Introduction

Abetalipoproteinemia, also known as Bassen-Kornzweig syndrome, is a rare autosomal recessive genetic disorder characterised by the body's inability to properly absorb dietary fats, cholesterol, and fat-soluble vitamins.¹ This condition is primarily caused by mutations in the MTTP (microsomal triglyceride transfer protein) gene, which plays a crucial role in the assembly and secretion of apolipoprotein B-containing lipoproteins. Individuals with abetalipoproteinemia typically exhibit a range of symptoms, including neurological abnormalities, muscle weakness, retinal degeneration, and gastrointestinal issues.¹ Managing this condition requires a comprehensive approach that combines dietary modifications and specific supplementation to address the deficiencies and complications associated with the disorder.

Pathophysiology of abetalipoproteinemia

The MTTP gene mutation leads to a deficiency or absence of apolipoprotein B (apoB), a critical component of chylomicrons and very low-density lipoproteins.¹⁰ Without apoB, these lipoproteins cannot form correctly, resulting in impaired intestinal absorption of dietary fats and fat-soluble vitamins. Consequently, individuals with abetalipoproteinemia have extremely low levels of triglycerides and cholesterol in their blood, leading to deficiencies in essential nutrients and energy sources.¹ Abetalipoproteinemia is diagnosed in a proband with very low LDL-cholesterol, triglyceride, and apoB levels, and biallelic pathogenic variants in MTTP identified by genetic testing.¹⁰

The clinical manifestations of abetalipoproteinemia are diverse. Neurological symptoms, such as ataxia, peripheral neuropathy, and cognitive impairments, are common due to deficiencies in essential fatty acids and fat-soluble vitamins.¹⁰ Retinal degeneration, specifically retinitis pigmentosa, can lead to progressive vision loss. Gastrointestinal symptoms, including steatorrhea (fatty stools), abdominal distension, and malabsorption, are direct consequences of impaired fat absorption.¹

Dietary modifications

Given the impaired fat absorption in individuals with abetalipoproteinemia, dietary modifications play a pivotal role in managing the condition. Cautions should be made about caloric intake to prevent impaired growth.¹ The primary goal is to provide adequate nutrition while minimising the intake of long-chain triglycerides (LCTs), which are poorly absorbed.

Low-fat diet

A low-fat diet (10%-20% of total calories from fat) is essential to reduce the symptoms associated with fat malabsorption.^1,10 Individuals with abetalipoproteinemia should avoid high-fat foods, particularly those rich in saturated and trans fats, which can exacerbate gastrointestinal symptoms and contribute to nutrient deficiencies. Instead, the diet should focus on low-fat alternatives and lean protein sources.

Medium-chain triglycerides

Unlike long-chain triglycerides, medium-chain triglycerides are absorbed directly into the bloodstream without the need for chylomicron formation. This makes MCTs a valuable energy source for individuals with abetalipoproteinemia.¹⁰ MCTs are found in coconut oil, palm kernel oil, and MCT oil supplements.

High-carbohydrate and protein diet

To compensate for the reduced fat intake, individuals with abetalipoproteinemia should consume a diet high in carbohydrates and proteins.¹⁰ Foods rich in complex carbohydrates, such as whole grains, fruits, and vegetables, provide sustained energy. Lean proteins, including poultry, fish, legumes, and low-fat dairy products, are essential for maintaining muscle mass and overall health.

Supplementation strategies

In addition to dietary modifications, targeted supplementation is crucial to address the specific nutrient deficiencies associated with abetalipoproteinemia. The focus is primarily on fat-soluble vitamins (A, D, E, and K) and essential fatty acids, which are typically deficient due to impaired absorption.

Fat-soluble vitamins

• Vitamin A: Deficiency can lead to night blindness and other vision problems. Supplementation with vitamin A (100-400 IU/kg/day) is recommended. Women who are pregnant or who are planning to become pregnant should reduce their vitamin A supplement dose by 50%¹
• Vitamin D: Deficiency can result in rickets in children and osteomalacia in adults. Supplementation with water-soluble vitamin D (800-1,200 IU/day) is advised to enhance absorption¹
• Vitamin E: Deficiency can cause neurological problems and muscle weakness. High doses of water-soluble vitamin E (100-300 IU/kg/day) are necessary to achieve therapeutic levels¹
• Vitamin K: Deficiency can lead to bleeding disorders. Water-soluble forms of vitamin K are recommended to ensure adequate absorption (5-35 mg/week)¹

Essential fatty acids

Essential fatty acids, particularly omega-3 and omega-6 fatty acids, are vital for brain function, inflammation regulation, and overall cellular health. Given the poor absorption of dietary fats, supplementation with essential fatty acids is necessary. Sources include fish oil supplements (up to 1 teaspoon per day, as tolerated) and flaxseed oil.¹

Other nutrients

In addition to fat-soluble vitamins and essential fatty acids, other nutrients such as vitamin C, zinc, and selenium play supportive roles in maintaining overall health. Vitamin C supports immune function and collagen synthesis, while zinc and selenium are important for immune health and antioxidant defence.¹⁰ Mild anaemia rarely requires treatment, although occasionally vitamin B12 or iron therapy may be considered.¹  

Monitoring and adjustments

Effective management of abetalipoproteinemia requires regular medical monitoring and adjustments to dietary and supplementation strategies based on individual needs and responses.

