Aberrant CpG-island methylation has non-random and tumour-type-specific patterns.

CpG islands frequently contain gene promoters or exons and are usually unmethylated in normal cells. Methylation of CpG islands is associated with delayed replication, condensed chromatin and inhibition of transcription initiation. The investigation of aberrant CpG-island methylation in human cancer has primarily taken a candidate gene approach, and has focused on less than 15 of the estimated 45,000 CpG islands in the genome. Here we report a global analysis of the methylation status of 1,184 unselected CpG islands in each of 98 primary human tumours using restriction landmark genomic scanning (RLGS). We estimate that an average of 600 CpG islands (range of 0 to 4,500) of the 45,000 in the genome were aberrantly methylated in the tumours, including early stage tumours. We identified patterns of CpG-island methylation that were shared within each tumour type, together with patterns and targets that displayed distinct tumour-type specificity. The expression of many of these genes was reactivated by experimental demethylation in cultured tumour cells. Thus, the methylation of particular subsets of CpG islands may have consequences for specific tumour types.

Polyamine biosynthesis impacts cellular folate requirements necessary to maintain S-adenosylmethionine and nucleotide pools.

Folate (vitamin B9) is utilized for synthesis of both S-adenosylmethionine (AdoMet) and deoxythymidine monophosphate (dTMP), which are required for methylation reactions and DNA synthesis, respectively. Folate depletion leads to an imbalance in both AdoMet and nucleotide pools, causing epigenetic and genetic damage capable of initiating tumorigenesis. Polyamine biosynthesis also utilizes AdoMet, but polyamine pools are not reduced under a regimen of folate depletion. We hypothesized that high polyamine biosynthesis, due to the high demand on AdoMet pools, might be a factor in determining sensitivity to folate depletion. We found a significant correlation (P<0.001) between polyamine biosynthesis and the amount of folate required to sustain cell line proliferation. We manipulated polyamine biosynthesis by genetic and pharmacological intervention and mechanistically demonstrated that we could thereby alter AdoMet pools and increase or decrease demand on folate availability needed to sustain cellular proliferation. Furthermore, growing a panel of cell lines with 100 nM folate led to imbalanced nucleotide and AdoMet pools only in cells with endogenously high polyamine biosynthesis. These data demonstrate that polyamine biosynthesis is a critical factor in determining sensitivity to folate depletion and may be particularly important in the prostate, where biosynthesis of polyamines is characteristically high due to its secretory function.

Pan-cancer analysis of transcriptional metabolic dysregulation using The Cancer Genome Atlas.

Understanding metabolic dysregulation in different disease settings is vital for the safe and effective incorporation of metabolism-targeted therapeutics in the clinic. Here, using transcriptomic data for 10,704 tumor and normal samples from The Cancer Genome Atlas, across 26 disease sites, we present a novel bioinformatics pipeline that distinguishes tumor from normal tissues, based on differential gene expression for 114 metabolic pathways. We confirm pathway dysregulation in separate patient populations, demonstrating the robustness of our approach. Bootstrapping simulations were then applied to assess the biological significance of these alterations. We provide distinct examples of the types of analysis that can be accomplished with this tool to understand cancer specific metabolic dysregulation, highlighting novel pathways of interest, and patterns of metabolic flux, in both common and rare disease sites. Further, we show that Master Metabolic Transcriptional Regulators explain why metabolic differences exist, can segregate patient populations, and predict responders to different metabolism-targeted therapeutics.

DNA copy number gains in head and neck squamous cell carcinoma.

Gene amplification, a common mechanism for oncogene activation in cancer, has been used as a tag for the identification of novel oncogenes. DNA amplification is frequently observed in head and neck squamous cell carcinoma (HNSCC) and potential oncogenes have already been reported. We applied restriction landmark genome scanning (RLGS) to study gene amplifications and low-level copy number changes in HNSCC in order to locate previously uncharacterized regions with copy number gains in primary tumor samples. A total of 63 enhanced RLGS fragments, indicative of DNA copy number changes, including gains of single alleles, were scored. Enhanced sequences were identified from 33 different chromosomal regions including those previously reported (e.g. 3q26.3 and 11q13.3) as well as novel regions (e.g. 3q29, 8q13.1, 8q22.3, 9q32, 10q24.32, 14q32.32, 17q25.1 and 20q13.33). Furthermore, our data suggest that amplicons 11q13.3 and 3q26.3-q29 may be divided into possibly two and three independent amplicons, respectively, an observation supported by published microarray expression data.

