Heterotaxy-spectrum heart defects in Zic3 hypomorphic mice

Congenital heart defects caused by FOXJ1

FOXJ1 is expressed in ciliated cells of the airways, testis, oviduct, central nervous system and the embryonic left-right organizer. Ablation or targeted mutation of Foxj1 in mice, zebrafish and frogs results in loss of ciliary motility and/or reduced length and number of motile cilia, affecting the establishment of the left-right axis. In humans, heterozygous pathogenic variants in FOXJ1 cause ciliopathy leading to situs inversus, obstructive hydrocephalus and chronic airway disease. Here, we report a novel truncating FOXJ1 variant (c.784_799dup; p.Glu267Glyfs*12) identified by clinical exome sequencing from a patient with isolated congenital heart defects (CHD) which included atrial and ventricular septal defects, double outlet right ventricle (DORV) and transposition of the great arteries. Functional experiments show that FOXJ1 c.784_799dup; p.Glu267Glyfs*12, unlike FOXJ1, fails to induce ectopic cilia in frog epidermis in vivo or to activate the ADGB promoter, a downstream target of FOXJ1 in cilia, in transactivation assays in vitro. Variant analysis of patients with heterotaxy or heterotaxy-related CHD indicates that pathogenic variants in FOXJ1 are an infrequent cause of heterotaxy. Finally, we characterize embryonic-stage CHD in Foxj1 loss-of-function mice, demonstrating randomized heart looping. Abnormal heart looping includes reversed looping (dextrocardia), ventral looping and no looping/single ventricle hearts. Complex CHDs revealed by histological analysis include atrioventricular septal defects, DORV, single ventricle defects as well as abnormal position of the great arteries. These results indicate that pathogenic variants in FOXJ1 can cause isolated CHD.

Loss of Zic3 impairs planar cell polarity leading to abnormal left-right signaling, heart defects and neural tube defects

Loss of function of ZIC3 causes heterotaxy (OMIM #306955), a disorder characterized by organ laterality defects including complex heart defects. Studies using Zic3 mutant mice have demonstrated that loss of Zic3 causes heterotaxy due to defects in establishment of left-right (LR) signaling, but the mechanistic basis for these defects remains unknown. Here, we demonstrate Zic3 null mice undergo cilia positioning defects at the embryonic node consistent with impaired planar cell polarity (PCP). Cell-based assays demonstrate that ZIC3 must enter the nucleus to regulate PCP and identify multiple critical ZIC3 domains required for regulation of PCP signaling. Furthermore, we show that Zic3 displays a genetic interaction with the PCP membrane protein Vangl2 and the PCP effector genes Rac1 and Daam1 resulting in increased frequency and severity of neural tube and heart defects. Gene and protein expression analyses indicate that Zic3 null embryos display disrupted expression of PCP components and reduced phosphorylation of the core PCP protein DVL2 at the time of LR axis determination. These results demonstrate that ZIC3 interacts with PCP signaling during early development, identifying a novel role for this transcription factor, and adding additional evidence about the importance of PCP function for normal LR patterning and subsequent heart development.

Application of CRISPR-Cas9 gene editing for congenital heart disease

Clustered regularly interspaced short palindromic repeats and CRISPR-associated protein 9 (CRISPR-Cas9) is an ancient prokaryotic defense system that precisely cuts foreign genomic DNA under the control of a small number of guide RNAs. The CRISPR-Cas9 system facilitates efficient double-stranded DNA cleavage that has been recently adopted for genome editing to create or correct inherited genetic mutations causing disease. Congenital heart disease (CHD) is generally caused by genetic mutations such as base substitutions, deletions, and insertions, which result in diverse developmental defects and remains a leading cause of birth defects. Pediatric CHD patients exhibit a spectrum of cardiac abnormalities such as septal defects, valvular defects, and abnormal chamber development. CHD onset occurs during the prenatal period and often results in early lethality during childhood. Because CRISPR-Cas9-based genome editing technology has gained considerable attention for its potential to prevent and treat diseases, we will review the CRISPR-Cas9 system as a genome editing tool and focus on its therapeutic application for CHD.

