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Keywords:

  • epigenesis;
  • DNA methylation;
  • methyltransferases;
  • histone code

Abstract

  1. Top of page
  2. Abstract
  3. Methylation Process and Cancer
  4. Mechanisms of Silencing
  5. DNA Methyltransferases (DNMTs)
  6. DNMT and Cancer
  7. DNMT Inhibitors
  8. Conclusions
  9. CONFLICT OF INTEREST DISCLOSURES
  10. REFERENCES

Epigenetic alterations such as DNA methylation have been implicated in the development and progression of various cancers. DNA methylation consists of the reversible addition of a methyl group to the carbon 5 position of cytosine in CpG dinucleotides and is considered essential for normal embryonic development. However, global genomic hypomethylation and aberrant hypermethylation of regulatory regions of tumor suppressor genes have been associated with chromosomal instability and transcription repression, respectively, providing neoplastic cells with a selective advantage. DNA methyltransferases are the enzymes responsible for the addition of methyl groups to CpG dinucleotides, which, together with histone modifiers, initiate the events necessary for transcription repression to occur. It has been demonstrated that increased expression of DNA methyltransferases may contribute to tumor progression through methylation-mediated gene inactivation in various human cancers. Given their importance, this article reviews the main epigenetic mechanisms for regulating transcription and its implications in cancer development. Cancer 2011. © 2010 American Cancer Society.

Heritable and reversible mechanisms known as epigenetic alterations do not require direct alterations of DNA sequences, but they can be responsible for modifying gene expression and are related to cancer development.1-6 Although the genetic information provides the sequence for protein synthesis, the epigenetic information provides instructions on how, where, and when the genetic information will be used.7

Several epigenetic mechanisms regulate gene expression: DNA methylation, modifications of histone proteins, and functional noncoding RNA.1 The major form present in mammalian cells is DNA methylation, which is the covalent addition of a methyl group to the carbon-5 position of cytosine predominantly in the CpG dinucleotide.7, 8 This cytosine modification pattern can be transmitted through cell division and may contribute to gene inactivation in cancer.3, 4

DNA methylation is essential for normal embryonic development and has a variety of important functions, such as the regulation of gene expression, control of cell differentiation and development, chromatin modification, mutation accumulation, silencing of endogenous retroviruses, preservation of chromosomal integrity, genomic imprinting control, and X chromosome inactivation.7, 9-13 Initially discovered as a mechanism for the control of development, it plays an important role in many tumor types.14 Genomic methylation patterns are frequently altered in tumor cells, with global hypomethylation accompanying region-specific hypermethylation sites. When hypermethylation occurs within the promoter of a tumor suppressor gene, it can silence expression of the associated gene and provide the cell with a growth advantage in a manner akin to deletions or mutations.7, 10, 15

This review summarizes the main epigenetic mechanisms involved in cancer development, with special focus on DNA methyltransferase enzymes and hypermethylation of tumor suppressor genes.

Methylation Process and Cancer

  1. Top of page
  2. Abstract
  3. Methylation Process and Cancer
  4. Mechanisms of Silencing
  5. DNA Methyltransferases (DNMTs)
  6. DNMT and Cancer
  7. DNMT Inhibitors
  8. Conclusions
  9. CONFLICT OF INTEREST DISCLOSURES
  10. REFERENCES

In mammalian cells, the DNA targets for modification through methylation are cytosine bases adjacent to guanine bases (CpG dinucleotides).16, 17 Sequences of CpG, when found at a high frequency in the genome, are referred to as CpG islands.18 Most of the 29,000 CpG islands found in the human genome are in the promoter regions of almost half the genes and are generally unmethylated in normal cells.3, 10, 19

The modification of cytosine is catalyzed by the enzymes DNA methyltransferases (DNMTs) using S-adenosyl-L-methionine (AdoMet) as the methyl donor (Fig. 1).9, 13, 18 The methyl group of AdoMet is bound to a sulfonium ion that thermodynamically destabilizes the molecule and makes the relatively inert methylthiol of the methionine moiety very reactive toward activated carbon atoms.9 The reaction involves DNMT DNA binding, flipping the target cytosine out of the double helix, and formation of a transient covalent complex with the cytosine residue.17 DNMT adds a cysteine thiolate to the 6-carbon of the substrate cytosine, followed by transfer of the methyl group to the 5-carbon.20 Glycine N-methyltransferase (GNMT) is the main enzyme responsible for the catabolism of excess hepatic AdoMet, and the absence of this enzyme has been associated with a greater methylation process, especially during hepatic tumorigenesis.21

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Figure 1. The structures of cytosine before and after the transfer of a methyl group from the cofactor AdoMet, catalyzed by DNA methyltransferases, are shown.

