Methylation of the rat glial fibrillary acidic protein gene shows tissue-specific domains

Abstract

The gene for glial fibrillary acidic protein (GFAP) was compared for CpG sites that are potential locations of methylated cytosine (mC). GFAP sequences in the 5'-upstream promoter and in exon 1 of rat, mouse, and human showed extensive similarity in the locations of CpG sites in the promoter and in exon 1, implying conservation. The methylation of mC at 9 CpG sites in the promoter and 10 sites in exon 1 was analyzed in F344 male rats by a quantitative application of ligation-mediated polymerase chain reaction (LMPCR). CpG sites with varying mC in different tissues were found in the GFAP promoter and in a CpG island in exon 1. In the brain, the promoter had about 40% less mC than in testis and liver. The degree of methylation varied strikingly between adjacent sites within and between tissues. Testis GFAP exon 1 had a gradient of mC from 5' to 3' across the exon that was absent in liver, brain, and cultured neurons and astrocytes. Among brain regions, the hippocampus had 10-40% less mC at 12 CpG sites than in hypothalamus; the other sites (7/19) showed smaller differences between these brain regions. In DNA from primary cultures, astrocytes had slightly less mC than neurons at all sites. Because neuron-rich hippocampal subregions and primary neurons cultures had less methylation than nonneural tissues, we hypothesize that neuroectodermal derivatives tend to be less methylated, whether or not GFAP is expressed. Four domains of methylated CpG sites are proposed on the basis of tissue and cell-type distribution: I) a constitutively methylated domain in the mid-upstream promoter; II) a testis-specific gradient of methylation in exon 1; III) a hypomethylated domain found in neuroectodermal derivatives; and IV) subsets of sites in the promoter and in exon 1 that have the least methylation in astrocytes, and therefore may be astrocyte-specific domains.