Band 3 Edmonton I, a novel mutant of the anion exchanger 1 causing spherocytosis and distal renal tubular acidosis

Abstract

dRTA (distal renal tubular acidosis) and HS (hereditary spherocytosis) are two diseases that can be caused by mutations in the gene encoding the AE1 (anion exchanger 1; Band 3). dRTA is characterized by defective urinary acidification, leading to metabolic acidosis, renal stones and failure to thrive. HS results in anaemia, which may require regular blood transfusions and splenectomy. Mutations in the gene encoding AE1 rarely cause both HS and dRTA. In the present paper, we describe a novel AE1 mutation, Band 3 Edmonton I, which causes dominant HS and recessive dRTA. The patient is a compound heterozygote with the new mutation C479W and the previously described mutation G701D. Red blood cells from the patient presented a reduced amount of AE1. Expression in a kidney cell line showed that kAE1 (kidney AE1) C479W is retained intracellularly. As kAE1 is a dimer, we performed co-expression studies and found that, in kidney cells, kAE1 C479W and G701D proteins traffic independently from each other despite their ability to form heterodimers. Therefore the patient carries one kAE1 mutant that is retained in the Golgi (G701D) and another kAE1 mutant (C479W) located in the endoplasmic reticulum of kidney cells, and is thus probably unable to reabsorb bicarbonate into the blood. We conclude that the C479W mutant is a novel trafficking mutant of AE1, which causes HS due to a decreased cell-surface AE1 protein and results in dRTA due to its intracellular retention in kidney.

Mutation Conferring Apical-Targeting Motif on AE1 Exchanger Causes Autosomal Dominant Distal RTA

“…16,17 Loss of polarized membrane expression has also been reported with the G609R mutant, whereas intracellular retention in polarized cells characterizes the R589H, S613F, and C479W mutants. 12,14,17 Additional investigation confirmed the tyrosine residue at position 904 within the C terminus as a basolateral determinant; the phosphorylation states of both this determinant and an N-terminal tyrosine seem important in governing trafficking to and from the membrane. [16][17][18] In addition, the extreme C terminus of AE1 conforms to the X-F-X-F amino acid sequence of a class II PDZ (postsynaptic density protein, Drosophila disc large tumor suppressor, Zonula occludens-1 protein) ligand; such interactions may also play a role in membrane targeting or retention.…”

“…9 To date, nine separate SLC4A1 mutations have been described in association with ddRTA. 3,[10][11][12][13][14] Where examined, the in vitro anion exchange function of these mutants was found to be at or near normal levels, 3,6,12,15 suggesting that disease may instead occur through mistargeting of the mutant protein within the polarized a-IC. 3 Expression of the Cterminal truncation mutant R901X in polarized MadinDarby canine kidney (MDCK) cells confirmed this suggestion with mislocalization of the mutant kAE1 to the apical membrane, indicating the presence of basolateral targeting motifs within the C-terminal domain.…”

Molecular Genetic Mechanisms of Hereditary Spherocytosis: Current Perspectives

“…Chu et al [28] found a TGC>TGG mutation in exon 13 and a GGC>GAC mutation in exon 17 of SLC4A1 in an HS patient with distal renal tubular acidosis. The mutation in exon 13 was detected in his father and that in exon 17 in his mother.…”

Substitution of transmembrane domain Cys residues alters pHo-sensitive anion transport by AE2/SLC4A2 anion exchanger

“…The mutant C479W polypeptide was retained in the endoplasmic reticulum of polarized MDCK cells, and contributed to reduced red cell AE1 abundance (12). Mutations to Cys of hAE1 residues R490, R518, and R808 also cause hereditary spherocytosis.…”