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Sterols

Sitosterolemia is characterized by:

  • Tendon or tuberous xanthomas in unusual locations (heels, knees, elbows and buttocks);
  • Premature atherosclerosis
  • Hemolytic anemia with stomatocytes and large platelets

Sitosterolemia is a rare autosomal recessive disorder caused by mutations in 2 ATP-binding cassette (ABC) transporter genes, ABCG5 and ABCG8. ABCG5 codes for the protein sterolin-1 and ABG8 codes for sterolin-2. Both proteins form a heterodimer that transports cholesterol and plant sterols from the intestines. Mutations in either of these genes may cause increased sterol absorption. Affected individuals have biallelic mutations in either ABCG5 or ABCG8. Asians primarily have mutations in ABCG5 and whites primarily have mutations in ABCG8.

Laboratory diagnosis of sitosterolemia is made by documenting elevations of the plant sterols sitosterol and campesterol in plasma or serum. Sitosterol and campesterol values may be mildly elevated in individuals whose diets include foods with high concentrations of plant sterols, such as some vegetable oils and infant formulas. Patients with sitosterolemia typically have campesterol values >40 mg/L and sitosterol values >80 mg/L.

False positive results may occur in infants ingesting commercial infant formula and patients with cholestasis receiving parenteral nutrition. False negative results may occur in individuals on diets lacking plant derived foods and in patients treated with ezetimibe or bile acid binding resin.

Because sitosterolemia is an autosomal recessive disorder, 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.

Genetic testing may be helpful when biochemical testing is not diagnostic. Carrier testing for at-risk family members and prenatal testing for pregnancies at increased risk are possible if the disease-causing mutations in the family have been identified.

Sterols are measured using gas chromatography-mass spectrometry (GC-MS). Genetic testing is performed by gene sequencing. Specimen requirement is a green top tube of blood collected from a patient after 12 hours of fasting.

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