CpG island methylator phenotype in colorectal cancer

No SMAD4 hypermethylation in colorectal cancer

The chromosome region 18q21 is frequently deleted in colorectal cancers. Three candidate tumour suppressor genes, DCC, SMAD4 and SMAD2, map to this region. The SMAD4(DPC4) gene was recently identified as a candidate pancreatic cancer suppressor gene. It is also a gene for juvenile polyposis tumour predisposition syndrome. Somatic SMAD4 mutations have been detected in some colorectal carcinomas. However, the frequency of these mutations is relatively low, and whether SMAD4 plays a key role in colorectal tumorigenesis is still unclear. In addition to loss of chromosomal material and intragenic mutations there is a third mechanism, DNA methylation, which may have an important role in gene inactivation. In the present study, we examined whether promoter hypermethylation could be a mechanism for SMAD4 inactivation. In total, 42 colorectal tumours were selected for the methylation analysis and no evidence of promoter hypermethylation was found. Our result suggests that hypermethylation of the SMAD4 promoter region is not a frequent event in colorectal tumorigenesis.

Hyperplastic polyps and DNA microsatellite unstable cancers of the colorectum

Although the scientific and clinical rationale for classifying colorectal cancer according to mechanisms underlying genetic instability is well supported, little is known of the early morphogenesis of sporadic cancer showing high levels of DNA microsatellite instability (MSI-H). Evidence is accumulating that the traditional adenoma-carcinoma sequence may not apply to sporadic MSI-H colorectal cancer. The serrated pathway comprising hyperplastic polyps, mixed polyps and serrated adenomas may serve as the missing link. This review relates the recently described CpG island methylator phenotype (CIMP) to the serrated pathway. Two rate-limiting genetic steps may underlie the neoplastic pathway associated with CIMP. A transmembrane receptor expressed by pericryptal myofibroblasts (HPP1) may be implicated in the transition from normal to hyperplasia whereas inactivation of hMLH1 is responsible for the conversion of hyperplasia to dysplasia through loss of DNA mismatch repair. These mechanisms fit with clinical observations relating to sporadic MSI-H colorectal cancer, specifically proximal location, multiplicity, higher frequency among females and rapid evolution of early cancer.

Frequent hypermethylation of the hMLH1 gene promoter in differentiated-type tumors of the stomach with the gastric foveolar phenotype

Hypermethylation of the hMLH1 mismatch repair gene promoter has been revealed to lead to microsatellite instability (MSI). Previously, we demonstrated a high prevalence of MSI in differentiated-type gastric tumors showing distinct features of gastric foveolar epithelium (foveolar type). To clarify the significance of hMLH1 promoter hypermethylation in the development of this tumor type, we studied promoter methylation status and expression of hMLH1 in foveolar-type tumors and their surrounding non-neoplastic mucosae, as well as in tumors with other cellular phenotypes. The results were compared to MSI status. After phenotypical analyses using mucin histochemistry and immunohistochemistry, 41 differentiated-type tumors with distinct cellular phenotypes were classified into three categories: foveolar type, intestinal type (tumors with the distinct cellular phenotype of the intestine), and combined type (tumors with both foveolar and intestinal phenotypes). Methylation-specific polymerase chain reaction (MSP) was performed to determine the methylation status of hMLH1 promoter. hMLH1 protein expression was immunohistochemically examined. MSI was detected in 57% of the foveolar type, 8% of the intestinal type, and 67% of the combined-type tumors. Hypermethylation of hMLH1 promoter was found in 74% of the foveolar type, 33% of the intestinal type, and 83% of the combined-type tumors. Of 18 MSI-positive tumors, all but one were hypermethylated. Methylation status of hMLH1 promoter correlated well with protein expression in foveolar-type tumors. Moreover, hypermethylation was also detected frequently (71%) in the non-neoplastic surrounding mucosa of the hypermethylated tumors. Hypermethylation of hMLH1 promoter is an initial, vital event in the development of foveolar-type tumors of the stomach.

