Sucrase-isomaltase deficiency in humans. Different mutations disrupt intracellular transport, processing, and function of an intestinal brush border enzyme

Genetic and acquired sucrase-isomaltase deficiency: A clinical review

Genetic sucrase-isomaltase deficiency (GSID) is an inherited deficiency in the ability to digest sucrose and potentially starch due to mutations in the sucrase-isomaltase (SI) gene. Congenital sucrase-isomaltase deficiency is historically considered to be a rare condition affecting infants with chronic diarrhea as exposure to dietary sucrose begins. Growing evidence suggests that individuals with SI variants may present later in life, with symptoms overlapping with those of irritable bowel syndrome. The presence of SI genetic variants may, either alone or in combination, affect enzyme activity and lead to symptoms of different severity. As such, a more appropriate term for this inherited condition is GSID, with a recognition of a spectrum of severity and onset of presentation. Currently, disaccharidase assay on duodenal mucosal tissue homogenates is the gold standard in diagnosing SI deficiency. A deficiency in the SI enzyme can be present at birth (genetic) or acquired later, often in association with damage to the enteric brush-border membrane. Other noninvasive diagnostic alternatives such as sucrose breath tests may be useful but require further validation. Management of GSID is based on sucrose and potentially starch restriction tailored to the individual patients' tolerance and symptoms. As this approach may be challenging, additional treatment with commercially available sacrosidase is available. However, some patients may require continued starch restriction. Further research is needed to clarify the true prevalence of SI deficiency, the pathobiology of single SI heterozygous mutations, and to define optimal diagnostic and treatment algorithms in the pediatric population.

The genetics of monogenic intestinal epithelial disorders

Monogenic intestinal epithelial disorders, also known as congenital diarrheas and enteropathies (CoDEs), are a group of rare diseases that result from mutations in genes that primarily affect intestinal epithelial cell function. Patients with CoDE disorders generally present with infantile-onset diarrhea and poor growth, and often require intensive fluid and nutritional management. CoDE disorders can be classified into several categories that relate to broad areas of epithelial function, structure, and development. The advent of accessible and low-cost genetic sequencing has accelerated discovery in the field with over 45 different genes now associated with CoDE disorders. Despite this increasing knowledge in the causal genetics of disease, the underlying cellular pathophysiology remains incompletely understood for many disorders. Consequently, clinical management options for CoDE disorders are currently limited and there is an urgent need for new and disorder-specific therapies. In this review, we provide a general overview of CoDE disorders, including a historical perspective of the field and relationship to other monogenic disorders of the intestine. We describe the genetics, clinical presentation, and known pathophysiology for specific disorders. Lastly, we describe the major challenges relating to CoDE disorders, briefly outline key areas that need further study, and provide a perspective on the future genetic and therapeutic landscape.

Loss of Sucrase-Isomaltase Function Increases Acetate Levels and Improves Metabolic Health in Greenlandic Cohorts

BACKGROUND & AIMS

The sucrase-isomaltase (SI) c.273_274delAG loss-of-function variant is common in Arctic populations and causes congenital sucrase-isomaltase deficiency, which is an inability to break down and absorb sucrose and isomaltose. Children with this condition experience gastrointestinal symptoms when dietary sucrose is introduced. We aimed to describe the health of adults with sucrase-isomaltase deficiency.

METHODS

The association between c.273_274delAG and phenotypes related to metabolic health was assessed in 2 cohorts of Greenlandic adults (n = 4922 and n = 1629). A sucrase-isomaltase knockout (Sis-KO) mouse model was used to further elucidate the findings.

RESULTS

Homozygous carriers of the variant had a markedly healthier metabolic profile than the remaining population, including lower body mass index (β [standard error], -2.0 [0.5] kg/m²; P = 3.1 × 10^-5), body weight (-4.8 [1.4] kg; P = 5.1 × 10^-4), fat percentage (-3.3% [1.0%]; P = 3.7 × 10^-4), fasting triglyceride (-0.27 [0.07] mmol/L; P = 2.3 × 10^-6), and remnant cholesterol (-0.11 [0.03] mmol/L; P = 4.2 × 10^-5). Further analyses suggested that this was likely mediated partly by higher circulating levels of acetate observed in homozygous carriers (β [standard error], 0.056 [0.002] mmol/L; P = 2.1 × 10^-26), and partly by reduced sucrose uptake, but not lower caloric intake. These findings were verified in Sis-KO mice, which, compared with wild-type mice, were leaner on a sucrose-containing diet, despite similar caloric intake, had significantly higher plasma acetate levels in response to a sucrose gavage, and had lower plasma glucose level in response to a sucrose-tolerance test.

CONCLUSIONS

These results suggest that sucrase-isomaltase constitutes a promising drug target for improvement of metabolic health, and that the health benefits are mediated by reduced dietary sucrose uptake and possibly also by higher levels of circulating acetate.

Rosa canina L. Can Restore Endoplasmic Reticulum Alterations, Protein Trafficking and Membrane Integrity in a Dextran Sulfate Sodium-Induced Inflammatory Bowel Disease Phenotype

Rosa canina L. is a natural polyphenol-rich medicinal plant that exhibits antioxidant and anti-inflammatory activities. Recent in vivo studies have demonstrated that a methanol extract of Rosa canina L. (RCME) has reversed an inflammatory bowel disease (IBD)-like phenotype that has been triggered by dextran sulfate sodium (DSS) in mice. In the current study, we investigated the effects of RCME on perturbations of cellular mechanisms induced by DSS-treatment of intestinal Caco-2 cells, including stress response in the endoplasmic reticulum (ER), protein trafficking and sorting as well as lipid rafts integrity and functional capacities of an intestinal enzyme. 6 days post-confluent cells were treated for 24 h with DSS (3%) or simultaneously with DSS (3%) and RCME (100 µg/mL) or exclusively with RCME (100 µg/mL) or not treated. The results obtained demonstrate the ability of RCME to counteract the substantial increase in the expression levels of several ER stress markers in DSS-treated cells. Concomitantly, the delayed trafficking of intestinal membrane glycoproteins sucrase-isomaltase (SI) and dipeptidyl peptidase 4 (DPP4) induced by DSS between the ER and the Golgi has been compromised by RCME. Furthermore, RCME restored the partially impaired polarized sorting of SI and DPP4 to the brush border membrane. An efficient sorting mechanism of SI and DPP4 is tightly associated with intact lipid rafts structures in the trans-Golgi network (TGN), which have been distorted by DSS and normalized by RCME. Finally, the enzymatic activities of SI are enhanced in the presence of RCME. Altogether, DSS treatment has triggered ER stress, impaired trafficking and function of membrane glycoproteins and distorted lipid rafts, all of which can be compromised by RCME. These findings indicate that the antioxidants in RCME act at two major sites in Caco-2 cells, the ER and the TGN and are thus capable of maintaining the membrane integrity by correcting the sorting of membrane-associated proteins.

Hypomorphic SI genetic variants are associated with childhood chronic loose stools

OBJECTIVE

The SI gene encodes the sucrase-isomaltase enzyme, a disaccharidase expressed in the intestinal brush border. Hypomorphic SI variants cause recessive congenital sucrase-isomaltase deficiency (CSID) and related gastrointestinal (GI) symptoms. Among children presenting with chronic, idiopathic loose stools, we assessed the prevalence of CSID-associated SI variants relative to the general population and the relative GI symptom burden associated with SI genotype within the study population.

