The Vacuolar H+-ATPase B1 Subunit Polymorphism p.E161K Associates with Impaired Urinary Acidification in Recurrent Stone Formers

Abstract

Mutations in the vacuolar-type H(+)-ATPase B1 subunit gene ATP6V1B1 cause autosomal-recessive distal renal tubular acidosis (dRTA). We previously identified a single-nucleotide polymorphism (SNP) in the human B1 subunit (c.481G>A; p.E161K) that causes greatly diminished pump function in vitro To investigate the effect of this SNP on urinary acidification, we conducted a genotype-phenotype analysis of recurrent stone formers in the Dallas and Bern kidney stone registries. Of 555 patients examined, 32 (5.8%) were heterozygous for the p.E161K SNP, and the remaining 523 (94.2%) carried two wild-type alleles. After adjustment for sex, age, body mass index, and dietary acid and alkali intake, p.E161K SNP carriers had a nonsignificant tendency to higher urinary pH on a random diet (6.31 versus 6.09; P=0.09). Under an instructed low-Ca and low-Na diet, urinary pH was higher in p.E161K SNP carriers (6.56 versus 6.01; P<0.01). Kidney stones of p.E161K carriers were more likely to contain calcium phosphate than stones of wild-type patients. In acute NH4Cl loading, p.E161K carriers displayed a higher trough urinary pH (5.34 versus 4.89; P=0.01) than wild-type patients. Overall, 14.6% of wild-type patients and 52.4% of p.E161K carriers were unable to acidify their urine below pH 5.3 and thus, can be considered to have incomplete dRTA. In summary, our data indicate that recurrent stone formers with the vacuolar H(+)-ATPase B1 subunit p.E161K SNP exhibit a urinary acidification deficit with an increased prevalence of calcium phosphate-containing kidney stones. The burden of E161K heterozygosity may be a forme fruste of dRTA.

Keywords: human genetics; kidney stones; renal tubular acidosis.

Figure 1.

p.E161K heterozygous SFs have higher UpH after acid loading. 48 WT and 21 p.E161K heterozygous (E161K) SFs underwent NH₄Cl loading, and 68.8% of WT and 71.4% of E161K heterozygous SFs tested were men (P=0.82). Median ages of WT and E161K patients were 41.8 (interquartile range, 34.5–50.6) and 39.5 years old (interquartile range, 25.4–44.8), respectively (P=0.17). (A) Urinary pH at the beginning of the test before ingestion of NH₄Cl capsules (time 0). (B) Trough urinary pH (nadir pH) reached during the test. (C) Trough urinary pH reached during the test adjusted for sex and median age. For this conditional plot, a linear regression model adjusted for sex and age was used. Age was set to the median of 41.8 years old, and sex was set to the most common category: men. (D) Urinary NH₄ at the beginning of the test before ingestion of NH₄Cl capsules (time 0). (E) Peak urinary NH₄ reached during the test. (F) Urinary NH₄ excretion profile (area under the curve). Urinary NH₄ levels were not available in all test participants. Between-group differences are determined by Mann–Whitney U test or chi-squared test where appropriate, and the corresponding P value is indicated. AUC, area under the curve; creat, creatinine.