DNA Methyltransferase 1 Function Research Articles

Phosphoproteomic profiling identifies DNMT1 as a key substrate of beta IV spectrin-dependent ERK/MAPK signaling in suppressing angiogenesis

βIV-spectrin is a membrane-associated cytoskeletal protein that maintains the structural stability of cell membranes and integral proteins such as ion channels and transporters. Its biological functions are best characterized in the brain and heart, although recently we discovered a fundamental new role in the vascular system. Using cellular and genetic mouse models, we reported that βIV-spectrin acts as a critical regulator of developmental and tumor-associated angiogenesis. βIV-spectrin was shown to selectively express in proliferating endothelial cells (EC) and suppress VEGF/VEGFR2 signaling by enhancing receptor internalization and degradation. Here we examined how these events impact the downstream kinase signaling cascades and target substrates. Based on quantitative phosphoproteomics, we found that βIV-spectrin significantly affects the phosphorylation of epigenetic regulatory enzymes in the nucleus, among which DNA methyltransferase 1 (DNMT1) was determined as a top substrate. Biochemical and immunofluorescence results showed that βIV-spectrin inhibits DNMT1 function by activating ERK/MAPK, which in turn phosphorylates DNMT1 at S717 to impede its nuclear localization. Given that DNMT1 controls the DNA methylation patterns genome-wide, and is crucial for vascular development, our findings suggest that epigenetic regulation is a key mechanism by which βIV-spectrin suppresses angiogenesis.

DNA Methyltransferase 1 (DNMT1) Function Is Implicated in the Age-Related Loss of Cortical Interneurons

Increased life expectancy in modern society comes at the cost of age-associated disabilities and diseases. Aged brains not only show reduced excitability and plasticity, but also a decline in inhibition. Age-associated defects in inhibitory circuits likely contribute to cognitive decline and age-related disorders. Molecular mechanisms that exert epigenetic control of gene expression contribute to age-associated neuronal impairments. Both DNA methylation, mediated by DNA methyltransferases (DNMTs), and histone modifications maintain neuronal function throughout lifespan. Here we provide evidence that DNMT1 function is implicated in the age-related loss of cortical inhibitory interneurons. Dnmt1 deletion in parvalbumin-positive interneurons attenuates their age-related decline in the cerebral cortex. Moreover, conditional Dnmt1-deficient mice show improved somatomotor performance and reduced aging-associated transcriptional changes. A decline in the proteostasis network, responsible for the proper degradation and removal of defective proteins, is implicated in age- and disease-related neurodegeneration. Our data suggest that DNMT1 acts indirectly on interneuron survival in aged mice by modulating the proteostasis network during life-time.

Chlorogenic Acid Decreases Malignant Characteristics of Hepatocellular Carcinoma Cells by Inhibiting DNMT1 Expression.

BackgroundHepatocellular carcinoma (HCC) is the most common malignant tumor of the adult liver, exhibiting rapid progression and poor prognosis. Chlorogenic acid (CGA), a polyphenol, has several biological activities, including the suppression of liver cancer cell invasion and metastasis. Increased levels or alterations in the function of DNMT1 are associated with the inactivation of tumor suppressor genes. However, the CGA-affected DNMT1 expression mediated mechanism is still unclear.MethodsThe human hepatocellular carcinoma (HCC) HepG2 cells were treated with a positive control drug (5-AZA) or varying doses of CGA. DNA methyltransferase 1 (DNMT1) protein levels and other relevant proteins were evaluated using Western blotting and immunocytochemistry. Cell-cycle analysis was performed by flow cytometry-based PI staining, and cell viability was assessed using 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay. The transwell invasion and wound healing assays were used to evaluate cell migration and invasion. In vivo proliferation of the HCC cells was detected. We investigated the expression of DNMT1, p53, p21, p-ERK, MMP-2, and MMP-9 in tumors using immunohistochemical analysis.ResultsOur results showed that CGA inhibited the proliferation, colony formation, invasion, and metastasis of HepG2 cells both in vitro and in vivo by down-regulating DNMT1 protein expression, which enhanced p53 and p21 activity, and resulting in a significant reduction in cell proliferation and metastasis. Moreover, CGA inactivated ERK1/2 and reduced MMP-2 and MMP-9 expression in HepG2 cells.ConclusionsCGA can suppress liver cancer cell proliferation, invasion, and metastasis through several pathways. CGA could serve as a candidate chemopreventive agent for HCC.

