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.

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.

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.

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

Images

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

Images

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.

Interspersed repetitive sequence (IRS)-PCR amplification of pulsed-field gel fractionated DNA to derive markers from the incontinentia pigmenti 1 (IP1) locus

Images

A radiation hybrid map of the proximal short arm of the human X chromosome spanning incontinentia pigmenti 1 (IP1) translocation breakpoints

Radiation hybrid mapping was used in combination with physical mapping techniques to order and estimate distances between 14 loci in the proximal region of the short arm of the human X chromosome. A panel of radiation hybrids containing human X-chromosomal fragments was generated from a Chinese hamster-human cell hybrid containing an X chromosome as its only human DNA. Sixty-seven radiation hybrids were screened by Southern hybridization with sets of probes that mapped to the region Xp11.4-Xcen to generate a radiation hybrid map of the area. A physical map of 14 loci was constructed based on the segregation of the loci in the hybrid clones. Using pulsed-field gel electrophoresis (PFGE) analyses and a somatic cell hybrid mapping panel containing naturally occurring X; autosome translocations, the order of the 14 loci was verified and the loci nearest to the X-chromosomal translocation breakpoints associated with the disease incontinentia pigmenti 1 (IP1) were identified. The radiation hybrid panel will be useful as a mapping resource for determining the location, order, and distances between other genes and polymorphic loci in this region as well as for generating additional region-specific DNA markers.

Isolation of DNA markers from a region between incontinentia pigmenti 1 (IP1) X-chromosomal translocation breakpoints by a comparative PCR analysis of a radiation hybrid subclone mapping panel

A strategy based on the use of human-specific interspersed repetitive sequence (IRS)-PCR amplification was used to isolate regional DNA markers in the vicinity of the incontinentia pigmenti 1 (IP1) locus. A radiation hybrid (RH) resulting from a fusion of an irradiated X-only somatic cell hybrid (C12D) and a thymidine kinase deficient (TK-) hamster cell line (a23) was identified as containing multiple X chromosome fragments, including DNA markers spanning IP1 X-chromosomal translocation breakpoints within region Xp11.21. From this RH, a panel of subclones was constructed and analyzed by IRS-PCR amplification to (a) identify subclones containing a reduced number of X chromosome fragments spanning the IP1 breakpoints and (b) construct a mapping panel to assist in identifying regional DNA markers in the vicinity of the IP1 locus. By using this strategy, we have isolated three different IRS-PCR amplification products that map to a region between IP1 X chromosome translocation breakpoints. A total of nine DNA sequences have now been mapped to this region; using these DNA markers for PFGE analyses, we obtained a probe order DXS14-DXS422-MTHFDL1-DXS705. These DNA markers provide a starting point for identifying overlapping genomic sequences spanning the IP1 translocation breakpoints; the availability of IP1 translocation breakpoints should assist the molecular analysis of this locus.

Assignment of human erythroid delta-aminolevulinate synthase (ALAS2) to a distal subregion of band Xp11.21 by PCR analysis of somatic cell hybrids containing X; autosome translocations

The erythroid-specific (ALAS2) and housekeeping (ALAS1) genes encoding delta-aminolevulinate synthase have recently been mapped to chromosomes Xp21.1----q21 and 3p21, respectively. The erythroid-specific gene is a candidate for mutations resulting in X-linked sideroblastic anemia. Analysis of DNA from hybrid clones containing translocations in the region Xp11.21----Xq21.3 permitted the finer localization of the ALAS2 gene with respect to other loci and breakpoints within this region. These studies localized the ALAS2 gene to the distal subregion of Xp11.21 in Interval 5 indicating the following gene order: Xpter-OATL2-[L62-3A, Xp11.21; A62-1A-4b, Xp11.21]-(ALAS2, DXS323)-[B13-3, Xp11.21; C9-5, Xp11.21]-(DXS14, DXS429)-DXS422-(DXZ1, Xcen). Thus, the reported linkage of acquired sideroblastic anemia and sideroblastic anemia with ataxia to Xq13 presumably results from genes other than ALAS2.