Distinct methylation patterns of two APC gene promoters in normal and cancerous gastric epithelia

The adenomatous polyposis coli (APC) tumor suppressor gene is mutationally inactivated in both familial and sporadic forms of colorectal cancers. In addition, hypermethylation of CpG islands in the upstream portion of APC, a potential alternative mechanism of tumor suppressor gene inactivation, has been described in colorectal cancer. Because a subset of both gastric and colorectal cancers display the CpG island methylator phenotype, we hypothesized that epigenetic inactivation of APC was likely to occur in at least some gastric cancers. APC exhibits two forms of transcripts from exons 1A and 1B in the stomach. Therefore, we investigated CpG island methylation in the sequences upstream of exons 1A and 1B, i.e., promoters 1A and 1B, respectively. We evaluated DNAs from 10 gastric cancer cell lines, 40 primary gastric cancers, and 40 matching non-cancerous gastric mucosae. Methylated alleles of promoter 1A were present in 10 (100%) of 10 gastric cancer cell lines, 33 (82.5%) of 40 primary gastric cancers, and 39 (97.5%) of 40 noncancerous gastric mucosae. In contrast, promoter 1B was unmethylated in all of these same samples. APC transcripts from exon 1A were not expressed in nine of the 10 methylated gastric cancer cell lines, whereas APC transcripts were expressed from exon 1B. Thus, expression from a given promoter correlated well with its methylation status. We conclude that in contrast to the colon, methylation of promoter 1A is a normal event in the stomach; moreover, promoter 1B is protected from methylation in the stomach and thus probably does not participate in this form of epigenetic APC inactivation.

PAX6 methylation and ectopic expression in human tumor cells

The mechanisms underlying the de novo methylation of CpG islands in human cancer remain almost completely unknown. We used a methylation-sensitive arbitrarily primed polymerase chain reaction (Ms AP-PCR) technique to scan genomic DNA for differential methylation patterns and identified a 550 bp band that was hypermethylated in a majority of colon and bladder cancer cells. This band corresponded to a CpG-rich region in exon 5 of PAX6, which is a highly conserved transcription regulatory factor involved in embryogenesis. Interestingly, exon 5 was very frequently methylated in solid tumors compared with adjacent normal tissues, 17 out of 27 (63%) in bladder, and colon, but the promoter remained unmethylated in all these cases. This methylation was not effective in blocking transcription since ectopic expression of PAX6 was seen in several tumors and cell lines with extensive exon 5 methylation. De novo methylation of the promoter was only seen in tumor cell lines and was associated with gene silencing since treatment with 5-aza-2'-deoxycytidine restored expression to the cells and resulted in a less methylated promoter. Thus, ectopic expression and hypermethylation of exon 5 of PAX6 demonstrate that methylation within a transcribed region, as opposed to promoter methylation, does not block gene expression.

Neoplastic progression occurs through mutator pathways in hyperplastic polyposis of the colorectum

AIM

Colorectal cancer has been described in association with hyperplastic polyposis but the mechanism underlying this observation is unknown. The aim of this study was to characterise foci of dysplasia developing in the polyps of subjects with hyperplastic polyposis on the basis of DNA microsatellite status and expression of the DNA mismatch repair proteins hMLH1, hMSH2, and hMSH6.

MATERIALS AND METHODS

The material was derived from four patients with hyperplastic polyposis and between one and six synchronous colorectal cancers. Normal (four), hyperplastic (13), dysplastic (13), and malignant (11) samples were microdissected and a PCR based approach was used to identify mutations at 10 microsatellite loci, TGFbetaIIR, IGF2R, BAX, MSH3, and MSH6. Microsatellite instability-high (MSI-H) was diagnosed when 40% or more of the microsatellite loci showed mutational bandshifts. Serial sections were stained for hMLH1, hMSH2, and hMSH6.

RESULTS

DNA microsatellite instability was found in 1/13 (8%) hyperplastic samples, in 7/13 (54%) dysplastic foci, and in 8/11 (73%) cancers. None of the MSI-low (MSI-L) samples (one hyperplastic, three dysplastic, two cancers) showed loss of hMLH1 expression. All four MSI-H dysplastic foci and six MSI-H cancers showed loss of hMLH1 expression. Loss of hMLH1 in MSI-H but not in MSI-L lesions showing dysplasia or cancer was significant (p<0.001, Fisher's exact test). Loss of hMSH6 occurred in one MSI-H cancer and one MSS focus of dysplasia which also showed loss of hMLH1 staining.

