Diethanolamine and Phenobarbital Produce an Altered Pattern of Methylation in GC-Rich Regions of DNA in B6C3F1 Mouse Hepatocytes Similar to That Resulting from Choline Deficiency

Xenobiotic metabolites modify immune responses of the cervicovaginal epithelium: potential mechanisms underlying barrier disruption

OBJECTIVE

Xenobiotic metabolites are exogenous biochemicals that can adversely impact reproductive health. We previously identified xenobiotics in cervicovaginal fluid during pregnancy in association with short cervix. In other organ systems, xenobiotics can modify epithelial barrier function. We hypothesise that xenobiotics dysregulate epithelial cell and macrophage immune responses as a mechanism to disrupt the cervicovaginal barrier.

DESIGN

In vitro cell culture system.

SETTING

Laboratory within academic institution.

SAMPLE

Vaginal, ectocervical and endocervical epithelial cell lines and primary macrophages.

METHODS

Cells were treated with diethanolamine (2.5 mM), ethyl glucoside (5 mM) or tartrate (2.5 mM) for 24 h.

MAIN OUTCOME MEASURES

Cytokines and matrix metalloproteinases were measured in cell supernatants (n = 3 per condition). One-way analysis of variance (ANOVA) with Dunnett's test for multiple comparisons was performed.

RESULTS

Diethanolamine induces inflammatory cytokines, whereas ethyl glucoside and tartrate generally exert anti-inflammatory effects across all cells. Diethanolamine increases interleukin 6 (IL-6), IL-8, interferon γ-induced protein 10 kDa (IP-10), growth-regulated oncogene (GRO), fractalkine, matrix metalloproteinase 1 (MMP-1), MMP-9 and MMP-10 (p < 0.05 for all), factors involved in acute inflammation and recruitment of monocytes, neutrophils and lymphocytes. Ethyl glucoside and tartrate decrease multiple cytokines, including RANTES and MCP-1 (p < 0.05 for all), which serve as chemotactic factors. Vaginal cells exhibit heightened inflammatory tone compared with cervical cells and macrophages, with a greater number of differentially expressed analytes after xenobiotic exposure.

CONCLUSIONS

Xenobiotic metabolites present in the cervicovaginal space during pregnancy modify immune responses, unveiling potential pathways through which environmental exposures may contribute to the pathogenesis of cervical remodelling preceding preterm birth. Future work identifying xenobiotic sources and routes of exposure offers the potential to modify environmental risks to improve pregnancy outcomes.

ToxCast chemical library Wnt screen identifies diethanolamine as an activator of neural progenitor proliferation

Numerous autism spectrum disorder (ASD) risk genes are associated with Wnt signaling, suggesting that brain development may be especially sensitive to genetic perturbation of this pathway. Additionally, valproic acid, which modulates Wnt signaling, increases risk for ASD when taken during pregnancy. We previously found that an autism-linked gain-of-function UBE3A T485A mutant construct hyperactivated canonical Wnt signaling, providing a genetic means to elevate Wnt signaling above baseline levels. To identify environmental use chemicals that enhance or suppress Wnt signaling, we screened the ToxCast Phase I and II libraries in cells expressing this autism-linked UBE3A T485A gain-of-function mutant construct. Using structural comparisons, we identify classes of chemicals that stimulated Wnt signaling, including ethanolamines, as well as chemicals that inhibited Wnt signaling, such as agricultural pesticides, and synthetic hormone analogs. To prioritize chemicals for follow-up, we leveraged predicted human exposure data, and identified diethanolamine (DEA) as a chemical that stimulates Wnt signaling in UBE3A T485A -transfected cells, and has a high potential for prenatal exposure in humans. DEA enhanced proliferation in primary human neural progenitor cell lines (phNPC), but did not affect expression of canonical Wnt target genes in NPCs or primary mouse neuron cultures. Instead, we found DEA increased expression of the H3K9 methylation sensitive gene CALB1, consistent with competitive inhibition of the methyl donor enzymatic pathways.

