DNA Hypomethylation Underlies Epigenetic Swapping between AGO1 and AGO1-V2 Isoforms in Tumors.

Human tumors progress in part by accumulating epigenetic alterations, which include gains and losses of DNA methylation in different parts of the cancer cell genome. Recent work has revealed a link between these two opposite alterations by showing that DNA hypomethylation in tumors can induce the expression of transcripts that overlap downstream gene promoters and thereby induce their hypermethylation. Preliminary in silico evidence prompted us to investigate if this mechanism applies to the locus harboring AGO1, a gene that plays a central role in miRNA biogenesis and RNA interference. Inspection of public RNA-Seq datasets and RT-qPCR experiments show that an alternative transcript starting 13.4 kb upstream of AGO1 (AGO1-V2) is expressed specifically in testicular germ cells, and becomes aberrantly activated in different types of tumors, particularly in tumors of the esophagus, stomach, and lung. This expression pattern classifies AGO1-V2 into the group of "Cancer-Germline" (CG) genes. Analysis of transcriptomic and methylomic datasets provided evidence that transcriptional activation of AGO1-V2 depends on DNA demethylation of its promoter region. Western blot experiments revealed that AGO1-V2 encodes a shortened isoform of AGO1, corresponding to a truncation of 75 aa in the N-terminal domain, and which we therefore referred to as "∆NAGO1". Interestingly, significant correlations between hypomethylation/activation of AGO1-V2 and hypermethylation/repression of AGO1 were observed upon examination of tumor cell lines and tissue datasets. Overall, our study reveals the existence of a process of interdependent epigenetic alterations in the AGO1 locus, which promotes swapping between two AGO1 protein-coding mRNA isoforms in tumors.

Identification of Tissue-Specific Gene Clusters Induced by DNA Demethylation in Lung Adenocarcinoma: More Than Germline Genes.

Genome-wide loss of DNA methylation is commonly observed in human cancers, but its impact on the tumor transcriptome remains ill-defined. Previous studies demonstrated that this epigenetic alteration causes aberrant activation of a germline-specific gene expression program. Here, we examined if DNA hypomethylation in tumors also leads to de-repression of gene clusters with other tissue specificities. To this end, we explored transcriptomic and methylomic datasets from human lung adenocarcinoma (LUAD) cell lines, normal lung, and lung alveolar type II cells, considered as the origin of LUAD. Interestingly, DNA demethylation in LUAD cell lines was associated with activation of not only germline-specific (CG) genes, but also gene clusters displaying specific expression in the gastrointestinal tract (GI), or in stratified epithelia (SE). Consistently, genes from all three clusters showed highly specific patterns of promoter methylation among normal tissues and cell types, and were generally sensitive to induction by a DNA demethylating agent. Analysis of TCGA datasets confirmed that demethylation and activation of CG, GI and SE genes also occurs in vivo in LUAD tumor tissues, in association with global genome hypomethylation. For genes of the GI cluster, we demonstrated that HNF4A is a necessary factor for transcriptional activation following promoter demethylation. Interestingly, expression of several SE genes, in particular FAM83A, correlated with both tumor grade and reduced patient survival. Together, our study uncovers novel cell-type specific gene clusters that become aberrantly activated in LUAD tumors in association with genome-wide hypomethylation.

Transcriptional overlap links DNA hypomethylation with DNA hypermethylation at adjacent promoters in cancer.

Tumor development involves alterations in DNA methylation patterns, which include both gains (hypermethylation) and losses (hypomethylation) in different genomic regions. The mechanisms underlying these two opposite, yet co-existing, alterations in tumors remain unclear. While studying the human MAGEA6/GABRA3 gene locus, we observed that DNA hypomethylation in tumor cells can lead to the activation of a long transcript (CT-GABRA3) that overlaps downstream promoters (GABRQ and GABRA3) and triggers their hypermethylation. Overlapped promoters displayed increases in H3K36me3, a histone mark deposited during transcriptional elongation and known to stimulate de novo DNA methylation. Consistent with such a processive mechanism, increases in H3K36me3 and DNA methylation were observed over the entire region covered by the CT-GABRA3 overlapping transcript. Importantly, experimental induction of CT-GABRA3 by depletion of DNMT1 DNA methyltransferase, resulted in a similar pattern of regional DNA hypermethylation. Bioinformatics analyses in lung cancer datasets identified other genomic loci displaying this process of coupled DNA hypo/hypermethylation, and some of these included tumor suppressor genes, e.g. RERG and PTPRO. Together, our work reveals that focal DNA hypomethylation in tumors can indirectly contribute to hypermethylation of nearby promoters through activation of overlapping transcription, and establishes therefore an unsuspected connection between these two opposite epigenetic alterations.

