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Millions of different genes play a role in cholesterol levels.

Most of those genetic variants have just a small impact. However, about 1 in 250 people have gene mutations that cause a severe form of high cholesterol that's known as familial hypercholesterolemia (FH).

The three key mutations are in the gene for the LDL cholesterol receptor that is supposed to help move "bad" low-density lipoprotein (LDL) out of the blood where it could leave damaging deposits on the vessel walls, the PCSK9 gene that helps regulate cholesterol levels in the bloodstream, and the gene for apolipoprotein B that helps transport LDL cholesterol.

Most people inherit only one mutated copy from a parent, which is called heterozygous FH (HeFH). The rare homozygous form of FH (HoFH) from inheriting two mutated copies is more severe. LDL levels can reach above 400 mg/dL in HoFH, whereas optimal levels are generally under 130 mg/dL.

Because these mutations are present from birth, guidelines recommend a one-time check for high cholesterol in childhood to catch FH early. Left untreated, heart disease may develop by age 30 in HoFH and in the first decade of life in HeFH.

The high levels of cholesterol found in FH may lead to symptoms other than heart disease. Cholesterol may form deposits in or around the eyes (premature arcus cornealis, xanthelasma palpebrarum, or planar xanthomas), under the skin (xanthomas), and in the tendons (Achilles xanthomas and tenosynovitis).

If your medical team suspects you may have FH based on high cholesterol and a family history of early heart disease or heart attacks, they might recommend genetic testing. Genetic testing finds the cause in about 60-80% of people thought to have FH.

You can be diagnosed with FH without genetic testing or even if testing doesn't identify the specific genetic problem. However, finding what caused the condition can help determine your future risk, inform your treatment, and allow family members to be checked for it too. Parents, siblings, nieces and nephews, and even cousins related by blood should be tested for the same mutation if you're found to have one.

Like non-inherited cases of high cholesterol, statins are an important treatment for FH. However, more aggressive treatments may be needed too, such as injectable PCSK9 inhibitors, cholesterol absorption blockers, or a dialysis-like procedure to filter cholesterol from the blood, called LDL apheresis.

Read previous installments in this series:

For Your Patients: Hypercholesterolemia: Understanding Your Diagnosis

For Your Patients: Why Do High Cholesterol Levels Matter?

For Your Patients: Who Gets Hypercholesterolemia?

For Your Patients: Screening and Diagnosis of High Cholesterol

"Medical Journeys" is a set of clinical resources reviewed by doctors, meant for physicians and other healthcare professionals as well as the patients they serve. Each episode of this journey through a disease state contains both a physician guide and a downloadable/printable patient resource. "Medical Journeys" chart a path each step of the way for physicians and patients and provide continual resources and support, as the caregiver team navigates the course of a disease.