Familial hypercholesterolemia
To view Lipoprotein disorders main page  Click here 
To view Hyperlipoproteinemia main page  Click here
Synonyms and keywords: FHC; FH; type IIA hyperlipoproteinemia; hyper-low-density-lipoproteinemia; familial hypercholesterolemic xanthomatosis; LDL receptor disorder

# Overview
Familial hypercholesterolemia is most commonly autosomal dominant disorder caused by mutations involving three genes and rarely an autosomal recessive disorder involving hypercholesterolemia adaptor protein.  It is characterized by very high LDL cholesterol with associated atherosclerotic plaque deposition in arteries and a markedly increased risk of premature coronary artery disease.

# Historical perspective
- The Norwegian physician Dr C Muller first associated the physical signs, high cholesterol levels and autosomal dominant inheritance in 1938.
- In the early 1970s and 1980s, the genetic cause for familial hypercholesterolemia was described by Dr Joseph L. Goldstein and Dr Michael S. Brown of Dallas, Texas .

# Classification
Familial hypercholesterolemia may be classified according to the severity of the mutation involving the LDL-cholesterol (LDL-C) receptor or depending on the mode of inheritance as follows:
- Heterozygous vs homozygous
This is particularly important because, phenotypically, familial hypercholesterolemia (FH) is characterized by increased plasma levels of total cholesterol and low-density lipoprotein cholesterol, tendinous xanthomata, and premature symptoms of coronary heart disease.
These are more pronounced in homozygous than heterozygous individuals.
- This is particularly important because, phenotypically, familial hypercholesterolemia (FH) is characterized by increased plasma levels of total cholesterol and low-density lipoprotein cholesterol, tendinous xanthomata, and premature symptoms of coronary heart disease.
These are more pronounced in homozygous than heterozygous individuals.
- These are more pronounced in homozygous than heterozygous individuals.
- Polygenic vs monogenic
Polygenic hypercholesterolemia is a major source of atherosclerotic cardiovascular disease.
Xanthelasma, premature CHD, tendon xanthomata, and childhood obesity are common in monogenic hypercholesterolemia.
Tendon xanthomata and childhood obesity are not observed in polygenic hypercholesterolemia.
- Polygenic hypercholesterolemia is a major source of atherosclerotic cardiovascular disease.
- Xanthelasma, premature CHD, tendon xanthomata, and childhood obesity are common in monogenic hypercholesterolemia.
- Tendon xanthomata and childhood obesity are not observed in polygenic hypercholesterolemia.

# Pathophysiology

## Genetics
- Familial hypercholesterolemia is a most commonly autosomal dominant disorder caused by mutations involving three genes:
The LDL receptor
The apolipoprotein B gene (ApoB)
ApoB is necessary for binding of LDL particles to the LDL receptor. Mutation in this gene leads to impaired binding to the LDL receptor.
Proprotein convertase subtilisin/kexin type 9 gene
Mutation in this gene leads to decreased LDL metabolism
- The LDL receptor
- The apolipoprotein B gene (ApoB)
ApoB is necessary for binding of LDL particles to the LDL receptor. Mutation in this gene leads to impaired binding to the LDL receptor.
- ApoB is necessary for binding of LDL particles to the LDL receptor. Mutation in this gene leads to impaired binding to the LDL receptor.
- Proprotein convertase subtilisin/kexin type 9 gene
Mutation in this gene leads to decreased LDL metabolism
- Mutation in this gene leads to decreased LDL metabolism
- There are over 1600 known mutations of the LDL receptor gene documented to cause familial hypercholesterolemia, accounting for about 85 to 90% of familial hypercholesterolemia cases.
- The apolipoprotein B gene mutation (Arg3500Gln mutation in APOB) is mostly observed in Northern European population but rare in other populations. It accounts for 5 to 10% of familial hypercholesterolemia cases.
- Mutations involving Proprotein convertase subtilisin/kexin type 9 gene is a gain-of-function mutation causing fewer than 5% of familial hypercholesterolemia cases.
- The LDL receptor gene is located on the short arm of chromosome 19 (19p13.1-13.3). It comprises 18 exons and spans 45kb, and the gene product contains 839 amino acids in mature form.
- Familial hypercholesterolemia is rarely an autosomal recessive disorder involving hypercholesterolemia adaptor protein. This adaptor protein contains a phosphotyrosine binding domain, which in other proteins binds LDL receptor. Hypercholesterolemia adaptor protein appears to have a tissue-specific role in LDL receptor function, as it is required in liver but not in fibroblasts.

