Functional analysis of a novel missense NBC1 mutation and of other mutations causing proximal renal tubular acidosis.
Mutations in the Na(+)-HCO(3)(-) cotransporter NBC1 cause severe proximal tubular acidosis (pRTA) associated with ocular abnormalities. Recent studies have suggested that at least some NBC1 mutants show abnormal trafficking in the polarized cells. This study identified a new homozygous NBC1 mutation (G486R) in a patient with severe pRTA. Functional analysis in Xenopus oocytes failed to detect the G486R activity due to poor surface expression. In ECV304 cells, however, G486R showed the efficient membrane expression, and its transport activity corresponded to approximately 50% of wild-type (WT) activity. In Madin-Darby canine kidney (MDCK) cells, G486R was predominantly expressed in the basolateral membrane domain as observed for WT. Among the previously identified NBC1 mutants that showed poor surface expression in oocytes, T485S showed the predominant basolateral expression in MDCK cells. On the other hand, L522P was exclusively retained in the cytoplasm in ECV304 and MDCK cells, and functional analysis in ECV304 cells failed to detect its transport activity. These results indicate that G486R, like T485S, is a partial loss of function mutation without major trafficking abnormalities, while L522P causes the clinical phenotypes mainly through its inability to reach the plasma membranes. Multiple experimental approaches would be required to elucidate potential disease mechanism by NBC1 mutations.
Department of Internal Medicine, Faculty of Medicine, Tokyo University, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8655, Japan.
Masashi
M
Suzuki
Maria Helena
MH
Vaisbich
Hideomi
H
Yamada
Shoko
S
Horita
Yuehong
Y
Li
Takashi
T
Sekine
Nobuo
N
Moriyama
Takashi
T
Igarashi
Yoko
Y
Endo
Thaís P
TP
Cardoso
Luis Carlos F
LC
de Sá
Vera H
VH
Koch
George
G
Seki
Toshiro
T
Fujita
D000141:Acidosis, Renal Tubular / Q000235:genetics* / Q000378:metabolism; D000818:Animals; D002460:Cell Line; D002462:Cell Membrane / Q000378:metabolism; D002648:Child; D003593:Cytoplasm / Q000378:metabolism; D004252:DNA Mutational Analysis; D004285:Dogs; D005260:Female; D020022:Genetic Predisposition to Disease; D006720:Homozygote; D006801:Humans; D020125:Mutation, Missense; D009865:Oocytes; D010641:Phenotype; D021381:Protein Transport; D012307:Risk Factors; D012720:Severity of Illness Index; D027982:Sodium-Bicarbonate Symporters / Q000235:genetics* / Q000378:metabolism; D014162:Transfection; D014982:Xenopus laevis
C498898:SLC4A4 protein, human; D027982:Sodium-Bicarbonate Symporters