Janus-faced EPHB4-associated disorders: novel pathogenic variants and unreported intrafamilial overlapping phenotypes

Purpose

Several clinical phenotypes including fetal hydrops, central conducting lymphatic anomaly or capillary malformations with arteriovenous malformations 2 (CM-AVM2) have been associated with EPHB4 (Ephrin type B receptor 4) variants, demanding new approaches for deciphering pathogenesis of novel variants of uncertain significance (VUS) identified in EPHB4, and for the identification of differentiated disease mechanisms at the molecular level.

Methods

Ten index cases with various phenotypes, either fetal hydrops, CM-AVM2, or peripheral lower limb lymphedema, whose distinct clinical phenotypes are described in detail in this study, presented with a variant in EPHB4. In vitro functional studies were performed to confirm pathogenicity.

Results

Pathogenicity was demonstrated for six of the seven novel EPHB4 VUS investigated. A heterogeneity of molecular disease mechanisms was identified, from loss of protein production or aberrant subcellular localization to total reduction of the phosphorylation capability of the receptor. There was some phenotype–genotype correlation; however, previously unreported intrafamilial overlapping phenotypes such as lymphatic-related fetal hydrops (LRFH) and CM-AVM2 in the same family were observed.

Conclusion

This study highlights the usefulness of protein expression and subcellular localization studies to predict EPHB4 variant pathogenesis. Our accurate clinical phenotyping expands our interpretation of the Janus-faced spectrum of EPHB4-related disorders, introducing the discovery of cases with overlapping phenotypes.

Further Evidence of Contrasting Phenotypes Caused by Reciprocal Deletions and Duplications: Duplication of NSD1 Causes Growth Retardation and Microcephaly

Microduplications of the Sotos syndrome region containing NSD1 on 5q35 have recently been proposed to cause a syndrome of microcephaly, short stature and developmental delay. To further characterize this emerging syndrome, we report the clinical details of 12 individuals from 8 families found to have interstitial duplications involving NSD1, ranging in size from 370 kb to 3.7 Mb. All individuals are microcephalic, and height and childhood weight range from below average to severely restricted. Mild-to-moderate learning disabilities and/or developmental delay are present in all individuals, including carrier family members of probands; dysmorphic features and digital anomalies are present in a majority. Craniosynostosis is present in the individual with the largest duplication, though the duplication does not include MSX2, mutations of which can cause craniosynostosis, on 5q35.2. A comparison of the smallest duplication in our cohort that includes the entire NSD1 gene to the individual with the largest duplication that only partially overlaps NSD1 suggests that whole-gene duplication of NSD1 in and of itself may be sufficient to cause the abnormal growth parameters seen in these patients. NSD1 duplications may therefore be added to a growing list of copy number variations for which deletion and duplication of specific genes have contrasting effects on body development.

Haploinsufficiency of SF3B4, a component of the pre-mRNA spliceosomal complex, causes Nager syndrome

Nager syndrome, first described more than 60 years ago, is the archetype of a class of disorders called the acrofacial dysostoses, which are characterized by craniofacial and limb malformations. Despite intensive efforts, no gene for Nager syndrome has yet been identified. In an international collaboration, FORGE Canada and the National Institutes of Health Centers for Mendelian Genomics used exome sequencing as a discovery tool and found that mutations in SF3B4, a component of the U2 pre-mRNA spliceosomal complex, cause Nager syndrome. After Sanger sequencing of SF3B4 in a validation cohort, 20 of 35 (57%) families affected by Nager syndrome had 1 of 18 different mutations, nearly all of which were frameshifts. These results suggest that most cases of Nager syndrome are caused by haploinsufficiency of SF3B4. Our findings add Nager syndrome to a growing list of disorders caused by mutations in genes that encode major components of the spliceosome and also highlight the synergistic potential of international collaboration when exome sequencing is applied in the search for genes responsible for rare Mendelian phenotypes.

