Tissue‐specific DNA methylation patterns of the rat glial fibrillary acidic protein gene

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Abstract
The glial fibrillary acidic protein (GFAP) is an intermediate filament protein, specific of the cytoskeleton of astrocytes in the central nervous system. In the present work, as a preliminary step to the study of glial‐specific gene expression, we cloned the rat GFAP gene, and we report the sequence of 1.9 kb of the 5′ flanking region, exon 1, and the majority of the first intron. By digestion with methylation‐sensitive restriction enzymes followed by Southern blot analysis, the methylation status of various CpG sites was examined in this genomic segment. We tested whether structural modification of the GFAP gene, such as DNA methylation, could be related to its tissue‐specific transcriptional activity. Therefore, we compared a GFAP‐expressing cell population (primary culture of astroglial cells), a mixed population of GFAP‐expressing and ‐nonexpressing cells (adult rat cerebral hemispheres), and a GFAP‐nonexpressing tissue (liver). In the 5′ flanking region we identified a CpG site at position −1176 whose level of methylation is inversely correlated to GFAP expression. In primary cultured astrocytes, 75% of the GFAP gene alleles were demethylated at this site, while the corresponding value obtained for the cerebral hemispheres was 45%, and for liver only 9%. On the basis of the sequence data, a CpG‐rich region (putative CpG island) was identified extending from −38 to + 347 and overlapping 80% of the first exon. HhaI and HpaII sites located in the putative CpG island showed a relatively high level of methylation in all the cell populations examined, and did not show any clear correlation with the level of GFAP gene expression or with the methylation status of the −1176 site. Further in vivo developmental studies and in vitro differentiation studies are necessary to better understand the functional differences of the various methylatable CpG sites in the 5′ end of the GFAP gene.