In FH, LDL receptor function is reduced or absent, and LDL circulates for an average duration of 4.5 days, resulting in significantly increased level of LDL cholesterol in the blood with normal levels of other lipoproteins. In mutations of ApoB, reduced binding of LDL particles to the receptor causes the increased level of LDL cholesterol. It is not known how the mutation causes LDL receptor dysfunction in mutations of PCSK9 and ARH.Although atherosclerosis occurs to a certain degree in all people, people with FH may develop accelerated atherosclerosis due to the excess level of LDL. The degree of atherosclerosis approximately depends on the number of LDL receptors still expressed and the functionality of these receptors. In many heterozygous forms of FH, the receptor function is only mildly impaired, and LDL levels will remain relatively low. In the more serious homozygous forms, the receptor is not expressed at all.Some studies of FH cohorts suggest that additional risk factors are generally at play when a person develops atherosclerosis. In addition to the classic risk factors such as smoking, high blood pressure, and diabetes, genetic studies have shown that a common abnormality in the prothrombin gene (G20210A) increases the risk of cardiovascular events in people with FH. Several studies found that a high level of lipoprotein(a) was an additional risk factor for ischemic heart disease. The risk was also found to be higher in people with a specific genotype of the angiotensin-converting enzyme (ACE). Screening
Cholesterol screening and genetic testing among family members of people with known FH is cost-effective.
A single abnormal copy (heterozygote) of FH causes cardiovascular disease by the age of 50 in about 40% of cases. Having two abnormal copies (homozygote) causes accelerated atherosclerosis in childhood, including its complications. The plasma LDL levels are inversely related to the activity of LDL receptor (LDLR). Homozygotes have LDLR activity of less than 2%, while heterozygotes have defective LDL processing with receptor activity being 2–25%, depending on the nature of the mutation. Over 1000 different mutations are known.There are five major classes of FH due to LDLR mutations:
Class I: LDLR is not synthesized at all. Class II: LDLR is not properly transported from the endoplasmic reticulum to the Golgi apparatus for expression on the cell surface. Class III: LDLR does not properly bind LDL on the cell surface because of a defect in either apolipoprotein B100 (R3500Q) or in LDL-R.
Class IV: LDLR bound to LDL does not properly cluster in clathrin-coated pits for receptor-mediated endocytosis (pathway step 2). Class V: LDLR is not recycled back to the cell surface (pathway step 5). Apolipoprotein B
Apolipoprotein B, in its ApoB100 form, is the main apolipoprotein, or protein part of the lipoprotein particle. Its gene is located on the second chromosome (2p24-p23) and is 46.2 kb long. FH is often associated with the mutation of R3500Q, which causes replacement of arginine by glutamine at position 3500. The mutation is located on a part of the protein that normally binds with the LDL receptor, and binding is reduced as a result of the mutation.