Skeletal-specific expression of Fgd1 during bone formation and skeletal defects in faciogenital dysplasia (FGDY; Aarskog syndrome)

FGD1 encodes a guanine nucleotide exchange factor (GEF) that specifically activates the Rho GTPase Cdc42; FGD1 mutations result in Faciogenital Dysplasia (FGDY, Aarskog syndrome), an X-linked developmental disorder that adversely affects the formation of multiple skeletal structures. To further define the role of FGD1 in skeletal development, we examined its expression in developing mouse embryos and correlated this pattern with FGDY skeletal defects. In this study, we show that Fgd1, the mouse FGD1 ortholog, is initially expressed during the onset of ossification during embryogenesis. Fgd1 is expressed in regions of active bone formation in the trabeculae and diaphyseal cortices of developing long bones. The onset of Fgd1 expression correlates with the expression of bone sialo-protein, a protein specifically expressed in osteoblasts at the onset of matrix mineralization; an analysis of serial sections shows that Fgd1 is expressed in tissues containing calcified and mineralized extracellular matrix. Fgd1 protein is specifically expressed in cultured osteoblast and osteoblast-like cells including MC3T3-E1 cells and human osteosarcoma cells but not in other mesodermal cells; immunohistochemical studies confirm the presence of Fgd1 protein in mouse calvarial cells. Postnatally, Fgd1 is expressed more broadly in skeletal tissue with expression in the perichondrium, resting chondrocytes, and joint capsule fibroblasts. The data indicate that Fgd1 is expressed in a variety of regions of incipient and active endochondral and intramembranous ossification including the craniofacial bones, vertebrae, ribs, long bones and phalanges. The observed pattern of Fgd1 expression correlates with FGDY skeletal manifestations and provides an embryologic basis for the prevalence of observed skeletal defects. The observation that the induction of Fgd1 expression coincides with the initiation of ossification strongly suggests that FGD1 signaling plays a role in ossification and bone formation; it also suggests that FGD1 signaling does not play a role in the earlier phases of skeletogenesis. With the observation that FGD1 mutations result in the skeletal dysplasia FGDY, accumulated data indicate that FGD1 signaling plays a critical role in ossification and skeletal development.

Isolation, characterization, and mapping of the mouse Fgd3 gene, a new Faciogenital Dysplasia (FGD1; Aarskog Syndrome) gene homologue

FGD1 gene mutations result in faciogenital dysplasia (FGDY, Aarskog syndrome), an X-linked developmental disorder that adversely affects the formation of multiple skeletal structures. FGD1 encodes a guanine nucleotide exchange factor (GEF) that specifically activates the Rho GTPase Cdc42. By way of Cdc42, FGD1 regulates the actin cytoskeleton and activates the c-Jun N-terminal kinase signaling cascade to regulate cell growth and differentiation. Previous work shows that FGD1 is the founding member of a family of related genes including the mouse Fgd2 gene and the rat Frabin gene. Here, we report on the isolation, characterization, and mapping of the mouse Fgd3 gene, a new and novel member of the FGD1 gene family. Fgd3 cDNA encodes a 733-amino-acid protein with a predicted mass of 81 kDa. Fgd3 and FGD1 share a high degree of sequence identity that spans >560 contiguous amino acid residues. Like FGD1, Fgd3 contains adjacent RhoGEF and pleckstrin homology (PH) domains, a second carboxy-terminal PH domain, and a distinctive FYVE domain. Together, these domains appear to form a canonical core structure for FGD1 family members. In addition, compared to other FGD1 family members, Fgd3 contains different structural regions that may be involved in distinct signaling interactions. Microinjection studies show that Fgd3 stimulates fibroblasts to form filopodia, actin microspikes formed upon the stimulation of Cdc42. Fgd3 transcripts are present in several diverse tissues and during mouse embryogenesis, suggesting a developmentally regulated pattern of expression and a potential role in embryonic development. Genetic linkage and radiation hybrid mapping data show that Fgd3 and the human FGD3 ortholog map to syntenic regions of murine chromosome 13 and human chromosome 9q22, respectively. We conclude that Fgd3 is a new and novel member of the FGD1 family of RhoGEF proteins.