METHODS

A prospective study conducted at 18 centers enrolled 308 non-Hispanic white children ≤18 years old who were experiencing chronic, idiopathic, loose stools at least once per week for >4 weeks. Data on demographics, GI symptoms, and genotyping for 37 SI hypomorphic variants were collected. Race/ethnicity-matched SI data from the Exome Aggregation Consortium (ExAC) database was used as the general population reference.

RESULTS

Compared with the general population, the cumulative prevalence of hypomorphic SI variants was significantly higher in the study population (4.5% vs. 1.3%, P < .01; OR = 3.5 [95% CI: 6.1, 2.0]). Within the study population, children with a hypomorphic SI variant had a more severe GI symptom burden than those without, including: more frequent episodes of loose stools (P < .01), higher overall stool frequency (P < .01), looser stool form (P = .01) and increased flatulence (P = .02).

CONCLUSION

Non-Hispanic white children with chronic idiopathic loose stools have a higher prevalence of CSID-associated hypomorphic SI variants than the general population. The GI symptom burden was greater among the study subjects with a hypomorphic SI variant than those without hypomorphic SI variants.

Clinical Characteristics of Disaccharidase Deficiencies Among Children Undergoing Upper Endoscopy

OBJECTIVES

The epidemiology and clinical significance of disaccharidase deficiencies have not been thoroughly characterized. Recent work suggests at least genetic sucrase-isomaltase deficiency is more prevalent than previously believed. Because lactase deficiency (LD) is well described, the present study focuses on the clinical characteristics of children with disaccharidase deficiencies determined by esophagogastroduodenoscopy.

METHODS

Endoscopic records were reviewed from patients undergoing esophagogastroduodenoscopies with biopsies assayed for disaccharidase activity performed by 13 pediatric gastroenterologists during 5 years (2010-2014). Presenting symptoms, clinical and histological diagnosis, treatment, disaccharidase results, and demographic variables were obtained from medical and endoscopic records of those with maltase and sucrase deficiency (SD).

RESULTS

Among 963 patients undergoing intestinal disaccharidase testing, 73 (7.6%) had SD on biopsy (enzyme activity <25 μmol · min · g). Thirty-four (34/73; 47%) had normal duodenal histology and are the focus of this report. Four patients had SD without LD. Pan-disaccharidase deficiency was observed in 24 patients when maltase and palatinase assays were obtained (n = 646), and 11 had SD + LD when just those 2 enzymes were analyzed (n = 317). Those with SD without LD were younger 4.6 ± 6.1 versus 14.1 ± 3.6 years and uniformly presented with diarrhea. Patients with pan-disaccharidase deficiency or SD + LD primarily reported abdominal pain (33/35; 94%), diarrhea (16/35; 46%), nausea (14/35; 40%); and poor weight gain/weight loss (10/35; 29%); constipation, flatulence, and bloating were also noted. Maltase deficiency is less common (8/963; 0.8%), presenting with similar symptoms.

CONCLUSIONS

Genetic sucrase-isomaltase deficiency often occurs together with lactase or pan-disaccharide deficiency. Disaccharidase deficiency should be considered a potential cause of abdominal pain and/or diarrhea in children and adolescents.

Advances in Evaluation of Chronic Diarrhea in Infants

Diarrhea is common in infants (children less than 2 years of age), usually acute, and, if chronic, commonly caused by allergies and occasionally by infectious agents. Congenital diarrheas and enteropathies (CODEs) are rare causes of devastating chronic diarrhea in infants. Evaluation of CODEs is a lengthy process and infrequently leads to a clear diagnosis. However, genomic analyses and the development of model systems have increased our understanding of CODE pathogenesis. With these advances, a new diagnostic approach is needed. We propose a revised approach to determine causes of diarrhea in infants, including CODEs, based on stool analysis, histologic features, responses to dietary modifications, and genetic tests. After exclusion of common causes of diarrhea in infants, the evaluation proceeds through analyses of stool characteristics (watery, fatty, or bloody) and histologic features, such as the villus to crypt ratio in intestinal biopsies. Infants with CODEs resulting from defects in digestion, absorption, transport of nutrients and electrolytes, or enteroendocrine cell development or function have normal villi to crypt ratios; defects in enterocyte structure or immune-mediated conditions result in an abnormal villus to crypt ratios and morphology. Whole-exome and genome sequencing in the early stages of evaluation can reduce the time required for a definitive diagnosis of CODEs, or lead to identification of new variants associated with these enteropathies. The functional effects of gene mutations can be analyzed in model systems such as enteroids or induced pluripotent stem cells and are facilitated by recent advances in gene editing procedures. Characterization and investigation of new CODE disorders will improve management of patients and advance our understanding of epithelial cells and other cells in the intestinal mucosa.

13C-Labeled-Starch Breath Test in Congenital Sucrase-isomaltase Deficiency

BACKGROUND AND HYPOTHESES

Human starch digestion is a multienzyme process involving 6 different enzymes: salivary and pancreatic α-amylase; sucrase and isomaltase (from sucrose-isomaltase [SI]), and maltase and glucoamylase (from maltase-glucoamylase [MGAM]). Together these enzymes cleave starch to smaller molecules ultimately resulting in the absorbable monosaccharide glucose. Approximately 80% of all mucosal maltase activity is accounted for by SI and the reminder by MGAM. Clinical studies suggest that starch may be poorly digested in those with congenital sucrase-isomaltase deficiency (CSID). Poor starch digestion occurs in individuals with CSID and can be documented using a noninvasive C-breath test (BT).

METHODS

C-Labled starch was used as a test BT substrate in children with CSID. Sucrase deficiency was previously documented in study subjects by both duodenal biopsy enzyme assays and C-sucrose BT. Breath CO2 was quantitated at intervals before and after serial C-substrate loads (glucose followed 75 minutes later by starch). Variations in metabolism were normalized against C-glucose BT (coefficient of glucose absorption). Control subjects consisted of healthy family members and a group of children with functional abdominal pain with biopsy-proven sucrase sufficiency.

RESULTS

Children with CSID had a significant reduction of C-starch digestion mirroring that of their duodenal sucrase and maltase activity and C-sucrase BT.

CONCLUSIONS

In children with CSID, starch digestion may be impaired. In children with CSID, starch digestion correlates well with measures of sucrase activity.

Molecular pathogenicity of novel sucrase-isomaltase mutations found in congenital sucrase-isomaltase deficiency patients

BACKGROUND & AIMS

Congenital sucrase-isomaltase deficiency (CSID) is a genetic disorder associated with mutations in the sucrase-isomaltase (SI) gene. The diagnosis of congenital diarrheal disorders like CSID is difficult due to unspecific symptoms and usually requires invasive biopsy sampling of the intestine. Sequencing of the SI gene and molecular analysis of the resulting potentially pathogenic SI protein variants may facilitate a diagnosis in the future. This study aimed to categorize SI mutations based on their functional consequences.

METHODS

cDNAs encoding 13 SI mutants were expressed in COS-1 cells. The molecular pathogenicity of the resulting SI mutants was defined by analyzing their biosynthesis, cellular localization, structure and enzymatic functions.

RESULTS

Three biosynthetic phenotypes for the novel SI mutations were identified. The first biosynthetic phenotype was defined by mutants that are intracellularly transported in a fashion similar to wild type SI and with normal, but varying, levels of enzymatic activity. The second biosynthetic phenotype was defined by mutants with delayed maturation and trafficking kinetics and reduced activity. The third group of mutants is entirely transport incompetent and functionally inactive.

CONCLUSIONS

The current study unraveled CSID as a multifaceted malabsorption disorder that comprises three major classes of functional and trafficking mutants of SI and established a gradient of mild to severe functional deficits in the enzymatic functions of the enzyme.

