Phenotypic and functional analysis of human SLC26A6 variants in patients with familial hyperoxaluria and calcium oxalate nephrolithiasis

Abstract

Background: Urinary oxalate is a major risk factor for calcium oxalate stones. Marked hyperoxaluria arises from mutations in 2 separate loci, AGXT and GRHPR, the causes of primary hyperoxaluria (PH) types 1 (PH1) and 2 (PH2), respectively. Studies of null Slc26a6(-/-) mice have shown a phenotype of hyperoxaluria, hyperoxalemia, and calcium oxalate urolithiasis, leading to the hypothesis that SLC26A6 mutations may cause or modify hyperoxaluria in humans.

Study design: Cross-sectional case-control.

Setting & participants: Cases were recruited from the International Primary Hyperoxaluria Registry. Control DNA samples were from a pool of adult subjects who identified themselves as being in good health.

Predictor: PH1, PH2, and non-PH1/PH2 genotypes in cases.

Outcomes & measures: Homozygosity or compound heterozygosity for SLC26A6 variants. Functional expression of oxalate transport in Xenopus laevis oocytes.

Results: 80 PH1, 6 PH2, 8 non-PH1/PH2, and 96 control samples were available for SLC26A6 screening. A rare variant, c.487C-->T (p.Pro163Ser), was detected solely in 1 non-PH1/PH2 pedigree, but this variant failed to segregate with hyperoxaluria, and functional studies of oxalate transport in Xenopus oocytes showed no transport defect. No other rare variant was identified specifically in non-PH1/PH2. Six additional missense variants were detected in controls and cases. Of these, c.616G-->A (p.Val206Met) was most common (11%) and showed a 30% reduction in oxalate transport. To test p.Val206Met as a potential modifier of hyperoxaluria, we extended screening to PH1 and PH2. Heterozygosity for this variant did not affect plasma or urine oxalate levels in this population.

Limitations: We did not have a sufficient number of cases to determine whether homozygosity for p.Val206Met might significantly affect urine oxalate.

Conclusions: SLC26A6 was effectively ruled out as the disease gene in this non-PH1/PH2 cohort. Taken together, our studies are the first to identify and characterize SLC26A6 variants in patients with hyperoxaluria. Phenotypic and functional analysis excluded a significant effect of identified variants on oxalate excretion.

Figure 1. Segregation Analysis of the c.487C>T (p.Pro163Ser) Variant in Non-PH1/PH2 Pedigree # 6

The proband is indicated by the arrow. P, proline allele; S, serine allele.

Figure 1. Segregation Analysis of the c.487C>T (p.Pro163Ser) Variant in Non-PH1/PH2 Pedigree # 6

The proband is indicated by the arrow. P, proline allele; S, serine allele.

Figure 2. 2A–2B Oxalate transport activity of c.487C>T (p.Pro163Ser) (A) and c.616G>A (p.Val206Met) (B) sequence variants of SLC26A6 expressed in Xenopus oocytes

Oxalate transport activity of oocytes injected with cRNA encoding wild-type (WT) and variant SLC26A6 is compared to water-injected control oocytes. Data represent means ± SE for n = 20–24 oocytes in each group (A), or 50–60 oocytes in each group (B). The difference between wild-type and p.Val206Met transport activity (B) was highly significant (P < 10⁻¹³ by Student’s t-test).

Figure 3. 3A–3B Levels of expression of total (A) and surface biotinylated (B) SLC26A6 in oocytes injected with cRNA encoding wild-type and p.Val206Met mutant transporters

Expression of HA-tagged transporters was assessed by immunoblotting. Representative Western blots are shown. In addition, band density was quantified with NIH Scion Image software for 4 similar experiments, and the mean results are expressed as arbitrary units. Data represent means ± SE.

Figure 4. Plasma and Urine Oxalate in Primary Hyperoxaluria Patients (PH1, PH2, nonPH1/PH2) With and Without the p.Val206Met SLC26A6 Variant

Rank-sums are given above the graphs. For Uox, only measurements at diagnosis were included (n= 9 for GFR<40, n=38 for GFR>40). For Pox, any available value was included (n=12 for GFR<40, n=15 for GFR>40).

Figure 4. Plasma and Urine Oxalate in Primary Hyperoxaluria Patients (PH1, PH2, nonPH1/PH2) With and Without the p.Val206Met SLC26A6 Variant

Rank-sums are given above the graphs. For Uox, only measurements at diagnosis were included (n= 9 for GFR<40, n=38 for GFR>40). For Pox, any available value was included (n=12 for GFR<40, n=15 for GFR>40).