Breaking symmetry: a clinical overview of left-right patterning

Monoallelic Mutations in CC2D1A Suggest a Novel Role in Human Heterotaxy and Ciliary Dysfunction

BACKGROUND

Human heterotaxy is a group of congenital disorders characterized by misplacement of one or more organs according to the left-right axis. The genetic causes of human heterotaxy are highly heterogeneous.

METHODS

We performed exome sequencing in a cohort of 26 probands with heterotaxy followed by gene burden analysis for the enrichment of novel rare damaging mutations. Transcription activator-like effector nuclease was used to generate somatic loss-of-function mutants in a zebrafish model. Ciliary defects were examined by whole-mount immunostaining of acetylated α-tubulin.

RESULTS

We identified a significant enrichment of novel rare damaging mutations in the CC2D1A gene. Seven occurrences of CC2D1A mutations were found to affect 4 highly conserved amino acid residues of the protein. Functional analyses in the transcription activator-like effector nuclease-mediated zebrafish knockout models were performed, and heterotaxy phenotypes of the cardiovascular and gastrointestinal systems in both somatic and germline mutants were observed. Defective cilia were demonstrated by whole-mount immunostaining of acetylated α-tubulin. These abnormalities were rescued by wild-type cc2d1a mRNA but not cc2d1a mutant mRNA, strongly suggesting a loss-of-function mechanism. On the other hand, overexpression of cc2d1a orthologous mutations cc2d1a P559L and cc2d1a G808V (orthologous to human CC2D1A P532L and CC2D1A G781V) did not affect embryonic development.

CONCLUSIONS

Using a zebrafish model, we were able to establish a novel association of CC2D1A with heterotaxy and ciliary dysfunction in the F2 generation via a loss-of-function mechanism. Future mechanistic studies are needed for a better understanding of the role of CC2D1A in left-right patterning and ciliary dysfunction.

Chemokine signaling links cell-cycle progression and cilia formation for left-right symmetry breaking

Zebrafish dorsal forerunner cells (DFCs) undergo vigorous proliferation during epiboly and then exit the cell cycle to generate Kupffer's vesicle (KV), a ciliated organ necessary for establishing left-right (L-R) asymmetry. DFC proliferation defects are often accompanied by impaired cilia elongation in KV, but the functional and molecular interaction between cell-cycle progression and cilia formation remains unknown. Here, we show that chemokine receptor Cxcr4a is required for L-R laterality by controlling DFC proliferation and KV ciliogenesis. Functional analysis revealed that Cxcr4a accelerates G1/S transition in DFCs and stabilizes forkhead box j1a (Foxj1a), a master regulator of motile cilia, by stimulating Cyclin D1 expression through extracellular regulated MAP kinase (ERK) 1/2 signaling. Mechanistically, Cyclin D1-cyclin-dependent kinase (CDK) 4/6 drives G1/S transition during DFC proliferation and phosphorylates Foxj1a, thereby disrupting its association with proteasome 26S subunit, non-ATPase 4b (Psmd4b), a 19S regulatory subunit. This prevents the ubiquitin (Ub)-independent proteasomal degradation of Foxj1a. Our study uncovers a role for Cxcr4 signaling in L-R patterning and provides fundamental insights into the molecular linkage between cell-cycle progression and ciliogenesis.

The V-ATPase accessory protein Atp6ap1b mediates dorsal forerunner cell proliferation and left-right asymmetry in zebrafish

Asymmetric fluid flows generated by motile cilia in a transient 'organ of asymmetry' are involved in establishing the left-right (LR) body axis during embryonic development. The vacuolar-type H(+)-ATPase (V-ATPase) proton pump has been identified as an early factor in the LR pathway that functions prior to cilia, but the role(s) for V-ATPase activity are not fully understood. In the zebrafish embryo, the V-ATPase accessory protein Atp6ap1b is maternally supplied and expressed in dorsal forerunner cells (DFCs) that give rise to the ciliated organ of asymmetry called Kupffer's vesicle (KV). V-ATPase accessory proteins modulate V-ATPase activity, but little is known about their functions in development. We investigated Atp6ap1b and V-ATPase in KV development using morpholinos, mutants and pharmacological inhibitors. Depletion of both maternal and zygotic atp6ap1b expression reduced KV organ size, altered cilia length and disrupted LR patterning of the embryo. Defects in other ciliated structures-neuromasts and olfactory placodes-suggested a broad role for Atp6ap1b during development of ciliated organs. V-ATPase inhibitor treatments reduced KV size and identified a window of development in which V-ATPase activity is required for proper LR asymmetry. Interfering with Atp6ap1b or V-ATPase function reduced the rate of DFC proliferation, which resulted in fewer ciliated cells incorporating into the KV organ. Analyses of pH and subcellular V-ATPase localizations suggested Atp6ap1b functions to localize the V-ATPase to the plasma membrane where it regulates proton flux and cytoplasmic pH. These results uncover a new role for the V-ATPase accessory protein Atp6ap1b in early development to maintain the proliferation rate of precursor cells needed to construct a ciliated KV organ capable of generating LR asymmetry.

