A mutational analysis of the 5′ HoxD genes: dissection of genetic interactions during limb development in the mouse

Homeobox genes and disease

To date, not many disorders have been associated with homeobox genes, especially with those belonging to the HOX family. This is particularly surprising, considering the body of evidence accumulated for a role of these genes in the control of mammalian development. Recently, this situation has changed and some congenital or somatic defects have been demonstrated to involve mutations in homeobox genes of the HOX, EMX, PAX, and MSX families, as well as in other novel genes containing either a paired- or bicoid-type homeobox.

Retinoid signaling is required for the establishment of a ZPA and for the expression of Hoxb-8, a mediator of ZPA formation

We show that retinoid receptor antagonists applied to the presumptive wing region block the formation of a zone of polarizing activity (ZPA). This suggests a direct relationship between retinoid signaling and the establishment of the ZPA. We provide evidence that the Hox gene, Hoxb-8, is a direct target of retinoid signaling since exogenously applied RA rapidly induces this gene in the absence of protein synthesis and, moreover, retinoid receptor antagonists down-regulate Hoxb-8 expression. In addition, we find that, in the lateral plate mesoderm, the domains of Hoxb-8 expression and of polarizing activity are coextensive. Taken together, these findings support the hypothesis that retinoids are required for the establishment of a ZPA, and that retinoids act, at least in part, through Hoxb-8, a gene associated with ZPA formation (Charite et al., 1994).

Evidence for genetic control of Sonic hedgehog by Gli3 in mouse limb development

Sonic hedgehog (Shh) expression in the developing limb is associated with the zone of polarising activity (ZPA), and both are restricted to the posterior part of the limb bud. We show that the expression patterns of Shh and Gli3, a member of the Gli-family believed to function in transcriptional control, appear to be mutually exclusive in limb buds of mouse embryos. In the polydactyly mouse mutant extra toes (Xt), possessing a null mutation of Gli3, Shh is additionally expressed in the anterior region of the limb bud. The transcript of Ptc, the putative receptor for Shh protein, can be detected anteriorly as well. Other genes known to be involved in limb outgrowth and patterning, like Fibroblast growth factor (Fgf), Bone morphogenetic protein (Bmp), and Hoxd are misexpressed in relation to the ectopic Shh expression domain in Xt limb buds. This data suggest that Gli3 is a regulator of Shh expression in mouse limb development.

Defining the skeletal elements

A recent study of mice carrying different combinations of mutations in the genes for two bone morphogenetic factors (BMPs), BMP5 and GDF5, indicates that BMPs have specific and synergistic functions in the regulation of skeleton development.

Analysis of Hoxd-13 and Hoxd-11 misexpression in chick limb buds reveals that Hox genes affect both bone condensation and growth

Hox genes are important regulators of limb pattern in vertebrate development. Misexpression of Hox genes in chicks using retroviral vectors provides an opportunity to analyze gain-of-function phenotypes and to assess their modes of action. Here we report the misexpression phenotype for Hoxd-13 and compare it to the misexpression phenotype of Hoxd-11. Hoxd-13 misexpression in the hindlimb results in a shortening of the long bones, including the femur, the tibia, the fibula and the tarsometatarsals. Mutations in an alanine repeat region in the N-terminus of Hoxd-13 have recently been implicated in human synpolydactyly (Muragaki, Y., Mundlos, S., Upton, J. and Olsen, B. R. (1996) Science 272, 548–551). N-terminal truncations of Hoxd-13 which lack this repeat were constructed and were found to produce a similar, although slightly milder, misexpression phenotype than the full-length Hoxd-13. The stage of bone development regulated by Hox genes has not previously been examined. The changes in bone lengths caused by Hoxd-13 misexpression are late phenotypes that first become apparent during the growth phase of the bones. Analysis of tritiated thymidine uptake by the tibia and fibula demonstrates that Hox genes can pattern the limb skeleton by regulating the rates of cell division in the proliferative zone of growing cartilage. Hoxd-11, in contrast to Hoxd-13, acts both at the initial cartilage condensation phase in the foot and during the later growth phase in the lower leg. Ectopic Hoxd-13 appears to act in a dominant negative manner in regions where it is not normally expressed. We propose a model in which all Hox genes are growth promoters, regulating the expression of the same target genes, with some Hox genes being more effective promoters of growth than other Hox genes. According to this model, the overall rate of growth in a given region is the result of the combined action of all of the Hox genes expressed in that region competing for the same target genes.

