Identification of a novel natriuretic protein in patients with cerebral-renal salt wasting—implications for enhanced diagnosis

Background: The most vexing problem in hyponatremic conditions is to differentiate the syndrome of inappropriate secretion of antidiuretic hormone (SIADH) from cerebral/renal salt wasting (C-RSW). Both have identical clinical parameters but diametrically opposite therapeutic goals of water- restricting water-logged patients with SIADH or administering salt and water to dehydrated patients with C-RSW. While C-RSW is considered a rare condition, the report of a high prevalence of C-RSW in the general hospital wards creates an urgency to differentiate one syndrome from the other on first encounter. We decided to identify the natriuretic factor (NF) we previously demonstrated in plasma of neurosurgical and Alzheimer diseases (AD) who had findings consistent with C-RSW. 

Methods: We performed the same rat renal clearance studies to determine natriuretic activity (NA) in serum from a patient with a subarachnoid hemorrhage (SAH) and another with AD and demonstrated NA in their sera. The sera were subjected to proteomic and SWATH (Sequential Windowed Acquisition of All) analyses which identified increased levels of haptoglobin related protein (Hpr) without signal peptide (Hpr-WSP). 

Results: Recombinant Hpr with His tag at the N terminus had no NA. Hpr-WSP had a robust NA in a dose-dependent manner when injected into rats. Serum after recovery from C-RSW in the SAH patient had no NA. 

Conclusions: Hpr-WSP may be the NF in C-RSW which should be developed as a biomarker to differentiate C-RSW from SIADH on first encounter, introduces a new syndrome of C-RSW in AD and can serve as a proximal diuretic to treat congestive heart failure.