In Vivo and In Vitro Genetic Models of Congenital Heart Disease

Congenital cardiovascular malformations represent the most common type of birth defect and encompass a spectrum of anomalies that range from mild to severe. The etiology of congenital heart disease (CHD) is becoming increasingly defined based on prior epidemiologic studies that supported the importance of genetic contributors and technological advances in human genome analysis. These have led to the discovery of a growing number of disease-contributing genetic abnormalities in individuals affected by CHD. The ever-growing population of adult CHD survivors, which are the result of reductions in mortality from CHD during childhood, and this newfound genetic knowledge have led to important questions regarding recurrence risks, the mechanisms by which these defects occur, the potential for novel approaches for prevention, and the prediction of long-term cardiovascular morbidity in adult CHD survivors. Here, we will review the current status of genetic models that accurately model human CHD as they provide an important tool to answer these questions and test novel therapeutic strategies.

Genetics of Transposition of Great Arteries: Between Laterality Abnormality and Outflow Tract Defect

Transposition of great arteries (TGA) is a complex congenital heart disease whose etiology is still unknown. This defect has been associated, at least in part, with genetic abnormalities involved in laterality establishment and heart outflow tract development, which suggest a genetic heterogeneity. In animal models, the evidence of association with certain genes is strong but, surprisingly, genetic anomalies of its human orthologues are found only in a low proportion of patients and in nonaffected subjects, so that the underlying causes remain as an unexplored field. Evidence related to TGA suggests different pathogenic mechanisms involved between patients with normal organ disposition and isomerism. This article reviews the most important genetic abnormalities related to TGA and contextualizes them into the mechanism of embryonic development, comparing them between humans and mice, to comprehend the evidence that could be relevant for genetic counseling. Graphical abstract.

Rare Extracardiac Anomalies Presented with Right Heterotaxy Syndrome in a Newborn Baby: A Case Report

BACKGROUND Heterotaxy is a syndrome of abnormal arrangement of the internal thoracic-abdominal structures across the left-right axis of the body. It is a primary disorder with 2 main settings - bilateral left sidedness (polysplenia syndrome) or right sidedness (asplenia syndrome) - although some overlapping or uncertainties may occur. Patients with right heterotaxy typically present with asplenia, complex heart disease, and other thoracoabdominal organ situs abnormalities. CASE REPORT We present a unique case of congenital asplenia syndrome with complex heart disease, annular pancreas, and other extra-heterotaxic anomalies (e.g., musculoskeletal) in the form of a radius aplasia and partial syndactyly of the thumb and index finger of the left hand. These associated anomalies have not been reported before. CONCLUSIONS This case shows the need for paying increased attention to the implications of other extracardiac anomalies that can be associated with heterotaxy syndrome.

A novel ZIC3 gene mutation identified in patients with heterotaxy and congenital heart disease

Heterotaxy syndrome (HTX) is characterized by left-right (LR) asymmetry disturbances associated with severe heart malformations. However, the exact genetic cause of HTX pathogenesis remains unclear. The aim of this study was to investigate the pathogenic mechanism underlying heterotaxy syndrome. Targeted next-generation sequencing (NGS) was performed for twenty-two candidate genes correlated with LR axis development in sixty-six HTX patients from unrelated families. Variants were filtered from databases and predicted in silico using prediction programs. A total of twenty-one potential disease-causing variants were identified in seven genes. Next, we used Sanger sequencing to confirm the identified variants in the family pedigree and found a novel hemizygous mutation (c.890G > T, p.C297F) in the ZIC3 gene in a male patient that was inherited from his mother, who was a carrier. The results of functional indicated that this ZIC3 mutation decreases transcriptional activity, affects the affinity of the GLI-binding site and results in aberrant cellular localization in transfected cells. Moreover, morpholino-knockdown experiments in zebrafish demonstrated that zic3 mutant mRNA failed to rescue the abnormal phenotype, suggesting a role for the novel ZIC3 mutation in heterotaxy syndrome.

