown in embryonic stem cells, germline stem cells and various somatic tissue stem cells. These findings reveal a new dimension of gene regulation in controlling stem cell fate and behaviour.

Consistent deregulation of gene expression between human and murine MLL rearrangement leukemias

Important biological and pathologic properties are often conserved across species. Although several mouse leukemia models have been well established, the genes deregulated in both human and murine leukemia cells have not been studied systematically. We performed a serial analysis of gene expression in both human and murine MLL-ELL or MLL-ENL leukemia cells and identified 88 genes that seemed to be significantly deregulated in both types of leukemia cells, including 57 genes not reported previously as being deregulated in MLL-associated leukemias. These changes were validated by quantitative PCR. The most up-regulated genes include several HOX genes (e.g., HOX A5, HOXA9, and HOXA10) and MEIS1, which are the typical hallmark of MLL rearrangement leukemia. The most down-regulated genes include LTF, LCN2, MMP9, S100A8, S100A9, PADI4, TGFBI, and CYBB. Notably, the up-regulated genes are enriched in gene ontology terms, such as gene expression and transcription, whereas the down-regulated genes are enriched in signal transduction and apoptosis. We showed that the CpG islands of the down-regulated genes are hypermethylated. We also showed that seven individual microRNAs (miRNA) from the mir-17-92 cluster, which are overexpressed in human MLL rearrangement leukemias, are also consistently overexpressed in mouse MLL rearrangement leukemia cells. Nineteen possible targets of these miRNAs were identified, and two of them (i.e., APP and RASSF2) were confirmed further by luciferase reporter and mutagenesis assays. The identification and validation of consistent changes of gene expression in human and murine MLL rearrangement leukemias provide important insights into the genetic base for MLL-associated leukemogenesis.

X chromosome inactivation in the absence of Dicer

Dicer is central to the RNA interference (RNAi) pathway, because it is required for processing of double-stranded RNA (dsRNA) precursors into small RNA effector molecules. In principle, any long dsRNA could serve as a substrate for Dicer. The X inactive specific transcript (Xist) is an untranslated RNA that is required for dosage compensation in mammals. It coats and silences 1 of the 2 X chromosomes in female cells and initiates a chromosomewide change in chromatin structure that includes the recruitment of Polycomb proteins, but it is largely unknown how Xist RNA mediates these processes. To investigate a potential link between the RNAi pathway and X inactivation, we generated and analyzed Dicer-deficient embryonic stem (ES) cells. In the absence of Dicer, coating by Xist RNA, initiation of silencing, and recruitment of Polycomb proteins occur normally. Dicer ablation had modest effects on the steady-state levels of spliced Xist RNA. Together our data indicate that the RNAi machinery is not essential for the initiation of X inactivation.

Small RNA-directed heterochromatin formation in the context of development: what flies might learn from fission yeast

A link between the RNAi system and heterochromatin formation has been established in several model organisms including Schizosaccharomyces pombe and Arabidopsis thaliana. However, the data to support a role for small RNAs and the associated machinery in transcriptional gene silencing in animal systems is more tenuous. Using the S. pombe system as a model, we analyze the role of small RNA pathway components and associated small RNAs in regulating transposable elements and potentially directing heterochromatin formation at these elements in Drosophila melanogaster.

Epigenetics of acute lymphocytic leukemia

The term epigenetics refers to the study of a number of biochemical modifications of chromatin that have an impact on gene expression regulation. Aberrant epigenetic lesions, in particular DNA methylation of promoter associated CpG islands, are common in acute lymphocytic leukemia (ALL). Recent data from multiple laboratories indicate that several hundred genes, involving dozens of critical molecular pathways, are epigenetically suppressed in ALL. Because these lesions are potentially reversible, the reactivation of these pathways using, for instance, hypomethylating agents may have therapeutic potential in this disease. Furthermore, the analysis of epigenetic alterations in ALL may allow: (1) identification of subsets of patients with poor prognosis when treated with conventional therapy; (2) development of new techniques to evaluate minimal residual disease; (3) better understanding of the differences between pediatric and adult ALL; and (4) new therapeutic interventions by incorporating agents with hypomethylating activity to conventional chemotherapeutic programs. In this review, we describe the role of epigenetic alterations in ALL from a translational perspective.

Chromatin and the cell cycle meet in Madrid

At the end of June 2008, researchers from diverse fields, ranging from chromatin remodeling to cell cycle control, gathered in Madrid at a Cantoblanco Workshop entitled 'Chromatin at the Nexus of Cell Division and Differentiation'. The work discussed at this meeting, which was co-organized by Crisanto Gutierrez, Ben Scheres and Ueli Grossniklaus, highlighted the emerging connections that exist between cell cycle regulation and chromatin in both animals and plants.