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The distribution of methylated and nonmethylated CpG dinucleotides is not random; rather it conforms to a pattern.16 Certain genomic sites, such as pericentromeric regions, imprinted regions, and genes on the inactive X chromosome in females, are hypermethylated, whereas other sites, such as CpG islands, which are often associated with gene promoter regions, are hypomethylated.7, 13, 22

Cancers exhibit at least 2 types of methylation defects: hypomethylation, characterized by a global loss of methylation, and hypermethylation of CpG islands of regulatory regions of tumor suppressor genes.23, 24

Hypomethylation of nonpromoter regions of DNA (known as global hypomethylation) may cause genomic instability and structural changes in chromosomes in cancer, although the relationship between the 2 processes is not clear.10, 25 Two resulting effects of losses of methylation in tumorigenesis have been proposed. First, weakening of transcriptional repression in normally silent regions of the genome could cause the potentially harmful expression of inserted viral genes and of normally silenced genes, such as imprinted genes and genes on the inactive X chromosome. Second, losses of methylation of nuclear structures other than genes could affect the functional stability of chromosomes, such as pericentromeric regions,26 although the loss-of-methylation mechanism in tumors is not known.

Genes affected by hypomethylation includes growth regulatory genes, enzymes, developmentally critical genes, and tissue-specific genes such as germ cell-specific tumor antigen genes.27 The cell-cell adhesion glycoprotein P-cadherin has been found to be overexpressed in breast cancer, whereas its expression is only restricted to the myoepithelial cells in normal breast tissue. Paredes et al stated that this gene promoter methylation could be a putative molecular mechanism responsible for its transcriptional regulation and found a higher percentage of P-cadherin unmethylation in contrast to normal tissue (42% versus 0%). Others genes have been shown to become promoter-hypomethylated during carcinogenesis such as cyclin D2 in gastric cancer28 and MAGE in melanomas.29 It have been demonstrated that cancer cells with hypomethylated oncogenes overexpress its protein, whereas treatment with a methylation inhibitor induces its expression in cell lines not expressing the gene.30 These observations indicate that aberrant hypomethylation in cancer may contribute to oncogene expression.

Hypermethylation of CpG islands in gene promoter regions may be involved in carcinogenesis as a result of 3 possible mechanisms: cytosine methylation facilitates gene mutation as 5-methylcytosine is deaminated to thymine,8, 31 aberrant DNA methylation may be associated with allelic loss,7, 32 and tumor suppressor genes may be inactivated by DNA hypermethylation.3, 33

Cytosine methylation can increase mutation rates because of the spontaneous hydrolytic deamination of methylated cytosine, which causes C[RIGHTWARDS ARROW]T transition mutation.8, 10, 18 This phenomenon was used to explain the high incidence of CpG to TpG transition mutations observed in the p53 tumor suppressor gene.34 The epigenetic silencing of the DNA repair enzyme O6-methylguanine DNA methyltransferase (O6-MGMT) is another example of how abnormal methylation may lead to increased rates of mutation.35 The O6-MGMT protein removes carcinogen-induced O6-methylguanine adducts from DNA, which produce G[RIGHTWARDS ARROW]A transition mutations if left unrepaired.10 Tumors with silenced O6-MGMT alleles, such as p5335 and K-ras,36 seem to be predisposed to mutation in key genes.

The hypermethylation of gene promoter regions blocks its transcription, contributing to inactivation of tumor suppressor genes in cancer. Many tumor suppressor genes in cancer have been found to have promoter hypermethylation (Table 1). Cell cycle-regulating genes such as p15 and p16 have been shown to be affected by CpG island hypermethylation. The inactivation of these 2 genes results in Rb phosphorylation, entry into the S phase, and subsequently in cell proliferation.38 DAP-kinase expression, a positive mediator of apoptosis, is frequently lost in metastatic lung tumors. In addition, restoration of its protein to the physiologic levels suppressed their ability to form metastases.43, 57 This observation indicates that tumor suppressor gene promoter hypermethylation contributes to cancer development.

