Abnormal laterality and congenital cardiac anomalies. Relations of visceral and cardiac morphologies in the iv/iv mouse

Clinical implications of respiratory ciliary dysfunction in heterotaxy patients with congenital heart disease: elevated risk of postoperative airway complications

Objective

Cardiac surgery in Congenital Heart Disease-Heterotaxy (CHD-HTX) patients often leads to increased postoperative airway complications. Abnormal respiratory ciliary function, resembling primary ciliary dyskinesia, has been observed. We expanded the sample size by retrospectively reviewing Ciliary Dysfunction (CD) in CHD-HTX patients to verify the increased risk of post-surgical respiratory complications.

Methods

We conducted a retrospective review of 69 CHD-HTX patients undergoing cardiac surgery, assessing abnormal respiratory function using nasal nitric oxide (nNO) levels and nasal ciliary motion observed in video microscopy. Data collected included demographics, surgical details, postoperative complications, length of stay, ICU hours, salvage procedures, intubation duration, and mortality.

Results

The CD and no-CD cohorts exhibited notable similarities in risk adjustment in Congenital Heart Surgery-1 (RACHS-1) risk categories, age at the time of surgery, and the duration of follow-up evaluations. We observed a trend toward an increased length of post-operative stay in the CD group (15.0 vs. 14.0; P = 0.0017). CHD-HTX patients with CD showed significantly higher rates of respiratory complications (70% vs. 44.4%; P = 0.008). There were no notable variances observed in postoperative hospitalization duration, mechanical ventilation period, or surgical mortality.

Conclusion

Our findings suggest that CHD-HTX patients with CD may face an elevated risk of respiratory complications. These results offer guidance for perioperative management and serve as a reference for further pathological studies.

Functions of cilia in cardiac development and disease

Errors in embryonic cardiac development are a leading cause of congenital heart defects (CHDs), including morphological abnormalities of the heart that are often detected after birth. In the past few decades, an emerging role for cilia in the pathogenesis of CHD has been identified, but this topic still largely remains an unexplored area. Mouse forward genetic screens and whole exome sequencing analysis of CHD patients have identified enrichment for de novo mutations in ciliary genes or non-ciliary genes, which regulate cilia-related pathways, linking cilia function to aberrant cardiac development. Key events in cardiac morphogenesis, including left-right asymmetric development of the heart, are dependent upon cilia function. Cilia dysfunction during left-right axis formation contributes to CHD as evidenced by the substantial proportion of heterotaxy patients displaying complex CHD. Cilia-transduced signaling also regulates later events during heart development such as cardiac valve formation, outflow tract septation, ventricle development, and atrioventricular septa formation. In this review, we summarize the role of motile and non-motile (primary cilia) in cardiac asymmetry establishment and later events during heart development.

Analysis of outcomes in patients with abnormal laterality undergoing congenital heart surgery

OBJECTIVE

Laterality anomalies are almost always associated with severe cardiac anomalies. Demographic properties, type of the procedures, associated anomalies, and early and mid-term prognosis of four types of laterality anomalies were analysed.

METHODS

A total of 64 consecutive patients with laterality anomalies were enrolled between July 2014 and July 2020. We grouped the patients as situs solitus dextrocardia (SSD) (n = 12; 18.7%); situs inversus (SI) (n = 16; 25%); right atrial isomerism (RAI) (n = 29; 45.3%); and left atrial isomerism (LAI) (n = 7; 10.9%). TAPVC was only present in the RAI group (31%). Incidence of mitral or tricuspid atresia was higher in the SSD group (25%). All the patients were followed up with a mean of 19.06 ± 17.6 (0.1-72) months.

RESULTS

Early postoperative mortality was 17 patients, among 107 procedures (15.8%). Twelve patients were in the neonatal period. All ten patients survived after isolated ductal stenting. Fourteen of the deaths were in the RAI group (48.3%). The 3-year survival rates were 85% in LAI, 78.7% in SI, 55.8% in SSD, and 38% in RAI groups. According to the multivariable Cox regression model, mechanical ventilation, kidney injury, RAI, and complex surgery in the neonatal period were independent risk factors for early mortality.

CONCLUSION

Laterality anomalies are one of the most challenging patients who commonly had univentricular physiology. The most prevalent anomaly was RAI, and RAI had the worst outcome and survival. Ductal stent is an acceptable first intervention during the neonatal period in suitable patients. Complex procedures may carry a high risk of death in the neonatal period.

Surgical Management of Heterotaxy Syndrome: Current Challenges and Opportunities

Introduction:

Heterotaxy syndrome presents a unique challenge in surgical management, even in the current era. We hypothesized that certain anatomic subsets merit novel strategies.

Methods:

We analyzed morphologic details, surgeries, comorbidities, subsequent admissions, and survival using Kaplan-Meier methods and multivariable risk models from a single-institution experience of 103 consecutive patients with heterotaxy who underwent cardiac surgery between January 1, 1990, and May 31, 2016.

Results:

Of the 103 patients (50 males and 53 females), 31 had left atrial isomerism, 64 had right atrial isomerism (RAI), and 8 patients’ isomerism was indeterminate (IND), with first cardiac operation at a mean 1.0 year (standard deviation ±3.0 years) of age. Kaplan-Meier overall survival estimate was 83.1% at six months, 77.8% at one year, 65.9% at five years, and 52.1% at ten years. Survival was particularly low among RAI following repair of total anomalous pulmonary venous connection (TAPVC) at first operation, with one- and five-year survival of 57% and 46%, respectively. By multivariable analysis, the only risk factor for death during the early phase (hazard model) was repair of TAPVC at the first cardiac operation (hazard ratio [HR]: 4.4, P = .01), and risk factors during the longer term constant phase were atrioventricular valve (AVV) regurgitation (HR: 4.2, P < .01), male gender (HR: 3.7, P < .01), and two-ventricle repair (HR: 3.0, P = .02). Patients with heterotaxy undergoing the Fontan procedure had excellent subsequent survival (85% at ten years).

Conclusions:

This analysis of over 100 patients with heterotaxy identified TAPVC requiring initial repair as the major risk factor for early death and important AVV regurgitation as the major risk factor in the longer term. Survival with RAI and early repair of TAPVC were poor, with one-year mortality exceeding 40%. Patients with single ventricle completing the Fontan operation enjoyed outstanding ten-year survival (85%). Initial management of RAI requiring early repair of TAPVC remains challenging. For this high-risk subset, alternative strategies such as early referral for cardiac transplantation evaluation warrant consideration.

Loss of ciliary transition zone protein TMEM107 leads to heterotaxy in mice

Cilia in most vertebrate left-right organizers are involved in the original break in left-right (L-R) symmetry, however, less is known about their roles in subsequent steps of the cascade - relaying the signaling and maintaining the established asymmetry. Here we describe the L-R patterning cascades in two mutants of a ciliary transition zone protein TMEM107, revealing that near-complete loss of cilia in Tmem107^null leads to left pulmonary isomerism due to the failure of the midline barrier. Contrary, partially retained cilia in the node and the midline of a hypomorphic Tmem107^schlei mutant appear sufficient for the formation of the midline barrier and establishment and maintenance of the L-R asymmetry. Despite misregulation of Shh signaling in both mutants, the presence of normal Lefty1 expression and midline barrier formation in Tmem107^schlei mutants, suggests a requirement for cilia, but not necessarily Shh signaling for Lefty1 expression and midline barrier formation.

Spontaneous Left Cardiac Isomerism in Chick Embryos: Case Report, Review of the Literature, and Possible Significance for the Understanding of Ventricular Non-Compaction Cardiomyopathy in the Setting of Human Heterotaxy Syndromes

The outer shape of most vertebrates is normally characterized by bilateral symmetry. The inner organs, on the other hand, are normally arranged in bilaterally asymmetric patterns. Congenital deviations from the normal organ asymmetry can occur in the form of mirror imagery of the normal arrangement (situs inversus), or in the form of arrangements that have the tendency for the development of bilateral symmetry, either in a pattern of bilateral left-sidedness (left isomerism) or bilateral right-sidedness (right isomerism). The latter two forms of visceral situs anomalies are called “heterotaxy syndromes”. During the past 30 years, remarkable progress has been made in uncovering the genetic etiology of heterotaxy syndromes. However, the pathogenetic mechanisms causing the spectrum of cardiovascular defects found in these syndromes remain poorly understood. In the present report, a spontaneous case of left cardiac isomerism found in an HH-stage 23 chick embryo is described. The observations made in this case confirmed the existence of molecular isomerism in the ventricular chambers previously noted in mouse models. They, furthermore, suggest that hearts with left cardiac isomerism may have the tendency for the development of non-compaction cardiomyopathy caused by defective development of the proepicardium.

