Pulmonary arterial hypertension (PAH) is a rare disease initiated by dysfunction of the pulmonary vascular endothelium. Mutations in BMPR2, which encodes the type-II bone morphogenic protein (BMP) receptor BMPR-II, explain the majority of heritable PAH. BMPR-II forms a signalling complex, with ALK1 and endoglin, specific to endothelial cells which maintains vascular quiescence. BMP9 and BMP10 are the high affinity ligands for this signalling complex. Administering exogenous BMP9 has been shown to reverse disease in animal models of PAH, so the Morrell group is investigating the use of BMP9 as a therapy for PAH. Nonetheless, mutations in GDF2, which encodes BMP9, or BMP10 had not been conclusively associated with PAH. BMP9 and BMP10 are produced as inactive ProBMPs, which are processed by proteases to leave an active, non-covalent Pro:BMP complex of the prodomains and growth-factor domain. The circulating levels of processed and unprocessed BMP9 and BMP10 published in current reports are inconsistent. There is also some evidence circulating BMP9 and BMP10 may be co-complexed and otherwise coregulated. BMP9 is highly expressed in livers, and high doses can regulate lipogenesis and glucogenesis in vitro and in vivo. BMP9 deficiency is reported to protect against hepatic fibrosis and damage after inducement with severe surgical and chemical interventions. In contrast, BMP9 deficient animals do not appear to develop a phenotype reminiscent of PAH.
Recently, the Morrell group discovered loss-of-function mutations in GDF2 were associated with PAH with genome-wide significance in a rare-variant analysis. Most of the identified mutations were missense. To validate their effect in vitro, I compared a selection of those predicted to be pathogenic in silico to those predicted to be benign. Pathogenic variants were poorly secreted due to destabilisation of the interaction between the prodomains and growth-factor domain, reducing their activity. Patients carrying loss-of-function GDF2 mutations had reduced circulating levels of both BMP9 and BMP10. Data from a combination of ELISAs, immunoprecipitations and activity assays suggested that active BMP9 circulates in healthy people, whilst inactive, unprocessed BMP10 circulates at higher levels. Circulating levels of BMP9 and BMP10 were tightly correlated, and data from immunoprecipitations and size-exclusion chromatography suggests they may circulate in macromolecular complexes. Exogenous BMP9 administered to mice was rapidly cleared from the circulation but was able to bind the lungs and activate signalling there, further demonstrating its therapeutic utility. Hepatic transcriptomes from Gdf2 knockout mice showed evidence of reduced lipogenesis and oxidative metabolism, confirming a role for endogenous BMP9 in metabolism. We developed a protocol to measure pulmonary vascular leak in BMP9 deficient mice.