Father-to-daughter transmission of focal dermal hypoplasia associated with nonrandom X-inactivation: support for X-linked inheritance and paternal X chromosome mosaicism

Focal dermal hypoplasia (FDH) is a rare syndrome of severe developmental anomalies of the tissues and organs derived from ectoderm and mesoderm. Though data have suggested that FDH is an X-linked dominant trait associated with male hemizygote lethality, a hypothesis supported by the observation of three unrelated infants with FDH manifestations and de novo chromosome rearrangements involving Xp22, observations of father-to-daughter transmission have suggested possible genetic heterogeneity and autosomal dominant inheritance with sex limitation. We hypothesize that, if FDH is an X-linked disorder, cells expressing an active disease locus might experience a selective disadvantage resulting in a nonrandom pattern of X-inactivation in patient tissue. To test this hypothesis, we studied one of the two previously described families demonstrating father-to-daughter inheritance of FDH. To determine if the affected daughter had a skewed pattern of X-inactivation consistent with X-linked inheritance of FDH, somatic cell hybrids were constructed by fusing hypoxanthine phosphoribosyl transferase (HPRT)-deficient rodent fibroblasts with either patient dermal fibroblasts or peripheral white blood cells (WBCs); hybrid clones retaining an active X chromosome were analyzed to determine the parental origin of the active X chromosome. Analyses of resulting hybrid clones showed that while hybrids constructed from skin fibroblasts contained an active X chromosome inherited from either of the patient's parents, hybrids constructed from WBCs showed a skewed pattern of X-inactivation; 11 of 11 hybrids contained an active maternal X chromosome (chi 2 = 12.2, P = .001).(ABSTRACT TRUNCATED AT 250 WORDS)

Human-specific amplification of radiation hybrid DNA fractionated by pulsed-field gel electrophoresis

Images

Localization of DNA sequences to a region within Xp11.21 between incontinentia pigmenti (IP1) X-chromosomal translocation breakpoints

Incontinentia pigmenti (IP) is an X-linked dominant disorder characterized by developmental anomalies of the tissues and organs derived from embryonic ectoderm and neuroectoderm. An IP locus, designated IP1, probably resides in Xp11.21, since five unrelated patients with nonfamilial IP have been identified who possess constitutional de novo reciprocal X;autosome translocations involving Xp11.21. We have used a series of somatic cell hybrids containing the rearranged chromosomes derived from three of the five IP1 patients, along with other hybrid cell lines, to map probes in the vicinity of the IP1 locus. Five anonymous DNA loci--DXS422, DXS14, DXS343, DXS429, and DXS370--have been mapped to a region within Xp11.21, between two IP1 X-chromosomal translocation breakpoints; the IP1 t(X;17) breakpoint is proximal (centromeric) to this region, and the IP1 t(X;13) and t(X;9) X-chromosomal breakpoints lie distal to it. While no IP1 translocation breakpoint has yet been identified by pulsed-field gel electrophoretic (PFGE) analysis, an overlap between three probes--p58-1, 7PSH3.5, and cpX210--has been detected, placing these probes within 125 kb. Four probes--p58-1, 7PSH3.5, cpX210, and 30CE2.8--have been helpful in constructing a 1,250-kb PFGE map of the region between the breakpoints; these results suggest that the IP1 X-chromosomal translocation breakpoints are separated by at least this distance. The combined somatic cell hybrid and PFGE analyses we report here favor the probe order DXS323-(IP1 t(X;13), IP1, t(X;9]-(DXS422, DXS14, DXS343, DXS429, DXS370)-(IP1 t(X;17), DXZ1). These sequences provide a starting point for identifying overlapping genomic sequences that span the IP1 translocation breakpoints; the availability of IP1 translocation breakpoints should now assist the cloning of this locus.

