A Truncated Mutant – The Most Common Sucrase‐Isomaltase Deficiency in the Inuit Population: Cellular and Biochemical Characterization

Abstract

Sucrase isomaltase (SI) is the most prominent disaccharidase in the small intestine responsible for the final steps of carbohydrate digestion. Mutations in the SI gene can lead to a drastic reduction or loss of the catalytic activity required for digestion of disaccharides, and thus can be associated with maldigestion and malabsorption of carbohydrates. Carbohydrate malabsorption is one of the most common gastrointestinal problems, where 70% of the population are affected. Congenital sucrase‐isomaltase deficiency (CSID) is an autosomal recessive disorder caused by defective SI activity; and it is clinically characterized by abdominal pain, flatulence, bloating, and diarrhea. Recently, a homozygous frameshift mutation, c.273_274delAG (p.Gly92Leufs*8), was identified in the Inuit population with an observed allele frequency of 17.2%. In this study, Cos‐1 cells were transiently transfected with cDNA encoding the Gly92Leufs*8 mutation or SI wild‐type (SI‐WT) as a control. The assessment of this new variant phenotype was investigated through enzymatic activity measurements, cellular localization and trafficking behavior. The truncation is associated with the elimination of all N‐glycosylation sites, whereas the Ser/Thr‐rich stalk region is retained pointing to a potential O‐glycosylation. In fact, treatment with benzyl 2‐acetamido‐2‐deoxy‐α‐D‐galactopyranoside, an inhibitor of O‐glycosylation in the Golgi, resulted in a substantial shift in the size of the mutant concomitant with transport competence of the mutant to O‐glycosylation in the Golgi Apparatus. The mutant is further trafficked to the cell surface as assessed by the detection of biotinylated forms of the mutant at the cell surface. This result was also corroborated by immunofluorescence images, which revealed the mutant at the cell surface. However, this mutant does not exhibit SI activity due to the absence of the sucrase and isomaltase domains. While our data unraveled the molecular basis for the onset of the clinical symptoms in patients homozygous to this mutation, further work is needed to determine whether heterozygotes with this mutation are also affected and to which extent.