CONCLUSION

Neoplastic changes in hyperplastic polyposis may occur within a hyperplastic polyp. Neoplasia may be driven by DNA instability that is present to a low (MSI-L) or high (MSI-H) degree. MSI-H but not MSI-L dysplastic foci are associated with loss of hMLH1 expression. At least two mutator pathways drive neoplasia in hyperplastic polyposis. The role of the hyperplastic polyp in the histogenesis of sporadic DNA microsatellite unstable colorectal cancer should be examined.

The epigenetics of colorectal cancer

Colorectal cancer has provided an excellent model for studying the genetic basis of cancer and is one of the better-understood malignancies in this regard. The orderly progression of the disease, with distinct genetic alterations at each step, is a useful framework for deciphering the molecular basis of neoplasia. Epigenetics, the study of clonal changes in gene expression without associated genetic lesions, has raised increased interest recently, in part because of the identification of DNA methylation as a potential molecular mediator of the process. Several tumor-suppressor genes are silenced in various neoplasms in association with aberrant promoter methylation, and in the absence of coding region mutations. The study of DNA methylation changes in colorectal cancer has now provided additional clues into the pathogenesis of the disease. This review presents evidence for a model whereby DNA methylation changes play two distinct roles in the molecular evolution of colorectal cancer. Initially, progressive methylation and silencing of a subset of genes takes place in normal tissues as a function of age or time-dependent events and predisposes these normal cells to neoplastic transformation. At a later stage of disease progression, DNA methylation plays an important role in a subset of tumors affected by the CpG island methylator phenotype (CIMP), a recently identified pathway that results in a form of epigenetic instability through the simultaneous silencing of multiple genes. DNA methylation changes have important interactions with genetic lesions in this cancer type. CIMP+ cancers include the majority of tumors with sporadic mismatch repair deficiency through hypermethylation of the hMLH1 promoter, and also account for the majority of tumors with Ki-ras mutations through an unknown mechanism. By contrast, CIMP- cases evolve along a more classic genetic instability pathway, with a high rate of p53 mutations and chromosomal changes. Thus, the integration of epigenetic and genetic information provides a more complete molecular understanding of colorectal cancer and may have implications for the diagnosis, prognosis, and treatment of patients affected by this disease.

How does DNA methyltransferase cause oncogenic transformation?

Global hypomethylation of genes and repetitive sequences, as well as hypermethylation of certain genes known to be involved in tumor suppression, are observed concurrently in cancer cells. Aberrant expression of DNA methyltransferase 1 (dnmt1) is a downstream effector of multiple tumorigenic pathways, and several data suggest that dnmt1 plays a causal role in these pathways. These data raise two critical questions: Why does ectopic expression of dnmt1 transform cells? and How can global hypomethylation exist in a cell that bears high levels of DNMT1 activity? It is proposed that DNMT1 induces cellular transformation by a mechanism that does not involve DNA methylation and that the low level of methylation in cancer cells is a result of induction of a DNA demethylase in these cells. Both DNMT1 and the demethylase play a causal role in cellular transformation and are candidate anticancer targets.

Different pathways of colorectal carcinogenesis and their clinical pictures

An entertaining debate in cancer genetics concerns the relative importance of selection for a growth advantage and genomic instability in tumorigenesis. Below, I present evidence that shows that selection is more important, particularly in the early stages of tumorigenesis, but that genomic instability is also an indisputable feature of many cancers. One attractive possibility is that some mutations may have pleiotropic effects both on cell replication or apoptosis and on genomic instability.

Association of tumour site and sex with survival benefit from adjuvant chemotherapy in colorectal cancer

BACKGROUND

Adjuvant chemotherapy can improve 5-year survival in Dukes' C colorectal carcinoma. Improved selection of patients who will respond to adjuvant treatments is required. We investigated whether site of tumour origin, sex, and presence of microsatellite instability (MSI) phenotype were associated with a survival benefit from adjuvant chemotherapy.

METHODS

We analysed data for 656 consecutive patients with Dukes' C colorectal carcinoma, with median follow-up of 54 months (range 7-104) and mean age 66.7 years (SD 12.9). We screened tumour samples by PCR for deletions in the BAT-26 mononucleotide repeat to establish MSI status. Details of chemotherapy and survival were obtained by review of hospital and health-department records. Adjuvant chemotherapy (fluorouracil and levamisole) was given with curative intent to 272 (42%) patients.

