Residency · Residency · Medical Genetics Genomics
Familial Hypercholesterolemia: Underdiagnosed and Undertreated
Introduction
Familial hypercholesterolemia (FH) is an autosomal dominant disorder of LDL cholesterol metabolism affecting approximately 1 in 250 individuals for heterozygous FH (HeFH) and 1 in 300,000-500,000 for homozygous FH (HoFH). Despite being one of the most common monogenic disorders worldwide, FH remains profoundly underdiagnosed, with fewer than 10% of affected individuals identified in most countries. Untreated, HeFH carries a 20-fold increased risk of premature coronary artery disease.
Molecular Genetics
Causative Genes
LDLR (LDL receptor) accounts for 85-90% of molecularly confirmed FH, with over 2,000 variants described. APOB (apolipoprotein B) accounts for 5-10%, typically through the p.R3527Q variant affecting the LDL receptor binding domain. PCSK9 (proprotein convertase subtilisin/kexin type 9) accounts for 1-3% through gain-of-function variants that increase LDL receptor degradation. LDLRAP1 causes autosomal recessive hypercholesterolemia and is rare.
Functional Impact
All pathogenic mechanisms converge on reduced hepatic LDL receptor-mediated clearance of LDL-C from plasma. LDLR variants are classified by functional class: null alleles (receptor-negative) versus defective alleles (receptor-defective). Homozygous or compound heterozygous patients have severely reduced or absent LDL receptor activity.
Genotype-Phenotype Correlation
LDLR-null variants are associated with a more severe phenotype than LDLR-defective variants. APOB and PCSK9 variants generally produce a milder phenotype than LDLR variants. Polygenic hypercholesterolemia, resulting from accumulation of common LDL-raising variants, can mimic the FH phenotype clinically.
Clinical Diagnosis
Clinical Criteria
The Dutch Lipid Clinic Network (DLCN) score integrates family history, personal history of CVD, physical findings, LDL-C level, and molecular testing, with definite FH scored above 8, probable FH at 6-8, and possible FH at 3-5. The Simon Broome criteria are used in the UK, while MEDPED criteria apply age-specific and family-specific LDL-C cutoffs.
Heterozygous FH (HeFH)
HeFH presents with LDL-C typically 190-400 mg/dL when untreated, premature coronary artery disease (men before age 55, women before 60), tendon xanthomas (pathognomonic but present in fewer than 50% of adults with HeFH), corneal arcus before age 45, and family history of early CVD or elevated LDL-C.
Homozygous FH (HoFH)
HoFH presents with LDL-C typically above 500 mg/dL untreated, cutaneous xanthomas in childhood (interdigital, planar, tuberous), aortic valve and supravalvular aortic stenosis from cholesterol deposits, and coronary artery disease in the first or second decade of life. Without treatment, death from cardiovascular disease occurs by age 20-30.
Genetic Testing
Indications
Genetic testing is indicated for clinical diagnosis of FH (DLCN score 6 or greater), LDL-C of 190 mg/dL or greater in adults (or 160 mg/dL or greater in children) after excluding secondary causes, family members of individuals with known FH mutations, and early-onset CAD with elevated LDL-C.
Testing Approach
A targeted gene panel covering LDLR, APOB, PCSK9, and optionally LDLRAP1 is standard. Full sequencing plus deletion/duplication analysis of LDLR is important, as large rearrangements account for approximately 10% of LDLR mutations. Cascade testing of first-degree relatives is the most cost-effective identification strategy and is recommended by all guidelines.
Value of Molecular Diagnosis
Molecular confirmation distinguishes monogenic FH from polygenic hypercholesterolemia, enables cascade family screening (identifying approximately 50% of tested first-degree relatives as affected), improves treatment adherence and intensification, and clarifies cardiovascular risk stratification.
Treatment
Lifestyle Modifications
A heart-healthy diet with reduced saturated fat and trans fat elimination, along with regular aerobic exercise, is recommended. However, lifestyle alone is insufficient -- pharmacotherapy is always required in FH.