Neuronal Lhx1 expression is regulated by DNMT1-dependent modulation of histone marks

ABSTRACT Apart from the conventional view of repressive promoter methylation, the DNA methyltransferase 1 (DNMT1) was recently described to modulate gene expression through a variety of interactions with diverse epigenetic key players. We here investigated the DNMT1-dependent transcriptional control of the homeobox transcription factor LHX1, which we previously identified as an important regulator in cortical interneuron development. We found that LHX1 expression in embryonic interneurons originating in the embryonic pre-optic area (POA) is regulated by non-canonic DNMT1 function. Analysis of histone methylation and acetylation revealed that both epigenetic modifications seem to be implicated in the control of Lhx1 gene activity and that DNMT1 contributes to their proper establishment. This study sheds further light on the regulatory network of cortical interneuron development including the complex interplay of epigenetic mechanisms.

The role of DNMT1/hsa-miR-124-3p/BCAT1 pathway in regulating growth and invasion of esophageal squamous cell carcinoma

BackgroundEsophageal squamous cell carcinoma (ESCC) is the major subtype of esophageal cancer with high aggressiveness and poor prognosis. There is an urgent need for understanding the molecular mechanism underlying the development and progression of ESCC.MethodsESCC tissues and corresponding non-neoplastic tissues were collected. The expression and function of miR-124-3p and BCAT1 in two cell lines KYSE-150 and Eca109 were determined.ResultsWe show downregulation of miR-124-3p expression in ESCC tissues, which is highly correlated with proliferation and migration of ESCC cell lines KYSE-150 and Eca109. miR-124-3p show high correlation with TNM stage and differentiation grade. Furthermore, miR-124-3p directly targets mRNA 3’UTR region of BCAT1, which results in upregulation of BCAT1 expression as observed in ESCC tissues and cell lines. Also, our data indicates that BCAT1 high expression is strongly linked to the disease-free survival, tumor size, pathologic stage, T classification and differentiation grade. On the other hand, we clarified the upstream mechanism regulating miR-124-3p expression in ESCC, which involves in the hypermethylation-silencing regulation mediated by DNA methyltransferase 1(DNMT1), which is of high expression in ESCC tissues and cell lines in the present study. In addition, DNMT1 knockdown or inhibition of DNMT1 function contributes to downregulation of miR-124-3p and BCAT1 expression.ConclusionsOur study thus clarifies a new mechanism that DNMT1/miR-124/BCAT1 axis regulates the development and progression of ESCC.

DNMT1 ablation suppresses tumorigenesis by inhibiting the self-renewal of esophageal cancer stem cells.

Cancer stem cells (CSCs) have been isolated from many tumors and considered as the main reason of cancer recurrence and metastasis. DNA methyltransferase 1 (DNMT1) mediates DNA methylation and plays an important role in CSCs maintenance. However, the function of DNMT1 in CSCs of esophageal squamous cell carcinoma (ESCC) remains unclear. In this study, we examined the role of DNMT1 in regulating self-renewal in CSCs of ESCC. We found a high expression of DNMT1 in both side population (SP) cells and sphere formation cells that represented as substitutes for CSCs in KYSE150 and EC109 ESCC cell lines. We performed the knockdown of DNMT1 using lentivirus-mediated RNA interference (RNAi) methods. We revealed that ablation of DNMT1 resulted in the numbers and self-renewal abilities of CSCs refrained significantly in ESCC cells. As a result of the CSCs inhibition, the malignant phenotypes such as cell proliferation, colony formation, migration and drug resistance abilities were dramatically inhibited in ESCC cells. Treatment of 5-aza-2'-deoxycytidine (5-aza-dC), a DNMT inhibitor, also resulted in the inhibition of CSCs and malignant profiles in ESCC cells. Our findings also provided the first evidence that 5-aza-dC inhibited the colony and sphere formation of CSCs. Thus, our results indicated that DNMT1 was important for the self-renewal maintenance of CSCs in ESCC, and 5-aza-dC could be a potential therapy for the CSCs of ESCC.

