Nodal asymmetry and hedgehog signaling during vertebrate left–right symmetry breaking

Association of copy number variation in X chromosome-linked PNPLA4 with heterotaxy and congenital heart disease

BACKGROUND

Heterotaxy (HTX) is a thoracoabdominal organ anomaly syndrome and commonly accompanied by congenital heart disease (CHD). The aim of this study was to analyze rare copy number variations (CNVs) in a HTX/CHD cohort and to examine the potential mechanisms contributing to HTX/CHD.

METHODS

Chromosome microarray analysis was used to identify rare CNVs in a cohort of 120 unrelated HTX/CHD patients, and available samples from parents were used to confirm the inheritance pattern. Potential candidate genes in CNVs region were prioritized via the DECIPHER database, and PNPLA4 was identified as the leading candidate gene. To validate, we generated PNPLA4 -overexpressing human induced pluripotent stem cell lines as well as pnpla4 -overexpressing zebrafish model, followed by a series of transcriptomic, biochemical and cellular analyses.

RESULTS

Seventeen rare CNVs were identified in 15 of the 120 HTX/CHD patients (12.5%). Xp22.31 duplication was one of the inherited CNVs identified in this HTX/CHD cohort, and PNPLA4 in the Xp22.31 was a candidate gene associated with HTX/CHD. PNPLA4 is expressed in the lateral plate mesoderm, which is known to be critical for left/right embryonic patterning as well as cardiomyocyte differentiation, and in the neural crest cell lineage. Through a series of in vivo and in vitro analyses at the molecular and cellular levels, we revealed that the biological function of PNPLA4 is importantly involved in the primary cilia formation and function via its regulation of energy metabolism and mitochondria-mediated ATP production.

CONCLUSIONS

Our findings demonstrated a significant association between CNVs and HTX/CHD. Our data strongly suggested that an increased genetic dose of PNPLA4 due to Xp22.31 duplication is a disease-causing risk factor for HTX/CHD.

Natural reversal of cavefish heart asymmetry is controlled by Sonic Hedgehog effects on the left-right organizer

The direction of left-right visceral asymmetry is conserved in vertebrates. Deviations of the standard asymmetric pattern are rare, and the underlying mechanisms are not understood. Here, we use the teleost Astyanax mexicanus, consisting of surface fish with normal left-oriented heart asymmetry and cavefish with high levels of reversed right-oriented heart asymmetry, to explore natural changes in asymmetry determination. We show that Sonic Hedgehog (Shh) signaling is increased at the posterior midline, Kupffer's vesicle (the teleost left-right organizer) is enlarged and contains longer cilia, and the number of dorsal forerunner cells is increased in cavefish. Furthermore, Shh increase in surface fish embryos induces asymmetric changes resembling the cavefish phenotype. Asymmetric expression of the Nodal antagonist Dand5 is equalized or reversed in cavefish, and Shh increase in surface fish mimics changes in cavefish dand5 asymmetry. Shh decrease reduces the level of right-oriented heart asymmetry in cavefish. Thus, naturally occurring modifications in cavefish heart asymmetry are controlled by the effects of Shh signaling on left-right organizer function.

Molecular anatomy of emerging Xenopus left-right organizer at successive developmental stages

BACKGROUND

Vertebrate left-right symmetry breaking is preceded by formation of left-right organizer. In Amphibian, this structure is formed by gastrocoel roof plate, which emerges from superficial suprablastoporal cells. GRP is subdivided into medial area, which generates leftward flow by rotating monocilia and lateral Nodal1 expressing areas, which are involved in sensing of the flow. After successful symmetry breaking, medial cells are incorporated into a deep layer where they contribute to the axial mesoderm, while lateral domains join somitic mesoderm.

RESULTS

Here, we performed detailed analysis of spatial and temporal gene expression of important markers and the corresponding morphology of emerging GRP. Endodermal marker Sox17 and markers of superficial mesoderm display complementary patterns at all studied stages. At early stages, GRP forms Tekt2 positive epithelial domain clearly separated from underlying deep layers, while at later stages, this separation disappears. Marker of early somitic mesoderm MyoD1 was absent in emerging GRP and was induced together with Nodal1 during early neurulation. Decreasing morphological separation is accompanied by lateral to medial covering of GRP by endoderm.

CONCLUSION

Our data supports continuous link between superficial mesoderm at the start of gastrulation and mature GRP and suggests late induction of somitic fate in lateral GRP.