Abetalipoproteinemia

Definition

Abetalipoproteinemia (ABL) is a rare inherited disorder characterized by difficulty in absorbing fat during digestion. The result is absence of betalipoproteins in the blood, abnormally shaped red blood cells, and deficiencies of vitamins A, E, and K. Symptoms include intestinal, neurological, muscular, skeletal, and ocular problems, along with anemia and prolonged bleeding in some cases.

Description

An unusual sign first described in ABL is the presence of star-shaped red blood cells, which were dubbed "acanthocytes" (literally, thorny cells). Thus, ABL is also known by the name acanthocytosis. Less commonly, ABL may be referred to as Bassen-Kornzweig syndrome.

The underlying problem in ABL is a difficulty in absorbing fats (lipids) in the intestine. Most people with ABL first develop chronic digestive problems, and then progress to neurological, muscular, skeletal, and ocular disease. Disorders of the blood may also be present. Severe vitamin deficiency causes many of the symptoms in ABL. Treatments include restricting fat intake in the diet and vitamin supplementation. Even with early diagnosis and treatment, though, ABL is progressive and cannot be cured.

Genetic profile

Fats are important components of a normal diet, and their processing, transport, and use by the body are critical to normal functioning. Lipids bind to protein (lipo-protein) so they can be absorbed in the intestine, transferred through the blood, and taken up by cells and tissues throughout the body. There are many different lipoprotein complexes in the body. One group, the betalipoproteins, must combine with another protein, microsomal triglyceride transfer protein (MTP). ABL is caused by abnormalities in the gene that codes for MTP. When MTP is nonfunctional or missing, then betalipoproteins will also be decreased or absent. The MTP gene has been localized to chromosome 4.

ABL is an autosomal recessive genetic disorder. This means that both copies of the MTP gene are abnormal in a person affected with the disorder. Since all genes are present at conception, a person cannot "acquire" ABL. Each parent of an affected child carries the abnormal MTP gene but also has a normally functioning gene of that pair. Enough functional MTP is produced by the normal gene so that the parent is unaffected (carrier). When both parents are carriers of the same recessive gene, there is a one in four chance in each pregnancy that they will have an affected child.