We recently reported a novel disorder, which we named HIPKD, an acronym for the key manifestations of HyperInsulinism and Polycystic Kidney Disease. In addition, patients can have liver cysts. As underlying cause, we identified a promoter mutation in a gene called PMM2. The encoded enzyme Phosphomannomutase2 is a key enzyme in protein glycosylation and ubiquitously expressed. Bi-allelic coding mutations in PMM2 cause a severe and typically lethal multiorgan disorder with obligate neurological involvement called congenital disorder of glycosylation type 1a (CDG1A).
In contrast, the promoter mutation, either homozygous or in trans with a PMM2 coding mutation causes the organ-specific HIPKD. Such organ-specificity conferred by a promoter mutation has not been described previously and suggests a critical role for this part of the promoter for expression of PMM2 in the key organs affected by HIPKD: pancreatic beta-cells, kidney and liver. This provides a unique opportunity to better understand the time and place-specific regulation of gene expression. Our preliminary data suggest that the promoter mutation affects chromatin-looping and we hypothesise that it interferes with transactivation by HNF4A, a transcription factor specifically expressed in pancreatic beta cells, kidney and liver. Indeed, several HNF4 binding sites are present in the predicted chromatin loop.
We have obtained peripheral blood monocytes from patients and heterozygous promoter mutation carriers (parents), which are currently used to derive patients’ and parents’ induced pluripotent stem cells (iPSC). iPS cells can be first induced to a stage corresponding to a common pancreatic progenitor that we can expand in vitro thus allowing a detailed molecular analysis of a well-defined population [4,5].
The aims of the project are:
- to differentiate these iPSC into pancreatic beta cells
- to investigate the effects of the promoter mutation on glycosylation of key proteins involved in insulin secretion (e.g. ABCC8 and KCNJ11)
- to assess chromatin conformation and transcription factor binding (especially HNF4A) of wildtype versus mutant promoter sequence
In summary, this project involves several exciting and highly topical aspects:
- a novel disorder with a novel mechanism of organ-specific manifestations due to a promoter mutation
- differentiation of iPSC into pancreatic beta-cells
- understanding the complex regulation of time- and tissue specific gene expression through chromatin looping
The partner institution for this project is UCL.
- Cabezas, O.R., et al., Polycystic Kidney Disease with Hyperinsulinemic Hypoglycemia Caused by a Promoter Mutation in Phosphomannomutase 2. J Am Soc Nephrol, 2017. 28(8): p. 2529-2539.
- Carney, E.F., Polycystic kidney disease: PMM2 mutation causes PKD and hyperinsulinism. Nat Rev Nephrol, 2017. 13(6): p. 321.
- Ihara, A., et al., Functional characterization of the HNF4alpha isoform (HNF4alpha8) expressed in pancreatic beta-cells. Biochem Biophys Res Commun, 2005. 329(3): p. 984-90.
- Huch M., Bonfanti P., et al. Unlimited in vitro expansion of adult bi-potent pancreas progenitors through the Lgr5/Rspondin axis. EMBO J. 2013 16;32(20):2708-21
- Cai Q., Bonfanti P., et al. Prospectively isolated NGN3-expressing progenitors from human embryonic stem cells give rise to endocrine hormone-expressing cells. Stem Cells Transl Med. 2014;3(4):489-99