CHARACTERIZATION OF A GENE TRAP INSERTION INTO A NOVEL GENE, CORDON-BLEU, EXPRESSED IN AXIAL STRUCTURES OF THE GASTRULATING MOUSE EMBRYO

We have used a gene trap (GT) vector and embryonic stem (ES) cell chim eras to screen for insertions of the lacZ reporter gene into transcrip tion units that are spatially and temporally regulated during early mo use embryogenesis. GT vectors which can act as both a reporter and a m utagen have been previously used to isolate new genes that are essenti al for mouse development. In this paper we describe a GT insertion whi ch displays a very restricted pattern of expression in the gastrulatin g embryo. beta-Galactosidase activity was first detected at 7.5 days p ost-coitum (E7.5) in the node region of the embryo and extended to the midline structures at E8.0. At E9.5 expression was restricted to the floor plate, the notochord, the roof of the gut, and the liver anlage. Expression appeared in the somites at E10.0 and later became more wid espread. We used rapid amplification of cDNA ends-polymerase chain rea ction (RACE-PCR) to clone a partial 360 basepair (bp) cDNA representin g an endogenous sequence and containing an open reading frame (ORF) fu sed in frame to the lacZ reporter gene. The sequence showed no homolog y to any known protein or protein domain. An overlapping 1,200 bp frag ment from a wild-type cDNA library was cloned and it detected the same pattern of expression as the reporter gene in E7.5, E8.5, and E9.5 wi ld-type embryos. It hybridized to a 5.4 kb lacZ fusion transcript and to an endogenous transcript of 6.5 kb. The gene was mapped to chromoso me 11 and was named cordon-bleu (cobl). No phenotype was detected in m ice homozygous for the insertion. However, the insertion may not cause a complete disruption of the gene function. The pattern of expression of cobl is very similar to that of hepatic nuclear factor 3 beta (HNF 3 beta) and sonic hedgehog (Shh), both of which are involved in axial patterning. Therefore, the product of the cobl gene may also prove to be an important component of the genetic pathway regulating vertebrate axis formation. (C) 1995 Wiley-Liss, Inc.