Specific and redundant functions of Gli2 and Gli3 zinc finger genes in skeletal patterning and development

The correct patterning of vertebrate skeletal elements is controlled by inductive interactions. Two vertebrate hedgehog proteins, Sonic hedgehog and Indian hedgehog, have been implicated in skeletal development. During somite differentiation and limb development, Sonic hedgehog functions as an inductive signal from the notochord, floor plate and zone of polarizing activity. Later in skeletogenesis, Indian hedgehog functions as a regulator of chondrogenesis during endochondral ossification. The vertebrate Gli zinc finger proteins are putative transcription factors that respond to Hedgehog signaling. In Drosophila, the Gli homolog cubitus interruptus is required for the activation of hedgehog targets and also functions as a repressor of hedgehog expression. We show here that Gli2 mutant mice exhibit severe skeletal abnormalities including cleft palate, tooth defects, absence of vertebral body and intervertebral discs, and shortened limbs and sternum. Interestingly, Gli2 and Gli3 (C.-c. Hui and A. L. Joyner (1993). Nature Genet. 3, 241–246) mutant mice exhibit different subsets of skeletal defects indicating that they implement specific functions in the development of the neural crest, somite and lateral plate mesoderm derivatives. Although Gli2 and Gli3 are not functionally equivalent, double mutant analysis indicates that, in addition to their specific roles, they also serve redundant functions during skeletal development. The role of Gli2 and Gli3 in Hedgehog signaling during skeletal development is discussed.

Targeted disruption of the Hoxb-2 locus in mice interferes with expression of Hoxb-1 and Hoxb-4

Mice with a disruption in the hoxb-2 locus were generated by gene targeting. 75% of the hoxb-2 mutant homozygotes died within 24 hours of birth. While a majority of these mice had severe sternal defects that compromised their ability to breathe, some had relatively normal sternum morphology, suggesting that one or more additional factor(s) contributed to neonatal lethality. At 3–3.5 weeks of age, half of the remaining hoxb-2 homozygotes became weak and subsequently died. All of the mutants that survived to 3 weeks of age showed marked facial paralysis similar to, but more severe than, that reported for hoxb-1 mutant homozygotes (Goddard, J. M., Rossel, M., Manley, N. R. and Capecchi, M. R. (1996) Development 122, 3217–3228). As for the hoxb-1 mutations, the facial paralysis observed in mice homozygous for the hoxb-2 mutation results from a failure to form the somatic motor component of the VIIth (facial) nerve which controls the muscles of facial expression. Features of this phenotype closely resemble the clinical signs associated with Bell's Palsy and Moebius Syndrome in humans. The sternal defects seen in hoxb-2 mutant mice are similar to those previously reported for hoxb-4 mutant mice (Ramirez-Solis, R., Zheng, H., Whiting, J., Krumlauf, R. and Bradley. A. (1993) Cell 73, 279–294). The above results suggest that the hoxb-2 mutant phenotype may result in part from effects of the hoxb-2 mutation on the expression of both hoxb-1 and hoxb-4. Consistent with this proposal, we found that the hoxb-2 mutation disrupts the expression of hoxb-1 in cis. In addition, the hoxb-2 mutation changes the expression of hoxb-4 and the hoxb-4 mutation, in turn, alters the pattern of hoxb-2 expression. Hoxb-2 and hoxb-4 appear to function together to mediate proper closure of the ventral thoracic body wall. Failure in this closure results in severe defects of the sternum.