GENERAL SIGNIFICANCE

This novel concept and the existence of mild consequences in a number of SI mutants strongly propose that CSID is an underdiagnosed and a more common intestinal disease than currently known.

The clinical consequences of sucrase-isomaltase deficiency

Primary sucrase-isomaltase deficiency, originally thought to be a homozygous recessive disorder, has been found to have numerous genetic variants that alone or in combination (compound heterozygosity) express varying degrees of clinical illness, most commonly causing chronic diarrhea, abdominal pain, and bloating. These symptoms are also present with secondary sucrase-isomaltase deficiency. Recent investigations are providing evidence that sucrase-isomaltase deficiency is more prevalent and of greater clinical significance than previously suspected. Further research is required to correlate the specific genotypes and phenotypes with their clinical expressions and to determine the most appropriate treatment algorithm for these patients.

Functional analysis of F508del CFTR in native human colon

The major cystic fibrosis mutation F508del has been classified by experiments in animal and cell culture models as a temperature-sensitive mutant defective in protein folding, processing and trafficking, but literature data on F508del CFTR maturation and function in human tissue are inconsistent. In the present study the molecular pathology of F508del CFTR was characterized in freshly excised rectal mucosa by bioelectric measurement of the basic defect and CFTR protein analysis by metabolic labelling or immunoblot. The majority of investigated F508del homozygous subjects expressed low amounts of complex-glycosylated mature F508del CFTR and low residual F508del CFTR-mediated chloride secretory activity in the rectal mucosa. The finding that some F508del CFTR escapes the ER quality control in vivo substantiates the hope that the defective processing and trafficking of F508del CFTR can be corrected by pharmacological agents.

Impact of glycosylation and detergent-resistant membranes on the function of intestinal sucrase-isomaltase

Sucrase-isomaltase (SI) is a highly N- and O-glycosylated intestinal brush border membrane protein. SI is sorted with high fidelity to the apical membrane via O-linked glycans that mediate its association with lipid rafts or detergent-resistant membranes (DRMs). Here, we show that N- and O-glycosylation and DRMs are implicated in the regulation of the function of SI in intestinal Caco-2 cells. The activities of sucrase (SUC) and isomaltase (IM) increase substantially in DRMs when N- and O-glycosylation patterns are intact. Disruption of DRMs by solubilization with Triton X-100 at 37°C substantially reduces the activities of SUC and IM. Furthermore, modulation of O-glycosylation by benzyl-2-acetamido-2-deoxy-α-d-galactopyranoside and N-glycosylation by deoxymannojirimycin is linked to a decreased capacity of SI to associate with DRMs, with a subsequent reduction of the enzymatic activities of SUC and IM. This is the first report that reveals a direct role of N- and O-glycans in association with DRMs in regulating the function of a membrane glycoprotein.

Impaired trafficking and subcellular localization of a mutant lactase associated with congenital lactase deficiency

BACKGROUND & AIMS

Congenital lactase deficiency (CLD) is a cause of disaccharide intolerance and malabsorption characterized by watery diarrhea in infants fed breast milk or lactose-containing formulas. The molecular basis of CLD is unknown. Mutations in the coding region of the brush border enzyme lactase phlorizin hydrolase (LPH) were found to cause CLD in a study of 19 Finnish families. We analyzed the effects of one of these mutations, G1363S, on LPH folding, trafficking, and function.

METHODS

We introduced a mutation into the LPH complementary DNA that resulted in the amino acid substitution G1363S. The mutant gene was transiently expressed in COS-1 cells, and the effects were assessed at the protein, structural, and subcellular levels.

RESULTS

The mutant protein LPH-G1363S was misfolded and could not exit the endoplasmic reticulum. Interestingly, the mutation creates an additional N-glycosylation site that is characteristic of a temperature-sensitive protein. The intracellular transport and enzymatic activity, but not correct folding, of LPH-G1363S were partially restored by expression at 20 degrees C. However, a form of LPH that contains the mutations G1363S and N1361A, which eliminates the N-glycosylation site, did not restore the features of wild-type LPH. Thus, the additional glycosyl group is not required for the LPH-G1363S defects.

CONCLUSIONS

This is the first characterization, at the molecular and subcellular levels, of a mutant form of LPH that is involved in the pathogenesis of CLD. Mutant LPH accumulates predominantly in the endoplasmic reticulum but can partially mature at a permissive temperature; these features are unique for a protein involved in a carbohydrate malabsorption defect implicating LPH.

13C-breath tests for sucrose digestion in congenital sucrase isomaltase-deficient and sacrosidase-supplemented patients

BACKGROUND

Congenital sucrase-isomaltase deficiency (CSID) is characterized by absence or deficiency of the mucosal sucrase-isomaltase enzyme. Specific diagnosis requires upper gastrointestinal biopsy with evidence of low to absent sucrase enzyme activity and normal histology. The hydrogen breath test (BT) is useful, but is not specific for confirmation of CSID. We investigated a more specific 13C-sucrose labeled BT.

OBJECTIVES

Determine whether CSID can be detected with the 13C-sucrose BT without duodenal biopsy sucrase assay, and if the 13C-sucrose BT can document restoration of sucrose digestion by CSID patients after oral supplementation with sacrosidase (Sucraid).

METHODS

Ten CSID patients were diagnosed by low biopsy sucrase activity. Ten controls were children who underwent endoscopy and biopsy because of dyspepsia or chronic diarrhea with normal mucosal enzymes activity and histology. Uniformly labeled 13C-glucose and 13C-sucrose loads were orally administered. 13CO2 breath enrichments were assayed using an infrared spectrophotometer. In CSID patients, the 13C-sucrose load was repeated adding Sucraid. Sucrose digestion and oxidation were calculated as a mean percent coefficient of glucose oxidation averaged between 30 and 90 minutes.

RESULTS

Classification of patients by 13C-sucrose BT percent coefficient of glucose oxidation agreed with biopsy sucrase activity. The breath test also documented the return to normal of sucrose digestion and oxidation after supplementation of CSID patients with Sucraid.

CONCLUSIONS

13C-sucrose BT is an accurate and specific noninvasive confirmatory test for CSID and for enzyme replacement management.

Compound heterozygous mutations affect protein folding and function in patients with congenital sucrase-isomaltase deficiency

BACKGROUND & AIMS

Congenital sucrase-isomaltase (SI) deficiency is an autosomal-recessive intestinal disorder characterized by a drastic reduction or absence of sucrase and isomaltase activities. Previous studies have indicated that single mutations underlie individual phenotypes of the disease. We investigated whether compound heterozygous mutations, observed in some patients, have a role in disease pathogenesis.

METHODS

We introduced mutations into the SI complementary DNA that resulted in the amino acid substitutions V577G and G1073D (heterozygous mutations found in one group of patients) or C1229Y and F1745C (heterozygous mutations found in another group). The mutant genes were expressed transiently, alone or in combination, in COS cells and the effects were assessed at the protein, structural, and subcellular levels.

RESULTS

The mutants SI-V577G, SI-G1073D, and SI-F1745C were misfolded and could not exit the endoplasmic reticulum, whereas SI-C1229Y was transported only to the Golgi apparatus. Co-expression of mutants found on each SI allele in patients did not alter the protein's biosynthetic features or improve its enzymatic activity. Importantly, the mutations C1229Y and F1745C, which lie in the sucrase domains of SI, prevented its targeting to the cell's apical membrane but did not affect protein folding or isomaltase activity.

CONCLUSIONS

Compound heterozygosity is a novel pathogenic mechanism of congenital SI deficiency. The effects of mutations in the sucrase domain of SIC1229Y and SIF1745C indicate the importance of a direct interaction between isomaltase and sucrose and the role of sucrose as an intermolecular chaperone in the intracellular transport of SI.