Table 1. Most Studied Hypermethylated Promoters of Genes Implicated in Carcinogenesis
GeneTissueReference
p15 (cell-cycle regulator)Oral cancerOgi37
Hepatic cancerOh38
p16 (cell proliferation inhibitor)Oral cancerKulkarni, Saranath39; Shaw40; Ogi37
Saliva of oral cancer patientsRosas41
Saliva of leukoplakia patientsLopez42
Head and neck cancerSanchez-Cespedes43; Maruya44
Hepatic cancerOh38
Colorectal cancerEads45
Renal cancerArai12
Lung cancerLin23
RASSF1 (cell cycle regulator)Nasopharyngeal cancerFendri46
Hepatic cancerOh38
Bladder cancerFriedrich47; Abbosh48
MLH1 (cell-cycle regulator)Colorectal cancerArnold49; Eads45; Herman50
Renal cancerArai12
MGMT (DNA repair)Oral cancerKulkarni; Saranath.39
Saliva of oral cancer patientsRosas41
Saliva of leukoplakia patientsLopez42
Head and neck cancerSanchez-Cespedes43; Maruya44
Bladder cancerAbbosh48
Lung cancerVallbohmer51
FHIT (DNA replication regulator)Lung cancerLin23; Kim52
DAP-K (proapoptotic protein)Oral cancerKulkarni; Saranath.39; Ogi37
Saliva of oral cancer patientsRosas41
Nasopharyngeal cancerFendri46
Head and neck cancerSanchez-Cespedes43; Maruya44
Pancreatic cancerDansranjavin53
Renal cancerChristoph54
Lung cancerVallböhmer51
APC (cell adhesion)Colorectal cancerEads45; Esteller55
Lung cancerVallbohmer51
Cadherin (cell adhesion)Hepatic cancerKanai32; Oh38
Pancreatic cancerDansranjavin53
Salivary gland cancerShieh56
Lung cancerKim52
RAR (retinoic acid receptor)Nasopharyngeal cancerFendri46
Head and neck cancerMaruya.44
Lung cancerKim52; Lin23

According to the Knudson 2-hit hypothesis, promoter methylation may occur on a gene acting on the wild-type allele, while the other is mutated, contributing to the biallelic inactivation of tumor suppressor genes, either as a primary or a second hit in both familial and sporadic forms of cancer.26, 58 In this case, genetic and epigenetic changes can collaborate to prevent the expression of a functional gene product in cancer cells.59 Otherwise, hypermethylation of both alleles may also be present in some cases (Fig. 2).26

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Figure 2. Genetic and epigenetic events that can block the expression of tumor suppressor genes in cancer are shown. When only 1 allele is hit either by genetic or by epigenetic alteration, the other can still express the protein that controls cell growth (first hit). However, if the other allele is inactivated (second hit), gene expression is blocked, contributing to the development of cancer (LOH, loss of heterozygosity).

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It is important to note that only methylation within or around the promoter region is associated with gene silencing. Dense methylation within the body of a gene, even within CpG islands, does not hinder transcription.26, 60

It has been recognized that aberrant hypermethylation events can occur early in tumorigenesis, predisposing cells to malignant transformation.3, 7, 14 Renal tumors were demonstrated to have their average number of methylated CpG islands increase significantly and progressively from precancerous conditions to invasive tumors,12 and thus, precursor lesions of oral,42 liver,61 and uterine cervix62 cancers have been the focus of studies. Moreover, methylation of genes such as APC in the formation of intestinal polyps, H19 in preneoplastic kidney parenchyma of Wilms tumor patients, and RB1 in familial cases of unilateral retinoblastoma is almost certainly implicated in the earliest stages of tumorigenesis.3

Mechanisms of Silencing

  1. Top of page
  2. Abstract
  3. Methylation Process and Cancer
  4. Mechanisms of Silencing
  5. DNA Methyltransferases (DNMTs)
  6. DNMT and Cancer
  7. DNMT Inhibitors
  8. Conclusions
  9. CONFLICT OF INTEREST DISCLOSURES
  10. REFERENCES

