Disorders of Lipoprotein Metabolism

Lipoprotein Basics - Fat Shuttles 101

• Function: Transport insoluble lipids (triglycerides ($TG$), cholesterol) in blood.
• Structure: Core (hydrophobic $TG$, cholesteryl esters ($CE$)) & Shell (amphipathic phospholipids, free cholesterol, apolipoproteins).
• Classification, Density & Key Features:
  • 📌 Density order (least to most dense): Chylomicrons < VLDL < IDL < LDL < HDL.

• Key Apolipoproteins & Functions:
  • Apo A-I: HDL structure; LCAT activator.
  • Apo B-100: VLDL, IDL, LDL; LDL receptor ligand.
  • Apo B-48: Chylomicrons (intestinal origin).
  • Apo C-II: Lipoprotein Lipase (LPL) activator.
  • Apo E: Ligand for remnant receptors (chylomicron remnants, IDL).

⭐ Apo B-48 is exclusive to chylomicrons and results from mRNA editing of the Apo B gene; Apo B-100 is the full-length protein.

Hyperlipidemias - Fredrickson's Lineup

Phenotypic classification of hyperlipidemias based on specific elevated lipoprotein patterns. Essential for initial diagnosis and guiding therapy.

• Type I (Hyperchylomicronemia): ↑ Chylomicrons (CM).
  • Defect: Lipoprotein lipase (LPL) or ApoC-II deficiency.
  • Features: Acute pancreatitis, eruptive xanthomas, creamy supernatant on standing plasma.
• Type IIa (Familial Hypercholesterolemia): ↑ Low-density lipoprotein (LDL).
  • Defect: LDL receptor deficiency.
  • Features: Tendon xanthomas, premature coronary artery disease (CAD), corneal arcus.
• Type IIb (Familial Combined Hyperlipidemia): ↑ LDL, ↑ Very-low-density lipoprotein (VLDL).
  • Defect: Overproduction of ApoB-100.
  • Features: Premature CAD, often associated with metabolic syndrome.
• Type III (Dysbetalipoproteinemia / Broad Beta Disease): ↑ Intermediate-density lipoprotein (IDL) / Remnants.
  • Defect: Homozygous ApoE2 genotype (ApoE2/E2).
  • Features: Palmar xanthomas (xanthoma striata palmaris), tuberoeruptive xanthomas, premature CAD & peripheral vascular disease (PVD).
• Type IV (Familial Hypertriglyceridemia): ↑ VLDL.
  • Defect: Increased VLDL production or decreased VLDL clearance.
  • Features: Often asymptomatic; risk of pancreatitis if triglycerides >1000 mg/dL, CAD risk. Turbid plasma.
• Type V (Mixed Hypertriglyceridemia): ↑ CM, ↑ VLDL.
  • Defect: Multiple factors, often LPL deficiency combined with ↑VLDL production.
  • Features: Similar to Type I & IV; pancreatitis, eruptive xanthomas. Creamy supernatant + turbid infranatant.

📌 Mnemonic (Elevated Lipoproteins by Type I-V): Children Love Licking Icy Vanilla Cones Very much. (CM; LDL; LDL+VLDL; IDL; VLDL; CM+VLDL)

⭐ > Type III dysbetalipoproteinemia is strongly associated with the ApoE2/E2 genotype and characteristic palmar xanthomas (xanthoma striata palmaris).

Key Genetic Defects - Rogue Lipid Runners

• Familial Hypercholesterolemia (FH): LDLR (LDL receptor) defect → ↑↑LDL-C, xanthomas, early CAD.
• Lipoprotein Lipase (LPL) Deficiency: LPL defect → ↑Chylomicrons, ↑VLDL (TG >1000 mg/dL); eruptive xanthomas, pancreatitis.
• Apo C-II Deficiency: APOC2 (LPL activator) defect → LPL-like, ↑TGs.
• Abetalipoproteinemia: MTTP (MTP) defect → ↓ApoB lipoproteins (CM, VLDL, LDL absent); fat malabsorption, acanthocytes, RP. 📌 "A-beta" = Absent Beta-lipoproteins.
• Tangier Disease: ABCA1 defect → ↓↓↓HDL; orange tonsils, HSM, neuropathy.
• LCAT Deficiency: LCAT defect → ↓HDL esterification; corneal opacities, renal issues.

⭐ Acanthocytes (spur cells) on peripheral blood smear are characteristic in Abetalipoproteinemia due to altered RBC membrane lipids.

Secondary Causes & Rx - Lifestyle & Lipid Lowerers

Secondary Causes:

• Conditions: DM, Hypothyroidism, Nephrotic syndrome, Cholestasis, Obesity.
• Drugs: Thiazides, β-blockers, Protease inhibitors, Corticosteroids, Estrogens.

Rx - Lifestyle (TLC):

• Diet: ↓Sat/trans fat, ↑fiber.
• Weight: BMI < 25 kg/m².
• Exercise: ≥150 min/wk moderate.
• Quit smoking; limit alcohol.

Rx - Lipid Lowerers:

• Statins: ↓$LDL-C$ (1st line). SE: Myopathy.
• Ezetimibe: ↓$LDL-C$.
• Fibrates: ↓↓TG. SE: Myopathy (esp. w/ statins), gallstones.
• Bile Acid Sequestrants (BAS): ↓$LDL-C$. May ↑TG.
• Niacin: ↑↑HDL, ↓TG. SE: Flushing.
• PCSK9 Inhibitors: ↓↓$LDL-C$.

⭐ Statins are first-line for elevated $LDL-C$, offering cardiovascular protection via lipid-lowering and pleiotropic effects.

High‑Yield Points - ⚡ Biggest Takeaways

• Familial Hypercholesterolemia (Type IIa): LDL receptor defect, ↑LDL, tendon xanthomas, premature atherosclerosis.
• LPL/ApoC-II Deficiency (Type I): ↑Chylomicrons, creamy plasma, pancreatitis, eruptive xanthomas, no ↑atherosclerosis.
• Abetalipoproteinemia: MTP gene defect, ↓ApoB, acanthocytes, steatorrhea, retinitis pigmentosa, ataxia.
• Dysbetalipoproteinemia (Type III): ApoE2 homozygosity, ↑Remnants, palmar & tuboeruptive xanthomas, premature CAD/PVD.
• Tangier Disease: ABCA1 defect, very ↓HDL, orange tonsils, neuropathy, cholesterol ester storage.
• Familial Hypertriglyceridemia (Type IV): Isolated ↑VLDL, ↑triglycerides, risk of pancreatitis, linked to diabetes/obesity.