Abnormal induction of 3-hydroxy-3-methylglutaryl coenzyme A reductase in leukocytes from subjects with heterozygous familial hypercholesterolemia.

Human leukocytes isolated from fresh defibinated blood were shown to utilize acetate and mevalonate for sterol synthesis. The capacity of the leukocytes to synthesize sterols is limited severely as compared to their ability to convert mevalonate into farnesyl pyrophosphate (which they hydrolyze rapidly to free farnesol) and into squalene. When leukocytes are incubated in a medium containing lipid-free serum, synthesis of sterols from acetate, but not from mevalonate, is much enhanced. It was shown that this increased synthesis resulted from increased levels of 3-hydroxy-3-methylglutaryl-CoA reductase activity in the cells. A comparison was made of the activation of sterol synthesis from acetate in leukocytes of normal individuals and of heterozygous familial hypercholesterolemics. The latter group responded to incubation in lipid-free sera with a significantly higher activation than the cells of normocholesterolemics. This activation was shown to be well correlated with a higher induction of 3-hydroxy-3-methylglutaryl-CoA reductase in the heterozygous cells than in the normals. The leukocytes of a heterozygous familial hypercholesterolemic individual were found to release, into a lipid-free incubation medium, more endogenously synthesized [3H]sterol (but not [3H]squalene) than the cells of a normal person. It is suggested that the genetic abnormality in heterozygous familial hypercholesterolemia could be accounted for by a mutation resulting in a weaker binding of a sterol repressor by heterozygous cells than by normal cells.