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Aims/hypothesis Genome-wide studies have uncovered multiple independent signals at the RREB1 locus associated with altered type 2 diabetes risk and related glycaemic traits. However, little is known about the function of the zinc finger transcription factor Ras-responsive element binding protein 1 (RREB1) in glucose homeostasis or how changes in its expression and/or function influence diabetes risk. Methods A zebrafish model lacking rreb1a and rreb1b was used to study the effect of RREB1 loss in vivo. Using transcriptomic and cellular phenotyping of a human beta cell model (EndoC-βH1) and human induced pluripotent stem cell (hiPSC)-derived beta-like cells, we investigated how loss of RREB1 expression and activity affects pancreatic endocrine cell development and function. Ex vivo measurements of human islet function were performed in donor islets from carriers of RREB1 type 2 diabetes risk alleles. Results CRISPR/Cas9-mediated loss of rreb1a and rreb1b function in zebrafish supports an in vivo role for the transcription factor in beta cell mass, beta cell insulin expression and glucose levels. Loss of RREB1 also reduced insulin gene expression and cellular insulin content in EndoC-βH1 cells and impaired insulin secretion under prolonged stimulation. Transcriptomic analysis of RREB1 knockdown and knockout EndoC-βH1 cells supports RREB1 as a novel regulator of genes involved in insulin secretion. In vitro differentiation of RREB1KO/KO hiPSCs revealed dysregulation of pro-endocrine cell genes, including RFX family members, suggesting that RREB1 also regulates genes involved in endocrine cell development. Human donor islets from carriers of type 2 diabetes risk alleles in RREB1 have altered glucose-stimulated insulin secretion ex vivo, consistent with a role for RREB1 in regulating islet cell function. Conclusions/interpretation Together, our results indicate that RREB1 regulates beta cell function by transcriptionally regulating the expression of genes involved in beta cell development and function. Graphical abstract

There is increasing genetic evidence for the role of microglia in neurodegenerative diseases, including Alzheimer’s, Parkinson’s, and motor neuron disease. Therefore, there is a need to generate authenticin vitromodels to study human microglial physiology. Various methods have been developed using human induced Pluripotent Stem Cells (iPSC) to generate microglia, however, systematic approaches to identify which media components are actually essential for functional microglia are mostly lacking. Here, we systematically assess medium components, coatings, and growth factors required for iPSC differentiation to microglia. Using single-cell RNA sequencing, qPCR, and functional assays, with validation across two labs, we have identified several medium components from previous protocols that are redundant and do not contribute to microglial identity. We provide an optimised, defined medium which produces both transcriptionally and functionally relevant microglia for modelling microglial physiology in neuroinflammation and for drug discovery.

Aims/hypothesisGenome-wide studies have uncovered multiple independent signals at the RREB1 locus associated with altered type 2 diabetes risk and related glycemic traits. However, little is known about the function of the zinc finger transcription factor RREB1 in glucose homeostasis or how changes in its expression and/or function influence diabetes risk.MethodsA zebrafish model lacking rreb1a and rreb1b was used to study the effect of RREB1 loss in vivo. Using transcriptomic and cellular phenotyping of a human beta cell model (EndoC-βH1) and human induced pluripotent stem cell (hiPSC)-derived beta-like cells, we investigated how loss of RREB1 expression and activity affects pancreatic endocrine cell development and function. Ex vivo measurements of human islet function were performed in donor islets from carriers of RREB1 T2D-risk alleles.ResultsCRISPR-Cas9-mediated loss of rreb1a and rreb1b function in zebrafish supports an in vivo role for the transcription factor in beta cell mass, beta cell insulin expression, and glucose levels. Loss of RREB1 reduced insulin gene expression and cellular insulin content in EndoC-βH1 cells, and impaired insulin secretion under prolonged stimulation. Transcriptomic analysis of RREB1 knockdown and knockout EndoC-βH1 cells supports RREB1 as a novel regulator of genes involved in insulin secretion. In vitro differentiation of RREB1KO/KO hiPSCs revealed a dysregulation of pro-endocrine cell genes, including RFX family members, suggesting that RREB1 also regulates genes involved in endocrine cell development. Human donor islets from carriers of T2D-risk alleles in RREB1 have altered glucose-stimulated insulin secretion ex vivo, consistent with RREB1 regulating islet cell function.Conclusions/interpretationTogether, our results indicate that RREB1 regulates beta cell function by transcriptionally regulating the expression of genes involved in beta cell development and function.Research in contextWhat is already known about this subject?Human genetic variation in RREB1 is associated with altered diabetes risk, variation in glycemic, and anthropometric traitsRREB1 is a transcription factor that binds to Ras-responsive elements and is expressed in multiple diabetes relevant tissues, including pancreatic isletsWhat is the key question?How does altered expression or function of RREB1 influence diabetes risk?What are the new findings?Knockdown and knockout of RREB1 in mature human EndoC-βH1 cells reduces expression of insulin transcript and cellular content, as well as insulin secretion under prolonged stressCarriers of the T2D-risk RREB1 coding allele trend towards reduced insulin content, but have improved glucose-stimulated insulin secretionA loss-of-function zebrafish model suggests that RREB1 is required for insulin expressionHow might this impact on clinical practice in the foreseeable future?RREB1 controls beta cell function and whole-body glucose homeostasis by transcriptionally regulating the development and function of pancreatic beta cells

