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Elimination of a long-range cis-regulatory module causes complete loss of limb-specific Shh expression and truncation of the mouse limb.

Mutations in a conserved non-coding region in intron 5 of the Lmbr1 locus, which is 1 Mb away from the sonic hedgehog (Shh) coding sequence, are responsible for mouse and human preaxial polydactyly with mirror-image digit duplications. In the mouse mutants, ectopic Shh expression is observed in the anterior mesenchyme of limb buds. Furthermore, a transgenic reporter gene flanked with this conserved non-coding region shows normal polarized expression in mouse limb buds. This conserved sequence has therefore been proposed to act as a long-range, cis-acting regulator of limb-specific Shh expression. Previous phylogenetic studies have also shown that this sequence is highly conserved among tetrapods, and even in teleost fishes. Paired fins of teleost fishes and tetrapod limbs have evolved from common ancestral appendages, and polarized Shh expression is commonly observed in fins. In this study, we first show that this conserved sequence motif is also physically linked to the Shh coding sequence in a teleost fish, the medaka, by homology search of a newly available genomic sequence database. Next, we show that deletion of this conserved intronic sequence by targeted mutation in the mouse results in a complete loss of Shh expression in the limb bud and degeneration of skeletal elements distal to the stylopod/zygopod junction. This sequence contains a major limb-specific Shh enhancer that is necessary for distal limb development. These results suggest that the conserved intronic sequence evolved in a common ancestor of fishes and tetrapods to control fin and limb development.

Animals↗

Analysis of Msx1; Msx2 double mutants reveals multiple roles for Msx genes in limb development.

The homeobox-containing genes Msx1 and Msx2 are highly expressed in the limb field from the earliest stages of limb formation and, subsequently, in both the apical ectodermal ridge and underlying mesenchyme. However, mice homozygous for a null mutation in either Msx1 or Msx2 do not display abnormalities in limb development. By contrast, Msx1; Msx2 double mutants exhibit a severe limb phenotype. Our analysis indicates that these genes play a role in crucial processes during limb morphogenesis along all three axes. Double mutant limbs are shorter and lack anterior skeletal elements (radius/tibia, thumb/hallux). Gene expression analysis confirms that there is no formation of regions with anterior identity. This correlates with the absence of dorsoventral boundary specification in the anterior ectoderm, which precludes apical ectodermal ridge formation anteriorly. As a result, anterior mesenchyme is not maintained, leading to oligodactyly. Paradoxically, polydactyly is also frequent and appears to be associated with extended Fgf activity in the apical ectodermal ridge, which is maintained up to 14.5 dpc. This results in a major outgrowth of the mesenchyme anteriorly, which nevertheless maintains a posterior identity, and leads to formation of extra digits. These defects are interpreted in the context of an impairment of Bmp signalling.

Animals↗

Mouse intraflagellar transport proteins regulate both the activator and repressor functions of Gli transcription factors.

