Metabolic Studies in an Unusual Case of Asymptomatic Familial Hypobetalipoproteinemia with Hypoalphalipoproteinemia and Fasting Chylomicronemia

Abstract A new kindred with asymptomatic hypobetalipoproteinemia is reported. The proband, age 67, differs from previously described cases in several respects: (a) unusually low levels of low density lipoprotein (LDL) cholesterol (4-8 mg/dl); (b) normal triglyceride levels; (c) low levels of high density lipoprotein; (d) mild fat malabsorption; and (e) a defect in chylomicron clearance. On a high-carbohydrate diet his plasma triglyceride levels, instead of rising, actually fell. Turnover of triglycerides in very low density lipoproteins (VLDL) was low (2.8 mg/kg per h). Fractional catabolic rate of LDL protein was just above the normal range (0.655/d) but net turnover was <10% of normal (0.65 mg/kg per d). The half-life of his chylomicrons was 29 min, five times the normal value. Postheparin lipoprotein lipase activity was normal and apolipoprotein C-II, the activator protein for lipoprotein lipase, was present and functional. Apolipoprotein C-III1, however, was not detected in the VLDL fraction, a finding previously reported in patients with abetalipoproteinemia. Fecal excretion of cholesterol was almost twice normal; total sterol balance was increased by 40%. The unusual features in the proband that distinguish him from previously described cases and from his affected first-degree relatives suggested that, in addition to the basic gene defect affecting LDL metabolism, he might have a second abnormality affecting clearance of chylomicrons and VLDL. The ratio of apolipoprotein E3 to E2 in his VLDL fraction was 0.93, just below the lower limit of normal, suggesting heterozygosity for E3 deficiency. Whether or not this contributes to his hypertriglyceridemia remains to be established.