Infant pulmonary hypertension (PH) is a major cause of newborn death. Congenital diaphragmatic hernia (CDH), a relatively common anomaly, produces mechanical pressure on the lungs and remains a considerable contributor to PH-associated infant mortality. Mutations in the PBX family of transcription factors have been implicated in patients who develop CDH. Now, work from Xin Sun’s lab at UCSD reveals that the PBX transcription factors also play a crucial role in early lung development and function independent of their effects on the diaphragm.
In the study, McCulley et al. observed that PBX1 is expressed in lung mesenchymal tissue beginning in early embryonic development and is maintained through the early postnatal period in mice. Selective inactivation of Pbx genes in the lung mesenchyme (Pbx1/2 CKO mice) did not affect diaphragm development. However, the mutant mice exhibited tachypnea and died between 2-3 weeks of age after developing severe PH. The formation of alveoli, a process that occurs in late stages of lung development, was defective in Pbx1/2 CKO mice. PBX loss also altered the expression of genes that regulate smooth muscle contraction, which led to inadequate relaxation of the pulmonary vasculature after birth. Finally, the researchers determined that pharmacologic disruption of smooth muscle myosin phosphorylation, which is increased following Pbx1/2 inactivation, ameliorates aberrant pulmonary vasoconstriction in the mutant mice. This observation suggests that approaches targeting certain pathways downstream of PBX transcription factors could potentially treat some forms of infant PH.
The featured images are 3-D reconstructions of PECAM-expressing alveolar capillaries in the lungs of control (left) and Pbx1/2 CKO mice (right) at postnatal day 3. The reductions in PECAM density and volume in mutant mice are indicative of defective alveoli formation.
A critical event in the adaptation to extrauterine life is relaxation of the pulmonary vasculature at birth, allowing for a rapid increase in pulmonary blood flow that is essential for efficient gas exchange. Failure of this transition leads to pulmonary hypertension (PH), a major cause of newborn mortality associated with preterm birth, infection, hypoxia, and malformations including congenital diaphragmatic hernia (CDH). While individual vasoconstrictor and dilator genes have been identified, the coordination of their expression is not well understood. Here, we found that lung mesenchyme–specific deletion of CDH-implicated genes encoding pre–B cell leukemia transcription factors (Pbx) led to lethal PH in mice shortly after birth. Loss of Pbx genes resulted in the misexpression of both vasoconstrictors and vasodilators in multiple pathways that converge to increase phosphorylation of myosin in vascular smooth muscle (VSM) cells, causing persistent constriction. While targeting endothelin and angiotensin, which are upstream regulators that promote VSM contraction, was not effective, treatment with the Rho-kinase inhibitor Y-27632 reduced vessel constriction and PH in Pbx-mutant mice. These results demonstrate a lung-intrinsic, herniation-independent cause of PH in CDH. More broadly, our findings indicate that neonatal PH can result from perturbation of multiple pathways and suggest that targeting the downstream common effectors may be a more effective treatment for neonatal PH.
David J. McCulley, Mark D. Wienhold, Elizabeth A. Hines, Timothy A. Hacker, Allison Rogers, Ryan J. Pewowaruk, Rediet Zewdu, Naomi C. Chesler, Licia Selleri, Xin Sun