Regular medical check-ups

Assess growth at each visit. Annually check CBC, INR, reticulocyte count, liver function tests, fat-soluble vitamins, serum calcium, phosphate, uric acid, and TSH.¹⁰ Perform lipid profile every few years, liver ultrasound every three years, and ophthalmology and neurology evaluations every six to twelve months.¹

Genetic test

Abetalipoproteinemia is autosomal recessive. Each sibling of an affected individual has a 25% chance of being affected, a 50% chance of being an asymptomatic carrier, and a 25% chance of being unaffected and not a carrier. Carrier testing for at-risk relatives, as well as prenatal and preimplantation genetic testing, are possible if the pathogenic MTTP variants in the family are known.^1,10

Adjusting diet and supplementation

Diet and supplementation plans should be tailored to each individual’s needs. Adjustments may be necessary based on symptoms, blood test results, and overall health status.

Potential challenges and solutions

Managing abetalipoproteinemia through diet and supplementation can be challenging. However, several strategies can help overcome these challenges.

Compliance with dietary restrictions

Most complications can be prevented through the institution of a low-fat diet with supplementation of fat-soluble vitamins.¹ Educating patients and their families about the importance of dietary compliance and providing practical tips for meal planning can enhance adherence.

Managing side effects

Some individuals may experience gastrointestinal side effects from supplements, such as bloating or diarrhoea. These side effects can often be managed by adjusting the dosage or form of the supplement.¹⁰ 

Access to specialised foods and supplements

Accessing specialised foods and supplements can be a logistical and financial challenge. Insurance coverage for medical foods and supplements varies, and out-of-pocket costs can be significant.¹⁰ Exploring resources such as patient assistance programs, nonprofit organisations, and insurance appeals can help alleviate these burdens.

Case studies and clinical evidence

Examples of successful management

A 46-year-old Northern European woman was diagnosed with abetalipoproteinemia at age 11 after presenting with right ptosis and acanthocytosis. She had infant diarrhoea that resolved with fat restriction and developed ataxia and paresthesia in her teens. In 1999, a homozygous frameshift mutation in MTP was identified. She began high doses of fat-soluble vitamins nine years after diagnosis. She married at 26 and had a healthy boy at 34, stopping vitamin A during pregnancy to avoid teratogenicity. Postpartum, she developed a corneal ulcer requiring a transplant, which failed, and she awaits a second one. She couldn't conceive again despite attempts. Her history included basal cell carcinoma and oligomenorrhea. Her parents were first cousins, and two sisters had Alport syndrome, but no family history of ABL. In 2007, 34 years post-diagnosis, she was on various vitamins and supplements. Examination showed mild scleral icterus, decreased right visual acuity, wide-based gait, positive Romberg test, and decreased limb sensation. Labs were mostly normal except for slightly elevated bilirubin, low vitamin E, and elevated creatine kinase. Her cardiovascular exam was unremarkable, and no treatment changes were recommended.²

In 1976, a 16-year-old girl with a history of infant diarrhoea controlled by a low-fat diet was diagnosed with abetalipoproteinemia after tests revealed acanthocytosis and undetectable cholesterol and triglycerides. She was prescribed vitamins A, D, and K. At 16, she experienced weakness and balance issues, with neurological symptoms like muscle weakness, absent reflexes, and ataxic gait. Tests showed low creatine kinase, undetectable triglycerides, cholesterol, and vitamin E. High-dose vitamin E therapy was started. By 2007, at age 47, her health was stable. Genetic testing confirmed compound heterozygosity for MTP mutations. Neurological symptoms had improved or stabilized, with better gait and muscle strength, though some sensory deficits and absent reflexes remained. Electromyography showed mild sensory neuropathy, and her vitamin E levels were slightly below normal.²

Successful pregnancies in women with ABL have also been reported, with varying outcomes linked to vitamin levels.⁸

Review of clinical research

In 1974, it was first reported that high-dose oral vitamin E (100 IU/kg) could improve vitamin levels in ABL patients.³ In 2001, another study of 10 ABL patients showed that high-dose vitamin E and A therapy can significantly slow retinal degeneration and stabilise neurological symptoms.⁴ While gastrointestinal issues like diarrhoea and fat-soluble vitamin deficiencies are common, vitamin E therapy is effective, though serum levels only reach 30% of normal.⁹ 

Hepatic issues such as steatosis and elevated transaminases may occur but were not observed in all patients.⁵ Neurological involvement can be severe, but early vitamin E treatment can prevent significant disability.³ Muscle involvement and cardiomyopathy have been reported.⁶ Death related to cardiomyopathy has been reported in a 10-year old male ABL patient.⁷

In 2001, another study of 10 ABL patients for a mean of 11.7 years showed that combined vitamin A and E supplementation that was initiated prior to 2 years of age markedly attenuated the severe retinal degeneration.⁴ 

Conclusion

In conclusion, managing abetalipoproteinemia involves strict adherence to a low-fat diet, supplemented with high doses of fat-soluble vitamins (A, D, E, K) and essential fatty acids to mitigate neurological, retinal, and gastrointestinal complications. Early intervention and consistent monitoring of vitamin levels and overall health are essential for improving patient outcomes. Advances in genetic research and tailored nutritional therapies continue to enhance the quality of life for those affected by this rare condition.

FAQs

Can abetalipoproteinemia be detected prenatally?

Yes, prenatal genetic testing can detect abetalipoproteinemia if the pathogenic MTTP variants are known in the family.

What is the life expectancy for someone with ABL?

With proper management, individuals with ABL can live into adulthood, but the quality of life and life expectancy can vary based on the severity of complications and adherence to treatment.

Is there a connection between ABL and other genetic disorders?

While ABL itself is not linked to other genetic disorders, individuals with ABL may have family members with different genetic conditions due to the underlying genetic predisposition.