Apolipoprotein E gene mutations in subjects with mixed hyperlipidemia and a clinical diagnosis of familial combined hyperlipidemia
Highlights
► Rare APOE mutations are responsible for approximately 3.5% of FCHL cases in our population. ► APOE R136S mutation induces autosomal dominant familial dysbetalipoproteinemia. ► APOE p.Leu149del induces a phenotype indistinguishable from familial combined hyperlipidemia.
Introduction
Familial combined hyperlipidemia (FCHL) is a common disorder of lipid metabolism characterized by elevated levels of total cholesterol and triglycerides, vertical transmission of a hyperlipidemic phenotype within the family, and high risk of premature cardiovascular disease (CVD) [1]. FCHL is the most common genetic lipid abnormality found in patients with premature coronary artery disease [2] and a major risk factor for the occurrence of myocardial infarction (MI) at a young age. In a recent study, as many as 38% patients with MI below age 40 years had a diagnosis of FCHL [3]. FCHL was described by Goldstein et al. as a dominant genetic disease [4], although current knowledge about the disease supports the notion that FCHL is highly heterogeneous in its genetic background, penetrance, and lipid phenotype [5]. Although some FCHL cases are monogenic, FCHL must be considered a complex disease in most patients, with interaction of several causal genes, mostly unknown, with multiple variations in modifier genes and environmental factors, noticeably visceral adiposity, insulin resistance, or full-blown diabetes [1].
FCHL is the most common cause of primary mixed hyperlipidemia in the population, with a prevalence of nearly 6% [6]. Other causes of the FCHL phenotype include familial dysbetalipoproteinemia (FD) and some hypertriglyceridemic forms of familial hypercholesteromia (FH) due to mutations in the genes coding for apolipoprotein E (APOE) and the low-density lipoprotein receptor (LDLR), respectively. We have recently described the overlapping lipid phenotype between FCHL and FH in subjects with a clinical diagnosis of FCHL in whom a LDLR mutation could be demonstrated [7].
Traditionally, the differential diagnosis between FCHL and FD was based on the family history of hyperlipidemia. FCHL is characterized by a dominant presentation in the family and FD is usually sporadic because most cases are recessive and two APOE defective alleles (ɛ2) plus additional environmental factors are required for full expression of dyslipidemia [8]. Other clinical criteria used in the differential diagnosis include the rare presence of palmar xanthomas in FD and a milder lipid phenotype, but with higher serum apo B concentration, in FCHL [9]. The diagnosis of dysbetalipoproteinemia is based on the demonstration of elevated very-low-density lipoprotein (VLDL) remnants and intermediate-density lipoproteins (IDL) after separation of lipoproteins by ultracentrifugation. The presence of a VLDL-cholesterol (mg/dL)/total triglycerides (mg/dL) ratio >0.30 has been considered characteristic of the disease [8]. However, ultracentrifugation is costly and time-consuming, and it is not standardized or available in many laboratories. Furthermore, the lipid phenotype is highly variable in FCHL and FD and lipid remnants are often elevated also in FCHL [10]. By contrast, the genotype analysis of the 3 common polymorphisms of APOE: ɛ2, ɛ3 and ɛ4 that code for the apo E2, E3 and E4 isoforms is a simple method available in most laboratories and has substituted the traditional ultracentrifugation technique because the detection of an APOE ɛ2/ɛ2 genotype provides an unequivocal diagnosis of FD [8], [11].
Although less common, dominant mutations in APOE may also cause FD. These rare mutations are not detected by using the usual APOE genotyping methodology, thus subjects carrying them could be easily misclassified as FCHL bearers. The objective of this study was to identify APOE mutations associated with dominant combined hyperlipoproteinemia and to establish their frequency in subjects with a clinical diagnosis of FCHL.
Section snippets
Study subjects
Consecutive unrelated subjects (n = 312) aged 18–80 years (205 men, 107 women) with a clinical diagnosis of familial mixed hyperlipidemia attending the Lipid Clinic of the Hospital Universitario Miguel Servet (Zaragoza, Spain) were recruited from January 2008 to April 2011. A clinical diagnosis of FCHL was made in subjects with off-treatment total cholesterol and triglyceride serum concentrations above the age- and sex-specific 90th percentiles of a Spanish reference population [12] and existence
Results
From the group of 312 subjects with primary combined hyperlipidemia, 10 subjects were identified as ɛ2/ɛ2 genotype, and were therefore classified as having FD. Another 22 subjects were carriers of a functional mutation in the LDLR gene and they were classified as FH with hypertriglyceridemia. One subject was heterozygous for a mutation in the LDLR associated with an APOE ɛ2/ɛ2 genotype and was considered as a FH case. The remaining 279 subjects were classified as FCHL. APOE gene sequencing in
Discussion
This is the first study, to our knowledge, to fully sequence APOE in subjects with the FCHL phenotype. The novel finding is that rare APOE mutations associated with a dominant transmission pattern are responsible for some cases of FCHL in our population. Several studies have previously examined the contribution of common APOE genetic polymorphisms ɛ2, ɛ3 and ɛ4 to FCHL and, as in our study, showed that the ɛ4 allele was overrepresented in this phenotype [15], [16], [17]. The mechanism of this
Acknowledgements
Grants from the Spanish Ministry of Health FIS PS09/0673, FIS PS09/0665, FIS PI10/00387 and RETIC C06/01 RD06/0014/0029 (RECAVA) supported this work. CIBERer, CIBERobn and CIBERdem are initiatives of ISCIII, Spain.
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