Marfan Syndrome

• Marfan syndrome is an autosomal dominant connective-tissue disorder.
• It mainly affects:
  • skeleton
  • eyes
  • cardiovascular system
• It is caused by a defect in fibrillin, an extracellular glycoprotein.

Pathogenesis

• Fibrillin is produced by fibroblasts and is a major component of microfibrils in the extracellular matrix.
• Microfibrils act as a scaffold for tropoelastin → help form elastic fibers.
• Microfibrils are especially abundant in:
  • aorta
  • ligaments
  • ciliary zonules supporting the lens
• Therefore, these tissues are particularly affected in Marfan syndrome.
• Fibrillin is encoded by the FBN1 gene on chromosome 15q21.
• Marfan syndrome is associated with mutations in FBN1.
• More than 1000 different FBN1 mutations have been described, so diagnosis is mainly based on clinical findings.
• The mutant fibrillin usually acts as a dominant-negative protein:
  • abnormal fibrillin interferes with normal fibrillin
  • → defective microfibril assembly.
• Prevalence is about 1 in 5000.
• About 70%–85% of cases are familial.
• Remaining cases result from new (de novo) FBN1 mutations.
• Some features result directly from weak connective tissue, but others are related to excessive TGF-β activity.
• Normally:
  microfibrils bind/sequester TGF-β → limit its activity.
• In Marfan syndrome:
  ↓ normal microfibrils → ↑ free TGF-β → excessive TGF-β signaling.
• Excess TGF-β causes:
  • abnormal vascular smooth muscle development
  • weakening of the extracellular matrix
• Mutations in the TGF-β type II receptor can cause a related disorder called Marfan syndrome type 2 (MFS2).
• Cardiovascular complications may be reduced by:
  • angiotensin receptor blockers
  • β-blockers
• These help reduce the risk of serious cardiovascular complications.
• Clinical severity varies widely.
• Some patients mainly have cardiovascular disease, with only mild skeletal or eye changes.
• This variable expressivity is related to different FBN1 mutations.

KEY CONCEPT

• Marfan syndrome = autosomal dominant FBN1 mutation → abnormal fibrillin → defective microfibrils + excessive TGF-β activity.
• Main organs affected:
  skeleton + eyes + aorta/cardiovascular system.
• Dominant-negative effect → mutant fibrillin interferes with normal fibrillin.

CONCEPTUAL EXAMPLES

• FBN1 mutation → weak microfibrils in aorta → cardiovascular complications.
• Weak ciliary zonules → poor lens support → ocular abnormalities.
• ↓ Microfibril binding of TGF-β → ↑ TGF-β signaling → further connective-tissue damage.

MORPHOLOGY

• The most obvious changes in Marfan syndrome involve the skeleton.
• Typical skeletal features include:
  • tall, slender body
  • very long arms, legs, and fingers → arachnodactyly
  • high-arched palate
  • hyperextensible joints
  • spinal deformities such as severe kyphoscoliosis
  • chest deformity:
    • pectus excavatum → depressed sternum
    • pigeon-breast deformity
• The most characteristic eye abnormality is ectopia lentis:
  • bilateral dislocation or subluxation of the lens
  • caused by weak suspensory ligaments
  • bilateral ectopia lentis strongly suggests Marfan syndrome
• The most serious abnormalities involve the cardiovascular system.
• In the aorta:
  • elastic fibers in the tunica media fragment
  • this predisposes to aortic dilation and aortic dissection
  • these medial changes are called cystic medionecrosis
  • similar changes can also occur with hypertension and aging
• Loss of support in the aortic media → dilation of the aortic valve ring → aortic incompetence.
• Cardiac valves, especially the mitral valve, may become excessively distensible and regurgitant.
  • This can cause mitral valve prolapse
  • and may lead to congestive cardiac failure
• Aortic rupture is the most common cause of death and can occur at any age.
• Less commonly, cardiac failure is the terminal event.