## Pathogenesis
- Familial disorders of cholesterol metabolism may result from one of the following:
Overproduction of lipoproteins
Impaired removal of lipoproteins (this may result from primary defect with the lipoprotein or its receptor) as described above.
- Overproduction of lipoproteins
- Impaired removal of lipoproteins (this may result from primary defect with the lipoprotein or its receptor) as described above.
- LDL cholesterol normally circulates in the body for 2.5 days, after which it is cleared by the liver.
- In Familial hypercholesterolemia (FH), the half-life of an LDL particle is almost doubled to 4.5 days.
This leads to markedly elevated LDL levels, with the other forms of cholesterol remaining normal, most notably HDL.
- This leads to markedly elevated LDL levels, with the other forms of cholesterol remaining normal, most notably HDL.
- The classic form of familial hypercholesterolemia results from defects in the cell surface receptor that normally removes LDL particles from the blood plasma.
- The excess circulating LDL is taken up by cells all over the bodymost notably, by macrophages, and particularly the ones in a primary streak (the earliest stage of atherosclerosis).
- Oxidation of LDL increases its uptake by foam cells.
- Although atherosclerosis can occur in all people, many familial hypercholesterolemia patients develop accelerated atherosclerosis due to the presence of excess LDL. Some studies of familial hypercholesterolemia cohorts suggest that additional risk factors are generally present when a familial hypercholesterolemic patient develops atherosclerosis.
The degree of atherosclerosis roughly depends of the amount of LDL receptors still expressed by the cells in the body, as well as the functionality of these receptors. In heterozygous FH, there is at least 50% of the normal LDL receptor activity however, in homozygous FH, both alleles are damaged to some degree, which may lead to extremely high levels of LDL. Children with this form of FH may develop severe premature heart disease. A further complication is the ineffectiveness of statins.
- The degree of atherosclerosis roughly depends of the amount of LDL receptors still expressed by the cells in the body, as well as the functionality of these receptors. In heterozygous FH, there is at least 50% of the normal LDL receptor activity however, in homozygous FH, both alleles are damaged to some degree, which may lead to extremely high levels of LDL. Children with this form of FH may develop severe premature heart disease. A further complication is the ineffectiveness of statins.

# Causes
Familial hypercholesterolemia is predominantly an autosomal dorminant  but rarely autosomal recessive disease caused by mutations involving three genes (LDL receptor gene, apolipoprotein B gene and Proprotein convertase subtilisin/kexin type 9 gene) .

# Epidemiology and demographics

## Prevalence
Homozygous familial hypercholesterolemiai is less prevalent (prevalence 1:160,000 to 1:1,000,000) compared to heterozygous hypercholesterolemia.
The prevalence of familial hypercholesterolemia varies dependeing on the population under study as shown below:

## Age
Most individuals with homozygous FH experience severe CHD by their mid-20s. The rate of either death or coronary bypass surgery by the teenage years is high. Severe aortic stenosis is also common.

## Race
There is no racial predilection to acquiring FH however some individuals are found to have higher prevalence of the disease as shown in the table of prevalence above.

# Screening
Universal screening for elevated serum cholesterol is recommended.

## General population screening
Familial hypercholesterolemia (FH) should be suspected when untreated fasting LDL cholesterol or non HDL cholesterol levels are at or above the following:
- Adults ( 20 years):
LDL cholesterol  190 mg/dL or non-HDL cholesterol  220 mg/dL
- LDL cholesterol  190 mg/dL or non-HDL cholesterol  220 mg/dL
- Children, adolescents and young adults (< 20 years):
LDL cholesterol 160 mg/dL or non- HDL cholesterol  190 mg/dL
- LDL cholesterol 160 mg/dL or non- HDL cholesterol  190 mg/dL
Cholesterol screening should be considered beginning at age 2 for children with a family history of premature cardiovascular disease or elevated cholesterol. All individuals should be screened by age 20.
Although not present in many individuals with familial hypercholesterolemia (FH), the following physical findings should prompt the clinician to strongly suspect FH and obtain necessary lipid measurements if not already available:
- Tendon xanthomas at any age (most common in Achilles tendon and finger extensor tendons, but can also occur in patellar and triceps tendons). B Arcus corneae in a patient under age 45.
- Tuberous xanthomas or xanthelasma in a patient under age 20 to 25
At the LDL cholesterol levels listed below the probability of FH is approximately 80% in the setting of general population screening.
- These LDL cholesterol levels should prompt the clinician to strongly consider a diagnosis of FH and obtain further family information:
LDL cholesterol  250 mg/dL in a patient aged 30 or more
LDL cholesterol  220 mg/dL for patients aged 20 to 29
LDL cholesterol  190 mg/dL in patients under age 20
- LDL cholesterol  250 mg/dL in a patient aged 30 or more
- LDL cholesterol  220 mg/dL for patients aged 20 to 29
- LDL cholesterol  190 mg/dL in patients under age 20

## Screening in children and adolescents
Lipid screening recommendations for familial hypercholesterolemia in children varies by age and risk factors as shown below: 

## Child-parent familial hypercholesterolemia screening in primary care
- Recent studies show the feasibility and efficacy of child-parent familial hypercholesterolemia screening in primary care setting.
- The conclusion remains that childparent familial hypercholesterolemia screening is a simple, practical, and effective way of screening the population to identify and prevent a common inherited cause of premature cardiovascular disease.

