Fetal Situs, Isomerism, Heterotaxy Syndrome: Diagnostic Evaluation and Implication for Postnatal Management

Zebrafish Congenital Heart Disease Models: Opportunities and Challenges

Congenital heart defects (CHDs) are common human birth defects. Genetic mutations potentially cause the exhibition of various pathological phenotypes associated with CHDs, occurring alone or as part of certain syndromes. Zebrafish, a model organism with a strong molecular conservation similar to humans, is commonly used in studies on cardiovascular diseases owing to its advantageous features, such as a similarity to human electrophysiology, transparent embryos and larvae for observation, and suitability for forward and reverse genetics technology, to create various economical and easily controlled zebrafish CHD models. In this review, we outline the pros and cons of zebrafish CHD models created by genetic mutations associated with single defects and syndromes and the underlying pathogenic mechanism of CHDs discovered in these models. The challenges of zebrafish CHD models generated through gene editing are also discussed, since the cardiac phenotypes resulting from a single-candidate pathological gene mutation in zebrafish might not mirror the corresponding human phenotypes. The comprehensive review of these zebrafish CHD models will facilitate the understanding of the pathogenic mechanisms of CHDs and offer new opportunities for their treatments and intervention strategies.

Molecular Pathways and Animal Models of Atrioventricular Septal Defect

The development of a fully functional four-chambered heart is critically dependent on the correct formation of the structures that separate the atrial and ventricular chambers. Perturbation of this process typically results in defects that allow mixing of oxygenated and deoxygenated blood. Atrioventricular septal defects (AVSD) form a class of congenital heart malformations that are characterized by the presence of a primary atrial septal defect (pASD), a common atrioventricular valve (cAVV), and frequently also a ventricular septal defect (VSD). While AVSD were historically considered to result from failure of the endocardial atrioventricular cushions to properly develop and fuse, more recent studies have determined that inhibition of the development of other components of the atrioventricular mesenchymal complex can lead to AVSDs as well. The role of the dorsal mesenchymal protrusion (DMP) in AVSD pathogenesis has been well-documented in studies using animal models for AVSDs, and in addition, preliminary data suggest that the mesenchymal cap situated on the leading edge of the primary atrial septum may be involved in certain situations as well. In this chapter, we review what is currently known about the molecular mechanisms and animal models that are associated with the pathogenesis of AVSD.

Getting to the Heart of Left-Right Asymmetry: Contributions from the Zebrafish Model

The heart is laterally asymmetric. Not only is it positioned on the left side of the body but the organ itself is asymmetric. This patterning occurs across scales: at the organism level, through left-right axis patterning; at the organ level, where the heart itself exhibits left-right asymmetry; at the cellular level, where gene expression, deposition of matrix and proteins and cell behaviour are asymmetric; and at the molecular level, with chirality of molecules. Defective left-right patterning has dire consequences on multiple organs; however, mortality and morbidity arising from disrupted laterality is usually attributed to complex cardiac defects, bringing into focus the particulars of left-right patterning of the heart. Laterality defects impact how the heart integrates and connects with neighbouring organs, but the anatomy of the heart is also affected because of its asymmetry. Genetic studies have demonstrated that cardiac asymmetry is influenced by left-right axis patterning and yet the heart also possesses intrinsic laterality, reinforcing the patterning of this organ. These inputs into cardiac patterning are established at the very onset of left-right patterning (formation of the left-right organiser) and continue through propagation of left-right signals across animal axes, asymmetric differentiation of the cardiac fields, lateralised tube formation and asymmetric looping morphogenesis. In this review, we will discuss how left-right asymmetry is established and how that influences subsequent asymmetric development of the early embryonic heart. In keeping with the theme of this issue, we will focus on advancements made through studies using the zebrafish model and describe how its use has contributed considerable knowledge to our understanding of the patterning of the heart.