KEY CONCEPT

• Marfan morphology mainly affects:
  skeleton + eyes + cardiovascular system.
• Arachnodactyly + ectopia lentis + aortic disease are major characteristic findings.
• The most dangerous complication is:
  aortic medial weakness → dilation/dissection → rupture.

CONCEPTUAL EXAMPLES

• Weak connective tissue in fingers → long, slender digits = arachnodactyly.
• Weak lens-supporting ligaments → lens shifts from normal position → ectopia lentis.
• Weak aortic media → wall dilates and may split → aortic dissection or rupture.
• Floppy mitral valve → backward blood flow → mitral regurgitation and possible heart failure.

Ehlers-Danlos Syndromes

• Ehlers-Danlos syndromes (EDS) are a group of single-gene connective-tissue disorders caused by defects in collagen synthesis or structure.
• Different genes may be affected, but all ultimately produce abnormal collagen.
• Inheritance may be:
  • autosomal dominant
  • autosomal recessive
• About 30 different collagen types exist, each with its own tissue distribution and gene.
• Therefore, mutations in different collagen genes help explain the clinical variation among EDS types.
• At least 13 clinical/genetic variants are recognized.
• Combined frequency is about 1 in 5000 births.
• Common features result from weak collagen:
  • Joint hypermobility
    • Skin, ligaments, and joints are commonly affected.
    • Weak collagen → ↓ tensile strength → joints become excessively flexible.
    • This also increases the risk of joint dislocation.
  • Skin fragility
    • Skin becomes highly stretchable and fragile.
    • Minor injury may produce large, gaping wounds.
    • Surgery is difficult because tissues lack normal tensile strength.
  • Structural failure of tissues/organs
    • Vascular EDS → rupture of large arteries or colon.
    • Kyphoscoliotic EDS → ocular fragility, corneal rupture, retinal detachment.
    • Classical EDS → may cause diaphragmatic hernia.
• Important molecular forms:
  • Vascular EDS
    • Mutation in COL3A1
    • ↓ type III collagen
    • Autosomal dominant
    • Weak blood vessels and bowel wall → risk of rupture.
  • Kyphoscoliotic EDS
    • Deficiency of lysyl hydroxylase
    • ↓ hydroxylation of lysine in type I and III collagen
    • ↓ collagen cross-linking
    • Autosomal recessive
    • Common features → congenital scoliosis + ocular fragility.
  • Classical EDS
    • Mutations in COL5A1 or COL5A2
    • ↓ type V collagen
    • Autosomal dominant.

KEY CONCEPT

• EDS = defective collagen → weak connective tissue.
• Main effects:
  hypermobile joints + stretchy fragile skin + tissue/organ rupture.
• COL3A1 → vascular EDS
• Lysyl hydroxylase deficiency → kyphoscoliotic EDS
• COL5A1/COL5A2 → classical EDS

CONCEPTUAL EXAMPLES

• Weak ligament collagen → joint bends excessively → hypermobility.
• Weak skin collagen → minor trauma causes a wide wound → skin fragility.
• Weak type III collagen in artery wall → vessel may rupture → vascular EDS.

Diseases Caused by Mutations in Genes Encoding Receptor Proteins or Channels

Familial Hypercholesterolemia

• Familial hypercholesterolemia (FH) is a receptor disease.
• In about 80%–85% of cases, it is caused by loss-of-function mutations in the LDL receptor gene.
• LDL receptors normally help transport and metabolize cholesterol.
• Defective LDL receptors → impaired cholesterol handling + loss of normal feedback control → markedly elevated cholesterol.
• High cholesterol → premature atherosclerosis → greatly increased risk of myocardial infarction.
• FH is one of the more common Mendelian disorders.
• Heterozygous FH occurs in about 1 in 500 people worldwide.

