Chapter 419: Disorders of Lipoprotein Metabolism¶
Chapter 419 | Part 12: Endocrinology and Metabolism
KEY CLINICAL POINTS¶
- Lipoproteins are essential for transporting cholesterol, triglycerides, and fat-soluble vitamins. Dyslipidemias are major risk factors for atherosclerotic cardiovascular disease (ASCVD).
- Primary genetic disorders like Familial Chylomicronemia Syndrome (FCS) and Familial Hypercholesterolemia (FH) are critical to diagnose due to their association with severe hypertriglyceridemia, pancreatitis, and premature ASCVD.
- Secondary causes of dyslipidemia include obesity, insulin resistance, diabetes, hypothyroidism, and medications (e.g., corticosteroids, beta-blockers).
- Lipid-lowering therapies (statins, PCSK9 inhibitors, fibrates, ezetimibe) are cornerstone treatments for managing dyslipidemia and reducing cardiovascular risk.
- Genetic testing is vital for diagnosing monogenic disorders like FH, FCS, and Tangier disease, enabling targeted management and family screening.
1. DEFINITION & OVERVIEW¶
Lipoproteins are complexes of lipids and proteins that transport cholesterol, triglycerides, and fat-soluble vitamins. Dyslipidemias (elevated LDL-C, TGs, or reduced HDL-C) are major contributors to ASCVD. Primary genetic disorders (e.g., FH, FCS) and secondary factors (e.g., obesity, diabetes) both play roles. Early diagnosis and intervention are critical to reduce cardiovascular risk.
Table 419-1: Major Apolipoproteins¶
| APOLIPOPROTEIN | PRIMARY SOURCE | LIPOPROTEIN ASSOCIATION | FUNCTION |
|---|---|---|---|
| ApoA-I | Intestine, liver | HDL, chylomicrons | Core structural protein for HDL, promotes lipid efflux via ABCA1, activates LCAT |
| ApoA-II | Liver | HDL, chylomicrons | Structural protein for HDL |
| ApoA-V | Liver | VLDL, chylomicrons | Promotes LPL-mediated triglyceride lipolysis |
| Apo(a) | Liver | Lp(a) | Structural protein for Lp(a) |
| ApoB-48 | Intestine | Chylomicrons, remnants | Core structural protein for chylomicrons |
| APOLIPOPROTEIN | PRIMARY SOURCE | LIPOPROTEIN ASSOCIATION | FUNCTION |
|---|---|---|---|
| ApoB-100 | Liver | VLDL, IDL, LDL, Lp(a) | Core structural protein for VLDL, LDL, IDL, Lp(a); ligand for LDL receptor |
| ApoC-II | Liver | Chylomicrons, VLDL, HDL | Cofactor for LPL |
| ApoC-III | Liver, intestine | Chylomicrons, VLDL, HDL | Inhibitor of LPL activity and remnant lipoprotein binding to receptors |
| ApoE | Liver | Chylomicron remnants, IDL, HDL | Ligand for LDL receptor and other receptors |
1.1 Lipoprotein Structure and Classification¶
Lipoproteins are classified by density: chylomicrons (least dense), VLDL, IDL, LDL, and HDL (most dense). Apolipoproteins (e.g., apoB, apoA-I) are structural and functional components. LDL is the primary atherogenic particle, while HDL mediates reverse cholesterol transport.
1.2 Lipoprotein Metabolism¶
Lipoprotein metabolism involves lipolysis by lipoprotein lipase (LPL), receptor-mediated uptake (e.g., LDLR), and hepatic processing. ApoC-II and apoA-I are critical cofactors for LPL and reverse cholesterol transport, respectively.
2. EPIDEMIOLOGY¶
Dyslipidemias are common, with hypercholesterolemia affecting ~1 in 500 individuals (FH) and hypertriglyceridemia affecting ~1 in 1000. Secondary causes (e.g., obesity, diabetes) are more prevalent. Genetic disorders like FCS and FH have variable prevalence depending on ethnicity and founder populations.
2.1 Risk Factors¶
Primary risk factors include genetic mutations (e.g., LDLR, APOB), while secondary factors include obesity, insulin resistance, diabetes, hypothyroidism, and medications (e.g., corticosteroids, beta-blockers).
2.2 Demographics¶
FH is more common in certain populations (e.g., South African Afrikaners, French Canadians). FCS is rare but more prevalent in individuals with specific genetic backgrounds.
3. ETIOLOGY & PATHOPHYSIOLOGY¶
Dyslipidemias arise from genetic defects (e.g., LDLR mutations in FH, LPL deficiency in FCS) or environmental factors (e.g., diet, lifestyle). Pathophysiology involves impaired lipolysis, altered receptor function, or defective lipid metabolism.