Safety Assessment of Diethanolamine and Its Salts as Used in Cosmetics

The Cosmetic Ingredient Review (CIR) Expert Panel assessed the safety of diethanolamine and its salts as used in cosmetics. Diethanolamine functions as a pH adjuster; the 16 salts included in this rereview reportedly function as surfactants, emulsifying agents, viscosity increasing agents, hair or skin conditioning agents, foam boosters, or antistatic agents. The Panel reviewed available animal and clinical data, as well as information from previous CIR reports. Since data were not available for each individual ingredient, and since the salts dissociate freely in water, the Panel extrapolated from previous reports to support safety. The Panel concluded that diethanolamine and its salts are safe for use when formulated to be nonirritating. These ingredients should not be used in cosmetic products in which N-nitroso compounds can be formed.

Mode of action and human relevance analysis for nuclear receptor-mediated liver toxicity: A case study with phenobarbital as a model constitutive androstane receptor (CAR) activator

The constitutive androstane receptor (CAR) and pregnane X receptor (PXR) are important nuclear receptors involved in the regulation of cellular responses from exposure to many xenobiotics and various physiological processes. Phenobarbital (PB) is a non-genotoxic indirect CAR activator, which induces cytochrome P450 (CYP) and other xenobiotic metabolizing enzymes and is known to produce liver foci/tumors in mice and rats. From literature data, a mode of action (MOA) for PB-induced rodent liver tumor formation was developed. A MOA for PXR activators was not established owing to a lack of suitable data. The key events in the PB-induced liver tumor MOA comprise activation of CAR followed by altered gene expression specific to CAR activation, increased cell proliferation, formation of altered hepatic foci and ultimately the development of liver tumors. Associative events in the MOA include altered epigenetic changes, induction of hepatic CYP2B enzymes, liver hypertrophy and decreased apoptosis; with inhibition of gap junctional intercellular communication being an associative event or modulating factor. The MOA was evaluated using the modified Bradford Hill criteria for causality and other possible MOAs were excluded. While PB produces liver tumors in rodents, important species differences were identified including a lack of cell proliferation in cultured human hepatocytes. The MOA for PB-induced rodent liver tumor formation was considered to be qualitatively not plausible for humans. This conclusion is supported by data from a number of epidemiological studies conducted in human populations chronically exposed to PB in which there is no clear evidence for increased liver tumor risk.

Genome-wide study of DNA methylation alterations in response to diazinon exposure in vitro

Pesticide exposure has repeatedly been associated with cancers. However, molecular mechanisms are largely undetermined. In this study, we examined whether exposure to diazinon, a common organophosphate that has been associated with cancers, could induce DNA methylation alterations. We conducted genome-wide DNA methylation analyses on DNA samples obtained from human hematopoietic K562 cell exposed to diazinon and ethanol using the Illumina Infinium HumanMethylation27 BeadChip. Bayesian-adjusted t-tests were used to identify differentially methylated gene promoter CpG sites. We identified 1069 CpG sites in 984 genes with significant methylation changes in diazinon-treated cells. Gene ontology analysis demonstrated that some genes are tumor suppressor genes, such as TP53INP1 (3.0-fold, q-value <0.001) and PTEN (2.6-fold, q-value <0.001), some genes are in cancer-related pathways, such as HDAC3 (2.2-fold, q-value=0.002), and some remain functionally unknown. Our results provided direct experimental evidence that diazinon may modify gene promoter DNA methylation levels, which may play a pathological role in cancer development.

DNA methylation alterations in response to pesticide exposure in vitro

Although pesticides are subject to extensive carcinogenicity testing before regulatory approval, pesticide exposure has repeatedly been associated with various cancers. This suggests that pesticides may cause cancer via nonmutagenicity mechanisms. The present study provides evidence to support the hypothesis that pesticide-induced cancer may be mediated in part by epigenetic mechanisms. We examined whether exposure to seven commonly used pesticides (i.e., fonofos, parathion, terbufos, chlorpyrifos, diazinon, malathion, and phorate) induces DNA methylation alterations in vitro. We conducted genome-wide DNA methylation analyses on DNA samples obtained from the human hematopoietic K562 cell line exposed to ethanol (control) and several organophosphate pesticides (OPs) using the Illumina Infinium HumanMethylation27 BeadChip. Bayesian-adjusted t-tests were used to identify differentially methylated gene promoter CpG sites. In this report, we present our results on three pesticides (fonofos, parathion, and terbufos) that clustered together based on principle component analysis and hierarchical clustering. These three pesticides induced similar methylation changes in the promoter regions of 712 genes, while also exhibiting their own OP-specific methylation alterations. Functional analysis of methylation changes specific to each OP, or common to all three OPs, revealed that differential methylation was associated with numerous genes that are involved in carcinogenesis-related processes. Our results provide experimental evidence that pesticides may modify gene promoter DNA methylation levels, suggesting that epigenetic mechanisms may contribute to pesticide-induced carcinogenesis. Further studies in other cell types and human samples are required, as well as determining the impact of these methylation changes on gene expression.

