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The glucose-regulated protein GRP94 is a molecular chaperone that is located in the endoplasmic reticulum (ER). Here, we demonstrate in pull down experiments an interaction between GRP94 and sucrase-isomaltase (SI), the most prominent disaccharidase of the small intestine. GRP94 binds to SI exclusively via its mannose-rich form compatible with an interaction occurring in the ER. We have also examined the interaction GRP94 to a panel of SI mutants that are associated with congenital sucrase-isomaltase deficiency (CSID). These mutants exhibited more efficient binding to GRP94 than wild type SI underlining a specific role of this chaperone in the quality control in the ER. In view of the hypoxic milieu of the intestine, we probed the interaction of GRP94 to SI and its mutants in cell culture under hypoxic conditions and observed a substantial increase in the binding of GRP94 to the SI mutants. The interaction of GRP94 to the major carbohydrate digesting enzyme and regulating its folding as well as retaining SI mutants in the ER points to a potential role of GRP94 in maintenance of intestinal homeostasis by chaperoning and stabilizing SI.

Purpose Congenital sucrase-isomaltase deficiency (CSID) is a rare genetic disorder characterized by a deficiency of the sucrase-isomaltase (SI) enzyme complex within the brush border membrane of the small intestine. Mutations in the SI gene result in abnormal synthesis and/or incorrect transport of the SI enzyme. Patients with CSID generally have reduced sucrase activity, but levels of isomaltase activity range from absent to almost normal. This study sought to better understand the experience of patients with CSID prior to, during, and after their diagnosis and its subsequent treatment with sacrosidase. Methods This was a cross-sectional interview study conducted in conjunction with a longitudinal, observational study of US patients prescribed and taking sacrosidase for at least three consecutive months as treatment for CSID. The observational study included both children and adults. Results This qualitative interview study explored the experiences of 43 adult and pediatric patients (n = 8 adults and n = 35 children/adolescents) with CSID pre-, during, and post-diagnosis. Findings suggest that a CSID diagnosis is particularly problematic given the disparate range of more commonly understood gastrointestinal (GI) disorders. After diagnosis and treatment with sacrosidase, participants reported considerable improvement in symptoms and health-related quality of life (HRQL), yet symptoms persist that continue to affect daily life, indicating areas of potential unmet need. Conclusion Educating clinicians about CSID may help improve the overall diagnosis experience. As this research is the first of its kind in CSID, additional research, qualitative and quantitative, will be important to furthering the understanding of HRQL impact and unmet need experienced by this population and identifying ways to best meet those needs.

Background: Congenital sucrase-isomaltase deficiency (CSID) is an autosomal recessive inherited disease that leads to the maldigestion of disaccharides and is associated with mutation of the sucrase-isomaltase (SI) gene. Cases of CSID are not very prevalent in China or worldwide but are gradually being identified and reported.Case Presentation: We report a case involving a 14-month-old male who presented with failure to thrive that had begun after food diversification and was admitted for chronic diarrhea. We used a whole-exome sequencing (WES) approach to identify mutations in this patient's genome. WES revealed two novel heterozygous mutations in the SI gene, c.2626C > T (p.Q876*) and c.2872C > T (p.R958C), which were confirmed by Sanger DNA sequencing. With a strict sucrose- and starch-restricted diet, the patient's diarrhea was resolved, and he began to gain weight.Conclusions: We report a case of novel variants in the SI gene that caused CSID. This report provides valuable information for the clinical field, especially in China.

Congenital sucrase isomaltase deficiency (CSID) is an autosomal recessive disorder which leads to chronic intestinal malabsorption of nutrients from ingested starch and sucrose. Symptoms usually present after consumption of fruits, juices, grains, and starches, leading to failure to thrive and malnutrition. Diagnosis is suspected on detailed patient history and confirmed by a disaccharidase assay using small intestinal biopsies or sucrose hydrogen breath test. Treatment of CSID consists of limiting sucrose in diet and replacement therapy with sacrosidase. Due to its nonspecific symptoms, CSID may be undiagnosed in many patients for several years. We present a case of a 50-year-old woman with persistent symptoms of bloating in spite of extensive evaluation and treatment.