In higher eukaryotes, DNA methylation and histone modifications appear to be the main events responsible for the formation of transcriptionally active or inactive chromatin.63

DNA methylation inhibits transcription by interfering with its initiation.3 Because 5-methylcytosine is in the major groove of the DNA helix,9 it is possible that this modified cytosine interferes directly with the binding of transcription factors.22, 64 Many factors are known to bind CpG-containing sequences, and some of these fail to bind when the CpG is methylated.22 However, it is unlikely to be a widespread mechanism for transcriptional silencing because most transcription factors do not have CpG dinucleotides within their DNA binding sites.7 Another possible mechanism is that specific transcriptional repressors may recognize methyl-CpG and turn off transcription.64

Four proteins with a methyl-CpG-binding domain (MeCP2, MBD1, MBD2, and MBD3) recognize methylated DNA and are implicated in transcriptional repression. These proteins also have an affinity for histone-modifying enzymes that cause chromatin condensation and gene silencing.65 MeCP2 contains both a methyl-CpG-binding domain and a transcriptional repression domain that can be tethered to another protein called Sin3A, which interacts with histone deacetylase, a member of another transcriptional repression system.66

Histones, nuclear proteins that interact with DNA to form nucleosomes, are also, in addition to being responsible for packing DNA within chromosomes, essential for transcription regulation.11, 67 Histone modifications such as acetylation and methylation may be read by the DNA methylation machinery, leading to either methylation of or failure to methylate a particular CpG dinucleotide.22 Histone acetylation occurs at sites where transcription takes place, resulting in chromatin decondensation (euchromatin) to permit binding of transcription factors to DNA.18, 66 Acetylation is controlled by histone acetylases and histone deacetylases (HDACs).68 Deacetylation of these proteins (in particular H3 and H4) by HDAC leads to a tighter nucleosomal packing and the formation of a compacted chromatin environment (heterochromatin) that inhibits transcription.3, 11 Moreover, histone methylation is linked to euchromatic and heterochromatic states.68, 69 Methylation of lysine 9 in the core histone H3 is associated with silenced genes,70 whereas methylation of lysine 4 in histone H3 is a feature of active genes.22

DNA methylation and histone modifications are intricately connected with each other.8, 25 Thus, the methylation level is connected to the broad organization of chromatin, with unmethylated DNA usually being part of euchromatin, whereas heavily methylated DNA is part of heterochromatin.9 HDAC may play an important role, in cooperation with DNA methyltransferases, in maintaining tumor suppressor gene silencing.33 DNA methylation recruits methyl-CpG-binding proteins and their associated corepressors and HDACs, resulting in tighter packaging of DNA and reduced access of transcription factors to their binding sites (Fig. 3).11, 71, 72

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Figure 3. Mechanism of transcriptional silencing by DNA methylation is shown. (A) Promoter of the gene in a transcriptionally active state is shown. Chromatin in this phase is occupied by spaced nucleosomes composed of acetylated histone complexes and with tails of histone H3 methylated at lysine 4, which configures the euchromatin, making the region accessible to components of the transcription machinery. (B) DNA methyltransferase adds the methyl group to the cytosine of CpG islands. (C) Methylated CpG sites attract methyl-binding proteins such as MeCP2 that, in turn, attract Sin3A and HDAC to the region. The chromatin structure is modified, with deacetylated histone and methylated lysine 9 of histone H3, configuring the heterochromatin. Once these changes have occurred, the transcription factors are repelled, and transcription is blocked.

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Indeed, DNA methylation appears to be the dominant mechanism of silencing genes.26 Drugs that inhibit HDAC can increase the expression of genes with unmethylated promoters, but they cannot induce the reexpression of hypermethylated genes in cancer cells. However, if some demethylation is first effected by low doses of demethylating drugs, histone deacetylase inhibitors act synergistically in reexpressing the silent gene.73, 74 Thus, the use of a combination of inhibitors of DNMTs and histone deacetylases is an attractive therapeutic strategy.26

DNA Methyltransferases (DNMTs)

  1. Top of page
  2. Abstract
  3. Methylation Process and Cancer
  4. Mechanisms of Silencing
  5. DNA Methyltransferases (DNMTs)
  6. DNMT and Cancer
  7. DNMT Inhibitors
  8. Conclusions
  9. CONFLICT OF INTEREST DISCLOSURES
  10. REFERENCES

