Genetic susceptibility to the metabolic syndrome

Tableau d’honneur de la Faculté des études supérieures et postdoctorales, 2004-2005Le syndrome métabolique est caractérisé par un regroupement de facteurs de risque présents chez un même individu et augmentant ainsi ses chances de développer le diabète de type 2 et les maladies cardiovasculaires. Il est donc important de comprendre l’étiologie génétique de ce trait. Dans cette thèse, une multitude d’approches génétiques ont été utilisées afin d’apporter un brin de connaissance sur l’architecture génétique du syndrome métabolique et de ses composantes individuelles. Trois gènes candidats ont été testés incluant le récepteur activé par les proliférateurs de péroxisomes (PPAR) α et PPARγ ainsi que la protéine de transfert des phospholipides (PLTP). Les gènes PPARα et PLTP ont tous deux été associés significativement avec plusieurs variables d’adiposité. Des effets significatifs d’interaction entre les gènes PPARα et PPARγ ont été obtenus pour les paramètres de glucose et d’insuline. Il a aussi été démontré que le polymorphisme PPARα L162V influence les changements de cholestérol-HDL2 suite à un traitement au gemfibrozil. Par la suite, des criblages génomiques ont été effectués sur les concentrations de lipides et de lipoprotéines plasmatiques. Plusieurs régions chromosomiques ont été identifiées incluant 1q43, 11q13 q24, 15q26.1, et 19q13.32 pour le cholestérol-LDL, 12q14.1 pour le cholestérol-HDL, 2p14, 11p13, et 11q24.1 pour les triglycérides, 18q21.32 pour l’apolipoprotéine (apo) B-LDL, et 3p25.2 pour l’apoAI. La contribution génétique à la variation du diamètre principal des particules LDL (DP-LDL) a aussi été étudiée. Les résultats démontrent une forte ressemblance familiale avec des coefficients d’héritabilité de plus de 50%, la présence d’un gène à effet majeur, et une forte évidence de liaison sur le chromosome 17q. Le gène de l’apoH, localisé à cet endroit, a par la suite été significativement associé au DP-LDL, suggérant que ce gène est responsable du signal de liaison observé sur le chromosome 17. Finalement, une variable quantitative du syndrome métabolique a été construite à l’aide d’une analyse factorielle. Un criblage génomique effectué sur cette variable a démontré une évidence de liaison sur le chromosome 15q, suggérant la présence d’un gène à cet endroit contribuant au regroupement des facteurs de risques caractérisant le syndrome métabolique. Plusieurs de ces résultats devront être répliqués, alors que d’autres méritent d’être suivis.The metabolic syndrome is a cluster of interrelated cardiovascular risk factors co-occurring in the same individual. People with this syndrome are at increased risk for developing diabetes mellitus and cardiovascular diseases. Accordingly, it is important to elucidate the genetic aetiology governing this trait in order to better comprehend its pathogenesis. In the present thesis, heritability and complex segregation analyses as well as candidate gene and genome-wide scan approaches have been applied to shed some lights on the genetic architecture of the metabolic syndrome and its individual components. A total of three candidate genes have been investigated including peroxisome proliferator-activated receptor (PPAR) α and PPARγ as well as phospholipid transfer protein (PLTP). It has been shown that polymorphisms in both PPARα and PLTP genes are significantly associated with several indices of adiposity. In addition, significant gene-gene interactions have been observed between PPARα and PPARγ on glucose/insulin parameters. It has also been shown that the HDL2-cholesterol response to gemfibrozil therapy is modulated by the PPARα L162V polymorphism. Genome-wide linkage scans have been performed on lipid and lipoprotein concentrations. Many chromosome regions harbouring lipoprotein/lipid genes have been identified including 1q43, 11q13 q24, 15q26.1, and 19q13.32 for LDL-cholesterol, 12q14.1 for HDL-cholesterol, 2p14, 11p13, and 11q24.1 for triglycerides, 18q21.32 for LDL-apolipoprotein (apo) B, and 3p25.2 for apoAI. The genetic contribution of the variation in LDL peak particle diameter (LDL-PPD) has been also investigated. Overall, the results indicate: 1) that LDL-PPD strongly aggregates within families with heritability estimate above 50%; 2) the existence of a major gene effect affecting the phenotype; and 3) the presence of a major quantitative trait locus located on chromosome 17q. The apo H gene, a positional candidate gene, was then significantly associated with LDL-PPD, suggesting that this gene is responsible for the linkage signal observed on 17q. Finally, factor analyses have been used to construct a quantitative metabolic syndrome variable and a genome-wide linkage scan has been conducted to identify the genomic regions underlying this trait. A major quantitative trait locus has been observed on chromosome 15q suggesting a gene within this region contributing to the clustering of the metabolic syndrome-related phenotypes. Many of these findings must go through independent replication, while others produced new leads that deserve follow-up