Abstract 1901: DNMT1 is involved in esophageal squamous cell carcinoma (ESCC) and self-renewal ability of ESCC-cancer stem cells

Abstract DNA methylation mediated by DNA methyltransferase 1 (DNMT1) plays an important role in carcinogenesis and self-renewal ability of cancer stem cells (CSCs). However, the function of DNMT1 in esophageal squamous cell carcinoma (ESCC) carcinogenesis, especially self-renewal ability of ESCC-CSCs remains unclear. In this study, we found a high expression of DNMT1 in both side population (SP) cells and sphere formation cells that represented as substitutes for CSCs in KYSE150 and EC109 ESCC cell lines. In addition, the expression of DNMT1 was decreased during the differentiation from SP to None-SP (NSP) in these ESCC cells. These results suggested that DNMT1 might have a role in regulating self-renewal and/or differentiation of ESCC-CSCs. To further investigate DNMT1 in ESCC carcinogenesis and self-renewal ability of ESCC-CSCs, we silenced the expression of DNMT1 in KYSE150 and EC109 ESCC cells using lentivirus-mediated RNA interference (RNAi). Our results showed that ablation of DNMT1 expression in KYSE150 and EC109 ESCC cells resulted in decreased their CSCs by SP analysis and sphere formation assay. Meanwhile, ablation of DNMT1 expression inhibited malignant phenotypes in KYSE150 and EC109 cells, including cell proliferation, colony formation, migration and drug resistance abilities. Treatment of 5-aza-2'-deoxycytidine (5-aza-dC), a DNMT inhibitor that led to the degradation of DNMT1 protein by proteasome, revealed that numbers of CSCs and the malignant phenotypes of KYSE150 and EC109 ESCC cells were refrained significantly, including a dramatic inhibition of self-renewal ability of these ESCC-CSCs. Thus, our results indicated that DNMT1 was involved in ESCC carcinogenesis, especially in the maintenance of ESCC-CSCs, suggesting that DNMT1 could be a potential target for ESCC, especially ESCC-CSCs, therapy. Citation Format: Ying Teng, Xiying Yu, Hui Yuan, Liping Guo, Wei Jiang, Shih-Hsin Lu. DNMT1 is involved in esophageal squamous cell carcinoma (ESCC) and self-renewal ability of ESCC-cancer stem cells [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 1901. doi:10.1158/1538-7445.AM2017-1901

Zebularine enhances apoptosis of human osteosarcoma cells by suppressing methylation of ARHI.

ARHI is an imprinted tumor suppressor gene and its methylation suppresses ARHI transcription levels to cause the development and progression of malignant tumors. Zebularine exerts a demethylation function for tumor suppressor genes. Our study aims to investigate the effect and mechanism of action of zebularine on the epigenetic modification of the ARHI gene, and whether this effect may modulate the viability and apoptosis of human osteosarcoma cells. We found that zebularine inhibited the viability and promoted apoptosis in osteosarcoma cells. Zebularine potentiated the expression of ARHI at both the protein and mRNA level. This was related to the downregulation of methylation of ARHI caused by zebularine. Zebularine suppressed the interaction of DNA methyltransferase 1 (DNMT1) with histone methyltransferase G9a, but had no effect on G9a alone. Knockdown of DNMT1 or G9a can induce a reduction of ARHI methylation. Therefore, we inferred that zebularine was likely to directly repress DNMT1 alone, but G9a was necessary to regulate the function of DNMT1 on ARHI methylation. Moreover, knockdown of ARHI rescued cell viability and apoptosis under the zebularine‐treated condition. We showed that zebularine inhibited viability and promoted apoptosis by disturbing the interaction between DNMT1 and G9a, thereby resulting in lower ARHI methylation and elevated ARHI expression in osteosarcoma cells.

Structural optimization of non-nucleoside DNA methyltransferase inhibitor as anti-cancer agent.

Inhibition of DNA methyltransferase 1 (DNMT1) can reverse the malignant behavior of cancer cells by restoring expression of aberrantly silenced genes that are required for differentiation, senescence, and apoptosis. Clinically used DNMT1 inhibitors decitabine and azacitidine inhibit their target by covalent trapping after incorporation into DNA as azacytidine analogs. These nucleoside compounds are prone to rapid enzymatic inactivation in blood, posing challenges to the development of purely epigenetic dosing schedules. Non-nucleoside compounds that suppress expression or function of DNMT1 may overcome this problem. Using a high-throughput PCR-based site specific chromatin condensation assay, we identified a compound that reactivated Cyclin-Dependent Kinase Inhibitor 2A (CDKN2A) in myeloma cells and suppressed expression of DNMT1 from a library of 5120 chemically diverse small molecules. Lead optimization was performed to generate 26 new analogs with lung cancer proliferation and DNMT1 expression as activity readout. Two of the new derivatives showed 2 fold improvement of growth inhibiting potency and also decreased DNMT1 protein levels in lung cancer cells.

