Disorders of lipid metabolism

Disorders of lipid metabolism

Disorders of lipid metabolism

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Lipoprotein Metabolism - The Lipid Taxi System

Transports insoluble lipids (triglycerides, cholesterol) through blood.

  • Chylomicron: Delivers dietary TGs to periphery.
  • VLDL: Delivers hepatic TGs to periphery.
  • LDL: Delivers hepatic cholesterol to tissues. (📌 Lousy)
  • HDL: Reverse cholesterol transport from tissues to liver. (📌 Healthy)

Key Enzymes & Apoproteins:

  • Lipoprotein Lipase (LPL): Degrades TGs in chylomicrons/VLDL. Activated by ApoC-II.
  • ApoB-100: Ligand for LDL receptor binding.
  • ApoE: Mediates remnant uptake by liver.

⭐ Familial hypercholesterolemia is commonly caused by defects in the LDL receptor (coded by the LDLR gene), leading to elevated LDL levels and premature atherosclerosis.

Lipid, cholesterol, and lipoprotein metabolic pathways

Familial Dyslipidemias - Cholesterol Chaos

  • Type I: Hyperchylomicronemia (AR)

    • Defect: ↓ Lipoprotein Lipase (LPL) or ApoC-II deficiency.
    • Labs: ↑↑ Chylomicrons, ↑ Triglycerides (>1000 mg/dL).
    • Clinical: Acute pancreatitis, eruptive xanthomas, creamy supernatant in blood sample. No increased risk for atherosclerosis.
  • Type IIa: Familial Hypercholesterolemia (AD)

    • Defect: ↓ or absent LDL receptors.
    • Labs: ↑↑ LDL, ↑ Cholesterol (Het: >300 mg/dL; Hom: >700 mg/dL).
    • Clinical: Tendinous xanthomas (Achilles), corneal arcus. Leads to premature atherosclerosis and MI.
  • Type IV: Hypertriglyceridemia (AD)

    • Defect: Hepatic overproduction of VLDL.
    • Labs: ↑ VLDL, ↑ Triglycerides.
    • Clinical: Mostly asymptomatic; can cause pancreatitis at high levels. Associated with insulin resistance.

⭐ Type IIa (Familial Hypercholesterolemia) is infamous for causing myocardial infarction in patients before the age of 20 in its homozygous form.

Lysosomal Storage Diseases - Cellular Traffic Jams

  • Pathophysiology: Inherited lysosomal enzyme deficiencies → accumulation of undigested substrates → organ damage.
  • Inheritance: Mostly Autosomal Recessive (AR), except Fabry disease (X-linked).
  • Key Sphingolipidoses:
    • Tay-Sachs: ↓ Hexosaminidase A → GM2 ganglioside buildup.
      • Features: "Cherry-red" macula, neurodegeneration, onion-skin lysosomes. No hepatosplenomegaly.
    • Fabry Disease: ↓ α-galactosidase A → Ceramide trihexoside buildup.
      • Features: Peripheral neuropathy, angiokeratomas, renal/heart failure.
    • Gaucher Disease: ↓ Glucocerebrosidase → Glucocerebroside buildup.
      • Features: Hepatosplenomegaly, pancytopenia, bone crises. Gaucher cells (lipid-laden macrophages).
    • Niemann-Pick Disease: ↓ Sphingomyelinase → Sphingomyelin buildup.
      • Features: "Cherry-red" macula, hepatosplenomegaly, neurodegeneration. Foam cells.

Gaucher disease is the most common lysosomal storage disease.

Gaucher cells with wrinkled tissue paper appearance

  • Type I (Familial Chylomicronemia): LPL or ApoC-II deficiency leads to ↑ chylomicrons, causing pancreatitis and eruptive xanthomas.
  • Type IIa (Familial Hypercholesterolemia): Absent LDL receptors cause ↑ LDL, leading to tendon xanthomas and premature CAD.
  • Type III (Dysbetalipoproteinemia): Defective ApoE results in ↑ chylomicron and VLDL remnants, with pathognomonic palmar xanthomas.
  • Type IV (Hypertriglyceridemia): Hepatic overproduction of VLDL increases triglycerides, raising the risk of acute pancreatitis.
  • Abetalipoproteinemia: MTP gene defect impairs ApoB formation, causing fat malabsorption and spinocerebellar degeneration.

Practice Questions: Disorders of lipid metabolism

Test your understanding with these related questions

An 8-year-old boy presents to his primary care pediatrician for routine check-up. During the visit, his mom says that she has noticed yellow bumps on his eyelids and was concerned about whether they were a problem. Upon hearing this concern, the physician inquires about parental health history and learns that both parents have high cholesterol despite adhering to a statin regimen. Furthermore, other family members have suffered early myocardial infarctions in their 30s. Physical exam reveals flat yellow patches on the patient's eyelids bilaterally as well as hard yellow bumps around the patient's ankles. Based on clinical suspicion an LDL level is obtained and shows a level of 300 mg/dL. What protein is most likely defective in this patient causing these findings?

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Flashcards: Disorders of lipid metabolism

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What substrate accumulates in the lysosomes of a patient with Niemann Pick disease? _____

TAP TO REVEAL ANSWER

What substrate accumulates in the lysosomes of a patient with Niemann Pick disease? _____

Sphingomyelin

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