DNAAF1 links heart laterality with the AAA+ ATPase RUVBL1 and ciliary intraflagellar transport

DNAAF1 (LRRC50) is a cytoplasmic protein required for dynein heavy chain assembly and cilia motility, and DNAAF1 mutations cause primary ciliary dyskinesia (PCD; MIM 613193). We describe four families with DNAAF1 mutations and complex congenital heart disease (CHD). In three families, all affected individuals have typical PCD phenotypes. However, an additional family demonstrates isolated CHD (heterotaxy) in two affected siblings, but no clinical evidence of PCD. We identified a homozygous DNAAF1 missense mutation, p.Leu191Phe, as causative for heterotaxy in this family. Genetic complementation in dnaaf1-null zebrafish embryos demonstrated the rescue of normal heart looping with wild-type human DNAAF1, but not the p.Leu191Phe variant, supporting the conserved pathogenicity of this DNAAF1 missense mutation. This observation points to a phenotypic continuum between CHD and PCD, providing new insights into the pathogenesis of isolated CHD. In further investigations of the function of DNAAF1 in dynein arm assembly, we identified interactions with members of a putative dynein arm assembly complex. These include the ciliary intraflagellar transport protein IFT88 and the AAA+ (ATPases Associated with various cellular Activities) family proteins RUVBL1 (Pontin) and RUVBL2 (Reptin). Co-localization studies support these findings, with the loss of RUVBL1 perturbing the co-localization of DNAAF1 with IFT88. We show that RUVBL1 orthologues have an asymmetric left-sided distribution at both the mouse embryonic node and the Kupffer's vesicle in zebrafish embryos, with the latter asymmetry dependent on DNAAF1. These results suggest that DNAAF1-RUVBL1 biochemical and genetic interactions have a novel functional role in symmetry breaking and cardiac development.

A Requirement for Zic2 in the Regulation of Nodal Expression Underlies the Establishment of Left-Sided Identity

ZIC2 mutation is known to cause holoprosencephaly (HPE). A subset of ZIC2 HPE probands harbour cardiovascular and visceral anomalies suggestive of laterality defects. 3D-imaging of novel mouse Zic2 mutants uncovers, in addition to HPE, laterality defects in lungs, heart, vasculature and viscera. A strong bias towards right isomerism indicates a failure to establish left identity in the lateral plate mesoderm (LPM), a phenotype that cannot be explained simply by the defective ciliogenesis previously noted in Zic2 mutants. Gene expression analysis showed that the left-determining NODAL-dependent signalling cascade fails to be activated in the LPM, and that the expression of Nodal at the node, which normally triggers this event, is itself defective in these embryos. Analysis of ChiP-seq data, in vitro transcriptional assays and mutagenesis reveals a requirement for a low-affinity ZIC2 binding site for the activation of the Nodal enhancer HBE, which is normally active in node precursor cells. These data show that ZIC2 is required for correct Nodal expression at the node and suggest a model in which ZIC2 acts at different levels to establish LR asymmetry, promoting both the production of the signal that induces left side identity and the morphogenesis of the cilia that bias its distribution.

Some Isolated Cardiac Malformations Can Be Related to Laterality Defects

Human beings are characterized by a left–right asymmetric arrangement of their internal organs, and the heart is the first organ to break symmetry in the developing embryo. Aberrations in normal left–right axis determination during embryogenesis lead to a wide spectrum of abnormal internal laterality phenotypes, including situs inversus and heterotaxy. In more than 90% of instances, the latter condition is accompanied by complex and severe cardiovascular malformations. Atrioventricular canal defect and transposition of the great arteries—which are particularly frequent in the setting of heterotaxy—are commonly found in situs solitus with or without genetic syndromes. Here, we review current data on morphogenesis of the heart in human beings and animal models, familial recurrence, and upstream genetic pathways of left–right determination in order to highlight how some isolated congenital heart diseases, very common in heterotaxy, even in the setting of situs solitus, may actually be considered in the pathogenetic field of laterality defects.

Development of the liver: Insights into organ and tissue morphogenesis

Recent development of improved tools and methods to analyse tissues at the three-dimensional level has expanded our capacity to investigate morphogenesis of foetal liver. Here, we review the key morphogenetic steps during liver development, from the prehepatic endoderm stage to the postnatal period, and consider several model organisms while focussing on the mammalian liver. We first discuss how the liver buds out of the endoderm and gives rise to an asymmetric liver. We next outline the mechanisms driving liver and lobe growth, and review morphogenesis of the intra- and extrahepatic bile ducts; morphogenetic responses of the biliary tract to liver injury are discussed. Finally, we describe the mechanisms driving formation of the vasculature, namely venous and arterial vessels, as well as sinusoids.

Vertebrate Left-Right Asymmetry: What Can Nodal Cascade Gene Expression Patterns Tell Us?

Laterality of inner organs is a wide-spread characteristic of vertebrates and beyond. It is ultimately controlled by the left-asymmetric activation of the Nodal signaling cascade in the lateral plate mesoderm of the neurula stage embryo, which results from a cilia-driven leftward flow of extracellular fluids at the left-right organizer. This scenario is widely accepted for laterality determination in wildtype specimens. Deviations from this norm come in different flavors. At the level of organ morphogenesis, laterality may be inverted (situs inversus) or non-concordant with respect to the main body axis (situs ambiguus or heterotaxia). At the level of Nodal cascade gene activation, expression may be inverted, bilaterally induced, or absent. In a given genetic situation, patterns may be randomized or predominantly lacking laterality (absence or bilateral activation). We propose that the distributions of patterns observed may be indicative of the underlying molecular defects, with randomizations being primarily caused by defects in the flow-generating ciliary set-up, and symmetrical patterns being the result of impaired flow sensing, on the left, the right, or both sides. This prediction, the reasoning of which is detailed in this review, pinpoints functions of genes whose role in laterality determination have remained obscure.

Cilia gene mutations cause atrioventricular septal defects by multiple mechanisms

Atrioventricular septal defects (AVSDs) are a common severe form of congenital heart disease (CHD). In this study we identified deleterious non-synonymous mutations in two cilia genes, Dnah11 and Mks1, in independent N-ethyl-N-nitrosourea-induced mouse mutant lines with heritable recessive AVSDs by whole-exome sequencing. Cilia are required for left/right body axis determination and second heart field (SHF) Hedgehog (Hh) signaling, and we find that cilia mutations affect these requirements differentially. Dnah11^avc4 did not disrupt SHF Hh signaling and caused AVSDs only concurrently with heterotaxy, a left/right axis abnormality. In contrast, Mks1^avc6 disrupted SHF Hh signaling and caused AVSDs without heterotaxy. We performed unbiased whole-genome SHF transcriptional profiling and found that cilia motility genes were not expressed in the SHF whereas cilia structural and signaling genes were highly expressed. SHF cilia gene expression predicted the phenotypic concordance between AVSDs and heterotaxy in mice and humans with cilia gene mutations. A two-step model of cilia action accurately predicted the AVSD/heterotaxyu phenotypic expression pattern caused by cilia gene mutations. We speculate that cilia gene mutations contribute to both syndromic and non-syndromic AVSDs in humans and provide a model that predicts the phenotypic consequences of specific cilia gene mutations.

