The Oak Ridge Polycystic Kidney (orpk) disease gene is required for left-right axis determination

Analysis of several mutations in the mouse is providing useful insights int o the nature of the genes required for the establishment of the left-right axis during early development. Here we describe a new targeted allele of th e mouse Tg737 gene, Tg737(Delta 2-3 beta Gal), which causes defects in left -right asymmetry and other abnormalities during embryogenesis. The Tg737 ge ne was originally identified based on its association with the mouse Oak Ri dge Polycystic Kidney (orpk) insertional mutation, which causes polycystic kidney disease and other defects. Complementation tests between the origina l orpk mutation and the new targeted knock-out mutation demonstrate that Tg 737(Delta 2-3 beta Gal) behaves as an allele of Tg737. The differences in t he phenotype between the two mutations suggest that the orpk mutation is a hypomorphic allele of the Tg737 gene. Unlike the orpk allele, where all hom ozygotes survive to birth, embryos homozygous for the Tg737(Delta 2-3 beta Gal) mutation arrest in development at mid-gestation and exhibit neural tub e defects, enlargement of the pericardial sac and, most notably, left-right asymmetry defects. At mid-gestation the direction of heart looping is rand omized, and at earlier stages in development lefty-2 and nodal, which are n ormally expressed asymmetrically, exhibit symmetrical expression in the mut ant embryos. Additionally, we determined that the ventral node cells in mut ant embryos fail to express the central cilium, which is a characteristic a nd potentially functional feature of these cells. The expression of both Sh h and Hnf3 beta is downregulated in the midline at E8.0, indicating that th ere are significant alterations in midline development in the Tg737(Delta 2 -3 beta Gal) homozygous embryos. We propose that the failure of ventral nod e cells to fully mature alters their ability to undergo differentiation as they migrate out of the node to contribute to the developing midline struct ures. Analysis of this new knockout allele allows us to define a critical r ole for the Tg737 gene during early embryogenesis. We have named the produc t of the Tg737 gene Polaris, which is based on the various polarity related defects associated with the different alleles of the Tg737 gene.