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Old 10-02-2011
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Question Lipid Genetics Question!

A 45-year-old male is hospitalized for treatment of myocardial infarction. His father and a paternal uncle also had heart attacks at an early age. His cholesterol is elevated, and lipoprotein electrophoresis demonstrates an abnormally high ratio of low- to high-density lipoproteins (LDL to HDL). Which of the following is the most likely explanation for this problem?
A. Mutant HDL is not responding to high cholesterol levels
B. Mutant LDL is not responding to high cholesterol levels
C. Mutant caveolae proteins are not responding to high cholesterol levels
D. Mutant LDL receptors are deficient in cholesterol uptake
E. Intracellular cholesterol is increasing the number of LDL receptors
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D.Mutant LDL receptors are deficient in cholesterol uptake
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Old 10-02-2011
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The answer is: D
This man has familial hypercholesterolemia , an autosomal dominant phenotype defined by studying men who experienced heart attacks at young ages. Mutations in the LDL receptor lead to decreased cellular cholesterol uptake and increased serum cholesterol. Since LDL has a high cholesterol content, the LDL fraction is elevated compared to the HDL fraction on lipoprotein electrophoresis. In normal individuals, the LDL is taken up by its specific receptor and imported via caveolae to the cell interior. Cholesterol then produces feedback inhibition on the rate-limiting enzyme of cholesterol synthesis (hydroxymethylglutaryl CoA reductase) and also leads to a decrease in the number of LDL receptors. In rare cases, two individuals with familial hypercholesterolemia marry and produce a child with homozygous familial hypercholesterolemia. These children develop severe atherosclerosis and xanthomas (fatty tumors) at an early age.
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