Genome-wide analysis of DNA methylation changes in human malignancies.

DNA methylation is an epigenetic modification of the DNA sequence and thus does not change the genetic code but affects chromosomal stability and gene expression. DNA methylation patterns are heritable and can be passed on to the daughter cell. In this review, we briefly summarize our current knowledge on normal DNA methylation patterns and move on to discuss the current state of the field with respect to altered DNA methylation in cancer. We make a special attempt to address current questions relating to genome-wide DNA methylation patterns. Since DNA methylation is used as a therapeutic target in clinical studies, it is of utmost importance to define potential target sequences that could be used as diagnostic or prognostic markers. We conclude the review by outlining possible scenarios that may explain tumor type-specific DNA methylation patterns described by assays evaluating genome-wide levels of DNA methylation.

Differential targets of CpG island hypermethylation in primary and metastatic head and neck squamous cell carcinoma (HNSCC).

Head and neck squamous cell carcinomas (HNSCC) often metastasise to the cervical lymph nodes. It is known for HNSCC as well as other cancers that progression from normal tissue to primary tumour and finally to metastatic tumour is characterised by an accumulation of genetic mutations. DNA methylation, an epigenetic modification, can result in loss of gene function in cancer, similar to genetic mutations such as deletions and point mutations. We have investigated the DNA methylation phenotypes of both primary HNSCC and metastatic tumours from 13 patients using restriction landmark genomic scanning (RLGS). With this technique, we were able to assess the methylation status of an average of nearly 1300 CpG islands for each tumour. We observed that the number of CpG islands hypermethylated in metastatic tumours is significantly greater than what is found in the primary tumours overall, but not in every patient. Interestingly, the data also clearly show that many loci methylated in a patient's primary tumour are no longer methylated in the metastatic tumour of the same patient. Thus, even though metastatic HNSCC methylate a greater proportion of CpG islands than do the primary tumours, they do so at different subsets of loci. These data show an unanticipated variability in the methylation state of loci in primary and metastatic HNSCCs within the same patient. We discuss two possible explanations for how different epigenetic events might arise between the primary tumour and the metastatic tumour of a person.

Gene amplification in PNETs/medulloblastomas: mapping of a novel amplified gene within the MYCN amplicon.

OBJECTIVES: The pathological entity of primitive neuroectodermal tumour/medulloblastoma (PNET/MB) comprises a very heterogeneous group of neoplasms on a clinical as well as on a molecular level. We evaluated the importance of DNA amplification in medulloblastomas and other primitive neuroectodermal tumours (PNETs) of the CNS. METHOD: Restriction landmark genomic scanning (RLGS), a method that allows the detection of low level amplification, was used. RLGS provides direct access to DNA sequences circumventing positional cloning efforts. Furthermore, we analysed several samples by CGH. DESIGN: Twenty primary medulloblastomas, five supratentorial PNETs, and five medulloblastoma cell lines were studied. RESULTS: Although our analysis confirms that gene amplification is generally a rare event in childhood PNET/MB, we found a total of 17 DNA fragments that were amplified in seven different tumours. Cloning and sequencing of several of these fragments confirmed the previous finding of MYC amplification in the cell line D341 Med and identified novel DNA sequences amplified in PNET/MB. We describe for the first time amplification of the novel gene, NAG, in a subset of PNET/MB. Despite genomic amplification, NAG was not overexpressed in the tumours studied. We have determined that NAG maps less than 50 kb 5' of DDX1 and approximately 400 kb telomeric of MYCN on chromosome 2p24. CONCLUSION: We found a similar but slightly higher frequency of amplification than previously reported. We present several DNA fragments that may belong to the CpG islands of novel genes amplified in a small subset of PNET/MB. As an example we describe for the first time the amplification of NAG in the MYCN amplicon in PNET/MB.

Methylation of the estrogen receptor-alpha gene promoter is selectively increased in proliferating human aortic smooth muscle cells.

