Genetic control of murine limb morphogenesis: relationships with human syndromes and evolutionary relevance

Over the past ten years, the discovery and functional characterisation of murine Hox genes has led to a better understanding of some of the molecular mechanisms underlying limb development. It has also shed some light on the potential genetic events which have accompanied the fin-to-limb transition, an evolutionary step of critical importance which opened the way to the evolution of higher vertebrates. This convergence between developmental biology and the sciences of evolution is one of the synergistic interface that has been established recently thanks to the use of genetic engineering and transgenic animals. The increasing number of human genetic syndromes which are derived from mutations in developmental control genes remind us that many human genetic diseases are nothing else but alterations in our developmental programme. Here, we illustrate these various issues by discussing the function of Hox genes during limb development.

Genetic analysis of a Hoxd-12 regulatory element reveals global versus local modes of controls in the HoxD complex

Vertebrate Hoxd genes are essential determinants of limb morphogenesis. In order to understand the genetic control of their complex expression patterns, we have used a combined approach involving interspecies sequence alignments in parallel with transgenic analyses, followed by in vivo mutagenesis. Here, we report on the identification of a regulatory element that is located in the vicinity of the Hoxd-12 gene. While this element is well conserved in tetrapods, little sequence similarity was scored when compared to the cognate fish DNA. The regulatory potential of this region XI (RXI) was first assayed in the context of a Hoxd-12/lacZ reporter transgene and shown to direct reporter gene expression in posterior limb buds. A deletion of this region was generated by targeted mutagenesis in ES cells and introduced into mice. Analyses of animals homozygous for the HoxDRXI mutant allele revealed the function of this region in controlling Hoxd-12 expression in the presumptive posterior zeugopod where it genetically interacts with Hoxa-11. Downregulation of Hoxd-12 expression was also detected in the trunk suggesting that RXI may mediate a rather general function in the activation of Hoxd-12. These results support a model whereby global as well as local regulatory influences are necessary to build up the complex expression patterns of Hoxd genes during limb development.

The evolution of 'bricolage'

The past ten years of developmental genetics have revealed that most of our genes are shared by other species throughout the animal kingdom. Consequently, animal diversity might largely rely on the differential use of the same components, either at the individual level through divergent functional recruitment, or at a more integrated level, through their participation in various genetic networks. Here, we argue that this inevitably leads to an increase in the interdependency between functions that, in turn, influences the degree to which novel variations can be tolerated. In this 'transitionist' scheme, evolution is neither inherently gradualist nor punctuated but, instead, progresses from one extreme to the other, together with the increased complexity of organisms.

Control of colinearity in AbdB genes of the mouse HoxD complex

During development, vertebrate Hox genes are activated in a temporal and spatial sequence colinear with the position of the genes within their clusters. To investigate the mechanistic basis of this phenomenon, we used the ES cell technology and the loxP/Cre system to engineer a conditional fusion of the 5' exon of Hoxd-13 with the 3' exon of Hoxd-12. This hybrid transcription unit was regulated like Hoxd-11, with expression limits in the trunk, limbs, intestinal, and urogenital systems more anterior than those expected for either Hoxd-13 or Hoxd-12. An in vivo interspecies replacement by the fish homologous DNA fragment showed that anteriorization was not due to a distance effect, thus suggesting the presence of a regulatory element between Hoxd-13 and Hoxd-12 that may contribute to the establishment, early on, of a repressive state over these two genes.

Regulation of number and size of digits by posterior Hox genes: a dose-dependent mechanism with potential evolutionary implications

The proper development of digits, in tetrapods, requires the activity of several genes of the HoxA and HoxD homeobox gene complexes. By using a variety of loss-of-function alleles involving the five Hox genes that have been described to affect digit patterning, we report here that the group 11, 12, and 13 genes control both the size and number of murine digits in a dose-dependent fashion, rather than through a Hox code involving differential qualitative functions. A similar dose-response is observed in the morphogenesis of the penian bone, the baculum, which further suggests that digits and external genitalia share this genetic control mechanism. A progressive reduction in the dose of Hox gene products led first to ectrodactyly, then to olygodactyly and adactyly. Interestingly, this transition between the pentadactyl to the adactyl formula went through a step of polydactyly. We propose that in the distal appendage of polydactylous short-digited ancestral tetrapods, such as Acanthostega, the HoxA complex was predominantly active. Subsequent recruitment of the HoxD complex contributed to both reductions in digit number and increase in digit length. Thus, transition through a polydactylous limb before reaching and stabilizing the pentadactyl pattern may have relied, at least in part, on asynchronous and independent changes in the regulation of HoxA and HoxD gene complexes.

