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.

Two cases with de novo 3q26.31 microdeletion suggest a role for FNDC3B in human craniofacial development.

Here, we report strong evidence for a role of the FNDC3B gene in craniofacial development. Chromosomal microarray identified deletions at 3q26.31 in two patients with dysmorphic facial features. Parental FISH studies demonstrated that they are de novo; therefore, these two 3q26.31 microdeletions likely contribute to the patients' dysmorphic features. Interestingly, the minimal region of overlap contains only the FNDC3B gene. Ffibronectin domain III-containing protein 3B (FNDC3B), also known as factor for adipocyte differentiation-104 (FAD104), was first identified as a positive regulator of adipogenesis in a mouse model. Excitingly, further studies in a mouse model have recently demonstrated that FNDC3B is required for normal calvarial bone formation and negatively regulated calvarial cell differentiation through inhibition of BMP/Smad signaling. fndc3b-deficient mice have multiple cranial and skeletal malformations, such as craniosynostosis-like premature calvarial ossification, and skeletal deformities in the anterior fontanel and femurs. In summary, we report the first two patients with de novo 3q26.31 microdeletions. Both have dysmorphic features, consistent with the phenotypes seen in fndc3b-deficient mice in animal studies, which imply a critical role of FNDC3B in human craniofacial development. © 2016 Wiley Periodicals, Inc.

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.

Clinical features associated with copy number variations of the 14q32 imprinted gene cluster.

Uniparental disomy (UPD) for imprinted chromosomes can cause abnormal phenotypes due to absent or overexpression of imprinted genes. UPD(14)pat causes a unique constellation of features including thoracic skeletal anomalies, polyhydramnios, placentomegaly, and limited survival; its hypothesized cause is overexpression of paternally expressed RTL1, due to absent regulatory effects of maternally expressed RTL1as. UPD(14)mat causes a milder condition with hypotonia, growth failure, and precocious puberty; its hypothesized cause is absence of paternally expressed DLK1. To more clearly establish how gains and losses of imprinted genes can cause disease, we report six individuals with copy number variations of the imprinted 14q32 region identified through clinical microarray-based comparative genomic hybridization. Three individuals presented with UPD(14)mat-like phenotypes (Temple syndrome) and had apparently de novo deletions spanning the imprinted region, including DLK1. One of these deletions was shown to be on the paternal chromosome. Two individuals with UPD(14)pat-like phenotypes had 122-154kb deletions on their maternal chromosomes that included RTL1as but not the differentially methylated regions that regulate imprinted gene expression, providing further support for RTL1 overexpression as a cause for the UPD(14)pat phenotype. The sixth individual is tetrasomic for a 1.7Mb segment, including the imprinted region, and presents with intellectual disability and seizures but lacks significant phenotypic overlap with either UPD(14) syndrome. Therefore, the 14q32 imprinted region is dosage sensitive, with deletions of different critical regions causing UPD(14)mat- and UPD(14)pat-like phenotypes, while copy gains are likely insufficient to recapitulate these phenotypes.

Further clinical and molecular delineation of the 15q24 microdeletion syndrome.

BACKGROUND: Chromosome 15q24 microdeletion syndrome is a rare genomic disorder characterised by intellectual disability, growth retardation, unusual facial morphology and other anomalies. To date, 20 patients have been reported; 18 have had detailed breakpoint analysis. AIM: To further delineate the features of the 15q24 microdeletion syndrome, the clinical and molecular characterisation of fifteen patients with deletions in the 15q24 region was performed, nearly doubling the number of reported patients. METHODS: Breakpoints were characterised using a custom, high-density array comparative hybridisation platform, and detailed phenotype information was collected for each patient. RESULTS: Nine distinct deletions with different breakpoints ranging in size from 266 kb to 3.75 Mb were identified. The majority of breakpoints lie within segmental duplication (SD) blocks. Low sequence identity and large intervals of unique sequence between SD blocks likely contribute to the rarity of 15q24 deletions, which occur 8-10 times less frequently than 1q21 or 15q13 microdeletions in our series. Two small, atypical deletions were identified within the region that help delineate the critical region for the core phenotype in the 15q24 microdeletion syndrome. CONCLUSION: The molecular characterisation of these patients suggests that the core cognitive features of the 15q24 microdeletion syndrome, including developmental delays and severe speech problems, are largely due to deletion of genes in a 1.1-Mb critical region. However, genes just distal to the critical region also play an important role in cognition and in the development of characteristic facial features associated with 15q24 deletions. Clearly, deletions in the 15q24 region are variable in size and extent. Knowledge of the breakpoints and size of deletion combined with the natural history and medical problems of our patients provide insights that will inform management guidelines. Based on common phenotypic features, all patients with 15q24 microdeletions should receive a thorough neurodevelopmental evaluation, physical, occupational and speech therapies, and regular audiologic and ophthalmologic screening.

