Missense mutations in SGLT1 cause glucose-galactose malabsorption by trafficking defects

Glucose-galactose malabsorption (GGM) is an autosomal recessive disorder ca used by defects in the Na+/glucose cotransporter (SGLT1). Neonates present with severe diarrhea while on any diet containing glucose and/or galactose [1]. This study focuses on a patient of Swiss and Dominican descent. All 15 exons of SGLT1 were screened using single stranded conformational polymorp hism analyses, and aberrant PCR products were sequenced. Two missense mutat ions, Gly318Arg and Ala468Val, were identified. SGLT1 mutants were expresse d in Xenopus laevis oocytes for radiotracer uptake, electrophysiological ex periments, and Western blotting. Uptakes of [C-14]alpha-methyl-D-glucoside by the mutants were 5% or less than that of wild-type. Two-electrode voltag e-clamp experiments confirmed the transport defects, as no noticeable sugar -induced current could be elicited from either mutant [2]. Western blots of cell protein showed levels of each SGLT1 mutant protein comparable to that of wild-type, and that both were core-glycosylated, Presteady-state curren t measurements indicated an absence of SGLT1 in the plasma membrane. We sug gest that the compound heterozygote missense mutations G318R and A468V lead to GGM in this patient by defective trafficking of mutant proteins from th e endoplasmic reticulum to the plasma membrane. (C) 1999 Elsevier Science B .V. All rights reserved.