Intraflagellar transport (IFT) is an active event in which cargo is transported along microtubules by motor proteins such as kinesin and dynein. IFT proteins are required for the formation and maintenance of flagella and cilia. We have previously shown that mouse mutants for two IFT proteins, IFT88 and IFT172, as well as Kif3a, a subunit of mouse kinesin 2, exhibit ventral spinal cord patterning defects that appear to result from reduced hedgehog (Hh) signaling. Although genetic epistasis experiments place IFT proteins downstream of the Hh receptor and upstream of the Gli transcription factors, the mechanism by which IFT regulates Gli function is unknown. The developing limb provides an excellent system to study Hh signaling, in particular as it allows a biological and molecular readout of both Gli activator and repressor function. Here we report that homozygous mutants for flexo (Fxo), a hypomorphic allele of mouse IFT88 generated in our ENU mutagenesis screen, exhibit polydactyly in all four limbs. Molecular analysis indicates that expression domains of multiple posteriorly restricted genes are expanded anteriorly in the mutant limbs, similar to loss of Gli3 transcriptional repressor function. Sonic hedgehog (Shh) expression is normal, yet Ptch1 and Gli1, two known targets of Hh signaling, are greatly reduced, consistent with loss of Shh signaling. Expression of Gli3 and Hand2 in the mutant limb indicates that the limb prepattern is abnormal. In addition, we show that partial loss-of-function mutations in another mouse IFT gene, Ift52 (Ngd5), result in similar phenotypes and abnormal Hh signaling as Fxo, indicating a general requirement for IFT proteins in Hh signaling and patterning of multiple organs. Analysis of Ift88 and Shh double mutants indicates that, in mouse, IFT proteins are required for both Gli activator and repressor functions, and Gli proteins are insensitive to Hh ligand in the absence of IFT proteins. Finally, our biochemical studies demonstrate that IFT proteins are required for proteolytic processing of Gli3 in mouse embryos. In summary, our results indicate that IFT function is crucial in the control of both the positive and negative transcriptional activities of Gli proteins, and essential for Hh ligand-induced signaling cascade.

Animals↗

Increasing Fgf4 expression in the mouse limb bud causes polysyndactyly and rescues the skeletal defects that result from loss of Fgf8 function.

A major function of the limb bud apical ectodermal ridge (AER) is to produce fibroblast growth factors (FGFs) that signal to the underlying mesenchyme. Previous studies have suggested that of the four FGF genes specifically expressed in the mouse AER, Fgf8 is unique not only in its expression pattern, but also because it is the only such FGF gene that causes limb skeletal abnormalities when individually inactivated. However, when both Fgf8 and Fgf4 are simultaneously inactivated in the AER, the limb does not develop. One possible explanation for these observations is that although both of these FGF family members contribute to limb development, Fgf8 has functions that Fgf4 cannot perform. To test this hypothesis, we used a novel method to substitute Fgf4 for Fgf8 expression in the developing limb bud by concomitantly activating a conditional Fgf4 gain-of-function allele and inactivating an Fgf8 loss-of-function allele in the same cells via Cre-mediated recombination. Our data show that when Fgf4 is expressed in place of Fgf8, all of the skeletal defects caused by inactivation of Fgf8 are rescued, conclusively demonstrating that FGF4 can functionally replace FGF8 in limb skeletal development. We also show that the increase in FGF signaling that occurs when the Fgf4 gain-of-function allele is activated in a wild-type limb bud causes formation of a supernumerary posterior digit (postaxial polydactyly), as well as cutaneous syndactyly between all the digits. These data underscore the importance of controlling the level of FGF gene expression for normal limb development.

Animals↗

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.

Animals↗

Hoxd-12 differentially affects preaxial and postaxial chondrogenic branches in the limb and regulates Sonic hedgehog in a positive feedback loop.

Several 5' members of the Hoxd cluster are expressed in nested posterior-distal domains of the limb bud suggesting a role in regulating anteroposterior pattern of skeletal elements. While loss-of-function mutants have demonstrated a regulatory role for these genes in the developing limb, extensive functional overlaps between various different Hox genes has hampered elucidation of the roles played by individual members. In particular, the function of Hoxd-12 in the limb remains obscure. Using a gain-of-function approach, we find that Hoxd-12 misexpression in transgenic mice produces apparent transformations of anterior digits to posterior morphology and digit duplications, while associated tibial hemimelia and other changes indicate that formation/growth of certain skeletal elements is selectively inhibited. If the digital arch represents an anterior bending of the main limb axis, then the results are all reconcilable with a model in which Hoxd-12 promotes formation of postaxial chondrogenic condensations branching from this main axis (including the anteriormost digit) and selectively antagonizes formation of 'true' preaxial condensations that branch from this main axis (such as the tibia). Hoxd-12 misexpression can also induce ectopic Sonic hedgehog (Shh) expression, resulting in mirror-image polydactyly in the limb. Misexpression of Hoxd-12 in other lateral plate derivatives (sternum, pelvis) likewise phenocopies several luxoid/luxate class mouse mutants that all share ectopic Shh signalling. This suggests that feedback activation of Shh expression may be a major function of Hoxd-12. Hoxd-12 can bind to and transactivate the Shh promoter in vitro. Furthermore, expression of either exogenous Hoxd-11 or Hoxd-12 in cultured limb bud cells, together with FGF, induces expression of the endogenous Shh gene. Together these results suggest that certain 5' Hoxd genes directly amplify the posterior Shh polarizing signal in a reinforcing positive feedback loop during limb bud outgrowth.

