8.1. Introduction
Complex congenital heart disease is clinically well described to be highly associated with heterotaxy, a birth defect involving randomization of left-right patterning [1]. The importance of left-right patterning in congenital heart disease is likely a reflection of the fact that the heart is the most left-right asymmetric organ in the body. This asymmetry is critical for establishing systemic and pulmonary circulation required for efficient oxygenation of blood. While heterotaxy is relatively rare, reported at approximately 1 in 10,000 live births, it is clinically of high importance given it is often associated with complex CHD with high morbidity and mortality [2].
8.1.1. Heterotaxy , Primary Ciliary Dyskinesia, and Motile Cilia Defects
Interestingly, heterotaxy and complex CHD have been reported in ~6 % of patients with primary ciliary dyskinesia (PCD), a sinopulmonary disease that arises from airway mucus clearance defects due to immotile or dyskinetic cilia in the respiratory epithelia [3]. Given PCD is also relatively rare at 1 in 16,000 [4], the co-occurrence of heterotaxy and PCD would suggest a mechanistic link for heterotaxy and PCD. This mechanistic link has now been demonstrated to involve a shared disturbance of motile function. Thus animal model studies have shown motile cilia play an important role in embryonic left-right patterning [5].
In the mouse embryo, motile cilia at the node generate nodal flow that helps specify the left-right axis (Fig. ). The nodal flow is sensed by nonmotile or primary cilia at the node periphery, resulting in activation of calcium sig naling at the embryo’s left, followed by left-sided activation of the nodal signaling cascade in the left lateral plate mesoderm. As motile cilia in the node and airway are constructed in a similar manner with many of the same proteins, it is not surprising that airway ciliary dysfunction might predict nodal cilia dysfunction. This likely accounts for the high prevalence of heterotaxy among PCD patients.
Motile and primary cilia in the mouse embryo. (a) Immunostaining with antibodies to acetylated tubulin (red) and γ-tubulin (blue) was used to visualize motile cilia in the E7.75 mouse embryonic node. (b–f) Immunostaining with antibodies (more...)
8.1.2. Motile Respiratory Cilia Defects in Other Ciliopathies
Given primary cilia also has been shown to play a role in the embryonic node to establish the left-right axis, this would suggest that mutations affecting primary cilia function may also contribute to complex CHD associated with heterotaxy. Given the extensive overlap between proteins found in the primary and motile cilia, this would suggest the clinical distinction may be blurred between patients with ciliopathies considered mediated by primary cilia defects vs. those with PCD that have motile cilia defects. Indeed in a recent study, we showed that a patient with cranioectodermal dysplasia, a ciliopathy thought to involve the primary cilia, has presentations consistent with PCD. This includes obstructive airway disease, low nasal NO, and abnormal respiratory ciliary motion [6].
8.1.3. Ciliary Dysfunction in Congenital Heart Disease Patients with Heterotaxy
Even as PCD patients were observed to have a 6 % incidence of heterotaxy, a study of CHD patients with heterotaxy revealed an even higher prevalence of respiratory ciliary dysfunction similar to that seen with PCD [7]. Two tests used for PCD assessment were used to evaluate heterotaxy patients. Ciliary motion analyzed using videomicroscopy of nasal tissue biopsy and nasal nitric oxide (nNO) was measured, which is typically low in patients with PCD. This analysis showed 42 % of the heterotaxy patients have ciliary dysfunction comprising abnormal ciliary motion and low nNO, presentations typically seen with PCD [7].
Interestingly, a mouse mutant exhibit ing complex congenital heart defect associated with heterotaxy was identified to have a pathogenic mutation in Dnah5, a cilium outer dynein arm component required for motile cilia function and a gene well described to cause PCD [8]. This mutant exhibited mostly immotile cilia in the airway and in the embryonic node [8], accounting for the laterality disturbance and airway clearance defects seen in PCD patients with DNAH5 mutations. Interestingly, these Dnah5 mutant mice exhibited either of three different laterality phenotypes: normal situs solitus, mirror symmetric situs inversus totalis, or randomized visceral organ situs known as heterotaxy (Fig. ). It is only with heterotaxy that complex congenital heart disease was observed, indicating that disturbance of the left-right patterning may play an important role in congenital heart disease. As the mouse Dnah5 mutants with heterotaxy were mostly inviable to term due to their complex congenital heart disease, this would suggest considerable ascertainment bias in the human population. Consistent with this, a study of PCD patients revealed most had either situs solitus or situs inversus, with only a small fraction exhibiting heterotaxy [4].
Situs anomalies, congenital heart defects and ciliogenesis defects in laterality mutants. (a–g) Ap1b1
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mutants exhibit situs solitus (a), situs inversus (b), or heterotaxy (c). Situs solitus, characterized by normal left-right visceral (more...)
