Congenital Sucrase‐Isomaltase Deficiency

Brush border membranes are the largest exposed surfaces in tissues. They constitute the interface between the “milieu exterieur” and the “milieu interieur” of the body in a variety of organs such as the gastrointestinal tract and bile canaliculi, where hydrolytic, absorptive, and secretory processes take place. The intestinal mucosa is the exclusive site for nutrient metabolism and subsequent uptake of the generated products, such as monosaccharides and amino acids. The hydrolysis and absorption of micronutrients are achieved by the concerted action of hydrolases and transporters that are predominantly located in the brush border membranes (BBMs) (1). The hydrolases are divided into 2 major families, the peptidases and the disaccharidases (2). The peptidases, such as aminopeptidases N (CD13), A, and W, carboxypeptidases P and M, dipeptidylpeptidase IV, or α-glutamyl transpeptidase, are expressed in many tissues, including the intestine and the kidney (3,4). The expression of many of the disaccharidases, by contrast, is limited to the intestinal BBM. Prominent members of this family are the enzymes sucrase-isomaltase (SI) (5), maltase-glucoamylase (6), and lactase-phlorizin hydrolase (7). SI and maltase-glucoamylase hydrolyze α-glycosidically linked starch, glycogen, sucrose, and maltose. The generated monosaccharides are eventually transported across the BBM of epithelial cells into the cell interior. Another major brush border disaccharidase is lactase-phlorizin hydrolase, which cleaves β-glycosidic linkage in lactose, the main carbohydrate in mammalian milk that constitutes the primary diet source for newborns (8). Several other glycosidases with similar enzymatic specificities toward cleaving α- or β-glycosidic linkages exert their function in intracellular compartments, such as the lysosomes. Examples of this group of carbohydrases are lysosomal α-glucosidase (9), β-glucosidase (10), and α-galactosidase (11) or glycosidases of the glycogen catabolism. Pathological conditions, most notably malabsorption of disaccharides and the subsequent symptoms, are associated with the absence of these enzymes in the intestinal lumen. Examples of these disorders are congenital SI deficiency (CSID), adult-type hypolactasia, congenital lactase deficiency, secondary lactase deficiency, and maltase-glucoamylase deficiency. This review focuses on the structural and biosynthetic features of SI and the molecular basis of sugar malabsorption in CSID. The diversity and heterogeneity of this disease is reflected in the existence of several mutant phenotypes of SI that vary in their posttranslational processing, cellular localization, and function. The pathogenetic mechanisms of CSID are unique for intestinal malabsorption disorders and have implications for the pathobiology of the intestinal mucosa. Unraveling the molecular basis of this disease revealed novel mechanisms of protein trafficking and polarized sorting. PATHOPHYSIOLOGY, CLINICAL FEATURES, AND DIAGNOSIS OF CSID CSID is an autosomal recessive intestinal disorder that was first described by Weijers et al in 1960 (12). It arises from mutations in the intestinal brush border enzyme complex SI. CSID occurs in 0.2% of individuals of European descent (13) and approximately 5% in indigenous Greenlanders (14). Heterozygotes with normal small intestinal morphology and with sucrase activity level below the lower limit for the normal population represent approximately 2% to 9% of Americans of European descent (15,16). SI comprises 2 activities: sucrase that hydrolyzes α-1,2- and α-1,4-glucosidic bonds and isomaltase that cleaves α-1,6 linkages. The sucrase activity overlaps with that of intestinal maltase-glucoamylase, which digests α-1,4-glucosidic linkages of the end and intermediary product of α-amylolysis of starches such as maltose, maltotriose and low- and high-molecular-weight branched dextrins. Patients with CSID experience vomiting, osmotic diarrhea, mild steatorrhea, chronic diarrhea, and crying spells upon ingestion of sugars (17). Occasionally dehydration, failure to thrive, developmental retardation, and muscular hypotonia were observed, which were compatible with broad clinical heterogeneity (13,18). CSID also has been reported to be associated with nephrocalcinosis, renal calculi, metabolic acidosis, and hypercalcemia (19,20). The clinical heterogeneity is supported by the findings that diverse mutant phenotypes of SI are responsible for the onset of CSID. Several factors contribute to the