Paralogous Hox genes: function and regulation

The Hox homeobox gene family plays a pivotal role in regulating patterning and axial morphogenesis in vertebrates. Molecular characterization of the four Hox clusters has shown that they are evolutionarily related with respect to sequence, organization, and expression, suggesting they arose by duplication and divergence. Transgenic analysis has clearly demonstrated the functional roles of individual genes in a broad range of embryonic tissues, and in compound mutants has addressed the issues of cooperativity and redundancy. There is an emerging picture of the cis-regulatory elements underlying Hox expression, and for the 3' members of the clusters there is a considerable degree of conservation between paralogous genes with respect to their functional roles and regulatory control.

The chick limbless mutation causes abnormalities in limb bud dorsal-ventral patterning: implications for the mechanism of apical ridge formation

In chick embryos homozygous for the limbless mutation, limb bud outgrowth is initiated, but a morphologically distinct apical ridge does not develop and limbs do not form. Here we report the results of an analysis of gene expression in limbless mutant limb buds. Fgf4, Fgf8, Bmp2 and Msx2, genes that are expressed in the apical ridge of normal limb buds, are not expressed in the mutant limb bud ectoderm, providing molecular support for the hypothesis that limb development fails in the limbless embryo because of the inability of the ectoderm to form a functional ridge. Moreover, Fgf8 expression is not detected in the ectoderm of the prospective limb territory or the early limb bud of limbless embryos. Since the early stages of limb bud outgrowth occur normally in the mutant embryos, this indicates that FGF8 is not required to promote initial limb bud outgrowth. In the absence of FGF8, Shh is also not expressed in the mutant limb buds, although its expression can be induced by application of FGF8-soaked beads. These observations support the hypothesis that Fgf8 is required for the induction of Shh expression during normal limb development. Bmp2 expression was also not detected in mutant limb mesoderm, consistent with the hypothesis that SHH induces its expression. In contrast, SHH is not required for the induction of Hoxd11 or Hoxd13 expression, since expression of both these genes was detected in the mutant limb buds. Thus, some aspects of mesoderm A-P patterning can occur in the absence of SHH and factors normally expressed in the apical ridge. Intriguingly, mutant limbs rescued by local application of FGF displayed a dorsalized feather pattern. Furthermore, the expression of Wnt7a, Lmx1 and En1, genes involved in limb D-V patterning, was found to be abnormal in mutant limb buds. These data suggest that D-V patterning and apical ridge formation are linked, since they show that the limbless mutation affects both processes. We present a model that explains the potential link between D-V positional information and apical ridge formation, and discuss the possible function of the limbless gene in terms of this model.

Synpolydactyly in mice with a targeted deficiency in the HoxD complex

The morphogenesis of mammalian digits requires the function of several genes of the HoxD complex during development of limb buds. Using embryonic stem (ES) cells and a site-specific recombination system (loxP/Cre), we have induced a deficiency that eliminates the products of the Hoxd-13, Hoxd-12 and Hoxd-11 genes simultaneously. A Hoxd-11/lacz reporter gene replaced the deleted region in order to monitor the effect of this triple inactivation at the cellular level. Mice homozygous for this deficiency showed small digit primordia, a disorganized cartilage pattern and impaired skeletal mass. These alterations are similar to the defects seen in a human synpolydactyly, suggesting that this syndrome, which is associated with a subtle mutation in HOXD13 (ref. 8), may involve the loss of function of several Hoxd genes. These results indicate the existence of a functional hierarchy among these genes and provide us with an animal model to study human digit malformations.