Gene expression and mutation assessment provide clues of genetic and epigenetic mechanisms in liver tumors of oxazepam-exposed mice

Liver tumors from a previous National Toxicology Program study were examined using global gene expression and mutation analysis to define the mechanisms of carcinogenesis in mice exposed to oxazepam. Five hepatocellular adenomas and 5 hepatocellular carcinomas from male B6C3F1 mice exposed to 5000 ppm oxazepam and 6 histologically normal liver samples from control animals were examined. One of the major findings in the study was upregulation of the Wnt/β-catenin signaling pathway. Genes that activate β-catenin, such as Sox4, were upregulated, whereas genes that inhibit Wnt signaling, such as APC and Crebbp, were downregulated. In addition, liver tumors from oxazepam-exposed mice displayed β-catenin mutations and increased protein expression of glutamine synthetase, a downstream target in the Wnt signaling pathway. Another important finding in this study was the altered expression of oxidative stress-related genes, specifically increased expression of cytochrome p450 genes, including Cyp1a2 and Cyp2b10, and decreased expression of genes that protect against oxidative stress, such as Sod2 and Cat. Increased oxidative stress was confirmed by measuring isoprostane expression using mass spectrometry. Furthermore, global gene expression identified altered expression of genes that are associated with epigenetic mechanisms of cancer. There was decreased expression of genes that are hypermethylated in human liver cancer, including tumor suppressors APC and Pten. Oxazepam-induced tumors also exhibited decreased expression of genes involved in DNA methylation (Crebbp, Dnmt3b) and histone modification (Sirt1). These data suggest that formation of hepatocellular adenomas and carcinomas in oxazepam-exposed mice involves alteration of the Wnt signaling pathway, oxidative stress, and potential epigenetic alterations.

Coupling global methylation and gene expression profiles reveal key pathophysiological events in liver injury induced by a methyl-deficient diet

SCOPE

A methyl-deficient diet induces liver injury similar to human nonalcoholic steatohepatitis, one of the main risk factors for the development of hepatocellular carcinoma. Previous studies have demonstrated that this diet perturbs DNA methylation by causing a profound loss of global cytosine methylation, predominantly at heavily methylated repetitive sequences. However, whether methyl deficiency affects the methylation status of gene promoters has not been explored.

METHODS AND RESULTS

Mouse gene expression and CpG island microarrays were used to characterize the gene expression and CpG island methylation profiles in the livers of C57BL/6J mice fed a methyl-deficient diet. We detected 164 genes that were differentially expressed and exhibited an inverse relationship between the gene expression and the extent of CpG island methylation. Furthermore, these genes were associated with altered lipid and glucose metabolism, DNA damage and repair, apoptosis, the development of fibrosis, and liver tissue remodeling. Although there were both increased and decreased levels of CpG island methylation, the number of hypomethylated genes was substantially greater than the number of hypermethylated genes.

CONCLUSION

The results this study demonstrate that pairing methylation profiles with gene expression profiles is a powerful approach to identify dysregulated high-priority fundamental pathophysiological pathways associated with disease development.

Effects of mixtures of polychlorinated biphenyls, methylmercury, and organochlorine pesticides on hepatic DNA methylation in prepubertal female Sprague-Dawley rats

