Impaired trafficking and instability of mutant kidney anion exchanger 1 proteins associated with autosomal recessive distal renal tubular acidosis

Background: Mutations in solute carrier family 4 member 1 (SLC4A1) encoding anion exchanger 1 (AE1) are the most common cause of autosomal recessive distal renal tubular acidosis (AR dRTA) in Southeast Asians. To explain the molecular mechanism of this disease with hematological abnormalities in an affected family, we conducted a genetic analysis of SLC4A1 and studied wild-type and mutant AE1 proteins expressed in human embryonic kidney 293T (HEK293T) cells.

Methods: SLC4A1 mutations in the patient and family members were analyzed by molecular genetic techniques. Protein structure modeling was initially conducted to evaluate the effects of mutations on the three-dimensional structure of the AE1 protein. The mutant kidney anion exchanger 1 (kAE1) plasmid construct was created to study protein expression, localization, and stability in HEK293T cells.

Results: We discovered that the patient who had AR dRTA coexisting with mild hemolytic anemia carried a novel compound heterozygous SLC4A1 mutations containing c.1199_1225del (p.Ala400_Ala408del), resulting in Southeast Asian ovalocytosis (SAO), and c.1331C > A (p.Thr444Asn). Homologous modeling and in silico mutagenesis indicated that these two mutations affected the protein structure in the transmembrane regions of kAE1. We found the wild-type and mutant kAE1 T444N to be localized at the cell surface, whereas the mutants kAE1 SAO and SAO/T444N were intracellularly retained. The half-life of the kAE1 SAO, T444N, and SAO/T444N mutants was shorter than that of the wild-type protein.

Conclusion: These results suggest impaired trafficking and instability of kAE1 SAO/T444N as the likely underlying molecular mechanism explaining the pathogenesis of the novel SLC4A1 compound heterozygous mutation identified in this patient.