Animals↗

Prx1 and Prx2 in skeletogenesis: roles in the craniofacial region, inner ear and limbs.

Prx1 and Prx2 are closely related paired-class homeobox genes that are expressed in very similar patterns predominantly in mesenchyme. Prx1 loss-of-function mutants show skeletal defects in skull, limbs and vertebral column (Martin, J. F., Bradley, A. and Olson, E. N. (1995) Genes Dev. 9, 1237-1249). We report here that mice in which Prx2 is inactivated by a lacZ insertion had no skeletal defects, whereas Prx1/Prx2 double mutants showed many novel abnormalities in addition to an aggravation of the Prx1 single mutant phenotype. We found defects in external, middle and inner ear, reduction or loss of skull bones, a reduced and sometimes cleft mandible, and limb abnormalities including postaxial polydactyly and bent zeugopods. A single, or no incisor was present in the lower jaw, and ectopic expression of Fgf8 and Pax9 was found medially in the mandibular arch. A novel method to detect &bgr ;-galactosidase activity in hydroxyethylmethacrylate sections allowed detailed analysis of Prx2 expression in affected structures. Our results suggest a role for Prx genes in mediating epitheliomesenchymal interactions in inner ear and lower jaw. In addition, Prx1 and Prx2 are involved in interactions between perichondrium and chondrocytes that regulate their proliferation or differentiation in the bones of the zeugopods.

Amino Acid Sequence↗

Gli3 (Xt) and formin (ld) participate in the positioning of the polarising region and control of posterior limb-bud identity.

During initiation of limb-bud outgrowth in vertebrate embryos, the polarising region (limb-bud organizer) is established upon activation of the Sonic Hedgehog (SHH) signaling molecule at the posterior limb-bud margin. Another hallmark of establishing anteroposterior limb-bud identities is the colinear activation of HoxD genes located at the 5' end of the cluster (5'HoxD genes). The unique and shared functions of Gli3 and formin in these determinative events were genetically analyzed using single and double homozygous Extra-toes (Xt; disrupting Gli3) and limb deformity (ld; disrupting formin) mouse embryos. Analysis of the limb skeletal phenotypes reveals genetic interaction of the two genes. In addition to loss of digit identity and varying degrees of polydactyly, proximal skeletal elements are severely shortened in Xt;ld double homozygous limbs. The underlying molecular defects affect both establishment of the polarising region and posterior limb-bud identity. In particular, the synergism between Gli3- and formin-mediated mesenchyme-AER interactions positions the SHH signaling center at the posterior limb-bud margin. The present study shows that establishment and positioning of the polarising region is regulated both by restriction of Shh through Gli3 and its positive feedback regulation through formin. Concurrently, Gli3 functions independently of formin during initial posterior nesting of 5'HoxD domains, whereas their subsequent distal restriction and anterior expansion depends on genetic interaction of Gli3 and formin.

Animals↗

Gli proteins encode context-dependent positive and negative functions: implications for development and disease.

