Congenital sucrase-isomaltase deficiency because of an accumulation of the mutant enzyme in the endoplasmic reticulum.
OBJECTIVE
Congenital sucrase-isomaltase deficiency (CSID) is an autosomal recessive human disorder characterized by reduced activities of the brush border enzyme sucrase-isomaltase (SI). Here, we elucidate the pathogenesis of a new variant of CSID at the cellular and molecular level.


METHODS
Assessment of the CSID phenotype was achieved by enzymatic activity measurements, biosynthetic labeling of intestinal biopsy specimens, immunoprecipitation of SI, and immunoelectronmicroscopy. The putative mutation was identified by sequencing of the SI cDNA isolated by RT-PCR from intestinal biopsy samples. The function of the mutation was verified by immunoprecipitation and confocal microscopy of transiently transfected cells.


RESULTS
Biosynthetic labeling and immunoelectron microscopy reveal a predominant localization of SI in the endoplasmic reticulum (ER) similar to phenotype I of CSID. Unlike phenotype I, however, a partial conversion of SI to a complex glycosylated mature form takes place. The SI cDNA in this phenotype revealed 3 mutations, 2 of which, Val to Phe at residue 15 and Ala to Thr at residue 231, had no effect on the structure or function of SI. By contrast, the third mutation resulted in an exchange of leucine by proline at position 620 (L620P) and revealed in transfected COS cells structural features and subcellular localization similar to the phenotype identified in the patient's enterocytes.


CONCLUSIONS
This is the first identification at the molecular and subcellular levels of a novel variant of CSID in which SI accumulates predominantly in the ER, and a minor proportion is further processed and transported to the apical membrane of enterocytes.
Ritz
Valentina
V
Institute of Physiological Chemistry, School of Veterinary Medicine Hannover, Germany.
Alfalah
Marwan
M
Zimmer
Klaus-Peter
KP
Schmitz
Jacques
J
Jacob
Ralf
R
Naim
Hassan Y
HY
D000818:Animals; D019556:COS Cells; D018076:DNA, Complementary / Q000302:isolation & purification; D004721:Endoplasmic Reticulum / Q000201:enzymology*; D006801:Humans; D017354:Point Mutation; D021381:Protein Transport; D013394:Sucrase-Isomaltase Complex / Q000032:analysis / Q000172:deficiency* / Q000235:genetics
D018076:DNA, Complementary; D013394:Sucrase-Isomaltase Complex