Atrioventricular septal defects among infants in Europe: a population-based study of prevalence, associated anomalies, and survival

OBJECTIVE

To describe the epidemiology of chromosomal and non-chromosomal cases of atrioventricular septal defects in Europe.

METHODS

Data were obtained from EUROCAT, a European network of population-based registries collecting data on congenital anomalies. Data from 13 registries for the period 2000-2008 were included.

RESULTS

There was a total of 993 cases of atrioventricular septal defects, with a total prevalence of 5.3 per 10,000 births (95% confidence interval 4.1 to 6.5). Of the total cases, 250 were isolated cardiac lesions, 583 were chromosomal cases, 79 had multiple anomalies, 58 had heterotaxia sequence, and 23 had a monogenic syndrome. The total prevalence of chromosomal cases was 3.1 per 10,000 (95% confidence interval 1.9 to 4.3), with a large variation between registers. Of the 993 cases, 639 cases were live births, 45 were stillbirths, and 309 were terminations of pregnancy owing to foetal anomaly. Among the groups, additional associated cardiac anomalies were most frequent in heterotaxia cases (38%) and least frequent in chromosomal cases (8%). Coarctation of the aorta was the most common associated cardiac defect. The 1-week survival rate for live births was 94%.

CONCLUSION

Of all cases, three-quarters were associated with other anomalies, both chromosomal and non-chromosomal. For infants with atrioventricular septal defects and no chromosomal anomalies, cardiac defects were often more complex compared with infants with atrioventricular septal defects and a chromosomal anomaly. Clinical outcomes for atrioventricular septal defects varied between regions. The proportion of termination of pregnancy for foetal anomaly was higher for cases with multiple anomalies, chromosomal anomalies, and heterotaxia sequence.

Congenital heart disease and heterotaxy: upper gastrointestinal fluoroscopy can be misleading and surgery in an asymptomatic patient is not beneficial

PURPOSE

Heterotaxy syndrome is associated with intestinal abnormalities. We sought to define the gastrointestinal anatomy and determine both the risk of volvulus and benefit of screening upper gastrointestinal fluoroscopy (UGI) in these patients.

METHODS

Medical records from 2003 until 2011 at Children's Hospital Los Angeles were reviewed in patients with heterotaxy for cardiovascular diagnosis, gastrointestinal symptoms, imaging and surgical arrangement of viscera, perioperative morbidities, and overall mortality.

RESULTS

224 patients were identified. Fifteen had polysplenia, 41 had asplenia, 50 had normal splenic morphology, 13 had inversus, and 104 were uncharacterized. UGI was performed in 4 patients for suspected volvulus and 20 for obstructive symptoms. Sixty-two had "screening" UGIs. Of 138 asymptomatic patients without imaging, none developed volvulus during the study period. In 30 patients with duodenojejunal malposition (DJM) who underwent surgery, none had malrotation or narrow mesentery. Eleven developed complications, with 8 requiring reoperation for obstruction. Of 8 patients with malrotation, 7 received a Ladd's procedure, and 2 had volvulus with viable bowel. One patient required reoperation and resection for obstruction.

CONCLUSION

While rotational abnormalities are common in heterotaxy, risk of volvulus is low. Following operation, the risk of bowel obstruction and of need for reoperation is higher. We advocate avoiding operation in the asymptomatic patient.

Common bile duct adenocarcinoma in a patient with situs inversus totalis: report of a rare case

BACKGROUND

Situs inversus totalis represents an unusual anomaly characterized by a mirror-image transposition of the abdominal and thoracic viscera. It often occurs concomitantly with other disorders that make difficult diagnosis and management of abdominal pathology. The relationship between situs inversus totalis and cancer remains unclear.