Several lines of evidence implicate zinc finger proteins of the Gli family in the final steps of Hedgehog signaling in normal development and disease. C-terminally truncated mutant GLI3 proteins are also associated with human syndromes, but it is not clear whether these C-terminally truncated Gli proteins fulfil the same function as full-length ones. Here, structure-function analyses of Gli proteins have been performed using floor plate and neuronal induction assays in frog embryos, as well as induction of alkaline phosphatase (AP) in SHH-responsive mouse C3H10T1/2 (10T1/2) cells. These assays show that C-terminal sequences are required for positive inducing activity and cytoplasmic localization, whereas N-terminal sequences determine dominant negative function and nuclear localization. Analyses of nuclear targeted Gli1 and Gli2 proteins suggest that both activator and dominant negative proteins are modified forms. In embryos and COS cells, tagged Gli cDNAs yield C-terminally deleted forms similar to that of Ci. These results thus provide a molecular basis for the human Polydactyly type A and Pallister-Hall Syndrome phenotypes, derived from the deregulated production of C-terminally truncated GLI3 proteins. Analyses of full-length Gli function in 10T1/2 cells suggest that nuclear localization of activating forms is a regulated event and show that only Gli1 mimics SHH in inducing AP activity. Moreover, full-length Gli3 and all C-terminally truncated forms act antagonistically whereas Gli2 is inactive in this assay. In 10T1/2 cells, protein kinase A (PKA), a known inhibitor of Hh signaling, promotes Gli3 repressor formation and inhibits Gli1 function. Together, these findings suggest a context-dependent functional divergence of Gli protein function, in which a cell represses Gli3 and activates Gli1/2 prevents the formation of repressor Gli forms to respond to Shh. Interpretation of Hh signals by Gli proteins therefore appears to involve a fine balance of divergent functions within each and among different Gli proteins, the misregulation of which has profound biological consequences.

Alkaline Phosphatase↗

Mouse Gli1 mutants are viable but have defects in SHH signaling in combination with a Gli2 mutation.

The secreted factor Sonic hedgehog (SHH) is both required for and sufficient to induce multiple developmental processes, including ventralization of the CNS, branching morphogenesis of the lungs and anteroposterior patterning of the limbs. Based on analogy to the Drosophila Hh pathway, the multiple GLI transcription factors in vertebrates are likely to both transduce SHH signaling and repress Shh transcription. In order to discriminate between overlapping versus unique requirements for the three Gli genes in mice, we have produced a Gli1 mutant and analyzed the phenotypes of Gli1/Gli2 and Gli1/3 double mutants. Gli3(xt) mutants have polydactyly and dorsal CNS defects associated with ectopic Shh expression, indicating GLI3 plays a role in repressing Shh. In contrast, Gli2 mutants have five digits, but lack a floorplate, indicating that it is required to transduce SHH signaling in some tissues. Remarkably, mice homozygous for a Gli1(zfd )mutation that deletes the exons encoding the DNA-binding domain are viable and appear normal. Transgenic mice expressing a GLI1 protein lacking the zinc fingers can not induce SHH targets in the dorsal brain, indicating that the Gli1(zfd )allele contains a hypomorphic or null mutation. Interestingly, Gli1(zfd/zfd);Gli2(zfd/+), but not Gli1(zfd/zfd);Gli3(zfd/+) double mutants have a severe phenotype; most Gli1(zfd/zfd);Gli2(zfd/+) mice die soon after birth and all have multiple defects including a variable loss of ventral spinal cord cells and smaller lungs that are similar to, but less extreme than, Gli2(zfd/zfd) mutants. Gli1/Gli2 double homozygous mutants have more extreme CNS and lung defects than Gli1(zfd/zfd);Gli2(zfd/+) mutants, however, in contrast to Shh mutants, ventrolateral neurons develop in the CNS and the limbs have 5 digits with an extra postaxial nubbin. These studies demonstrate that the zinc-finger DNA-binding domain of GLI1 protein is not required for SHH signaling in mouse. Furthermore, Gli1 and Gli2, but not Gli1 and Gli3, have extensive overlapping functions that are likely downstream of SHH signaling.

