Treatment of dyslipidemia: genetic interactions with diet and drug therapy

Loss of apolipoprotein E exacerbates the neonatal lethality of the Smith-Lemli-Opitz syndrome mouse

The Smith-Lemli-Opitz syndrome (SLOS) is caused by a genetic defect in cholesterol biosynthesis; mutations in the enzyme 3ss-hydroxysterol Delta7 reductase (Dhcr7) lead to a failure of cholesterol (and desmosterol) synthesis, with an accumulation of precursor sterols, such as 7-dehydrocholesterol. Extensive genotype-phenotype analyses have indicated that there is considerable variation in the severity of the disease, much of which is not explained by defects in the Dhcr7 gene alone. Factors ranging from variations in maternal-fetal cholesterol transfer during pregnancy, to other genetic factors have been proposed to account for this variability. Variations at the APOE locus affect plasma cholesterol levels in humans and this polymorphic gene has been found to be associated with cardiovascular as well as neurological disorders. This locus has recently been implicated in accounting for some of the variations in SLOS. To address whether maternal hypercholesterolemia can affect fetal outcome, we tested the ability of maternal hypercholesterolemia to rescue the neonatal lethality in a mouse model of SLOS. Maternal hypercholesterolemia, induced by ApoE or Ldl-r deficiency not only failed to ameliorate the postnatal lethality, it increased the prenatal mortality of Dhcr7 deficient pups. Thus the murine data suggest that maternal loss of ApoE or Ldl-r function further exacerbates the neonatal lethality, suggesting they may play a role in maternal transfer of cholesterol to the embryo.

Apolipoprotein E haplotyping by denaturing high-performance liquid chromatography

The apolipoprotein E ( APOE ) gene in humans contains two single-base polymorphisms in exon 4, which result in three common alleles, conventionally named epsilon2 , epsilon3 and epsilon4 . Numerous studies have shown an important association between the epsilon4 variant and an increased risk of Alzheimer's disease; other data suggest a possible linkage of APOE genetic heterogeneity with lipid profile and an increased risk of atherothrombotic stroke and coronary heart disease. APOE genotyping is therefore an increasingly common assay in laboratory medicine. The most widely used technique for APOE genotyping is based upon restriction isotyping, i.e., amplification of the fragment of exon 4 containing the most common sequence variations, followed by enzymatic digestion of the amplicon and fragments analysis by gel electrophoresis. We developed a novel, reliable and fast method that exploits the sensitivity and specificity of denaturing high-performance liquid chromatography in detecting single-nucleotide polymorphisms. We show that, in most cases, with a single chromatographic separation it is possible to correctly identify the six different APOE allelic patterns, without any manipulation after the polymerase chain reaction amplification. When compared to restriction isotyping, our method is much faster, less labor intensive and similarly inexpensive.

Does the level of LDL cholesterol moderate a relationship between DRD4 and novelty seeking?

This study examined the role of low density lipoprotein (LDL) cholesterol level in the association between Dopamine D4 receptor (DRD4) polymorphism and temperament dimension novelty seeking. From the on-going population based study of "Cardiovascular Risk in Young Finns", 78 men and women responded to the novelty seeking scale of the temperament and character inventory [Archives of General Psychiatry 44 (1987) 573] and were apolipoprotein E (apoE) and DRD4 genotyped. DRD4 polymorphism was related to disorderliness, a component of novelty seeking, in subjects with high LDL cholesterol level but not in subjects with low LDL cholesterol level. This finding did not change after adjustment for apoE polymorphism. Our findings suggest that the genetic determination of temperament may be dependent on biological factors, such as LDL cholesterol.

Pharmacogenetics of Lipid-lowering Therapies

Cardiovascular disease is associated with nonmodifiable risk factors such as age, gender, and genetic background, and with modifiable risk factors such as lipid concentrations. Lowering serum lipid levels has been demonstrated to slow the progression of, or even induce regression in, atherosclerosis. However, like any other drug treatment, the magnitude of plasma lipid responses to drug therapies varies considerably among individuals. Pharmacogenetics provides the experimental basis to understand the variability in response to drugs as a function of the individual genetic makeup. Information from small clinical trials reveals that several candidate genes may hold some promise in our quest to predict individual success to hypolipemic drug treatment. However, the current clinical relevance of this knowledge is quite limited due to the small effects observed for each of the genetic markers examined. Future progress in this area will be driven by studying gene-gene and gene-treatment interactions in much larger patient populations.