Interspecies exchange of a Hoxd enhancer in vivo induces premature transcription and anterior shift of the sacrum

The precise activation, in space and time, of vertebrate Hox genes is an essential requirement for normal morphogenesis. In order to assess for the functional potential of evolutionary conserved Hox regulatory sequences, a phylogenetically conserved bipartite regulatory element necessary for proper spatial and temporal activation of the Hoxd-11 gene was replaced by its fish counterpart in the HoxD complex of mice, using an ES cell-based targeted exchange. Fetuses carrying this replacement activated Hoxd-11 transcription prematurely, which led to a rostral shift of its expression boundary and a consequent anterior transposition of the sacrum. These results demonstrate the high phylogenetic conservation of regulatory mechanisms acting over vertebrate Hox complexes and suggest that minor time difference (heterochronies) in Hox gene activation may have contributed to important morphological variations in the course of evolution.

Ulnaless (Ul), a regulatory mutation inducing both loss-of-function and gain-of-function of posterior Hoxd genes

Ulnaless (Ul), an X-ray-induced dominant mutation in mice, severely disrupts development of forearms and forelegs. The mutation maps on chromosome 2, tightly linked to the HoxD complex, a cluster of regulatory genes required for proper morphogenesis. In particular, 5′-located (posterior) Hoxd genes are involved in limb development and combined mutations within these genes result in severe alterations in appendicular skeleton. We have used several engineered alleles of the HoxD complex to genetically assess the potential linkage between these two loci. We present evidence indicating that Ulnaless is allelic to Hoxd genes. Important modifications in the expression patterns of the posterior Hoxd-12 and Hoxd-13 genes at the Ul locus suggest that Ul is a regulatory mutation that interferes with a control mechanism shared by multiple genes to coordinate Hoxd function during limb morphogenesis.

Deletion of a HoxD enhancer induces transcriptional heterochrony leading to transposition of the sacrum

A phylogenetically conserved transcriptional enhancer necessary for the activation of Hoxd-11 was deleted from the HoxD complex of mice by targeted mutagenesis. While genetic and expression analyses demonstrated the role of this regulatory element in the activation of Hoxd-11 during early somitogenesis, the function of this gene in developing limbs and the urogenital system was not affected, suggesting that Hox transcriptional controls are different in different axial structures. In the trunk of mutant embryos, transcriptional activation of Hoxd-11 and Hoxd-10 was severely delayed, but subsequently resumed with appropriate spatial distributions. The resulting caudal transposition of the sacrum indicates that proper vertebral specification requires a precise temporal control of Hox gene expression, in addition to spatial regulation. A slight time delay in expression (transcriptional heterochrony) cannot be compensated for at a later developmental stage, eventually leading to morphological alterations.

Dorso-ventral limb polarity and origin of the ridge: on the fringe of independence?

Molecular and developmental studies of limb pattern formation have recently gained widespread attention. The fact that vertebrate limbs are amenable to both genetic and embryological manipulations has established this model system as a valuable paradigm for studying vertebrate development. Limb buds are polarised along all three major axes and the establishment of the dorso-ventral (DV) polarity is dependent upon cues localised in the trunk, where a DV ectodermal interface is produced by confrontation of dorsal and ventral identities. By analogy to Drosophila imaginal disc development, this interface has been proposed to determine and position an ectodermal organising centre, the Apical Ectodermal Ridge (AER), controlling limb bud outgrowth. Recent fate mapping studies and studies of genes regulating DV limb polarity, AER formation and differentiation suggest, however, that DV patterning and AER induction, though coordinately regulated during limb bud outgrowth, may early on be more dissociated than expected.

