Bone morphogenetic protein receptor 2 signalling pathway expression in different patient groups with pulmonary hypertension

Background:
Bone morphogenetic protein receptor 2 (BMPR2) signalling is disturbed in heritable pulmonary arterial hypertension (HPAH) patients. We could previously show a reduced BMPR2 mRNA expression in the blood of HPAH patients with pathogenic BMPR2 variants. The BMPR2 expression levels in other PH subgroups and the expression of further BMPR2 pathway members are still largely unknown. 

Objective:
The aim of this study was to investigate both the mRNA and protein expression of BMPR2 and other components of the signalling pathway in different subgroups of PH patients and healthy controls. 

Methods:
In this multicentre, exploratory, cross-sectional study, a total of 234 subjects were included in seven cohorts: 30 HPAH patients with BMPR2 mutations, 38 idiopathic PAH (IPAH) without and 40 IPAH with comorbidities without BMPR2 mutations, 38 systemic sclerosis associated PAH (SSc-APAH), 47 chronic thromboembolic pulmonary hypertension (CTEPH), 9 healthy BMPR2 pathogenic variant carriers and 32 healthy controls. Clinical parameters were recorded at routine visits. Whole blood mRNA was extracted and qPCR performed for BMP10, BMPR2, SMAD5, ID1, EIF2AK4. Plasma protein levels were measured by ELISAs for BMP10, BMPR2, SMAD5 and EIF2AK4. Between group levels were compared using univariate ANOVA. Correlations of expressions with clinical data were calculated with rank correlation (Spearman).

Results:
BMPR2 mRNA expression was significantly reduced in all of the PH subgroups compared to healthy controls (p<0.001) with HPAH patients showing the strongest reduction in BMPR2 mRNA expression. BMP10 mRNA levels were significantly higher in CTEPH patients (p=0.011) compared to healthy controls. SMAD5 mRNA was significantly lower in IPAH (p=0.011) and SSc-APAH (p=0.001) patients compared to healthy controls. Similarly, EIF2AK4 mRNA was significantly lower in IPAH (p=0.038), SSc-APAH patients (p=0.003) and PAH patients with comorbidities (p=0.025). ID1 was similarly expressed across cohorts. 

Of the 4 proteins only BMP10 showed significant changes. In concert with the mRNA levels, BMP10 protein levels were significantly higher in CTEPH (p=0.001), but also in SSc-APAH (p=0.032) and PAH patients with comorbidities (p<0.001) compared to healthy controls. In contrast, HPAH patients presented with significantly lower BMP10 protein levels (p=0.025) compared to healthy controls. 

We could confirm a previously reported correlation of lower BMPR2 mRNA expression and poorer haemodynamics. A completely new finding is that higher BMP10 protein levels correlated with lower cardiac output (p=0.002) and higher WHO functional class (p=0.002), higher NT-proBNP (p<0,0001) and shorter 6-minute walking distance (p<0.001). 

Conclusion:
We could show for the first time that the BMPR2 mRNA expression is not only significantly reduced in pathogenic BMPR2 variant carriers, but also in all other assessed PH subgroups. Hence, all PH subgroups appear to have a general impairment of BMPR2 signalling. This strengthens the use of new therapies addressing this pathway such as sotatercept. Lower BMPR2 mRNA levels and higher BMP10 levels correlated with worse clinical parameters. Thus, apart from BMPR2 mRNA also BMP10 protein expression in the blood could serve as a new biomarker in PAH to assess disease severity. Whether different mechanisms in HPAH patients with pathogenic BMPR2 variants are at play reducing BMP10 protein levels remains to be investigated.