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Mediator, a co-regulator complex required for RNA Polymerase II activity, interacts with tissue-specific transcription factors to regulate development and maintain homeostasis. We observe reduced Mediator subunit MED15 expression in endocrine hormone-producing pancreatic islets isolated from people living with type 2 diabetes and sought to understand how MED15 and Mediator control gene expression programs important for the function of insulin-producing β-cells. Here we show that Med15 is expressed during mouse β-cell development and maturation. Knockout of Med15 in mouse β-cells causes defects in β-cell maturation without affecting β-cell mass or insulin expression. ChIP-seq and co-immunoprecipitation analyses found that Med15 binds β-cell transcription factors Nkx6-1 and NeuroD1 to regulate key β-cell maturation genes. In support of a conserved role during human development, human embryonic stem cell-derived β-like cells, genetically engineered to express high levels of MED15, express increased levels of maturation markers. We provide evidence of a conserved role for Mediator in β-cell maturation and demonstrate an additional layer of control that tunes β-cell transcription factor function. The establishment of glucose-regulated insulin secretion from pancreatic β-cells requires multiple transcription factors but is incompletely understood. Here, the authors show that Mediator complex subunit MED15 is vital for this process of β-cell maturation, highlighting its role in coordinating transcriptional control of insulin production and secretion.

Aims/hypothesis Beta cells control glucose homeostasis via regulated production and secretion of insulin. This function arises from a highly specialised gene expression programme that is established during development and then sustained, with limited flexibility, in terminally differentiated cells. Dysregulation of this programme is seen in type 2 diabetes but mechanisms that preserve gene expression or underlie its dysregulation in mature cells are not well resolved. This study investigated whether methylation of histone H3 lysine 4 (H3K4), a marker of gene promoters with unresolved functional importance, is necessary for the maintenance of mature beta cell function. Methods Beta cell function, gene expression and chromatin modifications were analysed in conditional Dpy30 knockout mice, in which H3K4 methyltransferase activity is impaired, and in a mouse model of diabetes. Results H3K4 methylation maintains expression of genes that are important for insulin biosynthesis and glucose responsiveness. Deficient methylation of H3K4 leads to a less active and more repressed epigenome profile that locally correlates with gene expression deficits but does not globally reduce gene expression. Instead, developmentally regulated genes and genes in weakly active or suppressed states particularly rely on H3K4 methylation. We further show that H3K4 trimethylation (H3K4me3) is reorganised in islets from the Lepr db/db mouse model of diabetes in favour of weakly active and disallowed genes at the expense of terminal beta cell markers with broad H3K4me3 peaks. Conclusions/interpretation Sustained methylation of H3K4 is critical for the maintenance of beta cell function. Redistribution of H3K4me3 is linked to gene expression changes that are implicated in diabetes pathology. Graphical abstract

It is unclear how cells counteract the potentially harmful effects of uncoordinated DNA replication in the context of oncogenic stress. Here, we identify the WRAD (WDR5/RBBP5/ASH2L/DPY30) core as a modulator of DNA replication in pancreatic ductal adenocarcinoma (PDAC) models. Molecular analyses demonstrated that the WRAD core interacts with the replisome complex, with disruption of DPY30 resulting in DNA re-replication, DNA damage, and chromosomal instability (CIN) without affecting cancer cell proliferation. Consequently, in immunocompetent models, DPY30 loss induced T cell infiltration and immune-mediated clearance of highly proliferating cancer cells with complex karyotypes, thus improving anti-tumor efficacy upon anti-PD-1 treatment. In PDAC patients, DPY30 expression was associated with high tumor grade, worse prognosis, and limited response to immune checkpoint blockade. Together, our findings indicate that the WRAD core sustains genome stability and suggest that low intratumor DPY30 levels may identify PDAC patients who will benefit from immune checkpoint inhibitors.

In the progression of diabetes, pancreatic islet β-cells respond to increased metabolic demand with functional compensation, followed by pathogenic decompensation of mitochondria-dependent insulin secretion. It is not clear what mechanisms drive, or control, mitochondrial decompensation. Here, we report that anti-apoptotic Bcl-xL maintains mitochondrial integrity in β-cells under non-apoptotic levels of glucose stress. Prolonged glucose excess causes transcriptional reprogramming of glycolysis and β-cell identity genes, while sensitizing glucose-stimulated Ca2+ signaling and insulin secretion. Deletion of Bcl-xL amplifies this insulin hypersecretion and increases mitochondrial fusion, mitochondrial volume, and oxygen consumption, whereas ATP-coupled respiration and mitochondrial hyperpolarization become impaired. Of note, Bcl-xL-deficient β-cells have impaired Pgc-1α expression, and develop specific defects in the expression of Tfam, mitochondrial ribosomal genes, and OXPHOS components under glucose stress. Bcl-xL limits high glucose-induced mitochondrial ROS (mitoROS) levels and pharmacological normalization of mitoROS in Bcl-xL KO cells rescues glucose-induced defects in mitochondrial gene expression and changes to β-cell identity. Our data identify mitoROS as a primary retrograde driver of transcriptional re-wiring in β-cells exposed to excess glucose, and reveal Bcl-xL as an important safeguard against transcriptional and functional decompensation of β-cell mitochondria. Bcl-xL and mitoROS may thus be viable targets to prevent early β-cell dysfunction and the progression of diabetes.