Mosaic pattern of sucrase isomaltase deficiency in two brothers

The pathophysiology of mucosal changes observed in infants with chronic protracted diarrhea is poorly understood. We report on two brothers suffering from a special form of sucrase isomaltase (SI) deficiency. The children presented with weight loss and dyspepsia after sucrose exposition. We performed an H respiration test, which showed a pathologic result in the younger brother. Analysis of the brush border enzyme activities showed low expression of lactase and SI. Immunoelectron microscopy of duodenal biopsies showed an isolated SI deficiency in a mosaic pattern [e.g., 42% (14%) crypt enterocytes and 64% (59%) villus enterocytes with decreased amounts of SI on microvilli], whereas lactase and aminopeptidase n (ApN) were present at the apical membrane of all cells in a normal range. The SI mosaic pattern of these patients shows that the enterocytes contain low amounts of SI on the apical membrane but express normal quantities of other disaccharidases. These findings suggest the existence of different clonal expressions or specific (posttranslational) mechanisms of postGolgi transportation for individual brush border enzymes. It remains unresolved whether the mosaic distribution is part of a normal maturation process or caused by a lack of an overall control mechanism in the expression of brush border hydrolases.

Use of a monoclonal antibody to sucrase-isomaltase for evaluation of the columnar cuff after stapled restorative proctocolectomy

PURPOSE

Restorative proctocolectomy with a double-stapled pouch-anal anastomosis retains a cuff of diseased columnar mucosa (columnar cuff) in the upper anal canal that may require biopsy. Biopsying this can be difficult and colonic phenotypic change in the pouch can lead to errors interpreting the histology. This study was designed to investigate the use of a monoclonal antibody to sucrase-isomaltase for differentiating ileal pouch from columnar cuff mucosa. Then, by using this antibody, the ability to accurately take and report biopsies from the anal canal was examined.

METHODS

The technique of staining for sucrase-isomaltase was developed. From 113 patients who had a double-stapled pouch-anal anastomosis, 467 formalin-fixed biopsies and 177 fresh-frozen biopsies were taken from the ileal pouch, columnar cuff, or anal transitional zone. Biopsies were stained with a monoclonal antibody to sucrase-isomaltase, and fixed biopsies were routinely reported after staining with hematoxylin and eosin.

RESULTS

A monoclonal antibody to sucrase-isomaltase reliably discriminated between ileal from rectal mucosa. A biopsy of columnar cuff mucosa as reported by routine histology was obtained during 72 percent of attempted outpatient examinations. Sucrase-isomaltase staining of reported columnar cuff biopsies showed that biopsies were of pouch rather than columnar cuff in 4.4 percent (95 percent confidence interval, 2-8) of outpatient examinations.

CONCLUSIONS

The monoclonal antibody to sucrase-isomaltase used in this study may have a clinical role when interpreting columnar cuff biopsies from patients being investigated for pouch dysfunction, or in patients having surveillance biopsies to exclude neoplasia in the columnar cuff.

Expression of mutant Ins2C96Y results in enhanced tubule formation causing enlargement of pre-Golgi intermediates of CHO cells

Misfolded proteins are recognized by the protein quality control and eventually degraded by the ubiquitin-proteasome system. Previously, we demonstrated accumulation of a misfolded non-glycosylated protein, namely proinsulin, in enlarged pre-Golgi intermediates and dilated rough endoplasmic reticulum (ER) domains in pancreatic beta-cells of Akita mice. In order to exclude effects possibly due to coexisting wild type and mutant proinsulin in pancreatic beta-cells, CHO cells expressing singly wild type or mutant C96Y proinsulin 2 were now analyzed by electron microscopic morphometry and immunogold labeling as well as serial section 3D analysis. We found a significant increase in volume density of pre-Golgi intermediates in CHO Ins2(C96Y) cells which was principally due to an increase of its tubular elements, and no significant changes of the ER. The average diameter of the pre-Golgi intermediates of CHO Ins2(C96Y) cells was about twice that of CHO Ins2(wt) cells. The enlarged pre-Golgi intermediates and the ER of CHO Ins2(C96Y) cells were positive for proinsulin, which was not detectable in the significantly enlarged Golgi cisternal stack. Treatment of CHO Ins2(C96Y) cells with proteasome inhibitors resulted in the formation of proinsulin-containing aggresomes. We conclude that misfolded proinsulin causes enlargement of pre-Golgi intermediates which indicates their involvement in protein quality control.

Cell biology of membrane trafficking in human disease

Understanding the molecular and cellular mechanisms underlying membrane traffic pathways is crucial to the treatment and cure of human disease. Various human diseases caused by changes in cellular homeostasis arise through a single gene mutation(s) resulting in compromised membrane trafficking. Many pathogenic agents such as viruses, bacteria, or parasites have evolved mechanisms to subvert the host cell response to infection, or have hijacked cellular mechanisms to proliferate and ensure pathogen survival. Understanding the consequence of genetic mutations or pathogenic infection on membrane traffic has also enabled greater understanding of the interactions between organisms and the surrounding environment. This review focuses on human genetic defects and molecular mechanisms that underlie eukaryote exocytosis and endocytosis and current and future prospects for alleviation of a variety of human diseases.

Selective action of the iminosugar isofagomine, a pharmacological chaperone for mutant forms of acid-beta-glucosidase

Gaucher disease is a lysosomal glycolipid storage disorder characterized by defects in acid-beta-glucosidase (GlcCerase), the enzyme responsible for the catabolism of glucosylceramide. We recently demonstrated that isofagomine (IFG), an iminosugar that binds to the active site of GlcCerase, enhances the folding, transport and activity of the N370S mutant form of GlcCerase. In this study we compared the effects of IFG on a number of other glucosidases and glucosyltransferases. We report that IFG has little or no inhibitory activity towards intestinal disaccharidase enzymes, ER alpha-glucosidase II or glucosylceramide synthase at concentrations previously shown to enhance N370S GlcCerase folding and trafficking in Gaucher fibroblasts. Furthermore, treatment of wild type fibroblasts with high doses of IFG did not alter the processing of newly synthesized N-linked oligosaccharides. These findings support further evaluation of IFG as a potential therapeutic agent in the treatment of some forms of Gaucher disease.

Altered Folding, Turnover, and Polarized Sorting Act in Concert to Define a Novel Pathomechanism of Congenital Sucrase-Isomaltase Deficiency

Naturally occurring mutants of membrane and secretory proteins are often associated with the pathogenesis of human diseases. Here, we describe the molecular basis of a novel phenotype of congenital sucrase-isomaltase deficiency (CSID), a disaccharide malabsorption disorder of the human intestine in which several structural features and functional capacities of the brush-border enzyme complex sucrase-isomaltase (SI) are affected. The cDNA encoding SI from a patient with CSID reveals a mutation in the isomaltase subunit of SI that results in the substitution of a cysteine by an arginine at amino acid residue 635 (C635R). When this mutation is introduced into the wild type cDNA of SI a mutant enzyme, SI(C635R), is generated that shows a predominant localization in the endoplasmic reticulum. Nevertheless, a definite localization of SI(C635R) in the Golgi apparatus and at the cell surface could be also observed. Epitope mapping with conformation-specific mAbs protease sensitivity assays, and enzymatic activity measurements demonstrate an altered folding pattern of SI(C635R) that is responsible for a substantially increased turnover rate and an aberrant sorting profile. Thus, SI(C635R) becomes distributed also at the basolateral membrane in contrast to wild type SI. Concomitant with the altered sorting pattern, the partial detergent extractability of wild type SI shifts to a complete detergent solubility with Triton X-100. The mutation has therefore affected an epitope responsible for the apical targeting fidelity of SI. Altogether, the combined effects of the C635R mutation on the turnover rate, function, polarized sorting, and detergent solubility of SI constitute a unique and novel pathomechanism of CSID.