Normal Cholesterol Metabolism

• About 7% of body cholesterol circulates in plasma, mainly as LDL.
• Plasma cholesterol depends mainly on:
  • cholesterol synthesis
  • cholesterol breakdown/removal
• The liver plays a major role in both.
• Cholesterol comes from:
  • diet
  • endogenous synthesis
• Dietary triglycerides + cholesterol → packaged into chylomicrons in intestinal mucosa.
• Chylomicrons travel through:
  intestinal lymphatics → blood.
• In capillaries of muscle and fat:
  • lipoprotein lipase hydrolyzes chylomicrons.
  • Cholesterol-rich chylomicron remnants then go to the liver.
• In the liver:
  • some cholesterol enters the metabolic pool
  • some is excreted as free cholesterol or bile acids.
• Endogenous pathway begins in the liver (Fig. 4.5): Liver → VLDL → IDL → LDL
• Liver secretes triglyceride-rich VLDL into blood.
• In muscle and adipose capillaries:
  • VLDL loses triglycerides → becomes IDL.
• IDL contains:
  • less triglyceride
  • more cholesterol ester
  • apolipoproteins B-100 and E
• IDL then follows two paths:
  • most is taken up by the liver through the LDL receptor
  • some loses more triglyceride and apoE → becomes cholesterol-rich LDL
• The LDL receptor pathway removes about two-thirds of LDL.
• The remainder is handled mainly through scavenger receptors.
• LDL receptors bind apoB-100 and apoE, so they help transport both LDL and IDL.
• About 75% of LDL receptors are on hepatocytes → liver is central to LDL clearance.
• LDL uptake occurs as follows (Fig. 4.6):
  • LDL binds LDL receptor on cell surface
  • complex enters clathrin-coated pits
  • moves into endosomes
  • endosomes fuse with lysosomes
  • LDL separates from receptor
  • receptor is recycled back to the cell surface
  • LDL is degraded → releases free cholesterol
• PCSK9 reduces LDL-receptor recycling by promoting degradation of internalized LDL receptors.
• Movement of cholesterol out of lysosomes requires NPC1 and NPC2.
• Free intracellular cholesterol has several feedback effects:
  • inhibits HMG-CoA reductase → ↓ cholesterol synthesis
  • stimulates formation of cholesterol esters → storage
  • decreases synthesis of LDL receptors → limits further cholesterol uptake
  • increases PCSK9 → decreases LDL-receptor recycling
• These mechanisms protect cells from excessive cholesterol accumulation.
• Scavenger receptors on monocytes and macrophages take up modified LDL, especially oxidized or acetylated LDL.
• Uptake through the scavenger pathway increases as plasma cholesterol rises.

KEY CONCEPT

• FH = defective LDL receptor → ↓ LDL clearance → ↑ plasma cholesterol → premature atherosclerosis.
• Normal pathway:

VLDL → IDL → LDL → LDL receptor → endocytosis → lysosomal breakdown → free cholesterol

• Intracellular cholesterol then:
  ↓ HMG-CoA reductase + ↓ LDL receptors + ↑ cholesterol storage + ↑ PCSK9

CONCEPTUAL EXAMPLES

• LDL receptor defective → LDL remains in blood → hypercholesterolemia.
• Persistently high LDL → cholesterol accumulates in arteries → premature atherosclerosis.
• ↑ PCSK9 → fewer recycled LDL receptors → less LDL removal from blood.

FIG. 4.5 — VLDL → IDL → LDL Metabolism

🧠 Simplest idea

Liver sends triglycerides out as VLDL → tissues remove triglycerides → VLDL becomes IDL → IDL either returns to liver or becomes LDL → LDL mainly carries cholesterol back to cells/liver.

