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Genetics Home Reference: your guide to understanding genetic conditions
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SI

Reviewed July 2008

What is the official name of the SI gene?

The official name of this gene is “sucrase-isomaltase (alpha-glucosidase).”

SI is the gene's official symbol. The SI gene is also known by other names, listed below.

What is the normal function of the SI gene?

The SI gene provides instructions for producing the enzyme sucrase-isomaltase. This enzyme is found in the intestinal tract, where it is involved in breaking down the sugars sucrose (a sugar found in fruits, and also known as table sugar) and maltose (the sugar found in grains). Sucrose and maltose are called disaccharides because they are each made up of two simple sugar molecules. Disaccharides must be broken down into simple sugar molecules to be digested properly.

The sucrase-isomaltase enzyme is found on the surface of the intestinal epithelial cells, which are cells that line the walls of the intestine. These cells have fingerlike projections called microvilli that absorb nutrients from food as it passes through the intestine. Based on their appearance, groups of these microvilli are known collectively as the brush border. The role of the sucrase-isomaltase enzyme is to break down sucrose and maltose into simple sugars so that they can be absorbed by microvilli into intestinal epithelial cells.

How are changes in the SI gene related to health conditions?

congenital sucrase-isomaltase deficiency - caused by mutations in the SI gene

At least 10 mutations in the SI gene have been found to cause congenital sucrase-isomaltase deficiency. These mutations disrupt the folding and processing of the sucrose-isomaltase enzyme, transportation of the enzyme within the intestinal epithelial cells, the orientation of the enzyme to the cell surface, or its normal functioning. An impairment in any of these cell processes results in a sucrase-isomaltase enzyme that cannot effectively break down sucrose, maltose, or other compounds made from these sugar molecules (carbohydrates). The inability to digest these sugars causes the intestinal discomfort seen in people with congenital sucrase-isomaltase deficiency.

Where is the SI gene located?

Cytogenetic Location: 3q25.2-q26.2

Molecular Location on chromosome 3: base pairs 164,696,685 to 164,796,282

The SI gene is located on the long (q) arm of chromosome 3 between positions 25.2 and 26.2.

The SI gene is located on the long (q) arm of chromosome 3 between positions 25.2 and 26.2.

More precisely, the SI gene is located from base pair 164,696,685 to base pair 164,796,282 on chromosome 3.

See How do geneticists indicate the location of a gene? (http://ghr.nlm.nih.gov/handbook/howgeneswork/genelocation) in the Handbook.

Where can I find additional information about SI?

You and your healthcare professional may find the following resources about SI helpful.

  • Educational resources - Information pages

    • Biochemistry (fifith edition, 2002): Sucrose, Lactose, and Maltose Are the Common Disaccharides (http://www.ncbi.nlm.nih.gov/books/NBK22396/)
    • Molecular Cell Biology (fourth edition, 2000): The Intestinal Epithelium Is Highly Polarized (http://www.ncbi.nlm.nih.gov/books/NBK21502/)
    • The Cell: A Molecular Approach (second edition, 2000): Glucose transport by intestinal epithelial cells (http://www.ncbi.nlm.nih.gov/books/NBK9847/figure/A2010/)

  • Genetic Testing Registry - Repository of genetic test information

    • GTR: Genetic tests for SI (http://www.ncbi.nlm.nih.gov/gtr/tests/?term=6476%5Bgeneid%5D)

You may also be interested in these resources, which are designed for genetics professionals and researchers.

  • PubMed - Recent literature (http://www.ncbi.nlm.nih.gov/pubmed?term=((SI%5BTIAB%5D)%20OR%20(sucrase-isomaltase%5BTIAB%5D))%20AND%20((Genes%5BMH%5D)%20OR%20(Genetic%20Phenomena%5BMH%5D))%20AND%20english%5Bla%5D%20AND%20human%5Bmh%5D%20AND%20%22last%20360%20days%22%5Bdp%5D)
  • OMIM - Genetic disorder catalog (http://omim.org/entry/609845)

  • Research Resources - Tools for researchers

    • Entrez Gene (http://www.ncbi.nlm.nih.gov/gene/6476)
    • GeneCards (http://www.genecards.org/cgi-bin/carddisp.pl?id_type=entrezgene&id=6476)
    • HUGO Gene Nomenclature Committee (http://www.genenames.org/data/hgnc_data.php?hgnc_id=10856)

What other names do people use for the SI gene or gene products?

  • MGC131621
  • MGC131622
  • Oligosaccharide alpha-1,6-glucosidase
  • SUIS_HUMAN

See How are genetic conditions and genes named? (http://ghr.nlm.nih.gov/handbook/mutationsanddisorders/naming) in the Handbook.

What glossary definitions help with understanding SI?

brush border ; cell ; congenital ; deficiency ; enzyme ; epithelial ; gene ; intestine ; microvilli ; simple sugar

You may find definitions for these and many other terms in the Genetics Home Reference Glossary (http://www.ghr.nlm.nih.gov/glossary).

References

  • Entrez Gene (http://www.ncbi.nlm.nih.gov/gene/6476)
  • Jacob R, Zimmer KP, Schmitz J, Naim HY. Congenital sucrase-isomaltase deficiency arising from cleavage and secretion of a mutant form of the enzyme. J Clin Invest. 2000 Jul;106(2):281-7. (http://www.ncbi.nlm.nih.gov/pubmed/10903344?dopt=Abstract)
  • Keiser M, Alfalah M, Pröpsting MJ, Castelletti D, Naim HY. Altered folding, turnover, and polarized sorting act in concert to define a novel pathomechanism of congenital sucrase-isomaltase deficiency. J Biol Chem. 2006 May 19;281(20):14393-9. Epub 2006 Mar 16. (http://www.ncbi.nlm.nih.gov/pubmed/16543230?dopt=Abstract)
  • OMIM: SUCRASE-ISOMALTASE (http://omim.org/entry/609845)
  • Ritz V, Alfalah M, Zimmer KP, Schmitz J, Jacob R, Naim HY. Congenital sucrase-isomaltase deficiency because of an accumulation of the mutant enzyme in the endoplasmic reticulum. Gastroenterology. 2003 Dec;125(6):1678-85. (http://www.ncbi.nlm.nih.gov/pubmed/14724820?dopt=Abstract)
  • Sander P, Alfalah M, Keiser M, Korponay-Szabo I, Kovács JB, Leeb T, Naim HY. Novel mutations in the human sucrase-isomaltase gene (SI) that cause congenital carbohydrate malabsorption. Hum Mutat. 2006 Jan;27(1):119. (http://www.ncbi.nlm.nih.gov/pubmed/16329100?dopt=Abstract)

 

The resources on this site should not be used as a substitute for professional medical care or advice. Users seeking information about a personal genetic disease, syndrome, or condition should consult with a qualified healthcare professional. See How can I find a genetics professional in my area? (http://ghr.nlm.nih.gov/handbook/consult/findingprofessional) in the Handbook.

 
Reviewed: July 2008
Published: May 13, 2013