Table 419-2: Primary Dyslipoproteinemias Caused by Known Single-Gene Mutations¶
| GENETIC DISORDER | GENES MUTATED | LIPOPROTEINS AFFECTED | CLINICAL FINDINGS | GENETIC TRAN SMISSION | ESTIMATED PREVALENCE |
|---|---|---|---|---|---|
| Familial Chylomi cronemia Syndrome (FCS) | LPL, APOC2, APOA5, GPIHBP1, LMF1 | Chylomicrons, VLDL | Pancreatitis, eruptive xanthomas | Autosomal recessive | 1/200,000–300,0 00 |
| Familial Partial Lipodystrophy (FPLD) | LMNA, PPARG, PLIN1, AKT2, ADRA2A | Chylomicrons, VLDL, LDL | Insulin resistance, fatty liver | Autosomal dominant | 1/1,000,000 |
| Familial Hyperch olesterolemia (FH) | LDLR, APOB, PCSK9 | LDL | Tendon xanthomas, premature ASCVD | Autosomal dominant | 1/250 |
| Sitosterolemia | ABCG5, ABCG8 | LDL | Fatty liver, cholesterol gallstones | Autosomal recessive | 1/1,000,000 |
| Lysosomal Acid Lipase Deficiency (LALD) | LIPA | LDL | Fatty liver, hepatic fibrosis | Autosomal recessive | 1/1,000,000 |
3.1 Genetic Causes¶
Monogenic disorders like FH (LDLR, PCSK9, APOB mutations), FCS (LPL, GPIHBP1), and Tangier disease (ABCA1) are key. Polygenic factors (e.g., APOC3, ANGPTL3) contribute to complex dyslipidemias.
3.2 Secondary Causes¶
Obesity, insulin resistance, diabetes, hypothyroidism, and medications (e.g., anabolic steroids, beta-blockers) disrupt lipid metabolism. Liver disease and nephrotic syndrome also contribute.
4. CLINICAL FEATURES¶
Clinical manifestations vary by disorder. Severe hypertriglyceridemia (HTG) presents with pancreatitis, eruptive xanthomas, and abdominal pain. Hypercholesterolemia (e.g., FH) is associated with tendon xanthomas, premature ASCVD, and coronary artery disease.
Table 419-3: Secondary Causes of Altered Lipid and Lipoprotein Levels¶
| LDL-C | HDL-C | LP(a) | TG | ELEVATED | REDUCED | ELEVATED | REDUCED |
|---|---|---|---|---|---|---|---|
| High-carbo hydrate diet | Hypothyroid ism | Vegan diet | Alcohol | Hypertriglyc eridemia | Chronic kidney disease | Nephrotic syndrome | Malabsorpti on |
| Alcohol | Cholestasis | Malnutrition | Exercise | Vegan diet | Inflammatio n | Obesity | Menopause |
| Obesity | Nephrotic syndrome | Severe liver disease | Drugs: corti costeroids | Hypertriglyc eridemia | Acromegaly | Gaucher’s disease | Hypothyroid ism |
| Insulin resistance | Cushing’s syndrome | Drugs: phenytoin | Drugs: beta blockers | Nephrotic syndrome | Orchidecto my | Drugs: growth hormone | Acromegaly |
| Type 2 diabetes | Acute intermittent porphyria | Drugs: carb amazepine | Drugs: beta blockers | Viral hepatitis | Drugs: isotretinoin | Drugs: anabolic steroids | Glycogen storage disease |
4.1 Severe Hypertriglyceridemia¶
Defined by fasting TG >500 mg/dL. Features include acute pancreatitis, eruptive xanthomas, and abdominal pain. Severe cases may present with chylomicronemia and lipemia retinalis.
4.2 Hypercholesterolemia¶
Elevated LDL-C (>190 mg/dL) is a major risk factor for ASCVD. Clinical features include xanthomas, coronary artery disease, and increased cardiovascular mortality.
5. DIFFERENTIAL DIAGNOSIS¶
Differential diagnosis includes secondary causes of dyslipidemia (e.g., hypothyroidism, nephrotic syndrome) and other genetic disorders (e.g., FCS, FH). Clinical features like xanthomas, family history, and lipid profiles guide differentiation.
5.1 Severe Hypertriglyceridemia¶
Differential diagnoses include FCS, FPLD, and secondary causes (e.g., alcohol, diabetes). FCS is suspected in patients with fasting TG >500 mg/dL and pancreatitis.
5.2 Hypercholesterolemia¶
Differential diagnoses include FH, sitosterolemia, and secondary causes (e.g., hypothyroidism, nephrotic syndrome). FH is suspected in patients with family history of hypercholesterolemia and premature ASCVD.
