GSE40158
5-hmC in the brain: abundance in synaptic genes and differences at the exon-intron boundary (MRE-Seq)
5-hydroxymethylcytosine (5-hmC), a derivative of 5-methylcytosine (5-mC), is abundant in the brain for unknown reasons. We mapped the genomic distribution of 5-hmC and 5-mC in human and mouse tissues using glucosylation of 5-hmC coupled with restriction enzyme digestion, and interrogation on microarrays. We detected 5-hmC enrichment in genes with synapse-related functions in the brain. We also identified significant, tissue-specific differential distributions of these DNA modifications at the exon-intron boundary, in both human and mouse.  This boundary change was mainly due to 5-hmC in the brain, but due to 5-mC in non-neural contexts. This pattern was replicated in multiple independent datasets, and the brain-specific change in 5-hmC was validated using single-molecule sequencing. Moreover, in the brain, constitutive exons contained higher levels of 5-hmC, relative to alternatively-spliced exons. Our study suggests a novel role for 5-hmC in RNA splicing and synaptic function in the brain Overall design: The ß-glucosyltransferase (BGT) enzyme transfers a glucose molecule specifically to the hydroxymethyl group of 5-hmC, thus rendering it resistant to digestion by the methylation insensitive MspI enzyme at the ChmCGG target site;  5-hmC is thus detected by differential resistance to MspI-digestion with and without glucosylation of genomic DNA (gDNA).  HpaII (targets the same site, CCGG) cannot cut CmCGG or ChmCGG, and conceptually its difference with MspI digestion is a measure of both 5-mC and 5-hmC. Subtraction of 5-hmC from the HpaII-based estimate therefore measures 5-mC. These estimates were measured on respective Affymetrix whole-genome tiling arrays  (2.0 R ) MspI - APRIL_fc1.ch02_coverage.bed Undigested DNA - APRIL_fc1.ch01_coverage.bed GluMspI - APRIL_fc1.ch04_coverage_NONTARGET.bed GluMspI - APRIL_fc1.ch04_coverage.bed MspI - APRIL_fc1.ch02_coverage_NONTARGET.bed Undigested DNA - APRIL_fc1.ch01_coverage_NONTARGET.bed MspI - MARCH_fc1.ch02_coverage_NONTARGET.bed Undigested DNA - MARCH_fc1.ch01_coverage_NONTARGET.bed GluMspI - MARCH_fc1.ch04_coverage_NONTARGET.bed GluMspI - MARCH_fc1.ch04_coverage.bed MspI - MARCH_fc1.ch02_coverage.bed Undigested DNA - MARCH_fc1.ch01_coverage.bed MspI - MAY_fc1.ch02_coverage.bed GluMspI - MAY_fc1.ch04_coverage_NONTARGET.bed GluMspI - MAY_fc1.ch04_coverage.bed Undigested DNA - MAY_fc1.ch01_coverage.bed MspI - MAY_fc1.ch02_coverage_NONTARGET.bed Undigested DNA - MAY_fc1.ch01_coverage_NONTARGET.bed