Disorders Associated With Defects in Receptor Proteins

Slide 1 · Disorders Associated With

Defects in Receptor Proteins

Familial hypercholesterolemia as a model of impaired receptor-mediated transport

Dr Abu Suraih Sakhri

MBBS, MD (Pathology)

www.abusuraihsakhri.com

Slide 2 · Learning objectives

Explain why failure of an LDL receptor pathway elevates circulating cholesterol.

Trace normal LDL formation, hepatic uptake, receptor recycling, and feedback control.

Distinguish defects in LDLR, ApoB, and PCSK9 from one another.

Classify LDLR defects by the disrupted stage of receptor function.

Link molecular defects to xanthomas, atherosclerosis, and receptor-directed therapies.

Slide 3 · Receptor defects: A disease mechanism

Receptor pathway defect

Impaired ligand handling

Altered metabolite balance

Tissue injury

Familial hypercholesterolemia illustrates how a transport defect becomes vascular disease.

Slide 4 · The receptor pathway must do more than bind

Recognition

A ligand must bind a functioning extracellular receptor domain.

Transport

Receptors and ligands must enter the appropriate intracellular pathway.

Recovery

Receptors are normally returned to the cell surface for repeated use.

Slide 5 · LDL clearance is a multistep cellular process

EXTRACELLULAR / PLASMA

CYTOPLASM

LDL / ApoB-100

Endosome

Lysosome

Receptor returns to membrane

LDL cargo enters degradation pathway

Slide 6 · Familial hypercholesterolemia: Core defect

Reduced hepatic clearance of plasma LDL

LDL persists in the circulation; Chronically increased plasma cholesterol accelerates atherosclerosis and produces cholesterol-rich tissue deposits.

Slide 7 · Three genes converge on the LDL receptor pathway

LDLR

Encodes the LDL receptor itself. Mutations reduce receptor number or functional efficiency.

APOB

Encodes ApoB-100, the LDL particle ligand. Mutations reduce receptor binding.

PCSK9

Activating variants accelerate LDL receptor degradation and limit recycling.

Slide 8 · LDLR: The dominant genetic category

The textbook attributes approximately 80%–85% of familial hypercholesterolemia to LDLR mutations.

The receptor defect interferes directly with hepatic uptake of circulating LDL.

Loss of effective LDL receptors also compromises hepatic uptake of IDL.

More than 2,000 LDLR mutations are described in the supplied section.

Slide 9 · ApoB defects: The ligand is abnormal

Mutated ApoB-100

Weakened LDL–LDLR interaction

Reduced receptor-mediated uptake

Higher plasma LDL

The receptor may be present, but its ligand cannot engage it efficiently.

Slide 10 · PCSK9 gain of function: Receptor loss

PCSK9 binds LDLR

Internalized receptor degraded

Fewer receptors recycle

LDL clearance falls

Textbook estimate: Activating PCSK9 variants account for approximately 1%–2% of FH.

Slide 11 · Approximate gene contributions in the source

Defect

Approximate FH cases

Primary mechanism

LDLR

80%–85%

Receptor quantity or function

APOB

5%–10%

Ligand–receptor binding

PCSK9

1%–2%

Enhanced receptor degradation

Slide 12 · Heterozygous FH: Partial receptor capacity

Genotype

One disease-associated LDLR allele and one functional allele.

Functional effect

Approximately half the normal complement of high-affinity LDL receptors model.

Slide 13 · Heterozygous FH: Biochemical expression

The supplied source describes a two- to three-fold rise in plasma cholesterol from birth.

Persistently elevated LDL promotes tendinous xanthomas in adult life.

Premature atherosclerosis results from cumulative vascular exposure to cholesterol.

The textbook estimates heterozygous LDLR-related FH at approximately 1 in 200 individuals.

Slide 14 · Severe biallelic disease: A dose effect

Receptor activity

Two severely affected alleles can leave little or no normally functioning LDL receptor activity.

Clinical burden

The source describes five- to six-fold plasma cholesterol elevation with early tissue and vascular lesions.

Slide 15 · Xanthomas reveal excess cholesterol deposition

Elevated plasma LDL

Increased scavenger uptake

Lipid-laden phagocytic cells

Visible xanthomas

Tendinous xanthomas are emphasized in heterozygotes; Skin xanthomas may appear early in severe biallelic disease.