Genetics of LDL particle heterogeneity : from genetic epidemiology to DNA-based variations

Substantial evidence exists suggesting that small, dense LDL particles are associated with an increased risk of coronary heart disease. This disease-related risk factor is recognized to be under both genetic and environmental influences. Several studies have been conducted to elucidate the genetic architecture underlying this trait, and a review of this literature seems timely. The methods and strategies used to determine its genetic component and to identify the genes have greatly changed throughout the years owing to the progress made in genetic epidemiology and the influence of the Human Genome Project. Heritability studies, complex segregation analyses, candidate gene linkage and association studies, genome-wide linkage scans, and animal models are all part of the arsenal to determine the susceptibility genes. The compilation of these studies clearly revealed the complex genetic nature of LDL particles. This work is an attempt to summarize the growing evidence of genetic control on LDL particle heterogeneity with the aim of providing a concise overview in one read

NOTCH1 genetic variants in patients with tricuspid calcific aortic valve stenosis

BACKGROUND AND AIM OF THE STUDY: Calcific aortic valve stenosis (AS) affects 2-5% of the population aged > 65 years. Functional DNA variants at the NOTCH1 locus result in bicuspid aortic valve (BAV) and severe valve calcification. The contribution of these variants to AS in the population with tricuspid aortic valve (TAV) remains to be determined. METHODS: Fourteen genetic variants surrounding the NOTCH1 gene were genotyped, including rare mutations previously reported, and common polymorphisms. The study involved 457 French Canadian patients with severe tricuspid AS. Genotyping was carried out using the Illumina BeadXpress platform. Allele frequencies of common single nucleotide polymorphisms (SNPs) for patients with AS were compared to a shared control group of European ancestry (n = 3,294). In total, 88 ancestry-informative markers were used to correct for population stratification. RESULTS: The mutation R1107X, previously associated with AS and BAV, was identified in a relatively young patient (aged 58 years). The mutations R1279H and V2285I were detected in 18 and 14 heterozygotes, respectively. A common polymorphism (rs13290979) located in intron 2 was significantly associated with AS (p = 0.003), which remained significant after correction for multiple testing. However, this association was no longer significant after accounting for population stratification (p = 0.088). CONCLUSION: In this study, rare functional variants were found in the NOTCH1 gene in a French Canadian population of patients with severe tricuspid AS. This also suggests, for the first time, the presence of a common polymorphism in this gene conferring susceptibility to AS

Acetylsalicylic acid, aging and coronary artery disease are associated with ABCA1 DNA methylation in men

BACKGROUND: Previous studies have suggested that DNA methylation contributes to coronary artery disease (CAD) risk variability. DNA hypermethylation at the ATP-binding cassette transporter A1 (ABCA1) gene, an important modulator of high-density lipoprotein cholesterol and reverse cholesterol transport, has been previously associated with plasma lipid levels, aging and CAD, but the association with CAD has yet to be replicated. RESULTS: ABCA1 DNA methylation levels were measured in leucocytes of 88 men using bis-pyrosequencing. We first showed that DNA methylation at the ABCA1 gene promoter locus is associated with aging and CAD occurrence in men (P < 0.05). The latter association is stronger among older men with CAD (≥61 years old; n = 19), who showed at least 4.7% higher ABCA1 DNA methylation levels as compared to younger men with CAD (<61 years old; n = 19) or men without CAD (n = 50; P < 0.001). Higher ABCA1 DNA methylation levels in older men were also associated with higher total cholesterol (r = 0.34, P = 0.03), low-density lipoprotein cholesterol (r = 0.32, P = 0.04) and triglyceride levels (r = 0.26, P = 0.09). Furthermore, we showed that acetylsalicylic acid therapy is associated with 3.6% lower ABCA1 DNA methylation levels (P = 0.006), independent of aging and CAD status of patients. CONCLUSIONS: This study provides new evidence that the ABCA1 epigenetic profile is associated with CAD and aging, and highlights that epigenetic modifications might be a significant molecular mechanism involved in the pathophysiological processes associated with CAD. Acetylsalicylic acid treatment for CAD prevention might involve epigenetic mechanisms