FINDINGS

Striking survival benefits were seen for patients who had right-sided tumours and who received adjuvant chemotherapy compared with those who did not (48 vs 27% alive at end of study [95% CI 0.25-0.56], p<0.0001), for women (53 vs 33% [0.25-0.56], p<0.0001), and for patients with MSI tumours (90 vs 35% [0.01-0.53], p=0.0007). MSI-positive tumours were slightly more frequent in women than in men (10 vs 7%). Right-sided tumours were more frequently MSI positive than left-sided tumours (20 vs 1%). Men with right-sided tumours benefited from chemotherapy (37 vs 12% [0.24-0.69], p=0.0007) but men with left-sided tumours did not.

INTERPRETATION

The survival benefits seen in patients treated with adjuvant chemotherapy suggest that data from previous trials of adjuvant chemotherapy should be reassessed and the predictive value of MSI status confirmed. Validation of our results will allow better selection of patients for chemotherapy.

5-methylcytosine-DNA glycosylase activity is present in a cloned G/T mismatch DNA glycosylase associated with the chicken embryo DNA demethylation complex

We previously have shown that DNA demethylation by chicken embryo 5-methylcytosine DNA glycosylase (5-MCDG) needs both RNA and proteins. One of these proteins is a RNA helicase. Further peptides were sequenced, and three of them are identical to the mammalian G/T mismatch DNA glycosylase. A 3,233-bp cDNA coding for the chicken homologue of human G/T mismatch DNA glycosylase was isolated and sequenced. The derived amino acid sequence (408 aa) shows 80% identity with the human G/T mismatch DNA glycosylase, and both the C and N-terminal parts have about 50% identity. As for the highly purified chicken embryo DNA demethylation complex the recombinant protein expressed in Escherichia coli has both G/T mismatch and 5-MCDG activities. The recombinant protein has the same substrate specificity as the chicken embryo 5-MCDG where hemimethylated DNA is a better substrate than symmetrically methylated CpGs. The activity ratio of G/T mismatch and 5-MCDG is about 30:1 for the recombinant protein expressed in E. coli and 3:1 for the purified enzyme from chicken embryos. The incubation of a recombinant CpG-rich RNA isolated from the purified DNA demethylation complex with the recombinant enzyme strongly inhibits G/T mismatch glycosylase while slightly stimulating the activity of 5-MCDG. Deletion mutations indicate that G/T mismatch and 5-MCDG activities share the same areas of the N- and C-terminal parts of the protein. In reconstitution experiments RNA helicase in the presence of recombinant RNA and ATP potentiates the activity of 5-MCDG.

Genetic and epigenetic modification of MLH1 accounts for a major share of microsatellite-unstable colorectal cancers

Microsatellite instability (MSI) is a hallmark of hereditary nonpolyposis colorectal cancer, and in these patients, results from inherited defects in DNA mismatch repair genes, mostly MSH2 and MLH1. MSI also occurs in 15% of sporadic colorectal cancers, but in these tumors, its basis is less well characterized. We investigated 46 sporadic MSI+ colorectal cancers for changes in MSH2 and MLH1 protein expression, followed by the analysis of somatic mutation, loss of heterozygosity (LOH), and promoter hypermethylation as possible underlying defects. Most cases (36/46, 78%) showed lost or reduced MLH1 expression. Among these, a majority (83%) was associated with MLH1 promoter hypermethylation, whereas the rates of LOH and somatic mutation of MLH1 were 24% and 13%, respectively. Hypermethylation and LOH were inversely correlated, suggesting that they had alternative functions in the inactivation of MLH1. MSH2 expression was lost in 7/46 (15%), and of these, 2 (29%) showed LOH and/or somatic mutation of MSH2. We conclude that most sporadic MSI+ colorectal cancers have an MLH1-associated etiology and that epigenetic modification is a major mechanism of MLH1 inactivation. Moreover, we found a significantly lower prevalence for MLH1 promoter hypermethylation in hereditary nonpolyposis colorectal cancer tumors with MLH1 germline mutations (12/26, 46%), which might explain some differences that are known to occur in the clinicopathological characteristics and tumorigenic pathways between sporadic and hereditary MSI+ colorectal cancers.