DNA methylation is one of the epigenetic mechanisms that regulates gene expression, chromosome structure, and stability. Our objective was to determine whether the DNA methylation system could be a target following in utero and postnatal exposure to human blood contaminants. Pregnant rats were dosed daily from gestation day 1 until postnatal day 21 with 2 dose levels of either organochlorine pesticides (OCP; 0.019 or 1.9 mg/kg/day), methylmercury chloride (MeHg; 0.02 or 2 mg/kg/day), polychlorinated biphenyls (PCBs; 0.011 or 1.1 mg/kg/day), or a mixture (Mix; 0.05, or 5 mg/kg/day) including all 3 groups of chemicals. Livers from 1 female offspring per litter were collected at postnatal day 29. Hepatic analysis revealed that the mRNA abundance for DNA methyltransferase (DNMT)-1, -3a, and -3b were significantly reduced by the high dose of PCB, that the high dose of MeHg also reduced mRNA levels for DNMT-1, and -3b, but that OCP had no significant effects compared with control. The high dose of PCB and Mix reduced the abundance of the universal methyl donor S-adenosylmethionine, and Mix also reduced global genome DNA methylation (5-methyl-deoxycytidine/5-methyl-deoxycytidine + deoxycytidine). The latter is consistent with pyrosequencing methylation analysis, revealing that the high-dose groups (except OCP) generally decreased the methylation of CpG sites (position -63 to -29) in the promoter of the tumor suppressor gene p16(INK4a). Overall, these hepatic results suggest that the DNA methylation system can be affected by exposure to high doses of blood contaminants, and that OCP is the least potent chemical group from the investigated mixtures.

Multiple genes exhibit phenobarbital-induced constitutive active/androstane receptor-mediated DNA methylation changes during liver tumorigenesis and in liver tumors

The constitutive active/androstane receptor (CAR) mediates responses to the nongenotoxic rodent liver tumor promoter phenobarbital (PB), including certain gene expression changes, hepatomegaly, and tumor formation. Aberrant DNA methylation represents epigenetic events that can play multiple roles in tumorigenesis. Previously, 146 unique PB-induced regions of altered DNA methylation (RAMs) were observed in liver tumor-susceptible CAR wild-type (WT) mice (in 23 weeks, precancerous tissue, and 32 weeks, tumor tissue), as compared to the resistant knockout (KO). We believe that at least some of these might be key for tumorigenesis. In the current study, cloning and annotation of a subset (82%) of the unique RAMs revealed 47 genes exhibiting altered methylation; 17 are already implicated in cancer or related processes and, thus, we have identified 30 "new" candidate genes that might be involved in carcinogenesis due to an epigenetic alteration. These may contribute to tumor development through their involvement in angiogenesis, apoptosis, epithelial-mesenchymal cell transition, growth/survival, and invasion/migration/metastasis. We have also, previously, discerned unique PB-elicited RAMs in liver tumor-prone B6C3F1 mice, as compared to the relatively resistant C57BL/6 strain, at 2 or 4 weeks, and identified 51 genes exhibiting altered methylation. Importantly, 11 of these genes were identified from identical, unique RAMs discerned in both the sensitive B6C3F1 and CAR WT mice, thus representing an initial, potential candidate "fingerprint" which might serve as a biomarker for PB-induced tumorigenesis. These two studies reveal "new" genes whose epigenetic statuses changed uniquely in liver tumor-susceptible mice (B6C3F1 and CAR WT), as compared to their resistant counterparts (C57BL/6 and CAR KO, respectively), within a continuum of PB-induced tumorigenesis.

Inhalation of cigarette smoke induces regions of altered DNA methylation (RAMs) in SENCAR mouse lung

The development of early biomarkers, both of exposure and effect, would substantially improve science-based risk assessment with regard to cigarette smoke (CS)-associated toxicity. Altered DNA methylation, an epigenetic mechanism, is linked to CS-induced lung tumorigenesis. We have taken an unbiased approach (i.e. genomic regions are not pre-selected) to assess early methylation changes within lung DNA from female SENCAR mice treated with a single dose of 7,12-dimethylbenz[a]anthracene (DMBA), and then exposed to air alone, or CS for 4 or 8 weeks. Regions of altered DNA methylation (RAMs) were detected in mice treated with DMBA alone, or DMBA+0.16, 0.32 or 0.48 mg wet total particulate matter per liter (WTPM/L) CS, using methylation-sensitive restriction digestion, arbitrarily primed PCR and capillary electrophoresis. Comparison of the RAMs that formed in different treatment groups revealed: (1) RAMs which "carried forward" across time (i.e. occurred at both 4 and 8 weeks) in a particular dose group, in addition to unique RAMs observed only at 8 weeks, and (2) RAMs which "carried forward" across dose (i.e. occurred in at least 2 dose groups at a particular time point), in addition to unique RAMs observed only in 1 dose group. Furthermore, a subset of RAMs was observed, at both 4 and 8 weeks, in DMBA-treated and DMBA+CS-exposed groups; the presence of unique RAMs in the latter suggest that combined DMBA+CS treatment more than just "magnifies" a subset of cell populations bearing the methylation changes induced by DMBA alone. Importantly, only minimal histopathological changes were observed in the lungs of CS-treated mice. This study is the first to demonstrate changes in lung DNA methylation at early times following exposure to CS, e.g., prior to overt histopathology. Thus, altered methylation might serve as a biomarker of CS exposure, and, in light of the fact that methylation changes are linked to CS-induced lung tumorigenesis, might also be useful as biomarkers of effect.

