Originally published as Genetics Published Articles Ahead of Print on February 9, 2009.

Genetics, Vol. 181, 1359-1368, April 2009, Copyright © 2009
doi:10.1534/genetics.109.101055

Both the RGS Domain and the Six C-Terminal Amino Acids of Mouse Axin Are Required for Normal Embryogenesis

Ian V. Chia*, Min Jung Kim*, Keiji Itoh{dagger}, Sergei Y. Sokol{dagger} and Frank Costantini*,1

* Department of Genetics and Development, Columbia University Medical Center, New York, New York 10032 and {dagger} Department of Developmental and Regenerative Biology, Mount Sinai School of Medicine, New York, New York 10029

1 Corresponding author: Department of Genetics and Development, Columbia University Medical Center, 701 W. 168th St., New York, NY 10032.
E-mail: fdc3{at}columbia.edu

Axin is a negative regulator of canonical Wnt signaling, which promotes the degradation of β-catenin, the major effector in this signaling cascade. While many protein-binding domains of Axin have been identified, their significance has not been evaluated in vivo. Here, we report the generation and analysis of mice carrying modified Axin alleles in which either the RGS domain or the six C-terminal amino acids (C6 motif) were deleted. The RGS domain is required for APC-binding, while the C6 motif has been implicated in the activation of c-Jun N-terminal kinase, but is not required for the effects of Axin on the Wnt/β-catenin pathway, in vitro. Both mutant Axin alleles caused recessive embryonic lethality at E9.5–E10.5, with defects indistinguishable from those caused by a null allele. As Axin-{Delta}RGS protein was produced at normal levels, its inability to support embryogenesis confirms the importance of interactions between Axin and APC. In contrast, Axin-{Delta}C6 protein was expressed at only 25–30% of the normal level, which may account for the recessive lethality of this allele. Furthermore, many Axin{Delta}C6/{Delta}C6 embryos that were heterozygous for a β-catenin null mutation survived to term, demonstrating that early lethality was due to failure to negatively regulate β-catenin.