8.1.4. Respiratory Complications in Heterotaxy Patients with Ciliary Dysfunction
As the central hallmark of PCD is respiratory disease due to mucociliary clearance defects, the question arises as to whether heterotaxy patients may also have respiratory symptoms and disease. Indeed, heterotaxy patients with ciliary dysfunction are observed to have significantly more respiratory symptoms and disease [7]. Furthermore, those undergoing surgical procedures show increased pulmonary morbidity, including increased use of inhaled β-agonist [9]. β-agonist use is typically avoided in cardiac patients given its arrhythmogenic properties. Hence, the increased use of this medication is a strong indicator of serious respiratory complications.
These findings have important clinical translational ramifications, since respiratory complications in heterotaxy patients are usually attributed to the heart disease, and thus any airway clearance defects are not systematically add ressed clinically. In light of these findings, a change in the standard of care may be warranted with the presurgical screening of heterotaxy patients for mucociliary clearance defects and providing airway clearance therapy to help reduce postsurgical respiratory complications in those with airway ciliary dysfunction. This may help improve the prognosis for these patients who typically have to endure multiple high-risk cardiac surgeries to palliate their structural heart defects.
8.1.5. Left-Right Patterning and the Pathogenesis of Congenital Heart Disease
The importance of left-right patterning in the pathogenesis of congenital heart disease has also emerged from a large-scale mouse mutagenesis screen. High-throughput screening of ENU-mutagenized mice using fetal echocardiography allowed the ultrasound phenotyping of greater than 80,000 fetuses (Fig. ) [10, 11]. Fetal echocardiography is ideally suited for recovery of mutants with congenital heart defects, as it is an imaging modality developed in the clinical setting for the assessment of cardiac structure and function (Fig. ). Over 200 mutant mouse lines with a wide spectrum of congenital heart defect were recovered. Surprisingly, this included many mutant lines with laterality defects (~30 %), recovered based on the finding of complex CHD in mutants with heterotaxy. Given our screen was focused on congenital heart defects, not left-right patterning defects, this enrichment of laterality mutants would suggest the disturbance of left-right patterning plays an important role in the pathogenesis of congenital heart disease.
Ultrasound diagnoses of congenital heart disease and cilia defects in mouse mutants with congenital heart disease. Vevo 2100 color flow imaging showed crisscrossing of blood flow, indicating normal aorta (Ao) and pulmonary artery (PA) alignment (a Supplemental (more...)
This unexpected finding of a high prevalence of heterotaxy is actually in line with observations in the human fetal population. One clinical study using fetal echocardiography for CHD diagnosis reported that 16 % of human fetuses with congenital heart defects have heterotaxy [12]. This number is likely a minimal estimate, given several clinical studies have shown human fetuses with heterotaxy and congenital heart defect have very high rates of prenatal/intrauterine death (30–60 %) [13–15]. When combined with the fact that only 28 % of CHD is clinically diagnosed prenatally [16], these findings point to the prevalence of congenital heart disease associated with heterotaxy being significantly underestimated clinically. In our mouse screen, we also noted most mouse fetuses with congenital heart defects associated with heterotaxy died in utero. The recovery of these heterotaxy mutants rests entirely on our scr een having been conducted prenatally with fetal ultrasound imaging.
Of importance to note is the fact that many of these mutant lines with heterotaxy actually yielded three distinct phenotypes, similar to what had been observed for the Dnah5 mutants. Thus animals harboring the same mutation can have normal situs solitus, mirror symmetric situs inversus, or heterotaxy (Fig. ). As with the Dnah5 mutants, congenital heart defects were usually seen only in mutants with heterotaxy [11]. In a subset of these mutants, videomicroscopy of the tracheal epithelia in these mutants also showed immotile or dyskinetic cilia, suggesting they have mutations affecting motile cilia function and may be PCD mouse models [11].
8.1.6. Ciliome Gene Enrichment Among Mutations Causing Congenital Heart Disease
Whole-mouse exome-sequencing analysis was used to recover the pathogenic CHD-causing mutations in mutants recovered from the large-scale mouse mutagenesis screen. This was made possible given the screen was conducted in a C57BL6 inbred strain background. From this analysis, 91 pathogenic mutations were recovered in 61 genes (Fig. ). Of the 61 genes, 35 (58 %) are in cilia-related or ciliome genes (Fig. ); this included 12 genes (34 %) required for motile cilia function (Fig. ). Indeed 8 of these genes are known to cause PCD, including many alleles of Dnah5 and Dnah11 (Fig. ). Interestingly, 23 of the cilia genes are actually primary cilia related (66 %). Of these, half are found in mutant lines with laterality defects and half in lines without laterality defects (Fig. ) [11]. These findings suggest the link between cilia and CHD is broader, not merely a reflection of the role of cilia in left-right patterning. This is further supported by the recovery of 15 pathogenic mutations in genes involved in cilia-transduced cell signaling, including mutations in genes involved in Shh, Wnt, Tgfβ, and calcium signaling (Fig. ), all pathways known to play important role in cardiovascular development [11].
