Molecular Genetics of Familial

Introduction

Familial hypercholesterolemia (FH) is a genetic lipid disorder characterized by markedly elevated plasma low-density lipoprotein cholesterol (LDL-C) levels from birth, leading to premature atherosclerotic cardiovascular disease (ASCVD). Unlike polygenic or lifestyle-driven hypercholesterolemia, FH is a monogenic disorder, most commonly caused by mutations in one of three genes:

LDLR (Low-Density Lipoprotein Receptor gene) – the most frequently mutated gene in FH.
APOB (Apolipoprotein B gene) – the structural protein of LDL particles, necessary for receptor binding.
PCSK9 (Proprotein Convertase Subtilisin/Kexin Type 9 gene) – a protease regulating LDL receptor degradation.

Together, mutations in these genes impair clearance of LDL particles, resulting in lifelong hypercholesterolemia and a substantially increased risk of coronary artery disease (CAD), myocardial infarction (MI), and sudden death, often decades earlier than in the general population.

In this post, we will explore:

The molecular genetics of FH.
How LDLR, APOB, and PCSK9 mutations alter lipid metabolism.
Clinical phenotypes associated with different mutations.
Current diagnostic strategies and therapeutic interventions.

Section 1: Familial Hypercholesterolemia – An Overview

1.1 Epidemiology

FH is one of the most common monogenic disorders.
Prevalence of heterozygous FH (HeFH): ~1 in 200–250 worldwide.
Prevalence of homozygous FH (HoFH): ~1 in 160,000 to 300,000.

1.2 Clinical Features

Severely elevated LDL-C (often >190 mg/dL in HeFH, >500 mg/dL in HoFH).
Tendon xanthomas (cholesterol-rich nodules in tendons).
Xanthelasmas (cholesterol deposits around the eyes).
Arcus cornealis (cholesterol deposition in the cornea, often in young individuals).
Premature ASCVD (men <55 years, women <65 years).

1.3 Inheritance Pattern

Autosomal dominant inheritance in most cases.
Homozygous individuals have far more severe disease, often with CAD before the age of 20.

Section 2: LDL Metabolism and Molecular Pathophysiology

2.1 The Normal LDL Clearance Pathway

LDL particles transport cholesterol to peripheral tissues.
Apolipoprotein B-100 (apoB-100), the major protein on LDL, binds to the LDL receptor (LDLR) on hepatocytes.
LDLR-LDL complexes are internalized via endocytosis.
LDL is degraded in lysosomes → cholesterol released.
LDLR is normally recycled back to the hepatocyte surface to clear more LDL.

2.2 Disruption in FH

LDLR mutations: receptor cannot bind, internalize, or recycle LDL.
APOB mutations: apoB-100 cannot properly bind to LDLR.
PCSK9 mutations: increased degradation of LDLR, reducing receptor density.

Result: reduced LDL clearance → elevated plasma LDL-C → accelerated atherosclerosis.

Section 3: Genetics of LDLR Mutations

3.1 The LDLR Gene

Located on chromosome 19p13.2.
Encodes a transmembrane glycoprotein receptor responsible for LDL uptake.
Contains several functional domains: ligand-binding, epidermal growth factor (EGF)-like domain, O-linked sugars domain, transmembrane domain, and cytoplasmic tail.

3.2 Types of LDLR Mutations

Mutations in LDLR account for 60–80% of FH cases. They are divided into five functional classes:

Class I (Null mutations): No receptor protein produced.
Class II (Transport-defective): Misfolded receptor trapped in endoplasmic reticulum, not transported to Golgi.
Class III (Binding-defective): Receptor reaches surface but cannot bind LDL.
Class IV (Internalization-defective): Receptor binds LDL but cannot cluster in coated pits, preventing endocytosis.
Class V (Recycling-defective): Receptor binds and internalizes LDL but is degraded instead of recycled.

3.3 Clinical Implications

Class I/II mutations → more severe phenotype (very high LDL, early ASCVD).
Class III–V mutations → milder but still significant disease.

Section 4: Genetics of APOB Mutations

4.1 The APOB Gene

Located on chromosome 2p24.1.
Encodes apoB-100, the only protein ligand for LDLR.
ApoB-100 is essential for LDL recognition and uptake by hepatocytes.