Abnormalities, Multiple↗

Embryonic retinoic acid synthesis is required for forelimb growth and anteroposterior patterning in the mouse.

Numerous studies, often performed on avian embryos, have implicated retinoic acid (RA) in the control of limb bud growth and patterning. Here we have investigated whether the lack of endogenous RA synthesis affects limb morphogenesis in mutant mouse embryos deficient for the retinaldehyde dehydrogenase 2 (Raldh2/Aldh1a2). These mutants, which have no detectable embryonic RA except in the developing retina, die at E9.5-E10 without any evidence of limb bud formation, but maternal RA supplementation through oral gavage from E7.5 can extend their survival. Such survivors exhibit highly reduced forelimb rudiments, but apparently normal hindlimbs. By providing RA within maternal food, we found both a stage- and dose-dependency for rescue of forelimb growth and patterning. Following RA supplementation from E7.5 to 8.5, mutant forelimbs are markedly hypoplastic and lack anteroposterior (AP) patterning, with a single medial cartilage and 1-2 digit rudiments. RA provided until E9.5 significantly rescues forelimb growth, but cannot restore normal AP patterning. Increasing the RA dose rescues the hypodactyly, but leads to lack of asymmetry of the digit pattern, with abnormally long first digit or symmetrical polydactyly. Mutant forelimb buds are characterized by lack of expression or abnormal distal distribution of Sonic hedgehog (Shh) transcripts, sometimes with highest expression anteriorly. Downregulation or ectopic anterior expression of Fgf4 is also seen. As a result, genes such as Bmp2 or Hoxd genes are expressed symmetrically along the AP axis of the forelimb buds, and/or later, of the autopod. We suggest that RA signaling cooperates with a posteriorly restricted factor such as dHand, to generate a functional zone of polarizing activity (ZPA).

Aldehyde Oxidoreductases↗

Macrodystrophia lipomatosa: radiographic observations.

23 cases of macrodystrophia lipomatosa (MDL) are reported showing a wide spectrum of radiographic findings. Typical findings were hypertrophy of all the mesodermal tissues of the affected digits with dramatic overgrowth of fat. Phalanges were enlarged both in length and transverse diameter, but the trabecular pattern was maintained. In one patient, the phalanges and metatarsals were elongated but thinned. In another case, all the phalanges and metatarsals of the great toe were small. The little toe was also involved in two cases. Articular surfaces were slanting. There was a high incidence of palmar and plantar involvement. In a few cases the forearm and leg were also involved. Other uncommon features observed were early maturation of epiphyseal centres of ossification of phalanges and metatarsals, syndactyly, polydactyly, brachydactyly and symphalangism. Angiography was uncharacteristic.

Adolescent↗

The ultrasound markers of chromosomal disease: a retrospective study.

Over a 3 year period 37 pregnancies were complicated by a chromosomal abnormality. In the two cases of trisomy 13, holoprosencephaly, facial clefting, polydactyly and growth retardation were seen. In the seven cases of trisomy 18, abnormalities of the extremities, face and heart were common. Growth retardation and diaphragmatic hernia were also demonstrated. In the 21 cases of Down's syndrome the main abnormalities were cardiac, duodenal atresia and subtle digital anomalies. The two fetuses with triploidy showed a large hydropic placenta and holoprosencephaly respectively, and all five cases of Turner's syndrome demonstrated a cystic hygroma two of which were associated with hydrops. From the antenatal scans major anomalies were detected in 18 fetuses, however, chromosomal disease was suspected in only 15 cases. This was in part owing to a high false negative rate for cardiac anomalies (14 cases) in both routine and detailed scans. Owing to the diversity of anomalies present in chromosomal disease full assessment of the fetus is recommended with particular attention to the fetal heart, face, hands and feet. Specific anomalies are suggested for karyotype.

Abnormalities, Multiple↗

Rhombencephalosynapsis associated with hand anomalies.

