Haploinsufficiency of retinaldehyde dehydrogenase 2 decreases the severity and incidence of duodenal atresia in the fibroblast growth factor receptor 2IIIb−/− mouse model

The etiology of intestinal atresia remains undetermined. Atresias have been ascribed to a number of events, including in utero vascular accidents,1–3 mechanical compression,4 and disruptions in notochord development.5,6 In the case of the duodenum, atresias have been hypothesized to result from a failure of the intestinal lumen to recanalize after a period of exuberant endodermal growth.7 Over the last decade, genetic animal models of intestinal atresia have been developed. The best characterized of these genetic models result from homozygous mutation of either fibroblast growth factor 10 (Fgf10) or its cognate receptor, fibroblast growth factor receptor 2IIIb (Fgfr2IIIb) in mice.8,9 The homozygous mutation of either of these genes results in nonviable progeny at embryonic day (E) 18.5, in which 100% of the embryos have colonic atresias9,10 and 38% to 42% have duodenal atresias.8,11 Approximately 92% of the duodenal atresias observed in the Fgf10 mouse model are type III (gap in the intestine and mesentery), and 8% are type I (intestinal tube is intact externally but has a blockage in the lumen).8 In the Fgfr2IIIb model, the formation of colonic and duodenal atresia is preceded by an increase level of apoptosis in the endoderm.12 Interestingly, unlike human duodenal development, normal development of the mouse duodenum proceeds without a solid core phase in the lumen.12 These results suggest that in the Fgfr2IIIb mouse model, the formation of duodenal and colonic atresias result from the loss of endoderm during a critical phase in development, as opposed to a failure in recanalization of the lumen as proposed originally.7 We observed previously that retinaldehyde dehydrogenase 2 (Raldh2) is expressed in the duodenum during the temporal window of atresia formation (E 11.0–E 13.5).13 This gene encodes for an enzyme that catalyzes the final step of vitamin A conversion to its active metabolite, which is retinoic acid. Recent work has shown that this gene is critical for normal development in the pancreatic–duodenal region because homozygous mutations in Raldh2 result in disruptions in pancreatic development. These embryos fail to progress beyond E 9.5 in development, suggesting that Raldh2 is a critical gene in the development of numerous systems during embryogenesis.14 We hypothesized that haploinsufficiency of Raldh2 would affect the severity of duodenal atresias in the Fgfr2IIIb−/− model. In this model, 93% of the atresias are type III defects; however, the penetrance ranges from 38% to 42%.8,12 Therefore, if haploinsufficiency of Raldh2 were to increase the severity of the defects, we would expect to see a significant increase in atresia incidence. If haploinsufficiency of Raldh2 were to have the opposite effect, then we would expect both the severity and the incidence of atresias to decrease. To test this, we generated compound mutants using the hypoxanthine-guanine phosphoribosyltransferase-Cre (Hprt-Cre) system with conditional alleles for both Fgfr2IIIb and Raldh2.11 We observed that haploinsufficiency of Raldh2 decreased both the severity and incidence of duodenal atresia in comparison to mutant controls.