PIERCE1 is critical for specification of left-right asymmetry in mice

The specification of left-right asymmetry of the visceral organs is precisely regulated. The earliest breakage of left-right symmetry occurs as the result of leftward flow generated by asymmetric beating of nodal cilia, which eventually induces asymmetric Nodal/Lefty/Pitx2 expression on the left side of the lateral plate mesoderm. PIERCE1 has been identified as a p53 target gene involved in the DNA damage response. In this study, we found that Pierce1-null mice exhibit severe laterality defects, including situs inversus totalis and heterotaxy with randomized situs and left and right isomerisms. The spectrum of laterality defects was closely correlated with randomized expression of Nodal and its downstream genes, Lefty1/2 and Pitx2. The phenotype of Pierce1-null mice most closely resembled that of mutant mice with impaired ciliogenesis and/or ciliary motility of the node. We also found the loss of asymmetric expression of Cerl2, the earliest flow-responding gene in the node of Pierce1-null embryos. The results suggest that Pierce1-null embryos have defects in generating a symmetry breaking signal including leftward nodal flow. This is the first report implicating a role for PIERCE1 in the symmetry-breaking step of left-right asymmetry specification.

Determining bronchial morphology for the purposes of segregating so-called heterotaxy

Introduction Heterotaxy is a unique clinical entity in which lateralisation of the thoraco-abdominal organs is abnormal, typically with isomerism of the bronchial tree and atrial appendages. This study was carried out to determine whether routine clinical imaging such as chest radiographs, angiographic images, and CT/MRI can determine bronchial isomerism, and how sidedness of bronchial isomerism correlates with overall features anticipated in hearts with isomeric atrial appendages. Methods and results We identified 73 patients with heterotaxy, in whom imaging clearly demonstrated the bronchial tree, seen at our institution since 1998. We calculated bronchial angles and lengths using all the available imaging modalities to determine the presence and sidedness of bronchial isomerism. This was then compared with the anticipated presence of isomeric atrial appendages based on the overall clinical findings, as the appendages themselves had not specifically been imaged. The ratio of bronchial lengths revealed bronchial isomerism in all patients, with bronchial angles permitting distinction of right as opposed to left isomerism. We noted discordances between the identified bronchial isomerism and the presumed arrangement of the atrial appendages in nearly 20% of the patients in our cohort.

CONCLUSION

Routine clinical imaging with chest radiographs, angiographic imaging, and CT/MRI can determine the presence of bronchial isomerism in patients with so-called heterotaxy. Right as opposed to left isomerism can be distinguished based on bronchial angles. The finding of bronchial isomerism correlates well, but not totally, with the presumed isomerism of the atrial appendages as predicted from the identified intra-cardiac morphology.

Left Right Patterning, Evolution and Cardiac Development

Many aspects of heart development are determined by the left right axis and as a result several congenital diseases have their origins in aberrant left-right patterning. Establishment of this axis occurs early in embryogenesis before formation of the linear heart tube yet impacts upon much later morphogenetic events. In this review I discuss the differing mechanisms by which left-right polarity is achieved in the mouse and chick embryos and comment on the evolution of this system. I then discus three major classes of cardiovascular defect associated with aberrant left-right patterning seen in mouse mutants and human disease. I describe phenotypes associated with the determination of atrial identity and venous connections, looping morphogenesis of the heart tube and finally the asymmetric remodelling of the embryonic branchial arch arterial system to form the leftward looped arch of aorta and associated great arteries. Where appropriate, I consider left right patterning defects from an evolutionary perspective, demonstrating how developmental processes have been modified in species over time and illustrating how comparative embryology can aide in our understanding of congenital heart disease.

Transposition of great arteries: new insights into the pathogenesis

Transposition of great arteries (TGA) is one of the most common and severe congenital heart diseases (CHD). It is also one of the most mysterious CHD because it has no precedent in phylogenetic and ontogenetic development, it does not represent an alternative physiological model of blood circulation and its etiology and morphogenesis are still largely unknown. However, recent epidemiologic, experimental, and genetic data suggest new insights into the pathogenesis. TGA is very rarely associated with the most frequent genetic syndromes, such as Turner, Noonan, Williams or Marfan syndromes, and in Down syndrome, it is virtually absent. The only genetic syndrome with a strong relation with TGA is Heterotaxy. In lateralization defects TGA is frequently associated with asplenia syndrome. Moreover, TGA is rather frequent in cases of isolated dextrocardia with situs solitus, showing link with defect of visceral situs. Nowadays, the most reliable method to induce TGA consists in treating pregnant mice with retinoic acid or with retinoic acid inhibitors. Following such treatment not only cases of TGA with d-ventricular loop have been registered, but also some cases of congenitally corrected transposition of great arteries (CCTGA). In another experiment, the embryos of mice treated with retinoic acid in day 6.5 presented Heterotaxy, suggesting a relationship among these morphologically different CHD. In humans, some families, beside TGA cases, present first-degree relatives with CCTGA. This data suggest that monogenic inheritance with a variable phenotypic expression could explain the familial aggregation of TGA and CCTGA. In some of these families we previously found multiple mutations in laterality genes including Nodal and ZIC3, confirming a pathogenetic relation between TGA and Heterotaxy. These overall data suggest to include TGA in the pathogenetic group of laterality defects instead of conotruncal abnormalities due to ectomesenchymal tissue migration.

How insights from cardiovascular developmental biology have impacted the care of infants and children with congenital heart disease

To illustrate the impact developmental biology and genetics have already had on the clinical management of the million infants born worldwide each year with CHD, we have chosen three stories which have had particular relevance for pediatric cardiologists, cardiothoracic surgeons, cardiac anesthesiologists, and cardiac nurses. First, we show how Margaret Kirby's finding of the unexpected contribution of an ectodermal cell population - the cranial neural crest - to the aortic arch arteries and arterial pole of the embryonic avian heart provided a key impetus to the field of cardiovascular patterning. Recognition that a majority of patients affected by the neurocristopathy DiGeorge syndrome have a chromosome 22q11 deletion, have also spurred tremendous efforts to characterize the molecular mechanisms contributing to this pathology, assigning a major role to the transcription factor Tbx1. Second, synthesizing the work of the last two decades by many laboratories on a wide gamut of metazoans (invertebrates, tunicates, agnathans, teleosts, lungfish, amphibians, and amniotes), we review the >20 major modifications and additions to the ancient circulatory arrangement composed solely of a unicameral (one-chambered), contractile myocardial tube and a short proximal aorta. Two changes will be discussed in detail - the interposition of a second cardiac chamber in the circulation and the septation of the cardiac ventricle. By comparing the developmental genetic data of several model organisms, we can better understand the origin of the various components of the multicameral (multi-chambered) heart seen in humans. Third, Martina Brueckner's discovery that a faulty axonemal dynein was responsible for the phenotype of the iv/iv mouse (the first mammalian model of human heterotaxy) focused attention on the biology of cilia. We discuss how even the care of the complex cardiac and non-cardiac anomalies seen in heterotaxy syndrome, which have long seemed impervious to advancements in surgical and medical intensive care, may yet yield to strategies grounded in a better understanding of the cilium. The fact that all cardiac defects seen in patients with full-blown heterotaxy can also be seen in patients without obvious laterality defects hints at important roles for ciliary function not only in left-right axis specification but also in cardiovascular morphogenesis. These three developmental biology stories illustrate how the remaining unexplained mortality and morbidity of congenital heart disease can be solved.

High prevalence of respiratory ciliary dysfunction in congenital heart disease patients with heterotaxy

BACKGROUND

Patients with congenital heart disease (CHD) and heterotaxy show high postsurgical morbidity/mortality, with some developing respiratory complications. Although this finding is often attributed to the CHD, airway clearance and left-right patterning both require motile cilia function. Thus, airway ciliary dysfunction (CD) similar to that of primary ciliary dyskinesia (PCD) may contribute to increased respiratory complications in heterotaxy patients.