Isolation, characterization, and mapping of the mouse and human Fgd2 genes, faciogenital dysplasia (FGD1; Aarskog syndrome) gene homologues

FGD1 encodes a guanine nucleotide exchange factor (GEF) that specifically activates the Rho GTPase Cdc42. FGD1 gene mutations result in faciogenital dysplasia (FGDY, Aarskog syndrome), an X-linked developmental disorder that adversely affects the formation of multiple skeletal structures. Database searches show that the Caenorhabditis elegans genome contains an FGD1 homologue. Since C. elegans genes often have multiple vertebrate homologues, we hypothesized the existence of multiple mammalian FGD1-related sequences. Here we report the use of degenerate PCR to isolate and characterize the mouse and human Fgd2 genes, new members of the FGD1 gene family. Fgd2 cDNA encodes a 727-amino-acid protein with a predicted mass of 82 kDa. Fgd2 and FGD1 share a high degree of sequence identity that spans >560 contiguous amino acid residues. Fgd2, like FGD1, contains adjacent RhoGEF and PH domains, a second carboxy-terminal PH domain, and a distinctive FYVE domain. Genomic PCR studies indicate some degree of conserved gene structure between Fgd2 and FGD1. Fgd2 transcripts are present in several diverse tissues and during mouse embryogenesis, suggesting a role in embryonic development. Genetic linkage and radiation hybrid mapping data show that Fgd2 and the human FGD2 ortholog map to syntenic regions of murine chromosome 17 and human chromosome 6p21.2, respectively. The observation that all FGD1 gene family members contain equivalent signaling domains and a conserved structural organization strongly suggests that these signaling domains form a canonical core structure for members of the FGD1 family of RhoGEF proteins.

Familial Mondini dysplasia

OBJECTIVES/HYPOTHESIS

To determine the mode of inheritance of familial nonsyndromic Mondini dysplasia.

STUDY DESIGN

Correlative clinical genetic analysis of a single kindred.

METHODS

Clinical history, physical examination, audiologic analysis, computed tomography of the temporal bones, and cytogenetic analysis.

RESULTS

The male proband, three affected sisters, and an affected brother are offspring of unaffected parents. The mother and an unaffected brother have audiologic findings suggestive of heterozygous carrier status for a recessive hearing loss gene.

CONCLUSIONS

Pedigree analysis indicates autosomal recessive inheritance in this family. The observed inheritance and clinical, audiologic, and radiologic findings are different from those previously described for another family with nonsyndromic Mondini dysplasia. The phenotype in this study family therefore represents a distinct subtype, indicating clinical and genetic heterogeneity of this disorder. This information should facilitate future molecular linkage analyses and genetic counselling of patients with inner ear malformations.

CDC42 and FGD1 cause distinct signaling and transforming activities

Activated forms of different Rho family members (CDC42, Rac1, RhoA, RhoB, and RhoG) have been shown to transform NIH 3T3 cells as well as contribute to Ras transformation. Rho family guanine nucleotide exchange factors (GEFs) (also known as Dbl family proteins) that activate CDC42, Rac1, and RhoA also demonstrate oncogenic potential. The faciogenital dysplasia gene product, FGD1, is a Dbl family member that has recently been shown to function as a CDC42-specific GEF. Mutations within the FGD1 locus cosegregate with faciogenital dysplasia, a multisystemic disorder resulting in extensive growth impairments throughout the skeletal and urogenital systems. Here we demonstrate that FGD1 expression is sufficient to cause tumorigenic transformation of NIH 3T3 fibroblasts. Although both FGD1 and constitutively activated CDC42 cooperated with Raf and showed synergistic focus-forming activity, both quantitative and qualitative differences in their functions were seen. FGD1 and CDC42 also activated common nuclear signaling pathways. However, whereas both showed comparable activation of c-Jun, CDC42 showed stronger activation of serum response factor and FGD1 was consistently a better activator of Elk-1. Although coexpression of FGD1 with specific inhibitors of CDC42 function demonstrated the dependence of FGD1 signaling activity on CDC42 function, FGD1 signaling activities were not always consistent with the direct or exclusive stimulation of CDC42 function. In summary, FGD1 and CDC42 signaling and transformation are distinct, thus suggesting that FGD1 may be mediating some of its biological activities through non-CDC42 targets.