Disaccharide digestion: clinical and molecular aspects

Sugars normally are absorbed in the small intestine. When carbohydrates are malabsorbed, the osmotic load produced by the high amount of low molecular weight sugars and partially digested starches in the small intestine can cause symptoms of intestinal distention, rapid peristalsis, and diarrhea. Colonic bacteria normally metabolize proximally malabsorbed dietary carbohydrate through fermentation to small fatty acids and gases (ie, hydrogen, methane, and carbon dioxide). When present in large amounts, the malabsorbed sugars and starches can be excreted in the stool. Sugar intolerance is the presence of abdominal symptoms related to the proximal or distal malabsorption of dietary carbohydrates. The symptoms consist of meal-related abdominal cramps and distention, increased flatulence, borborygmus, and diarrhea. Infants and young children with carbohydrate malabsorption show more intense symptoms than adults; the passage of undigested carbohydrates through the colon is more rapid and is associated with detectable carbohydrates in copious watery acid stools. Dehydration often follows feeding of the offending sugar. In this review we present the clinical and current molecular aspects of disaccharidase digestion.

The early stages of the intracellular transport of membrane proteins: clinical and pharmacological implications

Intracellular transport mechanisms ensure that integral membrane proteins are delivered to their correct subcellular compartments. Efficient intracellular transport is a prerequisite for the establishment of both cell architecture and function. In the past decade, transport processes of proteins have also drawn the attention of clinicians and pharmacologists since many diseases have been shown to be caused by transport-deficient proteins. Membrane proteins residing within the plasma membrane are transported via the secretory (exocytotic) pathway. The general transport routes of the secretory pathway are well established. The transport of membrane proteins starts with their integration into the ER membrane. The ribosomes synthesizing membrane proteins are targeted to the ER membrane, and the nascent chains are co-translationally integrated into the bilayer, i.e., they are inserted while their synthesis is in progress. During ER insertion, the orientation (topology) of the proteins in the membrane is determined. Proteins are folded, and their folding state is checked by a quality control system that allows only correctly folded forms to leave the ER. Misfolded or incompletely folded forms are retained, transported back to the cytosol and finally subjected to proteolysis. Correctly folded proteins are transported in the membranes of vesicles through the ER/Golgi intermediate compartment (ERGIC) and the individual compartments of the Golgi apparatus ( cis, medial, trans) to the plasma membrane. In this review, the current knowledge of the first stages of the intracellular trafficking of membrane proteins will be summarized. This "early secretory pathway" includes the processes of ER insertion, topology determination, folding, quality control and the transport to the Golgi apparatus. Mutations in the genes of membrane proteins frequently lead to misfolded forms that are recognized and retained by the quality control system. Such mutations may cause inherited diseases like cystic fibrosis or retinitis pigmentosa. In the second part of this review, the clinical implications of the early secretory pathway will be discussed. Finally, new pharmacological strategies to rescue misfolded and transport-defective membrane proteins will be outlined.

Carbohydrate intolerance

PURPOSE OF REVIEW

The small intestinal mucosa is highly specialized for terminal digestion of nutrient polysaccharides and disaccharides and absorption of monosaccharides. However, in the case of digestive or absorptive deficiency, symptoms of carbohydrate intolerance result. Significant progress has been made toward defining the molecular genetic mechanisms responsible for several carbohydrate intolerances.

RECENT FINDINGS

This review summarizes monosaccharide and disaccharide intolerance conditions and recent clinical and basic science reports related to carbohydrate digestion and membrane transport. Genetic polymorphisms closely associated with lactase persistence/nonpersistence have been identified. Lactose intolerance is capable of preventing the achievement of adequate peak bone mass in susceptible young adults and may predispose to osteoporosis. Recent studies support previous reports that fructose malabsorption is associated with unexplained gastrointestinal symptoms. GLUT2 may be recruited from the basolateral to the apical membrane of enterocytes to facilitate small intestinal fructose absorption.

SUMMARY

Knowledge regarding the clinical aspects of and the physiologic mechanisms responsible for specific carbohydrate intolerances has allowed for improved diagnostic and treatment options and has contributed to continuing investigation of intestinal gene expression.

Congenital sucrase-isomaltase deficiency because of an accumulation of the mutant enzyme in the endoplasmic reticulum

BACKGROUND & AIMS

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.

A glutamine to proline exchange at amino acid residue 1098 in sucrase causes a temperature-sensitive arrest of sucrase-isomaltase in the endoplasmic reticulum and cis-Golgi

A striking feature of phenotype II in congenital sucrase-isomaltase deficiency is the retention of the brush border protein sucrase-isomaltase (SI) in the cis-Golgi. This transport block is the consequence of a glutamine to proline substitution at amino acid residue 1098 of the sucrase subunit. Here we provide unequivocal biochemical and confocal data to show that the SI(Q/P) mutant reveals characteristics of a temperature-sensitive mutant. Thus, correct folding, competent intracellular transport, and full enzymatic activity can be partially restored by expression of the mutant SI(Q/P) at the permissive temperature of 20 degrees C instead of 37 degrees C. The acquisition of normal trafficking and function appears to utilize several cycles of anterograde and retrograde steps between the endoplasmic reticulum and the Golgi implicating the molecular chaperones calnexin and heavy chain-binding protein. The data presented in this communication are to our knowledge the first to implicate a temperature-sensitive mutation in an intestinal enzyme deficiency or an intestinal disorder.

Disaccharidase activities in dyspeptic children: biochemical and molecular investigations of maltase-glucoamylase activity

BACKGROUND

Maltase-glucoamylase enzyme plays an important role in starch digestion. Glucoamylase deficiency is reported to cause chronic diarrhea in infants, but its role in dyspeptic children is unknown.

METHODS

Glucoamylase and other disaccharidase specific activities were assayed from duodenal biopsy specimens in 44 children aged 0.5-18 years (mean, 10 +/- 5 years) undergoing endoscopy to evaluate dyspeptic symptoms. All subjects had normal duodenal histology. Intestinal organ culture was used to evaluate synthesis and processing of maltase-glucoamylase. Sequencing of the maltase-glucoamylase coding region was performed in subjects with low activity or variation of isoform in organ culture.

RESULTS

Twenty-two of the dyspeptic children had one or more disaccharidases with low specific activity. Twelve subjects (28%) had low activity of glucoamylase. Eight subjects had low activities of glucoamylase, sucrase, and lactase. Low glucoamylase activity was not correlated with the isoform phenotype of maltase-glucoamylase as described by metabolic labeling and sodium dodecyl sulfate electrophoresis. Novel nucleotide changes were not detected in one subject with low glucoamylase activity or in two subjects with variant isoforms of maltase-glucoamylase peptides.

CONCLUSION

Twelve of 44 dyspeptic children had low specific activity of duodenal maltase-glucoamylase. Eight of these children had low specific activity of all measured disaccharidases.

Congenital maltase-glucoamylase deficiency associated with lactase and sucrase deficiencies

BACKGROUND

Multiple enzyme deficiencies have been reported in some cases of congenital glucoamylase, sucrase, or lactase deficiency. Here we describe such a case and the investigations that we have made to determine the cause of this deficiency.

