Supplementary Material for: L-Type Voltage-Dependent Calcium Channel Alpha Subunit 1C Is a Novel Candidate Gene Associated with Secondary Hyperparathyroidism: An Application of Haplotype-Based Analysis for Multiple Linked Single Nucleotide Polymorphisms

<p><i>Background and Methods:</i> We conducted a large-scale case-control study that explored the association of 358 single nucleotide polymorphisms (SNPs) in 185 patients with end-stage renal disease. A variety of SNPs were recognized as significant in simple association studies. In addition, haplotype analysis identified the gene for the alpha 1C subunit of the voltage-dependent L-type calcium channel (CACNA1C) as having a significant association with secondary hyperparathyroidism (intact parathyroid hormone level >200 pg/ml) among 61 haplotypes. Since CACNA1C is a relatively large molecule, we examined 84 SNP markers from the CACNA1C region located on chromosome 12 by haplotype case-control association analysis. <i>Results:</i> Sixteen SNPs of 14 genes were significant according to allelic and/or genotypic studies (p < 0.05 by Fisher’s exact test). Three different SNPs were from the CACNA1C gene. Next, we performed haplotype-based association testing with a focus on the CACNA1C region, revealing an odds ratio (OR) of 1.63 and 95% confidence interval (CI) of 1.05–2.52. The second major haplotype with a frequency of 27% was also significant and acted as a protective haplotype (p = 0.022 by Fisher’s exact test, with an OR of 0.55 and 95% CI of 0.33–0.90). <i>Conclusion:</i> These results suggest that CACNA1C may be associated with secondary hyperparathyroidism. In addition, the haplotype-based approach may be useful to screen for key molecules associated with complex traits.</p

Acute parathyroid hormone differentially regulates renal brush border membrane phosphate cotransporters

Renal phosphate reabsorption across the brush border membrane (BBM) in the proximal tubule is mediated by at least three transporters, NaPi-IIa (SLC34A1), NaPi-IIc (SLC34A3), and Pit-2 (SLC20A2). Parathyroid hormone (PTH) is a potent phosphaturic factor exerting an acute and chronic reduction in proximal tubule phosphate reabsorption. PTH acutely induces NaPi-IIa internalization from the BBM and lysosomal degradation, but its effects on NaPi-IIc and Pit-2 are unknown. In rats adapted to low phosphate diet, acute (30 and 60 min) application of PTH decreased BBM phosphate transport rates both in the absence and the presence of phosphonoformic acid, an inhibitor of SLC34 but not SLC20 transporters. Immunohistochemistry showed NaPi-IIa expression in the S1 to the S3 segment of superficial and juxtamedullary nephrons; NaPi-IIc was only detectable in S1 segments and Pit-2 in S1 and weakly in S2 segments of superficial and juxtamedullary nephrons. PTH reduced NaPi-IIa staining in the BBM with increased intracellular and lysosomal appearance. NaPi-IIa internalization was most prominent in S1 segments of superficial nephrons. We did not detect changes in NaPi-IIc and Pit-2 staining over this time period. Blockade of lysosomal protein degradation with leupeptin revealed NaPi-IIa accumulation in lysosomes, but no lysosomal staining for NaPi-IIc or Pit-2 could be detected. Immunoblotting of BBM confirmed the reduction in NaPi-IIa abundance and the absence of any effect on NaPi-IIc expression. Pit-2 protein abundance was also significantly reduced by PTH. Thus, function and expression of BBM phosphate cotransporters are differentially regulated allowing for fine-tuning of renal phosphate reabsorption