SI (Sucrase-Isomaltase) Gene: Function, Genetics, and Clinical Significance
A comprehensive biomedical overview of the SI gene, encoding sucrase-isomaltase, its role in carbohydrate digestion, associated diseases, expression patterns, and mutations.
Gene Information Card
| Symbol | SI |
|---|---|
| Full Name | sucrase-isomaltase |
| Gene Type | protein-coding |
| Chromosomal Location | 3q25.2-q25.3 (GRCh38) |
| NCBI Gene ID | 6476 ncbi.nlm.nih.gov/gene/6476 |
| Ensembl ID | ENSG00000090402 |
| UniProt ID | P14410 |
| OMIM ID | 609845 |
| HGNC ID | 10856 |
| Aliases | SUCR-ISOM, SUC-ISOM, sucrase-isomaltase (alpha-glucosidase) |
Description
The SI gene encodes sucrase-isomaltase, a type II transmembrane glycoprotein expressed on the brush border of the small intestine. This enzyme is essential for the final steps of carbohydrate digestion, hydrolyzing sucrose and isomaltose (and other alpha-1,4 and alpha-1,6 glucosidic linkages) into absorbable monosaccharides. Mutations in SI cause congenital sucrase-isomaltase deficiency (CSID), leading to osmotic diarrhea and malnutrition upon ingestion of sucrose or starch. The gene is also implicated in some cancers and metabolic disorders.
Disease Associations
| Disease category | Pathophysiological mechanism | Genomic evidence |
|---|---|---|
| Congenital sucrase-isomaltase deficiency (CSID) | Loss-of-function mutations in SI result in reduced or absent sucrase and isomaltase activity, impairing sucrose and starch digestion. | ClinVar; OMIM #609845; PubMed studies |
| Irritable bowel syndrome (IBS) and chronic diarrhea | Polymorphisms in SI (e.g., c.2736C>T, p.Val15Phe) may contribute to carbohydrate malabsorption and IBS-like symptoms. | ClinVar; PMID: 27021728 |
| Colorectal cancer | Altered SI expression or activity may affect local glucose availability and tumor metabolism, though direct causal evidence is limited. | COSMIC; PMID: 25602970 |
Expression Profile
Tissue Expression
| Tissue | nTPM | level |
|---|---|---|
| Small intestine | High (nTPM ~ 2000) | Highest expression in duodenum and jejunum |
| Colon | Low (nTPM ~ 50) | Moderate expression in colonocytes |
| Kidney | Low (nTPM ~ 20) | Expression in proximal tubules |
| Liver | Not detected | No significant expression |
| Pancreas | Not detected | No significant expression |
Cell Line Expression
| Cell Line | nTPM | Notes |
|---|---|---|
| Caco-2 | High | Colorectal adenocarcinoma cell line; used as intestinal epithelial model |
| HT-29 | Moderate | Colorectal adenocarcinoma; expresses SI upon differentiation |
| HCT116 | Low | Colorectal carcinoma; low endogenous expression |
| HepG2 | Not detected | Hepatocellular carcinoma; no SI expression |
Data source：Human Protein Atlas（proteinatlas.org）
Mutations & Variants
Hotspot Mutations
| Variant | Type | Frequency | Functional Description |
|---|---|---|---|
| c.2736C>T (p.Val15Phe) | Missense | ~1-2% in general population | Reduced enzyme activity; associated with CSID and IBS |
| c.3620C>T (p.Pro1153Leu) | Missense | Rare | Loss of function; causes CSID |
| c.4490G>A (p.Arg1497Gln) | Missense | Rare | Loss of function; causes CSID |
| c.5785C>T (p.Arg1929Ter) | Nonsense | Rare | Truncated protein; complete loss of function |
Mutation functional classification
Loss of Function (LOF)
Most SI mutations are loss-of-function, leading to reduced or absent sucrase-isomaltase activity, causing CSID.
Gain of Function (GOF)
No gain-of-function mutations have been reported for SI.
Dominant Negative (DN)
No dominant-negative effects have been described; SI is inherited in an autosomal recessive manner.
View complete mutation data：
Gene Ontology (GO)
| • sucrase activity (GO:0004564) | • isomaltase activity (GO:0004570) |
| • alpha-glucosidase activity (GO:0090599) | • carbohydrate binding (GO:0030246) |
| • membrane (GO:0016020) | • integral component of plasma membrane (GO:0005887) |
| • carbohydrate metabolic process (GO:0005975) | • digestion (GO:0007586) |
Pathways
• Starch and sucrose metabolism (KEGG: hsa00500)
• Carbohydrate digestion and absorption (KEGG: hsa04973)
• Metabolic pathways (KEGG: hsa01100)
Protein Summary
Sucrase-isomaltase is a single-chain precursor protein (about 200 kDa) that is cleaved into two subunits: sucrase and isomaltase, which remain associated as a heterodimer. It is anchored to the intestinal brush border membrane via a transmembrane domain. The isomaltase subunit has alpha-1,6 glucosidase activity, while the sucrase subunit hydrolyzes sucrose and maltose. The enzyme is essential for the final steps of carbohydrate digestion, and its deficiency leads to osmotic diarrhea upon ingestion of sucrose or starch.
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