Mapping the epigenome--impact for toxicology

Recent advances in technological approaches for mapping and characterizing the epigenome are generating a wealth of new opportunities for exploring the relationship between epigenetic modifications, human disease and the therapeutic potential of pharmaceutical drugs. While the best examples for xenobiotic-induced epigenetic perturbations come from the field of non-genotoxic carcinogenesis, there is growing evidence for the relevance of epigenetic mechanisms associated with a wide range of disease areas and drug targets. The application of epigenomic profiling technologies to drug safety sciences has great potential for providing novel insights into the molecular basis of long-lasting cellular perturbations including increased susceptibility to disease and/or toxicity, memory of prior immune stimulation and/or drug exposure, and transgenerational effects.

Identification of genes that may play critical roles in phenobarbital (PB)-induced liver tumorigenesis due to altered DNA methylation

Aberrant DNA methylation plays important roles in tumorigenesis, and the nongenotoxic rodent tumor promoter phenobarbital (PB) alters methylation patterns to a greater extent in liver tumor susceptible as compared to resistant mice (Watson and Goodman, 2002). Unique hepatic regions of altered DNA methylation (RAMs) were identified in sensitive B6C3F1, as compared to resistant C57BL/6, mice at 2 or 4 weeks of PB treatment using a novel approach involving methylation-sensitive restriction digestion, arbitrarily primed PCR, and capillary electrophoresis (Bachman et al., 2006b). PCR products representing 90 of 170 (53%) total unique B6C3F1 RAMs at 2 or 4 weeks were cloned and subjected to BLAST-like alignment tool searches that resulted in 51 gene matches; some of these have documented oncogenic or tumor suppressor roles. Importantly, uniquely hypomethylated genes play roles in angiogenesis (e.g., chymase 1, tyrosine kinase nonreceptor 2, and possibly ephrin B2 and triple functional domain, PTPRF interacting) and invasion and metastasis, including those involved in the epithelial-mesenchymal transition (transcription factor 4, transforming growth factor beta receptor II, and ral guanine nucleotide dissociation stimulator). Common cellular targets and regulators of the genes representing unique B6C3F1 RAMs were uncovered, indicating that they might act in concert to more efficiently promote tumorigenesis. Genes not previously associated with mouse liver tumorigenesis exhibited altered methylation at these very early times following PB treatment. We hypothesize that at least some of the unique PB-induced B6C3F1 RAMs represent key events facilitating transformation, which is consistent with a causative role of altered DNA methylation during early stages of tumorigenesis.

Epigenetic aspects of genotoxic and non-genotoxic hepatocarcinogenesis: studies in rodents

Hepatocellular carcinoma, which is one of the most prevalent life-threatening human cancers, is showing an increased incidence worldwide. Recent evidence indicates that the development of hepatocellular carcinoma is associated with not only genetic alterations, but also with profound epigenetic changes. This review summarizes the current knowledge about epigenetic alterations during rodent hepatocarcinogenesis, considers the similarities and differences in epigenetic effects of genotoxic and non-genotoxic rodent liver carcinogens, and discusses the possible role of these effects in the causality of liver tumor development.

Diethanolamine alters proliferation and choline metabolism in mouse neural precursor cells

Diethanolamine (DEA) is a widely used ingredient in many consumer products and in a number of industrial applications. It has been previously reported that dermal administration of DEA to mice diminished hepatic stores of choline and altered brain development in the fetus. The aim of this study was to use mouse neural precursor cells in vitro to assess the mechanism underlying the effects of DEA. Cells exposed to DEA treatment (3mM) proliferated less (by 5-bromo-2-deoxyuridine incorporation) at 48 h (24% of control [CT]), and had increased apoptosis at 72 h (308% of CT). Uptake of choline into cells was reduced by DEA treatment (to 52% of CT), resulting in diminished intracellular concentrations of choline and phosphocholine (55 and 12% of CT, respectively). When choline concentration in the growth medium was increased threefold (to 210 microM), the effects of DEA exposure on cell proliferation and apoptosis were prevented, however, intracellular phosphocholine concentrations remained low. In choline kinase assays, we observed that DEA can be phosphorylated to phospho-DEA at the expense of choline. Thus, the effects of DEA are likely mediated by inhibition of choline transport into neural precursor cells and by altered metabolism of choline. Our study suggests that prenatal exposure to DEA may have a detrimental effect on brain development.