Distribution of pathogenic congenital heart disease causing mutations recovered from large-scale mouse forward genetic screen.
Top: Distribution of pathogenic CHD mutations among different mutation types. Middle: Ciliome vs. non-ciliome CHD genes found (more...)
Congenital heart disease genes recovered from mouse mutagenesis screen. Diagrams illustrate biological context of CHD gene function (color-highlighting indicates CHD genes recovered; asterisk denotes CHD genes recovered from previous screen) (From Li (more...)
Relevant to this, we note cilia is bro adly expressed in the embryonic heart, including in the atrial and ventricular myocardium, the atrioventricular and outflow endocardial cushions (Fig. ). Importantly, in the Cc2d2a mutant recovered from our screen, atrioventricular septal defects were commonly observed, and this was associated with a selective loss of cilia only in the atrioventricular cushions, while the outflow cushions remain unaffected (Fig. ). The overall marked enrichment observed for cilia mutations in the context of a gene agnostic screen would point to the cilia as playing a central role in the pathogenesis of congenital heart disease, a role that is subserved by both motile and primary cilia and goes beyond the role of cilia in left-right patterning.
8.1.7. Ciliary Dysfunction in Congenital Heart Disease Patients Without Heterotaxy
The finding from our mouse studies that cilia defects play a central role in the pathogenesis of CHD would suggest the clinical impact of ciliary dysfunction in congenital heart disease may be broader and have relevance beyond heterotaxy. To examine this question, we conducted a large clinical study of over 200 patients with a broad spectrum of congenital heart disease mostly without heterotaxy to determine the prevalence of ciliary dysfunction [17]. As in our previous study of heterotaxy patients, we assessed for respiratory ciliary dysfunction using videomicroscopy of the nasal epithelia and measured nNO.
This study demonstrated a very high prevalence of ciliary dysfunction, in both the heterotaxy and nonheterotaxy congenital heart disease patients [17]. Moreover, patients with airway ciliary dysfunction had significantly more respiratory symptoms and disease and this did not correlate with heterotaxy status (Table ) [17]. The respiratory symptoms were largely localized to the lower airway and were also significantly associated with PCD symptoms such as chronic otitis media, chronic sinusitis, chronic wet cough, neonatal respiratory distress, pneumonia, and bronchiectasis. Together these findings suggest patients with congenital heart disease of a wide spectrum with or without heterotaxy may have high risk for respiratory ciliary dysfunction. These findings are in agreement with the mouse studies showing a central role for cilia in the pathogenesis of congenital heart disease.
Multivariate analysis and respiratory symptomsa
8.1.8. Future Directions and Clinical Implications
We identified a central role for cilia in the pathogenesis of congenital heart disease. This finding uncovered by our mouse mutagenesis screen was unexpected and shows the power of a non-gene-biased phenotype-driven genetic screen to uncover new insights into mechanisms of disease pathogenesis. The important role of cilia in congenital heart disease is supported by the human clinical studies, which also showed a high prevalence of ciliary dysfunction in congenital heart disease patients. Our mouse screen identified primary and motile cilia both playing essential roles in congenital heart disease. The high prevalence of laterality defects among congenital heart disease mutants is reminiscent of the clinicalx observation of a high prevalence of heterotaxy among human fetuses with congenital heart disease. This suggests that observations from our mouse fetal ultrasound screen may provide a window into the unborn human fetal population.
Our finding that congenital heart disease patients with ciliary dysfunction have more respiratory symptoms and disease has important clinical implications. Patients with complex congenital heart disease typically must endure multiple high-risk cardiac surgeries for palliation of their structural heart defects. Not infrequently, respiratory complications with high morbidity are observed postoperatively that can negatively impact outcome. Such respiratory problems are typically attributed to the heart disease, and the possibility of intrinsic airway clearance defects due to ciliary dysfunction is never considered and hence not tested nor treated. The observations from the human and mouse studies combined would strongly suggest congenital heart disease patients should be presurgically screened for respiratory ciliary dysfunction, and those with ciliary dysfunction should be provided perioperative pulmonary therapy to enhance airway clearance function. Instituting such change in the standard of care may have significant benefit in improving outcome, especially cumulatively through the multiple rounds of surgery that patients with critical congenital heart disease must en dure.
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