CASE PRESENTATION

We describe a 33-year old Guinean man with situs inversus totalis who presented with obstructive jaundice. Imaging and endoscopic modalities demonstrated a mass of distal common bile duct which biopsy identified an adenocarcinoma. The patient was successfully treated by cephalic pancreaticoduodenectomy followed by adjuvant chemoradiation and he is doing well without recurrence 8 months after surgery.

CONCLUSION

The occurrence of bile duct adenocarcinoma in patient with situs inversus totalis accounts as a rare coincidence. In this setting, when the tumor is resectable, surgical management should be considered without contraindication and must be preceded by a careful preoperative staging.

Association of PKD2 (polycystin 2) mutations with left-right laterality defects

Mutations in the PKD1 (polycystin 1) and PKD2 (polycystin 2) genes cause autosomal dominant polycystic kidney disease (ADPKD). Most Pkd2-null mouse embryos present with left-right laterality defects. For the first time, we report the association of ADPKD resulting from a mutation in PKD2 and left-right asymmetry defects. PKD1 and PKD2 were screened for mutations or large genomic rearrangements in 3 unrelated patients with ADPKD presenting with laterality defects: dextrocardia in one and situs inversus totalis in 2 others. A large gene deletion, a single-exon duplication, and an in-frame duplication respectively, were found in the 3 patients. These polymorphisms were found in all tested relatives with ADPKD, but were absent in unaffected related individuals. No left-right anomalies were found in other members of the 3 families. A possible association between heterotaxia and a PKD2 mutation in our 3 patients is suggested by: (1) the existence of laterality defects in Pkd2-null mouse and zebrafish models and (2) detection of a pathogenic PKD2 mutation in the 3 probands, although PKD2 mutations account for only 15% of ADPKD families. The presence of left-right laterality defects should be systematically screened in larger cohorts of patients with ADPKD harboring PKD2 mutations.

Disruption of planar cell polarity activity leads to developmental biliary defects

Planar cell polarity (PCP) establishes polarity within an epithelial sheet. Defects in PCP are associated with developmental defects involving directional cell growth, including defects in kidney tubule elongation that lead to formation of kidney cysts. Given the strong association between kidney cyst formation and developmental biliary defects in patients and in animal models, we investigated the importance of PCP in biliary development. Here we report that in zebrafish, morpholino antisense oligonucleotide-mediated knockdown of PCP genes including prickle-1a (pk1a) led to developmental biliary abnormalities, as well as localization defects of the liver and other digestive organs. The defects in biliary development appear to be mediated via downstream PCP targets such as Rho kinase, Jun kinase (JNK), and both actin and microtubule components of the cytoskeleton. Knockdown of pk1a led to decreased expression of vhnf1, a homeodomain gene previously shown to be involved in biliary development and in kidney cyst formation; forced expression of vhnf1 mRNA led to rescue of the pk1a morphant phenotype. Our results demonstrate that PCP plays an important role in vertebrate biliary development, interacting with other factors known to be involved in biliary morphogenesis.

Characterization of Pitx2c expression in the mouse heart using a reporter transgene

To aid in detection and tracking of cells targeted by the left-right (LR) pathway in the heart throughout morphogenesis, expression from a Pitx2c-lacZ transgene (P2Ztg) was analysed in detail. β-galactosidase expression from P2Ztg was robust, allowing reliable visualisation of low-level Pitx2c expression, and was virtually entirely dependent upon NODAL signalling in the heart. P2Ztg showed expression in trabecular and septal, as well as non-trabecular, myocardium, and a strong expression bias in myocardium associated with individual endocardial cushions of the atrioventricular canal and outflow tract, which are essential for cardiac septation. Expression on the ventral surface of the outflow tract evolved to a specific stripe that could be used to track the future aorta during outflow tract spiralling and remodelling. Our data show that the P2Ztg transgene is a useful resource for detection of molecular disturbances in the LR cascade, as well as morphogenetic defects associated with other cardiac congenital disorders.

A homozygous mutation in INVS causing juvenile nephronophthisis with abnormal reactivity of the Wnt/beta-catenin pathway

Mutations in the INVS gene coding for inversin have been identified in patients with nephronophthisis type 2 (NPHP2), typically causing infantile onset of ESRD and potentially associated with situs inversus. We report a novel family with a homozygous INVS mutation (c.2695 C > T; p.Arg899X) deleting the C-terminus of inversin. Both affected patients had juvenile ESRD and were discordant for situs inversus. The end-stage kidneys showed chronic interstitial nephritis with cysts and abnormal expression of β-catenin and Dishevelled-1 supporting up-regulated canonical Wnt pathway in tubular cells. This case shows that INVS mutation can cause juvenile nephronophthisis with abnormal reactivity of the Wnt/β-catenin pathway.

