High incidence of cardiac malformations in connexin40-deficient mice

Gap junctions are intercellular channels formed by oligomerization of a protein called connexin (Cx). The heart expresses at least three connexin isotypes: Cx40, Cx43, and Cx45. A possible role for Cx40 in cardiac morphogenesis remains to be determined. We have characterized the anatomy and histology of fetal and newborn hearts obtained from crossing Cx40-deficient mice of mixed genetic background (C57BL/6x129Sv). Hearts were serial-sectioned ( 5 mum) along the coronal plane, stained with hematoxylin-eosin, and visualized by conventional light microscopy. Cardiac malformations in mice lacking Cx40 in one allele (Cx40(+/-)) included bifid atrial appendage, ventricular septal defect, tetralogy of Fallot (TOF), and an aortic arch abnormality. In Cx40(-/-) mice resulting from crossing of Cx40(+/-) mice, the most common cardiac malformations were double-outlet right ventricle (DORV), TOF, and endocardial cushion defects. Overall incidence of cardiac malformations was 6/33 (18%) in Cx40(+/-) mice and 4/12 (33%) in Cx40(-/-) mice. No cardiac malformations were observed in 15 wild-type mice studied. In addition, we examined 39 hearts from offspring of Cx40(-/-) matings. Frequency of cardiac malformations was even higher in this group (44%). Over one third of the hearts ( 14 of 39) showed conotruncal malformations corresponding to either DORV or TOF. Endocardial cushion defects were found in 3 out of 39 hearts. Our results suggest that Cx40 participates in cardiac morphogenesis, likely in association with other ( unknown) products whose expression may vary with the genetic background of the mice.