A case of rhombencephalosynapsis, a very rare disorder characterized by agenesis or hypogenesis of the cerebellar vermis and fusion of the cerebellar hemispheres, is reported with magnetic resonance imaging features. Radiographs showed anomalies in both hands; namely phalangeal hypoplasia and occult polydactyly in the right hand and syndactyly in the left, previously unreported in association with this disorder.

Abnormalities, Multiple↗

PXO set of recombinant inbred strains of the rat: a new strain distribution pattern containing 448 markers.

A new PXO set of RIS represents a fixed F2 population derived from polydactylous (P) congenic strain SHR.Lx and oligodactylous (O) RI strain BXH2. The PXO strains were derived as a complementary set to current RIS (HXB, BXH) of the laboratory rat. All PXO strains are homozygous in the Lx allele and express different morphological phenotypes of the polydactyly-luxate syndrome (PLS) due to variable combinations of Lx modifying genes of either SHR or BN origin. The SDP is being built up by genotyping polymorphic microsatellite markers and several gene polymorphisms. The markers were ordered according to data from public mapping resources such as the Rat Genome Database (rgd.mcw.edu) and current SDP of the other RI strain sets (HXB, BXH). The resulting map corresponding to the common SDP of HXB, BXH RIS sets consists of 448 markers, from which 261 were proven to be polymorphic in the PXO set. The SDP of PXO strains with polymorphic markers arranged in approximately 5 cM intervals is ready for the association analysis and interval mapping in interconnection with the SDP of HXB/BXH strains.

Animals↗

Unusual magnetic resonance imaging findings of the sellar region in subjects with hypopituitarism: report of 4 cases.

Out of 323 consecutive growth hormone deficient patients who underwent magnetic resonance imaging (MRI), we describe the clinical and neuroradiological characteristics of four patients in whom MRI revealed unusual pictures of the sellar area. They were selected as unique in their morphological picture and representative of rare conditions. At presentation all subjects had short stature, growth hormone (GH) deficiency and complex phenotypical abnormalities. Patient 1. Female affected by vaginal atresia and sinus urogenitalis, polydactyly and syndactyly with Y-shaped metacarpals. MRI at age 11.2 years revealed normal pituitary, but hypothalamic mass occupying the suprasellar and interpeduncular cistern. The diagnosis of Hall-Pallister syndrome was made. Patients 2 and 3. Two sisters with a history of epilepsy both showing mild intellectual deficiency, midface hypoplasia and ectodermal dysplasia. MRI at age 8 and 12 years respectively displayed in both cases a round hypointensity protruding from the dorsum sellae into a normal pituitary. The diagnosis was sellar spine. Patient 4. Male with a history of postnatal hypoglycemia showing microphallus and clinical features of severe hypopituitarism. Hormonal evaluation at age 8 months confirmed multiple pituitary hormone deficiencies and MRI at age 6 years showed absent anterior lobe, rudimentary stalk and posterior lobe ectopia. The diagnosis was pituitary aplasia. The patients described show that MRI in pituitary dwarfs can reveal unusual intrasellar findings and allow the correct diagnosis of rare syndromes. Our patients also demonstrate the wide variability in the association of hypopituitarism with midline congenital abnormalities and the possible combination with complex syndromes.

Abnormalities, Multiple↗

Collodion baby concomitant with congenital hypothyroidism: a patient report and review of the literature.

Collodion baby is a rare congenital disorder resembling harlequin fetus but is milder in degree. Although it has been reported that harlequin fetus is associated with kidney abnormalities, malignant keratoma, micromelia, polydactyly, thymic atrophy and thyroid aplasia, there are few reports of collodion baby associated with congenital abnormality and/or disease in the literature. A newborn infant admitted with shedding of the skin and diagnosed as collodion baby concomitant with congenital hypothyroidism is presented here. This is the first case of collodion baby associated with congenital hypothyroidism in the literature to our knowledge.

Abnormalities, Multiple↗