Review of genetic factors in intestinal malrotation

Intestinal malrotation is well covered in the surgical literature from the point of view of operative management, but few reviews to date have attempted to provide a comprehensive examination of the topic from the point of view of aetiology, in particular genetic aetiology. Following a brief overview of molecular embryology of midgut rotation, we present in this article instances of and case reports and case series of intestinal malrotation in which a genetic aetiology is likely. Autosomal dominant, autosomal recessive, X-linked and chromosomal forms of the disorder are represented. Most occur in syndromic form, that is to say, in association with other malformations. In many instances, recognition of a specific syndrome is possible, one of several examples discussed being the recently described association of intestinal malrotation with alveolar capillary dysplasia, due to mutations in the forkhead box transcription factor FOXF1. New advances in sequencing technology mean that the identification of the genes mutated in these disorders is more accessible than ever, and paediatric surgeons are encouraged to refer to their colleagues in clinical genetics where a genetic aetiology seems likely.

Different gene expressions on the left and the right: a genotype/phenotype mismatch in need of attention

Discordance in monozygotic twins has traditionally been explained in terms of environmental influences. A recent investigation has found a difference in epigenetic markers in older but not in younger twins. However, phenotypic differences that depend on an individual's postnatal life style do not address the problem of discordance in congenital malformations, or the reason why malformations are frequently unilateral, often with a preference for one or the other side. One such condition, cleft lip with or without cleft palate, which is preferentially expressed on the left, is a multifactorial condition, that is caused by a failure of the critical timing necessary for different groups of cells to meet and develop into a normal face. This process is dependent on cell proliferation and migration, which are energy-dependent, while the additional requirement for apoptosis to allow cell fusion suggests the involvement of mitochondria. Recent progress in two separate areas of research could lead to a better understanding of the problem of facial clefts: (1) the recognition of an interaction between gene products and mitochondria in the aetiology of neurodegenerative diseases and (2) the discovery of an increasing number of genes, including transcription factors, growth factors and members of the TGF-beta signalling family, that are differentially expressed on the left and right side, thus pointing to a difference in their micro-environment. These findings emphasize the importance of investigating the activity of candidate genes for complex developmental processes separately on the left and right sides. Data presented in this review suggest that differential growth rates may lead to an inversion of laterality. A method is described to test for a possible mitochondrial difference between left and right sides, using a mouse model with cleft lip.

Two T-box genes play independent and cooperative roles to regulate morphogenesis of ciliated Kupffer's vesicle in zebrafish

The brain, heart and gastro-intestinal tract develop distinct left-right (LR) asymmetries. Asymmetric cilia-dependent fluid flow in the embryonic node in mouse, Kupffer's vesicle in zebrafish, notochordal plate in rabbit and gastrocoel roof plate in frog appears to be a conserved mechanism that directs LR asymmetric gene expression and establishes the orientation of organ asymmetry. However, the cellular processes and genetic pathways that control the formation of these essential ciliated structures are unknown. In zebrafish, migratory dorsal forerunner cells (DFCs) give rise to Kupffer's vesicle (KV), a ciliated epithelial sheet that forms a lumen and generates fluid flow. Using the epithelial marker atypical Protein Kinase C (aPKC) and other markers to analyze DFCs and KV cells, we describe a multi-step process by which DFCs form a functional KV. Using mutants and morpholinos, we show that two T-box transcription factors-No tail (Ntl)/Brachyury and Tbx16/Spadetail-cooperatively regulate an early step of DFC mesenchyme to epithelial transition (MET) and KV cell specification. Subsequently, each transcription factor independently controls a distinct step in KV formation: Tbx16 regulates apical clustering of KV cells and Ntl is necessary for KV lumen formation. By targeting morpholinos to DFCs, we show that these cell autonomous functions in KV morphogenesis are necessary for LR patterning throughout the embryo.