METHODS AND RESULTS

We assessed 43 CHD patients with heterotaxy for airway CD. Videomicrocopy was used to examine ciliary motion in nasal tissue, and nasal nitric oxide (nNO) was measured; nNO level is typically low with PCD. Eighteen patients exhibited CD characterized by abnormal ciliary motion and nNO levels below or near the PCD cutoff values. Patients with CD aged >6 years show increased respiratory symptoms similar to those seen in PCD. Sequencing of all 14 known PCD genes in 13 heterotaxy patients with CD, 12 without CD, 10 PCD disease controls, and 13 healthy controls yielded 0.769, 0.417, 1.0, and 0.077 novel variants per patient, respectively. One heterotaxy patient with CD had the PCD causing DNAI1 founder mutation. Another with hyperkinetic ciliary beat had 2 mutations in DNAH11, the only PCD gene known to cause hyperkinetic beat. Among PCD patients, 2 had known PCD causing CCDC39 and CCDC40 mutations.

CONCLUSIONS

Our studies show that CHD patients with heterotaxy have substantial risk for CD and increased respiratory disease. Heterotaxy patients with CD were enriched for mutations in PCD genes. Future studies are needed to assess the potential benefit of prescreening and prophylactically treating heterotaxy patients for CD.

Heterotaxy and complex structural heart defects in a mutant mouse model of primary ciliary dyskinesia

Primary ciliary dyskinesia (PCD) is a genetically heterogeneous disorder associated with ciliary defects and situs inversus totalis, the complete mirror image reversal of internal organ situs (positioning). A variable incidence of heterotaxy, or irregular organ situs, also has been reported in PCD patients, but it is not known whether this is elicited by the PCD-causing genetic lesion. We studied a mouse model of PCD with a recessive mutation in Dnahc5, a dynein gene commonly mutated in PCD. Analysis of homozygous mutant embryos from 18 litters yielded 25% with normal organ situs, 35% with situs inversus totalis, and 40% with heterotaxy. Embryos with heterotaxy had complex structural heart defects that included discordant atrioventricular and ventricular outflow situs and atrial/pulmonary isomerisms. Variable combinations of a distinct set of cardiovascular anomalies were observed, including superior-inferior ventricles, great artery alignment defects, and interrupted inferior vena cava with azygos continuation. The surprisingly high incidence of heterotaxy led us to evaluate the diagnosis of PCD. PCD was confirmed by EM, which revealed missing outer dynein arms in the respiratory cilia. Ciliary dyskinesia was observed by videomicroscopy. These findings show that Dnahc5 is required for the specification of left-right asymmetry and suggest that the PCD-causing Dnahc5 mutation may also be associated with heterotaxy.

Development of the left-right axis

Left-right is not an axis in the conventional sense but rather two mirror-image proximodistal axes, upon which a quantal piece of positional information (leftness or rightness) is superimposed for laterally asymmetric organ development. We are attempting to establish the stages at which left-right is specified and determined, but this is complicated by the apparent loss of normal handed development in embryos that are cultured from pre-neural plate stages. Experiments suggest that left-right is determined by the first somite stage. The loss of normal left-right development in early cultures is probably not due to removal of some maternal signal, even though embryos do develop in vivo with their axes in a specific orientation relative to the uterus. The fact that there are two random embryonic axis orientations, 180 degrees opposed to one another, and that the axes of the two uterine horns are mirror-images of each other make it unlikely that the uterus could impart a sense of left-right to the embryo. The right ovary produces more eggs than the left one; this is reversed in iv/iv situs inversus mice. Analysis of iv/iv mice shows a correlation of left-right abnormalities with sex and close relationships between the abnormal left-right development of some organs, for example the heart and spleen, that have no obvious developmental connection.

Human situs inversus of the thoracoabdominal structures

Situs inversus totalis is well known, but its comprehensive description has been rare, especially on the internal view of the heart. For discussions based on recent results in developmental biology, the present study demonstrates a region- or part-specific manner of the inverted morphology found in a male donated cadaver and discusses the pathogenetic mechanisms of situs inversus in the human. Therein, clearly inverted morphologies existed in the coronary vessels, the apex position (dextracardia), connections between the heart and great vessels, the internal view of the atrium, the aortic arch with the three major branches, lung and liver segments and abdominal gastrointestinal tract. However, the ventricular internal view suggested incomplete laterality, such as tricuspid atrioventricular valves for both ventricles. The cardiac conductive system appeared not to be inverted but abnormal. The thoracic aorta and pulmonary artery took an L-spiral position with modifications. The inferior vena cava was located on the right side of the abdominal aorta. However, the left-sided kidney was located superior to the right-sided kidney. Similarly, the testicular vessels did not exhibit a clearly inverted morphology, but were almost normal. Therefore, the posterior mediastinal and retroperitoneal structures appeared to exhibit neutral laterality, incomplete inverted morphology or even normal morphology. According to the personal history and present histology, this specimen was unlikely to correspond to Kartagener's syndrome. The present observations seem to be consistent with recent findings in mutant models of laterality disturbances, in which a single gene or molecule is responsible for the changes in a region-specific manner.

Right atrial isomerism in four siblings

Heterotaxy syndromes, right or left atrial isomerism, result from disruption of left-right axis determination and their manifestations include complicated heart defects. Recent studies in model organisms have revealed complex genetic pathways and several genes involved in this process. In affected humans, however, molecular studies have identified mutations in a small number of individuals, while in most the cause remains unknown. Furthermore, although family data suggest, autosomal recessive inheritance, such genes have not yet been identified. We have studied six members of a family, four children affected with right atrial isomerism (RAI) and their healthy parents, for disturbances of left-right axis development. The children, one female and three males who all had complicated heart defects, succumbed and had an autopsy. Their nonconsanguineous parents were examined by cardiac and abdominal ultrasound or MRI. In all four children the heart defects included single ventricle with dysplastic atrioventricular (AV) valve, total anomalous pulmonary venous drainage (TAPVD), and malposition of great arteries (MGA) with pulmonary stenosis (PS). All had asplenia; two also had dextrocardia and abdominal situs inversus. The diagnosis of RAI was made postnatally in the first child and prenatally in others. Two siblings had no surgery and died as a newborn, one with obstructed supracardiac TAPVD and the other with regurgitating AV valve. Two children underwent heart surgery. One had repair of obstructive infracardiac TAPVD but died in infancy. The other underwent both hemi-Fontan operation and heart transplantation but died at the age of 2 years. This is the first report describing four children with RAI in the same family. The occurrence of RAI in male and female siblings without any indication of left-right axis abnormalities in their parents suggests autosomal recessive inheritance of human isomerism.

Genetics of human laterality disorders: insights from vertebrate model systems

Many internal organs in the vertebrate body are asymmetrically oriented along the left-right (L-R) body axis. Organ asymmetry and some components of the molecular signaling pathways that direct L-R development are highly conserved among vertebrate species. Although individuals with full reversal of organ L-R asymmetry (situs inversus totalis) are healthy, significant morbidity and mortality is associated with perturbations in laterality that result in discordant orientation of organ systems and complex congenital heart defects. In humans and other vertebrates, genetic alterations of L-R signaling pathways can result in a wide spectrum of laterality defects. In this review we categorize laterality defects in humans, mice, and zebrafish into specific classes based on altered patterns of asymmetric gene expression, organ situs defects, and midline phenotypes. We suggest that this classification system provides a conceptual framework to help consolidate the disparate laterality phenotypes reported in humans and vertebrate model organisms, thereby refining our understanding of the genetics of L-R development. This approach helps suggest candidate genes and genetic pathways that might be perturbed in human laterality disorders and improves diagnostic criteria.

[Genetics of congenital heart diseases]

New insights into the genetics of congenital heart diseases in human beings have been drawn these past years. The identification of genes for heart defects have led to a new clinical approach of these malformations in children and their families. These progresses have been made with the help of positional cloning as well as with the analysis of mouse models. These findings also yielded a new complexity in understanding the development of cardiac defects and led to revise the different classifications for congenital heart defects. Pediatric cardiologists have also improved their efficiency in defining cardiac phenotypes in affected individuals and in pedigrees with recurrent malformations. Genetic heterogeneity has made the molecular approach of a given defect difficult. In addition, intrafamilial variability still has scarce explanations. Finally, the contribution of epigenetic factors has to be kept in mind in specific conditions such as twin gestations. Clinical consequences of these findings remain at the present time limited for the patients themselves but in particular cases, genetic counseling has been dramatically improved.