Statin Therapy
High-intensity statins (atorvastatin 40-80 mg, rosuvastatin 20-40 mg) are first-line treatment. Initiation in children with FH is recommended at age 8-10 years (some guidelines suggest age 6). The target is greater than 50% LDL-C reduction from baseline, with goal LDL-C below 70 mg/dL for high-risk patients and below 55 mg/dL for those with established CVD.
| FH Gene | Frequency | Mechanism | Phenotype Severity | Key Notes |
|---|---|---|---|---|
| LDLR | 85–90% | Reduced/absent LDL receptor | Variable (null > defective alleles) | >2,000 variants; 10% are large rearrangements |
| APOB | 5–10% | Defective LDL receptor binding | Generally milder than LDLR | p.R3527Q most common |
| PCSK9 | 1–3% | Gain-of-function increases LDLR degradation | Generally milder | Loss-of-function variants are cardioprotective |
| LDLRAP1 | Rare | Impaired LDL receptor internalization | AR inheritance; milder than HoFH from LDLR-null | Autosomal recessive |
Add-On Therapies
Ezetimibe inhibits intestinal cholesterol absorption and provides an additional 15-20% LDL-C reduction. PCSK9 inhibitors (evolocumab, alirocumab) are monoclonal antibodies providing an additional 50-60% LDL-C reduction via subcutaneous injection every 2-4 weeks. Inclisiran is an siRNA targeting PCSK9 mRNA given as a twice-yearly subcutaneous injection. Bempedoic acid is an ACL inhibitor available orally for statin-intolerant patients.
Homozygous FH-Specific Therapies
Lomitapide is an MTP inhibitor that reduces hepatic VLDL secretion, though it carries risk of hepatic steatosis. Evinacumab is an anti-ANGPTL3 antibody with an LDL receptor-independent mechanism, making it effective in HoFH. LDL apheresis provides extracorporeal LDL removal every 1-2 weeks and is essential for HoFH management. Liver transplantation is curative but reserved for refractory cases.
Emerging Approaches
CRISPR-based PCSK9 gene editing (VERVE-101) employs in vivo base editing and is in clinical trials. AAV-mediated LDLR gene therapy delivery is under investigation.
Cascade Screening
Cascade screening is the most cost-effective strategy for FH identification. Each index case leads to identification of 2-8 additional affected relatives on average. Genetic testing is preferred over lipid levels alone for cascade screening due to higher specificity. Barriers include family communication challenges, geographic dispersion, insurance coverage limitations, and lack of systematic programs in most countries.
Clinical Pearls
FH affects 1 in 250 people but fewer than 10% are diagnosed, and clinical geneticists play a critical role in identification and cascade testing. An LDL-C of 190 mg/dL or greater in an adult, after excluding secondary causes, should prompt FH evaluation and genetic testing. Tendon xanthomas are pathognomonic for FH, but their absence does not exclude the diagnosis. PCSK9 inhibitors and inclisiran have transformed management of statin-refractory FH, and in vivo gene editing approaches represent the next frontier in treatment.
References
- Nordestgaard BG, Chapman MJ, Humphries SE, et al. Familial hypercholesterolaemia is underdiagnosed and undertreated in the general population. Eur Heart J. 2013;34(45):3478-3490.
- Gidding SS, Champagne MA, de Ferranti SD, et al. The agenda for familial hypercholesterolemia: a scientific statement from the AHA. Circulation. 2015;132(22):2167-2192.
- Raal FJ, Rosenson RS, Reeskamp LF, et al. Evinacumab for homozygous familial hypercholesterolemia. N Engl J Med. 2020;383(8):711-720.
- Musunuru K, Chadwick AC, Mizoguchi T, et al. In vivo CRISPR base editing of PCSK9 durably lowers cholesterol in primates. Nature. 2021;593(7859):429-434.