Congenital sucrase-isomaltase deficiency (CSID) is a rare metabolic intestinal disorder with reduced or absent activity levels of sucrase-isomaltase (SI). Interestingly, the main symptoms of CSID overlap with those in irritable bowel syndrome (IBS), a common functional gastrointestinal disorder with unknown etiology. Recent advances in genetic screening of IBS patients have revealed rare SI gene variants that are associated with IBS. Here, we investigated the biochemical, cellular and functional phenotypes of several of these variants. The data demonstrate that the SI mutants can be categorized into three groups including immature, mature but slowly transported, and finally mature and properly transported but with reduced enzymatic activity. We also identified SI mutant phenotypes that are deficient but generally not as severe as those characterized in CSID patients. The variable effects on the trafficking and function of the mutations analyzed in this study support the view that both CSID and IBS are heterogeneous disorders, the severity of which is likely related to the biochemical phenotypes of the SI mutants as well as the environment and diet of patients. Our study underlines the necessity to screen for SI mutations in IBS patients and to consider enzyme replacement therapy as an appropriate therapy as in CSID.

Congenital sucrase-isomaltase deficiency (CSID) is an autosomal recessive disorder of carbohydrate maldigestion and malabsorption caused by mutations in the sucrase-isomaltase (SI) gene. SI, together with maltase-glucoamylase (MGAM), belongs to the enzyme family of disaccharidases required for breakdown of α-glycosidic linkages in the small intestine. The effects of homozygote and compound heterozygote inheritance trait of SI mutations in CSID patients have been well described in former studies. Here we propose the inclusion of heterozygote mutation carriers as a new entity in CSID, possibly presenting with milder symptoms. The hypothesis is supported by recent observations of heterozygote mutation carriers among patients suffering from CSID or patients diagnosed with functional gastrointestinal disorders. Recent studies implicate significant phenotypic heterogeneity depending on the character of the mutation and call for more research regarding the correlation of genetics, function at the cellular and molecular level and clinical presentation. The increased importance of SI gene variants in irritable bowel syndrome (IBS) or other functional gastrointestinal disorders FGIDs and their available symptom relief diets like fermentable oligo-, di-, mono-saccharides and polyols FODMAPs suggest that the heterozygote mutants may affect the disease development and treatment.

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.

BACKGROUND: Human starch digestion is a multi-enzyme process involving six 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). HYPOTHESES: Poor starch digestion occurs in individuals with CSID and can be documented using a noninvasive 13C-breath test (BT). METHODS: 13C-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 13C-sucrose BT. Breath 13CO2 was quantitated at intervals before and after serial 13C-substrate loads (glucose followed 75 minutes later by starch). Variations in metabolism were normalized against 13C-glucose BT (Coefficient of glucose absorption, CGO). 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 13C-starch digestion mirroring that of their duodenal sucrase and maltase activity and 13C-sucrase BT. CONCLUSION: In children with CSID, starch digestion may be impaired. In children with CSID, starch digestion correlates well with measures of sucrase activity.

