Chylomicron retention disease

Chylomicron retention disease is a disorder of fat absorption. It is associated with SAR1B. Mutations in SAR1B prevent the release of chylomicrons in the circulation which leads to nutritional and developmental problems. It is a rare autosomal recessive disorder with around 40 cases reported worldwide. Since the disease allele is recessive, parents usually do not show symptoms.

Without functional chylomicrons certain fat-soluble vitamins such as vitamin D and vitamin E cannot be absorbed. Chylomicrons have a crucial role in fat absorption and transport, thus deficiency in chylomicron functioning reduces available levels of dietary fats and fat-soluble vitamins.

Cause

The SAR1B gene has been identified as the cause of CRD. More than 14 different mutations in about 30 patients have been described. This gene encodes the Sar1b protein, which is involved in the transport of chylomicrons (carriers of dietary lipids) from the endoplasmic reticulum to the Golgi apparatus. This mutation results in accumulation of pre-chylomicron transport vesicles in the cytoplasm of enterocytes. Genotyping has revealed that Anderson's disease and CRD are in fact the same condition.

Inheritance

Autosomal recessive inheritance, a 25% chance

The disease follows an autosomal recessive pattern of inheritance.

Signs and symptoms

In the months following birth, signs and symptoms will appear. Some symptoms will manifest gradually during childhood.

• Failure to gain weight
• Failure to thrive
• Diarrhea
• Foul-smelling feces, steatorrhea
• Impaired nervous system functions
• Decreased reflexes, hyporeflexia

Diagnosis

• Diagnosis is often delayed because symptoms are nonspecific and hypocholesterolemia may be attributed to malnutrition secondary to chronic diarrhea.
• Diagnosis is based on a history of chronic diarrhea with fat malabsorption and a characteristic abnormal lipid profile: generally a 50% decrease in total cholesterol, LDL-cholesterol (LDL-C) and high-density lipoprotein-cholesterol (HDL-C) in the presence of normal triglycerides.
• Upper endoscopy and histology reveal fat-laden enterocytes. Elevated creatine kinase (CK) in patients with hypocholesterolemia may be suggestive of CRD.
• Genotyping makes it possible to identify the SAR1B gene mutations. Parental lipid screening may clarify the diagnosis. An absence of hypocholesterolemia in both parents favors CRD. Consanguinity is frequent in patients with the disorder.

Differential diagnosis Differential diagnosis includes abetalipoproteinemia and other genetic hypocholesterolemias characterized by decreased LDL-C, such as homozygous hypobetalipoproteinemia (HBL), and acquired disorders associated with low HDL-C.

Treatment

• Follow-up should be directed toward monitoring nutrition and growth, and treatment compliance. Management should focus on prevention and early detection of complications (hepatic, neuromuscular, retinal and bone). Control of vitamin E deficiency plays a key role in preventing neurological complications. Treatment includes fat-soluble vitamin supplements and large amounts of vitamin E.
• Vitamin A, in combination with vitamin E, may help to prevent ophthalmologic complications. Early vitamin D treatment makes it possible to prevent osteopenia. Vomiting, diarrhea and abdominal distension improve on a low-long chain fat diet. Dietary counseling is needed not only to monitor fat intake and improve symptoms, but also to maintain sufficient caloric and EFA intake.

Prognosis

Very long-term follow-up into adulthood is poorly documented.

References

Classification	ICD-10: E78.6 OMIM: 246700 MeSH: C535460 DiseasesDB: 33188
External resources	Orphanet: 71