Male accessory sex organ morphogenesis is altered by loss of function of Hoxd-13

The role of the Hox gene Hoxd-13 in postnatal morphogenesis of the male accessory sex organs was examined by correlating the distribution and temporal regulation of expression in the accessory sex organs of postnatal mice with morphologic abnormalities of Hoxd-13-deficient transgenic mice. Previous studies of Hoxd-13 expression in the perinatal period have shown a broad domain of expression in the lower genitourinary tract, with expression in both mesenchyme and epithelium; focal expression was also noted in the epithelium of the nascent ducts of the developing prostate. Quantitative RT-PCR studies of Hoxd-13 expression in the 5 day mouse confirm widespread expression in the accessory sex organs developing from both the Wolffian duct and the urogenital sinus. Expression is down-regulated with age, and a detailed time course of expression in the developing prostate shows that the level of Hoxd-13 expression correlates with morphogenetic activity in the development of the prostate ductal system. Transgenic Hoxd-13-deficient mice display multiple abnormalities in the male accessory sex organs. The most severe abnormalities were observed in organs exhibiting ductal branching during postnatal development and included diminished mesenchymal folding in the seminal vesicles, decreased size and diminished ductal branching in the ventral and dorsal prostate, and agenesis of the bulbourethral gland. We conclude that Hoxd-13 expression in the postnatal period correlates with a period of intense morphogenetic activity in accessory sex organ development and that the function of Hoxd-13 is evidenced by morphologic abnormalities in accessory sex organs of the Hoxd-13-deficient mutant.

[Control of limb morphogenesis by the Hox genes]

Vertebrate limbs are an amazing example of successful adaptation to various environmental conditions. In higher vertebrates, forelimbs help to fly, swim, walk, dig or grasp, yet their basic structure (the sequence and spatial arrangement of bony elements) is always the same. This implies the existence of a unique developmental strategy for building a limb (a limb plan) that imposes early on a basic scheme, on the top of which subsequent species-specific customizations will occur. The description of such a universal limb plan, hence the idea that the genetic and developmental processes that generate this plan are very ancient, has been controversial for about a century. It is worth asking whether recent discoveries of important genes involved in these processes can bring novel arguments to the debate.

Transgenic analysis of a potential Hoxd-11 limb regulatory element present in tetrapods and fish

Genes of the HoxD complex related to the Drosophila Abd-B gene are involved in the morphogenesis of vertebrate paired appendages. Hoxd-11, for instance, is necessary in combination with other Hox genes for the proper development of different parts of the tetrapod limbs. Sequence comparisons between the mouse, chicken, and zebrafish Hoxd-11 loci have revealed the conservation of several blocks of DNA sequence which may be of importance for the regulation of Hoxd-11 expression. We have used transgenic mice to show that one of these conserved elements specifically drives expression in a proximal-posterior part of developing forelimbs. Production of mice transgenic for a full fish Hoxd-11 construct as well as for mouse-fish Hoxd-11 chimeric constructs shows that the fish counterpart of this sequence is able to elicit expression in mouse forelimbs as well, though in a slightly different domain. However, this fish element requires the presence of the mouse promoter and does not work in its own context. These results are discussed in light of both the control of Hoxd gene expression during limb development and the use of a comparative interspecies approach to understand the regulation of genes involved in vertebrate development.

Function of the Evx-2 gene in the morphogenesis of vertebrate limbs

Vertebrate gene members of the HoxD complex are essential for proper development of the appendicular skeletons. Inactivation of these genes induces severe alterations in the size and number of bony elements. Evx-2, a gene related to the Drosophila even-skipped (eve) gene, is located close to Hoxd-13 and is expressed in limbs like the neighbouring Hoxd genes. To investigate whether this tight linkage reflects a functional similarity, we produced a null allele of Evx-2. Furthermore, and because Hoxd-13 function is prevalent over that of nearby Hoxd genes, we generated two different double mutant loci wherein both Evx-2 and Hoxd-13 were inactivated in cis. The analysis of these various genetic configurations revealed the important function of Evx-2 during the development of the autopod as well as its genetic interaction with Hoxd-13. These results show that, in limbs, Evx-2 functions like a Hoxd gene. A potential evolutionary scenario is discussed, in which Evx-2 was recruited by the HoxD complex in conjunction with the emergence of digits in an ancestral tetrapod.

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.