OBJECTIVE: Atherosclerosis is a multigenic process leading to the progressive occlusion of arteries of mid to large caliber. A key step of the atherogenic process is the proliferation and migration of vascular smooth muscle cells into the intimal layer of the arterial conduit. The phenotype of smooth muscle cells, once within the intima, is known to switch from contractile to de-differentiated, yet the regulation of this switch at the genomic level is unknown. Estrogen has been shown to regulate cell proliferation both for cancer cells and for vascular cells. However, methylation of the estrogen receptor-alpha gene (ERalpha) promoter blocks the expression of ERalpha, and thereby can antagonize the regulatory effect of estrogen on cell proliferation. We sought to determine whether methylation of the ERalpha is differentially and selectively regulated in contractile versus de-differentiated arterial smooth muscle cells. METHODS: We used Southern blot assay, combined bisulfite restriction analysis (Cobra) and restriction landmark genome scanning (RLGS-M) to determine the methylation status of ERalpha in human aortic smooth muscle cells, either in situ (normal aortic tissue, contractile phenotype), or the same cells explanted from the aorta and cultured in vitro (de-differentiated phenotype). RESULTS: We provide evidence that methylation of the ERalpha in smooth muscle cells that display a proliferative phenotype is altered relative to the same cells studied within the media of non-atherosclerotic aortas. Thus, the ERalpha promoter does not appear to be methylated in situ (normal aorta), but becomes methylated in proliferating aortic smooth muscle cells. Using a screening technique, RLGS-M, we show that alteration in methylation associated with the smooth muscle cell phenotypic switch does not seem to require heightened activity of the methyltransferase enzyme, and appears to be selective for the ERalpha and a limited pool of genes whose CpG island becomes either demethylated or de novo methylated. CONCLUSIONS: Our data support the concept that the genome of aortic smooth muscle cells is responsive to environmental conditions, and that DNA methylation, in particular methylation of the ERalpha, could contribute to the switch in phenotype observed in these cells.

Global methylation profiling of lung cancer identifies novel methylated genes.

Epigenetic changes, including DNA methylation, are a common finding in cancer. In lung cancers methylation of cytosine residues may affect tumor initiation and progression in several ways, including the silencing of tumor suppressor genes through promoter methylation and by providing the targets for adduct formation of polycyclic aromatic hydrocarbons present in combustion products of cigarette smoke. Although the importance of aberrant DNA methylation is well established, the extent of DNA methylation in lung cancers has never been determined. Restriction landmark genomic scanning (RLGS) is a highly reproducible two-dimensional gel electrophoresis that allows the determination of the methylation status of up to 2000 promoter sequences in a single gel. We selected 1184 CpG islands for RLGS analysis and determined their methylation status in 16 primary non-small cell lung cancers. Some tumors did not show methylation whereas others showed up to 5.3% methylation in all CpG islands of the profile. Cloning of 21 methylated loci identified 11 genes and 6 ESTs. We demonstrate that methylation is part of the silencing process of BMP3B in primary tumors and lung cancer cell lines.

Epigenetic changes in hypothalamic appetite regulatory genes may underlie the developmental programming for obesity in rat neonates subjected to a high-carbohydrate dietary modification.

Earlier, we showed that rearing of newborn rats on a high-carbohydrate (HC) milk formula resulted in the onset of hyperinsulinemia, its persistence in the post-weaning period and adult-onset obesity. DNA methylation of CpG dinucleotides in the proximal promoter region and modifications in the N-terminal tail of histone 3 associated with the neuropeptide Y (Npy) and pro-opiomelanocortin (Pomc) genes were investigated to decipher the molecular mechanisms supporting the development of obesity in HC females. Although there were no differences in the methylation status of CpG dinucleotides in the proximal promoter region of the Pomc gene, altered methylation of specific CpG dinucleotides proximal to the transcription start site was observed for the Npy gene in the hypothalami of 16- and 100-day-old HC rats compared with their methylation status in mother-fed (MF) rats. Investigation of histone tail modifications on hypothalamic chromatin extracts from 16-day-old rats indicated decreased acetylation of lysine 9 in histone 3 (H3K9) for the Pomc gene and increased acetylation for the same residue for the Npy gene, without changes in histone methylation (H3K9) in both genes in HC rats. These findings are consistent with the changes in the levels of Npy and Pomc mRNAs in the hypothalami of HC rats compared with MF animals. Our results suggest that epigenetic modifications could contribute to the altered gene expression of the Npy and Pomc genes in the hypothalami of HC rats and could be a mechanism leading to hyperphagia and the development of obesity in adult female HC rats.

Developmental acquisition of genome-wide DNA methylation occurs prior to meiosis in male germ cells.

