Identification of C2CD4A as a human diabetes susceptibility gene with a role in β cell insulin secretion

Abstract

Fine mapping and validation of genes causing β cell failure from susceptibility loci identified in type 2 diabetes genome-wide association studies (GWAS) poses a significant challenge. The VPS13C-C2CD4A-C2CD4B locus on chromosome 15 confers diabetes susceptibility in every ethnic group studied to date. However, the causative gene is unknown. FoxO1 is involved in the pathogenesis of β cell dysfunction, but its link to human diabetes GWAS has not been explored. Here we generated a genome-wide map of FoxO1 superenhancers in chemically identified β cells using 2-photon live-cell imaging to monitor FoxO1 localization. When parsed against human superenhancers and GWAS-derived diabetes susceptibility alleles, this map revealed a conserved superenhancer in C2CD4A, a gene encoding a β cell/stomach-enriched nuclear protein of unknown function. Genetic ablation of C2cd4a in β cells of mice phenocopied the metabolic abnormalities of human carriers of C2CD4A-linked polymorphisms, resulting in impaired insulin secretion during glucose tolerance tests as well as hyperglycemic clamps. C2CD4A regulates glycolytic genes, and notably represses key β cell “disallowed” genes, such as lactate dehydrogenase A. We propose that C2CD4A is a transcriptional coregulator of the glycolytic pathway whose dysfunction accounts for the diabetes susceptibility associated with the chromosome 15 GWAS locus.

Sexually dimorphic roles for the type 2 diabetes-associatedC2cd4bgene in murine glucose homeostasis

“…These questions have been pursued using mouse and fish knockout models and relevant cell lines. In contrast to earlier findings (15), we observed that global C2cd4a deletion in the mouse exerted no effects on insulin secretion in vitro or in vivo. These findings are consistent with the considerably (10-fold) lower expression of C2cd4a than C2cd4b in mouse islets and derived β cell lines (Supp.…”

“…Table 1) though we would emphasise that no attempt was made here to quantify protein (rather than mRNA) levels. The reasons for the differences between the present study and that of Kuo et al (15) with respect to C2cd4a are presently unclear, although this may reflect differences in the targeting strategies used to generate the different mouse mutants or genetic background and general housing conditions. Nonetheless, given that our work involved global inactivation of this gene we cannot exclude the possibility that compensatory changes occur in other tissues, or in islet cell other than β cells, that mitigate the effects of deleting C2cd4a in the β cell.…”

“…Our observation that neither C2CD4A nor C2CD4B are localised exclusively in the nucleus, as previously reported in COS7 cells (10), islets and MIN6 cells (15) was unexpected, but implies a more dynamic role for both C2CD4A, and possibly C2CD4B, in intracellular signalling. We note that the present studies explored the subcellular distribution of the H. sapiens protein, rather than the M. musculus homologue examined previously (15), providing a potential explanation for these differences. Interestingly, predictions from primary structure (11) indicate that neither C2CD4A nor C2CD4B…”

“…A recent study (15) has indicated that regulation of C2cd4a expression in islets by FOXO1 may be important for the control of insulin secretion. Thus, animals inactivated selectively in the β cell for C2cd4a displayed abnormal insulin secretion in response to glucose plus arginine (the effects on secretion stimulated by glucose alone were not reported), and glucose intolerance in vivo, and the abnormal expression of β cell signature and "disallowed" genes (16).…”

Antagonistic epistasis of Hnf4α and FoxO1 networks through enhancer interactions in β-cell function

“…An example of acquired transcriptional abnormalities leading to altered β-cell function is FoxO1, a nutrient-regulated transcription factor whose nutrient excess-driven failure leads first to metabolic inflexibility (Kim-Muller et al, 2014), and then to outright β-cell dedifferentiation (Cinti et al, 2016;Sun et al, 2019;Talchai et al, 2012). This is achieved in part by direct actions on glucose metabolism (Kuo et al, 2019b) and in part by regulating lineage stability (Kuo et al, 2019a).…”