Slide 16 · Premature atherosclerosis: Vascular consequence

Persistent LDL excess

Increased modified-LDL uptake

Lipid accumulation in vessel wall

Atherosclerotic lesions

The vascular mechanism links a receptor defect to coronary, cerebral, and peripheral disease.

Slide 17 · Severe disease may cause very early infarction

Early coronary, cerebral, and peripheral atherosclerosis is described in severe biallelic FH.

Myocardial infarction may occur before the age of 20 years in severely affected individuals.

In the cited large-scale studies, FH was reported in 3%–6% of myocardial infarction survivors.

These are observations, not newly verified 2026 prevalence estimates.

Slide 18 · Disease severity follows residual pathway function

Feature

Heterozygous LDLR-FH

Severe biallelic LDLR-FH

LDL receptor capacity

Partially retained

Very limited or absent

Cholesterol change

Two- to three-fold

Five- to six-fold

Typical consequences

Adult xanthomas; Premature disease

Early xanthomas and vascular disease

Slide 19 · Dietary cholesterol enters via chylomicrons

Intestinal mucosa

Chylomicrons

Blood circulation

Liver uptake of remnants

Capillary lipoprotein lipase

Dietary triglycerides and cholesterol

Via lymphatics

Lipolysis leaves cholesterol-rich remnants

Slide 20 · Lipoprotein lipase acts in peripheral capillaries

Chylomicron in blood

Capillary lipoprotein lipase

Triglyceride hydrolysis

Chylomicron remnant

This lipolysis occurs in the capillaries of adipose tissue and muscle.

Slide 21 · The liver is central to cholesterol traffic

Incoming

Chylomicron remnants bring dietary cholesterol from the circulation.

Production

The liver synthesizes cholesterol and secretes VLDL particles.

Disposal

Cholesterol can be secreted into bile as free cholesterol or bile acids.

Slide 22 · VLDL begins the endogenous LDL pathway

VLDL

Triglyceride rich

IDL

Less triglyceride; Retains ApoB and ApoE

LDL

Cholesterol rich; ApoB retained

Peripheral lipolysis

Further remodeling

Relative particle sizes are schematic; No quantitative scale is implied.

Slide 23 · VLDL contains several apolipoproteins

ApoB

Retained throughout VLDL → IDL → LDL processing.

ApoC

Present on VLDL; Lost during conversion to IDL.

ApoE

Retained in IDL, where it supports receptor recognition; Later largely removed.

Slide 24 · VLDL lipolysis forms IDL

VLDL

Triglyceride-rich particles carry ApoB, ApoC, and ApoE.

IDL

After lipolysis, the particles contain less triglyceride, more cholesteryl ester, and retain ApoB plus ApoE.

Slide 25 · IDL has two alternative fates

IDL in plasma

(ApoB and ApoE)

Hepatic LDLR uptake

Further processing into LDL

Approximately 50% of newly formed IDL enters the rapid hepatic uptake pathway in the source.

Slide 26 · LDLR recognizes ApoB and ApoE in IDL

IDL retains ApoB + ApoE

LDL receptor recognition

Hepatic receptor-mediated uptake

IDL removal from plasma

The same receptor is therefore important for both IDL and LDL homeostasis.

Slide 27 · LDL carries cholesterol and ApoB-100

Particle composition

IDL remodeling yields a cholesterol-rich LDL particle that retains ApoB-100.

Cell recognition

ApoB-100 serves as the ligand for high-affinity LDL receptors on susceptible cells.

Slide 28 · Hepatic LDL clearance is quantitatively dominant

About 70% of plasma LDL is cleared by the liver

Other LDLR-expressing tissues include fibroblasts, lymphocytes, smooth muscle cells, and adrenocortical cells.

Slide 29 · Surface binding occurs in coated-pit regions

EXTRACELLULAR / PLASMA

CYTOPLASM

LDL carrying ApoB-100

Recognition at extracellular domain

Coated-pit region recruits the receptor–LDL complex

Schematic — cell membrane and receptor are not drawn to molecular scale.

Slide 30 · Clathrin-mediated uptake forms coated vesicles

LDLR-bound LDL

Clathrin-coated pit

Membrane invagination

Intracellular vesicle

A defect in coated-pit clustering can stop uptake even when ligand binding is preserved.

