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A novel isoform of TET1 that lacks a CXXC domain is overexpressed in cancer

ABSTRACT: TET1 oxidizes methylated cytosine into 5-hydroxymethylcytosine, resulting in regulation of DNA methylation and gene expression. Full length TET1 (TET1FL) has a CXXC domain that binds to unmethylated stretches of CpG dinucleotides known as CpG islands (CGIs). This CXXC domain allows TET1 to protect CGIs from aberrant methylation but it also limits its ability to regulate genes outside of CGIs. Here we report a novel isoform of TET1 (TET1ALT) that has a unique transcription start site from an alternate promoter in intron 2, yielding a protein with a unique translation start site. Importantly, TET1ALT lacks the CXXC domain but retains the catalytic domain. TET1ALT is silenced in ESCs but becomes activated in embryonic and adult tissues while TET1FL is expressed in ESCs, but repressed in adult tissues. Overexpression of TET1ALT shows production of 5-hydroxymethylcytosine with distinct (and weaker) effects on DNA methylation or gene expression when compared to TET1FL. TET1ALT (but not TET1FL) is aberrantly activated in multiple cancer types including breast cancer, lung cancer and lymphomas. Our data suggest that the predominantly expressed isoform of TET1 in cancer cells does not protect from CGI methylation and likely mediates dynamic site specific demethylation outside of CGIs.

ORGANISM(S): Homo sapiens

PROVIDER: GSE93617 | GEO | 2017/05/12

SECONDARY ACCESSION(S): PRJNA361295

REPOSITORIES: GEO

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Project description:DNA methylation of C5-cytosine (5mC) in the mammalian genome is a key epigenetic event that is critical for various cellular processes. However, how the genome-wide 5mC pattern is dynamically regulated remains a fundamental question in epigenetic biology. The TET family of 5mC hydroxylases, which convert 5mC to 5-hydroxymethylcytosine (5hmC), have provided a new potential mechanism for the dynamic regulation of DNA methylation. The extent to which individual Tet family members contribute to the genome-wide 5mC and 5hmC patterns and associated gene network remains largely unknown. Here we report genome-wide mapping of Tet1 and 5hmC in mESCs and reveal a mechanism of action by which Tet1 controls 5hmC and 5mC levels in mESCs. In combination with microarray and mRNA-seq expression profiling, we identify a comprehensive yet intricate gene network influenced by Tet1. We propose a model whereby Tet1 controls DNA methylation both by binding to CpG-rich regions to prevent unwanted DNA methyltransferase activity, and by converting the existing 5mC to 5hmC through its enzymatic activity. This Tet1-mediated antagonism of CpG methylation imparts differential maintenance of DNA methylation status at Tet1 target loci, thereby providing a new regulatory mechanism for establishing the epigenetic landscape of mESCs, which ultimately contributes to mESC differentiation and the onset of embryonic development. To determine the genome-wide distribution of Tet1 and 5hmC in mouse ES cells, as well as identify the gene transcription changes after Tet1 depletion. GSM706669-GSM706671: We used GST pull-down followed by deep sequencing to map the DNA bound by the Tet1 CXXC domain in vitro. We made two mutants that have a single point mutation (Cys574 to Ala or Cys586 to Ala) in the core CXXC domain to ascertain the essential role of the CXXC domain in DNA binding by comparing the sequencing profile of DNA bound by wild type CXXC with the profiles of the CXXC mutants. GSM706672-GSM706673: Tet1 ChIP-seq was performed to identify the genome-wide distribution of Tet1 in mouse ES cells. GSM706674-GSM706679: We performed hydroxymethylated DNA immunoprecipitation (hMeDIP)-seq combined with a shRNA-mediated gene depletion strategy. To identify the loci specific 5hmC regulation by Tet1, we compared the 5hmC genome-wide distributions in control (Luc shRNA) and Tet1-depleted (Tet1 shRNA2863) mouse ES cells. GSM706680-GSM706682: To identify the gene regulation network by Tet1, we compared the gene expression profiles of control (scramble shRNA) and Tet1-depleted (Tet1 shRNA 2863 and Tet1 shRNA 3387) mouse ES cells determined by mRNA-seq.

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A novel isoform of TET1 that lacks a CXXC domain is overexpressed in cancer

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