40-17: Familial Hypercholesterolemia

Familial hypercholesterolemia (FH) is a group of autosomal dominant conditions that result in elevated low-density lipoprotein (LDL) levels in the blood. High LDL predisposes to atherosclerosis, which in turn leads to premature myocardial infarction or stroke. The incidence of these serious complications increases with age and when associated with the other common predispositions to atherosclerosis, such as smoking and hypertension. About 1 in 500 people in the United States have FH; worldwide, the prevalence is about 10 million. Only about 15% of people with FH are diagnosed and even fewer are treated effectively.

A. Symptoms and Signs

Yellow lipid deposits appear on tendons, especially the Achilles (tendon xanthoma).

B. Laboratory Findings

Total serum cholesterol with the LDL component is particularly high. A detailed family history and genetic testing should be obtained when individuals are younger than 40 years with an LDL level greater than 200 mg/mL and for individuals older than 40 years with a level greater than 250 mg/mL.

In most instances, the elevated LDL is inherited as an autosomal dominant trait. An affected individual in all likelihood inherited FH from one parent, and each of his or her children has a 50/50 chance of inheriting FH. In uncommon cases, both parents have a mutation in the LDL receptor and one-quarter of their children, on average, will inherit two mutant alleles and have homozygous FH, which is a much more serious disease with manifestations in childhood.

Mutations in the following four genes can cause FH: (1) LDLR, which encodes the LDL receptor located on the surface of cells and responsible for moving LDL into the cell for metabolism; the most common mutant gene in FH; (2) APOB, which encodes a component of LDL and mutations inhibit binding to LDL receptor; (3) PCSK9, which encodes a protein that normally reduces production of LDL receptors, so mutations actually protect from hypercholesterolemia; and (4) ARH, which requires mutations in both alleles (autosomal recessive inheritance) to cause FH.

Statins, usually at high doses, can reduce LDL levels, occasionally to acceptable levels (see Table 28–3). The earlier in life that treatment is begun, the better the outcome in reducing mortality from atherosclerosis. In homozygous FH, if high-dose statins do not reduce LDL sufficiently, treatment with a monoclonal antibody (eg, alirocumab and evolocumab) that blocks the action of the PCSK9 enzyme (which inactivates hepatic receptors that transport LDL into the liver for metabolism) can be an expensive adjunct to standard therapy (Chapter 28, Lipid Disorders). If all else fails, then plasmapheresis is needed to reduce LDL.

Another important need in effective management is to screen relatives who are at risk, certainly by measuring LDL levels, but increasingly by identifying the mutation in the family and utilizing that for screening.

• For comprehensive evaluation of infants for their lipid profile.

• For genetic counseling of the patient, his or her parents, siblings, and offspring.

• For signs of atherosclerotic cardiovascular disease.

For signs or symptoms of acute arterial occlusive events.