Bone alkaline phosphatase isoforms in chronic kidney disease : mineral and bone disorder

Abstract: Chronic kidney disease (CKD) is associated with increased mortality and cardiovascular complications. Disturbances in mineral metabolism occur early <luring the course of CKD and several components of the CKD-mineral and bone disorder (CKD-MBD) are independent predictors of mortality. Alkaline phosphatase (ALP) is necessary for skeletal mineralization and is also involved in the process of vascular calcification. In recent years, ALP has evolved as a strong predictor of mortality in the CKD population. The significant role of ALP in the mineralization process renders it a putative target for the treatment and prevention of vascular calcification. Three circulating isoforms of bone ALP (BALP) have been identified (B/I, B 1, and B2). A fourth isoform, Blx, has been identified exclusively in serum from patients with CKD. The aim of the present thesis was to further elucidate the role ofthe BALP isoforms in CKD with respect to bone abnormalities and vascular calcification.In study I we identified the novel BALP isoform Blx in 20% of patients with mild to moderate CKD. Blx was associated with lower glomerular filtration rate and higher serum phosphate and calcium x phosphate product, which are risk factors for cardiovascular mortality in CKD. We also identified the BALP isoforms B/I, Bl and B2 as predictors of total hip bone mineral density.Study II was an experimental study, investigating the role of the BALP isoforms in phosphate induced calcification of human aortic smooth muscle cells (HASMCs). We found that the ALP expressed in HASMCs is exclusively BALP. Phosphate induced calcification of HASMCs was associated with increased BALP isoforms B/I, Blx, and B2 activities, which implies functional differences between the BALP isoforms in HASMC calcification.In study III we investigated the association of BALP isoforms in serum and histomorphometric parameters of bone in patients on chronic hemodialysis. W e identified the BALP isoform Blx as a novel marker for reduced osteoblastic activity.Study IV was a prospective cohort study of the association of serum BALP isoforms with aortic calcification and vascular stiffness in prevalent chronic dialysis patients. Blx was associated with baseline and time varying vascular stiffness, determined by pulse wave velocity, but not with calcification of the abdominal aorta. We also found an association of Blx with better event-free survival.In conclusion, these studies demonstrate that the BALP isoforms, especially isoform Blx, are involved in different aspects of CKD-MBD. This opens up for further research to identify the BALP isoforms as diagnostic markers and possible treatment targets in CKD-MBD.