Patients with irritable bowel syndrome (IBS) often associate their symptoms to certain foods. In congenital sucrase-isomaltase deficiency (CSID), recessive mutations in the SI gene (coding for the disaccharidase digesting sucrose and 60% of dietary starch)1 cause clinical features of IBS through colonic accumulation of undigested carbohydrates, triggering bowel symptoms.2 Hence, in a previous study,3 we hypothesized that CSID variants reducing SI enzymatic activity may contribute to development of IBS symptoms. We detected association with increased risk of IBS for 4 rare loss-of-function variants typically found in (homozygous) CSID patients, because carriers (heterozygous) of these rare variants were more common in patients than in controls.1,4 Through a 2-step computational and experimental strategy, the present study aimed to determine whether other (dys-)functional SI variants are associated with risk of IBS in addition to known CSID mutations. We first aimed to identify all SI rare pathogenic variants (SI-RPVs) on the basis of integrated Mendelian Clinically Applicable Pathogenicity (M-CAP) and Combined Annotation Dependent Depletion (CADD) predictive (clinically relevant) scores; next, we inspected genotype data currently available for 2207 IBS patients from a large ongoing project to compare SI-RPV case frequencies with ethnically matched population frequencies from the Exome Aggregation Consortium (ExAC). ispartof: CLINICAL GASTROENTEROLOGY AND HEPATOLOGY vol:16 issue:10 pages:1673-1676 ispartof: location:United States status: published

OBJECTIVES: Congenital diarrheal disorders is a group of inherited enteropathies presenting in early life and requiring parenteral nutrition. In most cases, genetics may be the key for precise diagnosis. We present an infant girl with chronic congenital diarrhea that resolved after introduction of fructose-based formula but had no identified mutation in the SLC5A1 gene. Using whole exome sequencing (WES) we identified other mutations that better dictated dietary adjustments. METHODS: WES of the patient and her parents was performed. The analysis focused on recessive model including compound heterozygous mutations. Sanger sequencing was used to validate identified mutations and to screen the patient’s newborn sister and grandparents. Expression and localization analysis were performed in the patient’s duodenal biopsies using immunohistochemistry. RESULTS: Using WES we identified a new compound heterozygote mutation in sucrase-isomaltase (SI) gene; a maternal inherited known V577G mutation, and a novel paternal inherited C1531W mutation. Importantly, the newborn offspring carried similar compound heterozygous mutations. Computational predictions suggest that both mutations highly destabilize the protein. SI expression and localization studies determined that the mutated SI protein was not expressed on the brush border membrane in the patient’s duodenal biopsies, verifying the diagnosis of congenital sucrase-isomaltase deficiency (CSID). CONCLUSIONS: The novel compound heterozygote V577G/C1531W SI mutations lead to lack of SI expression in the duodenal brush border, confirming the diagnosis of CSID. These cases of CSID extend the molecular spectrum of this condition, further directing a more adequate dietary intervention for the patient and newborn sibling.

Background Congenital sucrase-isomaltase deficiency is a rare hereditary cause of chronic diarrhea in children. People with this condition lack the intestinal brush-border enzyme required for digestion of di- and oligosaccharides, including sucrose and isomaltose, leading to malabsorption. Although the condition is known to be highly prevalent (about 5%-10%) in several Inuit populations, the genetic basis for this has not been described. We sought to identify a common mutation for congenital sucrase-isomaltase deficiency in the Inuit population. Methods We sequenced the sucrase-isomaltase gene, SI, in a single Inuit proband with congenital sucrase-isomaltase deficiency who had severe fermentative diarrhea and failure to thrive. We then genotyped a further 128 anonymized Inuit controls from a variety of locales in the Canadian Arctic to assess for a possible founder effect. Results In the proband, we identified a novel, homozygous frameshift mutation, c.273_274delAG (p.Gly92Leufs*8), predicted to result in complete absence of a functional protein product. This change was very common among the Inuit controls, with an observed allele frequency of 17.2% (95% confidence interval [CI] 12.6%-21.8%). The predicted Hardy-Weinberg prevalence of congenital sucrase-isomaltase deficiency in Inuit people, based on this single founder allele, is 3.0% (95% CI 1.4%-4.5%), which is comparable with previous estimates. Interpretation We found a common mutation, SI c.273_274delAG, to be responsible for the high prevalence of congenital sucrase-isomaltase deficiency among Inuit people. Targeted mutation testing for this allele should afford a simple and minimally invasive means of diagnosing this condition in Inuit patients with chronic diarrhea.

