Abstract:
Hypocitraturia is a major metabolic risk factor for kidney stone development. Genetic heritability of urinary citrate excretion has been demonstrated. Reabsorption of citrate takes place in renal proximal tubules, which is principally responsible by sodium-dicarboxylate cotransporter-1 (NaDC-1). We investigated the association of genetic polymorphisms and NaDC-1 mRNA expression with hypocitraturic phenotype in patients with kidney stone. The exonic polymorphisms of NaDC-1 gene were screened in 13 patients with nephrolithiasis using direct genomic sequencing method. A missense SNP in exon 12 (I550V) was identified, rs11567842 (A/G). This SNP was genotyped in 114 nephrolithiasis patients (aged 21-77 years old, 46% males) and 62 healthy controls (aged 20-61 years old, 45% males) using PCR-RFLP. Logistic regression was performed to calculate adjusted odds ratios and 95% confidence intervals. Intrarenal mRNA expression of NaDC-1 was carried out in 29 nephrolithiasis patients using real-time RT-PCR. The patients excreted urinary citrate significantly lower than the controls. Hypocitraturia (<200 mg/day) was significantly associated with an increased risk of nephrolithiasis (adjusted OR = 6.89, 95%CI: 3.20-14.88). Allele and genotype frequencies of rs11567842 (A/G) SNP compared between nephrolithiasis and control subjects were not significantly different. The prevalence of hypocitaturia in individuals (all subjects) with homozygous GG was significantly lower than those carried A allele. In nephrolithiasis group, the risk of hypocitraturia in patients with homozygous GG (adjusted OR = 0.19, 95%CI: 0.06-0.62) was significantly lower than in those with homozygous AA (adjusted OR: 1.00), but risk of hypocitraturia in those with heterozygous AG (adjusted OR: 1.47, 95%CI: 0.50-4.34) was not significantly different from those with homozygous AA. No association of AA genotype with hypocitraturia in the healthy group was observed. A trend of increased intrarenal mRNA expression of NaDC-1 in nephrolithiasis patients associated with hypocitraturia was observed although it was not significant. Conclusion, we have demonstrated that the GG variant of rs11567842 SNP in NaDC-1 gene is associated with hypocitraturia in the nephrolithiasis population. Detection of this polymorphism may be useful for identifying individuals who are at risk of hypocitraturia. An increased expression of NaDC-1 in the kidneys of stone patients might contribute to the low urinary excretion of citrate