Therapy-induced DNA methylation inactivates MCT1 and renders tumor cells vulnerable to MCT4 inhibition.

Metabolic plasticity in cancer cells makes use of metabolism-targeting agents very challenging. Drug-induced metabolic rewiring may, however, uncover vulnerabilities that can be exploited. We report that resistance to glycolysis inhibitor 3-bromopyruvate (3-BrPA) arises from DNA methylation in treated cancer cells and subsequent silencing of the monocarboxylate transporter MCT1. We observe that, unexpectedly, 3-BrPA-resistant cancer cells mostly rely on glycolysis to sustain their growth, with MCT4 as an essential player to support lactate flux. This shift makes cancer cells particularly suited to adapt to hypoxic conditions and resist OXPHOS inhibitors and anti-proliferative chemotherapy. In contrast, blockade of MCT4 activity in 3-BrPA-exposed cancer cells with diclofenac or genetic knockout, inhibits growth of derived spheroids and tumors in mice. This study supports a potential mode of collateral lethality according to which metabolic adaptation of tumor cells to a first-line therapy makes them more responsive to a second-line treatment.

PAR-TERRA is the main contributor to telomeric repeat-containing RNA transcripts in normal and cancer mouse cells.

Telomeric repeat-containing RNA (TERRA) molecules play important roles at telomeres, from heterochromatin regulation to telomerase activity control. In human cells, TERRA is transcribed from subtelomeric promoters located on most chromosome ends and associates with telomeres. The origin of mouse TERRA molecules is, however, unclear, as transcription from the pseudoautosomal PAR locus was recently suggested to account for the vast majority of TERRA in embryonic stem cells (ESC). Here, we confirm the production of TERRA from both the chromosome 18q telomere and the PAR locus in mouse embryonic fibroblasts, ESC, and various mouse cancer and immortalized cell lines, and we identify two novel sources of TERRA on mouse chromosome 2 and X. Using various approaches, we show that PAR-TERRA molecules account for the majority of TERRA transcripts, displaying an increase of two to four orders of magnitude compared to the telomeric 18q transcript. Finally, we present a SILAC-based pull-down screen revealing a large overlap between TERRA-interacting proteins in human and mouse cells, including PRC2 complex subunits, chromatin remodeling factors, DNA replication proteins, Aurora kinases, shelterin complex subunits, Bloom helicase, Coilin, and paraspeckle proteins. Hence, despite originating from distinct genomic regions, mouse and human TERRA are likely to play similar functions in cells.

Epigenetic Coactivation of MAGEA6 and CT-GABRA3 Defines Orientation of a Segmental Duplication in the Human X Chromosome.

The human genome harbors many duplicated segments, which sometimes show very high sequence identity. This may complicate assignment during genome assembly. One such example is in Xq28, where the arrangement of 2 recently duplicated segments varies between genome assembly versions. The duplicated segments comprise highly similar genes, including MAGEA3 and MAGEA6, which display specific expression in testicular germline cells, and also become aberrantly activated in a variety of tumors. Recently, a new gene was identified, CT-GABRA3, the transcription of which initiates inside the segmental duplication but extends far outside. According to the latest genome annotation, CT- GABRA3 starts near MAGEA3, with which it shares a bidirectional promoter. In an earlier annotation, however, the duplicated segment was positioned in the opposite orientation, and CT-GABRA3 was instead coupled with MAGEA6. To resolve this discrepancy, and based on the contention that genes connected by a bidirectional promoter are almost always co-expressed, we decided to compare the expression profiles of CT-GABRA3, MAGEA3, and MAGEA6. We found that in tumor tissues and cell lines of different origins, the expression of CT-GABRA3 was better correlated with that of MAGEA6. Moreover, in a cellular model of experimental induction with a DNA demethylation agent, activation CT-GABRA3 was associated with that of MAGEA6, but not with that of MAGEA3. Together these results support a connection between CT-GABRA3 and MAGEA6 and illustrate how promoter-sharing genes can be exploited to resolve genome assembly uncertainties.