ZIC3 in Heterotaxy

Mutation of ZIC3 causes X-linked heterotaxy, a syndrome in which the laterality of internal organs is disrupted. Analysis of model organisms and gene expression during early development suggests ZIC3-related heterotaxy occurs due to defects at the earliest stage of left-right axis formation. Although there are data to support abnormalities of the node and cilia as underlying causes, it is unclear at the molecular level why loss of ZIC3 function causes such these defects. ZIC3 has putative roles in a number of developmental signalling pathways that have distinct roles in establishing the left-right axis. This complicates the understanding of the mechanistic basis of Zic3 in early development and left-right patterning. Here we summarise our current understanding of ZIC3 function and describe the potential role ZIC3 plays in important signalling pathways and their links to heterotaxy.

Cilia in vertebrate left-right patterning

Understanding how left-right (LR) asymmetry is generated in vertebrate embryos is an important problem in developmental biology. In humans, a failure to align the left and right sides of cardiovascular and/or gastrointestinal systems often results in birth defects. Evidence from patients and animal models has implicated cilia in the process of left-right patterning. Here, we review the proposed functions for cilia in establishing LR asymmetry, which include creating transient leftward fluid flows in an embryonic 'left-right organizer'. These flows direct asymmetric activation of a conserved Nodal (TGFβ) signalling pathway that guides asymmetric morphogenesis of developing organs. We discuss the leading hypotheses for how cilia-generated asymmetric fluid flows are translated into asymmetric molecular signals. We also discuss emerging mechanisms that control the subcellular positioning of cilia and the cellular architecture of the left-right organizer, both of which are critical for effective cilia function during left-right patterning. Finally, using mosaic cell-labelling and time-lapse imaging in the zebrafish embryo, we provide new evidence that precursor cells maintain their relative positions as they give rise to the ciliated left-right organizer. This suggests the possibility that these cells acquire left-right positional information prior to the appearance of cilia.This article is part of the themed issue 'Provocative questions in left-right asymmetry'.

Mechanism responsible for D-transposition of the great arteries: Is this part of the spectrum of right isomerism?

D-transposition of the great arteries (TGA) is one of the most common conotruncal heart defects at birth and is characterized by a discordant ventriculoarterial connection with a concordant atrioventricular connection. The morphological etiology of TGA is an inverted or arrested rotation of the heart outflow tract (OFT, conotruncus), by which the aorta is transposed in the right ventral direction to the pulmonary trunk. The rotational defect of the OFT is thought to be attributed to hypoplasia of the subpulmonic conus, which originates from the left anterior heart field (AHF) residing in the mesodermal core of the first and second pharyngeal arches. AHF, especially on the left, at the early looped heart stage (corresponding to Carnegie stage 10-11 in the human embryo) is one of the regions responsible for the impediment that causes TGA morphology. In human or experimentally produced right isomerism, malposition of the great arteries including D-TGA is frequently associated. Mutations in genes involving left-right (L-R) asymmetry, such as NODAL, ACTRIIB and downstream target FOXH1, have been found in patients with right isomerism as well as in isolated TGA. The downstream pathways of Nodal-Foxh1 play a critical role not only in L-R determination in the lateral plate mesoderm but also in myocardial specification and differentiation in the AHF, suggesting that TGA is a phenotype in heterotaxia as well as the primary developmental defect of the AHF.

Duplication and deletion of CFC1 associated with heterotaxy syndrome

Heterotaxy syndrome, which causes significant morbidity and mortality, is a class of congenital disorders, in which normal left-right asymmetry cannot be properly established. To explore the role of copy number variants (CNVs) in the occurrence of heterotaxy syndrome, we recruited 93 heterotaxy patients and studied 12 of them by the Affymetrix Genome-Wide Human SNP 6.0 Array. The results were confirmed in the remaining 81 patients and 500 healthy children by quantitative real-time polymerase chain reaction (qPCR). The analysis of the SNP6.0 array showed a duplication of chromosome 2q21.1, which was verified by qPCR. The result of qPCR in the other 81 patients showed that 8/81 patients had the CNVs of 2q21.1 and the only overlapping gene in these patients is CFC1. However, in the 500 healthy children, only one carried the duplication of CFC1 (p=3.5×10(-7)). The duplication and deletion of CFC1 may play key roles in the occurrence of heterotaxy syndrome. Moreover, the transposed great arteries, double outlet right ventricle, single atrium, and single ventricle may share a common genetic etiology with the heterotaxy syndrome.