A child with multiple congenital anomalies and karyotype 46,XY,del(14)(q31q32.3): further delineation of chromosome 14 interstitial deletion syndrome

We report on an infant with a multiple congenital anomaly syndrome and severe developmental delay in association with a previously undescribed de novo interstitial deletion of chromosome 14 [karyotype: 46,XY,del(14) (q31q32.3)]. Comparison of the presented patient with previously reported cases of interstitial and terminal chromosome 14q deletions provides a group of patients monosomic for various overlapping portions of the distal half of chromosome 14q and suggests a limited similarity in phenotype among patients with common deleted 14q segments. All patients with distal 14q deletions were developmentally delayed, most were microcephalic and failed to thrive. Most of the patient's anomalies were limited to the face and head. Few major internal congenital anomalies were observed. These comparisons serve to further clarify possible associations of subchromosomal aberrations with specific phenotypes.

Sequence and structure correlation of human ribosomal transcribed spacers

We report the sequences of the transcribed spacers of human rRNA that now allow us to piece together the entire primary transcript sequence of approximately 13.3 x 10(3) base-pairs. Comparison of transcribed spacer sequences with those of variable regions of rRNA and with those of the non-transcribed spacers supports the hypothesis that the variable regions are descended from transcribed spacers. Nucleotide sequence-derived secondary structures for the 5' external transcribed spacer and for internal transcribed spacers 1 and 2 match both the sizes and shapes of the structures that were visualized 15 years ago on electron micrographs. Parts of these structures are conserved in mammals and may be related to transcript processing.

Incontinentia pigmenti in a male infant with Klinefelter syndrome

Incontinentia pigmenti is a familial disorder affecting tissues derived from neuroectoderm. Statistical analysis of reported pedigrees is consistent with transmission of incontinentia pigmenti by an X-linked dominant gene with male hemizygote lethality. This report describes a male infant with the classic clinical features of this condition and a 47,XXY chromosomal constitution. These findings support the concept that incontinentia pigmenti is an X-linked dominant disorder. This case illustrates the importance of a full genetic investigation in all males with physical findings suggestive of an X-linked dominant disorder lethal in males.

Interstitial deletion of 2(q33q36) in a child with congenital abnormalities

Images

Terminal deletion of the long arm of chromosome 2 in a mildly dysmorphic hypotonic infant with karyotype 46,XY,del(2)(q37)

We describe a boy with severe hypotonia and minor facial anomalies with a terminal deletion of chromosome 2q (46,XY,del(2)(q37)). Comparison with previous cases in the literature indicates that this particular deletion results in infantile hypotonia, developmental delay, and minor craniofacial anomalies including frontal bossing and micrognathia. The absence of true malformations and few minor anomalies in this patient suggests that indications for obtaining a chromosome analysis from neurologically impaired individuals need to be reevaluated.

A somatic cell hybrid panel to facilitate identification of DNA sequences in the vicinity of the incontinentia pigmenti locus (IP1)

Somatic cell hybrids that retain derivative X chromosomes from women with sporadic incontinentia pigmenti (IP1) and de novo X/autosomal translocations with consistent breakpoints at Xp11.21 were constructed. An assembled hybrid panel was used to physically map DNA sequences in relationship to the IP breakpoint. DSX14 was found to map to region Xp11.21----p11.1. Regional assignments of 19 X-chromosomal loci were reviewed.

Bkm sequences from the human X chromosome contain large clusters of GATA/GACA repeats

In order to determine whether the regional localizations of Bkm repeats detected on the human X chromosome consisted of typical GATA/GACA repeats, clones were isolated, mapped, and sequenced. Nine Bkm-hybridizing clones from Kunkel's fluorescent-activated, cell-sorted X-chromosome library were all unique. Five were mapped in detail with restriction enzymes and the Bkm-hybridizing segments were localized. Confirmation of X chromosomal homology was obtained for 2 of the clones and Bkm segments from these 2 clones were sequenced. Seventeen contiguous GATA repeats were found in each clone and the overall repeat arrangement showed relatively few differences from previously sequenced Bkm sequences. These are the first sequences of human Bkm repeats. The results, when compared with previously published results, suggest that there may be significant differences between the organization of Bkm repeats on the human X and on the human Y chromosome.