METHODS AND RESULTS

A 2.5 month-old infant, admitted with congenital lactase deficiency, failed to gain weight on a glucose oligomer formula (Nutramigen). Jejunal mucosal biopsy at 4 and 12 months revealed normal histology with decreased maltase-glucoamylase, sucrase-isomaltase, and lactase-phlorizin hydrolase activities. Testing with a C-starch/breath CO loading test confirmed proximal starch malabsorption. Sequencing of maltase-glucoamylase cDNA revealed homozygosity for a nucleotide change (C1673T) in the infant, which causes an amino acid substitution (S542L) 12 amino acids after the N-terminal catalytic aspartic acid. The introduction of this mutation into "wildtype" N-terminus maltase-glucoamylase cDNA was not associated with obvious loss of maltase-glucoamylase enzyme activities when expressed in COS 1 cells and this amino-acid change was subsequently found in other people. Sequencing of the promoter region revealed no nucleotide changes. Maltase-glucoamylase, lactase, and sucrase-isomaltase were each normally synthesized and processed in organ culture.

CONCLUSIONS

The lack of evidence for a causal nucleotide change in the maltase-glucoamylase gene in this patient, and the concomitant low levels of lactase and sucrase activity, suggest that the depletion of mucosal maltase-glucoamylase activity and starch digestion was caused by shared, pleiotropic regulatory factors.

Molecular basis of aberrant apical protein transport in an intestinal enzyme disorder

The impaired sorting profile to the apical membrane of human intestinal sucrase-isomaltase is the underlying cause in the pathogenesis of a novel phenotype of intestinal congenital sucrase-isomaltase deficiency. Molecular characterization of this novel phenotype reveals a point mutation in the coding region of the sucrase-isomaltase (SI) gene that results in an amino acid substitution of a glutamine by arginine at residue 117 of the isomaltase subunit. This substitution is located in a domain revealing features of a trefoil motif or a P-domain in immediate vicinity of the heavily O-glycosylated stalk domain. Expression of the mutant SI phenotype in epithelial Madin-Darby canine kidney cells reveals a randomly targeted SI protein to the apical and basolateral membranes confirming an exclusive role of the Q117R mutation in generating this phenotype. Unlike wild type SI, the mutant protein is completely extractable with Triton X-100 despite the presence of O-glycans that serve in the wild type protein as an apical sorting signal and are required for the association of SI with detergent-insoluble lipid microdomains. Obviously the O-glycans are not adequately recognized in the context of the mutant SI, most likely due to altered folding of the P-domain that ultimately affects the access of the O-glycans to a putative sorting element.

Comparison of sucrase-free isomaltase with sucrase-isomaltase purified from the house musk shrew Suncus murinus

We purified sucrase-isomaltase and sucrase-free isomaltase from a normal and a sucrase-deficient line, respectively, of the house musk shrew Suncus murinus and examined the effects of mutation on enzyme structure and activities. Recent cDNA cloning studies have predicted that sucrase-free mutant isomaltase lacks the C-terminal 69 amino acids of normal isomaltase, as well as the entire sucrase. On SDS-polyacrylamide gel electrophoresis purified sucrase-free isomaltase gave a single protein band of 103 kDa, while sucrase-isomaltase gave two major protein bands of 106 and 115 kDa. The 115, but not 106, kDa band was quite similar to the 103 kDa band on Western blotting with Aleuria aurantia lectin and antibody against shrew sucrase-isomaltase, suggesting that the 115 and 103 kDa bands are due to normal and mutant isomaltases, respectively, in accordance with the above prediction. Purified isomaltase and sucrase-isomaltase were similar in Km and Vmax (based on isomaltase mass) values for isomaltose hydrolysis and in inhibition of isomaltase activity by antibody against rabbit sucrase-isomaltase, suggesting that the enzymatic properties of isomaltase are mostly unaffected by mutation.

Traffic jam: a compendium of human diseases that affect intracellular transport processes

As sequencing of the human genome nears completion, the genes that cause many human diseases are being identified and functionally described. This has revealed that many human diseases are due to defects of intracellular trafficking. This 'Toolbox' catalogs and briefly describes these diseases.

Congenital sucrase-isomaltase deficiency arising from cleavage and secretion of a mutant form of the enzyme

Congenital sucrase-isomaltase deficiency (CSID) is an autosomal recessive human intestinal disorder that is clinically characterized by fermentative diarrhea, abdominal pain, and cramps upon ingestion of sugar. The symptoms are the consequence of absent or drastically reduced enzymatic activities of sucrase and isomaltase, the components of the intestinal integral membrane glycoprotein sucrase-isomaltase (SI). Several known phenotypes of CSID result from an altered posttranslational processing of SI. We describe here a novel CSID phenotype, in which pro-SI undergoes an unusual intracellular cleavage that eliminates its transmembrane domain. Biosynthesis of pro-SI in intestinal explants and in cells transfected with the SI cDNA of this phenotype demonstrated a cleavage occurring within the endoplasmic reticulum due to a point mutation that converts a leucine to proline at residue 340 of isomaltase. Cleaved pro-SI is transported to and processed in the Golgi apparatus and is ultimately secreted into the exterior milieu as an active enzyme. To our knowledge this is the first report of a disorder whose pathogenesis results not from protein malfolding or mistargeting, but from the conversion of an integral membrane glycoprotein into a secreted species that is lost from the cell surface.

Anderson's disease: exclusion of apolipoprotein and intracellular lipid transport genes

Anderson's disease is a rare, hereditary hypocholesterolemic syndrome characterized by chronic diarrhea, steatorrhea, and failure to thrive associated with the absence of apo B48-containing lipoproteins. To further define the molecular basis of the disease, we studied 8 affected subjects in 7 unrelated families of North African origin after treatment with a low-fat diet. Lipid loading of intestinal biopsies persisted, but the pattern and extent of loading was variable among the patients. Electron microscopy showed lipoprotein-like particles in membrane-bound compartments, the densities (0.65 to 7.5 particles/mu(2)) and the mean diameters (169 to 580 nm) of which were, in general, significantly larger than in a normal fed subject (0.66 particles/mu(2), 209 nm mean diameter). There were also large lipid particles having diameters up to 7043 nm (average diameters from 368 to 2127 nm) that were not surrounded by a membrane. Rarely, lipoprotein-like particles 50 to 150 nm in diameter were observed in the intercellular spaces. Intestinal organ culture showed that apo B and apo AIV were synthesized with apparently normal molecular weights and that small amounts were secreted in lipid-bound forms (density <1.006 g/mL). Normal microsomal triglyceride transfer protein (MTP) and activity were also detected in intestinal biopsies. Segregation analyses of 4 families excluded, as a cause of the disease, significant regions of the genome surrounding the genes for apo AI, AIV, B, CI, CII, CIII, and E, as were the genes encoding 3 proteins involved in intracellular lipid transport, MTP, and fatty acid binding proteins 1 and 2. The results suggest that a factor other than apoproteins and MTP are important for human intestinal chylomicron assembly and secretion.

Dissociation between growth arrest and differentiation in Caco-2 subclone expressing high levels of sucrase

Growth arrest and cell differentiation are generally considered temporally and functionally linked phenomena in small intestinal crypt cells and colon tumor cell lines (Caco-2, HT-29). We have derived a Caco-2 subclone (NGI3) that deviates from such a paradigm. In striking contrast with the parental cells, proliferative and subconfluent NGI3 cells were found to express sucrase-isomaltase (SI) mRNA and to synthesize relatively high levels of SI, dipeptidyl peptidase IV, and aminopeptidase N (APN). In postconfluent cells, little difference was seen in SI mRNA levels between Caco-2 and NGI3 cells, but the latter still expressed much higher levels of SI that could be attributed to higher rates of translation. APN expression was also greatly enhanced in NGI3 cells. To determine whether high levels of brush-border enzymes correlated with expression of cell-cycle regulatory proteins, we investigated their relative cellular levels in growing and growth-arrested cells. The results showed that the cyclin-dependent kinase inhibitors (p21 and p27) and D-type cyclins (D1 and D3) were all induced in postconfluent cells, but NGI3 cells expressed much higher levels of p21. This study demonstrated that cell growth and expression of differentiated traits are not mutually exclusive in intestinal epithelial cells and provided evidence indicating that posttranscriptional events play an important role in regulation of SI expression.