Whole figure in one flow

Liver → VLDL → triglyceride removal → IDL → either liver uptake OR → LDL → LDL-receptor clearance🎨 COLOR KEY

• 🔴 Red circles = cholesterol esters
• 🟡 Yellow circles = triglycerides
• 🟣/green shapes on particles = apolipoproteins
• 🔴 Tubes = blood vessels/capillaries
• 🟠 Large cell = fat cell
• 🔴 elongated tissue = skeletal muscle

1️⃣ LIVER makes VLDL

🟣 Purple cell = liver cell

The liver packages mainly:

• lots of 🟡 triglycerides
• some 🔴 cholesterol esters

into:

VLDL = Very-Low-Density Lipoprotein

VLDL carries:

• ApoB
• ApoC
• ApoE

⬆️ Black arrow = VLDL is released from liver into blood.

Easy idea:

VLDL = liver’s triglyceride delivery truck.2️⃣ VLDL travels through blood

VLDL moves to capillaries supplying:

🟠 Adipose tissue

and

🔴 Skeletal muscle

⬇️ White dotted arrows show VLDL entering these capillary regions.

3️⃣ Lipolysis of VLDL

At capillary surfaces, lipoprotein lipase (LPL) breaks down VLDL triglycerides.

Important:

ApoC-II activates LPL.

So:

VLDL triglyceride
→ LPL
→ free fatty acids4️⃣ Fatty acids go to TWO places

🟠 Adipose tissue

Fatty acids enter fat cells.

➡️ re-formed into triglycerides
➡️ stored as fat

Think:

Adipose = STORAGE

🔴 Skeletal muscle

Fatty acids enter muscle.

➡️ oxidized
➡️ used to make energy

Think:

Muscle = BURNS fat

5️⃣ VLDL loses triglyceride → becomes IDL

After much triglyceride is removed:

VLDL → IDL

IDL = Intermediate-Density Lipoprotein

IDL has relatively:

• less 🟡 triglyceride
• more 🔴 cholesterol

and carries ApoB + ApoE.

6️⃣ IDL now has TWO possible fates

PATH A — IDL returns to liver

Black arrow:

IDL → liver

ApoE helps IDL bind hepatic receptors.

➡️ Receptor-mediated clearance of IDL

Memory:

ApoE = helps remnants Enter liver

7️⃣ PATH B — IDL → LDL

If more triglyceride is removed:

IDL → LDL

This is labeled:

Conversion of IDL to LDL

LDL becomes very rich in:

🔴 cholesterol esters

and mainly retains:

ApoB-100

8️⃣ LDL = cholesterol delivery particle

🔴 Red particle at bottom = LDL

Its main job:

Carry cholesterol to peripheral tissues and back to liver.

LDL binds the:

LDL receptor

through ApoB-100.

9️⃣ Receptor-mediated clearance of LDL

Green receptor on liver = LDL receptor

LDL ApoB-100
→ binds LDL receptor
→ LDL enters liver cell
→ cholesterol is cleared from blood.

Easy memory:

ApoB-100 binds LDL receptor.

🔟 Other clearance

Some LDL can also leave circulation through other uptake pathways.

But the major regulated route is:

LDL → LDL receptor → liver/cells

⭐ Arrow-by-arrow summary

Liver
⬇️
VLDL (TG-rich)
⬇️ LPL removes TG
↙️ ↘️
Fat → storage Muscle → energy
⬇️
IDL
↙️ ↘️
Liver uptake via ApoE → LDL
⬇️
ApoB-100 → LDL receptor
⬇️
LDL clearance Fastest exam recall

VLDL

TG-rich → made by liver → delivers triglycerides

IDL

VLDL remnant → ApoE helps liver uptake

LDL

Cholesterol-rich → ApoB-100 → LDL receptor

⭐ One-line memory

VLDL delivers TG → IDL is the remnant → LDL delivers cholesterol.✕Compare with Claude Opus 4.8