Correlation of p16 hypermethylation with p16 protein loss in sporadic gastric carcinomas

Hypermethylation of p16 has been detected frequently in a variety of cancer cells and is known to repress the level of p16 transcription. In human gastric carcinoma (GC) cells, p16 protein loss has often been detected, but genetic alterations of p16 are infrequent. To investigate the molecular mechanism of p16 gene inactivation in gastric carcinogenesis, we examined the methylation status of p16 in GC using methylation-specific PCR. Thirty-seven of eighty-eight (42%) GC showed p16 hypermethylation. Immunohistochemical analysis of 41 cases of GC showed a complete loss of p16 immunoreactivity in 19 of 22 (86%) methylation-positive cases, but in only 2 of 19 (11%) methylation-negative cases. Of 88 GC, 21 cases were previously identified as having microsatellite instability (MSI). Interestingly, 13 of 21 (62%) MSI-positive tumors and 24 of 67 (36%) MSI-negative tumors had hypermethylation on p16. The relatively high frequency of hypermethylation on p16 and the strong correlation between the immunoreactivity and methylation patterns suggest that methylation is an important mechanism for p16 gene inactivation in GC.

DNA hypermethylation in tumorigenesis: epigenetics joins genetics

Recently, the concept that epigenetic, as well as genetic, events might be central to the evolution of human cancer is re-emerging. Cancers often exhibit an aberrant methylation of gene promoter regions that is associated with loss of gene function. This DNA change constitutes a heritable state, not mediated by altered nucleotide sequence, that appears to be tightly linked to the formation of transcriptionally repressive chromatin. This epigenetic process acts as an alternative to mutations to disrupt tumor-suppressor gene function and can predispose to genetic alterations through inactivating DNA-repair genes. Dissecting the molecular processes that mediate these methylation changes will enhance our understanding of chromatin modeling and gene regulation and might present novel possibilities for cancer therapy. Methylation changes constitute potentially sensitive molecular markers to define risk states, monitor prevention strategies, achieve early diagnosis, and track the prognosis of cancer.

Quantitative DNA methylation analysis by fluorescent polymerase chain reaction single-strand conformation polymorphism using an automated DNA sequencer

A novel DNA methylation assay technique, termed bisulfite single-strand conformation polymorphism (bisulfite-SSCP), is a combination of sodium-bisulfite modification and fluorescence-based polymerase chain reaction (PCR)-SSCP. After bisulfite treatment followed by PCR amplification, methylated and unmethylated alleles can be simultaneously separated in a nondenaturing gel using an automated DNA sequencer. Using bisulfite-SSCP, methylation of hMLH1 was detected in a quantitative manner. This method is not only simple, quick, accurate, and quantitative, but detailed information about methylation is also available with less work. Methylation analysis of large numbers of samples for multiple loci will be facilitated by bisulfite-SSCP.

Microsatellite instability markers in breast cancer: a review and study showing MSI was not detected at 'BAT 25' and 'BAT 26' microsatellite markers in early-onset breast cancer

Microsatellite markers may provide evidence of faulty DNA mismatch repair (MMR) via the detection of microsatellite instability (MSI). The choice of microsatellite markers may impact on the MSI detection rate. In hereditary non-polyposis colon cancer (HNPCC), several informative microsatellite markers have been recommended. Two of these, BAT 25 and BAT 26, are quasi-homozygous, enabling analysis of tumour DNA in the absence of paired normal DNA. Sixty-six breast cancer patients under 45 years of age at diagnosis were examined for MSI at BAT 25 and BAT 26. Tumour DNA was extracted from paraffin-embedded tissue. No MSI was detected at the BAT 25 or BAT 26 loci. An additional five microsatellite markers, known to be informative for HNPCC, were examined for MSI in these patients. Apparently-normal profiles were achieved. A tabulated survey of 306 microsatellite markers used to detect MSI in breast cancer revealed that only 35.5% of markers detected MSI at an average rate of 2.9%. The MSI detection rate at the specific HNPCC markers varied from 0% to 10% in breast cancer, with D175250 and TP53 being the HNPCC markers most suitable for analysis of breast cancer. The size of the microsatellite marker's repeat unit did not impact on MSI detection rates. Compiled data from large studies (n > 100) revealed D115988 as the marker with the highest MSI detection rate. Genomic instability pathways of carcinogenesis, characterised by MMR defects and MSI, appear to play a role in the genesis of some breast cancer types.