Guanine nucleotide exchange factors regulate specificity of downstream signaling from Rac and Cdc42

The Rac and Cdc42 GTPases regulate diverse cellular behaviors involving the actin cytoskeleton, gene transcription, and the activity of multiple protein and lipid kinases. All of these pathways can potentially become activated when GTP-Rac or GTP-Cdc42 is formed in response to external cell signals, yet it is evident that each activity must also be able to be controlled individually. The mechanisms by which such specificity of GTPase signaling in response to upstream stimuli is achieved remains unclear. We investigated the action of several well characterized guanine nucleotide exchange factors (GEFRho) to activate Rac- and/or Cdc42-dependent kinase pathways. Coexpression studies in COS-7 cells revealed that the ability of individual guanine nucleotide exchange factors (GEFs) to activate the p21-activated kinase PAK1 could be dissociated from activation of c-Jun amino-terminal kinase, even though activation of both pathways requires the action of the GEFs on Rac and/or Cdc42. In contrast, expression of constitutively active forms of Rac or Cdc42 effectively stimulated both downstream kinases. We conclude that GEFs can be important determinants of downstream signaling specificity for members of the Rho GTPase family.

Activation of G1 progression, JNK mitogen-activated protein kinase, and actin filament assembly by the exchange factor FGD1

Cdc42 has been shown to control bifurcating pathways leading to filopodia formation/G1 cell cycle progression and to JNK mitogen-activated protein kinase activation. To dissect these pathways further, the cellular effects induced by a Cdc42 guanine nucleotide exchange factor, FGD1, have been examined. All exchange factors acting on the Rho GTPase family have juxtaposed Dbl homology (DH) and pleckstrin homology (PH) domains. We report here that FGD1 triggers G1 cell cycle progression and filopodia formation in Swiss 3T3 fibroblasts as well as JNK mitogen-activated protein kinase activation in COS cell transfection assays. FGD1-induced filopodia formation is Cdc42-dependent, and both the DH and PH domains are essential. Although expression of the FGD1 DH domain alone does not activate Cdc42 and induce filopodia, it does trigger both the JNK cascade in COS cells and G1 progression in quiescent Swiss 3T3 cells. We conclude that FGD1 can trigger G1 progression independently of actin polymerization or integrin adhesion complex assembly. Furthermore, since FGD1 activates JNK and G1 progression in a Cdc42-independent manner, it must have additional, as yet unidentified, targets.

XY sex reversal and gonadal dysgenesis due to 9p24 monosomy

We describe a case of XY sex reversal, gonadal dysgenesis, and gonadoblastoma in a patient with a deletion of 9p24 due to a familial translocation. The rearranged chromosome 9 was inherited from the father; the patient's karyotype was 46,XY,der(9)t(8;9) (p21;p24)pat. A review shows that 6 additional patients with 46,XY sex reversal associated with monosomy of the distal short arm of chromosome 9 have been observed. The observation that all 7 patients with sex reversal share a deletion of the distal short arm of chromosome 9 is consistent with the hypothesis that the region 9p24 contains a gene or genes necessary for male sex determination. This present case narrows the chromosome interval containing a critical sex determination gene to the relatively small region 9p24. A molecular analysis of this region will provide a means to identify a gene involved in male sex determination.

