Abetalipoproteinemia

Summary

Abetalipoprotein is a disease characterized by a lack of beta lipoproteins, or B lipoproteins. Specifically, there's a deficiency in Apo B-48 and Apo B-100, which leads to the inability to form chylomicrons and VLDL. It is caused by a deficiency in the enzyme MTP, which is normally responsible for the lipidation of the Apo B structural proteins. When MTP is not present, the Apo Bs are degraded and chylomicrons and VLDL are unable to be formed. The inability to form chylomicrons leads to fat remaining “stuck” in enterocytes, which leads to the finding of lipid-laden enterocytes on biopsy. This leads to malabsorption and steatorrhea, which subsequently leads to fat-soluble vitamin deficiencies. Vitamin E deficiency is particularly salient, and large oral doses of vitamin E are often given. Patients present with ataxia and neurological deficiencies secondary to vitamin E deficiency, and the presence of acanthocytes may also be observed. Retinitis pigmentosa may also be seen secondary to vitamin A deficiency.

Key Facts

• Abetalipoproteinemia
  • Genetics
    • Autosomal recessive
  • Pathophysiology
    • Caused by mutations in MTP
      • MTP normally lipidates Apo B-48 / 100; failure to lipidate results in Apo B degradation.
  • Labs
    • Deficiency of beta lipoproteins (apo B-containing lipoproteins)
      • No apo B-48 / B-100 → no chylomicrons / VLDL
        • All of these use apo Bs as key structural proteins
        • VLDLs metabolites (IDL, LDL) are also not seen
  • Clinical Features
    • Infants
      • Fat malabsorption → steatorrhea
        • Causes failure to thrive
    • Later
      • Retinitis pigmentosa
        • Likely due to vitamin A deficiency (fat-soluble)
          • Vitamin A is found in visual pigments
        • Also possibly related to vitamin E deficiency (fat-soluble)
      • Neurological abnormalities
        • Spinocerebellar degeneration due to vitamin E deficiency (fat-soluble)
          • Progressive ataxia
      • Acanthocytosis
        • Due to vitamin E deficiency (fat-soluble)
        • "Acantho-" means "thorn"
    • Lipid-laden enterocytes on intestinal biopsy
      • Chylomicrons are unable to be formed, so dietary lipids remain sequestered in enterocytes
  • Treatment
    • Large doses of oral vitamin E 
      • Can also supplement with other vitamins
    • Restriction of long-chain fatty acids
      • Long chain fatty acids are those primarily incorporated into chylomicrons
      • Smaller fatty acids can directly enter the portal system