Dnmt3a-mediated inhibition of Wnt in cardiac progenitor cells improves differentiation and remote remodeling after infarction.

Adult cardiac progenitor cells (CPCs) display a low capacity to differentiate into cardiomyocytes in injured hearts, strongly limiting the regenerative capacity of the mammalian myocardium. To identify new mechanisms regulating CPC differentiation, we used primary and clonally expanded Sca-1+ CPCs from murine adult hearts in homotypic culture or coculture with cardiomyocytes. Expression kinetics analysis during homotypic culture differentiation showed downregulation of Wnt target genes concomitant with increased expression of the Wnt antagonist, Wnt inhibitory factor 1 (Wif1), which is necessary to stimulate CPC differentiation. We show that the expression of the Wif1 gene is repressed by DNA methylation and regulated by the de novo DNA methyltransferase Dnmt3a. In addition, miR-29a is upregulated early during CPC differentiation and downregulates Dnmt3a expression, thereby decreasing Wif1 gene methylation and increasing the efficiency of differentiation of Sca-1+ CPCs in vitro. Extending these findings in vivo, transient silencing of Dnmt3a in CPCs subsequently injected in the border zone of infarcted mouse hearts improved CPC differentiation in situ and remote cardiac remodeling. In conclusion, miR-29a and Dnmt3a epigenetically regulate CPC differentiation through Wnt inhibition. Remote effects on cardiac remodeling support paracrine signaling beyond the local injection site, with potential therapeutic interest for cardiac repair.

Oncogenic roles of DNA hypomethylation through the activation of cancer-germline genes.

Global loss of DNA methylation is frequently observed in the genome of human tumors. Although this epigenetic alteration is clearly associated with cancer progression, the way it exerts its pro-tumoral effect remains incompletely understood. A remarkable consequence of DNA hypomethylation in tumors is the aberrant activation of "cancer-germline" genes (also known as "cancer-testis" genes), which comprise a diverse group of germline-specific genes that use DNA methylation as a primary mechanism for repression in normal somatic tissues. Here we review the evidence that such cancer-germline genes contribute to key processes of tumor development. Notably, several cancer-germline genes were found to stimulate oncogenic pathways involved in cell proliferation (SSX, DDX43, MAEL, PIWIL1), angiogenesis (DDX53), immortality (BORIS/CTCFL), and metastasis (CT-GABRA3). Others appear to inhibit tumor suppressor pathways, including those controlling growth inhibition signals (MAGEA11, MAGEB2), apoptosis (MAGEA2, MAGEC2), and genome integrity (HORMAD1, NXF2). Cancer-germline genes were also implicated in the regulation of tumor metabolism (MAGEA3/MAGEA6). Together, our survey substantiates the concept that DNA hypomethylation promotes tumorigenesis via transcriptional activation of oncogenes. Importantly, considering their highly restricted pattern of expression, cancer-germline genes may represent valuable targets for the development of anti-cancer therapies with limited side effects.

A gene expression signature identifying transient DNMT1 depletion as a causal factor of cancer-germline gene activation in melanoma.

BACKGROUND: Many human tumors show aberrant activation of a group of germline-specific genes, termed cancer-germline (CG) genes, several of which appear to exert oncogenic functions. Although activation of CG genes in tumors has been linked to promoter DNA demethylation, the mechanisms underlying this epigenetic alteration remain unclear. Two main processes have been proposed: awaking of a gametogenic program directing demethylation of target DNA sequences via specific regulators, or general deficiency of DNA methylation activities resulting from mis-targeting or down-regulation of the DNMT1 methyltransferase. RESULTS: By the analysis of transcriptomic data, we searched to identify gene expression changes associated with CG gene activation in melanoma cells. We found no evidence linking CG gene activation with differential expression of gametogenic regulators. Instead, CG gene activation correlated with decreased expression of a set of mitosis/division-related genes (ICCG genes). Interestingly, a similar gene expression signature was previously associated with depletion of DNMT1. Consistently, analysis of a large set of melanoma tissues revealed that DNMT1 expression levels were often lower in samples showing activation of multiple CG genes. Moreover, by using immortalized melanocytes and fibroblasts carrying an inducible anti-DNMT1 small hairpin RNA (shRNA), we demonstrate that transient depletion of DNMT1 can lead to long-term activation of CG genes and repression of ICCG genes at the same time. For one of the ICCG genes (CDCA7L), we found that its down-regulation in melanoma cells was associated with deposition of repressive chromatin marks, including H3K27me3. CONCLUSIONS: Together, our observations point towards transient DNMT1 depletion as a causal factor of CG gene activation in vivo in melanoma.

