Pancreas and Kidney organogenesis: developmental and disease molecular mechanisms – KiPancreas

The transcription factor vHNF1/ HNF1beta/ Tcf2 is emerging as an important regulator of early organogenesis in vertebrates. In humans, heterozygous mutations in the HNF1beta (further designed as HNF1b) gene are the cause of the multi-organ syndrome Maturity Onset Diabetes of the Young, subtype 5 (MODY5) or Renal Cysts And Diabetes (RCAD), characterized by impaired glucose-stimulated insulin secretion and liver dysfunction as well as a variety of developmental disorders including pancreas hypoplasia and abnormalities of the kidney and genital tract. Despite the increasing number of HNF1b human mutations identified, the underlying cellular and molecular defects are essentially unknown. This is in part because mice heterozygous for an Hnf1b null-allele exhibit a normal phenotype, while mice homozygous for a null allele, generated by constitutive or tissue-specific conditional inactivation, display respectively a more extreme or a partial phenotype compared to that of HNF1b-mutant carriers. Thus, the Hnf1b-homozygous deletion results in early embryonic death due to defective visceral endoderm formation, thus delaying our understanding of the pathogenic mechanisms of MODY5. By rescuing the early lethality through the generation of mouse tetraploid chimeras, we have shown that HNF1b is required for normal pancreas morphogenesis and regional specification of the gut. Rescued Hnf1b-/- embryos display also a severe liver hypoplasia and abnormal ureteric bud branching. In addition to these embryonic roles, Hnf1b-targeted inactivation in islet beta-cells produces an unexpectedly mild phenotype likely due to inefficient and delayed excision by the insulin2 cre-recombinase used, while renal-specific inactivation of Hnf1b in medullar tubules results in renal cyst formation and postnatal death. This is accompanied by a reduced transcription activation of genes involved in cystogenesis, including Pkd2, Pkhd1 and Umod. Our recent molecular characterization of the pancreas and kidney mutant phenotype of two fetuses carrying distinct HNF1b mutations demonstrates firstly that HNF1b plays in humans an important role in pancreas morphogenesis and pancreatic beta-cell differentiation and function. Secondly, it strongly suggests that the cystic phenotype in the two mutant fetuses, results from a deregulation of cell-cell adhesion and/or Wnt/beta-catenin signaling pathway, rather than decreased expression of cystoproteins, thus highlighting the need of creating mouse models that faithfully mimic the phenotypes of the MODY5/RCAD human disease. The main objective of this project is to obtain a more comprehensive view of HNF1b function during urogenital and pancreas development using the available and newly generated animal models and link this knowledge with observations from clinical studies of patients with mutations in this gene. The generation of mouse models that reproduce the phenotypes of the MODY5/RCAD disease is underway by using gene targeting in ES cells combined with Flp-Frt and cre-loxP technology to create allelic series of Hnf1b of varying severity. This approach, extensively used in invertebrates in the analysis of gene function, is based on the isolation of alleles of reduced biological activity. The engineering of hypomorphic mutations into the mouse Hnf1gene in combination with the conditional KO is expected to: - provide insights into molecular mechanisms of the pathogenesis associated with the MODY5/RCAD syndrome, i.e. pancreas hypoplasia, dorsal versus ventral pancreas specification, early onset of diabetes, cystic kidney disease, defective nephrogenesis and abnormal genital tract development. - help to and allow the development of therapeutic strategies to study the efficacy of novel treatments to prevent and treat debilitating disorders such as Diabetes and cystic kidney diseases, prior to conduct costly human clinical trials. - allow to address the implication of somatic HNF1b loss into development of certain canc..