Epigenetic regulation of the calcitonin gene-related peptide gene in trigeminal glia

Background: The neuropeptide calcitonin gene-related peptide (CGRP) plays a key role in migraine. CGRP gene expression involves an enhancer that is active in neurons, yet inactive in glia. In this report, we analyze epigenetic modifications that allow enhancer activation in glia. Methods: DNA methylation and histone acetylation states were measured in rat and human-model cell lines and primary cultures of rat trigeminal ganglia glia. The functional consequence of altering the chromatin state was determined by quantitative measurements of both calcitonin (CT) and CGRP mRNAs. Results: A hypermethylated CpG island flanking the enhancer was identified in glia and non-expressing cell lines. In addition, the chromatin was hypoacetylated. Treatment with the DNA methylation inhibitor 5-aza-2'-deoxycytidine induced CT mRNA similar to 30-fold in glial cultures. Treatment with a histone deacetylase inhibitor alone had little effect; however, the combination of inhibitors yielded a synergistic similar to 80-fold increase in CT and similar to threefold increase in CGRP mRNA. Treated glia contained CT precursor (pro-CT) immunoreactivity. Conclusions: Epigenetic modulation is sufficient to induce the CGRP gene in glia. Because the CGRP gene is systemically activated by inflammatory conditions, this suggests that glial pro-CT may be an unexplored biomarker during migraine.