The epigenetic landscape of the beta-cell in IUGR rats

Abstract Text: The abnormal intrauterine milieu of intrauterine growth retardation (IUGR) permanently alters gene expression and function of pancreatic b-cells, leading to the development of diabetes in adulthood. Expression of the pancreatic transcription factor Pdx1 is permanently reduced in IUGR, and epigenetic modifications are responsible for this decrease. Pdx1 encodes a homeobox transcription factor critically important for beta-cell function and development. The fetal IUGR state is characterized by loss of USF-1 binding at the proximal promoter of Pdx1, with deacetylation of histones H3 and H4 due to recruitment of the histone deacetylase HDAC1 and the co-repressor Sin3A. After birth, H3K4 is demethylated and H3K9 is methylated. During the neonatal period, the reduction in Pdx1 expression and these epigenetic changes can be reversed by HDAC inhibition. Finally, once diabetes occurs, DNA methylation of the CpG-island in the proximal promoter ensues, resulting in permanent silencing of the Pdx1 locus. Exendin-4 (Ex-4), a long-acting glucagon-like peptide 1 (GLP-1) analog, given on days 1-6 of life increases Pdx1 expression and prevents the development of diabetes in the IUGR rat. Ex-4 increases USF-1 and PCAF association at the proximal promoter of Pdx1, thereby increasing histone acetyl transferase (HAT) activity, which leads to a permanent increase in histone H3 acetylation and H3K4 methylation. Normalization of these histone modifications precludes DNA methylation, thereby preventing silencing of Pdx1 in islets of IUGR animals. These studies demonstrate a novel mechanism whereby a short treatment course of Ex-4 in the newborn period prevents diabetes in adulthood by restoring Pdx1 promoter chromatin structure thus preserving Pdx1 transcription. Finally, using the HELP assay, we generated the first DNA methylation map of the rat genome in normal and IUGR ß-cells. We validated candidate dysregulated loci with quantitative assays of cytosine methylation and gene expression. IUGR changes cytosine methylation at 1,400 loci in male rats at 7 weeks of age, preceding the development of diabetes and thus representing candidate loci for mediating the pathogenesis of metabolic disease that occurs later in life. Epigenetic dysregulation occurred preferentially at conserved intergenic sequences, frequently near genes regulating processes known to be abnormal in IUGR islets, such as vascularization, ß-cell proliferation, insulin secretion, and cell death, associated with concordant changes in mRNA expression. These results demonstrate that epigenetic dysregulation is a strong candidate for propagating the cellular memory of intrauterine events, causing changes in expression of nearby genes and long term susceptibility to type 2 diabetes.

Keywords: diabetes, programming, epigenetic