Methyltransferase Set7/9 Maintains Transcription and Euchromatin Structure at Islet-Enriched Genes

Abstract

Objective: The activation of beta-cell genes, particularly of those encoding preproinsulin, requires an appropriate euchromatin (or "open") DNA template characterized by hypermethylation of Lys4 of histone H3. We hypothesized that this modification is maintained in islet beta-cells by the action of the histone methyltransferase Set7/9. Research design and methods: To identify the role of Set7/9, we characterized its expression pattern and gene regulation and studied its function using RNA interference in both cell lines and primary mouse islets. Results: Within the pancreas, Set7/9 protein shows striking specificity for islet cells, including alpha- and beta-cells, as well as occasional cells within ducts. Consistent with these findings, the Set7/9 gene promoter contained an islet-specific enhancer located between -5,768 and -6,030 base pairs (relative to the transcriptional start site) that exhibited Pdx1-responsive activation in beta-cells. To study Set7/9 function, we depleted insulinoma cells and primary mouse islets of Set7/9 protein using siRNA. Following siRNA treatment, we observed striking repression of genes involved in glucose-stimulated insulin secretion, including Ins1/2, Glut2, and MafA. These changes in transcription were accompanied by loss of dimethylated H3 Lys4 and RNA polymerase II recruitment, particularly at the Ins1/2 and Glut2 genes. Consistent with these data, depletion of Set7/9 in islets led to defects in glucose-stimulated Ca(2+) mobilization and insulin secretion. Conclusions: We conclude that Set7/9 is required for normal beta-cell function, likely through the maintenance of euchromatin structure at genes necessary for glucose-stimulated insulin secretion.