Tracking clinical genetic services for newborns identified through newborn dried bloodspot screening in the United States-lessons learned

To determine how US newborn dried bloodspot screening (NDBS) programs obtain patient-level data on clinical genetic counseling services offered to families of newborns identified through newborn NDBS and the extent to which newborns and their families receive these services. These data should serve to inform programs and lead to improved NDBS follow-up services. Collaborations were established with three state NDBS programs that reported systematically tracking genetic counseling services to newborns and their families identified through NDBS. A study protocol and data abstraction form were developed and IRB approvals obtained. Data from three state NDBS programs on a total of 151 patients indicated that genetic services are documented systematically only by metabolic clinics, most often by genetic counselors. Data from 69 endocrinology patients indicated infrequent referrals for genetic services; as expected higher for congenital adrenal hyperplasia than congenital hypothyroidism. Endocrinology patients were often counseled by physicians. While systematic tracking of genetic counseling services may be desirable for quality assurance of NDBS follow-up services, current systems do not appear conducive to this practice. Clinical records are not typically shared with NDBS programs and tracking of follow-up clinical genetic services has not been generally defined as a NDBS program responsibility. Rather, tracking of clinical services, while recognized as useful data, has been viewed by NDBS programs as a research project. The associated IRB requirements for patient-related research may pose an additional challenge. National guidance for NDBS programs that define quality genetic service indicators and monitoring responsibilities are needed. US experiences in this regard may provide information that can assist developing programs in avoiding tracking issues.

Recurrent deletions and reciprocal duplications of 10q11.21q11.23 including CHAT and SLC18A3 are likely mediated by complex low-copy repeats

We report 24 unrelated individuals with deletions and 17 additional cases with duplications at 10q11.21q21.1 identified by chromosomal microarray analysis. The rearrangements range in size from 0.3 to 12 Mb. Nineteen of the deletions and eight duplications are flanked by large, directly oriented segmental duplications of >98% sequence identity, suggesting that nonallelic homologous recombination (NAHR) caused these genomic rearrangements. Nine individuals with deletions and five with duplications have additional copy number changes. Detailed clinical evaluation of 20 patients with deletions revealed variable clinical features, with developmental delay (DD) and/or intellectual disability (ID) as the only features common to a majority of individuals. We suggest that some of the other features present in more than one patient with deletion, including hypotonia, sleep apnea, chronic constipation, gastroesophageal and vesicoureteral refluxes, epilepsy, ataxia, dysphagia, nystagmus, and ptosis may result from deletion of the CHAT gene, encoding choline acetyltransferase, and the SLC18A3 gene, mapping in the first intron of CHAT and encoding vesicular acetylcholine transporter. The phenotypic diversity and presence of the deletion in apparently normal carrier parents suggest that subjects carrying 10q11.21q11.23 deletions may exhibit variable phenotypic expressivity and incomplete penetrance influenced by additional genetic and nongenetic modifiers.

The Cdc42 guanine nucleotide exchange factor FGD1 regulates osteogenesis in human mesenchymal stem cells

Loss of function mutations in FGD1 result in faciogenital dysplasia, an X-linked human developmental disorder that adversely affects the formation of multiple skeletal structures. FGD1 encodes a guanine nucleotide exchange factor that specifically activates Cdc42, a Rho family small GTPase that regulates a variety of cellular behaviors. We have found that FGD1 is expressed in human mesenchymal stem cells (hMSCs) isolated from adult bone marrow. hMSCs are multipotent cells that can differentiate into many cell types, including fibroblasts, osteoblasts, adipocytes, and chondrocytes, and are thought to play a role in maintaining musculoskeletal tissues throughout life. We demonstrate an active role of FGD1 in osteogenic differentiation of hMSCs. During osteogenic differentiation of hMSCs in culture, we observed up-regulation of both FGD1 expression and Cdc42 activity. Activating FGD1/Cdc42 signaling by overexpression of either FGD1 or constitutively active Cdc42 promoted hMSC osteogenesis, while inhibiting Cdc42 signaling by either dominant negative mutants of FGD1 or Cdc42 suppressed osteogenesis. These results demonstrate an important role for FGD1/Cdc42 signaling in hMSC osteogenesis and suggest that the defects in bone remodeling in faciogenital dysplasia may persist throughout adult life and serve as a potential pathway that may be targeted for enhancing bone regeneration.

Unilateral frontosphenoidal craniosynostosis with achondroplasia: a case report

Isolated, premature fusion of the frontosphenoidal suture is rare. This report describes an unusual combination of frontosphenoidal craniosynostosis and achondroplasia. Although craniosynostosis is known to occur in allelic conditions such as thanatophoric dysplasia, craniosynostosis in individuals with achondroplasia is exceedingly rare. Due to the distracting diagnosis of achondroplasia or inadequate knowledge of craniosynostosis, the abnormal head shape was initially treated by other physicians with helmet molding. Plastic surgery consultation was obtained at 2 years of age and surgical care was provided. An acceptable head shape was obtained, but the delay in appropriate evaluation was disconcerting. To our knowledge this is the first reported case of isolated frontosphenoidal craniosynostosis associated with achondroplasia.

