Methyltransferase set7/9 maintains transcription and euchromatin structure at islet-enriched genes

Abstract

OBJECTIVE-The activation of β-cell genes, particularly of those encoding preproinsulin, requires an appropriate euchroma-tin(or "open")DNA template characterized by hypermethylation of Lys4 of histone H3. We hypothesized that this modification is maintained in islet β-cells by the action of the histone methyl-transferase 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 α-and β-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 β-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 β-cell function, likely through the maintenance of euchro-matin structure at genes necessary for glucose-stimulated insulin secretion. © 2009 by the American Diabetes Association.