RFTS-deleted DNMT1 enhances tumorigenicity with focal hypermethylation and global hypomethylation

Site-specific hypermethylation of tumor suppressor genes accompanied by genome-wide hypomethylation are epigenetic hallmarks of malignancy. However, the molecular mechanisms that drive these linked changes in DNA methylation remain obscure. DNA methyltransferase 1 (DNMT1), the principle enzyme responsible for maintaining methylation patterns is commonly dysregulated in tumors. Replication foci targeting sequence (RFTS) is an N-terminal domain of DNMT1 that inhibits DNA-binding and catalytic activity, suggesting that RFTS deletion would result in a gain of DNMT1 function. However, a substantial body of data suggested that RFTS is required for DNMT1 activity. Here, we demonstrate that deletion of RFTS alters DNMT1-dependent DNA methylation during malignant transformation. Compared to full-length DNMT1, ectopic expression of hyperactive DNMT1-ΔRFTS caused greater malignant transformation and enhanced promoter methylation with condensed chromatin structure that silenced DAPK and DUOX1 expression. Simultaneously, deletion of RFTS impaired DNMT1 chromatin association with pericentromeric Satellite 2 (SAT2) repeat sequences and produced DNA demethylation at SAT2 repeats and globally. To our knowledge, RFTS-deleted DNMT1 is the first single factor that can reprogram focal hypermethylation and global hypomethylation in parallel during malignant transformation. Our evidence suggests that the RFTS domain of DNMT1 is a target responsible for epigenetic changes in cancer.

Loss of LSD1 (lysine-specific demethylase 1) suppresses growth and alters gene expression of human colon cancer cells in a p53- and DNMT1(DNA methyltransferase 1)-independent manner

Epigenetic silencing of gene expression is important in cancer. Aberrant DNA CpG island hypermethylation and histone modifications are involved in the aberrant silencing of tumour-suppressor genes. LSD1 (lysine-specific demethylase 1) is a H3K4 (histone H3 Lys4) demethylase associated with gene repression and is overexpressed in multiple cancer types. LSD1 has also been implicated in targeting p53 and DNMT1 (DNA methyltransferase 1), with data suggesting that the demethylating activity of LSD1 on these proteins is necessary for their stabilization. To examine the role of LSD1 we generated LSD1 heterozygous (LSD1+/−) and homozygous (LSD1−/−) knockouts in the human colorectal cancer cell line HCT116. The deletion of LSD1 led to a reduced cell proliferation both in vitro and in vivo. Surprisingly, the knockout of LSD1 in HCT116 cells did not result in global increases in its histone substrate H3K4me2 (dimethyl-H3K4) or changes in the stability or function of p53 or DNMT1. However, there was a significant difference in gene expression between cells containing LSD1 and those null for LSD1. The results of the present study suggested that LSD1 is critical in the regulation of cell proliferation, but also indicated that LSD1 is not an absolute requirement for the stabilization of either p53 or DNMT1.

Why is DNA methylation of Igf2 CpG island shore affected during ageing?