A novel cancer-germline transcript carrying pro-metastatic miR-105 and TET-targeting miR-767 induced by DNA hypomethylation in tumors.

Genome hypomethylation is a common epigenetic alteration in human tumors, where it often leads to aberrant activation of a group of germline-specific genes, commonly referred to as "cancer-germline" genes. The cellular functions and tumor promoting potential of these genes remain, however, largely uncertain. Here, we report identification of a novel cancer-germline transcript (CT-GABRA3) displaying DNA hypomethylation-dependent activation in various tumors, including melanoma and lung carcinoma. Importantly, CT-GABRA3 harbors a microRNA (miR-105), which has recently been identified as a promoter of cancer metastasis by its ability to weaken vascular endothelial barriers following exosomal secretion. CT-GABRA3 also carries a microRNA (miR-767) with predicted target sites in TET1 and TET3, two members of the ten-eleven-translocation family of tumor suppressor genes, which are involved in the conversion of 5-methylcytosines to 5-hydroxymethylcytosines (5hmC) in DNA. Decreased TET activity is a hallmark of cancer; here, we provide evidence that aberrant activation of miR-767 contributes to this phenomenon. We demonstrate that miR-767 represses TET1/3 mRNA and protein expression and regulates genomic 5hmC levels. Additionally, we show that high CT-GABRA3 transcription correlates with reduced TET1 mRNA levels in vivo in lung tumors. Together, our study identified a cancer-germline gene that produces microRNAs with oncogenic potential. Moreover, our data indicate that DNA hypomethylation in tumors can contribute to reduced 5hmC levels via activation of a TET-targeting microRNA.

Epigenetic regulations of immediate early genes expression involved in memory formation by the amyloid precursor protein of Alzheimer disease.

We previously demonstrated that APP epigenetically regulates Egr1 expression both in cultured neurons and in vivo. Since Egr1 is an immediate early gene involved in memory formation, we wondered whether other early genes involved in memory were regulated by APP and we studied molecular mechanisms involved. By comparing prefrontal (PF) cortex from wild type (APP+/+) and APP knockout mice (APP-/-), we observed that APP down regulates expression of four immediate early genes, Egr1, c-Fos, Bdnf and Arc. Down regulation of Egr1, c-Fos and Bdnf transcription resulted from a decreased enrichment of acetylated histone H4 on the corresponding gene promoter. Further characterization of H4 acetylation at Egr1 and c-Fos promoters revealed increased acetylation of H4K5 and H4K12 residues in APP-/- mice. Whereas APP affected Egr1 promoter activity by reducing access of the CREB transcription factor, its effect on c-Fos appeared to depend on increased recruitment of HDAC2 histone deacetylase to the gene promoter. The physiological relevance of the epigenetic regulation of Egr1 and c-Fos gene transcription by APP was further analyzed following exposure of mice to novelty. Although transcription of Egr1 and c-Fos was increased following exposure of APP+/+ mice to novelty, such an induction was not possible in APP-/- mice with a high basal level of expression of these immediate early genes. Altogether, these results demonstrate that APP-mediated regulation of c-Fos and Egr1 by different epigenetic mechanisms is needed for their induction during exposure to novelty.

Epigenetic induction of EGR-1 expression by the amyloid precursor protein during exposure to novelty.

Following transcriptome comparison of primary cultures isolated from brain of mice expressing or not the amyloid precursor protein APP, we found transcription of the EGR-1 gene to be regulated by APP. In primary cultures of cortical neurons, APP significantly down regulated EGR-1 expression at both mRNA and protein levels in a γ-secretase independent manner. The intracellular domain of APP did not interact with EGR-1 gene promoter, but enrichment of acetylated histone H4 at the EGR-1 promoter region was measured in APP-/- neurons, as well as in brain of APP-/- mice, in which increase in EGR-1 expression was also measured. These results argue for an important function of APP in the epigenetic regulation of EGR-1 gene transcription both in vitro and in vivo. In APP-/- mice, constitutive overexpression of EGR-1 in brain impaired epigenetic induction of this early transcriptional regulator during exposure to novelty. Altogether, these results indicate an important function of APP in the epigenetic regulation of the transcription of EGR-1, known to be important for memory formation.