development and extent of symptoms in patients with CSID: residual enzymatic activities of sucrase and isomaltase, amount of fed carbohydrate (in association with other foods), gastric emptying, small-bowel transit, degree of fermentation of unabsorbed carbohydrates by colonic bacteria, and absorption of the colon. Furthermore, the CSID symptoms also depend on the patient's age. Symptoms, and particularly starch tolerance, spontaneously improve with age. Onset of symptoms in adulthood with diagnosis up to age 59 years has been reported (19,21,22). The diagnosis of CSID is often delayed or perhaps missed because the symptoms are erroneously recognized as being related to diseases such as cystic fibrosis and celiac disease or to other causes of recurrent diarrhea and food allergy. A major step in diagnosing CSID is to recognize the complaints and clinical features of the patients in relation to age-dependent alterations and composition of nutrition. An increase in blood glucose of <20 mg/dL after a 2.0-g/kg sucrose load as well as an increase in breath hydrogen are compatible with sucrose intolerance (23–25). The diagnosis of CSID requires the determination of SI activity in mucosa with normal histology. Enterocytes of patients with CSID lack the sucrase activity of the enzyme SI, whereas the isomaltase activity can vary from absent to practically normal. The disaccharidase activities in the duodenum are reduced by almost 40% as compared with the proximal jejunum (25,26). In some SI-deficient patients, the activity of maltase-glucoamylase is reduced, as is the isomaltase activity (27). The activities of other brush border disaccharidases such as lactase-phlorizin hydrolase and maltase-glucoamylase and peptidases such as aminopeptidase N, dipeptidyl peptidase IV, and meprin are usually within the normal range. THERAPY OF CSID Lifelong sucrose restriction is an effective therapeutic option for patients with CSID. The degree of restriction, however, depends on the individual complaints of a patient because patients with CSID show variable tolerances toward sucrose. Sucrose concentrations between 3 and 6 g/100 g in nutrients (onion, honey, soybean flour) are considered to be high. In many cases of CSID the isomaltase activity is also affected. Therefore, the diet of patients with CSID should also exclude starch and glucose polymers, such as wheat and potatoes. Saccharomyces cerevisiae possesses sucrase activity and a low isomaltase and maltase activity and can be used in CSID therapy. The use of lyophilized preparations of S cerevisiae reduced hydrogen excretion by 70%, with loss or reduction of clinical symptoms in CSID (28,29). Sacrosidase or invertase (Sucraid), a liquid preparation produced from S cerevisiae, has been used successfully in the treatment of patients with CSID (30). STRUCTURAL FEATURES AND TRAFFICKING OF SI SI is a type II integral membrane glycoprotein that is exclusively expressed in the small intestinal microvillus membrane and is responsible for the terminal digestion of dietary sucrose and starch. The glycoprotein comprises 2 subunits that are highly homologous and are thought to be derived from the same ancestral gene (31,32). These 2 subunits are associated with each other by strong noncovalent, ionic interactions (33). SI is synthesized in the rough endoplasmic reticulum (ER) as a single-chain mannose-rich precursor comprising both subunits (pro-SIh, 210 kDa) (5) (Fig. 1). It is transported to the Golgi apparatus at a relatively slow rate and does not form homodimers before ER exit (5,34). The strong homologies between the 2 main domains suggest that quasidimers or pseudodimers are formed, which are presumably sufficient for the acquisition of transport competence from the ER to the Golgi apparatus and further to the cell surface. After modification of the N-linked glycans and O-glycosylation in the Golgi apparatus, SI is sorted to the apical membrane with high fidelity. In fact, 90% to 95% of the de novo synthesized protein is transported to the apical membrane. In the apical membrane pro-SI is cleaved in situ by luminal pancreatic proteases to its 2 active subunits, sucrase and isomaltase (33). On its way to the apical membrane, SI associates with cholesterol- and sphingolipid-enriched membrane microdomains (lipid rafts), which act as platforms to warrant an efficient sorting of SI (35). Interestingly, the association of SI with lipid rafts substantially increases the activity of sucrase and isomaltase by a factor of almost 3-fold (36).FIGURE 1: Pro-sucrase-isomaltase (1827 amino acids).MOLECULAR BASIS OF CSID Conformational modifications of membrane and secretory proteins commence during their translocation across the ER membrane and continue in the ER lumen (Fig. 1). Cotranslational glycosylation (37), intermolecular or intramolecular disulfide bond formation (38), and subunit assembly or oligomerization are examples of early modifications directly implicated in the protein maturation events, and are rate limiting along the exocytic pathway (39). Further posttranslational modifications in the Golgi apparatus, such as acquisition of a complex type of N-linked glycans and O-glycosylation (37), also affect protein trafficking to the cell surface and secretion into the external milieu and polarized protein sorting in epithelial cells. The dissection of molecular mechanisms required for efficient cellular transport of membrane and secretory proteins to the cell surface has greatly benefited from molecular analyses of genetic diseases directly associated with misfolded proteins and impaired protein targeting. Examples of these diseases are cystic fibrosis (CFTR protein) (40), familial hypercholesterolemia (low-density lipoprotein receptor) (41), Wilson disease (a P-type adenosine triphosphatase) (42), nephrogenic diabetes insipidus (aquaporin 2) (43), Liddell's syndrome (amilroide-sensitive epithelial Na+ channel) (44,45), and CSID (intestinal SI) (13). Early research on the molecular basis of CSID has suggested the presence of an enzymatically inactive SI in CSID (46,47). Hauri et al demonstrated a trafficking defect and an intracellular accumulation of SI in the trans-Golgi as an underlying cause of CSID (48). In a multicenter collaborative study, biopsy specimens from patients with CSID were analyzed at the biochemical and cellular levels (49). The study defined CSID as a heterogeneous carbohydrate malabsorption disorder that exists in several phenotypes relevant to altered trafficking, cellular localization, or function of SI (Table 1). This study further proposed that the CSID phenotypes are elicited by different mutations in the coding region of the SI gene. In fact, the first successful cloning and characterization of a cDNA-encoding SI from a biopsy sample of a patient with CSID led to the identification of a point mutation that is responsible for the impaired intracellular transport behavior of SI (50). In the meantime, the mutations and their consequences on the trafficking and function of SI in 7 different phenotypes of CSID have been identified (Table 1, Figs. 2 and 3) (48,49,51). A survey of the features of the individual phenotypes follows.TABLE 1: Naturally occurring phenotypes of congenital sucrase-isomaltase deficiencyFIGURE 2: Isomaltase-based mutations in CSID.FIGURE 3: Sucrase-based mutations in CSID.Phenotypes of SI in CSID Intracellular Arrest of SI in the ER Characterizes Phenotype I Here, SI revealed characteristics of a misfolded, and enzymatically inactive SI that is not of the in the ER and is as a mannose-rich protein that is This is the most of the CSID Another CSID with similar to of I has been analyzed at the molecular and cellular levels biosynthetic of an intestinal biopsy and revealed of SI in the ER and are similar to a of the SI protein to a complex form a of this as a of The SI in this revealed a point mutation that in an of a by a at of the isomaltase subunit (Fig. a of other mutations have been identified that were at the and levels and have been to These are the in isomaltase (Fig. 2) and the and mutations in the sucrase subunit (Fig. 3) SI in the and the in Phenotype II The II of CSID is by an intracellular of SI in the the and the in the enzymatic activities of sucrase and isomaltase are below This was the first in which a mutation in the SI gene has been identified and in a of a by a in the sucrase subunit of SI at amino (50). In this the mutant SI its mannose-rich form and is during In a of mutant SI is predominantly located in the ER and in the The on mutant SI In fact, enzymatic and intracellular transport can be by expression of the mutant SI at the of of function and normal trafficking and function is associated with several of and between the ER and the the molecular and the protein This SI is the first of its in an intestinal disease that a The is located in a region of the sucrase subunit (Fig. 3) that with the isomaltase subunit and other related such as lysosomal α-glucosidase and of the of this mutation into the homologous region of lysosomal α-glucosidase elicited similar cellular