Duplications of the critical Rubinstein-Taybi deletion region on chromosome 16p13.3 cause a novel recognisable syndrome

BACKGROUND

The introduction of molecular karyotyping technologies facilitated the identification of specific genetic disorders associated with imbalances of certain genomic regions. A detailed phenotypic delineation of interstitial 16p13.3 duplications is hampered by the scarcity of such patients.

OBJECTIVES

To delineate the phenotypic spectrum associated with interstitial 16p13.3 duplications, and perform a genotype-phenotype analysis.

RESULTS

The present report describes the genotypic and phenotypic delineation of nine submicroscopic interstitial 16p13.3 duplications. The critically duplicated region encompasses a single gene, CREBBP, which is mutated or deleted in Rubinstein-Taybi syndrome. In 10 out of the 12 hitherto described probands, the duplication arose de novo.

CONCLUSIONS

Interstitial 16p13.3 duplications have a recognizable phenotype, characterized by normal to moderately retarded mental development, normal growth, mild arthrogryposis, frequently small and proximally implanted thumbs and characteristic facial features. Occasionally, developmental defects of the heart, genitalia, palate or the eyes are observed. The frequent de novo occurrence of 16p13.3 duplications demonstrates the reduced reproductive fitness associated with this genotype. Inheritance of the duplication from a clinically normal parent in two cases indicates that the associated phenotype is incompletely penetrant.

A recurrent 16p12.1 microdeletion supports a two-hit model for severe developmental delay

We report the identification of a recurrent 520-kbp 16p12.1 microdeletion significantly associated with childhood developmental delay. The microdeletion was detected in 20/11,873 cases vs. 2/8,540 controls (p=0.0009, OR=7.2) and replicated in a second series of 22/9,254 cases vs. 6/6,299 controls (p=0.028, OR=2.5). Most deletions were inherited with carrier parents likely to manifest neuropsychiatric phenotypes (p=0.037, OR=6). Probands were more likely to carry an additional large CNV when compared to matched controls (10/42 cases, p=5.7×10^-5, OR=6.65). Clinical features of cases with two mutations were distinct from and/or more severe than clinical features of patients carrying only the co-occurring mutation. Our data suggest a two-hit model in which the 16p12.1 microdeletion both predisposes to neuropsychiatric phenotypes as a single event and exacerbates neurodevelopmental phenotypes in association with other large deletions or duplications. Analysis of other microdeletions with variable expressivity suggests that this two-hit model may be more generally applicable to neuropsychiatric disease.

Faciogenital dysplasia protein (FGD1) regulates export of cargo proteins from the golgi complex via Cdc42 activation

Mutations in the FGD1 gene are responsible for the X-linked disorder known as faciogenital dysplasia (FGDY). FGD1 encodes a guanine nucleotide exchange factor that specifically activates the GTPase Cdc42. In turn, Cdc42 is an important regulator of membrane trafficking, although little is known about FGD1 involvement in this process. During development, FGD1 is highly expressed during bone growth and mineralization, and therefore a lack of the functional protein leads to a severe phenotype. Whether the secretion of proteins, which is a process essential for bone formation, is altered by mutations in FGD1 is of great interest. We initially show here that FGD1 is preferentially associated with the trans-Golgi network (TGN), suggesting its involvement in export of proteins from the Golgi. Indeed, expression of a dominant-negative FGD1 mutant and RNA interference of FGD1 both resulted in a reduction in post-Golgi transport of various cargoes (including bone-specific proteins in osteoblasts). Live-cell imaging reveals that formation of post-Golgi transport intermediates directed to the cell surface is inhibited in FGD1-deficient cells, apparently due to an impairment of TGN membrane extension along microtubules. These effects depend on FGD1 regulation of Cdc42 activation and its association with the Golgi membranes, and they may contribute to FGDY pathogenesis.