Computerized three-dimensional analysis of the heart and great vessels in normal and holoprosencephalic human embryos

The developing heart and great vessels undergo drastic morphogenetic changes during the embryonic period. To analyze the normal and abnormal development of these organs, it is essential to visualize their structures in three and four dimensions, including the changes occurring with time. We have reconstructed the luminal structure of the hearts and great vessels of staged human embryos from serial histological sections to demonstrate their sequential morphological changes in three dimensions. The detailed structures of the embryonic heart and major arteries in normal and holoprosencephalic (HPE) human embryos could be reconstructed and visualized, and anatomical structures were analyzed using 3D images. By 3D analysis, cardiac anomalies such as double-outlet right ventricle and malrotation of the heart tube were identified in HPE embryos, which were not easily diagnosed by histological observation. Reconstruction and analysis of 3D images are useful for the study of anatomical structures of developing embryos and for identifying their abnormalities.

Symmetry of the submandibular glands in humans—a postmortem study assessing the linear morphometric parameters

BACKGROUND

The analysis of morphometric right-left symmetry of the salivary glands is important for assessing unilateral changes discovered in these organs during diagnostic imaging.

STUDY DESIGN

A study was carried out on 18 adult human autopsy neck and maxillofacial specimens from both sexes (14 males, 4 females), with age range of 17 to 73 years (mean age 49.9 years). The submandibular duct was injected with contrast medium and images were made in two projection planes.

RESULTS

Morphometry performed using the obtained radiographs included: (a) caliber (mean proximal, middle, and distal = 1.73 mm, 1.82 mm, and 1.77 mm), (b) length (mean = 37.2 mm); (c) genu of the submandibular duct (mean = 114 degrees ); and (d) longitudinal and transverse diameters of the gland (mean = 44.1 mm and 25.9 mm). Comparison of these morphometric parameters between the right and the left glands revealed no statistically significant differences.

CONCLUSION

There is a high level of right-left symmetry in linear morphometric parameters of the submandibular gland.

Asymmetry of skeletal effects of Dominant hemimelia

BACKGROUND

Dominant hemimelia (Dh) is a dominant mutation that arose spontaneously in mice; Dh animals exhibit reduced numbers of lumbar vertebrae and preaxial hindlimb defects. Absence of spleen occurs in both Dh/+ and Dh/Dh animals. This study was undertaken to characterize asymmetry of skeletal defects in the Dh mouse, specifically hindlimb asymmetries in association with axial defects.

METHODS

A total of 29 Dh/+ and 100 +/+ fetuses (gestational day [GD] 18) were identified by phenotype and linked DNA and their skeletons were analyzed.

RESULTS

The results revealed an asymmetry of hindlimb skeletal defects in Dh/+ animals. In +/+ fetuses, the left and right tibia were symmetrical with 99.0% of the animals possessing 6 lumbar vertebrae. However, Dh/+ fetuses showed asymmetry in length of left and right tibia and a reduction to 5 lumbar vertebrae in 86.2% of animals. There was a range from mild to severe asymmetry as evidenced by direct comparison of the length of the left to the right tibia of each animal. Tibial shortening was greater on the left than the right in 65.5% of Dh/+ fetuses; only 20.7% had symmetrical tibia. Oligodactyly, defined as absence of the first or second toe, was similarly more frequent on the left.

CONCLUSIONS

Asymmetry is characteristic of many human limb malformations, although analysis of the molecular basis is difficult. Therefore, Dh/+ mice, which exhibit reduced numbers of lumbar vertebrae, asymmetric hindlimb defects, and complete absence of spleen, provide an important model for studying the relationship between axial patterning and asymmetric skeletal defects.

Situs anomalies and gastrointestinal abnormalities

BACKGROUND/PURPOSE

The aim of the study was to review the gastrointestinal abnormalities occurring in association with situs anomalies.

METHODS

Patients with situs anomalies were identified from the medical records of pediatric patients of Seoul National University Children's Hospital from January 1980 to July 2004. Retrospective study was undertaken. Diagnosis was made on the basis of the information obtained from a combination of echocardiography, angiography, abdominal ultrasonography, liver scan, upper gastrointestinal study, or abdominal computed tomography.

