Single Nucleotide Polymorphisms (SNPs) in Human Homologs of Genes Up-Regulated in the Kidney of S+S− Antilles Transgenic Mice.

Renal damage is a common complication of sickle cell anemia as current management allows most patients to survive beyond the 5th decade. We previously reported differential gene expression in the kidney of the S+S− Antilles transgenic mouse, a mouse characterized by renal defects similar to those seen in sickle cell anemia, including congested glomeruli, medullary fibrosis, renal enlargement, vasoocclusion, and a concentrating defect. BOLD-MRI showed that the kidneys of S+S− Antillles mice had more deoxyhemoglobin than those of control (C57Bl) mice. Gene expression microarrays identified genes highly up-regulated in the kidneys that were validated by real-time PCR. Among the up-regulated genes were cytochrome P450 4a14 ( cyp411 ), mitochondrial hydroxyl-methylglutaryl CoA synthase (reductase) ( hmgcr ), cytokine inducible SH-2 containing protein ( cish ), retinol dehydrogenase type III, arginase II (a rg2 ), 2-hydroxy oxidase ( hao2 ), renin-1 ( ren ), and alkaline phosphatase 2 ( alplp) . An increase in enzyme activity was also shown for arginase II. These genes can be integrated into several different responses to organ damage: response to hypoxia, a replacement cascade involving the loss of renal proteins in the urine; and a tissue damage-ameliorating cascade in which damage repair is the main goal. We hypothesized that polymorphisms in these genes might permit them to modulate the development of renal failure and other organ damage in patients with sickle cell anemia. Accordingly, we examined SNPs in the human homologs of these selected genes in 198 cases of sickle cell anemia with renal failure, defined as a serum creatinine greater than the 75th percentile adjusted for the patients’ age and sex and in 645 sickle cell anemia patients without renal insufficiency, defined as a serum creatinine less than the 25th percentile, also adjusted for age and sex. SNPs were analyzed in 7 genes where hypoxia-induced up-regulation in sickle mouse kidneys: ALPL (seven SNPs), CYP411 (4 SNPs), HAO2 ( 5 SNPs), REN (1 SNP), CISH (2 SNPs), HMGCR (5 SNPs), ARG2 (2 SNPs). No significant associations were found between these SNPs and renal failure, as defined above. We next evaluated 129 SNPs in 44 candidate genes that mediated inflammation, oxidant injury, NO biology, vasoregulation; cell-cell interaction, blood coagulation, hemostasis, growth factors and cytokines. One SNP in Klotho ( KL ) was associated with renal failure. Polymorphisms in KL have been associated with other sub-phenotypes of sickle cell anemia. Recently, kl has been noted to be up-regulated in subtotal-nephrectomized rats, suggesting that this gene plays a role in renal adaptation and kl protein has been implicated in calcium and phosphorus homeostasis and the regulation of renal stanniocalcin gene expression in vivo, at least partly, through the control of calcium and phosphate concentrations. The lack of other associations between SNPs and the phenotype of renal failure may be due to insufficient numbers of SNPs studied, a poor choice of candidate genes (the highest up-regulated) and our definition of renal failure, that does not include renal damage without glomerular disease, like tubular damage, urine concentrating ability, renal hypertrophy, renal hypoxia and medullary damage.