Oculo-facio-cardio-dental syndrome: skewed X chromosome inactivation in mother and daughter suggest X-linked dominant Inheritance

Oculo-facio-cardio-dental syndrome (OFCD) is an uncommon multiple congenital anomaly syndrome that is characterized by congenital cataracts, multiple minor facial dysmorphic features, congenital heart defects, and dental anomalies including canine radiculomegaly and oligodontia. Although most cases of OFCD are sporadic, since all reported OFCD individuals have been female, it has been suggested that OFCD is an X-linked dominant trait. Here we report two affected female patients with OFCD, a mother and daughter, who both had congenital cataracts, microphthalmia, characteristic dental anomalies, and typical facial dysmorphisms. These features were diagnostic for OFCD; thus, these cases represent the second documented instance of mother-to-daughter OFCD transmission. In addition to the clinical features typically seen in OFCD individuals, the affected daughter exhibited several additional congenital anomalies including intestinal malrotation and hypoplastic thumbs. Thus, these cases further define and expand the OFCD clinical phenotype. These two individuals also displayed a skewed pattern of X chromosome inactivation. Together, these data strongly support the hypothesis that OFCD is inherited as an X-linked dominant condition.

Effect of Fgd1 on cortactin in Arp2/3 complex-mediated actin assembly

Mutations in faciogenital dysplasia protein (Fgd1) result in the human disease faciogenital dysplasia (FGDY). Fgd1 contains a RhoGEF domain specific for Cdc42. Fgd1 also contains a Src homology (SH3) binding domain (SH3-BD) that binds directly to the SH3 domain of cortactin, which promotes actin assembly by actin-related protein (Arp)2/3 complex. Here, we report the effect of ligation of cortactin's SH3 domain by the Fgd1 SH3-BD on actin polymerization in vitro. Glutathione S-transferase (GST)-fused Fgd1 SH3-BD enhanced the ability of cortactin to stimulate Arp2/3-mediated actin polymerization. However, a synthetic peptide containing only the SH3-BD sequence had no effect. The SH3-BD peptide bound to cortactin and inhibited the effect of GST-Fgd1 SH3-BD, suggesting that GST dimerization was responsible for the stimulating effect of GST-Fgd1 SH3-BD. When GST-Fgd1 SH3-BD was prepared as a heterodimer with a control GST fusion protein (GST-Pac1), no stimulatory effect on actin polymerization was observed. In addition, when cortactin was dimerized via its N-terminus, away from the C-terminal SH3 domain, actin polymerization with Arp2/3 complex increased markedly, compared to free cortactin. Thus, cortactin ligated by Fgd1 is fully active, indicating that the cell can use Fgd1 to target actin assembly. Moreover, if Fgd1 is multimerized, then cortactin's activity should be enhanced. Fgd1 and cortactin may participate as scaffolds and signal transducers in a positive feedback cycle to promote actin assembly at the cell cortex.

Fgd1, the Cdc42 GEF responsible for Faciogenital Dysplasia, directly interacts with cortactin and mAbp1 to modulate cell shape

FGD1 mutations result in Faciogenital Dysplasia (FGDY), an X-linked human disease that affects skeletal formation and embryonic morphogenesis. FGD1 and Fgd1, the mouse FGD1 ortholog, encode guanine nucleotide exchange factors (GEF) that specifically activate Cdc42, a Rho GTPase that controls the organization of the actin cytoskeleton. To further understand FGD1/Fgd1 signaling and begin to elucidate the molecular pathophysiology of FGDY, we demonstrate that Fgd1 directly interacts with cortactin and mouse actin-binding protein 1 (mAbp1), actin-binding proteins that regulate actin polymerization through the Arp2/3 complex. In yeast two-hybrid studies, cortactin and mAbp1 Src homology 3 (SH3) domains interact with a single Fgd1 SH3-binding domain (SH3-BD), and biochemical studies show that the Fgd1 SH3-BD directly binds to cortactin and mAbp1 in vitro. Immunoprecipitation studies show that Fgd1 interacts with cortactin and mAbp1 in vivo and that Fgd1 SH3-BD mutations disrupt binding. Immunocytochemical studies show that Fgd1 colocalizes with cortactin and mAbp1 in lamellipodia and membrane ruffles, and that Fgd1 subcellular targeting is dynamic. By using truncated cortactin proteins, immunocytochemical studies show that the cortactin SH3 domain targets Fgd1 to the subcortical actin cytoskeleton, and that abnormal Fgd1 localization results in actin cytoskeletal abnormalities and significant changes in cell shape and viability. Thus, this study provides novel in vitro and in vivo evidence that Fgd1 specifically and directly interacts with cortactin and mAbp1, and that these interactions play an important role in regulating the actin cytoskeleton and, subsequently, cell shape.