DNA methylation in development and human disease

DNA methylation is a heritable and stable epigenetic mark associated with transcriptional repression. Changes in the patterns and levels of global and regional DNA methylation regulate development and contribute directly to disease states such as cancer. Recent findings provide intriguing insights into the epigenetic crosstalk between DNA methylation, histone modifications, and small interfering RNAs in the control of cell development and carcinogenesis. In this review, we summarize the recent studies in DNA methylation primarily focusing on the interplay between different epigenetic modifications and their potential role in gene silencing in development and disease. Although the molecular mechanisms involved in the epigenetic crosstalk are not fully understood, unraveling their precise regulation is important not only for understanding the underpinnings of cellular development and cancer, but also for the design of clinically relevant and efficient therapeutics using stem cells and anticancer drugs that target tumor initiating cells.

Epigenomics and breast cancer

Breast carcinogenesis involves genetic and epigenetic alterations that cause aberrant gene function. Recent progress in the knowledge of epigenomics has had a profound impact on the understanding of mechanisms leading to breast cancer, and consequently the development of new strategies for diagnosis and treatment of breast cancer. Epigenetic regulation has been known to involve three mutually interacting events--DNA methylation, histone modifications and nucleosomal remodeling. These processes modulate chromatin structure to form euchromatin or heterochromatin, and in turn activate or silence gene expression. Alteration in expression of key genes through aberrant epigenetic regulation in breast cells can lead to initiation, promotion and maintenance of carcinogenesis, and is even implicated in the generation of drug resistance. We currently review known roles of the epigenetic machinery in the development and recurrence of breast cancer. Furthermore, we highlight the significance of epigenetic alterations as predictive biomarkers and as new targets of anticancer therapy.

Embryonic stem cell-specific microRNAs regulate the G1-S transition and promote rapid proliferation

Dgcr8 knockout embryonic stem (ES) cells lack microprocessor activity and hence all canonical microRNAs (miRNAs). These cells proliferate slowly and accumulate in G1 phase of the cell cycle. Here, by screening a comprehensive library of individual miRNAs in the background of the Dgcr8 knockout ES cells, we report that multiple ES cell-specific miRNAs, members of the miR-290 family, rescue the ES cell proliferation defect. Furthermore, rescued cells no longer accumulate in the G1 phase of the cell cycle. These miRNAs function by suppressing several key regulators of the G1-S transition. These results show that post-transcriptional regulation by miRNAs promotes the G1-S transition of the ES cell cycle, enabling rapid proliferation of these cells. Our screening strategy provides an alternative and powerful approach for uncovering the role of individual miRNAs in biological processes, as it overcomes the common problem of redundancy and saturation in the miRNA system.

Dicer regulates Xist promoter methylation in ES cells indirectly through transcriptional control of Dnmt3a

BACKGROUND

X chromosome inactivation is the mechanism used in mammals to achieve dosage compensation of X-linked genes in XX females relative to XY males. Chromosome silencing is triggered in cis by expression of the non-coding RNA Xist. As such, correct regulation of the Xist gene promoter is required to establish appropriate X chromosome activity both in males and females. Studies to date have demonstrated co-transcription of an antisense RNA Tsix and low-level sense transcription prior to onset of X inactivation. The balance of sense and antisense RNA is important in determining the probability that a given Xist allele will be expressed, termed the X inactivation choice, when X inactivation commences.

RESULTS

Here we investigate further the mechanism of Xist promoter regulation. We demonstrate that both sense and antisense transcription modulate Xist promoter DNA methylation in undifferentiated embryonic stem (ES) cells, suggesting a possible mechanistic basis for influencing X chromosome choice. Given the involvement of sense and antisense RNAs in promoter methylation, we investigate a possible role for the RNA interference (RNAi) pathway. We show that the Xist promoter is hypomethylated in ES cells deficient for the essential RNAi enzyme Dicer, but that this effect is probably a secondary consequence of reduced levels of de novo DNA methyltransferases in these cells. Consistent with this we find that Dicer-deficient XY and XX embryos show appropriate Xist expression patterns, indicating that Xist gene regulation has not been perturbed.

CONCLUSION

We conclude that Xist promoter methylation prior to the onset of random X chromosome inactivation is influenced by relative levels of sense and antisense transcription but that this probably occurs independent of the RNAi pathway. We discuss the implications for this data in terms of understanding Xist gene regulation and X chromosome choice in random X chromosome inactivation.

X-chromosome inactivation: the molecular basis of silencing

X-chromosome inactivation occurs randomly for one of the two X chromosomes in female cells during development. Inactivation occurs when RNA transcribed from the Xist gene on the X chromosome from which it is expressed spreads to coat the whole X chromosome. In the first issue of Epigenetics and Chromatin, Nesterova and colleagues investigate the role of the RNA interference pathway enzyme Dicer in DNA methylation of the Xist promoter.