Epigenetic hierarchy within the MAGEA1 cancer-germline gene: promoter DNA methylation dictates local histone modifications.

Gene MAGEA1 belongs to a group of human germline-specific genes that rely on DNA methylation for repression in somatic tissues. Many of these genes, termed cancer-germline (CG) genes, become demethylated and activated in a wide variety of tumors, where they encode tumor-specific antigens. The process leading to DNA demethylation of CG genes in tumors remains unclear. Previous data suggested that histone acetylation might be involved. Here, we investigated the relative contribution of DNA methylation and histone acetylation in the epigenetic regulation of gene MAGEA1. We show that MAGEA1 DNA hypomethylation in expressing melanoma cells is indeed correlated with local increases in histone H3 acetylation (H3ac). However, when MAGEA1-negative cells were exposed to a histone deacetylase inhibitor (TSA), we observed only short-term activation of the gene and detected no demethylation of its promoter. As a more sensitive assay, we used a cell clone harboring a methylated MAGEA1/hph construct, which confers resistance to hygromycin upon stable re-activation. TSA induced only transient de-repression of the transgene, and did not lead to the emergence of hygromycin-resistant cells. In striking contrast, transient depletion of DNA-methyltransferase-1 in the reporter cell clone gave rise to a hygromycin-resistant population, in which the re-activated MAGEA1/hph transgene displayed not only marked DNA hypomethylation, but also significant reversal of histone marks, including gains in H3ac and H3K4me2, and losses of H3K9me2. Collectively, our results indicate that DNA methylation has a dominant role in the epigenetic hierarchy governing MAGEA1 expression.

DNA hypomethylation and activation of germline-specific genes in cancer.

DNA methylation, occurring at cytosines in CpG dinucleotides, is a potent mechanism of transcriptional repression. Proper genomic methylation -patterns become profoundly altered in cancer cells: both gains (hypermethylation) and losses (hypomethylation) of methylated sites are observed. Although DNA hypomethylation is detected in a vast majority of human tumors and affects many genomic regions, its role in tumor biology remains elusive. Surprisingly, DNA hypomethylation in cancer was found to cause the aberrant activation of only a limited group of genes. Most of these are normally expressed exclusively in germline cells and were grouped under the term "cancer-germline" (CG) genes. CG genes represent unique examples of genes that rely primarily on DNA methylation for their tissue-specific expression. They are also being exploited to uncover the mechanisms that lead to DNA hypomethylation in tumors. Moreover, as CG genes encode tumor-specific antigens, their activation in cancer highlights a direct link between epigenetic alterations and tumor immunity. As a result, clinical trials combining epigenetic drugs with anti-CG antigen vaccines are being considered.

Cancer-linked satellite 2 DNA hypomethylation does not regulate Sat2 non-coding RNA expression and is initiated by heat shock pathway activation.

Epigenetic dysfunctions, including DNA methylation alterations, play major roles in cancer initiation and progression. Although it is well established that gene promoter demethylation activates transcription, it remains unclear whether hypomethylation of repetitive heterochromatin similarly affects expression of non-coding RNA from these loci. Understanding how repetitive non-coding RNAs are transcriptionally regulated is important given that their established upregulation by the heat shock (HS) pathway suggests important functions in cellular response to stress, possibly by promoting heterochromatin reconstruction. We found that, although pericentromeric satellite 2 (Sat2) DNA hypomethylation is detected in a majority of cancer cell lines of various origins, DNA methylation loss does not constitutively hyperactivate Sat2 expression, and also does not facilitate Sat2 transcriptional induction upon heat shock. In melanoma tumor samples, our analysis revealed that the HS response, frequently upregulated in tumors, is probably the main determinant of Sat2 RNA expression in vivo. Next, we tested whether HS pathway hyperactivation may drive Sat2 demethylation. Strikingly, we found that both hyperthermia and hyperactivated RasV12 oncogene, another potent inducer of the HS pathway, reduced Sat2 methylation levels by up to 27% in human fibroblasts recovering from stress. Demethylation occurred locally on Sat2 repeats, resulting in a demethylation signature that was also detected in cancer cell lines with moderate genome-wide hypomethylation. We therefore propose that upregulation of Sat2 transcription in response to HS pathway hyperactivation during tumorigenesis may promote localized demethylation of the locus. This, in turn, may contribute to tumorigenesis, as demethylation of Sat2 was previously reported to favor chromosomal rearrangements.

