Regulation of gene expression by dietary Ca²⁺ in kidneys of 25-hydroxyvitamin D₃-1α-hydroxylase knockout mice

Background. Pseudovitamin D deficiency rickets (PDDR) is an autosomal disease, characterized by undetectable levels of 1,25-dihydroxyvitamin D ₃ (1,25(OH)₂D₃), rickets and secondary hyperparathyroidism. Mice in which the 25-hydroxy-vitamin D₃- 1α-hydroxylase (1α-OHase) gene was inactivated, presented the same clinical phenotype as patients with PDDR. Methods. cDNA Microarray technology was used on kidneys of 1α-OHase knockout mice to study the expression profile of renal genes in this Ca²⁺-related disorder. Genome wide molecular events that occur during the rescue of these mice by high dietary Ca²⁺ intake were studied by the use of 15K cDNA microarray chips. Results. 1α-OHase knockout mice fed a normal Ca²⁺ diet developed severe hypocalcemia, rickets and died with an average life span of 12 ± 2 weeks. Intriguingly, 1α-OHase^-/- mice supplemented with an enriched Ca²⁺ diet were normocalcemic and not significantly different from wild-type mice. Inactivation of the 1α-OHase gene resulted in a significant regulation of ± 1000 genes, whereas dietary Ca ²⁺ supplementation of the 1α-OHase^-/- mice revealed ± 2000 controlled genes. Interestingly, 557 transcripts were regulated in both situations implicating the involvement in the dietary Ca ²⁺-mediated rescue mechanism of the 1α-OHase^-/- mice. Conspicuous regulated genes encoded for signaling molecules like the PDZ-domain containing protein channel interacting protein, FK binding protein type 4, kinases, and importantly Ca²⁺ transporting proteins including the Na⁺-Ca²⁺ exchanger, calbindin-D _28K and the Ca²⁺ sensor calmodulin. Conclusion. Dietary Ca²⁺ intake normalized disturbances in the Ca²⁺ homeostasis due to vitamin D deficiency that were accompanied by the regulation of a subset of renal genes, including well-known renal Ca²⁺ transport protein genes, but also genes not previously identified as playing a role in renal Ca²⁺ handling.