Familial hypercholesterolemia: the lipids or the genes?

Abstract

Familial Hypercholesterolemia (FH) is a common cause of premature cardiovascular disease and is often undiagnosed in young people. Although the disease is diagnosed clinically by high LDL cholesterol levels and family history, to date there are no single internationally accepted criteria for the diagnosis of FH. Several genes have been shown to be involved in FH; yet determining the implications of the different mutations on the phenotype remains a hard task. The polygenetic nature of FH is being enhanced by the discovery of new genes that serve as modifiers. Nevertheless, the picture is still unclear and many unknown genes contributing to the phenotype are most likely involved. Because of this evolving polygenetic nature, the diagnosis of FH by genetic testing is hampered by its cost and effectiveness.In this review, we reconsider the clinical versus genetic nomenclature of FH in the literature. After we describe each of the genetic causes of FH, we summarize the known correlation with phenotypic measures so far for each genetic defect. We then discuss studies from different populations on the genetic and clinical diagnoses of FH to draw helpful conclusions on cost-effectiveness and suggestions for diagnosis.

Figure 1

Molecular Pathways of Disease in Familial Hypercholesterolemia (1) The LDL receptor on the surface of hepatocytes binds ApoB-100 of the LDL particle forming a complex. (2) A clathrin-coated pit is formed and the ligand-receptor complex is endocytosed via interactions involving the LDLR Adaptor Protein 1 (LDLRAP1). (3) Inside the hepatocyte, the complex dissociates, the LDLR recycles to the cell membrane, (4) and free cholesterol is used inside the cell. (5) PCSK9 serves as a post-transcriptional inhibitor of LDLR. It is secreted and inhibits LDLR through cell-surface interactions. (6) The presence of an intracellular pathway for PCSK9-mediated LDLR inhibition is still a subject of controversy. (7) In response to decreased cholesterol such as during treatment with statins, Steroid Response Element Binding Protein (SREBP) binds to the Steroid Response Element (SRE) on the DNA and induces the transcription of the LDLR. (8) The sterol-responsive nuclear receptor LXR on the other hand responds to increased intracellular cholesterol inducing the transcription of IDOL, a recently discovered molecule that induces the ubiquitin-mediated degradation of the LDLR. Clouds in the figure refer to genes in which mutations have been associated with increased LDL-C levels.