Three DNA methyltransferases (DNMT1, DNMT3a, and DNMT3b) are responsible for adding methyl groups to CpG dinucleotides.7, 14

DNMT1

DNMT1 is often referred as the maintenance methyltransferase because it is believed to be the primary enzyme responsible for copying methylation patterns after DNA replication.7, 23, 61, 75 Its predominant splicing isoform in somatic cells in humans comprises 1616 amino acids, with a large N-terminal regulatory domain and a smaller C-terminal catalytic domain.9 It is the most abundant DNMT targeted to replication foci and, under experimental conditions, has up to a 50-fold preference for hemimethylated DNA substrate.15, 76

This enzyme can maintain CpG methylation after DNA replication by methylating the daughter DNA strand, using the methylation pattern of the parental strand as a template (Fig. 4A).9, 16, 77 Its inactivation produces global demethylation, which is consistent with the finding that DNMT1 is required for maintenance methylation.77 The structural and mechanistic basis for the specificity of the enzyme for CpG sites as well as its preference for hemimethylated DNA is still unknown.9

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Figure 4. Maintenance versus de novo methylation is shown. (A) After semiconservative DNA replication, the daughter strand was base-paired with one of the methylated parental strands. The enzyme DNMT1 was responsible for maintaining the methylation pattern by completing half-methylated sites. (B) De novo methylation of unmethylated sequences, catalyzed by DNMT3 family enzymes, is shown.

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Three sequences in the N-terminal increase the precision of maintenance methylation and give the enzyme direct access to the nuclear replication site: the proliferating cell nuclear antigen (PCNA) binding domain,78 the replication foci targeting sequence79 and the polybromo homology domain.17, 80 PCNA is required for DNA replication, and the DNMT1-PCNA interaction may allow the newly synthesized daughter strands to be rapidly remethylated before being packaged into chromatin.7, 78 This tight association of the DNMT1 with the replication machinery allows DNMT1 to bind newly replicated and still naked DNA.17

Otherwise, some genes may make this interaction difficult with replicating foci. The cell-cycle regulator p21 can disrupt the DNMT-PCNA interaction, suggesting that p21 may negatively regulate methylation by blocking access of DNMT to PCNA,78 particularly during DNA damage, when p21 protein is induced.81 It was also demonstrated that p21 may inhibit DNMT1 gene expression.81 The retinoblastoma gene product Rb, another cell-cycle regulator protein, can bind to DNMT1 and inhibit its methyltransferase activity during DNA replication in the cell cycle.82 Loss of functional Rb may grant DNMT1 free access to the genome, which could allow for aberrant de novo methylation of CpG.7 These observations point to a complicated network of connections between DNMT1 and several cellular proteins involved in gene regulation and epigenetic signaling during cell replication.63

DNMT3 family

Although DNMT1 is the major DNMT in humans, 2 other enzymes, DNMT3a and DNMT3b, have also been shown to possess DNMT activity.12 They catalyze DNA methylation at CpG dinucleotides in unmethylated genomic sequences.33

Because DNMT3a and DNMT3b cannot differentiate between unmethylated and hemimethylated CpG sites, they obviously cannot copy a specific pattern of methylation or contribute to the maintenance of methylation pattern.9 Because they show no preference for hemimethylated DNA, both enzymes appear to function as de novo methyltransferases77 and show a disperse distribution throughout the nucleus not associated with replication sites, even during S-phase (Fig. 4B).17 This finding suggests that these DNMTs utilize a different mechanism for accessing the densely packed chromatin and for interacting with their target sites that may involve auxiliary factors such as chromatin remodeling complexes.17

DNMT3a and DNMT3b are highly expressed in early embryonic cells, the stage in which most programmed de novo methylation events occur, are downregulated after differentiation and in adult somatic tissues, and are overexpressed in tumor cells.3, 22, 77, 83, 84 DNMT3b has been shown to play a crucial role in incorporating de novo hypermethylation of promoter CpG islands, a possible mechanism for tumor suppressor gene inactivation within human cancer cells.8, 18

Another member of the DNMT3 family is DNMT3L, a regulatory factor for de novo methylation without methylation capacities. Its amino acid sequence is very similar to that of DNMT3a and DNMT3b but lacks the residues required for DNA methyltransferase activity in the C-terminal domain.8

