Congenital sucrase-isomaltase deficiency is a rare genetic disorder that affects an individual's ability to digest certain sugars. People with this condition cannot break down the sugars sucrose and maltose. Sucrose (a sugar found in fruits, and also known as table sugar) and maltose (the sugar found in grains) are called disaccharides because they are made of two simple sugars. Disaccharides are broken down into simple sugars during digestion. Sucrose is broken down into glucose and another simple sugar called fructose, and maltose is broken down into two glucose molecules. People with congenital sucrase-isomaltase deficiency cannot break down the sugars sucrose and maltose, and other compounds made from these sugar molecules (carbohydrates).

Congenital sucrase-isomaltase deficiency usually becomes apparent after an infant is weaned and starts to consume fruits, juices, grains, and other starchy food. After ingestion of sucrose or maltose, an affected individual will typically experience stomach cramps, bloating, excess gas production, and diarrhea. These digestive problems can lead to failure to gain weight and grow at the expected rate (failure to thrive) and malnutrition.

Variants (also known as mutations) in the SI gene cause congenital sucrase-isomaltase deficiency. The SI gene provides instructions for producing the enzyme sucrase-isomaltase. This enzyme is found in the small intestine and is responsible for breaking down sucrose and maltose into their simple sugar components. These simple sugars are then absorbed by the small intestine. Variants that cause this condition alter the structure, disrupt the production, or impair the function of sucrase-isomaltase. These changes prevent the enzyme from breaking down sucrose and maltose. Rather than being absorbed by the small intestine, the undigested sugars move to the large intestine (colon). Here, they attract water and are consumed by normal bacteria in the colon, causing the intestinal discomfort seen in individuals with congenital sucrase-isomaltase deficiency.

Normal Function

The SI gene provides instructions for producing the enzyme sucrase-isomaltase. This enzyme is made in the cells that line the small intestine, 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). This enzyme is also important for digesting starches, which are first broken down into sucrose and maltose. 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 used by the body.

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.

Health Conditions Related to Genetic Changes

Congenital sucrase-isomaltase deficiency

Many variants (also known as mutations) in the SI gene have been found to cause congenital sucrase-isomaltase deficiency. These variants disrupt the folding and processing of the sucrase-isomaltase enzyme, transportation of the enzyme within the intestinal epithelial cells, localization of the enzyme at the correct 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 sugars (carbohydrates) from starchy food. Rather than being absorbed by the small intestine, the undigested sugars move to the large intestine (colon). Here, they attract water and are consumed by normal bacteria in the colon, causing intestinal discomfort in people with congenital sucrase-isomaltase deficiency.

Other Names for This Gene

• MGC131621
• MGC131622
• sucrase-isomaltase (alpha-glucosidase)
• SUIS_HUMAN

Genomic Location

Congenital sucrase-isomaltase deficiency is inherited in an autosomal recessive pattern, which means both copies of the gene in each cell have variants. The parents of an individual with an autosomal recessive condition each carry one copy of the altered gene, but they may not show signs and symptoms of the condition.

The prevalence of congenital sucrase-isomaltase deficiency is estimated to be 1 in 5,000 people of European descent. This condition is much more prevalent in the native populations of Greenland, Alaska, and Canada, where as many as 1 in 20 people may be affected.