In an effort to identify disease-causing mechanisms, numerous genome-wide studies across different ancestries have discovered genetic variants influencing type 2 diabetes (T2D) risk. Amongst identified diabetes-associated loci, a PAX4 coding variant (p.Arg192His) is uniquely and reproducibly associated with altered T2D-risk in East Asian populations. PAX4 encodes a transcription factor, and in mice Pax4 is essential for beta cell development. Whether PAX4 plays a similar role in humans and how diabetes associated PAX4 variants impact beta cell development and/or function is unknown. We combined in vivo studies in human variant carriers with disease modelling in human stem cell models to address these questions. Carriers of the PAX4 p.Arg192His allele have reduced islet cell function compared to non-carriers (AIRg: p.Arg192 756.9 vs. p.His192 530.8 p In conclusion, we have demonstrated the importance of PAX4 for human endocrine cell development and provide a potential mechanism for diabetes-associated alleles through de-repression of alpha cell gene expression. Disclosure N. Krentz: None. S. Hoon: None. D. Gardner: None. S. Kao: None. E. Tai: Advisory Panel; Amgen Inc., Merck Sharp & Dohme Corp., Novartis AG, Novo Nordisk, Stock/Shareholder; Abbott, AbbVie Inc. A. L. Gloyn: Other Relationship; Genentech, Inc., Roche Pharmaceuticals. A. Teo: Stock/Shareholder; BetaLife Pte Ltd. H. Lau: None. F. Abaitua: Employee; Vertex Pharmaceuticals Incorporated. M. Perez-alcantara: None. J. Chan: None. J. N. Ajeian: None. B. Champon: None. H. Sun: None. A. Jha: None. Funding Wellcome (200837) ; National Institutes of Health (U01-DK105535, U01-DK085545, UM1DK126185, U01DK123743, U24DK098085, P30DK116074) ; Lee Foundation Grant (SHTX/LFG/002/2018) Paris-NUS 2021-06-R/UP-NUS (ANR-18-IDEX-0001)

Heterozygous loss of function mutations in HNF1A, encoding the transcription factor hepatocyte nuclear factor 1 alpha (HNF-1A) are the most common cause of monogenic diabetes. HNF-1A is involved in both beta cell development and mature cell function. However, model organisms have often failed to faithfully recapitulate the human phenotypes. As such, we used a patient-derived HNF1A Pro291fsinsC iPSC model and CRISPR-Cas9 genome editing to correct the mutation and generate an isogenic control. iPSC clones (n=6) were differentiated towards the endocrine lineage in triplicate, with RNA-seq (bulk and single-cell) and ATAC-seq performed at definitive endoderm (DE), pancreatic endoderm (PE) and beta-like cells (BLC) stages. RNA-seq data confirmed HNF1A haploinsuffiency in the patient cell lines, with corrected cell lines demonstrating restored HNF1A expression at both transcript and protein level. At the BLC stage, there was a significant overlap in differentially expressed genes and HNF1A targets identified in an independent HNF1A knock-down experiment in the EndoC-βh1 cell line (p=4.4e-132). Corrected clones demonstrated a higher expression of INS (p=7.2e-03), insulin secretion genes and effects on pancreas progenitor cell genes indicating that HNF1A may mediate the cellular composition of the pancreatic islet. Bulk RNA-seq data were also supported in the preliminary analysis of the single-cell RNA-seq data. Open chromatin, assessed by ATAC-seq was significantly different between patient and corrected lines, with >90% of the sites being more accessible at both PE and BLC stages in the corrected clones. These sites were enriched for HNF1A binding motifs, implicating HNF1A in the establishment of open chromatin. In conclusion, our patient-derived HNF1A-MODY iPSC model recapitulates a number of features of HNF1A-MODY, as well as providing insights into the cellular and molecular phenotypes caused by HNF-1A deficiency. Disclosure C. Duff: Research Support; Self; Novo Nordisk A/S. R. Jensen: None. M. Perez-Alcantara: None. M. Rasmussen: None. A. Grotz: None. M. Jansen: None. B. Holst: None. C. Clausen: None. N.A.J. Krentz: None. M. Hansson: Employee; Self; Novo Nordisk A/S. M.I. McCarthy: Advisory Panel; Self; Illumina. Consultant; Self; Eli Lilly and Company, Novo Nordisk Inc., Zoe Global Ltd. Employee; Self; Genentech, Inc. Research Support; Self; AbbVie Inc., Merck & Co., Inc., Sanofi-Aventis, Servier, Takeda UK. A.L. Gloyn: Consultant; Self; Merck Sharp & Dohme Corp. Employee; Spouse/Partner; Genentech, Inc. Research Support; Self; Novo Nordisk A/S. A. Wesolowska-Andersen: None. C. Honore: Employee; Self; Novo Nordisk A/S. Stock/Shareholder; Self; Novo Nordisk A/S. Funding Innovative Medicines Initiative (115439)