The development of germ cells is a highly ordered process that begins during fetal growth and is completed in the adult. Epigenetic modifications that occur in germ cells are important for germ cell function and for post-fertilization embryonic development. We have previously shown that male germ cells in the adult mouse have a highly distinct epigenetic state, as revealed by a unique genome-wide pattern of DNA methylation. Although it is known that these patterns begin to be established during fetal life, it is not known to what extent DNA methylation is modified during spermatogenesis. We have used restriction landmark genomic scanning (RLGS) and other techniques to examine DNA methylation at multiple sites across the genome during postnatal germ cell development in the mouse. Although a significant proportion of the distinct germ cell pattern is acquired prior to the type A spermatogonial stage, we find that both de novo methylation and demethylation occur during spermatogenesis, mainly in spermatogonia and spermatocytes in early meiotic prophase I. Alterations include predominantly non-CpG island sequences from both unique loci and repetitive elements. These modifications are progressive and are almost exclusively completed by the end of the pachytene spermatocyte stage. These studies better define the developmental timing of genome-wide DNA methylation pattern acquisition during male germ cell development.

A unique configuration of genome-wide DNA methylation patterns in the testis.

In the mammalian lifecycle, the two periods of genome-wide epigenetic reprogramming are in the early embryo, when somatic patterns are set, and during germ cell development. Although some differences between the reprogrammed states of somatic and germ cells have been reported, overall patterns of genomic methylation are considered to be similar. Using restriction landmark genomic scanning to examine approximately 2,600 loci distributed randomly throughout the genome, we find that the methylation status of testicular DNA is highly distinct, displaying eightfold the number of hypomethylated loci relative to somatic tissues. Identification and analysis of >300 loci show that these regions are generally located within nonrepetitive sequences that are away from CpG islands and 5' regions of genes. We show that a contributing factor for these differences is that the methylation state of non-CpG-island DNA is correlated with the regional level of GC content within chromosomes and that this relationship is inverted between the testis and somatic tissues. We also show that in Dnmt3L-deficient mice, which exhibit infertility associated with abnormal chromosomal structures in germ cells, this unique testicular DNA methylation pattern is not established. These special properties of testicular DNA point to a broad, distinct epigenetic state that may be involved in maintaining a unique chromosomal structure in male germ cells.

Physical characterization of the chromosomal rearrangements that accompany the transgene insertion in the chakragati mouse mutant.

The circling phenotype of the chakragati mouse is a result of a transgenic insertional mutation. The absence of the phenotype in mice heterozygous for the transgene insertion suggests that this is due to a loss of function of an endogenous gene. Efforts to identify this gene have led to a previous report that sequences flanking the transgene, D16Ros1 and D16Ros2, map 10 cM apart in wildtype mice. We present here physical mapping data indicating that the proximity of D16Ros1 and D16Ros2 in the ckr mouse is explained by a duplication of D16Ros2 and its insertion along with the transgene at D16Ros1. We further demonstrate that D16Ros1 sequences are also duplicated and that this duplication is also part of the insertion at the endogenous D16Ros1 locus.

Genetic characterization of the chromosomal rearrangements that accompany the transgene insertion in the chakragati mouse mutant.

We have previously reported that the circling phenotype of the chakragati mouse segregates with the transgene integration event as an autosomal recessive trait. It was unclear, however, whether the phenotype was linked to the transgene integration point near D16Ros1 or to a potential disruption at D16Ros2, 10 cM away. We report here that animals recombinant between D16Ros1 and D16Ros2, homozygous for the transgene insertion at D16Ros1, but wildtype for D16Ros2, do indeed show the phenotype. We conclude that any potential disruption at the D16Ros2 locus is not responsible for the circling phenotype. We further show that recombination between D16Ros1 and D16Ros2 occurs at a greatly reduced level in the chakragati mouse compared to wildtype strains. Detailed genetic analysis of recombinants indicates that the proximal-most 4.5 cM shows no recombination in over 1400 meioses. We propose that this is due to an inversion in this region, and we genetically define the proposed distal inversion break point to a 1.3-cM region between D16Mit63 and D16Mit169.

Aberrant hypermethylation of the major breakpoint cluster region in 17p11.2 in medulloblastomas but not supratentorial PNETs.

