Heterozygous missense variant in GLI2 impairs human endocrine pancreas development.

Missense variants are the most common type of coding genetic variants. Their functional assessment is fundamental for defining any implication in human diseases and may also uncover genes that are essential for human organ development. Here, we apply CRISPR-Cas9 gene editing on human iPSCs to study a heterozygous missense variant in GLI2 identified in two siblings with early-onset and insulin-dependent diabetes of unknown cause. GLI2 is a primary mediator of the Hedgehog pathway, which regulates pancreatic ß-cell development in mice. However, neither mutations in GLI2 nor Hedgehog dysregulation have been reported as cause or predisposition to diabetes. We establish and study a set of isogenic iPSC lines harbouring the missense variant for their ability to differentiate into pancreatic ß-like cells. Interestingly, iPSCs carrying the missense variant show altered GLI2 transcriptional activity and impaired differentiation of pancreatic progenitors into endocrine cells. RNASeq and network analyses unveil a crosstalk between Hedgehog and WNT pathways, with the dysregulation of non-canonical WNT signaling in pancreatic progenitors carrying the GLI2 missense variant. Collectively, our findings underscore an essential role for GLI2 in human endocrine development and identify a gene variant that may lead to diabetes.

A whole organism small molecule screen identifies novel regulators of pancreatic endocrine development.

An early step in pancreas development is marked by the expression of the transcription factor Pdx1 within the pancreatic endoderm, where it is required for the specification of all endocrine cell types. Subsequently, Pdx1 expression becomes restricted to the ß-cell lineage, where it plays a central role in ß-cell function. This pivotal role of Pdx1 at various stages of pancreas development makes it an attractive target to enhance pancreatic ß-cell differentiation and increase ß-cell function. In this study, we used a newly generated zebrafish reporter to screen over 8000 small molecules for modulators of pdx1 expression. We found four hit compounds and validated their efficacy at different stages of pancreas development. Notably, valproic acid treatment increased pancreatic endoderm formation, while inhibition of TGFß signaling led to α-cell to ß-cell transdifferentiation. HC toxin, another HDAC inhibitor, enhances ß-cell function in primary mouse and human islets. Thus, using a whole organism screening strategy, this study identified new pdx1 expression modulators that can be used to influence different steps in pancreas and ß-cell development.