Genomic organization of the faciogenital dysplasia (FGD1; Aarskog syndrome) gene

Faciogenital dysplasia (FGDY; MIM 305400), or Aarskog syndrome, is an X-linked developmental disorder that adversely affects the formation of specific skeletal structures including elements of the face, the cervical vertebrae, and the distal extremities. FGD1, the gene responsible for faciogenital dysplasia, encodes a guanine nucleotide exchange factor that specifically activates Cdc42, a member of the Rho (Ras homology) family of p21 GTPases. By activating Cdc42, FGD1 stimulates fibroblasts to form filopodia, cytoskeletal elements involved in cellular signaling and migration, and through Cdc42, FGD1 also activates the stress-activated protein kinase/c-Jun N-terminal kinase signaling cascade, a pathway that regulates cell growth and differentiation. Here, we report a detailed characterization of the genomic organization of the FGD1 gene. The FGD1 gene is composed of 18 exons that range in size from 31 to 1240 bp. These exons span over 51 kb of genomic DNA within region Xp11.21. Flanking intronic sequences and the sequence of the 5' and 3' untranslated regions were determined to facilitate the detection of FGDY patient mutations. Analyses show that FGD1 transcripts are differentially spliced; in brain and placenta an alternatively spliced form of the FGD1 transcript removes part of the Cdc42GEF domain to encode a null Cdc42 activator.

The faciogenital dysplasia gene product FGD1 functions as a Cdc42Hs-specific guanine-nucleotide exchange factor

The Rho family of small GTP-binding proteins plays important roles in the regulation of actin cytoskeleton organization and cell growth. Activation of these GTPases involves the replacement of bound GDP with GTP, a process catalyzed by the Dbl-like guanine-nucleotide exchange factors, all of which seem to share a putative catalytic motif termed the Dbl homology (DH) domain, followed by a pleckstrin homology (PH) domain. Here we have examined the role of a Dbl-like molecule, the faciogenital dysplasia gene product (FGD1), which when mutated in its Dbl homology domain, cosegregates with the developmental disease Aarskog-Scott syndrome. We report that a polypeptide of FGD1 encompassing the DH and PH domains can bind specifically to the Rho family GTPase Cdc42Hs and stimulates the GDP-GTP exchange of the isoprenylated form of Cdc42Hs. Microinjection of this FGD1 polypeptide into Swiss 3T3 fibroblast cells induces the formation of peripheral actin microspikes, similar to that previously observed when cells were injected with a constitutively active form of Cdc42Hs. This effect of FGD1 on actin organization is readily inhibited by coinjection of a dominant-negative mutant of Cdc42Hs. Examination of NIH 3T3 cells expressing the FGD1 fragment revealed that similar to cells expressing Dbl, two independent elements downstream of Cdc42Hs, the Jun NH2-terminal kinase and the p70 S6 kinase, became activated. Hence, our results indicate that FGD1, through its DH and PH domains, acts as a Cdc42Hs-specific guanine-nucleotide exchange factor and suggest that the Cdc42Hs GTPase may have a role in mammalian development.

Faciogenital dysplasia protein (FGD1) and Vav, two related proteins required for normal embryonic development, are upstream regulators of Rho GTPases

BACKGROUND

Dbl, a guanine nucleotide exchange factor (GEF) for members of the Rho family of small GTPases, is the prototype of a family of 15 related proteins. The majority of proteins that contain a DH (Dbl homology) domain were isolated as oncogenes in transfection assays, but two members of the DH family, FGD1 (the product of the faciogenital dysplasia or Aarskog-Scott syndrome locus) and Vav, have been shown to be essential for normal embryonic development. Mutations to the FGD1 gene result in a human developmental disorder affecting specific skeletal structures, including elements of the face, cervical vertebrae and distal extremities. Homozygous Vav-/- knockout mice embryos are not viable past the blastocyst stage, indicating an essential role of Vav in embryonic implantation.

RESULTS

Here, we show that the microinjection of FGD1 and Vav into Swiss 3T3 fibroblasts induces the polymerization of actin and the assembly of clustered integrin complexes. FGD1 activates Cdc42, whereas Vav activates Rho, Rac and Cdc42. In addition, FGD1 and Vav stimulate the mitogen activated protein kinase cascade that leads to activation of the c-Jun kinase SAPK/JNK1.

CONCLUSIONS

We conclude that FGD1 and Vav are regulators of the Rho GTPase family. Along with their target proteins Cdc42, Rac and Rho, FGD1 and Vav control essential signals required during embryonic development.

