A novel missense mutation in the sodium bicarbonate cotransporter (NBCe1/SLC4A4) causes proximal tubular acidosis and glaucoma through ion transport defects

Abstract

In humans and terrestrial vertebrates, the kidney controls systemic pH in part by absorbing filtered bicarbonate in the proximal tubule via an electrogenic Na+/HCO3- cotransporter (NBCe1/SLC4A4). Recently, human genetics revealed that NBCe1 is the major renal contributor to this process. Homozygous point mutations in NBCe1 cause proximal renal tubular acidosis (pRTA), glaucoma, and cataracts (Igarashi, T., Inatomi, J., Sekine, T., Cha, S. H., Kanai, Y., Kunimi, M., Tsukamoto, K., Satoh, H., Shimadzu, M., Tozawa, F., Mori, T., Shiobara, M., Seki, G., and Endou, H. (1999) Nat. Genet. 23, 264-266). We have identified and functionally characterized a novel, homozygous, missense mutation (S427L) in NBCe1, also resulting in pRTA and similar eye defects without mental retardation. To understand the pathophysiology of the syndrome, we expressed wild-type (WT) NBCe1 and S427L-NBCe1 in Xenopus oocytes. Function was evaluated by measuring intracellular pH (HCO3- transport) and membrane currents using microelectrodes. HCO3- -elicited currents for S427L were approximately 10% of WT NBCe1, and CO2-induced acidification was approximately 4-fold faster. Na+ -dependent HCO3- transport (currents and acidification) was also approximately 10% of WT. Current-voltage (I-V) analysis reveals that S427L has no reversal potential in HCO3-, indicating that under physiological ion gradient conditions, NaHCO3 could not move out of cells as is needed for renal HCO3- absorption and ocular pressure homeostasis. I-V analysis without Na+ further shows that the S427L-mediated NaHCO3 efflux mode is depressed or absent. These experiments reveal that voltage- and Na+ -dependent transport by S427L-hkNBCe1 is unfavorably altered, thereby causing both insufficient HCO3- absorption by the kidney (proximal RTA) and inappropriate anterior chamber fluid transport (glaucoma).