Genetic and biochemical factors associated with variation in blood pressure in a genetic isolate

Pedigree structure and kinship measurements of a mid-Michigan community: a new North American population isolate identified

Previous studies identified a cluster of individuals with an autosomal recessive form of deafness that reside in a small region of mid-Michigan. We hypothesized that affected members from this community descend from a defined founder population. Using public records and personal interviews, we constructed a genealogical database that includes the affected individuals and their extended families as descendants of 461 settlers who emigrated from the Eifel region of Germany between 1836 and 1875. The genealogical database represents a 13-generation pedigree that includes 27,747 descendants of these settlers. Among these descendants, 13,784 are presumed living. Many of the extant descendants reside in a 90-square-mile area, and 52% were born to parents who share at least one common ancestor. Among those born to related parents, the median kinship coefficient is 3.7 × 10(-3). While the pedigree contains 2,510 founders, 344 of the 461 settlers accounted for 67% of the genome in the extant population. These data suggest that we identified a new population isolate in North America and that, as demonstrated for congenital hearing loss, this rural mid-Michigan community is a new resource to discover heritable factors that contribute to common health-related conditions.

Genetic determinants of blood pressure regulation

Hypertension is a multifactorial disorder that probably results from the inheritance of a number of susceptibility genes and involves multiple environmental determinants. Existing evidence suggests that the genetic contribution to blood pressure variation is about 30-50%. Although a number of candidate genes have been studied in different ethnic populations, results from genetic analysis are still inconsistent and specific causes of hypertension remain unclear. Furthermore, the abundance of data in the literature makes it difficult to piece together the puzzle of hypertension and to define candidate genes involved in the dynamic of blood pressure regulation. In this review, we attempt to highlight the genetic basis of hypertension pathogenesis, focusing on the most important existing genetic variations of candidate genes and their potential role in the development of this disease. Our objective is to review current knowledge and discuss limitations to clinical applications of genotypic information in the diagnosis, evaluation and treatment of hypertension. Finally, some principles of pharmacogenomics are presented here along with future perspectives of hypertension.

Association of two apolipoprotein A-I gene MspI polymorphisms with lipid and blood pressure levels

INTRODUCTION

Two MspI polymorphisms in the ApoA-I gene (G-75A and C83T) have been shown to be associated with plasma HDL-cholesterol levels.

METHODS

We used a PCR-based RFLP method to determine the association of these polymorphisms with lipid parameters in 271 non-diabetic, normotriglyceridaemic Chinese subjects, of whom 104 were patients with hypertension, with 10.2% having hypercholesterolaemia and the remainder were controls.

RESULTS

As expected, the hypertensive group had higher blood pressure and indices of obesity, and a more adverse lipid profile. No differences in the ApoA-I G-75A genotype or allele frequency distributions between the controls and patients were identified. However, there was a significantly lower frequency of the CT genotype (p=0.012) and T allele (p=0.011) in the affected subjects with hypercholesterolaemia or hypertension. Similarly, blood pressure and triglyceride levels were significantly lower and HDL-cholesterol levels significantly higher in the subjects with the CT genotype compared to those with the CC genotype (p<0.05). However, the G-75A genotypes did not appear to influence the lipid or blood pressure levels. The -75A allele frequency was higher in our healthy controls than an equivalent Caucasian population (31.1% vs. 18.3%, p<0.001), whereas the 83T allele frequency was similar between the healthy Chinese and Caucasian groups.

CONCLUSION

The 83T allele may be associated with a better lipid profile and blood pressure levels in this group of Chinese subjects.

Genetic influences on blood pressure within the Stanislas Cohort

OBJECTIVE

To investigate the association of 21 polymorphisms within 13 genes, APOE, APOB, APOC3, CETP, LPL, PON1, MTHFR, FGB, F5, GPIIIa, SELE, ACE and AGT, with inter-individual blood pressure (BP) variation.

PARTICIPANTS

Seven hundred and seventy-six men and 836 women, free of antihypertensive and lipid-lowering medications, were selected from the Stanislas Cohort.

RESULTS

ANOVA on blood pressure values after adjustment for covariates [age, body mass index (BMI), contraceptive pill, tobacco and alcohol] showed that lipoprotein lipase (LPL) Ser447Ter and glycoprotein IIIA (GpIIIa) Pl polymorphisms were significantly associated with BP in women (0.01 < or = P < or = 0.05), whereas BP levels in men were significantly different according to apolipoprotein CIII (APOC3) 3206T/G and -482C/T polymorphisms (P < or = 0.05). In women, compared to the most common allele, the GpIIIa Pl allele was associated with increased mean arterial pressure (MAP) (P < 0.05) and pulse pressure (PP) (P < 0.001), and the LPL 447Ter allele was associated with decreased systolic blood pressure (SBP) and PP levels (0.001 < or = P < or = 0.05). These two polymorphisms appeared to act independently. In men, the APOC3 3206GG genotype was related to decreased diastolic blood pressure (DBP) and MAP levels (P < or = 0.01), and the APOC3 -482T allele with decreased PP levels (P < or = 0.05). The presence of both the -482C allele and the 3206GG genotype was related to decreased DBP, suggesting that specific haplotypes might be involved.