# Natural history, complication and prognosis
If left untreated, the majority of affected individuals will have symptomatic coronary artery disease by 50-60 years and half of the men and 15%-30% of the women will have died. On the other hand, patients who start attending a lipid clinic before they develop clinical CAD may enjoy a normal life expectancy if well managed.

# Diagnosis

## History and Symptoms
History is usually suggestive of:
- Premature cardiovascular disease, such as:
Angina pectoris, leading to PTCA or CABG
Myocardial infarction
Transient ischemic attacks (TIA's)
Cerebrovascular accidents/Strokes
Peripheral artery disease (PAD)
- Angina pectoris, leading to PTCA or CABG
- Myocardial infarction
- Transient ischemic attacks (TIA's)
- Cerebrovascular accidents/Strokes
- Peripheral artery disease (PAD)
- Family history of premature atherosclerosis

## Physical examinations
Ptients with familial hypercholesterolemia may present with the following:

### Eyes
- Xanthelasma palpabrum (yellowish patches above the eyelids)
- Arcus senilis corneae, whitish discoloration of the iris

### Extremities
- Tendon xanthomas (thickening of tendons due to accumulation of macrophages filled with cholesterol).

## Laboratory findings
In FH, a genetic diagnosis is important for family screening, to establish the diagnosis in patients with borderline LDL-C and to improve patient adherence to therapy.
Genetic testing is generally undertaken when:
- A family member has been shown to have a mutation
- High cholesterol is found in a young patient with atherosclerotic disease
- Tendon xanthomas are found in a patient with high cholesterol
FH are best identified by a definite or probable phenotypic diagnosis of FH based on the DLCN criteria as shown below:
The table below shows the interpretation of the DLCN diagnostic criteria.

# Treatment

## Medical therapy

### Heterozygous FH
The initial drug of choice are high-dose statin therapy especially atorvastatin, simvastatin or rosuvastatin. 
Statins work by forcing the liver to produce more LDL receptor to maintain the amount of cholesterol in the cell. This requires at least one functioning copy of the gene.
In cases where monotherapy with statins are ineffective, combination therapy with statin and other lipid lowering medication can be considered although their utility is not well established.
The following can be added to statins:
- Ezetimibe
- Fibrate
The combination of fibrates and statins is associated with a markedly increased risk of myopathy and rhabdomyolysis (breakdown of muscle tissue, leading to acute renal failure), so patients must be monitored closely.
- Bile acid sequestrant
- Niacin/acipimox

### Homozygous FH
According to the 2015 AHA scientific statement on FH, early diagnosis of homozygous FH and prompt initiation of diet and lipid-lowering therapy are critical.
As previously mentioned, the LDL levels are much higher and the most effective treatments (statins) require at least one copy of the functional LDL receptor gene. In this case, high amounts of bile acid sequestrants are often given. Occasionally, high-dosed statins can help express a dysfunctional (but some times working) LDL receptor. Other treatments used are LDL apheresis (clearing LDL by blood filtration, similar to dialysis) with liver transplant  as last result. The last option will introduce liver cells with working LDL receptors, effectively curing the condition.

## National Lipid Association Guidelines for Identifying Patients Eligible for Apheresis:
In patients who, after six months, do not have an adequate response to maximum tolerated drug therapy, LDL apheresis is indicated according to these guidelines:
1.Functional homozygous FH patients with LDL cholesterol 300 mg/dL (or non-HDL cholesterol 330 mg/dL).
2.Functional heterozygous FH patients with LDL cholesterol 300 mg/dL (or non-HDL cholesterol 330 mg/dL) and 0-1 risk factors.
3.Functional heterozygous FH patients with LDL cholesterol 200 mg/dL (or non-HDL cholesterol 230 mg/dL) and high risk characteristics such as 2 risk factors or high lipoprotein (a) 50 mg/dL using an isoform insensitive assay.
4. Functional heterozygotes with LDL cholesterol 160 mg/dL (or non-HDL cholesterol 190 mg/dL) and very high risk characteristics (established CHD, other cardiovascular disease, or diabetes).

# Prevention
Prevention of complications of familial hypercholesterolemia are as follows:
- Early genetic diagnosis
- Early attendance at a lipid clinic before they develop clinical CAD
- Adherence to therapy