Deletions of 17p have been consistently reported in up to 50% of medulloblastomas (MBs), and the major breakpoint interval has been localized to chromosome segment 17p11.2. Based on several reports linking aberrant DNA methylation and chromosomal disruption, we examined the methylation pattern in this region by employing restriction landmark genomic scanning (RLGS). Several CpG islands located in the major breakpoint cluster region were identified using a bacterial artificial chromosome (BAC) contig of the breakpoint region. A long-range methylation map was established for 20 MBs and 5 supratentorial primitive neuroectodermal tumors (stPNETs). Selected CpG islands were examined using Southern and bisulfite sequencing analysis. Aberrantly hypermethylated CpG islands in 17p11. 2 were found in 33% of MBs. Interestingly, one CpG island was methylated in MBs, but not in any of the examined stPNETs. A BAC clone covering three of the methylated CpG islands was partially sequenced in the search for a potential tumor suppressor gene. None of the expressed sequence tag sequences and full-length mouse/human cDNAs that were associated with aberrant methylation showed a change in expression levels due to methylation. The potential link between chromosomal instability in 17p11.2 and hypermethylation in this region is discussed.

A new tool for the rapid cloning of amplified and hypermethylated human DNA sequences from restriction landmark genome scanning gels.

Restriction landmark genome scanning (RLGS) is an effective genome-scanning technique capable of identifying DNA amplification and aberrant DNA methylation. Previously published methods for the cloning of human DNA fragments from RLGS gels have been successful only for high-copy-number fragments (repetitive elements or DNA amplifications). We present here the first technique capable of efficiently cloning single-copy human DNA fragments ("spots") identified in RLGS profiles. This technique takes advantage of a plasmid-based, human genomic DNA, NotI/EcoRV boundary library. The library is arrayed in microtiter plates. When clones from a single plate are pooled and mixed with genomic DNA, the resultant RLGS gel is a normal profile with a defined set of spots showing enhanced intensity for that particular plate. This was performed for a set of 32 plates as well as their pooled rows and columns. Thus, we have mapped individual RLGS spots to exact plate, row, and column addresses in the library and have thereby obtained immediate access to these clones. The feasibility of the technique is demonstrated in examples of cloning methylated DNA fragments identified in human breast tumor and testicular tumor RLGS profiles and in the cloning of an amplified DNA fragment identified in a human medulloblastoma RLGS profile.

Novel methylation targets in de novo acute myeloid leukemia with prevalence of chromosome 11 loci.

Aberrant DNA methylation is believed to be important in tumorigenesis by causing either transcriptional inactivation of genes or chromosomal instability. Several laboratories have identified promoter hypermethylation of tumor suppressor genes in acute myeloid leukemia (AML). However, these studies do not provide a global assessment of overall methylation changes and do not allow the identification of novel methylated sequences. Previously, nonrandom CpG island methylation was reported in 17 adult de novo AML diagnostic samples when compared with the corresponding remission samples by means of restriction landmark genomic scanning (RLGS). That study has been expanded on by an analysis of a larger set of CpG islands (1740 vs 1184), which now provides details of 33 cloned methylated loci, including 21 known genes or expressed sequence tags. Five of these cloned loci appear to be methylated only in AML and not in the 6 solid tumors studied in this study (more than 98 samples analyzed). Chromosomal location was available for 30 of the 33 loci, and 5 of these 30 (17%) are localized to chromosome 11, suggesting a trend toward overrepresentation of methylation events on this chromosome. These results provide evidence for widespread aberrant methylation in AML, with identification of novel methylation targets, epigenetic changes that appear unique to AML, and apparent preferential methylation on chromosome 11.

Excessive CpG island hypermethylation in cancer cell lines versus primary human malignancies.

Cancer cell lines are widely used in many types of cancer research, including studies aimed at understanding DNA hypermethylation of gene promoters in cancer. Hypermethylation of promoters is capable of repressing the expression of tumor suppressor genes and may play a role in the development and/or progression of cancer. Although both primary malignancies and cancer cell lines exhibit this epigenetic phenomenon, there has been no direct comparison between them. In order to address this question, we have utilized restriction landmark genomic scanning to measure the hypermethylation phenotypes of cancer cell lines and compared these data with the same analysis performed on primary malignancies. In all cases, cancer cell lines exhibit significantly higher levels of CpG island hypermethylation than the primary malignancies they represent. Colon cancer cell lines are most similar to their respective tumors, with only a 5-fold increase in hypermethylation, while head and neck squamous cell carcinoma cell lines show a 93-fold increase in hypermethylation. Furthermore, >57% of the loci methylated in cell lines are never methylated in 114 primary malignancies studied. Seventy percent of loci hypermethylated in cell lines are hypermethylated in lines from more than one type of cancer. These data indicate that most CpG island hypermethylation observed in cancer cell lines is due to an intrinsic property of cell lines as opposed to the malignant tissue from which they originated.