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.

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.

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.

Refinement of causative genes in monosomy 1p36 through clinical and molecular cytogenetic characterization of small interstitial deletions.

Monosomy 1p36 is the most common terminal deletion syndrome seen in humans, occurring in approximately 1 in 5,000 live births. Common features include mental retardation, characteristic dysmorphic features, hypotonia, seizures, hearing loss, heart defects, cardiomyopathy, and behavior abnormalities. Similar phenotypes are seen among patients with a variety of deletion sizes, including terminal and interstitial deletions, complex rearrangements, and unbalanced translocations. Consequently, critical regions harboring causative genes for each of these features have been difficult to identify. Here we report on five individuals with 200-823 kb overlapping deletions of proximal 1p36.33, four of which are apparently de novo. They present with features of monosomy 1p36, including developmental delay and mental retardation, dysmorphic features, hypotonia, behavioral abnormalities including hyperphagia, and seizures. The smallest region of deletion overlap is 174 kb and contains five genes; these genes are likely candidates for some of the phenotypic features in monosomy 1p36. Other genes deleted in a subset of the patients likely play a contributory role in the phenotypes, including GABRD and seizures, PRKCZ and neurologic features, and SKI and dysmorphic and neurologic features. Characterization of small deletions is important for narrowing critical intervals and for the identification of causative or candidate genes for features of monosomy 1p36 syndrome.

Molecular and clinical delineation of the 17q22 microdeletion phenotype.

Deletions involving 17q21-q24 have been identified previously to result in two clinically recognizable contiguous gene deletion syndromes: 17q21.31 and 17q23.1-q23.2 microdeletion syndromes. Although deletions involving 17q22 have been reported in the literature, only four of the eight patients reported were identified by array-comparative genomic hybridization (array-CGH) or flourescent in situ hybridization. Here, we describe five new patients with 1.8-2.5-Mb microdeletions involving 17q22 identified by array-CGH. We also present one patient with a large karyotypically visible deletion involving 17q22, fine-mapped to ~8.2 Mb using array-CGH. We show that the commonly deleted region in our patients spans 0.24 Mb and two genes; NOG and C17ORF67. The function of C17ORF67 is not known, whereas Noggin, the product of NOG, is essential for correct joint development. In common with the 17q22 patients reported previously, the disease phenotype of our patients includes intellectual disability, attention deficit hyperactivity disorder, conductive hearing loss, visual impairment, low set ears, facial dysmorphology and limb anomalies. All patients displayed NOG-related bone and joint features, including symphalangism and facial dysmorphology. We conclude that these common clinical features indicate a novel clinically recognizable, 17q22 contiguous microdeletion syndrome.

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.

A copy number variation morbidity map of developmental delay.

To understand the genetic heterogeneity underlying developmental delay, we compared copy number variants (CNVs) in 15,767 children with intellectual disability and various congenital defects (cases) to CNVs in 8,329 unaffected adult controls. We estimate that ∼14.2% of disease in these children is caused by CNVs >400 kb. We observed a greater enrichment of CNVs in individuals with craniofacial anomalies and cardiovascular defects compared to those with epilepsy or autism. We identified 59 pathogenic CNVs, including 14 new or previously weakly supported candidates, refined the critical interval for several genomic disorders, such as the 17q21.31 microdeletion syndrome, and identified 940 candidate dosage-sensitive genes. We also developed methods to opportunistically discover small, disruptive CNVs within the large and growing diagnostic array datasets. This evolving CNV morbidity map, combined with exome and genome sequencing, will be critical for deciphering the genetic basis of developmental delay, intellectual disability and autism spectrum disorders.

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

We report the identification of a recurrent, 520-kb 16p12.1 microdeletion associated with childhood developmental delay. The microdeletion was detected in 20 of 11,873 cases compared with 2 of 8,540 controls (P = 0.0009, OR = 7.2) and replicated in a second series of 22 of 9,254 cases compared with 6 of 6,299 controls (P = 0.028, OR = 2.5). Most deletions were inherited, with carrier parents likely to manifest neuropsychiatric phenotypes compared to non-carrier parents (P = 0.037, OR = 6). Probands were more likely to carry an additional large copy-number variant when compared to matched controls (10 of 42 cases, P = 5.7 x 10(-5), OR = 6.6). The clinical features of individuals with two mutations were distinct from and/or more severe than those of individuals carrying only the co-occurring mutation. Our data support 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 indicates that this two-hit model might 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.

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.