Congenital sucrase–isomaltase (SI) deficiency is a rare genetic condition characterised by a deficiency in the brush-border SI enzyme, resulting in an inability to metabolise sucrose and starches. Six cases of congenital SI deficiency treated with Sucraid (sacrosidase, a yeast-derived enzyme that facilitates sucrose digestion) are described. Typical presenting symptoms were watery diarrhoea, abdominal pain and bloating, sometimes noticeably worse after ingestion of fruit. Diagnosis is challenging since conventional hydrogen breath testing after an oral sucrose load is impractical in young children, and many laboratories no longer look for maldigested sucrose using faecal sugar chromatography. Confirmation is by disaccharidase assay of duodenal or jejunal mucosa obtained endoscopically. All six patients showed little improvement following advice regarding dietary management, but experienced a marked reduction in symptoms with sacrosidase administration; no adverse events were reported. Sacrosidase is an effective and well-tolerated treatment for patients with congenital SI deficiency. Gene testing and clinical trial of sacrosidase may become an alternative to endoscopic biopsies for diagnosis.

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.

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.

Chronic nonspecific diarrhea in children, or toddler's diarrhea, is a frequently encountered entity in pediatric clinical practice. This disorder remains poorly understood. Suggested etiologies include malabsorption, dietary intake and motility abnormalities. We investigated the use of sacrosidase (a yeast sucrase supplement) in children with toddler's diar- rhea. The study outcome was clinical response to sacrosidase supplementation. Children, 1-6 years of age, with toddler's diarrhea were enrolled in an open prospective trial. Stooling patterns were obtained at baseline and while on supplementa- tion. Twelve patients were enrolled out of 40 patients who presented with chronic diarrhea. Eight patients did not respond to standard diet changes and were entered into the trial. With supplementation, 4 out of the 8 patients responded clinically with a decrease in mean daily stool frequency and an improvement in the mean daily stool consistency. Sacrosidase sup- plementation demonstrated a potential benefit in a subset of children with toddler's diarrhea. Response to sacrosidase may suggest excessive sucrose intake or unrecognized partial or complete sucrase deficiency in some children with toddler's diarrhea.

Osmotic diarrhea and abdominal pain in humans are oftentimes associated with carbohydrate malabsorption in the small intestine due to loss of function of microvillar disaccharidases. Disaccharidases are crucial for the digestion and the subsequent absorption of carbohydrates. This review focuses on sucrase-isomaltase as the most abundant intestinal disaccharidase and the primary or induced pathological conditions that affect its physiological function. Congenital defects are primary factors which directly influence the transport and function of sucrase-isomaltase in a healthy epithelium. Based on the mutation type and the pattern of inheritance, a mutation in the sucrase-isomaltase gene may exert a variety of symptoms ranging from mild to severe. However, structure and function of wild type sucrase-isomaltase can be also affected by secondary factors which influence its structure and function either specifically via certain inhibitors and therapeutic agents or generally as a part of intestinal pathogenesis, for example in the inflammatory responses. Diagnosis of sucrase-isomaltase deficiency and discriminating it from other gastrointestinal intolerances can be latent in the patients because of common symptoms observed in all of these cases. Here, we summarize the disorders that implicate the digestive function of sucrase-isomaltase as well as the diagnostic and therapeutic strategies utilized to restore normal intestinal function.

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.

