The correct establishment of anterior-posterior polarity in the vertebrate embryonic heart tube during embryogenesis is crucial

Early heart patterning

The correct establishment of anterior-posterior polarity in the vertebrate embryonic heart tube during embryogenesis is crucial for the proper morphogenesis of the mature heart. Molecular details of this process are poorly understood. There is now considerable evidence that retinoic acid (RA) is a morphogen that communicates anterior-posterior polarity to the heart (reviewed in Duester, 2008; Niederreither and Dolle, 2008). Using several approaches to examine the contribution of the SHF to pharyngeal mesoderm, atria and OFT in RA-deficient mouse embryos, we have recently shown that RA is required to restrict the SHF posteriorly (Ryckebusch et al., 2008). Mouse embryos lacking RALDH2 exhibit an increase in Fgf8 and Isl1 cardiac expression posteriorly (Fig. 1). This study suggests that RA regulates SHF organisation and furthermore indicate that this occurs within a precise time frame. We continue this study by exploring the role of Hox genes as RA-target genes during heart development.

Figure 1: Retinoic acid signalling during early organogenesis. Retinoic acid (RA) generated by retinaldehyde deshydrogen-ase 2 (Raldh2) in the lateral mesoderm travels anteriorly where it provides a signal that establishes the posterior border of the heart field. RA may function as a repressor of cardiac progenitors.

Genetic interaction between RA and Tbx1

The common clinical features associated with DiGeorge/velo-cardio-facial syndrome (DGS/VCFS) are congenital cardiovascular malformations, thymic and parathyroid aplasia or hypoplasia and craniofacial defects. However, despite the vast majority of patients diagnosed with DGS/VCFS having the same region of 22q11.2 deleted, there is wide phenotypic variation. The causes of this phenotype variability remain unknown. Genetic studies have identified the transcription factor encoding gene Tbx1 as a major candidate gene for DGS/VCFS. Tbx1 heterozygous mutant mice display absence or hypoplasia of the 4th pharyngeal arch artery (PAA) at E10.5 and associated great artery anomalies at later stages of development. Retinoic acid (RA) interacts with Tbx1, as Tbx1 expression was increased in Raldh2^-/- deficient embryos and reduction of RA synthesis modulates development of the 4th PAA in Tbx1^+/- embryos by accelerating the recovery of the arterial growth delay (Ryckebusch et al., 2010). RA deficiency therefore induces earlier recovery of DiGeorge-related aortic arch defects, supporting the hypothesis that differences in levels of embryonic RA may contribute to the phenotypic variability observed in DGS/VCFS patients (Fig 2). Genes involved in RA synthesis, metabolism and signaling should be considered candidate modifiers of the DGS/VCFS phenotype associated with 22q11deletion.

What Is Known?

∑ Abnormal development of the pharyngeal apparatus has been implicated in the pathogenesis of the DGS/VCFS syndrome, the most common genetic deletion syndrome in humans.

∑ DGS/VCFS syndrome is characterized by a high clinical variability.

∑ In the mouse, loss of Tbx1 and aberrant RA synthesis results in cardiovascular defects similar to the phenotype found in DGS/VCFS patients.

What New Information Does Our Work Contribute?

∑ It reveals a genetic interaction between Tbx1 and RA signaling.

∑ It reveals that reduction of RA-levels decreases cardiovascular malformations found normally in Tbx1 heterozygous embryos.

Figure 2: Percentage of Raldh2^+/-, Tbx1^+/- and Raldh2^+/-;Tbx1^+/- embryos with 4th PAAs or 4th derived defects at E10.5, E11.5 and fetal stages. The percentage of embryos that overcomes the primary 4th PAA defects at E11.5 is significantly increased in compound heterozygous mutants compare to single Tbx1^+/-(*p<0.01). Surprisingly, the difference observed at E11.5 is decreased at later stages when a majority of Tbx1^+/- embryos overcome the primary 4th PAA defect.

These works are supported by the AFM grants 13517/14134 and the ANR-07-MRAR-003