Genome-wide association studies have identified multiple independent signals for type 2 diabetes risk at the RREB1 locus. Coding variant fine-mapping (D1171N, rs9379084) indicated RREB1 as the effector transcript. To investigate the role of this transcription factor (TF) in beta cell function and development, we performed transcriptome profiling and cellular phenotyping of genome-edited EndoC-βH1 beta cells and human induced pluripotent stem cells (hiPSC). CRISPR/Cas9-mediated knockout (KO) of RREB1 in EndoC-βH1 cells reduced INS expression (log2FC=-0.4, q=0.01) and insulin content (-44.2%, p=2x10-4). Motif activity response analysis (ISMARA) of RREB1 KO and wild type (WT) transcriptome expression data highlighted RFX2 and RFX3 as key TFs mediating gene expression variation across samples. RFX2 expression was upregulated in RREB1 KO cells at the transcript (log2FC=0.4,q=6.5x10-4) and protein level (+125.7%, p=8x10-4). Consistent with this, RFX2/3 target genes CAMK2A (log2FC=3.9, q=8.7x10-109) and GPR56 (log2FC=3.1, q=5.2x10-283) were among the strongest upregulated differentially expressed genes (DEG). Transcriptome profiling of CRISPR/Cas9-generated RREB1 KO hiPSC, differentiated along the pancreatic endocrine lineage, revealed that RREB1 was expressed throughout development (CV=10.7%). In total 5476 DEG (q In conclusion, our combined data support a novel role for RREB1 in both beta cell function and development likely via RFX2/3. Disclosure K.K. Mattis: None. F. Abaitua: None. A. Grotz: None. A. Wesolowska-Andersen: None. M. Perez-Alcantara: None. M. McCarthy: Advisory Panel; Self; European Association for the Study of Diabetes, Pfizer Inc. Consultant; Self; Eli Lilly and Company, Merck & Co., Inc. Consultant; Spouse/Partner; Merck & Co., Inc. Research Support; Self; AbbVie Inc., Boehringer Ingelheim International GmbH. Research Support; Spouse/Partner; Diabetes UK. Research Support; Self; Janssen Pharmaceuticals, Inc., Merck & Co., Inc., National Institutes of Health. Research Support; Spouse/Partner; National Institutes of Health. Research Support; Self; Novo Nordisk A/S. Research Support; Spouse/Partner; Novo Nordisk A/S. Research Support; Self; Novo Nordisk Foundation, Roche Pharma, Sanofi-Aventis, Servier, Takeda Pharmaceutical Company Limited. A. Mahajan: None. B. Davies: None. A.L. Gloyn: Consultant; Spouse/Partner; Eli Lilly and Company, Merck & Co., Inc. Consultant; Self; Merck & Co., Inc. Consultant; Spouse/Partner; Novo Nordisk A/S, Pfizer Inc. Research Support; Self; Novo Nordisk A/S. Speaker’s Bureau; Self; Novo Nordisk A/S. Other Relationship; Self; Diabetes UK, European Foundation for the Study of Diabetes. Funding UK Wellcome Trust

Aims/hypothesis Most type 2 diabetes-associated genetic variants identified via genome-wide association studies (GWASs) appear to act via the pancreatic islet. Observed defects in insulin secretion could result from an impact of these variants on islet development and/or the function of mature islets. Most functional studies have focused on the latter, given limitations regarding access to human fetal islet tissue. Capitalising upon advances in in vitro differentiation, we characterised the transcriptomes of human induced pluripotent stem cell (iPSC) lines differentiated along the pancreatic endocrine lineage, and explored the contribution of altered islet development to the pathogenesis of type 2 diabetes. Methods We performed whole-transcriptome RNA sequencing of human iPSC lines from three independent donors, at baseline and at seven subsequent stages during in vitro islet differentiation. Differentially expressed genes (q 1) were assigned to the stages at which they were most markedly upregulated. We used these data to characterise upstream transcription factors directing different stages of development, and to explore the relationship between RNA expression profiles and genes mapping to type 2 diabetes GWAS signals. Results We identified 9409 differentially expressed genes across all stages, including many known markers of islet development. Integration of differential expression data with information on transcription factor motifs highlighted the potential contribution of REST to islet development. Over 70% of genes mapping within type 2 diabetes-associated credible intervals showed peak differential expression during islet development, and type 2 diabetes GWAS loci of largest effect (including TCF7L2; log2FC = 1.2; q = 8.5 × 10−10) were notably enriched in genes differentially expressed at the posterior foregut stage (q = 0.002), as calculated by gene set enrichment analyses. In a complementary analysis of enrichment, genes differentially expressed in the final, beta-like cell stage of in vitro differentiation were significantly enriched (hypergeometric test, permuted p value Conclusions/interpretation The present study characterises RNA expression profiles during human islet differentiation, identifies potential transcriptional regulators of the differentiation process, and suggests that the inherited predisposition to type 2 diabetes is partly mediated through modulation of islet development.