DNA methylator and mismatch repair phenotypes are not mutually exclusive in colorectal cancer cell lines

A potential link between DNA repair and de novo methylation of exogenous sequences in colorectal cancer cell lines suggested that cells deficient in mismatch repair (MMR-) had an increased ability to silence the introduced virus promoter by DNA methylation due to the presence of a methylator phenotype (MET+) (Lengauer et al., 1997a). We explored this relationship in more detail and found that although there was a clear difference in the abilities of MMR+ cells to express the viral promoter compared to their MMR- counterparts, this difference was not consistently explained by levels of methylation in the viral promoter. Furthermore, we were unable to distinguish differences between the levels of methylation of six endogenous known CpG islands or 100 random DNA fragments containing CCGG sites within the cells. No consistent differences between the abilities of the cells to methylate the CpG island in exon 2 of the p16 gene were observed after transient demethylation by 5-aza-2'-deoxycytidine nor in the levels of expression of three human methyltransferase enzymes. Our results do not therefore support the existence of mutually exclusive DNA methylation (MET) and DNA repair (MMR) phenotypes. Oncogene (2000) 19, 943 - 952.

Distinct genetic profiles in colorectal tumors with or without the CpG island methylator phenotype

Colorectal cancers (CRCs) are characterized by multiple genetic (mutations) and epigenetic (CpG island methylation) alterations, but it is not known whether these evolve independently through stochastic processes. We have recently described a novel pathway termed CpG island methylator phenotype (CIMP) in CRC, which is characterized by the simultaneous methylation of multiple CpG islands, including several known genes, such as p16, hMLH1, and THBS1. We have now studied mutations in K-RAS, p53, DPC4, and TGFbetaRII in a panel of colorectal tumors with or without CIMP. We find that CIMP defines two groups of tumors with significantly different genetic lesions: frequent K-RAS mutations were found in CIMP(+) CRCs (28/41, 68%) compared with CIMP(-) cases (14/47, 30%, P = 0.0005). By contrast, p53 mutations were found in 24% (10/41) of CIMP(+) CRCs vs. 60% (30/46) of CIMP(-) cases (P = 0.002). Both of these differences were independent of microsatellite instability. These interactions between CIMP, K-RAS mutations, and p53 mutations were preserved in colorectal adenomas, suggesting that they occur early in carcinogenesis. The distinct combinations of epigenetic and genetic alterations in each group suggest that activation of oncogenes and inactivation of tumor suppressor genes is related to the underlying mechanism of generating molecular diversity in cancer, rather than simply accumulate stochastically during cancer development.

Suppression of metallothionein gene expression in a rat hepatoma because of promoter-specific DNA methylation

Metallothionein I can be induced in response to a variety of agents that include heavy metals and oxidative stress. On the contrary, its induction was suppressed in some lymphoid-derived cancer cells. The mechanism of this repression has not been elucidated. Here, we show silencing of MT-I gene in a solid transplanted rat tumor as a result of promoter methylation at all the 21 CpG dinucleotides that span the region from -225 bp to +1 bp. By contrast, none of these CpG dinucleotides were methylated in the livers from the rats bearing the tumor, which was consistent with the efficient induction of the gene in this tissue by zinc sulfate. Genomic footprinting revealed lack of access of the transcriptional activators to the respective cis-acting elements of the methylated MT-I promoter in the hepatoma. The absence of footprinting was not due to inactivation of the metal regulatory transcription factor MTF-1, because it was highly active in the hepatoma. Treatment of the hepatoma bearing rats with 5-azacytidine, a demethylating agent, induced basal as well as heavy metal-activated MT-I gene expression in the hepatoma, implying that methylation was indeed responsible for silencing the gene. Bisulfite genomic sequencing showed significant (>90%) demethylation of CpG dinucleotides spanning MT-I promoter in the hepatoma following treatment with 5-AzaC. The hypermethylation of MT-I promoter was probably caused by significantly higher (as much as 7-fold) level of DNA methyl transferase activity as well as enhanced expression of its gene in the hepatoma relative to the host liver. These data elucidated for the first time the molecular mechanism for the silencing of a highly inducible gene in a solid tumor transplanted in an animal, as compared with the robust induction in the corresponding parental tissue and have discussed the probable reasons for the suppression of this gene in some tumors.