Gene-dosage-sensitive genetic interactions between inversus viscerum (iv), nodal, and activin type IIB receptor (ActRIIB) genes in asymmetrical patterning of the visceral organs along the left-right axis

We have shown previously that mice deficient in the activin type IIB receptor (ActRIIB) exhibit right isomerism, which is characterized by mirror-image symmetrical right lungs, complex cardiac malformations, and hypoplasia of the spleen. These observations led us to hypothesize that the signaling of a TGF-beta family member by means of ActRIIB is necessary for the determination of the left-sidedness of the visceral organs. To test this hypothesis, we examined laterality defects in mice carrying mutations in both ActRIIB and inversus viscerum (iv) genes, because iv(-/-) mice display a spectrum of laterality defects, including situs inversus, right isomerism, and left isomerism. We found that all mice homozygous for both iv and ActRIIB mutations displayed the right isomerism. The phenotype of right isomerism in the double mutants was also more severe than that in ActRIIB(-/-) mice as shown by persistent left inferior vena cava, right atrial isomerism, and hypoplasia of spleen. Interestingly, the incidence of right isomerism also increased significantly in iv(-/-);ActRIIB(+/-) and iv(+/-);ActRIIB(-/-) mice compared with homozygous mice carrying either of single gene mutations. A mechanism of the genetic modulation between ActRIIB and iv genes may be that iv modulates the asymmetric expression of a TGF-beta family member that signals through activin type II receptors, ActRIIA and ActRIIB, to specify the "left-sidedness." Nodal is the most likely candidate. We show here that the penetrance and severity of the right isomerism is significantly elevated in nodal(+/-); ActRIIB(-/-) mice, compared with ActRIIB(-/-) mice. Furthermore, the chimeric mice derived from nodal(-/-) ES cells displayed right isomerism, indistinguishable from that in (iv(-/-);ActRIIB(-/-)) mice. We propose that iv functions to establish asymmetric expression of nodal in a gene-dosage-sensitive manner and that nodal signals through the activin type II receptors to specify the left-sidedness by means of a threshold mechanism.

Intrauterine diagnosis of heterotaxy syndrome

BACKGROUND

Heterotaxy syndrome, including right isomerism and left isomerism, is characterized by an abnormal symmetry of the viscera and veins and is frequently associated with complex cardiac anomalies. We sought to define the feasibility of in utero diagnosis and the postnatal outcome.

METHODS

Patients with heterotaxy syndrome were identified from 579 fetal echocardiograms performed from January 1994 to December 1998. The diagnosis was made on the basis of the fetal echocardiographic findings and was confirmed with autopsy or postnatal evaluation.

RESULTS

A total of 25 fetuses with right isomerism and 4 with left isomerism constitute the study population. The pregnancies of 7 fetuses (6 right and 1 left isomerism) were terminated before the 24th gestational week and subjected to autopsy. Twelve fetuses (10 right and 2 left isomerism) were lost to follow-up. Nine with right isomerism and 1 with left isomerism were delivered and underwent palliation. Among them, 5 patients (56%) with right isomerism died and more than half of the deaths occurred during infancy. The major cardiac anomalies detected and confirmed with postnatal evaluation or autopsy in fetuses with right isomerism were total anomalous pulmonary venous connection (6/15; 40%), common atrium (15/15; 100%), complete atrioventricular canal (15/15; 100%), double outlet right ventricle (15/15; 100%), and pulmonary stenosis (11/15; 73%). The major cardiac anomalies in fetuses with left isomerism were interruption of inferior vena cava (2/2; 100%), common atrium (1/2; 50%), and complete atrioventricular canal (1/2; 50%). Undetected lesions with fetal echocardiogram were abnormal pulmonary venous return to systemic veins in 1 case (sensitivity, 83%; 5/6; and specificity, 90%; 9/10) and outflow obstruction in 1 case (sensitivity, 91%; 11/12; and specificity, 67%; 2/3). Different patterns of rhythm disturbances were identified: supraventricular tachycardia in 1 case with right isomerism and sinus bradycardia with junctional rhythm in 3 cases with left isomerism (2 of them lost to follow-up). After birth, another 2 patients with right isomerism had supraventricular tachycardia, and 1 with left isomerism had sinus bradycardia develop at age 2 years.

CONCLUSION

Heterotaxy syndrome is usually detected in fetuses with the sonographic cardiac abnormalities. Visualization of the pulmonary venous return and outflow obstruction and characterization of the rhythm disturbances are feasible. However, in spite of prenatal diagnosis, the prognosis remains poor.

Classification of left-right patterning defects in zebrafish, mice, and humans

Numerous genes and developmental processes have been implicated in the establishment of the vertebrate left-right axis. Although the mechanisms that initiate left-right patterning may be distinct in different classes of vertebrates, it is clear that the asymmetric gene expression patterns of nodal, lefty, and pitx2 in the left lateral plate mesoderm are conserved and that left-right development of the brain, heart, and gut is tightly linked to the development of the embryonic midline. This review categorizes left-right patterning defects based on asymmetric gene expression patterns, midline phenotypes, and situs phenotypes. In so doing, we hope to provide a framework to assess the genetic bases of laterality defects in humans and other vertebrates.

Diffusion of nodal signaling activity in the absence of the feedback inhibitor Lefty2

The role of Lefty2 in left-right patterning was investigated by analysis of mutant mice that lack asymmetric expression of lefty2. These animals exhibited various situs defects including left isomerism. The asymmetric expression of nodal was prolonged and the expression of Pitx2 was upregulated in the mutant embryos. The absence of Lefty2 conferred on Nodal the ability to diffuse over a long distance. Thus, Nodal-responsive genes, including Pitx2, that are normally expressed on the left side were expressed bilaterally in the mutant embryos, even though nodal expression was confined to the left side. These results suggest that Nodal is a long-range signaling molecule but that its range of action is normally limited by the feedback inhibitor Lefty2.

Retinoic acid administration is associated with changes in the extracellular matrix and cardiac mesenchyme within the endocardial cushion

Retinoic acid has been associated with a number of cardiac defects, some of which seem to be related to changes in the endocardial cushions. Studies in mice and older chick embryos have suggested that these defects may be associated with a decrease in mesenchymal cell formation within the cushion. In a previous report we showed that retinoic acid lowered the number of mesenchymal cells in a culture bioassay of mesenchyme formation and that this response was due to retinoic acid modifying the production of particulate matrix from the myocardium. In this study, we have extended these observations to the embryo by implanting a retinoic acid coated bead into the embryo and examined the effect on cardiac mesenchyme formation and in the production of the particulate matrix. In all cases the addition of retinoic acid resulted in a decrease in the number of mesenchymal cells invading the endocardial cushions. In addition retinoic acid increased the production of hLAMP-1 and fibronectin but not transferrin, confirming our earlier report. Finally, we measured the volume of the cushion and calculated the cell density of both the inferior and superior cushions. The results suggest that the superior cushion is more sensitive to retinoic acid treatment than the inferior cushion. Collectively, these results support our earlier work that suggests that the mechanism of retinoic acid cardiac abnormalities involves a disruption in the production of particulate matrix from the myocardium and a subsequent decrease in cardiac mesenchyme cells that results in a malformed cardiac cushions.

