Mouse Model of Heterotaxy with Single Ventricle Spectrum of Cardiac Anomalies

Abstract

Heterotaxy arises from a failure of the embryo to establish normal left-right asymmetry and is known to affect 3% of infants with congenital heart disease. A recessive mutation causing heterotaxy was recovered in a mouse mutagenesis screen focused on congenital heart defects. Homozygote mutants exhibit abnormal situs in the thoracic and abdominal cavities. Dextrocardia, levocardia, or mesocardia was seen together with right pulmonary isomerism and complex structural heart defects in the single ventricle spectrum. A dominant chamber of left ventricular morphology positioned on the left or right is seen together with transposition of the great arteries. Right atrial isomerism with or without total anomalous pulmonary venous connection was observed in half of the mutants. Because ciliary motion at the embryonic node is required for the specification of laterality, we examined the tracheal epithelia of newborn mice as a proxy for the nodal cilia. However, videomicroscopy showed no defect in ciliary motion. Genome scanning using polymorphic microsatellite markers mapped the mutation to a 3.3 Mb interval on mouse chromosome 7. None of the genes previously described for familial heterotaxy were found in this interval, indicating a novel mutation in this mouse model of heterotaxy. (Pediatr Res 63: 9-14, 2008) T he specification of laterality is essential for survival of air breathing mammals because left-right asymmetry associated with the four chamber heart and great vessels is required for the normal separation of pulmonary and systemic circulation. Heterotaxy syndromes involve birth defects in which organ situs in the thoracic and abdominal cavity are altered in a discordant manner. They are associated with 3% of all congenital heart defects (CHD) (1), with the prevalence estimated at 1.4/10,000 births (2). Because a wide spectrum of complex structural heart malformations are frequently seen in conjunction with heterotaxy, the overall prognosis is often poor. Some of the cardiovascular malformations associated with heterotaxy syndromes include transposition of the great arteries, single ventricle defects, anomalous pulmonary venous connection, atrial septal defects, ventricular septal defects, and right ventricular outflow tract obstruction (3). Other associated findings include left-sided or bilobed liver, rightsided stomach with gut malrotation, and a high incidence of midline associated defects such as cleft lip/palate, encephalocele, and renal abnormalities (1,3,4).Despite detailed knowledge of the clinical presentations, the cause(s) of heterotaxy remains largely unknown. Data collected from the Baltimore-Washington Infant Study from 1980 to 1989 showed a potential association with a familial history of malformations (odds ratio ϭ 5.1, 95% confidence interval ϭ 2.0 -12.9), maternal diabetes, and geographic location, suggesting both environmental and genetic contributions to heterotaxy syndromes (5). In addition, it has been reported that approximately 10% of infants with heterotaxy have a family histor...

Far from solved: A perspective on what we know about early mechanisms of left–right asymmetry

“…First, the ciliary model cannot explain results from several studies suggesting that consistent LR asymmetry can be produced in the absence of a node, confined fluid flow, or even multicellularity (Xu et al, 2007;Gros et al, 2009). Additionally, just as the cytoplasmic model predicts, we have identified at least six examples where ciliary phenotypes can be dissociated from problems with LR asymmetry, indicating that a causal link between cilia and LR asymmetry is less than solid (Zhao and Malicki, 2007;Aune et al, 2008;Kishimoto et al, 2008;Serluca et al, 2009;Tian et al, 2009;Zeng et al, 2010). Finally, a role for cilia as initiators of laterality is inconsistent with the existence of asymmetries that are consistent, and both physiological and morphological during early cleavage stages (Adams et al, 2006;Danilchik et al, 2006;Levin and Palmer, 2007).…”

“…The chromatid segregation model initially arose from the discovery of the chromosome-specific segregation phenomenon among mouse chromosome 7 homologs (Klar, 2008;Armakolas et al, 2010). This model is then especially interesting considering a recently identified mouse with a mutation on Chromosome 7 and LR asymmetry defects, yet apparently normal cilia (Aune et al, 2008); although tracheal, not nodal, cilia were examined in this study, their phenotypes are quite often linked (e.g., in Kartageners syndrome). If the node ciliary function indeed mirrors tracheal in these animals, the data would fit predictions made by the cytoplasmic model (which presumes the existence of mutants in which ciliary flow problems are dissociated from LR defects).…”

Massively parallel sequencing identifies the gene Megf8 with ENU-induced mutation causing heterotaxy

“…Typically these mutants exhibited thoracoabdominal organ situs anomalies that included dextracardia/mesocardia, right pulmonary isomerism, transposition of the great arteries, abnormal pulmonary venous connections, right-sided stomach and asplenia/ polysplenia (Fig. S1 A-C) (11). We reported the mapping of this mutation to a 3.3-Mb interval on mouse chromosome 7.…”

“…The latter embryos often exhibited a shortened heart tube ( Fig. 2C), indicative of a role for Megf8 in cardiac morphogenesis (11). Approximately half of the embryos with abnormal heart looping showed reverse looping of the gut.…”

“…In this study, we used massively parallel sequencing to recover an ENU-induced mutation causing a single-ventricle spectrum of complex structural heart defects recovered in our mouse fetal echocardiography screen (11). This mutant exhibits transposition of the great arteries, randomized left-right cardiopulmonary and visceral organ situs, a constellation of phenotype referred to as heterotaxy.…”