Background & aims: Defects in the Na+-dependent glucose transporter (SGLT1) are associated with the disorder glucose-galactose malabsorption, characterized by severe diarrhea. This study focused on a unique proband with glucose-galactose malabsorption who was investigated 30 years ago, and the aims of the study were to identify mutations in the SGLT1 gene and to determine the defect in sugar transport.
Methods: Mutations were identified by sequencing, and each mutant protein was then studied using a Xenopus oocyte heterologous expression system. Analysis included Western, freeze fracture, radiotracer uptake, and electrophysiological assays.
Results: Two heterozygous missense mutations (Cys355Ser and Leu147Arg) were identified that entirely eliminated Na+/sugar cotransport activity. Western blot analysis showed that the levels of both mutant proteins in the oocyte were comparable to wild-type SGLT1, but no complex glycosylation was detected. No SGLT1 charge movements were observed with the mutant proteins, and freeze fracture data showed that neither mutant protein reached the plasma membrane.
Conclusions: The Cys355Ser and Leu147Arg mutations eliminate the Na+/sugar cotransport by blocking the transfer of SGLT1 protein from the endoplasmic reticulum to the plasma membrane. This is consistent with earlier studies on phlorizin binding to the brush border membrane of duodenal biopsy specimens from this patient.