and biochemical characteristics to II of SI. A is that is a of a within SI that to a of SI in the within the of a that in the or A be amino that the has been suggested to be implicated in the and subsequent trafficking of SI from the ER to the Golgi The in this is most required for a in the of SI, of a by a P at of sucrase this and of in and in II of CSID (Table 1). in Phenotype SI is transported to the cell surface with similar as the It is because with different and is sorted to the apical membrane (Table (49). These are to that structural alterations not in this of CSID. The defect in this with the site of sucrase that is not whereas the isomaltase subunit normal further the of the mutation in this is is to that the mutation is located or in the of the of sucrase of SI to the and the Phenotype SI is with high to the BBM in intestinal epithelial cells where its function (Table 1). trafficking of SI be associated with malabsorption because of reduced levels of the enzyme in the BBM. of 2 cases of CSID demonstrated an altered of SI from an exclusive apical to a at the apical and membranes This is elicited by the amino of to at in the isomaltase subunit (Fig. 2) that is located in to the that is implicated in the sorting of SI. In SI, the is directly in the SI to the apical membrane an of its carbohydrate with a that SI to lipid microdomains (lipid (35). It is that the mutation a misfolded the region to an of the in SI by such a sorting or an This CSID an to be used in the of this Intracellular of SI at 2 in and SI is transported to the BBM as a single-chain that is cleaved in the intestinal lumen by pancreatic to isomaltase and sucrase In the pro-SI precursor is cleaved in the trans-Golgi the sucrase subunit is and the isomaltase subunit is transported to the apical membrane This of CSID the first that isomaltase of the required for apical transport of SI (Fig. this was and the for apical sorting were identified in the region and the membrane both domains are located in the isomaltase subunit of mutant SI in the ER occurs in which is elicited by a point mutation in the isomaltase subunit that a to at (Table 1, 2) Interestingly, cleaved SI is transported along the secretory in the Golgi apparatus, and into the milieu as an active The pathogenetic underlying CSID is elicited by the of an integral membrane glycoprotein into a protein that exert its function in the BBM. and Phenotype Another mutation relevant to polarized sorting of SI to the apical membrane has been identified in CSID. This is located in the isomaltase subunit and has been to of mutant SI to the membrane It a disulfide bond and a protein in isomaltase that is presumably for of the apical sorting of SI. of mutant SI in a mammalian cell revealed an altered with subsequent intracellular and an transport of mutant SI to the apical membrane (Fig. with the altered sorting the of association of mutant SI with the membrane is altered and the protein from a protein with that is associated with lipid rafts to a The mutation has an implicated in the apical of SI. the of the mutation on the polarized and of SI constitute a unique and novel of CSID. of the molecular of CSID from cases in which patients were for mutations in the SI a of a of patients in with symptoms of malabsorption has suggested in several patients with CSID analyses at the cellular and molecular levels of several of the mutations that the mutations on the individual act in to CSID Here, 2 major of mutations were that in the amino and in patient and and in These individual mutations in an intracellular of SI in the ER and or in the Golgi apparatus It is that each of the mutations have elicited CSID in a The of the individual mutations in domains of sucrase or isomaltase the that an altered of a of sucrase or isomaltase by an individual mutation be by the of the protein product of the in the same this is the the be an of the SI of both a that at the and cell levels from the individual an the of the SI deficiency. This was not supported however, because of 2 derived from patient not the or function as compared with the individual OF SI AND TRAFFICKING The CSID phenotypes of the that both subunits, the sucrase and isomaltase, are within the SI enzyme the trafficking and alterations of SI in CSID relevant to the subunit of the mutations are of of each of the subunits in a trafficking the mutations located in the isomaltase and (Fig. 3) impaired trafficking of SI to the apical membrane compatible with an of this subunit with the polarized sorting of SI. This is