Familial hypercholesterolemia/autosomal dominant hypercholesterolemia: Molecular defects, the LDL-C continuum, and gradients of phenotypic severity

Abstract

Familial hypercholesterolemia (FH) is a common inherited disorder in which the severity of atherosclerosis is generally proportional to the extent and duration of elevated plasma low-density lipoprotein cholesterol (LDL-C) levels. Homozygous FH (HoFH) is generally considered the most severe condition and results in very high LDL-C levels that respond only partially to statin therapy. The diagnosis of HoFH is complicated by its presentation as a phenotypic spectrum involving multiple genes. The objective here is to review the genetics, continuum of LDL-C concentrations, and phenotypic severity of FH. Multiple PubMed searches were conducted as described in the main text of this article. Traditionally, FH has been considered an autosomal co-dominant disorder whereby both heterozygotes (HeFH) and homozygotes are affected. Recently, additional genes and loci for monogenic FH have been characterized that allow for the identification of double mutations in the known genes and loci and the description of novel forms of double heterozygous FH. Phenotypic expression and clinical severity of untreated HeFH, double HeFH, compound HeFH, and HoFH vary with some overlap both between and within the genotypes. In addition, there is overlap in LDL-C levels of treated HeFH and treated HoFH. These discoveries raise the possibility that new combinations of molecular defects could modulate the severity of hypercholesterolemia. These defects are unlikely to represent true homozygosity. However, they are likely to result in a phenotype consistent with HoFH or severe HeFH, which will be important as new therapies become available with indications specifically for HoFH.