Pathogenesis

• In familial hypercholesterolemia (FH), defective LDL receptors impair:
  • receptor expression on the cell surface
  • LDL binding and endocytosis
• Result:
  ↓ LDL clearance from blood → ↑ plasma LDL cholesterol.
• Loss of LDL receptors on hepatocytes also reduces IDL uptake by the liver.
• Therefore, more IDL is converted into LDL.
• So FH causes high cholesterol by two combined mechanisms:
  ↓ LDL catabolism + ↑ formation of LDL from IDL → severe hypercholesterolemia (Fig. 4.5).
• Excess LDL is taken up increasingly by monocytes, macrophages, and vessel walls through scavenger receptors.
• This produces:
  • xanthomas
  • premature atherosclerosis
• Causes of FH include:
  • LDL receptor mutations → about 80%–85%
  • ApoB-100 loss-of-function mutations → about 5%–10%
    • ApoB-100 is the ligand on LDL that binds the LDL receptor.
  • PCSK9 gain-of-function mutations → about 1%–2%
    • ↑ PCSK9 → ↑ LDL receptor degradation → ↓ receptor recycling
• All these abnormalities ultimately cause:
  ↓ hepatic LDL clearance → ↑ plasma LDL.
• More than 2000 LDL receptor mutations are known.
• A common type causes abnormal receptor folding → receptor cannot reach the cell surface.

Clinical Features

• FH is an autosomal dominant disorder.
• Heterozygotes:
  • plasma cholesterol rises about 2–3 times normal
  • cholesterol is elevated from birth
  • symptoms often appear in adulthood
  • may develop:
    • tendon xanthomas
    • premature atherosclerosis
    • coronary artery disease
• Homozygotes:
  • plasma cholesterol may exceed 5 times normal
  • disease is much more severe
  • develop cutaneous xanthomas in childhood
  • may die from myocardial infarction before age 20
• Statins lower cholesterol by inhibiting HMG-CoA reductase.
• This promotes increased production of LDL receptors (Fig. 4.6) → more LDL removed from blood.
• However, increased LDL receptors are accompanied by ↑ PCSK9, which promotes receptor degradation and reduces part of the statin effect.
• Therefore, resistant hypercholesterolemia may be treated with agents that inhibit PCSK9, including:
  • PCSK9 antibodies
  • siRNAs that reduce PCSK9 production

KEY CONCEPT

• FH = defective LDL clearance → very high LDL → xanthomas + premature atherosclerosis.
• Main mechanisms:
  LDL receptor defect / ApoB defect / ↑ PCSK9 → ↓ hepatic LDL uptake.
• Heterozygote → 2–3× cholesterol; homozygote → >5× and much more severe.
• Statins → ↓ HMG-CoA reductase → ↑ LDL receptors.
• PCSK9 inhibition → preserves LDL receptors → improves LDL clearance.

CONCEPTUAL EXAMPLES

• LDL receptor absent → LDL cannot enter liver efficiently → LDL accumulates in blood.
• Excess LDL enters macrophages → cholesterol deposits form → xanthomas and atherosclerosis.
• Gain-of-function PCSK9 → more LDL receptors destroyed → plasma LDL rises further.

FIG. 4.6 — LDL Receptor Pathway & Cholesterol Regulation

🧠 Simplest idea

LDL brings cholesterol into the cell → LDL receptor is usually recycled → released cholesterol then tells the cell: “Stop making and importing more cholesterol; store the excess.”

Whole figure in one flow

LDL–ApoB100 → LDL receptor → clathrin-coated pit → endosome → lysosome → free cholesterol → ↓ cholesterol synthesis + ↓ LDL receptors + ↑ cholesterol storage

1️⃣ TOP — LDL binds LDL receptor

🔴 Cluster = cholesterol esters inside LDL
🟢 surface protein = ApoB-100
🟢 Y-shaped structure = LDL receptor

First step:

ApoB-100 on LDL → binds LDL receptor

🧠 Memory:

ApoB-100 = LDL’s “address label” for the LDL receptor.