The role of DNA hypermethylation in human neoplasia

Cancer development and progression is dictated by a series of alterations in genes such as oncogenes, tumor suppressor genes, DNA repair genes, and others. DNA methylation is an epigenetic modification that is profoundly altered in most cancers. Recently, hypermethylation of CpG-rich areas located in the promoter of genes (CpG islands) has been shown to be commonly implicated in silencing tumor suppressor genes in cancer. By cloning and characterizing a large number of such CpG islands hypermethylated in colon cancer, we found that two processes explain most of these events. Age-related CpG island methylation in a subset of cells in normal tissues, followed by intensification of methylation in cancer cells explains the majority of hypermethylation events in colon cancer and may provide a mechanistic link between aging and cancer formation. Most of the other CpG islands methylated in colon cancer can be explained by a newly described phenotype, the CpG island methylator phenotype (CIMP) which results in multiple methylation events in a subset of cancers. CIMP accounts for the majority of sporadic colon cancers characterized by microsatellite instability, as well as most tumors with k-ras mutations. Understanding further the factors that lead to, and modulate, aberrant methylation in cancer may provide novel avenues for prevention and treatment of this disease.

Epigenetic phenotypes distinguish microsatellite-stable and -unstable colorectal cancers

Aberrant DNA methylation is a common phenomenon in human cancer, but its patterns, causes, and consequences are poorly defined. Promoter methylation of the DNA mismatch repair gene MutL homologue (MLH1) has been implicated in the subset of colorectal cancers that shows microsatellite instability (MSI). The present analysis of four MspI/HpaII sites at the MLH1 promoter region in a series of 89 sporadic colorectal cancers revealed two main methylation patterns that closely correlated with the MSI status of the tumors. These sites were hypermethylated in tumor tissue relative to normal mucosa in most MSI(+) cases (31/51, 61%). By contrast, in the majority of MSI(-) cases (20/38, 53%) the same sites showed methylation in normal mucosa and hypomethylation in tumor tissue. Hypermethylation displayed a direct correlation with increasing age and proximal location in the bowel and was accompanied by immunohistochemically documented loss of MLH1 protein both in tumors and in normal tissue. Similar patterns of methylation were observed in the promoter region of the calcitonin gene that does not have a known functional role in tumorigenesis. We propose a model of carcinogenesis where different epigenetic phenotypes distinguish the colonic mucosa in individuals who develop MSI(+) and MSI(-) tumors. These phenotypes may underlie the different developmental pathways that are known to occur in these tumors.

CpG island methylator phenotypes in aging and cancer

CpG islands are short stretches of CpG rich regions that are frequently associated with the promoter region of genes. Aberrant methylation of CpG islands is one mechanism of inactivating tumor suppressor genes (TSGs) in neoplasia, and there is growing evidence that altered cytosine methylation play important roles in cancer development. However, the differences in global CpG island methylation patterns between normal and cancer cells remain poorly understood. By examining a large number of loci in a series of cancers, global methylation profiles can be constructed. Such studies revealed that in colorectal cancer, there appears to be two types of methylation that are associated with cancer progression: type A (for age-related) methylation, and type C (for cancer-specific) methylation. Initially, type A methylation arises as a function of age in normal colorectal epithelial cells. By affecting genes that regulate the growth and/or differentiation of these cells, such methylation may result in a predisposition state that precedes tumor formation in the colon. Type C methylation, by contrast, was found exclusively in a subset of cancers, which display a CpG island methylator phenotype (CIMP). CIMP is a novel molecular instability pathway that appears to be responsible for most cases of aberrant TSG methylation in colorectal cancer, and which has important interactions with genetic pathways as well. In fact, CIMP+ tumors account for the majority of sporadic colorectal cancers with microsatellite instability, through methylation of the mismatch repair gene hMLH1. This model whereby age-related methylation increases cell-susceptibility to transformation and cancer-specific methylation results in neoplastic progression in a subset of cases may be applicable to many human neoplasms.