RESULTS

A total 67 patients diagnosed as having situs anomalies were identified. There were 40 males and 26 females (1.54:1). Of these 67 patients, 45 patients (67%) were diagnosed as having situs inversus, 16 patients (24%) as having right isomerism, and 6 patients (9%) as having left isomerism. Of 45 patients with situs inversus, there were 26 patients (58%) who had intraabdominal abnormalities. These were duodenal atresia, biliary atresia, gastroschisis with malrotation, congenital hepatic fibrosis, tracheoesophageal fistula (type C), Currarino's triad, and pheochromocytoma. Of 16 patients with right isomerism, there were 14 patients (88%) who had intraabdominal abnormalities. These were hiatal hernia and diaphragmatic hernia. Of 6 patients with left isomerism, there were 4 patients (67%) who had intraabdominal abnormalities. These were malrotation and biliary atresia.

CONCLUSION

When a patient is noted to have congenital heart disease as part of situs anomalies, or if an atypical position of organs is noted at imaging evaluation, we recommend that the patient undergo chest radiography, abdominal ultrasonography, upper gastrointestinal study, and abdominal computed tomography.

Wnt3a links left-right determination with segmentation and anteroposterior axis elongation

The alignment of the left-right (LR) body axis relative to the anteroposterior (AP) and dorsoventral (DV) axes is central to the organization of the vertebrate body plan and is controlled by the node/organizer. Somitogenesis plays a key role in embryo morphogenesis as a principal component of AP elongation. How morphogenesis is coupled to axis specification is not well understood. We demonstrate that Wnt3a is required for LR asymmetry. Wnt3a activates the Delta/Notch pathway to regulate perinodal expression of the left determinant Nodal, while simultaneously controlling the segmentation clock and the molecular oscillations of the Wnt/beta-catenin and Notch pathways. We provide evidence that Wnt3a, expressed in the primitive streak and dorsal posterior node, acts as a long-range signaling molecule, directly regulating target gene expression throughout the node and presomitic mesoderm. Wnt3a may also modulate the symmetry-breaking activity of mechanosensory cilia in the node. Thus, Wnt3a links the segmentation clock and AP axis elongation with key left-determining events, suggesting that Wnt3a is an integral component of the trunk organizer.

Genetics of human heterotaxias

The past decade has seen remarkable advances in defining the molecular mechanisms underlying formation of the embryonic left right (LR) axis. This information is slowly transforming our understanding of human birth defects that are caused by disturbed LR axis patterning. Reversals, isomerisms, or segmental discordances of thoraco-abdominal organ position, that is, classic heterotaxy, clearly indicate embryonic disruption of normal LR patterning. Other isolated birth defects, particularly cardiovascular malformations, may be caused by deficiencies in the same pathways. Here, we review the distinctive clinical features of human heterotaxias and try to summarize the known connections between them and the corresponding developmental pathways.

Cited2 is required both for heart morphogenesis and establishment of the left-right axis in mouse development

Establishment of the left-right axis is a fundamental process of vertebrate embryogenesis. Failure to develop left-right asymmetry leads to incorrect positioning and morphogenesis of numerous internal organs, and is proposed to underlie the etiology of several common cardiac malformations. The transcriptional modulator Cited2 is essential for embryonic development: Cited2-null embryos die during gestation with profound developmental abnormalities, including cardiac malformations, exencephaly and adrenal agenesis. Cited2 is also required for normal establishment of the left-right axis; we demonstrate that abnormal heart looping and right atrial and pulmonary isomerism are consistent features of the left-right-patterning defect. We show by gene expression analysis that Cited2 acts upstream of Nodal, Lefty2 and Pitx2 in the lateral mesoderm,and of Lefty1 in the presumptive floor plate.

Although abnormal left-right patterning has a major impact on the cardiac phenotype in Cited2-null embryos, laterality defects are only observed in a proportion of these embryos. We have therefore used a combination of high-resolution imaging and three-dimensional (3D) modeling to systematically document the full spectrum of Cited2-associated cardiac defects. Previous studies have focused on the role of Cited2in cardiac neural crest cell development, as Cited2 can bind the transcription factor Tfap2, and thus affect the expression of Erbb3 in neural crest cells. However, we have identified Cited2-associated cardiac defects that cannot be explained by laterality or neural crest abnormalities. In particular, muscular ventricular septal defects and reduced cell density in the atrioventricular (AV) endocardial cushions are evident in Cited2-null embryos. As we found that Cited2 expression tightly correlated with these sites, we believe that Cited2 plays a direct role in development of the AV canal and cardiac septa. We therefore propose that, in addition to the previously described reduction of cardiac neural crest cells, two other distinct mechanisms contribute to the spectrum of complex cardiac defects in Cited2-null mice; disruption of normal left-right patterning and direct loss of Cited2 expression in cardiac tissues.