6. INVESTIGATIONS & DIAGNOSIS¶
Diagnostic workup includes lipid panel (total cholesterol, LDL-C, HDL-C, TG), apoB measurement, and genetic testing for monogenic disorders. Fasting lipid profiles and advanced tests (e.g., apoE genotyping) are critical for diagnosis.
Table 419-4: Drugs Used to Treat Dyslipidemia¶
| DRUG | MAJOR INDICATIONS | STARTING DOSE | MAXIMAL DOSE | MECHANISM | ADVERSE EFFECTS |
|---|---|---|---|---|---|
| Statins (e.g., atorvastatin) | Elevated LDL-C, increased CV risk | 20–80 mg daily | 80 mg daily | Inhibit HMG-CoA reductase, upregulate LDL receptors | Myalgia, elevated transaminases, increased diabetes risk |
| Ezetimibe | Elevated LDL-C | 10 mg daily | 10 mg daily | Inhibits cholesterol absorption | Bloating, constipation |
| PCSK9 inhibitors (evo | Elevated LDL-C | 140 mg SC every 2 weeks | 420 mg SC every 1 month | Inhibit PCSK9, increase LDL receptors | Injection site reactions |
| Fibrates (e.g., fenofibrate) | Elevated TG | 40–160 mg daily | 40–160 mg daily | Activate PPARa, increase LPL activity | Myopathy, myalgia, cholelithiasis |
| Omega-3 fatty acids | Elevated TG | 4 g daily | 4 g daily | Increase TG catabolism | Dyspepsia |
6.1 Laboratory Tests¶
Standard lipid panel (total cholesterol, LDL-C, HDL-C, TG). Friedewald formula estimates LDL-C (if TG <400 mg/dL). ApoB measurement is essential for risk stratification.
6.2 Imaging and Special Tests¶
Liver ultrasound for fatty liver, abdominal imaging for pancreatitis. Genetic testing for FH, FCS, and other monogenic disorders is recommended in high-risk patients.
7. MANAGEMENT & TREATMENT¶
Management includes lifestyle modifications (diet, exercise), pharmacotherapy (statins, fibrates, PCSK9 inhibitors), and genetic counseling. Severe HTG requires aggressive intervention to prevent pancreatitis.
Table 419-4 (continued)¶
| DRUG | MAJOR INDICATIONS | STARTING DOSE | MAXIMAL DOSE | MECHANISM | ADVERSE EFFECTS |
|---|---|---|---|---|---|
| Bempedoic acid | Elevated LDL-C | 180 mg daily | 180 mg daily | Inhibits ATP citrate lyase | Uric acid, gout, cholelithiasis |
| Mipomersen | HoFH | 200 mg SC weekly | 200 mg SC weekly | Inhibits apoB synthesis | Injection site reactions, hepatic steatosis |
| Inclisiran | Elevated LDL-C | 300 mg SC every 6 months | 300 mg SC every 6 months | Silences PCSK9 mRNA | Injection site reactions |
7.1 Lifestyle Modifications¶
Dietary fat restriction, weight loss, and exercise are foundational. Patients with HTG should avoid alcohol and high-carb diets. Statins are first-line for hypercholesterolemia.
7.2 Pharmacologic Therapy¶
Statins are first-line for LDL-C reduction. Fibrates and omega-3s target TGs. PCSK9 inhibitors are used in FH and severe HTG. Ezetimibe and bile acid sequestrants are adjuncts.
8. PROGNOSIS & COMPLICATIONS¶
Prognosis depends on severity of dyslipidemia and underlying conditions. Complications include acute pancreatitis, ASCVD, and hepatic fibrosis. Early intervention improves outcomes.
8.1 Severe Hypertriglyceridemia¶
Complications include acute pancreatitis, chylomicronemia, and lipemia retinalis. Mortality is high without intervention.
8.2 Hypercholesterolemia¶
Long-term risks include premature ASCVD, coronary artery disease, and increased cardiovascular mortality. FH patients have a 50% risk of MI before age 60.
9. SPECIAL CONSIDERATIONS¶
Special populations require tailored management. Pregnancy, diabetes, and renal disease influence lipid metabolism. Genetic counseling is essential for familial disorders.
9.1 Pregnancy¶
Hormonal changes may worsen dyslipidemia. Statins are generally safe, but some (e.g., simvastatin) are contraindicated in late pregnancy.
9.2 Diabetes¶
Insulin resistance exacerbates dyslipidemia. GLP-1 agonists and statins improve TG and LDL-C in diabetic patients.
10. KEY POINTS & CLINICAL PEARLS¶
- LDL-C is the primary target for ASCVD prevention. 2. Severe HTG requires urgent intervention to prevent pancreatitis. 3. Genetic testing is critical for diagnosing FH, FCS, and other monogenic disorders. 4. Lifestyle modification and statins are first-line for most dyslipidemias. 5. PCSK9 inhibitors and fibrates are adjuncts for refractory cases.