Analysis of multiple cardiac abnormalities of a mouse mutant Hoxb3lacZ

Abstract

We examined the functional roles of Hoxb3 by generating a Hoxb3lacZ mutant with in-frame insertion of lacZ and neo reporter cassette after the start codon of Hoxb3. In this mutant the coding exons of Hoxb3 have been interrupted by the reporter cassette. Heterozygous Hoxb3lacZ appeared normal, whereas homozygous Hoxb3lacZ/lacZ could not survive till term. There was apparent systemic edema and poor systemic circulation observed in the homozygous mutants at around 15.5 dpc. Whole mount preparation at 9.5 dpc showed a subtle heart tube looping defect in homozygous mutant. Histological analysis of 14.5 dpc Hoxb3lacZ/lacZ showed a range of cardiac abnormalities, including ventricular septal defect (VSD), thinning of the myocardium compact layer and swollen inlet valves. Immunohistochemisty using antibodies against a-smooth muscle actin (a-SMA), a-sarcomeric actin, desmin and Troponin I showed no changes in Hoxb3lacZ/lacZ. Similarity in the expression pattern of both types of actins suggested that the contractility of the mutant heart was not affected. Hoxb3lacZ/lacZ showed remarkedly similar mutant phenotype to TGF-b2 knockout mice. However, analysis of the TGF-b2 expression pattern in homozygous mutants did not show significant difference to the wild type controls, suggesting that other signaling pathways might be involved. As cardiac neural crest contributes to the formation of the outflow tract septation, the abnormal phenotypes observed in the mutant suggest that Hoxb3 may affect cardiac neural crest cells which contribute to cardiac defects of the mutant embryos.