CONTRIBUTION OF RETINOIC ACID RECEPTOR-BETA ISOFORMS TO THE FORMATIONOF THE CONOTRUNCAL SEPTUM OF THE EMBRYONIC HEART

To investigate the relative contribution of retinoic acid receptor (RA R)beta isoforms in conotruncal septation, RAR beta 1 and beta 3 were i nactivated in the mouse. Mice lacking RAR beta 1 and beta 3 appear nor mal. Disruption of these isoforms in RAR alpha or RAR gamma null genet ic backgrounds results in a high postpartum lethality. However, except for ocular defects found in RAR beta 1-3/RAR gamma compound mutants, the double null mutants display only abnormalities seen in single null mutants. This probably reflects a functional redundancy with other RA Rs, most notably with RAR beta 2 which is five- to sixfold more abunda nt than RAR beta 1 and beta 3 and whose domain of expression is largel y overlapping. The conotruncal ridges form normally in retinoid X rece ptor (RXR)alpha/RAR beta 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 RAR beta 1 and beta 3 are expressed specifically in the conotruncal ridges, failure of fusion of these structures is not more frequent in RXR alpha/RAR beta 1-3 double null mutants than in RX R alpha single null mutants. Similarly, the disruption of the sole RAR beta 2 isoform in a RXR alpha null genetic background does not result in an increase of the frequency of conotruncal septum agenesis. Howev er, this agenesis is fully penetrant in RYR alpha/RAR beta(+/-) mutant s, which reflects distinct roles of RXR alpha:RAR beta 1 (and beta 3) and RXR alpha:RAR beta 2 heterodimers in promoting the survival of con otruncal mesenchymal cells. Unexpectedly, we discovered that, in wild- type embryos, the conotruncal mesenchyme is a major site of morphogene tic cell death and that conotruncal myocytes are occasionally necrotic . Thus, excessive cell death in the conotruncus is a potential cause o f ventricular septal defects in humans. (C) 1998 Academic Press.