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RESEARCH PAPER

The mouse homeobox gene Not is required for caudal notochord development and affected by the truncate mutation

¹ Institute for Molecular Biology OE5250, Medizinische Hochschule Hannover, D-30625 Hannover, Germany; ² Samuel Lunenfeld Research Institute, Mount Sinai Hospital, Toronto M5G 1X5, Ontario, Canada; ³ Max-Planck-Institute for Molecular Genetics, D-14195 Berlin, Germany; ⁴ Max-Planck-Institute for Biophysical Chemistry, D-37077 Göttingen, Germany

The floating head (flh) gene in zebrafish encodes a homeodomain protein, which is essential for notochord formation along the entire body axis. flh orthologs, termed Not genes, have been isolated from chick and Xenopus, but no mammalian ortholog has yet been identified. Truncate (tc) is an autosomal recessive mutation in mouse that specifically disrupts the development of the caudal notochord. Here, we demonstrate that truncate arose by a mutation in the mouse Not gene. The truncate allele (Not^tc) contains a point mutation in the homeobox of Not that changes a conserved Phenylalanine residue in helix 1 to a Cysteine (F20C), and significantly destabilizes the homeodomain. Reversion of F20C in one allele of homozygous tc embryonic stem (ES) cells is sufficient to restore normal notochord formation in completely ES cell-derived embryos. We have generated a targeted mutation of Not by replacing most of the Not coding sequence, including the homeobox with the eGFP gene. The phenotype of Not^eGFP/eGFP, Not^eGFP/tc, and Not^tc/tc embryos is very similar but slightly more severe in Not^eGFP/eGFP than in Not^tc/tc embryos. This confirms allelism of truncate and Not, and indicates that tc is not a complete null allele. Not expression is abolished in Foxa2 and T mutant embryos, suggesting that Not acts downstream of both genes during notochord development. This is in contrast to zebrafish embryos, in which flh interacts with ntl (zebrafish T) in a regulatory loop and is essential for development of the entire notochord, and suggests that different genetic control circuits act in different vertebrate species during notochord formation.

[Keywords: Notochord development; Not gene; homeodomain protein]

Received March 22, 2004; revised version accepted May 13, 2004.

⁵ These authors contributed equally to this work.

⁶ Present address: Nura Inc., 1124 Columbia St., Seattle, WA 98104, USA

⁷ Present address: Institute of Psychiatry, Box P082, De Crespigny Park, London SE5 8AF, UK

⁸ Present address: Centro Regional de Estudios Genomicos, Laboratorio de Biofisica Molecular y Proteomica, Av. Calchaqui Km 23,5, 1888 Florencio Varela, Argentina

⁹ Present address: Centro Regional de Estudios Genomicos (CREG), Universidad Nacional de La Plata, Calle 7 No 776, 1900 La Plata, Argentina.

¹⁰ Corresponding author.
E-MAIL Gossler.Achim{at}mh-hannover.de; FAX 49-511-532-4283.

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