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      Aberrantly methylated genes in human papillary thyroid cancer and their association with BRAF/ RAS mutation

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          Cancer arises through accumulation of epigenetic and genetic alteration. Aberrant promoter methylation is a common epigenetic mechanism of gene silencing in cancer cells. We here performed genome-wide analysis of DNA methylation of promoter regions by Infinium HumanMethylation27 BeadChip, using 14 clinical papillary thyroid cancer samples and 10 normal thyroid samples. Among the 14 papillary cancer cases, 11 showed frequent aberrant methylation, but the other three cases showed no aberrant methylation at all. Distribution of the hypermethylation among cancer samples was non-random, which implied existence of a subset of preferentially methylated papillary thyroid cancer. Among 25 frequently methylated genes, methylation status of six genes ( HIST1H3J, POU4F2, SHOX2, PHKG2, TLX3, HOXA7) was validated quantitatively by pyrosequencing. Epigenetic silencing of these genes in methylated papillary thyroid cancer cell lines was confirmed by gene re-expression following treatment with 5-aza-2′-deoxycytidine and trichostatin A, and detected by real-time RT-PCR. Methylation of these six genes was validated by analysis of additional 20 papillary thyroid cancer and 10 normal samples. Among the 34 cancer samples in total, 26 cancer samples with preferential methylation were significantly associated with mutation of BRAF/ RAS oncogene ( P = 0.04, Fisher's exact test). Thus, we identified new genes with frequent epigenetic hypermethylation in papillary thyroid cancer, two subsets of either preferentially methylated or hardly methylated papillary thyroid cancer, with a concomitant occurrence of oncogene mutation and gene methylation. These hypermethylated genes may constitute potential biomarkers for papillary thyroid cancer.

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          Most cited references 42

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          CpG island methylator phenotype in colorectal cancer.

          Aberrant methylation of promoter region CpG islands is associated with transcriptional inactivation of tumor-suppressor genes in neoplasia. To understand global patterns of CpG island methylation in colorectal cancer, we have used a recently developed technique called methylated CpG island amplification to examine 30 newly cloned differentially methylated DNA sequences. Of these 30 clones, 19 (63%) were progressively methylated in an age-dependent manner in normal colon, 7 (23%) were methylated in a cancer-specific manner, and 4 (13%) were methylated only in cell lines. Thus, a majority of CpG islands methylated in colon cancer are also methylated in a subset of normal colonic cells during the process of aging. In contrast, methylation of the cancer-specific clones was found exclusively in a subset of colorectal cancers, which appear to display a CpG island methylator phenotype (CIMP). CIMP+ tumors also have a high incidence of p16 and THBS1 methylation, and they include the majority of sporadic colorectal cancers with microsatellite instability related to hMLH1 methylation. We thus define a pathway in colorectal cancer that appears to be responsible for the majority of sporadic tumors with mismatch repair deficiency.
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            High prevalence of BRAF mutations in thyroid cancer: genetic evidence for constitutive activation of the RET/PTC-RAS-BRAF signaling pathway in papillary thyroid carcinoma.

            Thyroid papillary cancers (PTCs) are associated with activating mutations of genes coding for RET or TRK tyrosine kinase receptors, as well as of RAS genes. Activating mutations of BRAF were reported recently in most melanomas and a small proportion of colorectal tumors. Here we show that a somatic mutation of BRAF, V599E, is the most common genetic change in PTCs (28 of 78; 35.8%). BRAF(V599E) mutations were unique to PTCs, and not found in any of the other types of differentiated follicular neoplasms arising from the same cell type (0 of 46). Moreover, there was no overlap between PTC with RET/PTC, BRAF, or RAS mutations, which altogether were present in 66% of cases. The lack of concordance for these mutations was highly unlikely to be a chance occurrence. Because these signaling proteins function along the same pathway in thyroid cells, this represents a unique paradigm of human tumorigenesis through mutation of three signaling effectors lying in tandem.
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              Genome-scale analysis of aberrant DNA methylation in colorectal cancer.

              Colorectal cancer (CRC) is a heterogeneous disease in which unique subtypes are characterized by distinct genetic and epigenetic alterations. Here we performed comprehensive genome-scale DNA methylation profiling of 125 colorectal tumors and 29 adjacent normal tissues. We identified four DNA methylation-based subgroups of CRC using model-based cluster analyses. Each subtype shows characteristic genetic and clinical features, indicating that they represent biologically distinct subgroups. A CIMP-high (CIMP-H) subgroup, which exhibits an exceptionally high frequency of cancer-specific DNA hypermethylation, is strongly associated with MLH1 DNA hypermethylation and the BRAF(V600E) mutation. A CIMP-low (CIMP-L) subgroup is enriched for KRAS mutations and characterized by DNA hypermethylation of a subset of CIMP-H-associated markers rather than a unique group of CpG islands. Non-CIMP tumors are separated into two distinct clusters. One non-CIMP subgroup is distinguished by a significantly higher frequency of TP53 mutations and frequent occurrence in the distal colon, while the tumors that belong to the fourth group exhibit a low frequency of both cancer-specific DNA hypermethylation and gene mutations and are significantly enriched for rectal tumors. Furthermore, we identified 112 genes that were down-regulated more than twofold in CIMP-H tumors together with promoter DNA hypermethylation. These represent ∼7% of genes that acquired promoter DNA methylation in CIMP-H tumors. Intriguingly, 48/112 genes were also transcriptionally down-regulated in non-CIMP subgroups, but this was not attributable to promoter DNA hypermethylation. Together, we identified four distinct DNA methylation subgroups of CRC and provided novel insight regarding the role of CIMP-specific DNA hypermethylation in gene silencing.

                Author and article information

                Front Genet
                Front Genet
                Front. Genet.
                Frontiers in Genetics
                Frontiers Media S.A.
                05 December 2013
                : 4
                1Genome Science Division, Research Center for Advanced Science and Technology, The University of Tokyo Tokyo, Japan
                2Department of Metabolic Care and Endocrine Surgery, Graduate School of Medicine, The University of Tokyo Tokyo, Japan
                3Department of Pathology, Graduate School of Medicine, The University of Tokyo Tokyo, Japan
                4Department of Breast and Endocrine Surgery, Kawasaki Medical University Okayama, Japan
                5Department of Molecular Oncology, Graduate School of Medicine, Chiba University Chiba, Japan
                6CREST, Japan Science and Technology Agency Saitama, Japan
                Author notes

                Edited by: Yoshimasa Saito, Keio University Faculty of Pharmacy, Japan

                Reviewed by: Craig A. Cooney, Central Arkansas Veterans Healthcare System, USA; Michèle Amouyal, Centre National de la Recherche Scientifique, France

                *Correspondence: Atsushi Kaneda, Department of Molecular Oncology, Graduate School of Medicine, Chiba University, Inohana 1-8-1, Chuo-ku, Chiba-City 260-8670, Japan e-mail: kaneda@ 123456chiba-u.jp

                This article was submitted to Epigenomics and Epigenetics, a section of the journal Frontiers in Genetics.

                Copyright © 2013 Kikuchi, Tsuji, Yagi, Matsusaka, Tsuji, Kurebayashi, Ogawa, Aburatani and Kaneda.

                This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) or licensor are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

                Page count
                Figures: 7, Tables: 4, Equations: 1, References: 53, Pages: 11, Words: 7790
                Original Research Article


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