Slide 31 · Endosomal sorting separates ligand and receptor

EXTRACELLULAR / PLASMA

CYTOPLASM

LDL–LDLR enters endosome

Endosome

Receptor recycling

LDL → lysosome

Ligand and receptor dissociate in the endosome

Slide 32 · Receptor recycling sustains repeated clearance

Receptor dissociates

Recycling machinery

Return to cell surface

New round of LDL uptake

PCSK9 reduces receptor recycling by directing internalized receptors toward degradation.

Slide 33 · Lysosomes release usable cholesterol

Internalized LDL

Lysosomal degradation

Protein → amino acids

Esters → free cholesterol

LDL apoproteins and cholesteryl esters undergo different lysosomal breakdown processes.

Slide 34 · NPC1 and NPC2 allow lysosomal cholesterol exit

Lysosome

Free cholesterol

NPC2 / NPC1

Cytoplasmic cholesterol

NPC1 and NPC2 are required for movement of free cholesterol out of lysosomes.

Slide 35 · Intracellular cholesterol has two major roles

Cellular substrate

Supports synthesis and maintenance of cellular membranes.

Feedback regulator

Adjusts cholesterol synthesis, cholesterol storage, and LDL receptor availability.

Slide 36 · Cholesterol limits de novo synthesis

Intracellular cholesterol rises

HMG-CoA reductase inhibited

Endogenous synthesis falls

HMG-CoA reductase is the rate-limiting enzyme of cellular cholesterol synthesis.

Slide 37 · Excess cholesterol is esterified for storage

Intracellular cholesterol rises

ACAT activity promoted

Cholesteryl esters stored

ACAT: Acyl-coenzyme A:cholesterol acyltransferase.

Slide 38 · Cholesterol reduces LDLR synthesis

Cellular cholesterol excess

LDLR expression suppressed

Less further LDL uptake

Feedback downregulation helps protect cells from excessive cholesterol accumulation.

Slide 39 · Cholesterol induces PCSK9-mediated receptor loss

Cholesterol excess

PCSK9 expression rises

LDLR recycling reduced

Further uptake limited

This as an additional feedback barrier to cholesterol overload.

Slide 40 · Four coordinated responses to cholesterol excess

Regulated process

Response to intracellular cholesterol

Functional result

HMG-CoA reductase

Inhibition

Less de novo synthesis

ACAT

Activation

More ester storage

LDLR production

Suppression

Reduced receptor uptake

PCSK9 expression

Upregulation

Reduced LDLR recycling

Slide 41 · Reduced LDLR activity also diverts IDL into LDL

IDL in circulation

Normal hepatic IDLR/LDLR removal

IDL continues to LDL

Reduced in FH

Increased precursor availability for LDL production

Slide 42 · LDLR and scavenger pathways differ

Characteristic

LDL receptor pathway

Scavenger pathway

Principal particles

Native LDL; IDL

Chemically modified LDL

Dominant setting

Normal LDL clearance

Prominent with LDL excess

Relevant cells

Especially hepatocytes

Monocytes and macrophages

Disease link

FH clearance defect

Xanthomas and atherogenesis

Slide 43 · Modified LDL uptake can overwhelm phagocytes

Hypercholesterolemia

Modified LDL accumulation

Scavenger receptor uptake

Foam-cell lipid storage

In the source, increased scavenger traffic explains xanthoma formation and contributes to atherosclerosis.

Slide 44 · LDLR mutations fall into six functional classes

Class

Impaired function

Where the problem occurs

I

Receptor synthesis

Protein production

II

Receptor transport

ER → Golgi

III

Ligand binding

Cell surface

IV

Coated-pit clustering

Plasma membrane

V

Receptor recycling

Endosome

VI

Hepatocyte targeting

Basolateral membrane

Slide 45 · Class I: Failure to synthesize LDLR

LDLR null allele

No functional receptor protein

No LDLR-mediated capture

Class I as relatively uncommon.

Slide 46 · Class II: Receptor retained in endoplasmic reticulum

Folding defect / truncation

Protein accumulates in ER

Transport to Golgi blocked

Little surface LDLR

In contrast to class I, the receptor protein is synthesized but fails to traffic normally.

Slide 47 · Class III: Defective ApoB binding

LDLR produced

Surface transport preserved

ApoB binding deficient

LDL capture impaired

The LDLR reaches the plasma membrane but interacts poorly—or not at all—with LDL.

Slide 48 · Class IV: Binding without internalization

Normal surface expression

LDL binding preserved

Coated-pit clustering fails

Complex remains at surface

An uptake defect can arise downstream of perfectly adequate ligand binding.