CONCLUSION

The APOC3, LPL and GpIIIa genes were found to be associated with BP levels. The contributions of these genes, although modest, are consistent with the polygenic nature of BP levels.

Association of the D8S282 marker near the lipoprotein lipase gene locus with systolic blood pressure in healthy Chinese subjects

OBJECTIVE

To investigate the association between the marker D8S282 near the lipoprotein lipase (LPL) gene locus, and blood pressure, anthropometric and biochemical parameters in 229 healthy Chinese subjects. METHOD Genotyping was performed using an automated DNA sequencer and the Base ImageIR software. Eight different alleles were identified (272-286 bp) resulting in 15 genotypes in our population. We investigated the association between the common (28.8%) 278 bp allele and the anthropometric and biochemical parameters.

RESULTS

In a tertile analysis, the frequency of the 278 bp allele increased linearly ( P = 0.003) with increasing systolic blood pressure (SBP). The relationship was most evident in the females ( n = 141); SBP was higher in homozygotes for the 278 bp allele (117 +/- 10 mmHg, = 12) than those without this allele (109 +/- 9 mmHg, = 77, 0.05) and was gene-dose dependent, and this difference was more significant after adjusting for age (P = 0.004). No relationship between the locus and the anthropometric or biochemical parameters investigated was observed.

CONCLUSION

The D8S282 marker near the LPL gene locus contributes to the variance of SBP in healthy Hong Kong Chinese subjects, particularly in females.

Genetics of population isolates

Genetic isolates, as shown empirically by the Finnish, Old Order Amish, Hutterites, Sardinian and Jewish communities among others, represent a most important and powerful tool in genetically mapping inherited disorders. The main features associated with that genetic power are the existence of multigenerational pedigrees which are mostly descended from a small number of founders a short number of generations ago, environmental and phenotypic homogeneity, restricted geographical distribution, the presence of exhaustive and detailed records correlating individuals in very well ascertained pedigrees, and inbreeding as a norm. On the other hand, the presence of a multifounder effect or admixture among divergent populations in the founder time (e.g. the Finnish and the Paisa community from Colombia) will theoretically result in increased linkage disequilibrium among adjacent loci. The present review evaluates the historical context and features of some genetic isolates with emphasis on the basic population genetic concepts of inbreeding and genetic drift, and also the state-of-the-art in mapping traits, both Mendelian and complex, on genetic isolates.

Variable association between genetic variation in the CYP7 gene promoter and plasma lipoproteins in three Canadian populations

The promoter sequence variant -278A in the CYP7 gene, which encodes cholesterol 7-alpha hydroxylase, was previously reported to be associated with reduced plasma low density lipoprotein (LDL) cholesterol concentration. We tested for association of CYP7-278A with plasma lipoprotein traits in samples taken from three distinct Canadian populations: 594 Alberta Hutterites, 325 Ontario Oji-Cree and 190 Keewatin Inuit. The CYP7-278A allele frequencies in these three groups were 0.708, 0.466 and 0.490, respectively. The frequencies of CYP7-278A/A homozygotes were 0.481, 0.215 and 0.247, respectively. In the Hutterites, CYP7-278A was associated with reduced plasma HDL-cholesterol and apolipoprotein AI concentration. In the Oji-Cree, CYP7-278A was not significantly associated with any plasma lipoprotein trait. In the Inuit CYP7-278A was associated with elevated plasma total and LDL-cholesterol. There was no consistent relationship between the population mean plasma LDL-cholesterol concentration and the population CYP7-278A frequency. Our findings suggest that the common -278A promoter variant of CYP7 was inconsistently associated with variation in plasma LDL- and HDL-cholesterol in samples from three independent populations. The inconsistencies could be due to differences in genetic background or to unspecified environmental or genetic factors.

Lipoprotein lipase (C/G)447 polymorphism and blood pressure in the Stanislas Cohort

OBJECTIVE

Association between blood pressure and triglyceride levels, and between lipoprotein lipase (LPL) (C/G)447 polymorphism and triglyceride levels has been described. We investigated whether the LPL (C/G)447 polymorphism was associated with blood pressure (BP) levels and longitudinal changes.

DESIGN AND PARTICIPANTS

For cross-sectional analysis, 767 men and 816 women (29-55 years) were selected from the Stanislas Cohort, a cohort of volunteers for a free health check-up. Only subjects without anti-hypertensive or lipid-lowering medication were included in the study. A subset of this sample population, 359 men and 337 women, had been followed during the 11 years prior to recruitment in the Stanislas Cohort and was used for longitudinal analysis.

RESULTS

The cross-sectional study showed that serum triglyceride levels differed significantly according to LPL genotypes in both genders, the G447 allele being associated with the lowest triglyceride levels (P < or = 0.01). Univariate and multivariate analysis found that LPL polymorphism was not related to BP levels in men. In contrast, women with the LPL-G447 allele had lower systolic (SBP) and pulse (PP) pressure levels than those with the LPL-CC genotype (P < or = 0.01 and P < or = 0.05, respectively); this association being independent of triglyceride level. The longitudinal study showed LPL genotype was an independent predictor of PP and SBP follow-up levels in women; changes over 11 years being lower for LPL-G447 allele carriers (P < or = 0.05). These associations were independent of triglyceride level.

