Contribution of retinoic acid receptor β isoforms to the formation of the conotruncal septum of the embryonic heart

To investigate the relative contribution of retinoic acid receptor (RAR)β isoforms in conotruncal septation, RARβ1 and β3 were inactivated in the mouse. Mice lacking RARβ1 and β3 appear normal. Disruption of these isoforms in RARα or RARγ null genetic backgrounds results in a high postpartum lethality. However, except for ocular defects found in RARβ1-3/RARγ compound mutants, the double null mutants display only abnormalities seen in single null mutants. This probably reflects a functional redundancy with other RARs, most notably with RARβ2 which is five- to sixfold more abundant than RARβ1 and β3 and whose domain of expression is largely overlapping. The conotruncal ridges form normally in retinoid X receptor (RXR)α/RARβ compound mutants but fail to fuse, apparently as a result of excessive apoptosis of mesenchymal cells. Additionally, many cardiomyocytes in the conotruncal wall of these mutants appear necrotic. Although RARβ1 and β3 are expressed specifically in the conotruncal ridges, failure of fusion of these structures is not more frequent in RXRα/RARβ1–3 double null mutants than in RXRα single null mutants. Similarly, the disruption of the sole RARβ2 isoform in a RXRα null genetic background does not result in an increase of the frequency of conotruncal septum agenesis. However, this agenesis is fully penetrant in RXRα/RARβ +/− mutants, which reflects distinct roles of RXRα:RARβ1 (and β3) and RXRα:RARβ2 heterodimers in promoting the survival of conotruncal mesenchymal cells. Unexpectedly, we discovered that, in wild-type embryos, the conotruncal mesenchyme is a major site of morphogenetic cell death and that conotruncal myocytes are occasionally necrotic. Thus, excessive cell death in the conotruncus is a potential cause of ventricular septal defects in humans.