supported by of in which isomaltase is sorted to the apical membrane the of sucrase in the Golgi The mutation in this has not been however, because of the lack of from the The of the mutations of and in the of the region in isomaltase particularly strong to the that the region is the main required for the sorting of SI to the apical membrane. In fact, of O-glycosylation of the region of SI by to a of the protein on the apical and The sorting that these glycans is however, several of have suggested that function in the of a by an active of the a that eventually SI to cholesterol- and sphingolipid-enriched membrane microdomains or lipid The lipid rafts are trafficking platforms and major of a of apical that in the from population of lipid apical The of these is however, a group of 3 and have been proposed to be implicated in the sorting of brush border proteins to the apical membrane The of of these proteins to proteins of the secretory pathway is of this 1, has been suggested to be use of surface that as its The relevant sorting mutations in CSID are that can be used to the sorting of SI and its The of the sucrase subunit can be at the level of the ER to Golgi trafficking of SI. It perhaps as a that the isomaltase to the apical membrane. This is demonstrated in the SI the mutations and (Fig. 3) in which isomaltase as a whereas the mutations of a and an enzymatically active isomaltase are not of being to the apical membrane a function of sucrase within SI which an between sucrase and isomaltase is required in the intracellular transport of SI. mutations such as and this An of sucrase enzymatic activity the activity of isomaltase by approximately a of sucrase and isomaltase in activities of the enzyme complex several from the protein that associates or does not with lipid The lipid rafts association of these proteins depends in on the apical the are diverse in their and These can be in the domains of proteins or in in domains or in the of the sorted proteins SI associates with that protein such as and II is particularly because its not to accumulation of SI in intracellular of cells also the morphology in intestinal cells. Here, of II to alterations in the apical membrane, as revealed by surface and reduction in the of OF CSID AND has been demonstrated in several diseases such as cystic factor deficiency, or familial and CSID can be to this as the first intestinal disaccharidase disorder with this of It is to that the normal activity levels of the disaccharidases SI, maltase-glucoamylase, and lactase-phlorizin hydrolase a in which the levels are the normal These activity levels be by a genetic of SI that with for the normal and 2 for the normal is that coding or mutations in these be thought and that the elicited by these mutations are for by the these is that CSID does not as a as was 2 are and is to be of impaired protein transport and altered intracellular in many diseases has greatly of the mechanisms and their protein and structural cases of a mild intestinal brush border which several of the trafficking between the ER and the Golgi apparatus, as well as the polarized apical have been defined and A novel has been proposed by II in which a mutant SI is synthesized and between the ER and It to be this is limited to SI and similar proteins or a have to be The sorting phenotypes to apical sorting of SI to O-glycosylation and the and have demonstrated that an active of perhaps a is required for association of SI with lipid main of these sorting mechanisms is the association or of the proteins with lipid microdomains in and Furthermore, proteins be identified that were in the of the SI along lactase-phlorizin hydrolase, a in which II with the membrane and the and the protein The of of CSID is on mutations on both or by that CSID is a disease It is to that that of the patients described in this review have similar clinical symptoms, with different mutations in the sucrase or isomaltase coding a between the and in the clinical symptoms requires a sample to which of the described The in the molecular basis of CSID constitutes a strong for novel in which protein implicated in the onset of the disease be this be to the of CSID in the population of where the of CSID for almost 5% of the and to CSID is elicited by novel mutations for this are on the onset of CSID in In however, the deficiency was the of dietary carbohydrate restriction characterization of the genetic of these cases and their with the mutations on the of mutations in the of of carbohydrate are to the and of members of are where