DNMT and Cancer

  1. Top of page
  2. Abstract
  3. Methylation Process and Cancer
  4. Mechanisms of Silencing
  5. DNA Methyltransferases (DNMTs)
  6. DNMT and Cancer
  7. DNMT Inhibitors
  8. Conclusions
  9. CONFLICT OF INTEREST DISCLOSURES
  10. REFERENCES

Although the DNMT1 and DNMT3 families have been considered maintenance and de novo methyltransferases, respectively, it is likely that all 3 DNMTs possess both functions in vivo, particularly during carcinogenesis.7

Excessive amounts of DNMT1, which cannot target replication foci, may participate in the de novo methylation of CpG islands that are not methylated in normal cells,85 supporting the idea that DNMT can contribute to tumor progression through CpG island methylation-mediated gene inactivation.86 Thus, its increased expression may play an important role in the malignant progression of cancer, leading to aberrant methylation in many important tumor suppressor genes.23

Increased expression of DNMT protein may be an early and significant event in urothelial,85 hepatic,38, 61, 87-89 gastric,90 pancreatic,91 lung,23, 51, 52 breast,92 and uterine cervix62 carcinogenesis (Table 2).

Table 2. Summary of DNA Methyltransferase Studies in Human Cancer Tissues and Cell Lines
DNMTTissueSampleMethodExpressionReference
  1. NA, not available; IHC, immunohistochemistry; qRT-PCR, real-time PCR; NBH, Northern blot hybridization; [UPWARDS ARROW], increased expression; [DOWNWARDS ARROW], decreased expression.

DNMT1Hepatic cancer59 patientsIHC[UPWARDS ARROW]Choi87
42 patientsIHC/qRT-PCR[UPWARDS ARROW]Fan88
53 patientsIHC[UPWARDS ARROW]Saito61
27 patientsqRT-PCR[UPWARDS ARROW]Sun89
25 patientsIHC/qRT-PCR[UPWARDS ARROW]Oh38
 Gastric cancer38 patientsIHC[UPWARDS ARROW]Ding90
 Pancreatic cancer100 patientsIHC[UPWARDS ARROW]Peng91
NAqRT-PCR[UPWARDS ARROW]Robertson84
 Colorectal cancer25 patientsqRT-PCR[UPWARDS ARROW]Eads45
36 patientsIHCHeterogeneous expressionDe Marzo93
  48 patientsIHC/qRT-PCR[UPWARDS ARROW]Zhu23
  Cell linesqRT-PCR[UPWARDS ARROW]Jin94
 Renal cancer110 patientsIHC[UPWARDS ARROW]Arai12
NAqRT-PCR[UPWARDS ARROW]Robertson84
 Bladder cancer102 patientsIHC[UPWARDS ARROW]Nakagawa85
NAqRT-PCR[UPWARDS ARROW]Robertson84
 Lung cancer153 patientsqRT-PCR[UPWARDS ARROW]Kwon95
102 patientsqRT-PCR[UPWARDS ARROW]Kim52
91 patientsqRT-PCR[UPWARDS ARROW]Vallbohmer51
100 patientsIHC[UPWARDS ARROW]Lin23
 Cervix cancer127 patientsIHC[UPWARDS ARROW]Sawada62
      
DNMT3aHepatic cancer59 patientsIHC[UPWARDS ARROW] nuclear staining; [DOWNWARDS ARROW] cytoplasmic stainingChoi87
25 patientsIHC/qRT-PCR[UPWARDS ARROW]Oh38
 Gastric cancer38 patientsIHC[UPWARDS ARROW]Ding90
 Pancreatic cancerNAqRT-PCR[UPWARDS ARROW]Robertson84
 Colorectal cancer25 patientsqRT-PCR[UPWARDS ARROW]Eads45
 Renal cancerNAqRT-PCR[UPWARDS ARROW]Robertson84
 Bladder cancerNAqRT-PCR[UPWARDS ARROW]Robertson84
 Lung cancer91 patientsqRT-PCR[UPWARDS ARROW]Vallbomer51
100 patientsIHC[UPWARDS ARROW]Lin23
 Tumor cell lines (not specified)NANBH[UPWARDS ARROW]Xie77
      