In post-genomic era attention has been focused on the characterization of the human DNA “methylome” at single nucleotide resolution. It is a particularly difficult goal to achieve as methylation profiling is different in tissues and cells of the same individual. 5- Methylcytosine is considered the “fifth base” of DNA as it introduces an epigenetic code on genome which is defined during development. Nutrition, environment and style of life have an impact on the “methylome” and the introduction of anomalous methyl groups in sequences that have to be maintained unmethylated and/or demethylation of chromatin regions which are usually methylated drive the biological events associated with pathologies. The same anomalies in methylation patterns occur with age so an in-depth knowledge of these changes will help us to individuate what leads to the passage from successful ageing to one in which there is the onset of disease [1]. Data from Franceschi's group are focused on ageing and the cohorts of individuals are selected in order to establish whether variability in DNA methylation patterns is to ascribe to age of individuals rather than to their geographical dimension. Notably, in this study monozygotic and dizygotic twins of ages ranging from 22 to 97 years are also enrolled. Four regions, located at the Igf2/H19 imprinted locus, were analyzed including the CpG island present in differentially methylated region 2 (DMR2) and the nearby 5’ shore, two fragments never investigated up to now. Authors conclude that the shore is the one in which the scatter in methylation variability during ageing is more evident. Shore regions, which localize in proximity of CpG islands at 200-2000 kb away from them, are of particular interest as involved in gene expression regulation through the DMRs often located within them [2]. A long-range looping interaction is involved in the control of imprinting at maternally inherited allele at the Igf2/H19 gene locus. An interesting hypothesis for susceptibility to epimutation of this shore region in aged individuals is that its position inside the loop is topologically responsible for its availability to change the methylation profile. Although the higher order chromatin organization at Igf2/H19 locus is well assessed in mice, less information is available in humans. Anyway, the analysis of data about chromatin structure from the ENCODE consortium, accessible at the UCSC Genome Browser (http://genome.ucsc.edu/) [3, 4], indicates a distinctive chromatin feature of this shore region which corresponds to an open, DNAaseI hypersensitive chromatin domain characterized by an early S-phase replication. As chromatin structure affects the susceptibility to DNA damage, the chromatin shape of the shore could be the reason for its susceptibility to epimutation. An important link was found between DNA damage, homology directed repair and DNA methylation [5], suggesting that DNA-methyltransferase 1(DNMT1), which is recruited to repair sites following double-strand DNA breaks [6], marks the repaired segments superimposing novel methylation profile. Another interesting hypothesis rises from the early replication timing of this shore. The maintenance of DNA methylation patterns during DNA replication requires the function of DNMT1 which is brought to the replication fork through a direct interaction with Proliferating Cell Nuclear Antigen protein (PCNA). Interestingly, DNMT1 forms a complex at replication foci only during late stages of S-phase when methylated DNA is replicated. Thus, the controlled recruitment of DNMT1 into replication forks is an important component in the perpetuation of DNA methylation patterns and S-phase abnormalities could affect methylation patterns by altering the replication timing of sequences. The biological event able to disrupt DNMT1 function in S phase could be poly(ADP-ribosyl)ation. It has been found that competitive inhibition of PARPs leads to hyperexpression of DNMT1 in G1/early S phase, increases the amount of DNMT1 that co-immunoprecipitates with PCNA in this phase and causes DNA hypermethylation [7]. Interestingly, reduced levels of poly(ADP-ribosyl)ation enzymes PARP1 and 2 and poly(ADP-ribosyl)ation activity have been associated with ageing [8, 9]. In conclusion, changes of methylation profile at Igf2 CpG island shore observed during ageing could be due to the loss of local epigenetic control of this region. The understanding of molecular mechanism(s) involved in the protection of the methylation pattern of this shore could provide important information about the participation of epigenetics in ageing and disease.

HNaa10p contributes to tumorigenesis by facilitating DNMT1-mediated tumor suppressor gene silencing

Hypermethylation-mediated tumor suppressor gene silencing plays a crucial role in tumorigenesis. Understanding its underlying mechanism is essential for cancer treatment. Previous studies on human N-alpha-acetyltransferase 10, NatA catalytic subunit (hNaa10p; also known as human arrest-defective 1 [hARD1]), have generated conflicting results with regard to its role in tumorigenesis. Here we provide multiple lines of evidence indicating that it is oncogenic. We have shown that hNaa10p overexpression correlated with poor survival of human lung cancer patients. In vitro, enforced expression of hNaa10p was sufficient to cause cellular transformation, and siRNA-mediated depletion of hNaa10p impaired cancer cell proliferation in colony assays and xenograft studies. The oncogenic potential of hNaa10p depended on its interaction with DNA methyltransferase 1 (DNMT1). Mechanistically, hNaa10p positively regulated DNMT1 enzymatic activity by facilitating its binding to DNA in vitro and its recruitment to promoters of tumor suppressor genes, such as E-cadherin, in vivo. Consistent with this, interaction between hNaa10p and DNMT1 was required for E-cadherin silencing through promoter CpG methylation, and E-cadherin repression contributed to the oncogenic effects of hNaa10p. Together, our data not only establish hNaa10p as an oncoprotein, but also reveal that it contributes to oncogenesis through modulation of DNMT1 function.

