PON2 gene variants are associated with clinical manifestations of cardiovascular disease in familial hypercholesterolemia patients
Introduction
Familial hypercholesterolemia (FH) is a genetic disorder with an autosomal dominant mode of inheritance and is caused by defects in the gene coding for the low density lipoprotein (LDL) receptor, which decrease the capacity of this receptor to bind LDL. As a result of impaired cholesterol uptake, total cholesterol and LDL-cholesterol levels rise ≈2–3-fold, predisposing to premature cardiovascular disease (CVD) [1], [2]. It has repeatedly been demonstrated that important differences exist in the susceptibility for CVD among individuals affected with FH, even when these patients share the same LDL receptor gene defect [1], [3], [4], [5]. This suggests that besides the LDL-receptor defect, additional factors are involved in the expression of the clinical phenotype. Indeed, recently it has been reported that a frequent mutation in the lipoprotein lipase gene and polymorphisms of the angiotensin-converting enzyme modulate the clinical expression of familial hypercholesterolemic [6], [7]. Other additional factors could constitute factors influencing LDL oxidation, since peroxidation of LDL is recognized to play a central role in initiating atherosclerosis [8], [9]. It has been demonstrated that oxidative modification of LDL can be prevented by high-density lipoproteins (HDL), in vitro as well as in vivo [10], [11].
The HDL associated enzyme paraoxonase is capable of hydrolyzing lipid peroxides in vitro and is therefore, believed to be responsible, at least partly, for these anti-oxidative and anti-inflammatory properties [12], [13], [14], [15], [16]. This implicates paraoxonase (PON1) as a candidate gene involved in the development of atherosclerosis in FH subjects. Paraoxonase is exclusively bound to HDL in humans [17], [18]. It has largely been studied for its role in catalyzing the hydrolysis of certain non-physiological substrates like paraoxon and phenylacetate [17]. Variability of paraoxonase activity exists towards paraoxon caused by common polymorphisms in the enzyme. In the high activity isoform, glutamine (Gln allele) is replaced by arginine (Arg allele) at position 192 [19]. Another frequent polymorphism, at residue 55, involves a methionine (Met allele) to leucine (Leu allele) interchange [19]. This latter polymorphism has been reported to affect enzyme–protein concentration but not activity [20]. Several studies have revealed significant associations [20], [21], [22], [23], [24], [25], while conversely others reported a lack of association [26], [27], [28], [29], [30] between these polymorphisms and risk for CVD.
Besides PON1, the paraoxonase gene family contains at least two other members, PON2 and PON3, located on chromosome 7q21.3-22.1, exhibiting a high degree of homology with PON1 [31], [32]. A common polymorphism at codon 311 (Cys→Ser) in the PON2 gene has been described and is associated with CVD in Asian Indians [25]. In contrast to PON1, which is mainly expressed in the liver, PON2 is ubiquitously expressed [31], [32]. Although the physiological role of the PON2 gene product is unknown, its tissue distribution suggests a role different from that of paraoxonase in the atherosclerotic process.
The aim of this study was to investigate whether the PON1 and PON2 polymorphisms are associated with clinical manifestations of CVD in subjects at very high risk of premature CVD. We therefore investigated the frequency and genotype distributions of the two PON1 polymorphisms and the PON2 polymorphism in FH subjects with and without documented CVD and compared this to a healthy control group.
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Subjects
The control population comprised 201 unrelated voluntary blood donors at the bloodbank of Utrecht, The Netherlands. Consecutive subjects with definite classical FH, seen at the Lipid Research Clinics of the Academic Medical Center and the Slotervaart Hospital in Amsterdam, The Netherlands were included for the study. Diagnosis of heterozygous FH was made based on the presence of a documented LDL receptor mutation, or if the patients met the following criteria: an LDL-cholesterol level above the
Characteristics of the study subjects
A total of 201 controls and 197 FH subjects were included in the study. Clinical and biochemical characteristics of the FH- cohort and their respective controls are summarized in Table 1. No differences were found in age and gender between the groups. The mean cholesterol levels were significantly lower in controls compared to the FH-cohort (5.1±1.0 vs. 9.6±2.0 mmol/l, P<0.001).
The FH-cohort was subdivided according to the presence or absence of clinically manifest CVD. Of the 197 FH subjects
Discussion
In this study, PON1 and PON2 variants were investigated in a FH cohort to assess the possible association between these mutations and the susceptibility for CVD. For this reason we divided the FH population in to groups of subjects with and without clinical manifestations of CVD. With regard to PON1, we could not find a significant association between either the 55 or 192 polymorphisms and the presence of clinical manifestations of CVD. This lack of association is in agreement with several
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