Phenotypically Silent Bone Morphogenetic Protein Receptor 2 Mutations Predispose Rats to Inflammation-Induced Pulmonary Arterial Hypertension by Enhancing the Risk for Neointimal Transformation.

Abstract

<i>Bmpr2</i> (bone morphogenetic protein receptor 2) mutations are critical risk factors for hereditary pulmonary arterial hypertension (PAH) with approximately 20% of carriers developing disease. There is an unmet medical need to understand how environmental factors, such as inflammation, render <i>Bmpr2</i> mutants susceptible to PAH. Overexpressing 5-LO (5-lipoxygenase) provokes lung inflammation and transient PAH in <i>Bmpr2</i>^<i>+/</i>- mice. Accordingly, 5-LO and its metabolite, leukotriene B₄, are candidates for the second hit. The purpose of this study was to determine how 5-LO-mediated pulmonary inflammation synergized with phenotypically silent <i>Bmpr2</i> defects to elicit significant pulmonary vascular disease in rats. Monoallelic <i>Bmpr2</i> mutant rats were generated and found phenotypically normal for up to 1 year of observation. To evaluate whether a second hit would elicit disease, animals were exposed to 5-LO-expressing adenovirus, monocrotaline, SU5416, SU5416 with chronic hypoxia, or chronic hypoxia alone. <i>Bmpr2</i>-mutant hereditary PAH patient samples were assessed for neointimal 5-LO expression. Pulmonary artery endothelial cells with impaired BMPR2 signaling were exposed to increased 5-LO-mediated inflammation and were assessed for phenotypic and transcriptomic changes. Lung inflammation, induced by intratracheal delivery of 5-LO-expressing adenovirus, elicited severe PAH with intimal remodeling in <i>Bmpr2</i>^+/- rats but not in their wild-type littermates. Neointimal lesions in the diseased <i>Bmpr2</i>^+/- rats gained endogenous 5-LO expression associated with elevated leukotriene B₄ biosynthesis. <i>Bmpr2</i>-mutant hereditary PAH patients similarly expressed 5-LO in the neointimal cells. In vitro, BMPR2 deficiency, compounded by 5-LO-mediated inflammation, generated apoptosis-resistant and proliferative pulmonary artery endothelial cells with mesenchymal characteristics. These transformed cells expressed nuclear envelope-localized 5-LO consistent with induced leukotriene B₄ production, as well as a transcriptomic signature similar to clinical disease, including upregulated nuclear factor Kappa B subunit (NF-κB), interleukin-6, and transforming growth factor beta (TGF-β) signaling pathways. The reversal of PAH and vasculopathy in <i>Bmpr2</i> mutants by TGF-β antagonism suggests that TGF-β is critical for neointimal transformation. In a new 2-hit model of disease, lung inflammation induced severe PAH pathology in <i>Bmpr2</i>^+/- rats. Endothelial transformation required the activation of canonical and noncanonical TGF-β signaling pathways and was characterized by 5-LO nuclear envelope translocation with enhanced leukotriene B₄ production. This study offers an explanation of how an environmental injury unleashes the destructive potential of an otherwise silent genetic mutation.