Cardiopulmonary malformations in the inv/inv mouse

The inv/inv mouse carries an insertional mutation in the inversin gene, (inv, for inversion of embryonic turning). Previously it had been reported that almost 100% of the homozygous offspring (inv/inv) were characterized by situs inversus totalis. In this report we identify the spectrum of cardiopulmonary anatomical abnormalities in inv/inv mice surviving to birth to determine whether the abnormalities seen are of the categories classically associated with human situs abnormalities. Stillborn mice, offspring that died unexpectedly (within 48 hr after birth), and neonates with phenotypic characteristics of situs inversus (right-sided stomachs, growth failure or jaundice) were processed for standard histological examination. Of 173 offspring, 34 (20%) neonates (11 stillborn, 9 unexpected deaths, and 14 mice with situs inversus phenotype) were examined, 27 of which were genotyped to be inv/inv. Interestingly, three inv/inv mice (11%) were found to have situs solitus. Twenty-four had situs inversus with normal, mirror-image cardiac anatomy (dextrocardia with atrioventricular concordance, ventriculoarterial concordance and a right aortic arch). The overall incidence of cardiovascular anomalies observed was 10 out of 27 (37%). The most frequent severe malformation, identified in 3 out of 27 animals, was a complex consisting of pulmonary infundibular stenosis/atresia with absence of pulmonary valve tissue and a ventricular septal defect. The pulmonary phenotype in inv/inv mice was situs inversus with occasional minor lobar abnormalities. We conclude that 1) cardiopulmonary malformations in inv/inv mice are not rare (37%), 2) the cardiopulmonary malformations observed in inv/inv specimens are not of the spectrum typically associated with human heterotaxia. In particular, inv/inv mice have a propensity for defects in the development of the right ventricular outflow tract and the interventricular septum, and 3) approximately one out of ten inv/inv mice is born with situs solitus and shows cardiac anomalies that correspond to those observed in inv/inv specimens with situs inversus. Our data therefore suggest that inversin, the product of the inv locus, may have specific roles in cardiac morphogenesis independent of its role in situs determination.

Pitx2 expression defines a left cardiac lineage of cells: evidence for atrial and ventricular molecular isomerism in the iv/iv mice

The homeobox gene Pitx2 has been characterized as a mediator of left-right signaling in heart, gut, and lung morphogenesis. However, the relationship between the developmental role of Pitx2 and its expression pattern at the organ level has not been explored. In this study we focus on the role of Pitx2 in heart morphogenesis. Chicken Pitx2 transcripts are present in the left portion of the cardiac crescent and in the left side of the heart tube. Through looping Pitx2 is present in the left atrium, in the ventral portion of the ventricles and in the left-ventral part of the outflow tract. Mouse Pitx2 shows a similar developmental profile of expression. To test whether Pitx2 represents a lineage marker we have tagged the left portion of the chicken cardiac tube with fluorescent DiD. Labeled cells were found at HH16 in the left atrium and in the ventral region of the ventricles and the outflow tract. In the iv/iv mouse model of cardiac heterotaxia Pitx2 was abnormally expressed in the atrial and in the ventricular chambers. Furthermore, altered Pitx2 expression correlated with the occurrence of DORV. Our data reveal the existence of molecular isomerism not only in the atrial, but also in the ventricular compartment of the heart.

MLC3F transgene expression iniv mutant mice reveals the importance of left-right signalling pathways for the acquisition of left and right atrial but not ventricular compartment identity

Abstract Transcriptional differences between left and right cardiac chambers are revealed by an nlacZ reporter transgene controlled by regulatory sequences of the MLC3F gene, which is expressed in the left ventricle (LV), atrioventricular canal (AVC), and right atrium (RA). To examine the role of left-right signalling in the acquisition of left and right chamber identity, we have investigated MLC3F transgene expression in iv mutant mice. iv/iv mice exhibit randomised direction of heart looping and an elevated frequency of associated laterality defects, including atrial isomerism. At fetal stages, 3F-nlacZ-2E transgene expression remains confined to the morphological LV, AVC, and RA in L-loop hearts, although these appear on the opposite side of the body. In cases of morphologically distinguishable right atrial appendage isomerism, both atrial appendages show strong transgene expression. Conversely, specimens with morphological left atrial appendage isomerism show only weak expression in both atrial appendages. The earliest left-right atrial differences in the expression of the 3F-nlacZ-2E transgene are observed at E8.5. DiI labelling experiments confirmed that transcriptional regionalisation of the 3F-nlacZ-2E transgene at this stage reflects future atrial chamber identity. In some iv/iv embryos at E8.5, the asymmetry of 3F-nlacZ-2E expression was lost, suggesting atrial isomerism at the transcriptional level prior to chamber formation. These data suggest that molecular specification of left and right atrial but not ventricular chambers is dependent on left-right axial cues.

Arrangement of the systemic and pulmonary venous components of the atrial chambers in hearts with isomeric atrial appendages

BACKGROUND

The morphological definition of atrial chambers, and the determination of atrial laterality, are based on analysis of the structure of the atrial appendages. The systemic and pulmonary venous connections to the heart, nonetheless, are important in the management of patients having isomeric appendages. In this study, therefore, we analysed the morphology of the postero-superior walls of the atrial chambers so as to provide evidence concerning the morphogenetic background of those hearts, and to improve operative management.

METHODS

We reviewed 15 autopsied specimens with isomeric right appendages, and 10 with isomeric left appendages, paying particular attention to the morphology of the systemic and pulmonary venous connections. The postero-superior walls of the atrial chambers can be made up of the atrial body, the systemic venous components, or the pulmonary venous component. We analysed the contributions made by each of these components.

RESULTS

The postero-superior walls of the atrial chambers were markedly variable, but could be grouped into five patterns. Bilaterally well-developed systemic venous components and absence of the pulmonary venous component within the hypoplastic atrial body were present in 9 hearts with extracardiac pulmonary venous connections in the setting of right isomerism. Bilaterally well-developed systemic venous components, and a hypoplastic pulmonary venous component within the hypoplastic atrial body, were present in 5 hearts with intracardiac pulmonary venous connections in right isomerism. Bilaterally well-developed systemic venous components, and a hypoplastic pulmonary venous component within the sizable atrial body, were present in 1 heart with an intracardiac pulmonary venous connection in right isomerism. A well-developed pulmonary venous component within the atrial body, and hypoplasia of one systemic venous component, were present in 7 hearts with left isomerism. A well-developed pulmonary venous component within the atrial body, and hypoplasia of bilateral systemic venous components, were present in 3 hearts with left isomerism.

CONCLUSIONS

The postero-superior walls of the atrial chambers in hearts with isomeric atrial appendages can be analysed on the basis of a compound structure made of bilateral systemic venous components, a central pulmonary venous component, and the body of the atrium. Hearts with isomeric right appendages have absence or hypoplasia of the pulmonary venous component, while hearts with isomeric left appendages have hypoplastic systemic venous components.

Multiple pathways in the midline regulate concordant brain, heart and gut left-right asymmetry

The embryonic midline in vertebrates has been implicated in left-right development, but the mechanisms by which it regulates left-right asymmetric gene expression and organ morphogenesis are unknown. Zebrafish embryos have three domains of left-right asymmetric gene expression that are useful predictors of organ situs. cyclops (nodal), lefty1 and pitx2 are expressed in the left diencephalon; cyclops, lefty2 and pitx2 are expressed in the left heart field; and cyclops and pitx2 are expressed in the left gut primordium. Distinct alterations of these expression patterns in zebrafish midline mutants identify four phenotypic classes that have different degrees of discordance among the brain, heart and gut. These classes help identify two midline domains and several genetic pathways that regulate left-right development. A cyclops-dependent midline domain, associated with the prechordal plate, regulates brain asymmetry but is dispensable for normal heart and gut left-right development. A second midline domain, associated with the anterior notochord, is dependent on no tail, floating head and momo function and is essential for restricting asymmetric gene expression to the left side. Mutants in spadetail or chordino give discordant gene expression among the brain, heart and gut. one-eyed pinhead and schmalspur are necessary for asymmetric gene expression and may mediate signaling from midline domains to lateral tissues. The different phenotypic classes help clarify the apparent disparity of mechanisms proposed to explain left-right development in different vertebrates.

Suppression of atrial myosin gene expression occurs independently in the left and right ventricles of the developing mouse heart

Many cardiac genes are broadly expressed in the early heart and become restricted to the atria or ventricles as development proceeds. Additional transcriptional differences between left and right compartments of the embryonic heart have been described recently, in particular for a number of transgenes containing cardiac regulatory elements. We now demonstrate that three myosin genes which become transcriptionally restricted to the atria between embryonic day (E) 12.5 and birth, alpha-myosin heavy chain (MHC), myosin light chain (MLC) 1A and MLC2A, are coordinately downregulated in the compact myocardium of the left ventricle before that of the right ventricle. alpha-MHC protein also accumulates in the right, but not left, compact ventricular myocardium during this period, suggesting that this transient regionalization contributes to fktal heart function. dHAND and eHAND, basic helix-loop-helix transcription factors known to be expressed in the right and left ventricles respectively at E10. 5, remain regionalized between E12.5 and E14.5. Downregulation of alpha-MHC, MLC1A, and MLC2A in iv/iv embryos, which have defective left/right patterning, initiates in the morphological left (systemic) ventricle regardless of its anatomical position on the right or left hand side of the heart. This points to the importance of left/right ventricular differences in sarcomeric gene expression patterns during fktal cardiogenesis and indicates that these differences originate in the embryo in response to anterior-posterior patterning of the heart tube rather than as a result of cardiac looping. Dev Dyn 2000;217:75-85.

