#3325 EXTRARENAL CYP27B1 ACTIVITY CONTRIBUTES TO THE CIRCULATING POOL OF 1,25(OH)2D
Authors: Joergensen Hanne , Peersman Nele , Billen Jaak , Vermeersch Pieter , Ivison Fiona , Heijboer Annemieke , Bouillon Roger , Naesens Maarten , Kuypers Dirk , Evenepoel Pieter 
Published: June 2023

Abstract

Background and Aims
Alterations of vitamin D metabolism are ubiquitous in patients with chronic kidney disease (CKD) and a key feature of the syndrome of CKD-mineral and bone disorder. A central paradigm in vitamin D metabolism is that the kidneys are the only relevant source of activation of vitamin D for systemic effects. However, various cells and tissues exhibit CYP27B1 activity and are capable of producing 1.25(OH)2D, which could contribute to the circulating pool [1]. This longitudinal, observational study challenges the dogma of the kidneys as the only source of 1.25(OH)2D, by investigating vitamin D metabolites in anephric patients free of active vitamin D therapy.

Method
Anephric patients free of active vitamin D therapy were included at time of kidney transplantation. A subset had a second study visit at 12 months post-transplant. Liquid chromatography–tandem mass spectrometry was used to measure 25(OH)D2/3, 1,25(OH)2D3 and 24,25(OH)2D3. Metabolic ratios of vitamin D metabolites over precursors were used as measures of CYP24A1 (24,25(OH) 2D3/ 25(OH)D3) and CYP27B1(1,25(OH)2D/ 25(OH)D3) activity. Biointact PTH (1-84) was measured by an in-house immunoradiometric assay [2] and FGF23 was measured by ELISA (Kainos Laboratories, Tokyo, Japan). Interleukin 6 (IL6), tumor necrosis factor-Α (TNF- Α) and interferon-Γ (IFN-Γ) were measured using an electrochemiluminescence multiplex immunoassay (Human Proinflammatory 1 V-plex, MSD, Maryland, US). Two independent laboratories performed confirmatory analyses of 1,25(OH)2D.

Results
All anephric patients (n = 38) had detectable (&gt;2.5 pg/mL) levels of 1,25(OH)2D, with a median value of 13.5 pg/mL (range 3.9 to 35.7 pg/ml). There was a significant correlation between levels of 25(OH)D and 1,25(OH)2D (r = 0.583, p≪0.001), with 25(OH)D-levels explaining 34% of the variation in 1,25(OH)2D (Fig. 1). 1,25(OH)2D levels were not significantly correlated to PTH (rho = 0.279, p = 0.09) or FGF23 (rho = –0.280, p = 0.09), nor to any of the inflammatory markers; IL6 (rho = 0.068, p = 0.69), TNF-Α (rho = –0.264, p = 0.11), IFN-Γ (rho = –0.001, p = 0.99). To validate our findings, anonymized samples of 17 anephric patients and 17 CKD stage 5D with residual renal function were sent to two independent laboratories. The presence of 1,25(OH)2D was confirmed in all anephric patients. Twenty-five anephric patients underwent a successful kidney transplantation and were free of active vitamin D therapy at a study visit 12 months post-transplant. After kidney transplantation, 25(OH)D-level decreased, while levels of 1,25(OH)₂D and 24,25(OH) ₂D substantially increased. The metabolic ratios expressing CYP24A1 and CYP27B1 activity both increased (Fig. 2). At 12 months post-transplant, 1,25(OH)₂D-levels correlated with levels of biointact PTH (rho = 0.603, p = 0.002), but not with levels of FGF23 (rho = –0.262, p = 0.21).

Conclusion
Extrarenal CYP27B1 activity contributes to the circulating active vitamin D pool and may sustain low-normal levels of active vitamin D, even in anephric individuals. While renal CYP27B1 activity is hormonally regulated, extrarenal CYP27B1 activity seems to be mainly substrate dependent.