Low plasma stem cell factor combined with high transforming growth factor-a identifies high-risk patients in pulmonary arterial hypertension

In pulmonary arterial hypertension (PAH), severe vasoconstriction and remodelling of small pulmonary arteries result in high mortality. Receptor tyrosine kinases and their ligands, such as transforming growth factor (TGF)-alpha, modulate proliferation in PAH. Although the receptor tyrosine kinase c-Kit has been shown to be overexpressed in PAH, the expression and role of its ligand stem cell factor (SCF) remain unknown. However, low plasma SCF levels are known to be linked to higher cardiovascular mortality risk. Using proximity extension assays, we measured SCF and TGF-alpha in venous plasma from treatment-naive PAH patients and healthy controls. Patients were stratified into risk classes based on PAH guidelines. Plasma SCF was decreased (p=0.013) and TGF-alpha was increased (p<0.0001) in PAH patients compared to controls. SCF correlated to pulmonary vascular resistance (r=-0.66, p<0.0001), cardiac index (r=0.66, p<0.0001), venous oxygen saturation (r=0.47, p<0.0008), mean right atrial pressure (r=-0.44, p<0.002) and N-terminal pro-brain natriuretic protein (r=-0.39, p<0.006). SCF was lower in "high-risk" compared to "intermediate-risk" (p=0.0015) or "low-risk" (p=0.0009) PAH patients. SCF and TGF-alpha levels combined (SCF/TGF-alpha) resulted in 85.7% sensitivity and 81.5% specificity for detecting high-risk patients (p<0.0001). Finally, REVEAL (Registry to Evaluate Early and Long-Term Pulmonary Arterial Hypertension Disease Management) risk scores in PAH patients correlated to SCF/TGF-alpha levels (r=-0.50, p=0.0003). In conclusion, low plasma SCF combined with high TGF-alpha identifies high-risk PAH patients at baseline. Lower circulating SCF levels, which are associated with worse haemodynamics, may be related to the c-Kit accumulation previously observed in PAH.