Dominant-negative effect of Southeast Asian ovalocytosis anion exchanger 1 in compound heterozygous distal renal tubular acidosis

The human chloride/bicarbonate AE1 (anion exchanger) is a dimeric glycoprotein expressed in the red blood cell membrane, and expressed as an N-terminal (Delta 1-65) truncated form, kAEl (kidney AE1), in the basolateral membrane of a-intercalated cells in the distal nephron. Mutations in AE1 can cause SAO (Southeast Asian ovalocytosis) or dRTA (distal renal tubular acidosis), an inherited kidney disease resulting in impaired acid secretion. The dominant SAO mutation (Delta 400-408) that results in an inactive transporter and altered eythrocyte shape occurs in many dRTA families, but does not itself result in dRTA. Compound heterozygotes of four dRTA mutations (R602H, G701D, Delta V850 and A858D) with SAO exhibit dRTA and abnormal red blood cell properties. Co-expression of kAE1 and kAE1 SAO with the dRTA mutants was studied in polarized epithelial MDCK (Madin-Darby canine kidney) cells. Like SAO, the G701D and Delta V850 mutants were predominantly retained intracellularly, whereas the R602H and A858D mutants could traffic to the basolateral membrane. When co-expressed in transfected cells, kAE1 WT (wild-type) and kAE1 SAO could interact with the dRTA mutants. MDCK cells co-expressing kAE1 SAO with kAE1 WT, kAE1 R602H or kAE1 A858D showed a decrease in cell-surface expression of the co-expressed proteins. When co-expressed, kAE1 WT colocalized with the kAE1 R602H, kAE1 G701D, kAE1 Delta V850 and kAE1 A858D mutants at the basolateral membrane, whereas kAE1 SAO co-localized with kAE1 WT, kAE1 R602H, kAE1 G701D, kAE1 Delta V850 and kAE1 A858D in MDCK cells. The decrease in cell-surface expression of the dRTA mutants as a result of the interaction with kAE1 SAO would account for the impaired expression of functional kAE1 at the basolateral membrane of alpha-intercalated cells, resulting in dRTA in compound heterozygous patients.