2️⃣ Clathrin-coated pit

After LDL binds its receptor:

⬇️ Black arrow

The membrane folds inward into a:

Clathrin-coated pit

🩷 Red curved coat = clathrin

The pit pinches off and carries:

LDL + LDL receptor → inside the cell

3️⃣ Endosome

The vesicle becomes an:

Endosome

Inside the acidic endosome:

LDL separates from its LDL receptor.

Now the two take different paths.

4️⃣ 🟢 LDL receptor recycling

Most LDL receptors leave the endosome.

⬆️ Black arrow

Endosome → receptor-containing vesicle → plasma membrane

The receptor can be used again to capture another LDL particle.

Easy concept:

LDL receptor = reusable catcher

5️⃣ PCSK9 destroys LDL receptors

🟣 PCSK9 can bind the LDL receptor.

🔴 Dotted arrow shows:

PCSK9 + LDL receptor
→ prevents normal recycling
→ receptor goes to lysosome
→ LDL receptor degradation

Result:

↓ LDL receptors on liver surface
→ ↓ LDL removed from blood
→ ↑ blood LDL cholesterol

⭐ Important exam point:

PCSK9 raises plasma LDL by decreasing LDL-receptor recycling.

6️⃣ LDL goes to the lysosome

The endosome containing LDL fuses with:

🟤 Lysosome

Lysosomal enzymes break LDL apart.

LDL protein

→ amino acids

Cholesterol esters

→ free cholesterol

So:

LDL → lysosome → free intracellular cholesterol

7️⃣ NPC1 and NPC2

Inside the lysosome are:

• NPC1
• NPC2

These proteins help move cholesterol:

Lysosome → cytoplasm/cellular membranes

Easy idea:

NPC1/NPC2 = cholesterol exit system from lysosome

Defects cause cholesterol to become trapped in lysosomes.

8️⃣ Free cholesterol now controls the cell

🔴 Small red circles = free cholesterol

If cholesterol becomes abundant:

Three important feedback effects occur.

① 🔴 ↓ HMG-CoA reductase

HMG-CoA reductase is the key enzyme for making new cholesterol.

Excess cholesterol gives a negative feedback signal:

↑ intracellular cholesterol
→ inhibits HMG-CoA reductase
→ ↓ new cholesterol synthesis

🧠 Think:

“Already enough cholesterol → stop making more.”

② 🔴 ↓ LDL receptor synthesis

The nucleus normally makes RNA and protein for new LDL receptors.

But excess cholesterol inhibits this.

↑ intracellular cholesterol
→ ↓ LDL-receptor gene expression
→ fewer receptors produced
→ ↓ additional LDL entry

Think:

“Already full → stop importing more.”

③ 🟢 ↑ Cholesterol ester storage

Excess free cholesterol is converted into:

Cholesterol esters

and stored safely inside the cell.

↑ free cholesterol → ↑ esterification/storage

Think:

“Extra cholesterol → pack it away.”

9️⃣ LEFT SIDE — Making new LDL receptors

Inside nucleus:

DNA → RNA
⬇️
LDL receptor synthesized in ER
⬇️
receptor transported to cell surface

This supplies new receptors when the cell needs cholesterol.

🎨 Arrow / color guide

⭐ Three effects of HIGH intracellular cholesterol

1. ↓ Synthesis

↓ HMG-CoA reductase

2. ↓ Uptake

↓ LDL receptor synthesis

3. ↑ Storage

↑ Cholesterol ester formation

🧠 Fastest exam recall

LDL ApoB-100 → LDL receptor → endocytosis → lysosome → free cholesterol.

Then free cholesterol says:

STOP making it → ↓ HMG-CoA reductase
STOP importing it → ↓ LDL receptors
STORE extra → ↑ cholesterol esters

🔥 PCSK9 memory

PCSK9 destroys LDL receptors → fewer receptors → blood LDL rises.✕Compare with Claude Opus 4.8