Cosmids map two incontinentia pigmenti type 1 (IP1) translocation breakpoints to a 180-kb region within a 1.2-Mb YAC contig

Incontinentia pigmenti (IP) is an X-linked dominant disorder of neuroectodermal development. Based on the observation of six unrelated females with clinical features of nonfamilial IP with constitutional de novo reciprocal X;autosome translocations, a putative incontinentia pigmenti type 1 locus (IP1; MIM No. 308300) was localized to region Xp11.21. Using available regional DNA markers, we constructed a yeast artificial chromosome (YAC) contig that contained 1.2 Mb of distal Xp11.21 and spanned two IP1 X-chromosomal breakpoints. This contig was used to generate a detailed molecular map of the region and identify three regional CpG islands. YAC-derived cosmids were used to clone and map the IP1 breakpoints to a 180-kb interval that was flanked by DNA markers DXS705 and DXS741. The physical map and genomic clones should facilitate the isolation and characterization of transcripts associated with the IP1 translocation breakpoints.

Cytogenetic and molecular analysis of inv dup(15) chromosomes observed in two patients with autistic disorder and mental retardation

A variety of distinct phenotypes has been associated with supernumerary inv dup(15) chromosomes. Although different cytogenetic rearrangements have been associated with distinguishable clinical syndromes, precise genotype-phenotype correlations have not been determined. However, the availability of chromosome 15 DNA markers provides a means to characterize inv dup(15) chromosomes in detail to facilitate the determination of specific genotype-phenotype associations. We describe 2 patients with an autistic disorder, mental retardation, developmental delay, seizures, and supernumerary inv dup(15) chromosomes. Conventional and molecular cytogenetic studies confirmed the chromosomal origin of the supernumerary chromosomes and showed that the duplicated region extended to at least band 15q13. An analysis of chromosome 15 microsatellite CA polymorphisms suggested a maternal origin of the inv dup(15) chromosomes and biparental inheritance of the two intact chromosome 15 homologs. The results of this study add to the existing literature which suggests that the clinical phenotype of patients with a supernumerary inv dup(15) chromosome is determined not only by the extent of the duplicated region, but by the dosage of genes located within band 15q13 and the origin of the normal chromosomes 15.

An intragenic TaqI polymorphism in the faciogenital dysplasia (FGD1) locus, the gene responsible for Aarskog syndrome

A Taq1 polymorphism, located in intron 4 of the faciogenital dysplasia (FGD1) gene, the gene responsible for Aarskog syndrome, is described. FGD1 encodes a putative Rho/Rac guanine nucleotide exchange factor involved in mammalian morphogenesis. The identification of an intragenic polymorphism will facilitate the accurate carrier detection of individuals at risk for Aarskog syndrome.

Three genes that escape X chromosome inactivation are clustered within a 6 Mb YAC contig and STS map in Xp11.21-p11.22

In order to study the distribution of genes that escape X chromosome inactivation, a high density yeast artificial chromosome (YAC) contig and STS map spanning approximately 6 Mb has been constructed in Xp11.21-p11.22. The contig contains 113 YACs mapped with 53 markers, including 10 genes. Four genes have been assayed for their expression status on both the active and inactive human X chromosomes, and these data have been combined with previous results on two other genes in the contig. Three of these genes escape X inactivation and have been localized to a single YAC clone of approximately 1075 kb. The other three genes are subject to inactivation, with two of them lying among the genes that escape inactivation. These results suggest that there are both regional control signals as well as gene-specific elements that determine the X inactivation status of genes on the proximal short arm of the human X chromosome.