Acute extrapyramidal syndrome in mild ornithine transcarbamylase deficiency: metabolic stroke involving the caudate and putamen without metabolic decompensation

A 6-year-old male with partial ornithine transcarbamylase (OTC) deficiency had acute and rapidly progressive symmetrical swelling of the head of the caudate nuclei and putamina. Clinical presentation was ataxia and dysarthria progressing to seizures and coma; these symptoms gradually resolved with supportive management. Although he had been recently treated for mild hyperammonemia, there was no evidence of acute metabolic decompensation prior to presentation, and plasma ammonia and amino acids were consistent with good metabolic control. This case is novel in that the neurological insult affected the neostriatum of the basal ganglia and the episode occurred in the absence of an apparent metabolic abnormality, unique observations in a patient with OTC deficiency.

CONCLUSION

This case suggests that the pathophysiology of metabolic stroke is complicated. It also argues for an evaluation for metabolic stroke in patients with known inborn errors of metabolism who present with unusual neurological symptoms in the absence of biochemical abnormalities. Similarly, this case suggests that patients presenting with unexplained neurological insults might benefit from an evaluation for an inborn error of metabolism.

Interrupted aortic arch in a child with trisomy 5q31.1q35.1 due to a maternal (20;5) balanced insertion

Complex congenital heart defects (CHD) are associated with a variety of single gene abnormalities and chromosomal rearrangements. Of the various forms of CHD, aortic arch interruption, a conotruncal heart defect, is relatively uncommon. Here we report a male neonate with aortic arch interruption type B, secundum atrial septal defect, perimembranous ventricular septal defect, patent ductus arteriosus, aortic and subaortic stenosis, and trisomy 5q31.1q35.1 resulting from a maternal balanced insertion (20;5). Chromosomal deletions, including deletion 22q11, have been reported with interrupted aortic arch (IAA); however, to our knowledge this is the first report of a trisomy of distal chromosome 5q associated with aortic arch interruption. Here we compare this child's features to other cases of trisomy 5q31.1q35.1, and review other causes of IAA. We conclude that gene dosage in this chromosomal region likely influences aortic arch development.

The Caenorhabditis elegans homolog of FGD1, the human Cdc42 GEF gene responsible for faciogenital dysplasia, is critical for excretory cell morphogenesis

FGD1 mutations result in faciogenital dysplasia, an X-linked human disease that affects skeletogenesis. FGD1 encodes a guanine nucleotide exchange factor (GEF) that specifically activates the Rho GTPase Cdc42. To gain insight into the function of FGD1, we have isolated and characterized fgd-1, the Caenorhabditis elegans homolog of the human FGD1 gene. Comparative sequence analyses show that fgd-1 and FGD1 share a similar structural organization and a high degree of sequence identity throughout shared signaling domains. In nematodes, interference with fgd-1 expression results in excretory cell abnormalities and cystic dilation of the excretory cell canals. Molecular lesions associated with two exc-5 alleles affect the fgd-1 gene, and fgd-1 transgenic expression rescues the Exc-5 phenotype. Together, these data confirm that the fgd-1 transcript corresponds to the exc-5 gene. Transgenic expression studies show that fgd-1 has a limited pattern of expression that is confined to the excretory cell during development, a finding that suggests that the C.elegans FGD-1 protein might function in a cell autonomous manner. Serial observations indicate that fgd-1 mutations lead to developmental excretory cell abnormalities that cause cystic dilation and interfere with canal process extension. Based on these data, we conclude that fgd-1 is the C.elegans homolog of the human FGD1 gene, a new member of the FGD1-related family of RhoGEF genes, and that fgd-1 plays a critical role in excretory cell morphogenesis and cellular organization.

Retinitis pigmentosa, growth hormone deficiency, and acromelic skeletal dysplasia in two brothers: possible familial RHYNS syndrome

Here we report two brothers with retinitis pigmentosa, growth hormone deficiency, and acromelic skeletal dysplasia. We propose that their clinical picture is consistent with RHYNS syndrome (retinitis pigmentosa, hypopituitarism, nephronophthisis, and skeletal dysplasia) and that they represent the first instance of a familial occurrence of this syndrome. The presence of RHYNS in two siblings supports an autosomal recessive mode of inheritance; however, since all four known cases were male, an X-linked mode of inheritance cannot be excluded. The combination of clinical features found in these affected males is unique and supports the existence of RHYNS syndrome as a separate and distinct entity.

