Which enzyme deficiency results in Type III hyperlipoproteinemia (dysbetalipoproteinemia), characterized by accumulation of chylomicron and VLDL remnants?

Explanation

## Type III Hyperlipoproteinemia: ApoE Deficiency ### Pathophysiology of Type III HLP **Key Point:** Type III hyperlipoproteinemia (dysbetalipoproteinemia) results from defective or absent apolipoprotein E, which impairs the hepatic uptake of chylomicron and VLDL remnants via receptor-mediated endocytosis. ### Role of ApoE in Remnant Clearance 1. **ApoE as a ligand:** ApoE is the primary ligand for LDL receptors and LDL receptor-related proteins (LRP) on hepatocytes 2. **Remnant recognition:** Both chylomicron remnants and VLDL remnants require ApoE for recognition by hepatic receptors 3. **Consequence of deficiency:** Without functional ApoE, remnants accumulate in blood, causing severe dyslipidemia ### Clinical and Biochemical Features | Feature | Type III HLP | |---------|-------------| | **Lipid pattern** | ↑↑ Cholesterol, ↑↑ Triglycerides (both markedly elevated) | | **Lipoprotein pattern** | Accumulation of remnants (β-VLDL) | | **Apolipoprotein** | Defective ApoE (E2/E2 or E2/E3 genotype) | | **Age of onset** | Typically 20–40 years | | **Xanthomas** | Palmar/tuberous xanthomas (pathognomonic) | | **Atherosclerosis risk** | Very high; premature CAD and peripheral vascular disease | | **Inheritance** | Autosomal recessive | ### ApoE Isoforms and Type III Risk **High-Yield:** Type III HLP occurs almost exclusively in individuals with the **E2/E2 genotype** of apolipoprotein E. The E2 isoform has reduced binding affinity for hepatic receptors. - **E2/E2:** ~1–2% of population; highest risk for Type III HLP - **E3/E3:** Most common; normal lipid metabolism - **E4/E4 or E4 carriers:** Associated with higher LDL cholesterol ### Distinction from Other Lipoprotein Lipase Deficiencies | Type | Enzyme Defect | Remnant Accumulation | Chylomicrons | |------|---------------|----------------------|---------------| | **Type I** | Lipoprotein lipase | No | ↑↑↑ (massive) | | **Type III** | ApoE deficiency | Yes (remnants) | ↑ (mild) | | **Type IV** | Unknown (often VLDL overproduction) | No | Normal | | **Type V** | Lipoprotein lipase + VLDL overproduction | Mild | ↑ | **Mnemonic:** **E for Elimination** — ApoE is needed to eliminate (clear) remnants from the bloodstream. Without it, remnants accumulate. **Clinical Pearl:** Type III HLP patients characteristically present with palmar xanthomas (orange-yellow discoloration of palms) and tuberous xanthomas on elbows and knees — these are highly specific for this condition.