Absorptive hypercalciuria (AH) is a systemic disorder secondary to defective calcium homeostasis in intestine and bone but presents primarily as nephrolithiasis. AH is a complex trait (genetic and environmental) with a polygenic component (multiple disease genes in the same affected individual) and exhibits loci heterogeneity (different genes in different affected individuals). We used positional cloning to identify a novel human gene linked to AH. This gene is homologous to the rat soluble adenylyl cyclase and is referred to as the AH-related adenylyl cyclase (AHRAC). AHRAC has complex multiple transcriptional and translational variants yielding multiple gene products with distinct expression patterns in intestine, bone, and kidney. We identified multiple nucleotide sequence variations in AHRAC that are observed with high frequency in patients with AH but are also present in low frequency in phenotypically normal individuals. The number of base changes in AHRAC is strikingly correlated with lower spinal bone density and increased gut calcium absorption. We hypothesize that either activating mutations (exon mutations) of AHRAC or increase expression (intron mutations) leads to enhanced apical calcium entry to the intestinal cell via the calcium channel ECaC as well as impairment of osteoblastic bone formation culminating in hypercalciuria. This paradigm can explain the heterozygosity and the phenotypic picture of intestinal hyperabsorption and low bone density in AH.
Aim 1 will define the primary structure, localization, and function of the AHRAC polypeptide in the intestinal cell (apical membrane calcium entry through (ECaC) and osteoblast function (proliferation and differentiation).
Aim 2 will examine the effect mutant AHRAC has on the same intestinal cell and osteoblast functions.
Aim 3 will examine the effect of AHRAC n patients using genotype-phenotype correlations in adults with AH as well as children with hypercalciuria or calcium stones. The independent variable will be number and/or type of AHRAC mutations and the dependent variable will be intestinal calcium absorption, bone mineral density, bone histomorphometry, and molecular markers of bone physiology from biopsy samples. This combined bench and clinical approach will help us understand the role of the AHRAC gene product in AH and in calcium homeostasis overall. These data will lay the foundation for further pursuits of the biology of AHRAC.
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