Differentiation-Chronology Specific Function of DNMT1 and Selective Anti-Leukemia Stem-Cell Therapy

A therapeutically crucial sub-set of leukemia cells are those that self-renew (leukemia stem-cells or LSC). An open question is whether LSC self-renewal is mediated by the same pathways or factors that self-renew normal hematopoietic stem-cells (HSC). A difference in self-renewal mechanisms could form the basis for LSC-specific therapy. Hematopoietic differentiation is dependent on lineage-determining factors such as Pu.1. Pu.1 can repress or activate genes, depending on its interaction partners. We show that Pu.1 mediated terminal differentiation involves sequential repression of genes associated with self-renewal (pro-SR) (HoxB4, Bmi-1, c-Kit) followed-by activation of genes associated with differentiation (pro-DIFF)(Mcsfr, Gmcsfr, F4/80). DNA methyl-transferase 1 (DNMT1) is an arbiter of transcription repression. DNMT1 depletion concurrent with Pu.1 introduction prevents the first step of pro-SR repression and maintains self-renewal. However, DNMT1 depletion 6 hours after Pu.1 introduction (at which point pro-SR repression by Pu.1 is complete), increases differentiation. Therefore, the phenotypic consequences of DNMT1 depletion critically depend on the differentiation chronology of the cell. We then examined the effects of a leukemia first-hit event, Runx1 disruption, on Pu.1 mediated differentiation. Runx1 knock-down allowed Pu.1 mediated pro-SR repression but prevented pro-DIFF up-regulation and terminal differentiation, resulting in persistent dysregulated proliferation (self-renewal) despite pro-SR repression. DNMT1 depletion in these abnormally self-renewing cells, with an epigenetic and transcript profile distinct from parental self-renewing cells, relieved pro-DIFF repression and restored the terminal differentiation response. Therefore, DNMT1 depletion had opposite effects in the cells containing the leukemia-associated abnormality versus parental cells. Suggesting that these findings were relevant to clinical disease, leukemic bone marrow (n=130), although consisting largely of myeloblasts, demonstrated a DNA methylome (1505 CpGs, Illumina) profile that resembled differentiated cells and not precursors, with aberrant methylation concentrated at precursor signature (pro-SR) promoters and hypomethylation at differentiation signature (pro-DIFF) promoters. To therapeutically exploit these observations, we demonstrate that the nucleoside analogue Decitabine, given at doses that deplete DNMT1 without causing DNA damage, and given frequently but intermittently to allow for cell-division, produces an ideal therapeutic profile with terminal differentiation of leukemia cells but increased self-renewal of human CD34+ HSC. This effect was seen in different models of human LSC (primary human CD34+ cells transduced with either the MLL-AF9 or RUNX1-ETO leukemia fusion genes) and in primary leukemia cell samples (n=15) containing a variety of chromosome abnormalities. This therapeutic approach increased survival in a murine xeno-graft model of aggressive human MLL-AF9 leukemia. In conclusion, leukemia and LSC self-renewal, in a number of examples covering a variety of transformation-initiating abnormalities, is programmatically distinct from HSC selfrenewal. Leukemia self-renewal is associated with an epigenetic and transcript profile that reflects part-way commitment into differentiation with aberrant gene repression preventing completion of the process. This difference could explain the opposite effects of DNMT1 depletion on HSC and leukemia self-renewal, and provides the scientific foundation for differentiation (frequent metronomic low-dose) instead of cytotoxic (infrequent high-dose) regimens of epigenetically active and clinically available agents such as Decitabine.

Poly(ADP-ribose): a co-regulator of DNA methylation?

The formation of multiprotein complexes is l'ordre du jour in regulatory pathways. In this issue of Oncogene, Reale et al. report the formation of a particularly sophisticated complex of two important regulatory enzymes, DNMT1 (DNA methyltransferase-1) and PARP-1 (poly(ADP-ribose)polymerase-1). The former evolved with a specific sequence motif binding the enzymatic product of the latter. The product, poly(ADP-ribose), bonds the two partners into a heterodimeric complex and, as a consequence, the catalytic function of DNMT1 is silenced. Thus, PARP-1 becomes a conditional negative regulator of DNMT1. In a larger perspective, Reale et al. highlight the potential role of PARP-1 as a co-regulator of DNA methylation leading to epigenetic reprogramming of cancer cells.