Reproductive risks for carriers of complex chromosome rearrangements: analysis of 25 families

We have determined the empirical reproductive risks for heterozygous carriers of complex chromosome rearrangements (CCRs). CCRs are structural rearrangements involving at least three chromosomes and three or more chromosomal breakpoints. Pregnancy outcome, the frequency and type of chromosomal imbalance in the offspring, and the localization and distribution of chromosome breakpoints were analyzed in 25 CCR families ascertained by the birth of a malformed child or repeated spontaneous abortions. This study included two newly ascertained familial CCRs and a total of 67 informative pregnancies. Analysis of the data, after correction for ascertainment bias, showed that the incidence of spontaneous abortions in CCR families was 48.3%. Approximately one in ten pregnancies and 18.4% of all live births to CCR carriers resulted in phenotypically abnormal offspring. One-half of all CCR carrier liveborn offspring were also CCR carriers. There was a 53.7% incidence of an abnormal pregnancy outcome to CCR carriers. We failed to detect any evidence for a non-random involvement of specific chromosomes in CCRs. However, we did observe a non-random distribution of specific breakpoints at sites 1q25, 4q13, 6q27, 7p14, 9q12, 11p11, 11p15, 12q21, 13q31, and 18q21.

The secondary structure of human 28S rRNA: the structure and evolution of a mosaic rRNA gene

We have determined the secondary structure of the human 28S rRNA molecule based on comparative analysis of available eukaryotic cytoplasmic and prokaryotic large-rRNA gene sequences. Examination of large-rRNA sequences of both distantly and closely related species has enabled us to derive a structure that accounts both for highly conserved sequence tracts and for previously unanalyzed variable-sequence tracts that account for the evolutionary differences in size among the large rRNAs. Human 28S rRNA is composed of two different types of sequence tracts: conserved and variable. They differ in composition, degree of conservation, and evolution. The conserved regions demonstrate a striking constancy of size and sequence. We have confirmed that the conserved regions of large-rRNA molecules are capable of forming structures that are superimposable on one another. The variable regions contain the sequences responsible for the 83% increase in size of the human large-rRNA molecule over that of Escherichia coli. Their locations in the gene are maintained during evolution. They are G + C rich and largely nonhomologous, contain simple repetitive sequences, appear to evolve by frequent recombinational events, and are capable of forming large, stable hairpins. The secondary-structure model presented here is in close agreement with existing prokaryotic 23S rRNA secondary-structure models. The introduction of this model helps resolve differences between previously proposed prokaryotic and eukaryotic large-rRNA secondary-structure models.

Complex chromosomal rearrangement and multiple spontaneous abortions

We report on a woman with a balanced complex chromosomal rearrangement (CCR) involving chromosomes 7, 10, and 21. She is the third individual with an apparently de novo CCR to be ascertained by repeated fetal wastage. Both familial and de novo CCRs are associated with recurrent spontaneous abortions.

Cloning of the breakpoint of an X;21 translocation associated with Duchenne muscular dystrophy

Duchenne muscular dystrophy (DMD) is an X-linked recessive disorder which affects approximately 1 in 3,300 males, making it the most common of the neuromuscular dystrophies. The biochemical basis of the disease is unknown and as yet no effective treatment is available. A small number of females are also affected with the disease, and these have been found to carry X; autosome translocations involving variable autosomal sites but always with a breakpoint within band Xp21 of the X chromosome (implicated by other kinds of genetic evidence as the site of the DMD lesion). In these female patients the normal X chromosome is preferentially inactivated, which it is assumed silences their one normal DMD gene, leading to expression of the disease. In one such affected female the autosomal breakpoint lies in the middle of the short arm of chromosome 21, within a cluster of ribosomal RNA genes. Here we have used rRNA sequences as probes to clone the region spanning the translocation breakpoint. A sequence derived from the X-chromosomal portion of the clone detects a restriction fragment length polymorphism (RFLP) which is closely linked to the DMD gene and uncovers chromosomal deletions in some male DMD patients.