A mutation in the Glut2 glucose transporter gene of a diabetic patient abolishes transport activity

GLUT2, the facilitative glucose transporter isoform expressed in pancreatic beta cells, is believed to play a role in glucose-stimulated Insulin secretion. Two polymorphisms that result in amino acid substitutions have been reported in the human GLUT2 gene (Tanizawa, Y., Riggs, A. C., Chiu, K. C., Janssen, R. C., Bell, D. S. H., Go, R. P. C., Roseman, J. M., Acton, R. T., and Permutt, M. A. (1994) Diabetologia 37, 420-427). A threonine 110-->isoleucine substitution was present at equal frequency in diabetic and control populations, and a valine 197-->isoleucine substitution was discovered in a single allele of a patient with non-insulin-dependent diabetes. The effect of these amino acid changes on glucose transport activity was tested by expression of the mutant proteins in Xenopus oocytes. The polymorphism at threonine 110 had no effect on the expression of GLUT2 protein or the uptake of 2-deoxyglucose. Remarkably, however, the highly conservative valine 197-->isoleucine amino acid change abolished transport activity of the GLUT2 transporter expressed in Xenopus oocytes. This represents the first known dysfunctional mutation in a human facilitative glucose transporter protein. The presence of this mutation in a diabetic patient suggests that defects in GLUT2 expression may be causally involved in the pathogenesis of non-insulin-dependent diabetes.