High expression of the Pi-transporter SLC20A1/Pit1 in calcific aortic valve disease promotes mineralization through regulation of Akt-1

The regulation of phosphate (Pi) handling is crucial during calcification of the aortic valve. Gene profiling of Pi transporters revealed that VIC culture expresses SLC201A1/Pit1 and SLC20A2/Pit2. On exposure to a mineralizing medium (2 mM Pi), the expression of Pi transporters in VIC culture is increased several folds, with the highest magnitude for SLC20A1. By using siRNAs, we established that silencing SLC20A1 significantly reduced Pi-induced mineralization of VICs. In human pathological specimens, we found that the expression of SCL20A1 was increased in CAVD tissues compared to control nonmineralized aortic valves. Treatment of VIC culture with Pi promoted the loss of mitochondrial membrane potential (DYm) and cytochrome c release within the cytosol, leading to apoptosis. Inhibition of Pi transporters with phosphonoformic acid (PFA) prevented Pi-mediated apoptosis of VICs. Moreover, we discovered that the level of the Akt-1 transcript is diminished in CAVD tissues compared with control valves. Accordingly, treatment with Pi caused a reduction of the Akt-1 transcript in VIC culture, and treatment with PFA or siRNA against SLC20A1 restored the level of Akt-1. Overexpression of Akt-1 (pCMVAkt-1) prevented both Pi-induced apoptosis and mineralization of VIC culture. These results strongly suggest that overexpression of SLC20A1 promotes apoptosis and mineralization by altering the level of Akt-1

Gene expression variability in subcutaneous and omental adipose tissue of obese men

We investigated interindividual variability in gene expression in abdominal subcutaneous (SC) and omental (OM) adipose tissue of 10 massively obese men. Affymetrix human U133A microarrays were used to measure gene expression levels. A total of 6811 probesets generated significant signal in both depots in all samples. Interindividual variability in gene expression was rather low, with more than 90% of transcripts showing a coefficient of variation (CV) lower than 23.6% and 21.7% in OM and SC adipose tissues, respectively. The distributions of CV were similar between the two fat depots. A set of highly variable genes was identified for both tissues on the basis of a high CV and elevated gene expression level. Among the set of highly regulated genes, 18 transcripts were involved in lipid metabolism and 28 transcripts were involved in cell death for SC and OM samples, respectively. In conclusion, gene expression interindividual variability was rather low and globally similar between fat compartments, and the adipose tissue transcriptome appeared as relatively stable, although specific pathways were found to be highly variable in SC and OM depots

The T111I mutation in the EL gene modulates the impact of dietary fat on the HDL profile in women

The objective of the present study was to examine the impact of the T111I missense mutation in exon 3 of the endothelial lipase (EL) gene on HDL and its potential interaction effect with dietary fat. The study sample included 281 women and 216 men aged between 17 and 76 years from the Québec Family Study. Plasma HDL3-C levels of I111I homozygote women were higher compared with those of women carrying the wild-type allele (P 0.03). These differences were not attenuated when adjusted for levels of obesity and were not observed among men. Dietary PUFA interacted with the T111I mutation to modulate apolipoprotein A-I (apoA-I) and HDL3-C levels among women. Specifically, a diet rich in PUFA was associated with increased apoA-I levels among women carriers of the I111 allele and with decreased apoA-I among women homozygotes for the wild-type allele (P 0.002). A similar interaction was observed with plasma HDL3-C levels (P 0.003). These interactions were not observed among men. In conclusion, the EL T111I mutation appears to have a modest effect on plasma HDL levels. The gene-diet interaction among women, however, suggests that the T111I missense mutation may confer protection against the lowering effect of a high dietary PUFA intake on plasma apoA-I and HDL3-C levels.—Paradis, M-E., P. Couture, Y. Bossé, J-P. Després, L. Pérusse, C. Bouchard, M-C. Vohl, and B. Lamarche. The T111I mutation in the EL gene modulates the impact of dietary fat on the HDL profile in women