Growth, development and differentiation: a functional food science approach

Few other aspects of food supply and metabolism are of greater biological importance than the feeding of mothers during pregnancy and lactation, and of their infants and young children. Nutritional factors during early development not only have short-term effects on growth, body composition and body functions but also exert long-term effects on health, disease and mortality risks in adulthood, as well as development of neural functions and behaviour, a phenomenon called 'metabolic programming'. The interaction of nutrients and gene expression may form the basis of many of these programming effects and needs to be investigated in more detail. The relation between availability of food ingredients and cell and tissue differentiation and its possible uses for promoting health and development requires further exploration. The course of pregnancy, childbirth and lactation as well as human milk composition and the short- and long-term outcome of the child are influenced by the intake of foods and particularly micronutrients, e.g. polyunsaturated fatty acids, Fe, Zn and I. Folic acid supplementation from before conception through the first weeks of pregnancy can markedly reduce the occurrence of severe embryonic malformations; other potential benefits of modulating nutrient supply on maternal and child health should be further evaluated. The evaluation of dietary effects on child growth requires epidemiological and field studies as well as evaluation of specific cell and tissue growth. Novel substrates, growth factors and conditionally essential nutrients (e.g. growth factors, amino acids, polyunsaturated fatty acids) may be potentially useful as ingredients in functional foods and need to be assessed carefully. Intestinal growth, maturation, and adaptation as well as long-term function may be influenced by food ingredients such as oligosaccharides, gangliosides, high-molecular-mass glycoproteins, bile salt-activated lipase, pre- and probiotics. There are indications for some beneficial effects of functional foods on the developing immune response, for example induced by antioxidant vitamins, trace elements, fatty acids, arginine, nucleotides, and altered antigen contents in infant foods. Peak bone mass at the end of adolescence can be increased by dietary means, which is expected to be of long-term importance for the prevention of osteoporosis at older ages. Future studies should be directed to the combined effects of Ca and other constituents of growing bone, such as P, Mg and Zn, as well as vitamins D and K, and the trace elements F and B. Pregnancy and the first postnatal months are critical time periods for the growth and development of the human nervous system, processes for which adequate substrate supplies are essential. Early diet seems to have long-term effects on sensory and cognitive abilities as well as behaviour. The potential beneficial effects of a balanced supply of nutrients such as I, Fe, Zn and polyunsaturated fatty acids should be further evaluated. Possible long-term effects of early exposure to tastes and flavours on later food choice preferences may have a major impact on public health and need to be further elucidated. The use of biotechnology and recombinant techniques may offer the opportunity to include various bioactive substances in special dietary products, such as human milk proteins, peptides, growth factors, which may have beneficial physiological effects, particularly in infancy and early childhood.

Molecular cloning of sucrase-isomaltase cDNA in the house musk shrew Suncus murinus and identification of a mutation responsible for isolated sucrase deficiency

Isolated sucrase deficiency has been demonstrated in a line of house musk shrew, Suncus murinus (laboratory name: suncus). This animal belongs to the order Insectivore and is phylogenetically different from ordinarily used laboratory animals. They are believed to have evolved with mainly animal food without sucrose. To study the molecular basis of the sucrase deficiency in suncus, we cloned 6. 0-kilobase (kb) sucrase-isomaltase (SI, EC 3.2.1.48-10) cDNA from suncus intestinal cDNA library. The cDNA clone contained a 5442-base pair (bp)-long open reading frame preceded by an in frame termination codon. The deduced 1813-amino acid sequence showed 68.6, 71.2, and 74.7% similarity with those of rat, rabbit, and human, respectively. A cleavage site between isomaltase and sucrase as well as the region surrounding the catalytic sites for sucrase and isomaltase were conserved among the species. Out of 18 potential N-linked glycosylation sites, 5 were common among all 4 species. In the connecting segment which was enriched with O-linked glycosylation sites in the other species, only two sites were present in suncus. Northern blot analysis revealed that the 6.0-kb SI mRNA was expressed in the KAT line with intact sucrase-isomaltase activity. In contrast, 3.0-kb SI mRNA was expressed in suncus of the MI line with isolated sucrase deficiency. The 3.0-kb mRNA cosegregated with sucrase deficiency phenotype as an autosomal recessive trait. Sequence analysis revealed a 2-nucleotide deletion at position 2767-2768, which results in a frameshift and an immature termination codon. The cDNA of the MI line diverged from that of the KAT line at position 2865, having an 18-bp unique sequence followed by a poly(A) tail. The mutant cDNA encodes 922 amino acid residues which preserves the region for isomaltase but lacks that for whole sucrase. While the cells transfected with the plasmids expressing SI in the KAT line showed both sucrase and isomaltase activity, the plasmids expressing MI line cDNA showed only isomaltase activity. Thus it was concluded that the mutation in the SI gene was responsible for isolated sucrase deficiency in the MI line.

Analysis of a naturally occurring mutation in sucrase-isomaltase: glutamine 1098 is not essential for transport to the surface of COS-1 cells

A glutamine for proline substitution at position 1098 was previously shown to result in accumulation of brush-border sucrase-isomaltase in the Golgi apparatus. The substitution is present in a highly homologous region of the protein, and results in a comparable accumulation when introduced into the same region in lysosomal alpha-glucosidase. To study the importance of the glutamine-1098, we analyzed the transport compatibility of two mutants in which glutamine-1098 is substituted by lysine or alanine. Both mutants were transported to the cell surface and processed comparable to wild type. We concluded that glutamine-1098 is not essential for transport to the cell surface.

Ostrich intestinal glycohydrolases: distribution, purification and partial characterisation

Intestinal glycohydrolases are enzymes involved in assimilating carbohydrate for nutrition. The avian forms of these enzymes, in particular the maltase-glucoamylase complex (MG), are not well characterised. This study encompassed characterisation of these enzymes from ostrich intestines, and the first kinetic analysis of an avian MG. Proteolytically solubilised MG from ileal brush border membrane vesicles was purified by Sephadex G-200 gel filtration and Tris-affinity-chromatography, while jejunal sucrase-isomaltase (SI) and MG were purified by Toyopearl-Q650 and phenyl-Sepharose chromatography. Amino acid sequences and compositions of enzyme subunits, resulting from SDS-PAGE, were determined. Kinetics of hydrolysis of linear oligosaccharides was studied. Ostrich MG and SI showed the highest activity in the jejunum, followed by the ileum and duodenum. No lactase or trehalase activity could be detected. The jejunal MG and SI, resulting from brush-border membrane vesicles, could not be separated during purification. However, a minor form of ileal MG was purified using Sephadex G-200 chromatography. Ileal MG contained three subunits of M(r) 145,000, 125,000 and 115,000. Although the N-terminal amino acid sequences bear no homology to SI, the M(r) 115,000 subunit shows homology to porcine MG in both sequence and amino acid composition. The pH optimum of maltose-, starch- and isomaltose-hydrolysing activity was 6.5 and that of sucrose-hydrolysing activity 5.5. The glycohydrolases were most active at 58 degrees C, but were quickly denatured above 60 degrees C. Sucrose- and starch-hydrolysing activities were more thermostable than maltose- and isomaltose-hydrolysing activities. Kinetic parameters (K(m), kcat and kcat/K(m)) for the hydrolysis of maltooligosaccharides, starch and glycogen are reported for ileal MG. Maltotriose and maltotetraose displayed partial inhibition of ileal MG. The study revealed large similarities between ostrich SI and MG in charge, size, shape and hydrophobicity, based on their inseparability by several methods. Measurement of the specificity constants for maltooligosaccharide hydrolysis by ileal MG revealed less efficient hydrolysis of longer substrates as compared to maltose and maltotriose.