The APOE locus and the pharmacogenetics of lipid response

Genetic variation at the APOE locus has been associated with plasma lipoprotein concentrations in the fasting (low-density, and high-density lipoproteins and triglycerides), and in the postprandial (triglyceride-rich lipoproteins) states. Resulting from these associations, the APOE locus has been found to be a significant genetic determinant of cardiovascular disease in the general population. Beyond the traditional association studies, APOE genetic variation has been shown to play a significant role, which explains some of the individual variations in therapies aimed at normalizing plasma lipid concentrations. Thus, the APOE E4 allele has been shown in some studies to be associated with increased response to dietary intervention. Conversely, APOE E2 carriers appear to be more responsive to statin therapy. The mechanisms behind these observations, however, have not been elucidated. Moreover, several other gene:environment and gene:therapy interactions have recently been demonstrated, thus further increasing the interest in this remarkable apolipoprotein.

[Apolipoprotein E gene polymorphism and risk of hypercholesterolemia: a case control study in a working population of Valencia]

BACKGROUND

The apolipoprotein E (apoE) gene is polymorphic with three common alleles (epsilon 2, epsilon 3, epsilon 4) whose allelic frequency and association with lipid levels varies from population to population. The aim of this study was to estimate the association between these genetic variants and the risk of hypercholesterolemia in a Mediterranean Spanish population.

PATIENTS AND METHODS

A case-control study in a working population from Valencia was carried out. A total of 330 cases (148 men and 182 women) with moderate hypercholesterolemia (total cholesterol > 200 mg/dl or with lipid lowering treatment) and age range 20 to 60 years, were identified. 330 normocholesterolemic controls matched by age and sex were selected. From all of them data of apoE genotype, body mass index, lipid and lipoprotein levels, socioeconomic and life-style variables were obtained.

RESULTS

The epsilon 2 allele frequency was statistically lower in cases (0.033) than in controls (0.086). The epsilon 4 allele frequency was higher in cases (0.115) than in controls (0.039). In the crude logistic regression analysis, the apoE polymorphism was related (p < 0.001) to the risk of hypercholesterolemia. After adjustment by age, body mass index, educational level, tobacco smoking, alcohol consumption and physical activity the epsilon 2 allele was associated with a lower risk of hypercholesterolemia (odds ratio [OR] = 0.36; 95% confidence interval (CI): 0.20-0.64), and the epsilon 4 allele was associated with a higher risk (OR = 3.04; 95% CI: 1.82-5.06).

CONCLUSIONS

The apoE genotype was significantly related to the risk of moderate hypercholesterolemia in the Mediterranean Spanish population.

Genetic determinants of plasma lipid response to dietary intervention: the role of the APOA1/C3/A4 gene cluster and the APOE gene

Polymorphisms at the APOA1/C3/A4 gene cluster and the APOE gene have been extensively studied in order to examine their potential association with plasma lipid levels, coronary heart disease risk and more recently with inter-individual variability in response to dietary therapies. Although the results have not been uniform across studies, the current research supports the concept that variation at these genes explains a significant, but still rather small, proportion of the variability in fasting and postprandial plasma lipid responses to dietary interventions. This information constitutes the initial frame to develop panels of genetic markers that could be used to predict individual responsiveness to dietary therapy for the prevention of coronary heart disease. Future progress in this complex area will come from experiments carried out using animal models, and from carefully controlled dietary protocols in humans that should include the assessment of several other candidate gene loci coding for products that play a relevant role in lipoprotein metabolism (i.e. APOB, CETP, LPL, FABP2, SRBI, ABC1 and CYP7).

Genetic predictors of plasma lipid response to diet intervention

There is a growing interest in determining the genetic predictors of plasma lipid response to diet intervention. Several candidate gene loci, namely, apolipoprotein (APO) A1, APOA4, APOC3, APOB, APOE, CETP, LPL, and FABP2, have been shown to explain a significant, but still rather small, proportion of the interindividual variability in dietary response. Other gene loci code for products that play a relevant role in lipoprotein metabolism and are prime candidates for future studies (ie, CYP7). Future progress in this complex area will come from experiments carried out using animal models and from carefully controlled dietary protocols in humans.