Altered methylation in gene-specific and GC-rich regions of DNA is progressive and nonrandom during promotion of skin tumorigenesis

Altered DNA methylation, an epigenetic mechanism, likely contributes to tumorigenesis, with an inverse relationship existing between methylation in a promoter region and transcription. Using the SENCAR two-stage mouse skin tumorigenesis model, altered methylation was characterized in precancerous tissue and in tumor tissue. Mouse skin was initiated with 7,12-dimethylbenz[a]anthracene and promoted three times a week with 3, 9, 18, or 27 mg cigarette smoke condensate (CSC) for 4, 8, or 29 weeks; tumors were collected at 29 weeks. In addition, reversibility of changes in methylation was assessed following cessation of the promoting stimulus. DNA was isolated, and GC-rich methylation was assessed quantitatively via methylation-sensitive restriction digestion, arbitrarily primed PCR, and electrophoretic separation of PCR products. Analysis focused on regions of altered methylation (RAMs), which persisted from 4 to 8 weeks and from 8 weeks to tumor tissue. Persistent RAMs (i.e., seen in precancerous tissue and carried forward to tumors) are likely to play a key role in tumorigenesis. Twenty-two CpG sites in the upstream region of the Ha-ras promoter were unmethylated in control skin, 27 mg CSC, and tumor tissue. At two CpG sites closer to the transcriptional start site the incidence of hypomethylation increased with the dose of CSC. Hypomethylation was detected in all tumor samples. Expression of Ha-ras increased with 18 and 27 mg CSC promotion and more so in tumor tissue. These data support our hypothesis that tumor promotion involves instability of the epigenome, providing an environment where changes in the methylation status of specific regions of the genome accumulate progressively and contribute to the clonal expansion of initiated cells that leads to tumor formation.

Phenobarbital induces progressive patterns of GC-rich and gene-specific altered DNA methylation in the liver of tumor-prone B6C3F1 mice

Altered DNA methylation contributes to tumorigenesis by affecting gene expression in a heritable fashion. Phenobarbital (PB) is a nongenotoxic rodent carcinogen which induces global hypomethylation and regions of hypermethylation in mouse liver. Liver tumor-sensitive (B6C3F1) and -resistant (C57BL/6) male mice were administered 0.05% (wt/wt) PB in drinking water for 2 or 4 weeks, and a 2-week recovery was included following each dosing period. DNA was isolated from liver (target) and kidney (nontarget) tissues. The methylation status of GC-rich regions of DNA was assessed via methylation-sensitive restriction digestion, arbitrarily primedpolymerase chain reaction, and capillary electrophoretic separation of products. PB-induced regions of altered methylation (RAMs) which carry forward from an early to a later time point are more likely to be mechanistically relevant as compared to those that do not. Twelve of 69 RAMs (17%) present in B6C3F1 liver at 2 weeks were also seen at 4 weeks, while only 1 of the 123 RAMs (< 1%) present in C57BL/6 liver was seen at 4 weeks. In the B6C3F1 mice, 57 unique (as compared to the C57BL/6) regions of altered hepatic methylation (RAMs), predominantly hypomethylation, were observed at 2 weeks, increasing to 86 at 4 weeks. Changes in methylation were largely reversible. Altered methylation in liver was highly dissimilar to that of kidney. Following 4 weeks PB, bisulfite sequencing revealed hypomethylation of Ha-ras in B6C3F1, but not C57BL/6, which correlated with increased gene expression. These data indicate that (1) progressive, nonrandom changes in methylation provide an epigenetic mechanism underlying the ability of PB to cause mouse liver tumorigenesis and (2) susceptibility to tumorigenesis is related inversely to the capacity to maintain normal patterns of methylation.