Abetalipoproteinemia: Description, Causes and Risk Factors:
A disorder characterized by an absence of low-density beta-lipoprotein, presence of acanthocytes in blood, retinal pigmentary degeneration, malabsorption, engorgement of upper intestinal absorptive cells with dietary triglycerides, and neuromuscular abnormalities; autosomal recessive inheritance, caused by mutation in the gene encoding microsomal triglyceride transfer protein (MTTP) on chromosome 4q.
Abetalipoproteinemia is a very rare (<1:100,000) autosomal recessive metabolic disorder that results from mutations in the gene encoding microsomal triglyceride transfer protein (MTTP). MTTP catalyzes the transfer of lipids onto apolipoprotein B (ApoB), the major structural protein component of very low-density lipoproteins (VLDL) and chylomicrons in the liver and intestine, respectively. In the absence of MTTP, ApoB cannot be properly lipidated and undergoes pre-secretory degradation, preventing the assembly of chylomicrons and VLDL. The defective secretion of chylomicrons and VLDL causes lipid accumulation in the cytoplasm of intestinal epithelial cells and hepatocytes.
Microsomal triglyceride transfer protein deficiency in Abetalipoproteinemia results in almost undetectable concentrations of plasma ApoB-containing lipoproteins - chylomicrons, VLDL, and low-density lipoproteins (LDL) - and exceptionally low plasma concentrations of cholesterol and triglyceride. Affected patients may express a wide range of clinical features. In early childhood, typical clinical manifestations of ABL are intestinal fat malabsorption and failure to thrive. In later childhood or adolescence, ABL patients may develop ataxic neuropathy and retinopathy. The latter clinical manifestations are partly due to deficiency of fat-soluble vitamins, specifically vitamin E, as well as to other factors such as polyunsaturated fatty acid deficiency and oxidative stress. Other manifestations of ABL include fatty liver, acanthocytosis and anemia.
This condition is inherited in an autosomal recessive pattern, which means both copies of the gene in each cell have mutations. The parents of an individual with an autosomal recessive condition each carries one copy of the mutated gene, but they typically do not show signs and symptoms of the condition.
Some of the more common signs and symptoms of abetalipoproteinemia are seen in the first few months of life, including:
Failure to gain weight.
- Abnormal star shaped RBC (acanthocytosis).
- Fatty, foul-smelling stools (steatorrhea).
- Failure to grow in infancy
- Protruding abdomen.
- Mental retardation/developmental delay.
- Dyspraxia, evident by age ten.
- Muscle weakness.
- Slurred speech.
- Scoliosis (curvature of the spine).
- Progressive decreased vision.
- Retinitis Pigmentosa.
- Balance and coordination problems.
The diagnosis of ABL is suspected from the intestinal, neuromuscular, and ocular symptoms, and is confirmed by laboratory tests showing acanthocytes in the blood and absence of betalipoproteins and chylomicrons in the blood. Other diseases resulting in similar intestinal or neurological symptoms, and those associated with symptoms related to malnutrition and vitamin deficiency must be excluded. As of 2000, there was no direct test of the MTTP gene available for routine diagnostic testing. Accurate carrier testing and prenatal diagnosis are therefore not yet available. However, this could change at any time. Any couple whose child is diagnosed with Abetalipoproteinemia should be referred for genetic counseling to obtain the most up-to-date information.
The recommended treatments for ABL include diet restrictions and vitamin supplementation. Reduced triglyceride content in the diet is suggested if intestinal symptoms require it. Large supplemental doses of vitamin E (tocopherol) have been shown to lessen or even reverse the neurological, muscular, and retinal symptoms in many cases. Supplementation with a water-soluble form of vitamin A is also suggested. Vitamin K therapy should be considered if blood clotting problems occur.
Occupational and physical therapy can assist with any muscular and skeletal problems that arise. Physicians that specialize in orthopedics, digestive disorders, and eye disease should be involved. Support groups and specialty clinics for individuals with multisystem disorders such as Abetalipoproteinemia are available in nearly all metropolitan areas.
NOTE: The above information is for processing purpose. The information provided herein should not be used during any medical emergency or for the diagnosis or treatment of any medical condition.
DISCLAIMER: This information should not substitute for seeking responsible, professional medical care.
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