Silencing of cancer-germline genes in human preimplantation embryos: evidence for active de novo DNA methylation in stem cells.

Several human germline-specific genes rely principally on DNA methylation for repression in somatic tissues. Many of these genes, including MAGEA1, were qualified as cancer-germline (CG), as they become activated in tumors, where losses of DNA methylation are common. The developmental stage at which CG genes acquire DNA methylation marks is unknown. Here, we show that in human preimplantation embryos, transcription of CG genes increases up to the morula stage, and then decreases dramatically in blastocysts, suggesting that CG gene silencing occurs in blastocyst stem cells. Consistently, transfection studies with MAGEA1 constructs in embryonal carcinoma (EC) cells, which represent a malignant surrogate of blastocyst-derived stem cells, revealed active repression and marked de novo methylation of MAGEA1 transgenes in these cells. Active repression of the endogenous MAGEA1 gene in human EC cells was evidenced by its rapid re-silencing following prior induction with a DNA methylation inhibitor. Moreover, de novo DNA methyltransferases DNMT3A and DNMT3B appeared to contribute to the silencing of MAGEA1 and other CG genes in EC cells. Altogether our data indicate that CG genes like MAGEA1 are programmed for repression in the blastocyst, and suggest that de novo DNA methylation is a key event in this process.

Demethylation of the FOXP3 gene in human melanoma cells precludes the use of this epigenetic mark for quantification of Tregs in unseparated melanoma samples.

The human suppressive T cells that stably express transcription factor FOXP3, or regulatory T cells (Tregs), are thought to suppress antitumor immune responses. The most specific marker for human Tregs is the demethylation of CpG dinucleotides located in the first intron of FOXP3 (FOXP3i1). FOXP3i1 is completely methylated in other hematopoietic cells, including nonsuppressive T cells that transiently express FOXP3 after activation. Previously, we and others reported estimations of the frequency of Tregs in the blood of melanoma patients using a FOXP3i1 methylation-specific qPCR assay. Here, we attempted to quantify Tregs inside tumor samples using this assay. However, we found demethylated FOXP3i1 sequences in the melanoma cells themselves. This demethylation was not associated with substantial FOXP3 mRNA or protein expression, even though the demethylation extended to the promoter and terminal regions of the gene in some melanoma cells. Our results imply that analyzing Treg frequencies by quantification of demethylated FOXP3i1 will require that tumor-infiltrating T cells be separated from melanoma cells.

Aberrant demethylation of the recoverin gene is involved in the aberrant expression of recoverin in cancer cells.

The Ca(2+) -binding protein recoverin is normally specific for the retina. Recoverin aberrantly expressed in lung and melanoma tumors can trigger the host immune response followed by the development of a paraneoplastic neurological syndrome represented by cancer- and melanoma-associated retinopathy, respectively. The mechanisms, underlying the aberrant expression of recoverin in tumor cells, have remained unknown. The data obtained in this study suggest that (i) DNA methylation participates in the repression of synthesis of mRNA for recoverin in normal tissues and (ii) aberrant hypomethylation of the recoverin gene region, overlapping the promoter up-stream of the first exon and the first exon itself, is involved in the aberrant expression of recoverin in tumor cells.

DNA hypomethylation in cancer: epigenetic scars of a neoplastic journey.

Cytosine methylation is a heritable modification of DNA in mammalian cells, and has a determinant impact on long-term gene repression and genome stability. Genomic methylation patterns, which remain generally stable in the adult, become profoundly altered in most human tumors. While discrete DNA segments become hypermethylated in cancer cells, many more sequences become hypomethylated. This review discusses our current understanding of the mechanisms that lead to DNA hypomethylation in tumors. Evidence suggests that methylation losses are not random, but rather evolve into mosaic hypomethylation patterns. It is proposed that such hypomethylation patterns result from a historical event of transient DNA demethylation, and that transcriptional regulators contribute to determining which regions escape remethylation and remain therefore unmethylated. Finally, possible stages of tumor development during which the transient DNA demethylation process may take place will be discussed.