Isolation and characterization of the faciogenital dysplasia (Aarskog-Scott syndrome) gene: a putative Rho/Rac guanine nucleotide exchange factor

Faciogenital dysplasia (FGDY), also known as Aarskog-Scott syndrome, is an X-linked developmental disorder characterized by disproportionately short stature and by facial, skeletal, and urogenital anomalies. Molecular genetic analyses mapped FGDY to chromosome Xp11.21. To clone this gene, YAC clones spanning an FGDY-specific translocation breakpoint were isolated. An isolated cDNA, FGD1, is disrupted by the breakpoint, and FGD1 mutations cosegregate with the disease. FGD1 codes for a 961 amino acid protein that has strong homology to Rho/Rac guanine nucleotide exchange factors (GEFs), contains a cysteine-rich zinc finger-like region, and, like the RasGEF mSos, contains two potential SH3-binding sites. These results provide compelling evidence that FGD1 is responsible for FGDY and suggest that FGD1 is a Rho/RacGEF involved in mammalian development.

Clinical, cytogenetic, and molecular characterization of seven patients with deletions of chromosome 22q13.3

We have studied seven patients who have chromosome 22q13.3 deletions as revealed by high-resolution cytogenetic analysis. Clinical evaluation of the patients revealed a common phenotype that includes generalized developmental delay, normal or accelerated growth, hypotonia, severe delays in expressive speech, and mild facial dysmorphic features. Dosage analysis using a series of genetically mapped probes showed that the proximal breakpoints of the deletions varied over approximately 13.8 cM, between loci D22S92 and D22S94. The most distally mapped locus, arylsulfatase A (ARSA), was deleted in all seven patients. Therefore, the smallest region of overlap (critical region) extends between locus D22S94 and a region distal to ARSA, a distance of > 25.5 cM.

YAC subclone contig assembly by serial interspersed repetitive sequence (IRS)-PCR product hybridizations

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The molecular genetics of incontinentia pigmenti

Incontinentia pigmenti (IP) is an unusual and fascinating disorder of the developing neuroectoderm. IP is an X-linked dominant disease characterized by congenital and age-related dermatologic abnormalities and significant neurological, ophthalmologic, and dental anomalies. Two distinct IP gene loci, IP1, mapped to Xp11.21, and IP2, mapped to Xq28, have been identified. The necessary prerequisites for cloning the IP1 gene by a positional cloning approach are available. Ten DNA markers have been mapped to a region between IP1 X-chromosomal translocation breakpoints within region Xp11.21. Approximately 60% of the 2,500-kb region between IP1 X-chromosomal translocation breakpoints has been cloned in yeast artificial chromosome clones.

Nager acrofacial dysostosis

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Insidious onset of familial craniosynostosis

A mother, son, and daughter are presented, in whom serial photographs document an insidious and late onset of exorbitism and midfacial retrusion consistent with a diagnosis of familial nonsyndromic craniosynostosis. Papilledema was found in the 4.5-year-old daughter because of increased intracranial pressure secondary to a reduction in cranial vault size, whereas optic nerve sheath swelling on CT scan was found in the son.

Discordant phenotype of two overlapping deletions involving the PAX3 gene in chromosome 2q35

Waardenburg syndrome (WS), the most common form of inherited congenital deafness, is a pleiotropic, autosomal dominant condition with variable penetrance and expressivity. WS is clinically and genetically heterogeneous. The basis for the phenotypic variability observed among and between WS families is unknown. However, mutations within the paired-box gene, PAX3, have been associated with a subset of WS patients. In this report we use cytogenetic and molecular genetic techniques to study a patient with WS type 3, a form of WS consisting of typical WS type 1 features plus mental retardation, microcephaly, and severe skeletal anomalies. Our results show that the WS3 patient has a de novo paternally derived deletion, del (2)(q35q36), that spans the genetic loci PAX3 and COL4A3. A molecular analysis of a chromosome 2 deletional mapping panel maps the PAX3 locus to 2q35 and suggests the locus order: centromere-(INHA, DES)-PAX3-COL4A3-(ALPI, CHRND)-telomere. Our analyses also show that a patient with a cleft palate and lip pits, but lacking diagnostic WS features, has a deletion, del (2)(q33q35), involving the PAX3 locus. This result suggests that not all PAX3 mutations are associated with a WS phenotype and that additional regional loci may modify or regulate the PAX3 locus and/or the development of a WS phenotype.