Uncoupling CKD from MBD: Role of Proximal Tubule HNF4A in Mineral Metabolism and Kidney Function

Background: Hepatocyte nuclear factor 4 alpha (HNF4A) is a transcription factor that plays an essential role in nephron development as well as glucose and phosphate reabsorption in the proximal tubules. HNF4A expression is reduced in the kidneys of patients and animals with CKD; however, the direct contribution of kidney HNF4A deficiency to CKD progression and impaired mineral metabolism is largely unknown. Methods: We generated mice harboring a conditional deletion of Hnf4a in proximal tubules and studied kidney, mineral and bone phenotypes at 20 weeks of age in WT, Col4a3KO (CKD), Hnf4aPax8-cKO(CKO) and Col4a3KO/Hnf4aPax8-cKO (CPD). We measured biochemical, hematological parameters, and 3D bone microarchitecture. We also performed “bulk” and single-cell combined RNA and ATAC sequencing to identify genes and molecular pathways altered by HNF4A. Results: As expected, CKD mice showed reduced kidney function, hyperphosphatemia, and increased PTH and FGF23 levels. RNAseq revealed reduced kidney Hnf4a expression and impaired HNF4A-regulated molecular pathways in CKD, including mitochondrial dysfunction, oxidative phosphorylation, and cellular glycolysis. Multiomics analysis showed that reduction of HNF4A expression and activity inhibits the expression of Napi2a and 1α-hydroxylase in CKO compared to WT and leads to further inhibition in CPD mice compared to CKD. Consequently, genetic deletion of kidney Hnf4a promoted kidney fibrosis, calcitriol deficiency, and phosphaturia without impacting serum phosphate in CKO and further accentuated kidney disease progression in CPD. Conversely, despite impaired renal function, mutant mice had lower levels of FGF23 and PTH (-68% and -50%, respectively in CPD vs CKD). CKD mice had lower trabecular and cortical BMD, as well as higher cortical porosity, that were further accentuated in CPD mutants (-10% and +12%, respectively), despite the significant reduction in PTH levels. Conclusion: In aggregate, kidney HNF4A reduction is an initial adaptive response promoting phosphate excretion, leading to reduced FGF23, but becomes maladaptive, leading to mitochondrial dysfunction, accelerated CKD progression, and PTH-independent bone loss. Our results support HNF4A as a regulator of kidney function and a promising therapeutic target for CKD-MBD. Funding: NIDDK Support