Slide 49 · Class V: Dissociation and recycling fail

LDL binds LDLR

Complex internalized

Endosomal release fails

Receptor degraded

The required pH-dependent dissociation does not occur, trapping the receptor in the endosome.

Slide 50 · Class VI: Hepatocyte membrane mistargeting

Apical membrane (incorrect destination)

Basolateral / sinusoidal membrane (required)

Hepatocyte

Mislocalization prevents efficient LDL clearance from plasma at the sinusoidal interface.

Slide 51 · Classes I–III: The defect precedes uptake

Class

Present at cell surface?

Specific defect

I

No receptor synthesized

Protein production

II

Little or none

ER-to-Golgi transport

III

Yes

LDL binding

Slide 52 · Classes IV–VI: Receptor location and reuse

Class

What remains intact?

What fails?

IV

Synthesis, transport, binding

Clustering / internalization

V

Synthesis, binding, endocytosis

Endosomal dissociation / recycling

VI

Synthesis of receptor protein

Basolateral membrane targeting

Slide 53 · Similar LDL levels can arise through different defects

Receptor not available

Class I and class II mutations prevent effective receptor display.

Receptor cannot perform

Class III and class IV impair binding or coated-pit uptake.

Receptor cannot return or localize

Class V impairs recycling; Class VI misdirects hepatocyte membrane targeting.

Slide 54 · Statins work through receptor upregulation

HMG-CoA reductase inhibited

Intracellular cholesterol falls

LDLR synthesis increases

Plasma LDL clearance rises

The response depends on the pathway’s capacity to produce functional LDL receptors.

Slide 55 · PCSK9 antibodies preserve receptor availability

PCSK9 inhibited

Less LDLR degradation

More LDLR recycling

More plasma LDL clearance

The supplied textbook reports clinical benefit when statins alone provide insufficient LDL reduction.

Slide 56 · PCSK9 loss-of-function validates the pathway

Human genetic observation

Individuals with very low serum cholesterol carrying loss-of-function PCSK9 variants.

Causal interpretation

Less PCSK9-mediated degradation allows greater LDL receptor availability at the cell surface.

Slide 57 · Two interventions converge on surface LDLR

Statin inhibition of HMG-CoA reductase

PCSK9 antibody inhibition

More LDLR synthesized

Less LDLR degraded

Increased surface LDLR → increased plasma LDL clearance

Slide 58 · Case: Tendon xanthomas and premature coronary disease

CLINICAL SCENARIO

An adult with lifelong marked hypercholesterolemia develops tendon xanthomas and premature atherosclerotic disease.

Which receptor-mediated pathway is most likely impaired?

Apply the LDLR → hepatic clearance → elevated LDL → tissue-deposition sequence.

Slide 59 · Case: Severe childhood hypercholesterolemia

CLINICAL SCENARIO

A child develops skin xanthomas and early arterial disease with profoundly elevated circulating cholesterol.

Why does loss of receptor function on both alleles produce greater severity?

Compare residual LDLR activity in heterozygous versus severe biallelic disease.

Slide 60 · Case: Normal LDL binding but failed endocytosis

CLINICAL SCENARIO

A mutant LDL receptor is synthesized, reaches the cell surface, and binds LDL normally, but fails to enter coated pits.

Identify the LDLR mutation class and the affected process.

Class IV: Defective coated-pit clustering / internalization.

Slide 61 · Integrated causal model of receptor-protein disease

Inherited pathway variant

Defective LDLR function

Sustained plasma LDL excess

Xanthoma and plaque formation

LDLR, ApoB, and PCSK9 converge on the same physiological bottleneck: Hepatic LDL clearance.

Slide 62 · Synthesis: Six classes are one mechanistic framework

Synthesis, trafficking, binding, clustering, recycling, and membrane targeting are distinct receptor functions.

A defect at any of these steps can reduce effective hepatic LDL clearance.

An LDL receptor’s presence at the cell surface does not guarantee a functioning uptake pathway.

The same molecular map explains both the pathology and the rationale of receptor-directed treatment.

Slide 63 · Concept check: test the mechanism, not the label

Why can a class IV mutant receptor bind LDL but still fail to clear it?

How does defective LDLR increase plasma LDL by two distinct mechanisms?

Which familial hypercholesterolemia gene encodes the LDL receptor ligand?

What cellular process explains cholesterol lowering by PCSK9 inhibition?