Abetalipoproteinemia

Roshni Sonawane
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What is Abetalipoproteinemia?
Abetalipoproteinemia is also known as Bassen–Kornzweig Syndrome. It is a metabolic disorder with an autosomal recessive pattern of inheritance. It causes abnormalities in fat metabolism. It results in impaired absorption of lipid and lipid-soluble vitamins. If untreated, it can cause significant morbidity related to fat-soluble vitamin deficiencies.

There is genetic defect causing mutations in the microsomal triglyceride transfer protein gene (MTP). MTP participates in the process of transfer of lipids as follows:

Lipoproteins, VLDL (Very-Low-Density Lipoproteins) and CM (chylomicrons) facilitate the transport of nonpolar lipids (cholesterol and Triglycerides-TGs) to plasma and assist in the function of the digestive system. Apo B is an essential structural component of these lipoproteins. The synthesis and manufacture of VLDL and CM occur in the endoplasmic reticulum (ER) of hepatocytes and enterocytes respectively. The first step in this is the formation of ApoB-containing lipoprotein particle (primordial lipoprotein) by the fusion of nascent ApoB with TG in the ER lumen. MTP mediates the transfer of TG from the ER membrane to the ApoB in the ER lumen. The second step involves the addition of another TG to this particle to form matured lipoprotein, likely mediated by MTP again. ApoB is present as ApoB-100 in VLDL and as ApoB-48 in CM. After the assembly, lipoproteins CM and VLDL are secreted into plasma where they are converted to CM remnants and low-density lipoproteins (LDL) respectively by plasma lipases. They carry the lipids and lipid soluble nutrients to the tissues.

MTP mutation causes no availability of lipids at the initial phase of lipoprotein formation. Apo-B then gets degraded. There is subsequent impairment in the formation of lipoproteins and the associated lipid transfer to tissues. It leads to a deficiency of fat and fat-soluble vitamins, especially Vitamin E, which is almost entirely dependent on the lipoprotein pathway while vitamins A,D and K also have alternative pathways.
This is the reason that the serum levels of vitamin E rise only up to about 30% even after administering greater doses, while vitamin A and K levels can normalize and their deficiencies are relatively easily corrected in ABL.


References
Abetalipoproteinemia Abetalipoproteinemia 03/14/2016
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