In vivo targeted mutagenesis of a regulatory element required for positioning the Hoxd-11 and Hoxd-10 expression boundaries

Vertebrate Hox genes are required for the proper organization of structures along the rostrocaudal axis. Hoxd-11 is expressed in the posterior part of the embryo, up to the level of prevertebra 27, and its expression boundary is reproduced by a Hoxd-11/lacZ transgene. Expression of this transgene anterior to prevertebra 27 is prevented by the silencing activity of a cis-acting element, region IX. Using transgenic mice, we show that Hoxd-11 repression by region IX is necessary to position the sacrum properly. This silencing activity depends on phylogenetically conserved sequences able to bind in vitro retinoic acid receptors and COUP-TFs. ES cells were used to generate mice carrying a subtle mutation that abolishes binding of nuclear receptors to region IX. Mutant mice display an anterior shift of their lumbosacral transition inherited as a codominant trait. In mutant embryos, expression of both Hoxd-11 and Hoxd-10 mRNAs in the prevertebral column is anteriorized. These results illustrate the sharing, in cis, of a single regulatory element in order to establish the expression boundaries of two neighboring Hoxd genes.

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.

Gene transpositions in the HoxD complex reveal a hierarchy of regulatory controls

Vertebrate Hox genes are activated following a temporal sequence that reflects their linear order in the clusters. We introduced two Hoxd transcription units, labeled with lacZ, to an ectopic 5' position in the HoxD complex. Early expression of the relocated genes was delayed and resembled that of the neighboring Hoxd-13. At later stages, locus-dependent expression in distal limbs and the genital eminence was observed, indicating that common regulatory mechanisms are used for several genes. These experiments also illustrated that neighboring genes can share the same cis-acting sequence and that moving genes around in the complex induces novel regulatory interferences. These results suggest that high order regulation controls the activation of Hox genes and highlight three important constraints responsible for the conservation of Hox gene clustering.

Functional equivalence and rescue among group 11 Hox gene products in vertebral patterning

Hoxa-11 and Hoxd-11 are paralogous genes required for proper development of the vertebral column, the limbs, and the urogenital system. To further explore the functional relationship between these genes, as well as the potential rescue of one function by the other, we have introduced a Hoxd-11-expressing transgene into Hoxa-11/Hoxd-11 mutant genetic backgrounds. A range of phenotypes was observed, with transgenic mice displaying as few as four lumbar vertebrae while double mutant mice had as many as eight. When transgenic, double homozygote mutant animals showed six lumbar vertebrae, instead of the eight usually observed. The phenotypic rescue of these genotypes shows that the Hoxa-11 and Hoxd-11 products are functionally equivalent and that extra doses of Hoxd-11 can rescue Hoxa-11 loss of function.

Teleost HoxD and HoxA genes: comparison with tetrapods and functional evolution of the HOXD complex

In tetrapods, Hox genes are essential for the proper organization and development of axial structures. Experiments involving Hox gene inactivations have revealed their particularly important functions in the establishment of morphological transitions within metameric series such as the vertebral column. Teleost fish show a much simpler range of axial (trunk or appendicular) morphologies, which prompted us to investigate the nature of the Hox system in these lower vertebrates. Here, we show that fish have a family of Hox genes, very similar in both number and general organization, to that of tetrapods. Expression studies, carried out with HoxD and HoxA genes, showed that all vertebrates use the same general scheme, involving the colinear activation of gene expression in both space and time. Comparisons between tetrapods and fish allowed us to propose a model which accounts for the primary function of this gene family. In this model, a few ancestral Hox genes were involved in the determination of polarity in the digestive tract and were further recruited in more elaborate axial structures.

[Genetic control of limb development]

Vertebrate limbs are an amazing example of successful adaptation to various environmental conditions. In higher vertebrates, forelimbs help to fly, swim, walk, dig, grasp or play the Passacaille, yet their basic structure (the sequence and spatial arrangement of bony elements) is always the same. This implies the existence of a unique developmental strategy for building a limb (a limb plan) that imposes early on a basic scheme, on the top of which subsequent species-specific customizations will occur. The description of such a universal limb plan, hence the idea that the genetic and developmental processes that generate this plan are very ancient, has been controversial for about a century. It is worth asking whether recent discoveries of important genes involved in these processes can bring novel arguments to the debate.