Fgd1, the Cdc42 guanine nucleotide exchange factor responsible for faciogenital dysplasia, is localized to the subcortical actin cytoskeleton and Golgi membrane

FGD1, the gene responsible for the inherited disease faciogenital dysplasia, encodes a guanine nucleotide exchange factor (GEF) that specifically activates the p21 GTPase Cdc42. In order, FGD1 is composed of a proline-rich N-terminal region, adjacent GEF and pleckstrin homology (PH) domains, a FYVE-finger domain and a second C-terminal PH domain (PH2), structural motifs involved in signaling and subcellular localization. Fgd1, the mouse FGD1 ortholog, is expressed in regions of active bone formation within osteoblasts and in the osteoblast-like cell line MC3T3-E1, a finding consistent with its role in skeletal formation. Here, we use subcellular fractionation studies to show that endogenous Fgd1 protein is localized in the cytosolic and Golgi and plasma membrane fractions of mouse calvarial cells. Immunocytochemical studies performed with osteoblast-like MC3T3-E1 cells and other mammalian cell lines confirm the localization of Fgd1 and show that the proline-rich N-terminal region is necessary and sufficient for Fgd1 subcellular localization to the plasma membrane and Golgi complex. In contrast, the FYVE-finger and PH2 domains do not appear to direct the localization of Fgd1 or the activation of Cdc42. In addition, microinjection studies indicate that the N-terminal Fgd1 domain inhibits filopodia formation, suggesting that this region down-regulates GEF function. These results characterize the function of the Fgd1 domains for both protein localization and Cdc42 activation and indicate that the Fgd1 Cdc42GEF protein is involved in the regulation of Cdc42 activity at the subcortical actin cytoskeleton and Golgi complex.

CHARGE association with choanal atresia and inner ear hypoplasia in a child with a de novo chromosome translocation t(2;7)(p14;q21.11)

A 3-year-old boy was diagnosed with CHARGE association on the basis of bilateral choanal atresia, absence of the semicircular canals, hypoplastic cochleae, genital hypoplasia, growth and developmental delays, cranial nerve dysfunction, and facial anomalies. Ophthalmologic and cardiac evaluations were normal. He was found to have an apparently balanced t(2;7)(p14;q21.11) chromosomal translocation. Parental karyotypes were normal. Although there is evidence suggesting a genetic basis for CHARGE association, individuals with chromosomal abnormalities and CHARGE are rare. In the described patient, the presence of characteristic CHARGE features suggests that the t(2;7)(p14;q21.11) translocation breakpoints may cause a deletion or disruption of genes within the involved regions that are involved in the generation of the CHARGE association phenotype.

Mutations in FOXC2 (MFH-1), a forkhead family transcription factor, are responsible for the hereditary lymphedema-distichiasis syndrome

Lymphedema-distichiasis (LD) is an autosomal dominant disorder that classically presents as lymphedema of the limbs, with variable age at onset, and double rows of eyelashes (distichiasis). Other complications may include cardiac defects, cleft palate, extradural cysts, and photophobia, suggesting a defect in a gene with pleiotrophic effects acting during development. We previously reported neonatal lymphedema, similar to that in Turner syndrome, associated with a t(Y;16)(q12;q24.3) translocation. A candidate gene was not found on the Y chromosome, and we directed our efforts toward the chromosome 16 breakpoint. Subsequently, a gene for LD was mapped, by linkage studies, to a 16-cM region at 16q24.3. By FISH, we determined that the translocation breakpoint was within this critical region and further narrowed the breakpoint to a 20-kb interval. Because the translocation did not appear to interrupt a gene, we considered candidate genes in the immediate region that might be inactivated by position effect. In two additional unrelated families with LD, we identified inactivating mutations-a nonsense mutation and a frameshift mutation-in the FOXC2 (MFH-1) gene. FOXC2 is a member of the forkhead/winged-helix family of transcription factors, whose members are involved in diverse developmental pathways. FOXC2 knockout mice display cardiovascular, craniofacial, and vertebral abnormalities similar to those seen in LD syndrome. Our findings show that FOXC2 haploinsufficiency results in LD. FOXC2 represents the second known gene to result in hereditary lymphedema, and LD is only the second hereditary disorder known to be caused by a mutation in a forkhead-family gene.

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.