Zebrafish Hoxa and Evx-2 genes: cloning, developmental expression and implications for the functional evolution of posterior Hox genes

Vertebrate Hox genes are required for the establishment of regional identities along body axes. This gene family is strongly conserved among vertebrates, even in bony fish which display less complex ranges of axial morphologies. We have analysed the structural organization and expression of Abd-B related zebrafish HoxA cluster genes (Hoxa-9, Hoxa-10, Hoxa-11 and Hoxa-13) as well as of Evx-2, a gene closely linked to the HoxD complex. We show that the genomic organization of Hoxa genes in fish resembles that of tetrapods albeit intergenic distances are shorter. During development of the fish trunk, Hoxa genes are coordinately expressed, whereas in pectoral fins, they display transcript domains similar to those observed in developing tetrapod limbs. Likewise, the Evx-2 gene seems to respond to both Hox- and Evx-types of regulation. During fin development, this latter gene is expressed as the neighbouring Hox genes, in contrast to its expression in the central nervous system which does not comply with colinearity and extends up to anterior parts of the brain. These results are discussed in the context of the functional evolution of Hoxa versus Hoxd genes and their different roles in building up paired appendages.

Mice with targeted disruption of Hoxb-1 fail to form the motor nucleus of the VIIth nerve

Mice were generated with targeted disruptions in the hoxb-1 gene. Two separate mutations were created: the first disrupts only the homeodomain and the second inactivates the first exon as well as the homeodomain. The phenotypes associated with these two mutant alleles are indistinguishable in surviving adult mice. The predominant defect in these mutant mice is a failure to form the somatic motor component of the VIIth (facial) nerve, possibly through a failure to specify these neurons. The phenotype of hoxb-1 mutant homozygotes closely resembles features of the clinical profile associated with humans suffering from Bell's Palsy or Moebius Syndrome. These animals should therefore provide a useful animal model for these human diseases.

Hoxa-13 and Hoxd-13 play a crucial role in the patterning of the limb autopod

Members of the Abdominal-B-related Hox gene subfamily (belonging to homology groups 9 to 13) are coordinately expressed during limb bud development. Only two genes from homology group 13 (Hoxa-13 and Hoxd-13) are specifically expressed in the developing distal region (the autopod), which displays the most complex and evolutionarily flexible pattern among limb ‘segments’. We report here that targeted disruption of the Hoxa-13 gene leads to a specific forelimb and hindlimb autopodal phenotype, distinct from that of the Hoxd-13 paralogous gene inactivation. In both limbs, Hoxa-13 loss of function results in the lack of formation of the most anterior digit and to altered morphogenesis of some ‘preaxial’ carpal/tarsal elements. We have generated mice with all possible combinations of disrupted Hoxa-13 and/or Hoxd-13 alleles, which allowed us to investigate the degree of functional specificity versus redundancy of the corresponding gene products in the developing limb autopod. The phenotype of any double mutant was much more severe than the sum of the phenotypes seen in the corresponding single mutants, indicating that these genes act in a partially redundant manner. Our major findings were: (1) an abnormal autopodal phenotype in Hoxa-13+/−/Hoxd-13+/− double heterozygous mutants, which mostly consists of subsets of the alterations seen in each individual homozygous mutant, and therefore appears to result from quantitative, rather than qualitative, homeoprotein deficiency; (2) partly distinct alterations in mutants harboring a single non-disrupted allele of Hoxa-13 or Hoxd-13, indicating that the remaining reduced protein amounts are not functionally equivalent; (3) a polydactyly in the forelimbs of Hoxa-13+/−/Hoxd-13−/−double mutants, consisting of seven symmetrically arranged, truncated and mostly non-segmented digits; (4) an almost complete lack of chondrified condensations in the autopods of double homozygous mutants, showing that the activity of group 13 Hox gene products is essential for autopodal patterning in tetrapod limbs.

Function of posterior HoxD genes in the morphogenesis of the anal sphincter

Vertebrate 5′-located HoxD genes are expressed in the most caudal part of the digestive tract and their potential functions during gut development have been assessed by gene disruptions. We have inserted reporter lacZ sequences within the Hoxd-12 gene and analysed the morphology of the gut in these mice as well as in Hoxd-13 mutant animals. When homozygous, both mutations induce an important disorganization of the anorectal region. In particular, severe alterations of the smooth muscle layers of the rectum led to defective morphogenesis of the internal anal sphincter. Similarly, Hoxd-12 and Hoxd-13 functionally overlap during digit development. The function of these genes in the morphogenesis of the digestive system as well as their functional evolution are discussed.