Embryonic origins of spleen asymmetry

The spleen is a vertebrate organ that has both hematopoietic and immunologic function. The embryonic origins of the spleen are obscure, with most studies describing the earliest rudiment of the spleen as a condensation of mesodermal mesenchyme on the left side of the dorsal mesogastrium. The development of spleen handedness has not been described previously, presumably because of the difficulty in assaying spleen position in the embryo and the lack of early, organ-specific molecular markers. Here we show that expression of the homeobox gene Nkx2-5 serves as a marker for spleen precursor tissue. Pre-splenic tissue is initially located in symmetric domains on both sides of the embryo but, during subsequent development, only the left side goes on to form the mature spleen. Therefore, the final location of the spleen on the left side of the body axis appears to result from preferential development of the spleen precursor cells on the left side of the embryo. Our studies indicate that the spleen and heart become asymmetric via different cellular mechanisms. Nkx2-5 may function locally as part of the laterality cascade, downstream of nodal and Pitx2, or it may direct asymmetric morphogenesis after laterality has been determined.

Associations between chromosomal anomalies and congenital heart defects: a database search

Recent technical advances in molecular biology and cytogenetics, as well as a more developmental approach to congenital heart disorders (CHDs), have led to considerable progress in our understanding of their pathogenesis, especially of the important causative role of genetic factors. The complex embryology of the heart suggests the involvement of numerous genes, and hence, numerous chromosomal loci, such as the recently identified 22q11, in normal cardiomorphogenesis. In order to identify other loci, the Human Cytogenetics DataBase was searched for all chromosome anomalies associated with CHD. Through the application of several (arbitrary) criteria we have selected associations occurring so frequently that they may not be forfuituous, suggesting assignment of a gene or genes responsible for specific CHDs to certain chromosome regions. The results of this study may be a first step in the detection of specific chromosome defects responsible for CHD, which will be useful in daily patient care and may provide clues for further cytogenetic and molecular studies.

The role of the brachyury gene in heart development and left-right specification in the mouse

The midline has a theoretical role in the development of left-right asymmetry, and this is supported by both genetic analyses and experimental manipulation of midline structures in vertebrates. The mouse brachyury (T) gene encodes a transcription factor which is expressed in the developing notochord and is required for its development. T/T mice lack a mature notochord and have a dorsalised neural tube. We have examined the hearts of T/T mice and have found consistent morphological abnormalities, resulting in ventrally displaced ventricular loops, and a 50% incidence of inverted heart situs. Three TGF-beta related genes, lefty-1, lefty-2 and nodal, are expressed asymmetrically in mouse embryos, and are implicated in the development of situs. We find that nodal, which is normally expressed around the node and in left lateral plate mesoderm in early somite embryos, is completely absent at this stage in T/T embryos. In contrast, lefty-1 and lefty-2, which are normally expressed in the left half of prospective floorplate and left lateral plate mesoderm, respectively, are both expressed in T/T embryos only in a broad patch of ventral cells in, and just rostral to, the node region. These results implicate the node as a source of instructive signals driving expression of nodal and lefty-2 in the left lateral plate mesoderm, and being required for normal looping and situs of the heart.

Diagnosis of heterotaxy syndrome by fetal echocardiography

We report a series of 13 consecutive patients diagnosed in utero with heterotaxy syndrome in which we found a slight predominance (8 of 13) of fetuses with right atrial isomerism. In previous studies in which diagnosis of left or right isomerism was made based upon findings at fetal echocardiography, there has been a preponderance of infants (95%) diagnosed with left isomerism.

Defective lateralisation in children with congenitally malformed hearts

Defects in lateralization can be studied from the stance of populations, the individual, or the systems of organs within each individual. Unfortunately, and confusingly, the same terms are being applied to each of these situations, but inevitably with different meanings. Thus, there is presently no consensus on how we should use terms such as "heterotaxy" and "situs ambiguus". By far the least ambiguous use of these words is encountered when they are applied to the organs. In fact, each system of organs can accurately and simply be described in terms of its left-right morphology. All those organs which are paired then can be described, when interpreted on the basis of their intrinsic morphology, as being usually arranged, mirror-imaged, or as showing left or right isomerism. Within the heart, these changes are seen only in the atrial segment. The criterion for distinction of rightness or leftness within the atrial segment is the extent of the pectinate muscles relative to the atrioventricular junction. Application of this criterion permits unequivocal recognition of symmetry as opposed to lateralization. The same holds good for the other organs. Within any individual organ, therefore, the situation is neither ambiguous nor heterotaxic. Instead, it is lateralised or symmetrical. Within the individual, in contrast, there may well be discrepancies in the expected disposition of the systems of organs which produces potential ambiguity. To dispel this ambiguity, it is necessary to provide a full catalogue. For example, persons with otherwise normally arranged organs may have left bronchial isomerism. Other persons may have discordance between the thoracic organs, which are usually arranged, and the abdominal organs, which are mirror-imaged, but no evidence of isomerism. Within the population, however, we are unaware of any genetically or environmentally induced syndrome in which all individuals show evidence of mirror-imagery, or of isomerism, or of specific discordance between the systems. In fact, all known syndromes encompass all types of defective lateralization. When attempting to identify the genetic mechanisms for production of the syndromes, therefore, it could be positively misleading to attempt to separate isomerism from other perceived forms of"heterotaxy". Our preference is to consider any deviation from the usual arrangement as heterotaxy, and to specify the specific arrangement of the organs within each malformed individual.

Mutation of the mouse hepatocyte nuclear factor/forkhead homologue 4 gene results in an absence of cilia and random left-right asymmetry

Winged helix transcription factors play important roles in cellular differentiation and cell-specific gene expression. To define the role of the winged helix factor hepatocyte nuclear factor/forkhead homologue (HFH)-4, a targeted mutation was created in the mouse hfh-4 gene. No expression of HFH-4 was detected in hfh-4(-)/- mice by RNA blot analysis, in situ hybridization, or RT-PCR. hfh-4(-)/- mice were noted to have abnormalities of organ situs consistent with random determination of left-right asymmetry. In addition, a complete absence of cilia was noted in hfh-4(-)/- mice. The hfh-4 gene is thus essential for nonrandom determination of left-right asymmetry and development of ciliated cells. Homozygous mutant mice also exhibited prenatal and postnatal growth failure, perinatal lethality and, in some cases, hydrocephalus. RT-PCR revealed an absence of left-right dynein (lrd) expression in the embryonic lungs of hfh-4(-)/- mice, suggesting that HFH-4 may act by regulating expression of members of the dynein family of genes. The abnormalities in ciliary development and organ situs in hfh-4(-)/- mice are similar to those observed in human congenital syndromes such as Kartagener syndrome. Targeted mutation of hfh-4 thus provides a model for elucidating the mechanisms regulating ciliary development and determination of left-right asymmetry.

lefty-1 is required for left-right determination as a regulator of lefty-2 and nodal

lefty-1, lefty-2, and nodal are expressed on the left side of developing mouse embryos and are implicated in left-right (L-R) determination. The role of lefty-1 was examined by analyzing mutant mice lacking this gene. The lefty-1-deficient mice showed a variety of L-R positional defects in visceral organs. Unexpectedly, however, the most common feature of lefty-1-/- mice was thoracic left isomerism (rather than right isomerism). The lack of lefty-1 resulted in bilateral expression of nodal, lefty-2, and Pitx2 (a homeobox gene normally expressed on the left side). These observations suggest that the role of lefty-1 is to restrict the expression of lefty-2 and nodal to the left side, and that lefty-2 or nodal encodes a signal for "leftness."