Objectives: Individuals affected with sucrase isomaltase deficiency are unable to digest sucrose-containing foods. Primary symptoms include watery diarrhea, abdominal discomfort and vomiting. Secondary symptoms include retarded growth and malnutrition. Nonspecific symptoms may lead to lengthy delays in diagnosis. The objective of this report is to describe the clinical presentation, diagnosis and treatment of five Turkish children with sucrase isomaltase deficiency. Methods: This is a series of three male and two female patients that presented to the Department of Pediatric Gastroenterology, Hepatology, and Nutrition at Ege University between 2008 and 2014. Results: Patient ages ranged from 6 months to 9 years and symptoms began after the introduction of food or formula into the diet at 1 week to 6 months of age. Presenting symptoms included growth retardation, decreased weight gain, watery diarrhea and vomiting. Four patients were previously misdiagnosed with GERD, celiac disease, lactose intolerance and food allergy but they did not respond to standard treatment. Routine clinical laboratory assessments including hematology, renal, thyroid and hepatic function, renal tubular function were normal. A detailed history revealed the association between the onset of illness and dietary changes leading to the diagnosis of congenital sucrase isomaltase deficiency. All five patients responded to treatment with sacrosidase oral solution. Conclusion: This is the first case series of pediatric patients with sucrase isomaltase deficiency in Turkey. The actual prevalence of the disease in Turkey is unknown, probably due incorrect diagnoses. Four of our patients were previously misdiagnosed with other gastrointestinal disorders leading to delayed treatment. We suggest that sucrase isomaltase deficiency should be considered in pediatric patients presenting with chronic watery diarrhea.

Background: Congenital sucrase-isomaltase deficiency (CSID) is an autosomal recessive disease characterized by absent sucrase activity with variable decrease in isomaltase activity. The prevalence of CSID in Portuguese population is unknown and there are few reported cases. Case report: We report the case of a six-month-old male infant admitted for chronic profuse diarrhea and failure to thrive that began after food diversification. The investigation showed that he had CSID. The therapeutic option was the addition of baker’s yeast to the diet which was followed by complete resolution of symptoms and excellent weight recovery. Discussion: This case highlights the relevance of clinical observation and awareness in a condition where diagnosis is essentially clinical. The available therapeutic options are addressed with pragmatic use of baker’s yeast. Introdução: O défice congénito de sacarase-isomaltase é uma doença autossómica recessiva caraterizada pela ausência da atividade da sacarase e diminuição variável da atividade da isomaltase. A prevalência desta patologia na população portuguesa é desconhecida e há poucos casos divulgados. Relato de caso: Lactente masculino de 6 meses, internado para estudo de diarreia crónica profusa coincidente com o início da diversificação alimentar e associada a má evolução ponderal. A investigação mostrou tratar-se de uma deficiência congénita de sacarase-isomaltase. A medida terapêutica escolhida foi a adição de fermento de padeiro na dieta, com resolução da sintomatologia e excelente recuperação ponderal. Discussão: Discute-se a importância de uma elevada suspeição clínica numa patologia cujo diagnóstico é essencialmente clínico e também as opções terapêuticas disponíveis, com ênfase no uso do fermento de padeiro.

The purpose of this study was to determine if sacrosidase, a liquid produced from Saccharomyces cerevisiae containing 6000 IU of sucrase activity per mg protein, prevented symptoms of diarrhea, abdominal cramps, gas, and bloating in patients with congenital sucrase-isomaltase deficiency (CSID) consuming a normal sucrose and carbohydrate-containing diet.Twenty-eight children (aged 5 months to 11 years) underwent a randomized, double-blind trial consisting of two phases: 1) three sucrose breath H2 tests with three single-dose treatments (placebo, sacrosidase, and sacrosidase plus milk), and 2) a dose-response phase consisting of four multidose treatments, each for 10 days of full-strength sacrosidase, 1:10 dilution, 1:100 dilution, and 1:1000 dilution. Patients who weighed less than or equal to 15 kg received a dose of sacrosidase and those who weighed more than 15 kg received 2 ml. For the dose-response phase each patient consumed a normal diet. The number of stools and severity of symptoms were recorded daily for each concentration of sacrosidase administered and compared to a baseline period during which the patient took no sacrosidase and consumed a sucrose/starch-free diet. Data were analyzed using an ANOVA model and the nonparameter Wilcoxon signed-rank test.Breath H2 excretion decreased significantly when patients received sacrosidase or sacrosidase plus milk compared to placebo during sucrose breath tests. During the dose-response phase significant treatment differences were observed between the two higher concentrations and the two lower concentrations of sacrosidase for both total stools (p0.001) and total symptom score (p = 0.003). Higher concentrations of sacrosidase were associated with fewer stools and a greater number of formed or hard stools compared to lower concentrations and compared to the baseline period. Higher concentrations were also associated with fewer symptoms of gas, abdominal cramps, or bloating, but no differences in vomiting. The only significant adverse event was wheezing in one child with a history of asthma.Sacrosidase is a safe, effective, well-accepted treatment to prevent gastrointestinal symptoms in patients with CSID consuming a normal diet.

