The death of a 13-year-old boy from coronary atherosclerosis prompted the study of an Amish family. Five of his twelve sibs had tendon and tuberous xanthomas, and increased plasma plant sterols, particularly β-sitosterol. The plasma level of the major apoprotein of low density lipoprotein (LDL), the B protein, was very high (mean 173 mg/dl) in these five sibs, while the LDL cholesterol level was moderately increased (209 mg/dl). Four other sibs and both parents had an increased LDL B protein level with a normal or mildly raised plasma total and LDL cholesterol level (hyperapobetalipoproteinaemia). Evidence for coronary artery disease was found in both parents and three xanthomatous sibs. The original family with β-sitosterolaemia and xanthomatosis, described in 1974, was re-examined. The proband and her sister had persistent phytosterolaemia and normocholesterolaemia but increased LDL B protein levels. Both parents, two uncles, and three of four grandparents had increased LDL B protein levels and normal total and LDL cholesterol levels. The proband's father had atypical angina pectoris. People with the full syndrome (phytosterolaemia, xanthomas, and hyperapobetalipoproteinaemia) are most probably homozygous for a mutant allele. An increased LDL B protein level permits the identification of heterozygotes in these families, even though in the fasting state they show no phytosterolaemia. The homozygote and probably the heterozygote are at increased risk for cardiovascular atherosclerotic disease.
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