Mutations in the chloride-bicarbonate exchanger gene AE1 cause autosomal dominant but not autosomal recessive distal renal tubular acidosis

Abstract

Primary distal renal tubular acidosis (dRTA) is characterized by reduced ability to acidify urine, variable hyperchloremic hypokalemic metabolic acidosis, nephrocalcinosis, and nephrolithiasis. Kindreds showing either autosomal dominant or recessive transmission are described. Mutations in the chloride-bicarbonate exchanger AE1 have recently been reported in four autosomal dominant dRTA kindreds, three of these altering codon Arg589. We have screened 26 kindreds with primary dRTA for mutations in AE1. Inheritance was autosomal recessive in seventeen kindreds, autosomal dominant in one, and uncertain due to unknown parental phenotype or sporadic disease in eight kindreds. No mutations in AE1 were detected in any of the autosomal recessive kindreds, and analysis of linkage showed no evidence of linkage of recessive dRTA to AE1. In contrast, heterozygous mutations in AE1 were identified in the one known dominant dRTA kindred, in one sporadic case, and one kindred with two affected brothers. In the dominant kindred, the mutation Arg-589/Ser cosegregated with dRTA in the extended pedigree. An Arg-589/His mutation in the sporadic case proved to be a de novo mutation. In the third kindred, affected brothers both have an intragenic 13-bp duplication resulting in deletion of the last 11 amino acids of AE1. These mutations were not detected in 80 alleles from unrelated normal individuals. These findings underscore the key role of Arg-589 and the C terminus in normal AE1 function, and indicate that while mutations in AE1 cause autosomal dominant dRTA, defects in this gene are not responsible for recessive disease.

Figure 1

Proton and bicarbonate transport in the alpha-intercalated cell of the distal tubule. At the apical surface, hydrogen ions produced by the carbonic anhydrase-catalyzed hydration of CO₂ are secreted via the ATP-driven hydrogen pump and hydrogen-potassium exchanger. At the basolateral membrane, bicarbonate is reabsorbed in exchange for chloride via the anion exchanger AE1. Chloride exits the cell through a separate channel. CA, carbonic anhydrase.

Figure 2

Structures of autosomal recessive RTA kindreds in which linkage studies were performed. Affected and unaffected subjects are indicated by solid and open symbols, respectively. Dots indicate individuals unavailable for study. Double lines indicate parental consanguinity; all are first cousins except kindreds RTA23 and RTA24 who are second cousins. Genotypes at polymorphic loci closely flanking AE1 are shown.

Figure 3

Homozygosity mapping in the 14 consanguineous recessive kindreds. The result of multipoint comparison of affectation status with genotypes at polymorphic loci tightly linked to AE1 is shown under the stringent model of linkage. Genetic distances between these loci, and the region within which AE1 is thought to lie, are displayed.

Figure 4

Mutations in AE1 in dominant dRTA patients. The structure of each kindred is shown. Affected, unaffected, and phenotype-unknown subjects are indicated by solid, open, and shaded symbols, respectively. n = unrelated normal subjects. Below the diagram of each kindred in a–c are the results of single-strand conformational polymorphism analysis of exons 14, 14, and 20 of AE1, respectively. The corresponding DNA sequence of the sense strand of wild-type (Upper) and mutant alleles (Lower) are also shown. Arrows indicate variants specific to RTA patients. In a and b, asterisks above sequences indicate the variant bases in codon Arg-589. (a) In kindred RTA10, CGC is changed to AGC (589Ser) in all six affected members. (b) In kindred RTA8, only the affected index case shows the de novo mutation altering CGC to CAC (589His). (c) The bracket shows the 13-bp sequence present in single copy in wild-type sequence, but duplicated in tandem in both affected members of RTA6. The result leads to premature termination at codon 901 (underlined), truncating the protein by 11 amino acids. (d) Genotypes and haplotypes of kindred RTA8. Haplotypes flanking AE1 are shown, which confirmed biological parentage, and in addition show that the index case was identical with an unaffected sister. The results confirm the presence of a de novo mutation.