Mice doubly deficient in Six4 and Six5 show ventral body wall defects reproducing human omphalocele

Abstract

Omphalocele is a human congenital anomaly in ventral body wall closure and may be caused by impaired formation of the primary abdominal wall (PAW) and/or defects in abdominal muscle development. Here, we report that mice doubly deficient in homeobox genes Six4 and Six5 showed ventral body wall closure defects reproducing human omphalocele. SIX4 and SIX5 were localized in surface ectodermal cells and somatic mesoderm-derived mesenchymal and coelomic epithelial cells (CECs) in the PAW. Six4−/−;Six5−/− fetuses exhibited large omphalocele with protrusion of both the liver and intestine or small omphalocele with that of the intestine with complete penetrance. The umbilical ring of Six4−/−;Six5−/− embryos was shifted anteriorly and the lateral size of the ring was larger than that of normal embryos at the E11.5 stage, prior to the onset of myoblast migration into the PAW. The proliferation rates of surface ectodermal cells in the left and right PAW and somatic mesoderm-derived cells in the right PAW were lower in Six4−/−;Six5−/− embryos than those in wild-type embryos at E10.5. The transition from CECs of the PAW to rounded mesothelial progenitor cells was impaired and the inner coelomic surface of the PAW showed relatively smooth in Six4−/−;Six5−/− embryos at E11.25. Furthermore, Six4 overexpression in CECs of the PAW promoted ingression of CECs. Taken together, our results suggest that Six4 and Six5 are required for growth and morphological change of the PAW, and the impairment of these processes is linked to the abnormal positioning and expansion of the umbilical ring, resulting in omphalocele.