What causes fh

What is Familial Hypercholesterolemia (FH)?

Familial Hypercholesterolemia (FH) is a serious inherited condition characterized by extremely high levels of low-density lipoprotein (LDL) cholesterol in the blood. LDL cholesterol is often referred to as 'bad' cholesterol because high levels can lead to a buildup of cholesterol in the arteries, a process known as atherosclerosis. In individuals with FH, this buildup begins early in life, significantly increasing the risk of premature cardiovascular disease, including heart attacks and strokes, often occurring in young adulthood.

What Causes Familial Hypercholesterolemia?

FH is a genetic disorder, meaning it is caused by changes (mutations) in one or more genes. These gene mutations are inherited from one or both parents. The primary function of these genes is to regulate the body's ability to remove LDL cholesterol from the blood. When these genes are mutated, the LDL receptors, which are responsible for binding and removing LDL cholesterol from the bloodstream, do not function properly or are not present in sufficient numbers.

The Role of Genes in FH

The vast majority of FH cases are caused by mutations in genes involved in the LDL cholesterol pathway. The most common genetic cause, accounting for about 85% of all FH cases, is a mutation in the LDLR gene. This gene provides instructions for making the LDL receptor protein. These receptors are primarily located on the surface of liver cells and are crucial for clearing LDL cholesterol from the circulation. Without functional LDL receptors, LDL cholesterol remains in the blood at very high levels.

Other genes can also cause FH, although less frequently:

• APOB gene: Mutations in this gene can affect the structure of apolipoprotein B, a protein that is a component of LDL particles and is recognized by the LDL receptor. If the apolipoprotein B is altered, the LDL receptor may not be able to bind to it effectively.
• PCSK9 gene: Mutations in this gene can lead to an overactive PCSK9 protein. PCSK9 normally binds to LDL receptors and promotes their degradation, thereby reducing the number of receptors available to clear LDL cholesterol. When PCSK9 is overactive due to a genetic mutation, it leads to fewer LDL receptors on the cell surface and higher LDL cholesterol levels.
• LDLRAP1 gene: Mutations in this gene cause a recessive form of FH, known as autosomal recessive hypercholesterolemia (ARH). Unlike the dominant forms, individuals must inherit two mutated copies of this gene (one from each parent) to develop the condition. This gene is involved in the process where the LDL receptor-LDL complex is internalized into the cell after binding.

Inheritance Patterns

FH is typically inherited in an autosomal dominant pattern. This means that a person only needs to inherit one copy of the mutated gene from either their mother or their father to have the condition. If a parent has FH, each child has a 50% chance of inheriting the mutated gene and developing FH. In cases of homozygous FH (where both copies of a gene are mutated), the condition is much more severe and rare, often presenting in childhood. Heterozygous FH (one mutated copy) is more common and accounts for the majority of cases.

Prevalence and Impact

FH is one of the most common inherited disorders affecting lipid metabolism. It is estimated to affect approximately 1 in 250 individuals in populations of European descent, though prevalence can vary among different ethnic groups. Despite its prevalence, FH often goes undiagnosed or is diagnosed late, leading to preventable cardiovascular events. Early diagnosis and treatment are crucial to manage cholesterol levels and reduce the risk of heart disease.

Symptoms and Diagnosis

Symptoms of FH are not always obvious, especially in the early stages. However, individuals with FH often have very high LDL cholesterol levels from birth. Physical signs may include:

• Xanthomas: Cholesterol deposits under the skin, often appearing as yellowish bumps, particularly on the tendons (e.g., Achilles tendon) or around the eyelids (xanthelasma).
• Corneal arcus: A grayish-white ring around the cornea of the eye, which can appear in younger individuals than typically seen in age-related arcus senilis.

Diagnosis is usually suspected based on:

• Extremely high LDL cholesterol levels (often >190 mg/dL or 4.9 mmol/L in adults, and >160 mg/dL or 4.1 mmol/L in children).
• A personal or family history of premature cardiovascular disease (heart attack or stroke before age 55 in men, or before age 60 in women).
• A family history of high cholesterol.

Genetic testing can confirm the diagnosis by identifying mutations in the genes associated with FH.

Management and Treatment

The goal of treatment for FH is to lower LDL cholesterol levels and reduce the risk of cardiovascular disease. Management typically involves:

• Lifestyle modifications: While less effective alone than in common hypercholesterolemia, a heart-healthy diet (low in saturated and trans fats), regular exercise, and avoiding smoking are still recommended.
• Medications: Statins are the cornerstone of FH treatment and are often prescribed at high doses. Other medications, such as ezetimibe, PCSK9 inhibitors (e.g., evolocumab, alirocumab), bile acid sequestrants, and others, may be used in combination with statins to achieve target LDL cholesterol levels.
• Cascade screening: Once an individual with FH is identified, it is crucial to screen their close relatives (parents, siblings, children) for the condition, as it is inherited.

Early and aggressive management is essential for individuals with FH to live longer, healthier lives and prevent the devastating consequences of premature heart disease.