The Prenatal Origins of Human Brain Asymmetry: Lessons Learned from a Cohort of Fetuses with Body Lateralization Defects

Knowledge about structural brain asymmetries of human fetuses with body lateralization defects-congenital diseases in which visceral organs are partially or completely incorrectly positioned-can improve our understanding of the developmental origins of hemispheric brain asymmetry. This study investigated structural brain asymmetry in 21 fetuses, which were diagnosed with different types of lateralization defects; 5 fetuses with ciliopathies and 26 age-matched healthy control cases, between 22 and 34 gestational weeks of age. For this purpose, a database of 4007 fetal magnetic resonance imagings (MRIs) was accessed and searched for the corresponding diagnoses. Specific temporal lobe brain asymmetry indices were quantified using in vivo, super-resolution-processed MR brain imaging data. Results revealed that the perisylvian fetal structural brain lateralization patterns and asymmetry indices did not differ between cases with lateralization defects, ciliopathies, and normal controls. Molecular mechanisms involved in the definition of the right/left body axis-including cilium-dependent lateralization processes-appear to occur independently from those involved in the early establishment of structural human brain asymmetries. Atypically inverted early structural brain asymmetries are similarly rare in individuals with lateralization defects and may have a complex, multifactorial, and neurodevelopmental background with currently unknown postnatal functional consequences.

Cardiac Development: A Glimpse on Its Translational Contributions

Cardiac development is a complex developmental process that is initiated soon after gastrulation, as two sets of precardiac mesodermal precursors are symmetrically located and subsequently fused at the embryonic midline forming the cardiac straight tube. Thereafter, the cardiac straight tube invariably bends to the right, configuring the first sign of morphological left–right asymmetry and soon thereafter the atrial and ventricular chambers are formed, expanded and progressively septated. As a consequence of all these morphogenetic processes, the fetal heart acquired a four-chambered structure having distinct inlet and outlet connections and a specialized conduction system capable of directing the electrical impulse within the fully formed heart. Over the last decades, our understanding of the morphogenetic, cellular, and molecular pathways involved in cardiac development has exponentially grown. Multiples aspects of the initial discoveries during heart formation has served as guiding tools to understand the etiology of cardiac congenital anomalies and adult cardiac pathology, as well as to enlighten novels approaches to heal the damaged heart. In this review we provide an overview of the complex cellular and molecular pathways driving heart morphogenesis and how those discoveries have provided new roads into the genetic, clinical and therapeutic management of the diseased hearts.

Unusual long survival in a case of heterotaxy and polysplenia

Heterotaxy syndrome with polysplenia is an extremely rare congenital disorder caused by a disruption in the embryonic development that results in an abnormal arrangement of the abdominal and thoracic organs. We present the case of a 59-year-old female patient with invasive ductal carcinoma of the right breast (luminal A type) and CT findings of heterotaxy syndrome with polysplenia. The most remarkable anomalies identified were a left inferior vena cava draining into the hemiazygos vein, absent inferior vena cava at the thoracic level, and hepatic veins directly draining into the right atrium. Moreover, an atrial septal defect was identified, explaining the pulmonary hypertension of unknown cause previously detected in the patient. The relevance of this case lies in the unusual anatomical abnormalities found and the large patient survival, having in to account the great rate of heterotaxy syndrome mortality in the first years of life.

Fetal cardiac abnormalities: Genetic etiologies to be considered

Congenital heart diseases are a common prenatal finding. The prenatal identification of an associated genetic syndrome or a major extracardiac anomaly helps to understand the etiopathogenic diagnosis. Besides, it also assesses the prognosis, management, and familial recurrence risk while strongly influences parental decision to choose termination of pregnancy or postnatal care. This review article describes the most common genetic diagnoses associated with a prenatal finding of a congenital heart disease and a suggested diagnostic process.

Rethinking what constitutes a diagnosis in the genomics era: a critical illness perspective

PURPOSE OF REVIEW

The purpose of this review is to highlight the significant advances in the testing, interpretation, and diagnosis of genetic abnormalities in critically ill children and to emphasize that pediatric intensivists are uniquely positioned to search for genetic diagnoses in these patients.

RECENT FINDINGS

Ten years following the first clinical diagnosis made through whole exome sequencing, we remain in the dark about the function of roughly 75% of our genes. However, steady advancements in molecular techniques, particularly next-generation sequencing, have spurred a rapid expansion of our understanding of the genetic underpinnings of severe congenital diseases. This has resulted in not only improved clinical diagnostics but also a greater availability of research programs actively investigating rare, undiagnosed diseases. In this background, the scarcity of clinical geneticists compels nongeneticists to familiarize themselves with the types of patients that could benefit from genetic testing, interpretations of test results as well as the available resources for these patients.

