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Identification of methylation haplotype blocks aids in deconvolution of heterogeneous tissue samples and tissue-of-origin mapping from plasma DNA [WGBS]

ABSTRACT: Adjacent CpG sites in mammalian genomes tend to be co-methylated due to the processivity of enzymes responsible for adding or removing the methyl group. Yet discordant methylation patterns have also be observed, and found to be related to stochastic or uncoordinated molecular processes. Here we focused on a systematic search and investigation of regions in the human genome that exhibit highly coordinated methylation. By examining the co-methylation patterns of multiple adjacent CpG sites, termed methylation haplotypes, in single bisulfite sequencing reads, we applied a greedy-searching strategy to defined blocks of tightly coupled CpG sites, called Methylation Haplotype Blocks (MHBs), based on 53 sets of whole genome bisulfite sequencing (WGBS) data, including 43 published sets from human adult tissues, ESC and in vitro differentiated cell lines, as well as 10 sets from human adult tissues generated in this study. The MHBs were then further validated with 101 sets of RRBS ENCODE data, and 637 sets of Illumina 450k methylation array data from TCGA tumor and normal samples. Globally, MHBs are enriched in but only partially overlap with several well-known genomic features, including CpG islands, promoters, enhancers and VMRs. To perform quantitative analysis of the MHBs, we defined a metric called Methylation Haplotype Load (MHL), which is covered both average methylation level and methylation complexity and therefore more informative than average methylation level or Shannon entropy. Using a feature selection strategy, we identified a set of tissue-specific MHBs that cluster by developmental germ-layers. Interestingly, examination of these MHBs revealed two distinct mechanisms for fate commitment during development: epigenetic silencing of pluripotent genes, such as NANOG, for mesoderm induction; and epigenetic induction (or de-suppression) of lineage-specific factors for ectoderm commitment.

ORGANISM(S): Homo sapiens

PROVIDER: GSE79215 | GEO | 2017/02/19

SECONDARY ACCESSION(S): PRJNA315194

REPOSITORIES: GEO

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Project description:Genetic abnormalities of cholangiocarcinoma (CCA) have been widely studied; however, epigenomic changes related to cholangiocarcinogenesis have been less well characterized. We have profiled the DNA methylomes of 28 primary CCA and six matched adjacent normal tissues using Infinium’s HumanMethylation27 BeadChips with the aim of identifying gene sets aberrantly epigenetically regulated in CCA. Using a linear model for microarray data we identified 1610 differentially methylated autosomal CpG sites with 809 CpG sites (representing 603 genes) being hypermethylated and 801 CpG sites (representing 712 genes) being hypomethylated in CCA versus adjacent normal (false discovery rate ? 0.05). Gene Ontology and Gene Set Enrichment analyses identified gene sets significantly associated with hypermethylation at linked CpG sites in CCA including homeobox genes and target genes of PRC2, EED, SUZ12 and Histone H3 trimethylation at lysine 27. We confirmed frequent hypermethylation at the homeobox genes HOXA9 and HOXD9 by bisulfite pyrosequencing in a larger cohort of CCA (n = 103). Our findings indicate a key role for hypermethylation of multiple CpG sites at genes associated with a stem cell-like phenotype as a common molecular aberration in CCA. These data have implications for CCA carcinogenesis, as well as possible novel treatment options using histone methyltransferase inhibitors. Methods: Thirty-two primary CCA, 6 matched adjacent normal, 5 CCA cell lines and an immortal biliary cell line were analyzed for DNA methylation profiles using the Infinium HumanMethylation27 BeadChip. Differential methylation was statistically analyzed using LIMMA. Specific functional terms and Gene Set Enrichment Analysis were performed to identify specific function of aberrantly methylated genes. Results: We identified 711 and 590 autosomal CpG sites as being hypermethylated and hypomethylated representing 527 and 521 genes, respectively, in CCA compared to adjacent normal (false discovery rate 0.05). A number of gene sets were significantly associated with hypermethylation at linked CpG sites in CCA including homeobox genes and the target genes of PRC2, EED, SUZ12, and H3K27me3, whereas target genes of NOS and OCT4 were hypomethylated at linked CpG sites in tumors. Frequent aberrant DNA methylation at HOXA9 and HOXD9 was confirmed by bisulfite pyrosequencing in a larger cohort of CCA (n = 107). Conclusions: This is the first report of a DNA methylation profile of multiple CpG sites at genes associated with a stem cell-like phenotype as a common molecular aberration in CCA. These data have implications for CCA carcinogenesis, as well as possible novel treatment options using histone methyltransferase inhibitors. Array-based methylation profiling was performed using the Infinium HumanMethylation27 BeadChip in 28 primary cholangiocarcinomas and six matched adjacent normal tissues. The reproducibility of the Infinium HumanMethylation27 BeadChips was evaluated using five technical replicates of the ovarian cancer cell line PEO1. Differential methylation cutoff was estimated from five technical replicates by bootstrap resampling and set at ?? ? 0.15 corresponding to a FDR ? 0.05.

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Identification of methylation haplotype blocks aids in deconvolution of heterogeneous tissue samples and tissue-of-origin mapping from plasma DNA [WGBS]

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