Targeted Genomic Deletions Identify Diverse Enhancer Functions and Generate a Kidney-specific, Endocrine-deficient Pseudo-null Mouse

Vitamin D is terminally bioactivated in the kidney to 1α,25-dihydroxyvitamin D (1,25(OH)D) via cytochrome P450 family 27 subfamily B member 1 (CYP27B1), whose gene is regulated by parathyroid hormone (PTH), fibroblast growth factor 23 (FGF23), and 1,25(OH)D Our recent genomic studies in the mouse have revealed a complex kidney-specific enhancer module within the introns of adjacent methyltransferase-like 1 () and that mediate basal and PTH-induced expression of and FGF23- and 1,25(OH)D-mediated repression. Gross deletion of these segments in mice has severe effects on regulation and skeletal phenotype but does not affect expression in nonrenal target cells (NRTCs). Here, we report a bimodal activity in the intronic enhancer with components responsible for PTH-mediated induction and 1,25(OH)D-mediated repression and additional activities, including FGF23 repression, within the enhancers. Deletion of both submodules eliminated basal expression and regulation in the kidney, leading to systemic and skeletal phenotypes similar to those of -null mice. However, basal expression and lipopolysaccharide-induced regulation of in NRTCs was unperturbed. Importantly, dietary normalization of calcium, phosphate, PTH, and FGF23 rescued the skeletal phenotype of this mutant mouse, creating an ideal model to study nonrenal 1,25(OH)D production in health and disease. Finally, we confirmed a conserved chromatin landscape in human kidney that is similar to that in mouse. These findings define a finely balanced homeostatic mechanism involving PTH and FGF23 together with protection from 1,25(OH)D toxicity that is responsible for both adaptive vitamin D metabolism and mineral regulation.