Noncoding RNA in development

Non-protein-coding sequences increasingly dominate the genomes of multicellular organisms as their complexity increases, in contrast to protein-coding genes, which remain relatively static. Most of the mammalian genome and indeed that of all eukaryotes is expressed in a cell- and tissue-specific manner, and there is mounting evidence that much of this transcription is involved in the regulation of differentiation and development. Different classes of small and large noncoding RNAs (ncRNAs) have been shown to regulate almost every level of gene expression, including the activation and repression of homeotic genes and the targeting of chromatin-remodeling complexes. ncRNAs are involved in developmental processes in both simple and complex eukaryotes, and we illustrate this in the latter by focusing on the animal germline, brain, and eye. While most have yet to be systematically studied, the emerging evidence suggests that there is a vast hidden layer of regulatory ncRNAs that constitutes the majority of the genomic programming of multicellular organisms and plays a major role in controlling the epigenetic trajectories that underlie their ontogeny.

Circular reasoning: microRNAs and cell-cycle control

MicroRNAs (miRNAs) have attracted considerable attention because of their important roles in development, normal physiology, and disease states including cancer. Recent studies have identified specific miRNAs that regulate the cell cycle and have documented that the loss or gain of miRNA-mediated cell-cycle control contributes to malignancy. miRNAs regulate classic cell-cycle control pathways by directly targeting proteins such as E2F transcription factors, cyclin-dependent kinases (Cdks), cyclins and Cdk inhibitors. Moreover, from recent findings, it has been suggested that miRNAs themselves might be subject to cell-cycle dependent regulation. Together, these observations indicate that the reciprocal control of RNA silencing and the metazoan cell cycle impacts cellular behavior and disease.

DNA methylomes, histone codes and miRNAs: tying it all together

Our current knowledge of the deregulation that occurs during the onset and progression of cancer and other diseases leads us to recognize both genetic and epigenetic alterations as being at the core of the pathological state. The epigenetic landscape includes a variety of covalent modifications that affect the methylation status of DNA but also the post-translational modifications of histones, and determines the structural features of chromatin that ultimately control the transcriptional outcome of the cell to accommodate developmental, proliferative or environmental requirements. MicroRNAs are small non-coding RNAs that regulate the expression of complementary messenger RNAs and function as key controllers in a myriad of cellular processes, including proliferation, differentiation and apoptosis. In the last few years, increasing evidence has indicated that a substantial number of microRNA genes are subjected to epigenetic alterations, resulting in aberrant patterns of expression upon the occurrence of cancer. In this review we discuss microRNA genes that are epigenetically modified in cancer cells, and the role that microRNAs themselves can have as chromatin modifiers.

RNAi, heterochromatin and the cell cycle

For many decades after its initial characterization, heterochromatin was considered to be transcriptionally inert, but newer work indicates that this highly condensed chromosomal material is transcribed, and rapidly silenced, by an orchestrated sequence of events directed by RNA interference (RNAi). Recent studies shed light on the timely assembly and inheritance of heterochromatin within a short period during the cell cycle, thereby providing an explanation for how 'silent' heterochromatin can be transcribed during the S phase of the cell cycle. Together, these findings suggest a model of RNAi-directed epigenetic inheritance.

Telomeric chromatin: roles in aging, cancer and hereditary disease

Over the last several years there has been an explosion in our understanding of the organization of telomeric chromatin in mammals. As in other regions of the genome, chromatin composition at the telomere regulates structure, which defines function. Mammalian telomeres, similar to what has been demonstrated for telomeres of other eukaryotes, carry marks of heterochromatin and alteration in this underlying epigenetic code has effects on telomere replication and recombination. Experiments aimed at determining links between changes in telomeric chromatin and possible roles in aging and disease are beginning to emerge. The rapid refinement of our knowledge of the structure and alterations in telomeric chromatin over the last several years makes it likely that we are just seeing the tip of the iceberg.

MiRNAs, epigenetics, and cancer

By virtue of having multiple targets, a microRNA (miRNA) can have variable effects on oncogenesis by acting as tumor suppressor or oncogene in a context-dependent manner. Genome-wide epigenetic changes that occur in various cancers affect the transcription of many genes. Since the transcriptional regulation of miRNAs remains an unexplored field, it is still unknown how epigenetic changes will affect the regulation of miRNAs. Many miRNAs are intron-bound within the body of a protein-coding gene. Any change to the transcription of the "host" gene affects the transcription and genesis of the resident miRNA. It is therefore reasonable to deduce that epigenetic changes brought on by transformation can potentially affect miRNA expression in both direct and indirect ways. We have reviewed the literature pertaining to the epigenetic regulation of miRNA genes in the context of various cancers and have speculated on the potential role of epigenetic modifications on the transcriptional regulation and expression of these genes.