4.2 APOB Mutations in FH

Mutations in exon 26 (the LDLR-binding domain) are the most common.
The p.Arg3500Gln mutation (also known as R3500Q) is the classic FH-causing variant.
Other variants: p.Arg3500Trp, p.Arg3531Cys.

4.3 Pathophysiology

Mutant apoB has reduced affinity for LDLR.
LDL particles circulate longer in plasma.
LDL-C levels increase but usually not as high as in LDLR mutations.

4.4 Clinical Implications

APOB mutations account for 5–10% of FH cases.
Phenotype is often milder compared to LDLR mutations.
Premature ASCVD risk remains significantly elevated.

Section 5: Genetics of PCSK9 Mutations

5.1 The PCSK9 Gene

Located on chromosome 1p32.3.
Encodes a serine protease that binds LDLR and promotes its lysosomal degradation.

5.2 PCSK9 Function in LDL Metabolism

Normally, PCSK9 regulates the number of LDL receptors on the hepatocyte surface.
Increased PCSK9 activity → fewer LDLRs → reduced LDL clearance.
Decreased PCSK9 activity → more LDLRs → increased clearance, lower LDL-C.

5.3 PCSK9 Mutations in FH

Gain-of-function (GOF) mutations: Cause FH due to excessive LDLR degradation.
- Examples: p.Asp374Tyr, p.Ser127Arg, p.Phe216Leu.
Loss-of-function (LOF) mutations: Lead to lifelong low LDL-C and protection from ASCVD.

5.4 Clinical Implications

PCSK9 GOF mutations are rare (<1% of FH cases).
However, the discovery of PCSK9’s role in FH led to PCSK9 inhibitors, a breakthrough therapy for hypercholesterolemia.

Section 6: Genotype-Phenotype Correlations

LDLR mutations → most severe phenotype (especially null mutations).
APOB mutations → moderate LDL-C elevation, milder phenotype.
PCSK9 GOF mutations → variable but can resemble LDLR-FH.
Homozygous FH (two defective alleles) → extreme LDL-C (>500 mg/dL), childhood xanthomas, CAD before age 20.

Section 7: Diagnostic Approaches

7.1 Clinical Diagnostic Criteria

Dutch Lipid Clinic Network (DLCN) Criteria – assigns points for LDL levels, family history, physical signs, and DNA analysis.
Simon Broome Criteria (UK).
Make Early Diagnosis to Prevent Early Death (MEDPED) Criteria (US).

7.2 Genetic Testing

Identifies mutations in LDLR, APOB, and PCSK9.
Confirms diagnosis and aids in family cascade screening.

7.3 Biomarkers and Imaging

LDL-C levels – persistent and severe elevation from childhood.
Carotid intima-media thickness (IMT) or coronary artery calcium score – detect early atherosclerosis.

Section 8: Therapeutic Implications

8.1 Lifestyle Interventions

Diet low in saturated fats and cholesterol.
Regular physical activity.
Smoking cessation.

8.2 Pharmacological Therapy

Statins: Inhibit HMG-CoA reductase → upregulate LDLR. First-line treatment.
Ezetimibe: Inhibits intestinal cholesterol absorption.
PCSK9 inhibitors (alirocumab, evolocumab): Monoclonal antibodies preventing LDLR degradation.
Inclisiran: Small interfering RNA (siRNA) silencing PCSK9 expression.
Bempedoic acid: Inhibits cholesterol synthesis upstream of statins.

8.3 Therapies for Severe FH (HoFH)

Lomitapide: Inhibits microsomal triglyceride transfer protein.
Mipomersen: Antisense oligonucleotide targeting apoB mRNA.
Lipoprotein apheresis: Mechanical removal of LDL particles from blood.
Liver transplantation (rare cases): Restores LDLR function.

Section 9: Future Directions

Gene therapy: Delivering functional LDLR genes to hepatocytes.
CRISPR/Cas9 editing: Correcting pathogenic mutations.
RNA-based therapies: Expanding on PCSK9 siRNA strategies.
Biomarker discovery: Identifying modifiers of disease severity.

Section 10: Public Health and Genetic Counseling

Cascade screening: Testing relatives of FH patients to identify affected individuals early.
Early intervention: Treating from childhood in HeFH prevents premature CAD.
Awareness campaigns: Improve recognition and diagnosis rates (currently <20% of FH patients worldwide are diagnosed).