DNMT3bHepatic cancer25 patientsIHC/qRT-PCR[UPWARDS ARROW]Oh38
 Gastric cancer38 patientsIHC[UPWARDS ARROW]Ding90
 Colorectal cancer25 patientsqRT-PCR[UPWARDS ARROW]Eads45
Cell linesqRT-PCR[UPWARDS ARROW]Jin94
 Renal cancerNAqRT-PCR[UPWARDS ARROW]Robertson84
 Bladder cancerNAqRT-PCR[UPWARDS ARROW]Robertson84
 Lung cancer100 patientsIHC[UPWARDS ARROW]Lin23
102 patientsqRT-PCR[UPWARDS ARROW]Kim52
91 patientsqRT-PCR[UPWARDS ARROW]Vallbohmer51
 Breast cancer12 cell linesqRT-PCRAberrant expressionRoll92
 Tumor cell lines (not specified)NANBH[UPWARDS ARROW]Xie77

DNMT Inhibitors

  1. Top of page
  2. Abstract
  3. Methylation Process and Cancer
  4. Mechanisms of Silencing
  5. DNA Methyltransferases (DNMTs)
  6. DNMT and Cancer
  7. DNMT Inhibitors
  8. Conclusions
  9. CONFLICT OF INTEREST DISCLOSURES
  10. REFERENCES

In contrast to genetic alterations, epigenetic changes in cancer are potentially reversible, which has spurred the development of pharmacologic inhibitors of DNA methylation and histone deacetylation.96 Indeed, the reactivation of epigenetically silenced genes in cancer may have a profound antitumor effect, thereby being a rational target for therapy and prevention.26

DNA methylation inhibitors such as 5-aza-2′-deoxycytidine (decitabine) can be utilized to reverse the effects of methylation, including the reduction of mutations at methylated CpG sites, reactivation of genes suppressed by hypermethylation, and restoration of cell growth control.97, 98 Treatment of cultured cells with this drug has been shown to cause cell growth inhibition, G2/M arrest, and cell apoptosis.83 The disadvantage of this demethylating agent is its myelosuppressive effect, particularly when used at high doses.10

The combination of DNMTs and HDAC inhibitors may have an advantage in the treatment of cancer.26 The use of a histone deacetylase inhibitor such as trichostatin A and phenylbutyrate in combination with 5-aza-2′-deoxycytidine has resulted in a strong synergistic growth inhibition in both cell lines and tumor.99, 100

Aberrant methylation of genes controlling cell proliferation, metastasis, apoptosis, and drug susceptibility has been identified in multiple cancer types. The individual determination of the cases showing hypermethylation of these tumor suppressor genes by molecular techniques such as methylation-specific polymerase chain reaction may contribute to determining the best treatment strategy for each case. So, the addition of DNA methylation inhibitors to the current therapy shows great possibility for improving cancer therapy.

Conclusions

  1. Top of page
  2. Abstract
  3. Methylation Process and Cancer
  4. Mechanisms of Silencing
  5. DNA Methyltransferases (DNMTs)
  6. DNMT and Cancer
  7. DNMT Inhibitors
  8. Conclusions
  9. CONFLICT OF INTEREST DISCLOSURES
  10. REFERENCES

We have described the main epigenetic mechanisms for regulating gene transcription, mainly of tumor suppressor genes. Among these are DNA methylation and histone modification such as deacetylation, intricately related to chromatin configuration. This transcriptional control already exists in normal cells and is essential for adequate development. However, the aberrant epigenetic regulation of gene expression plays an important role in cancer development, as do the genetic alterations, making it an important topic for molecular oncology research.

Because it is a potentially reversible change, the epigenetic event represents new opportunities for the clinical management of cancer through the development of strategies to reverse gene silencing. Further, the associated molecular changes (such as DNMT/HDAC overexpression and gene promoter hypermethylation) may serve as markers for risk assessment, diagnosis and prognosis of cancer.

REFERENCES

  1. Top of page
  2. Abstract
  3. Methylation Process and Cancer
  4. Mechanisms of Silencing
  5. DNA Methyltransferases (DNMTs)
  6. DNMT and Cancer
  7. DNMT Inhibitors
  8. Conclusions
  9. CONFLICT OF INTEREST DISCLOSURES
  10. REFERENCES