OBJECTIVE: To describe four infants with protracted diarrhea caused by glucose polymer intolerance resulting from congenital sucrase-isomaltase deficiency. METHODS: The diagnosis of congenital sucrase-isomaltase deficiency was established by mucosal disaccharidase assay. In each case the clinical response to discontinuation of glucose polymer was documented. RESULTS: The median age at the onset of symptoms was 3 weeks (range, 2 to 16 weeks). In three infants the formula had been prescribed for presumed postgastroenteritis diarrhea, and in a fourth it was begun after resection of a short-segment congenital ileal atresia. In each infant watery diarrhea occurred and persisted for many months, and it was assumed that the original gastrointestinal disorder was responsible. In two cases, parenteral nutrition was administered for persistent failure to thrive. Ultimately, investigation revealed the underlying congenital sucrase-isomaltase deficiency, and elimination of glucose polymer from the diet led to immediate recovery in each case. CONCLUSION: Congenital sucrase-isomaltase deficiency should be considered a possible cause of protracted diarrhea in infants receiving glucose polymer-based feedings. (J PEDIATR 1996;128:753-6)

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.

Evaluation for congenital sucrase-isomaltase deficiency (CSID) historically has been performed using duodenal enzyme disaccharidase assays, in which the Dahlqvist method is used to assess the activities of the disaccharidases in vitro (1, 2). Meanwhile, noninvasive means have been developed to test for CSID (3). One of these methods is the 13C-based breath test, the results of which have been found to correlate well with duodenal mucosal enzyme analysis (4). Another approach is SI exome genetic sequencing, which has identified both homozygous mutations and compound heterozygote mutations that cause CSID (5–7).

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

Objective Congenital sucrase-isomaltase deficiency (CSID) is a rare hereditary cause of chronic diarrhoea in children. Persons with CSID lack the intestinal brush-border enzyme required for digestion of di- and oligosaccharides, including sucrose and isomaltose. Malabsorption results in abdominal pain, distention, copious diarrhoea, and failure to thrive. If recognised, dietary avoidance of the offending carbohydrates is highly effective. Although CSID is known to be highly prevalent (˜5–10%) in several Inuit populations, the genetic basis for this condition has not been described. Methods We sequenced the sucrase-isomaltase gene, SI, in a single Inuit CSID proband with severe fermentative diarrhoea and failure to thrive. We then genotyped a further 128 anonymized Inuit control individuals from a variety of circumpolar locales to assess for a possible founder effect. Results We identified a novel, homozygous frameshift mutation, c.273_274delAG (p. Gly92Leufs*8) in exon 4 of SI in the proband, that is predicted to result in complete absence of functional protein product. This change is indeed very common among Inuit control specimens, with an observed allele frequency of 0.17 (95% confidence interval 0.13–0.22). The predicted Hardy-Weinberg prevalence of CSID in the Inuit, based on this single founder allele is ˜3% (95% confidence interval 1.6–5.0%), comparable with previous estimates. Interpretation Targeted mutation testing for the c.273_274delAG allele should afford a simple and minimally invasive means of diagnosing CSID in persons of Inuit descent. Because CSID is a readily treatable disorder, such testing should be considered at an early stage in the assessment of Inuit patients with chronic diarrhoea.

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.

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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.

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 α-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.