Abetalipoproteinemia

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]

Synonyms and keywords: Acanthocytosis, Bassen-Kornzweig syndrome, apolipoprotein B deficiency, microsomal triglyceride transfer protein deficiency, MTP deficiency

Overview

It is a disease with autosomal recessive inheritance, affecting the etc. Abetalipoproteinemia and hypobetalipoproteinemia together are reffered as familial hypolipoproteinemia. These are a set of diseases which specifically have low LDL C levels.

Pathophysiology

• Autosomal Recessive Inheritance.
• Mutation which codes for the Microsomal Trigyceride transfer Protien, MTP.
• MTP mutation leads to the failure of formation and secretion of apolipoprotein B( Apo B)containing lipoproteins which include chylomicrons, LDL and VLDL from the intestine and liver.
• Chylomicron formation failure causes Steatorrhea and malabsorption of fat soluble vitamins.
• Spinocerebellar and posterior columns are affected due to low vitamin E levels resulting in neurological symptoms.

Clinical Features

• Patients present in the early childhood with chronic diaarhea, steatorrhea and failure to thrive.
• Symptoms progress with age and neurological symptoms appear in early adolescence include progressive ataxia, visual defects and peripheral neuropathy.

Physical Examination

Specific physical exam findings include as follows:

• EYE: Reduced Visual Acuity and degenerative changes in the retina secondary to Vitamin A deficiency.
• Neurological:
  • Truncal Ataxia due to the effect on spinocerebellar tracts.
  • Sensory Motor neuropathy presenting with weakness and muscular atrophy.
  • Loss of proprioception, vibration and temperature can be affected when the disease affects the posterior column.
  • Reflexes can be present or reduced.

Diagnosis

Characteristic labs include :

• Low Triglyceride and total cholesterol levels.
• Absent Beta-lipoprotien on electrophoresis is the diagnostic test, Genetic testing for MTP gene is not available.
• Peripheral Smear shows 50 to 90% of acanthocytes.
• Nerve conduction studies show reduced or absent action potential.
• Very low or Undetectable vitamin E levels.
• Elevated LFT's due to hepatic steatosis.
• Impaired ACTH response to cortisol in homozygotes.

Differential Diagnosis

• Hypobetalipoproteinemia: Clinical Features, peripheral smear and lipid analysis are similar.
• Severe Vitamin E deficiency: Neurological Symptoms improve significantly with supplementation of Vitamin E.
• Friedrich Ataxia.

Treatment

• Vitamin E Supplementation, 150mg/kg/day helps in preventing or reversal of the neurological symptoms. Dosing and efficacy can be assessed by checking the Vitamin E levels in the adipose tissue biopsy.
• Fat soluble vitamins( A, K and D) supplementation.
• Diet modification to control gastrointestinal symptoms.

Hypobetalipoproteinemia

It shares similar clinical and lab features with abetalipoproteinemia.

Pathophysiology and Lab Findings

• Mutations in the gene coding for Apolipoprotein B resulting in malabsorption, hepatic steatosis and fat souble vitamin deficiency.
• Genetics:
  • Based on a Study which involved genetic analysis in 2010, showed

• Patients commonly have low levels of plasma ApoB and LDL cholesterol.

Overview

Abetalipoproteinemia is a rare autosomal recessive genetic disorder that interferes with the normal absorption of fat and fat soluble vitamins from food. The syndrome causes the body not to make lipoproteins, including low-density lipoproteins, very-low-density lipoproteins, and chylomicrons. It is an autosomal recessive inherited disorder, which affects both sexes. It predominantly affects males. It is caused by mutations in the genes: apolipoprotein B (APOB) or microsomal triglyceride transfer protein (MTP).

Pathophysiology

Abetalipoproteinemia is an inherited disorder that affects the absorption of dietary fats, cholesterol, and certain vitamins. People affected by this disorder are not able to make certain lipoproteins, which are molecules that consist of proteins combined with cholesterol and particular fats called triglycerides. This leads to a multiple vitamin deficiency, affecting the fat soluble vitamin A, vitamin D, vitamin E, and vitamin K. However, many of the observed effects are due to vitamin E deficiency in particular.

Two genes have been identified in which mutations are associated with this disorder: microsomal triglyceride transfer protein (MTTP) and apolipoprotein B (ApoB).

The MTTP gene provides instructions for making a protein called microsomal triglyceride transfer protein, which is essential for creating beta-lipoproteins. These lipoproteins are necessary for the absorption of fats, cholesterol, and fat-soluble vitamins from the diet and the efficient transport of these substances in the bloodstream. Most of the mutations in this gene lead to the production of an abnormally short microsomal triglyceride transfer protein, which prevents the normal creation of beta-lipoproteins in the body. MTTP associated mutations are inherited in an autosomal recessive pattern, which means both copies of the gene must be faulty to produce the disease.

There is an absence of apolipoprotein B. On intestinal biopsy, vacuoles containing lipids are seen in enterocytes. Since there is no or little assimilation of chylomicrons, their levels in plasma remains low. This disorder may also result in fat accumulation in the liver (hepatic steatosis). Because the epithelial cells of the bowel lack the ability to place fats into chylomicrons, lipids accumulate at the surface of the cell, crowding the functions that are necessary for proper absorption.

Clinical History, Complications and Prognosis

This normally results in the affected person being extremely thin, and is normally, if untreated, fatal. It is usually diagnosed in infancy, and sometimes can develop later in life. The signs and symptoms of abetalipoproteinemia appear in the first few months of life. They can include failure to gain weight and grow at the expected rate (failure to thrive); diarrhea; abnormal star-shaped red blood cells (acanthocytosis); and fatty, foul-smelling stools (steatorrhea). Specifically the stool may contain large chunks of fat and or blood. Other features of this disorder may develop later in childhood and often impair the function of the nervous system. They can include poor muscle coordination, difficulty with balance and movement (ataxia), and progressive degeneration of the light-sensitive layer (retina) at the back of the eye that can progress to near-blindness. Adults in their thirties or forties may have increasing difficulty with balance and walking. Many of the signs and symptoms of abetalipoproteinemia result from a severe vitamin deficiency, especially vitamin E deficiency which typically results in eye problems with degeneration of the spinocerebellar and dorsal columns tracts.

Signs and Symptoms

Often symptoms will arise that indicate the body is not absorbing or making the lipoproteins that it needs. These symptoms usually appear en masse, meaning that they happen all together, all the time. These symptoms come as follows:

• Failure to grow in infancy
• Fatty, pale stools
• Frothy stools
• Foul smelling stools
• Protruding abdomen
• Mental retardation/developmental delay
• Dyspraxia, evident by age ten
• Muscle weakness
• Slurred speech
• Scoliosis (curvature of the spine)
• Progressive decreased vision
• Balance and coordination problems

Diagnosis

The inability to absorb fat in the ileum will result in steatorrhea, or fat in the stool. As a result, this can be clinically diagnosed when foul smelling stool is encountered. Low plasma chylomicron levels are also characteristic. Acanthocytes are seen on blood smear.

Shown below is an image depicting acanthocytes on blood smear.

Acanthocytes (Abetalipoproteinemia)

Treatment

• Treatment normally consists of rigorous dieting, involving mass amounts of vitamin E. Vitamin E helps the body restore and produce lipoproteins, which people with abetalipoprotenimia usually lack. Vitamin E also helps keep skin and eyes healthy, which studies show that many males whom are affected will have vision problems later on in life. Dyspraxia and muscle weakness is usually combated with psysiotherapy, or occupational therapy.
• Dietary restriction of triglycerides has also been useful.