Hox genes encode transcription factors governing complex developmental processes in several organs. A subset of Hox genes are expressed in the developing lung. Except for Hoxa5, the lack of overt lung phenotype in single mutants suggests that Hox genes may not play a predominant role in lung ontogeny or that functional redundancy may mask anomalies. In the Hox5 paralog group, both Hoxa5 and Hoxb5 genes are expressed in the lung mesenchyme whereas Hoxa5 is also expressed in the tracheal mesenchyme. Herein, we generated Hoxa5;Hoxb5 compound mutant mice to evaluate the relative contribution of each gene to lung development. Hoxa5;Hoxb5 mutants carrying the four mutated alleles displayed an aggravated lung phenotype, resulting in the death of the mutant pups at birth. Characterization of the phenotype highlighted the role of Hoxb5 in lung formation, the latter being involved in branching morphogenesis, goblet cell specification, and postnatal air space structure, revealing partial functional redundancy with Hoxa5. However, the Hoxb5 lung phenotypes were less severe than those seen in Hoxa5 mutants, likely because of Hoxa5 compensation. New specific roles for Hoxa5 were also unveiled, demonstrating the extensive contribution of Hoxa5 to the developing respiratory system. The exclusive expression of Hoxa5 in the trachea and the phrenic motor column likely underlies the Hoxa5-specific trachea and diaphragm phenotypes. Altogether, our observations establish that the Hoxa5 and Hoxb5 paralog genes shared some functions during lung morphogenesis, Hoxa5 playing a predominant role.

Summary: Cre-expressing mouse lines constitute an important asset to mammalian genetics, allowing the deletion of genes in a spatio-temporal specific manner. Our study on Hox gene function in lung development has led us to use a lung endoderm-specific deletion with the Sftpc-cre mouse line expressing the Cre recombinase gene under the control of human surfactant protein C regulatory sequences. In control experiments, the Cre recombinase faithfully activated the Rosa26-lacZ reporter gene in lung epithelium. However as early as e15.5, lungs from Sftp-Cre 1 embryos showed abnormal dilated cysts. This unexpected phenotype was also observed in mice carrying the conditional lung epithelial Hoxa5 deletion, indicating some bias due to Cre deleterious effects. Excessive apoptosis, likely due to Cre toxicity, could explain the abnormal cysts. Our findings illustrate the need for appropriate control experiments and careful interpretation of data to discriminate between the phenotype due to the targeted mutation and the confounding effects of the Cre recombinase. genesis 49:152-159, 2011. V V C 2011 Wiley-Liss, Inc.