Congenital chloride-losing diarrhoea: absence of the anion-exchange mechanism in the rectum

BACKGROUND

Congenital chloride-losing diarrhoea is characterized by a defect in chloride/bicarbonate exchange, which is normally present in the ileum and colon. Whether the defect is an absence or a reversal of such an exchange is unclear, and we have investigated two young children with the disorder to answer this question.

METHODS

We used a previously described nonequilibrium rectal dialysis method, using different dialysate anion concentrations, to investigate the movement of sodium, potassium, chloride, and bicarbonate in the rectum of the two children.

RESULTS

The results showed that chloride and bicarbonate movements were not linked in any active way, and both ions appeared to move passively in response to the electrochemical gradients generated.

CONCLUSIONS

In the two subjects studied, the defect in the rectum appears to be an absence of the normal anion exchange mechanism present in the bowel, rather than its reversal.

A mutation in a highly conserved region in brush-border sucrase-isomaltase and lysosomal alpha-glucosidase results in Golgi retention

A point mutation in the cDNA of human intestinal sucrase-isomaltase has been recently identified in phenotype II of congenital sucrase-isomaltase deficiency. The mutation results in a substitution of glutamine by proline at position 1098 (Q1098P) in the sucrase subunit. Expression of this mutant sucrase-isomaltase cDNA in COS-1 cells results in an accumulation of sucrase-isomaltase in the ER, intermediate compartment and the cis-Golgi cisternae similar to the accumulation in phenotype II intestinal cells. An interesting feature of the Q1098P substitution is its location in a region of the sucrase subunit that shares striking similarities with the isomaltase subunit and other functionally related enzymes, such as human lysosomal acid alpha-glucosidase and Schwanniomyces occidentalis glucoamylase. We speculated that the Q—&gt;P substitution in these highly conserved regions may result in a comparable accumulation. Here we examined this hypothesis using lysosomal alpha-glucosidase as a reporter gene. Mutagenesis of the glutamine residue at position 244 in the homologous region of alpha-glucosidase to proline results in a protein that is neither transported to the lysosomes nor secreted extracellularly but accumulates in the ER, intermediate compartment and cis-Golgi as a mannose-rich polypeptide similar to mutant sucrase-isomaltase in phenotype II. We propose that the Q1098P and Q244P mutations (in sucrase-isomaltase and alpha-glucosidase, respectively) generate structural alterations that are recognized by a control mechanism, operating beyond the ER in the intermediate compartment or cis-Golgi.

Chronic diarrhea: causes, presentation, and management

Important inroads are being made into understanding the pathophysiology of diarrhea. Clear understanding of key mechanisms should suggest new approaches to combat disease. Exciting developments are occurring in terms of super-ORS solutions, particularly with the promise of short chained glucose polymers and glutamine. Perhaps the most important development is the prospect of a good rotavirus vaccine being available before the end of the decade. Chronic diarrhea seems to be on the increase globally, probably because of the success of ORS. The mechanisms that lead to mucosal injury are elusive, and therapy still largely supportive and empiric. Celiac disease continues to be a puzzle, because of the uncomfortable feeling that a majority of cases may be missed because of atypical presentations. The successful use of long term parenteral nutrition has allowed survival and better characterization of cases that otherwise would have perished as 'lethal protracted diarrhea'. Microvillus inclusion disease may be the commonest congenital secretory diarrhea. The role of the recently reported high prevalence of glucoamlase deficiency may be important. Lastly, attention to micronutrients, particularly low vitamin A and probably zinc may prove to be important in prevention and amelioration of diarrhea and growth failure.

Sucrase-isomaltase deficiency: changing pattern over two decades

Twenty-two cases of sucrase-isomaltase deficiency (SID) were observed over a period of 20 years. Since 1977 delay of introduction of sucrose and its decrease in infants' diets have modified the symptomatology. In general, onset of diarrhea has not taken place immediately but 15 days to 2 months after introduction of sucrose. Out of 12 cases with dehydration, five occurred 3 to 7 months after the beginning of sucrose diet. Hypotrophy was not constant (11 of 22 cases), thus diagnosis was delayed in 17 of 22 cases. A yellow complexion due to rising carotene levels in the blood is a striking feature. Because of falsely positive sucrose load tests (four out of 14 nonSID infants) and failure of the hydrogene breath test (one out of five studied cases), disaccharidase determination remains the key to diagnosis. Despite the genetic difference symptoms seem to depend on infant feeding practices.

Congenital sucrase-isomaltase deficiency. Identification of a glutamine to proline substitution that leads to a transport block of sucrase-isomaltase in a pre-Golgi compartment

Congenital sucrase-isomaltase deficiency is an example of a disease in which mutant phenotypes generate transport-incompetent molecules. Here, we analyze at the molecular level a phenotype of congenital sucrase-isomaltase deficiency in which sucrase-isomaltase (SI) is not transported to the brush border membrane but accumulates as a mannose-rich precursor in the endoplasmic reticulum (ER), ER-Golgi intermediate compartment, and the cis-Golgi, where it is finally degraded. A 6-kb clone containing the full-length cDNA encoding SI was isolated from the patient's intestinal tissue and from normal controls. Sequencing of the cDNA revealed a single mutation, A/C at nucleotide 3298 in the coding region of the sucrase subunit of the enzyme complex. The mutation leads to a substitution of the glutamine residue by a proline at amino acid 1098 (Q1098P). The Q1098P mutation lies in a region that is highly conserved between sucrase and isomaltase from different species and several other structurally and functionally related proteins. This is the first report that characterizes a point mutation in the SI gene that is responsible for the transport incompetence of SI and for its retention between the ER and the Golgi.

Uncoordinated, transient mosaic patterns of intestinal hydrolase expression in differentiating human enterocytes

The heterogenous expression of brush border membrane hydrolases by the human enterocyte-like Caco-2 cell line during morphological and functional differentiation in vitro was investigated at the cellular level. Indirect immunofluorescence revealed that the heterogeneous ("mosaic") expression of sucrase-isomaltase, lactase, aminopeptidase N, and alkaline phosphatase was, in fact, transient in nature. The labeling indexes for each hydrolase gradually increased during culture at postconfluence in order to reach a maximum (> or = 90%) after 30 days, concomitant with an upregulation of their respective protein expression levels. In contrast, dipeptidylpeptidase IV labeling remained relatively constant. Backscattered electron imaging analysis in midstage (12 days postconfluence) monolayers demonstrated a lack of correlation between brush border membrane development and expression of each enzyme studied. Moreover, double immunostaining revealed that none of the other four hydrolases correlated directly with sucrase-isomaltase expression. Finally, immunodetection for the proliferation-associated antigen KI-67 revealed a transient mosaic pattern of proliferation which was inversely related to Caco-2 cell differentiation. These data indicate that enterocytic differentiation-related (as well as proliferation-related) gene expression in Caco-2 cells is regulated but uncoordinated at the cellular level, suggesting that an overall control mechanism is lacking.