Differential Diagnosis

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It is important in the differential diagnosis of hypoalphalipoproteinemia and in the management of patients with low HDL-C to distinguish between rare cases with a profound decrease in HDL-C (<10 mg/dL) from more common cases, in which there is only a modest decrease in HDL-C. Patients with a marked reduction in HDL-C such as the one presented here often have a genetic disorder in HDL metabolism, whereas patients with only a modest HDL-C decrease often have elevated serum triglycerides and/or other secondary causes for low HDL.

The primary genetic causes of hypoalphalipoproteinemia besides Tangier disease include deletions and or mutations in the apo-AI gene and lecithin:cholesterol-acyltransferase (LCAT) deficiency. Because apoA-I is the main structural protein on HDL, deletions or structural defects in the apoA-I gene often affect the level of HDL. Familial apolipoprotein AI deficiency is due to deletion of the apoA-I gene, and these patients have no detectable apoA-I. These patients also typically have corneal opacities due to lipid accumulation and may have cutaneous or planar xanthomas. Like Tangier disease patients, they are at an increased risk of developing premature coronary atherosclerosis, but typically develop more severe atherosclerosis and at an earlier age. In addition to gene deletions, many point mutations in apo-AI have been described that alter the structure of apoA-I, leading to increased catabolism and low HDL-C. Some forms of apoA-I mutants will spontaneously self-aggregate and can cause systemic amyloidosis.

LCAT plays a key role in HDL metabolism by converting free cholesterol on HDL to cholesteryl esters, which promotes reverse cholesterol transport by trapping on HDL any cholesterol that has effluxed from cells. Patients homozygous for LCAT deficiency have low HDL-C and develop corneal opacities from lipid accumulation but do not appear to

584 Case 80 A 5-Year-Old Boy with Yellow-Orange Tonsils: Hypoalphalipoproteinemia have a significantly increased risk for coronary artery disease. Because of the decreased content of cholesteryl esters in lipoproteins, they do form abnormal-shaped lipoproteins, which accumulate in the glomerulus, causing renal insufficiency. Heterozygous patients with only a partial deficiency of LCAT, which is sometimes called "Fish-eye disease," usually just have corneal opacities and only have a modest decrease in HDL-C.

There are many secondary causes of hypoalphalipoproteinemia, such as obesity, sedentary lifestyle, cigarette use, and low-fat diets. Low HDL-C is also often associated with diabetes, end-stage renal disease, and uremia. Hypertriglyceridemia from almost any cause also lowers HDL, due to the enrichment of triglycerides on HDL, which alters the physical properties of HDL and leads to its increased catabolism. Finally, certain medications, such as b-blockers, thiazide diuretics, retinoids, probucol, androgenic steroids, and many progestational drugs, can also lower HDL.

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