Anomalous looping, atrioventricular cushion dysplasia, and unilateral ventricular hypoplasia in the mouse embryos with right isomerism induced by retinoic acid

BACKGROUND

Visceroatrial heterotaxy syndrome is characterized by abnormality of visceral laterality and complex cardiovascular anomalies usually involving both the outflow and inflow tract. Morishima et al. (1995) showed that mouse embryos treated with all-trans retinoic acid at embryonic day 6.5 (primitive streak stage) induces this syndrome.

METHODS

To investigate the morphogenetic process of visceroatrial heterotaxy syndrome, we examined retinoic acid-treated mouse embryos at embryonic days 9-15 using scanning electron microscopy.

RESULTS

The sinoatrial connection was first distinguished for the determination of situs as early as at embryonic day 10.5. Normal visceroatrial situs was found in 57% of all treated embryos, and the rest had abnormal situs, in which right isomerism was found in 81%. In the right-isomeric mouse, the cardiac morphology was characterized by abnormal looping together with dysplasia of the inflow and outflow tract cushion; that is, the primitive right ventricle was usually deviated cranially to various degrees, the atrioventricular cushion appeared trilobed in a half of them, and unilateral ventricular hypoplasia was noted in about one-third of them after embryonic day 14.5.

CONCLUSIONS

An anomalous relation between the atrioventricular cushions and the interventricular septum appeared to have caused a restrictive inflow to the unilateral ventricle, leading to ventricular chamber hypoplasia on the ipsilateral side. Thus, we clarified that retinoic-acid treatment at the primitive streak stage disturbed cardiac looping and formation of atrioventricular cushion development, which secondarily influenced ventricular chamber development.

Usefulness of electron beam computed tomography in children with heterotaxy syndrome

Children with heterotaxy syndrome frequently have complex cardiac and noncardiac malformations requiring detailed diagnostic evaluation for management planning. Direct delineation of these structures by electron beam computed tomography (EBCT) is validated as a means of diagnosis. From July 1995 to March 1997, 32 patients (16 girls, 16 boys) with clinically impressed heterotaxy syndrome were enrolled in this study. After evaluation by echocardiography, EBCT studies were performed. Interpretation of cardiac anomalies was performed by sequential analysis based on these cross-sectional images. The diagnoses were subsequently confirmed by angiocardiography and surgical findings. Twenty-eight patients had bilateral trifurcated bronchi, and most of these (24 of 28) did not have a spleen. Four patients had bilateral bifurcated bronchi, 2 patients had polysplenia, and the other 2 patients had a lobulated single spleen. We found that laterality could be identified by EBCT in all patients. Comparison of diagnostic yield between echocardiography, catheterization, and EBCT showed that EBCT is superior to echocardiography and catheterization in demonstration of pulmonary venous anatomy and presence of a very small rudimentary ventricle. In addition, associated visceral, bronchopulmonary, mediastinal, and intracardiac anomalies could all be clearly delineated by EBCT at the same time. Thus, EBCT is a promising complementary modality for an overall understanding of heterotaxy syndrome.

The signaling pathway mediated by the type IIB activin receptor controls axial patterning and lateral asymmetry in the mouse

Vertebrate animals exhibit segmented axial skeletons and lateral asymmetry of the visceral organs. The segment identity of individual vertebrae is believed to be determined by a combination of functionally active Hox genes that have defined expression boundaries along the anteroposterior axis (known as the axial Hox code). Disturbance of the Hox code by ectopic expression or mutation of Hox genes often leads to homeotic transformation of the vertebrae. Largely unknown, however, are the signaling molecules that provide the positional cues for the precise establishment and maintenance of the Hox code. In this study we show that disruption of the type IIB activin receptor (ActRIIB) by gene targeting results in altered expression of multiple Hox genes and abnormal patterning of the vertebrae, similar to but severer than retinoic acid (RA)-induced anterior transformation. We further show that RA and ActRIIB mutation have synergistic effects on vertebral patterning. Activin, Vg-1 and, type II activin receptors have been implicated in regulation of lateral asymmetry during chick and Xenopus development. We show here that the ActRIIB-/- mice die after birth with complicated cardiac defects including randomized heart position, malposition of the great arteries, and ventricular and atrial septal defects. In addition, the heart anomalies are associated with right pulmonary isomerism and splenic abnormalities, recapitulating the clinical symptoms of the human asplenia syndrome. These findings provide genetic evidence that the ActRIIB-mediated signaling pathway plays a critical role in patterning both anteroposterior and left-right axes in vertebrate animals.

Homeodomain factor Nkx2-5 controls left/right asymmetric expression of bHLH gene eHand during murine heart development

One of the first morphological manifestations of left/right (L/R) asymmetry in mammalian embryos is a pronounced rightward looping of the linear heart tube. The direction of looping is thought to be controlled by signals from an embryonic L/R axial system. We report here that morphological L/R asymmetry in the murine heart first became apparent at the linear tube stage as a leftward displacement of its caudal aspect. Beginning at the same stage, the basic helix-loop-helix (bHLH) factor gene eHand was expressed in a strikingly left-dominant pattern in myocardium, reflecting an intrinsic molecular asymmetry. In hearts of embryos lacking the homeobox gene Nkx2-5, which do not loop, left-sided eHand expression was abolished. However, expression was unaffected in Sc1-/- hearts that loop poorly because of hematopoietic insufficiency, and was right-sided in hearts of inv/inv embryos that display situs inversus. The data predict that eHand expression is enhanced in descendants of the left heart progenitor pool as one response to inductive signaling from the L/R axial system, and that eHand controls intrinsic morphogenetic pathways essential for looping. One aspect of the intrinsic response to L/R information falls under Nkx2-5 homeobox control.

Fashioning the vertebrate heart: earliest embryonic decisions

Our goal here is to set out the types of unitary decisions made by heart progenitor cells, from their appearance in the heart field until they form the simple heart tube. This provides a context to evaluate cell fate, lineage and, finally, morphogenetic decisions that configure global heart form and function. Some paradigms for cellular differentiation and for pattern generation may be borrowed from invertebrates, but neither Drosophila nor Caenorhabditis elegans suffice to unravel higher order decisions. Genetic analyses in mouse and zebrafish may provide one entrance to these pathways.

Left-right asymmetry of a nodal-related gene is regulated by dorsoanterior midline structures during Xenopus development

Development of asymmetry along the left-right axis is a critical step in the formation of the vertebrate body plan. Disruptions of normal left-right patterning are associated with abnormalities of multiple organ systems, including significant congenital heart disease. The mouse nodal gene, and its homologues in chick and Xenopus, are among the first genes known to be asymmetrically expressed along the left-right axis before the development of organ asymmetry. Alterations in the expression pattern of mouse nodal and the chick homologue (cNR-1) have been associated with defects in the development of left-right asymmetry and cardiac looping (Levin, M., Johnson, R. L., Stern, C. D., Kuehn, M. and Tabin, C. (1995) Cell 82, 803–814; Collignon, J., Varlet, I. and Robertson, E. J. (1996) Nature 381, 155–158; Lowe, L. A., Supp, D. M., Sampath, K., Yokoyama, T., Wright, C. V. E., Potter, S. S., Overbeek, P. and Kuehn, M. R. (1996) Nature 381, 158–161). Here, we show that the normal expression patterns of the Xenopus nodal-related gene (Xnr-1) are variable in a large population of embryos and that Xnr-1 expression is altered by treatments that perturb normal left-right development. The incidence of abnormal Xnr-1 expression patterns correlates well with cardiac reversal rates in both control and experimentally treated Xenopus embryos. Furthermore, dorsal midline structures, including notochord and/or hypochord and neural floorplate, regulate Xnr-1 expression prior to the specification of cardiac left-right orientation by repression of Xnr-1 expression in the right lateral plate mesoderm during closure of the neural tube. The correlation of Xnr-1 expression and orientation of cardiac looping suggests that Xnr-1 is a component of the left-right signaling pathway required for the specification of cardiac orientation in Xenopus, and that dorsal midline structures normally act to repress the signaling pathway on the right side of the embryo.