The structural and functional organization of the murine HOX gene family resembles that of Drosophila homeotic genes.

This paper reports the cloning of the fourth major murine homeogene complex, HOX-5. The partial characterization of this gene cluster revealed the presence of two novel genes (Hox-5.2, Hox-5.3) located at the 5' extremity of this complex. In situ hybridization experiments showed that these two genes are transcribed in very posterior domains during embryonic and foetal development. We also show that Hox-1.6, the gene located at the 3' most position in the HOX-1 complex, has a very anterior expression boundary during early development. These results clearly support the recently proposed hypothesis that the expression of murine Antp-like homeobox-containing genes along the antero-posterior developing body axis follows a positional hierarchy which reflects their respective physical positions within the HOX clusters, similar to that which is found for the Drosophila homeotic genes. Such a structural and functional organization is likely conserved in most vertebrates. Moreover, on the basis of sequence comparisons, we propose that the ordering of homeobox-containing genes within clusters has been conserved between Drosophila and the house mouse. Thus, very different body plans might be achieved, both in insects and vertebrates, by evolutionarily conserved gene networks possibly displaying similar regulatory interactions.     

Hox-3.6: isolation and characterization of a new murine homeobox gene located in the 5' region of the Hox-3 cluster.

Most members of the murine Hox gene system can be grouped into two subclasses based on their structural similarity to either one of the Drosophila homeotic genes Antennapedia (Antp) or Abdominal B (AbdB). All the AbdB-like genes reported thus far are located in the 5' region of their respective cluster. We describe here the isolation, structural characterization and spatio-temporal expression pattern of a new AbdB-like homeobox gene designated Hox-3.6 that is located in the 5' region of the Hox-3 cluster. Hox-3.6 has an extreme posterior expression domain in embryos of 12.5 days of gestation, a feature that has thus far only been observed for the 5' most genes of the Hox-4 cluster. Like the other members of the AbdB subfamily, Hox-3.6 exhibits spatially restricted expression in the hindlimb bud, but the expression domain is antero-proximal in contrast to the postero-distal domain reported for its cognate gene Hox-4.5. Structural analysis of the 5' region revealed the presence of a 35 bp sequence which shares homology and relative 5' position with an upstream sequence present in its two nearest downstream neighbors, Hox-3.2 and -3.1.     

Murine genes related to the Drosophila AbdB homeotic genes are sequentially expressed during development of the posterior part of the body.

The cloning, characterization and developmental expression patterns of two novel murine Hox genes, Hox-4.6 and Hox-4.7, are reported. Structural data allow us to classify the four Hox-4 genes located in the most upstream (5') position in the HOX-4 complex as members of a large family of homeogenes related to the Drosophila homeotic gene Abdominal B (AbdB). It therefore appears that these vertebrate genes are derived from a selective amplification of an ancestral gene which gave rise, during evolution, to the most posterior of the insect homeotic genes so far described. In agreement with the structural colinearity, these genes have very posteriorly restricted expression profiles. In addition, their developmental expression is temporally regulated according to a cranio-caudal sequence which parallels the physical ordering of these genes along the chromosome. We discuss the phylogenetic alternative in the evolution of genetic complexity by amplifying either genes or regulatory sequences, as exemplified by this system in the mouse and Drosophila. Furthermore, the possible role of 'temporal colinearity' in the ontogeny of all coelomic (metamerized) metazoans showing a temporal anteroposterior morphogenetic progression is addressed.     

Mouse Hox-2.2 specifies thoracic segmental identity in Drosophila embryos and larvae

The mouse genome has a number of homeobox genes that are structurally similar to the Drosophila Antenapedia (Antp) gene. We find that one of the mouse Antp-like genes, Hox-2.2, when expressed in developing Drosophila cells under control of a heat shock promoter, can induce homeotic transformations that are nearly identical to those caused by ectopic expression of Antp. In larvae, the Hox-2.2-induced transformations include thoracic denticle belts in place of head structures; in adults, the Hox-2.2 transformations include thoracic legs in place of antennae. The phenotypic effects of Hox-2.2 do not depend on the endogenous Antp gene, whose spatial limits of expression are unaffected by Hox-2.2 expression. Thus, in the Drosophila embryo, Hox-2.2 can substitute for some of the segmental identity functions of Antp, presumably by regulating the same set of downstream genes.     

Dominant mutation of the murine Hox-2.2 gene results in developmental abnormalities.

Genes carrying the homeobox were originally identified in Drosophila, in which they are now known to play key roles in establishing segmentation patterns and in determining segment identities. A number of genes with striking homology to the Drosophila homeobox genes have now been found in the mouse genome, and mutational analysis is beginning to shed light on their function in mammalian development. To understand better the developmental significance of the murine Hox-2.2 gene, we have generated gain of function mutants by using the chicken beta-actin promoter to drive ubiquitous expression in transgenic mice. The resulting Hox-2.2 misexpression produces early postnatal lethality as well as craniofacial and axial skeletal perturbations that include open eyes at birth, cleft palate, micrognathia, microtia, skull bone deficiencies, and structural and positional alterations in the vertebral column. We repeatedly observe complete or partial absence of the supraoccipital bone and malformations of the exoccipital and the basioccipital bones. These results suggests a role for the Hox-2.2 gene in specifying positional identity along the anterior-posterior axis.     

Sequence analysis of the murine Hox-2.2, -2.3, and -2.4 homeo boxes: evolutionary and structural comparisons

We have determined the nucleotide sequences and deduced the amino acid sequences of three tandemly arranged murine boxes of the Hox-2 homeo box gene complex on mouse chromosome 11 (Hox-2.2, -2.3, and -2.4). The type and position of differences with other sequenced homeo boxes were analyzed. Hox-2.2 is nearly identical with its cognate human homeo box Hu-2. Hox-2.3 shares 59 of 61 amino acids with the Antennapedia homeo domain of Drosophila and the MM-3 homeo domain of Xenopus and shows 60 of 61 amino acid identity with human HuC1. Hox-2.3, MM-3, and HuC1 also share a stretch of six glutamic acid residues followed by a stop codon 15-20 amino acids 3' of the homeo domain. Hox-2.4 is relatively divergent from most of the other homeo boxes sequenced to date; however, it matches the Hox-3.1 murine homeo domain at 60 of 61 positions. Sequence comparisons with other murine homeo domains, together with previous studies of their genomic organization and chromosomal location, provide support for the hypothesis of a large-scale duplication resulting in the two major murine homeo box gene complexes Hox-1 and Hox-2.