SUMMARY

When caring for seriously ill children, critical care pediatricians should actively seek the possibility of an underlying genetic cause for their patients' conditions. This is true even in instances when a child has a descriptive diagnosis without a clear underlying molecular genetic mechanism. By promoting such diagnostics, in both clinical and research settings, pediatric intensivists can advance the care of their patients, improve the quality of information provided to families, and contribute to the knowledge of broad fields in medicine.

Successful Palliation via Kawashima Procedure of an Infant With Heterotaxy Syndrome and Left-Atrial Isomerism

Background

Heterotaxy is a condition of abnormal lateralization of organs across the body's left-right axis, causing multiple congenital malformations. The anatomic manifestations of heterotaxy syndrome generally follow one of two patterns, referred to as right atrial isomerism (with two similar right atria and duplication of right-sided features of multiple organs) and left atrial isomerism (with two similar left atria and duplication of left-sided features of multiple organs). Cardiac surgical intervention for patients with heterotaxy syndrome depends on ventricular physiology and circulatory balance. For patients with single-ventricle physiology, a Fontan operation, which directs systemic venous return to the pulmonary arteries, is the definitive intervention. Prior to a Fontan operation, many patients require one or more palliative surgeries (eg, a Blalock-Taussig-Thomas shunt or bidirectional Glenn/Kawashima procedure) to prepare them for definitive correction.

Case Report

We present the case of a term female neonate who was transferred to our pediatric cardiovascular intensive care unit for management of suspected congenital cardiac disease. Echocardiography confirmed the diagnosis of heterotaxy syndrome with left atrial isomerism, an interrupted inferior vena cava with azygos continuation, and a hypoplastic left ventricle with single-ventricle physiology. At 11 months of age, she underwent a Kawashima procedure with subtotal pulmonary artery ligation. She tolerated the procedure well and is anticipated to remain stable for the near future, possibly without the need for further cardiac surgery.

Conclusion

Patients with heterotaxy syndrome have congenital malformations in several organ systems, requiring lifelong coordination of care among health providers across multiple disciplines.

Effect of Prenatal Laterality Disturbance and Its Accompanying Anomalies on Survival

In this retrospective, observational study of fetuses diagnosed with a laterality disturbance we describe the findings and outcome of fetuses diagnosed between 1980 and 2017 at a tertiary fetal-pediatric cardiology unit. In addition we sought to identify features which impact on outcome. Left atrial isomerism (LAI) was diagnosed in 177 babies and right atrial isomerism (RAI) in 100. Major structural heart disease was present in all cases of RAI and 91% with LAI. Complete heart block (CHB) was present in 40% of LAI. For surviving live-born infants a biventricular circulation was feasible in 3% with RAI and 43% with LAI. The median survival for live-borns with LAI was 13 months (range 0 to 272 months) and for RAI 19 months (range 0.3 to 292 months). The median postnatal survival with CHB was 0.2 months (range 0 to 228 months) compared to 44 months with sinus rhythm (interquartile range 0 to 272 months; p <0.0001). The 5-year survival was 1980 to 1989, RAI 0%, LAI 0%; 1990 to 1999, RAI 62%, LAI 54%; 2000 to 2009, RAI 59%, LAI 53%; 2010 to 2017, RAI 67%, LAI 75% by era. The rate of intrauterine death remained. Risk factors for death/transplantation for RAI were total anomalous pulmonary venous drainage, left heart obstruction (hazard ratios 2.7, p = 0.048; 5.8, p = 0.03) and for LAI: CHB, anomalous pulmonary venous drainage and right heart obstruction (hazard ratios 11.5, 6.2, 3.8, respectively (p = 0.008, p = 0.003, p <0.001)). In conclusion, laterality disturbances represent a complex form of congenital heart disease and although survival is improved, it remains poor especially in the presence of anomalous pulmonary venous drainage, stenotic and/or atretic valves, and CHB.