Molecular basis of heterozygosity in congenital sucrase‐isomaltase deficiency and functional gastrointestinal disorders

Congenital sucrase‐isomaltase deficiency (CSID) is a genetically‐determined gastrointestinal disorder caused mainly by homozygous or compound heterozygous hypomorphic variants in the sucrase isomaltase (SI) gene. Patients suffering from CSID present with abdominal pain, diarrhea, bloating and alterations in bowel pattern. Similar symptoms have been also observed in patients, who express low activities of sucrase in functional gastrointestinal disorders (FGIDs) as well in CSID heterozygotes. In this study, we assessed the effects of SI mutants that occur in heterozygotes on the trafficking and function of wild type SI (SI‐WT). HA‐tagged SI gene variants were co‐expressed in COS‐1 cells with FLAG‐tagged SI‐WT. The intracellular localization of individually or co‐expressed SI isoforms was assessed by immunofluorescence and biochemically by endoglycosidase H treatment which primarily cleaves ER‐located N‐linked mannose‐rich isoforms of glycoproteins and provides thus information on their trafficking pattern. Both approaches revealed a pronounced mutation‐relevant diversity in the trafficking pattern of the SI mutants and a hierarchical pattern in their interaction with wild type SI. Highly pathogenic SI mutants, such as SI carrying the E801X mutation, override the transport competence of SI‐WT and elicit its retention in the ER. Other mutants behave in a variable manner depending on their pathogenicity levels. Finally, the digestive capacities of SI‐WT towards sucrose and palatinose were substantially reduced when co‐expressed with SI mutants and correlated directly with the pathogenicity of the SI mutation. Our study demonstrates unequivocally that SI mutants interact intracellularly with SI‐WT, negatively impact the biosynthetic, trafficking and functional properties of the resultant SI protein and provides a novel insight into the role of heterozygosity in the aetiology of CSID and FGID.