Pancreatic β-cells control glucose homeostasis via regulated production and secretion of insulin. This function arises from a highly specialized gene expression program which is established during development and then sustained, with limited flexibility, in terminally differentiated β-cells. Dysregulation of this program is seen in type 2 diabetes (T2D) but mechanisms that preserve gene expression or underlie its dysregulation in mature β-cells are not well resolved. Here we show that trithorax group-dependent histone H3 lysine 4 trimethylation (H3K4me3) maintains expression of genes important for insulin biosynthesis and glucose-responsiveness in β-cells. Transcriptional changes in H3K4me3-deficient β-cells lead to severe hyperglycemia in adult mice. We show that H3K4me3 deficiency leads to a less active and more repressed epigenome profile, which locally correlates with gene expression deficits but does not globally reduce gene expression. Instead, developmentally regulated genes and genes in weakly active or suppressed states particularly rely on H3K4 methylation. We then show that H3K4me3 is re-organized in diabetic Leprdb/db mouse islets in favour of weakly active and disallowed genes at the expense of terminal β-cell markers with broad H3K4me3 peaks. Our results point to key roles of H3K4me3 in maintaining mature β-cell function and establishing a dysfunctional transcriptome in diabetic islets.

Summary Histone 3 lysine 4 trimethylation (H3K4me3) is associated with promoters of actively expressed genes, with genes important for cell identity frequently having exceptionally broad H3K4me3-enriched domains at their TSS. While H3K4 methylation is implicated in contributing to transcription, maintaining transcriptional stability, facilitating enhancer-promoter interactions, and preventing irreversible silencing, some studies suggest it has little functional impact. Therefore, the function of H3K4 methylation is not resolved. Insufficient insulin release by β-cells is the primary etiology in type 2 diabetes (T2D) and is associated with the loss of expression of genes essential to normal β-cell function. We find that H3K4me3 is reduced in islets from mouse models of diabetes and from human donors with T2D. Using a genetic mouse model to impair H3K4 methyltransferase activity of TrxG complexes, we find that reduction of H3K4 methylation significantly reduces insulin production and glucose-responsiveness and increases transcriptional entropy, indicative of a loss of β-cell maturity. Genes that are downregulated by reduction to H3K4 methylation are concordantly downregulated in T2D. Loss of H3K4 methylation causes global dilution of epigenetic complexity but does not generally reduce gene expression – instead, genes related to β-cell function and/or in particular chromatin environments are specifically affected. While neither H3K4me3 nor H3K4me1 are strictly required for the expression of many genes, the expression of genes with critical roles in β-cell function becomes destabilized, with increased variance and decreased overall expression. Our data further suggests that, in absence of H3K4me3, promoter-associated H3K4me1 is sufficient to maintain expression. Together, these data implicate H3K4 methylation dysregulation as destabilizing β-cell gene expression and contributing to β-cell dysfunction in T2D. Competing Interest Statement The authors have declared no competing interest. Footnotes * Meilin An's name has been corrected in the Author list metadata.

Summary During pancreas development, endocrine progenitors differentiate into the islet-cell subtypes, which undergo further functional maturation in postnatal islet development. In islet β-cells, genes involved in glucose-stimulated insulin secretion are activated and glucose exposure increases the insulin response as β-cells mature. Here, we investigated the role of H3K4 trimethylation in endocrine cell differentiation and functional maturation by disrupting TrxG complex histone methyltransferase activity in mouse endocrine progenitors. In the embryo, genetic inactivation of TrxG component Dpy30 in NEUROG3+ cells did not affect the number of endocrine progenitors or endocrine cell differentiation. H3K4 trimethylation was progressively lost in postnatal islets and the mice displayed elevated random and fasting glycemia, as well as impaired glucose tolerance by postnatal day 24. Although postnatal endocrine cell proportions were equivalent to controls, islet RNA-sequencing revealed a downregulation of genes involved in glucose-stimulated insulin secretion and an upregulation of immature β-cell genes. Comparison of histone modification enrichment profiles in NEUROG3+ endocrine progenitors and mature islets suggested that genes downregulated by loss of H3K4 trimethylation more frequently acquire active histone modifications during maturation. Taken together, these findings suggest that H3K4 trimethylation is required for the activation of genes involved in the functional maturation of pancreatic islet endocrine cells. Competing Interest Statement The authors have declared no competing interest.