CONCLUSION

The LPL-G447 allele was found associated with lower PP and SBP independently of triglyceride level in women. This result suggests that the LPL gene may influence blood pressure.

Exclusion of angiotensinogen gene in molecular basis of human hypertension: sibpair linkage and association analyses in Australian anglo-caucasians

Linkage with essential hypertension has been claimed for a microsatellite marker near the angiotensinogen gene (AGT; chromosome 1q42), as has association for the AGT variants M235T, G(-6)A and A(-20)C. To more rigorously evaluate AGT as a candidate gene for hypertension we performed sibpair analysis with multiple microsatellite markers surrounding this locus and using more sophisticated analysis programs. We also performed an association study of the AGT variants in unrelated subjects with a strong family history (two affected parents). For the linkage study, single and multiplex polymerase chain reaction (PCRs) and automated genescan analysis were conducted on DNA from 175 Australian Anglo-Celtic Caucasian hypertensives for the following markers: D1S2880-(2.1 cM)-D1S213-(2.8 cM)-D1S251-(6.5 cM)-AGT-(2.0 cM) -D1S235. Statistical evaluation of genotype data by nonparametric methods resulted in the following scores: Single-point analysis - SPLINK, P > 0.18; APM method, P > 0.25; ASPEX, MLOD < 0.28; SIB-PAIR, P > 0. 24; Multipoint analysis - MAPMAKER/SIBS, MLOD < 0.24; GENEHUNTER, P > 0.35. Exclusion scores of Lod -4.1 to -5.1 were obtained for these markers using MAPMAKER/SIBS for a lambda(s) of 1.6. The association study of G(-6)A, A(-20)C and M235T variants in 111 hypertensives with strong family history and 190 normotensives with no family history showed significant linkage disequilibrium between particular haplotypes, but we could find no association with hypertension. The present study therefore excludes AGT in the etiology of hypertension, at least in the population of Australian Anglo-Celtic Caucasians studied.

Absence of association between genetic variation in the LIPC gene promoter and plasma lipoproteins in three Canadian populations

The promoter sequence variant -480T in the hepatic lipase gene (LIPC) has been shown to be significantly associated with low post-heparin hepatic lipase activity. Some studies have also found that the -480T variant is associated with elevation in plasma HDL cholesterol. We tested for associations of LIPC -480T with plasma lipoprotein traits in samples taken from three distinct Canadian populations: 657 Alberta Hutterites, 328 Ontario Oji-Cree and 210 Keewatin Inuit. Plasma HL activity was not available for analyses. The LIPC -480T allele frequencies in these three groups, respectively, were 0.219, 0.527 and 0.383, and the prevalence of LIPC -480T/T homozygotes was, respectively, 0.042, 0.274 and 0.167. No significant association was found between LIPC -480T and plasma HDL cholesterol or apolipoprotein AI concentration, after adjusting for covariates including gender and body mass index. There was no consistent relationship between the population mean plasma HDL cholesterol concentration and the population LIPC -480T frequency. Our findings are consistent with the idea that the common promoter variation in LIPC, which has been reported to be associated with variation in post heparin HL activity and HDL triglyceride concentration, is not always associated with variation in plasma HDL cholesterol concentration, possibly due to yet unspecified environmental or genetic factors.

Angiotensinogen variants and human hypertension

The research on molecular genetics of human hypertension aims to identify the loci involved in the regulation of blood pressure, detect gene variants within the identified loci, associate them with intermediate phenotypes, and ultimately estimate their quantitative effects on blood pressure level and their interaction with main environmental factors. So far, the angiotensinogen (AGT) gene is one of the few candidate genes that has been investigated using these multiple statistical, clinical, and biochemical strategies. A highly polymorphic dinucleotide GT repeat (80% heterozygosity) has been used in several linkage studies. Other diallelic polymorphisms, located in the 5' regulatory region of the gene in intronic and exonic sequences, have been described, which were then used in association studies in different clinical settings. Positive associations between the M235T and the G-6A polymorphisms and plasma angiotensinogen levels indicates a pathway by which the AGT locus could be involved in essential hypertension.

Genetic variation in paraoxonase-2 is associated with variation in plasma lipoproteins in Canadian Oji-Cree

We report studies of the association between genetic variation in PON2 and variation in plasma quantitative traits in a sample of 334 non-diabetic Oji-Cree. We detected associations between PON2 variation in codon 148 (Ala --> Gly) and variation in fasting plasma concentrations of total and low-density lipoprotein (LDL) cholesterol and apolipoprotein (apo) B. In particular, homozygotes for a PON2 allele that encoded A148 had significantly higher plasma total, LDL cholesterol and apo B than subjects having the other two genotypes (p < 0.01). Taken together, our results suggest that common genetic variation on chromosome 7q21.3-22.1 in PON2 is associated with significant variation of intermediate traits in plasma lipoprotein metabolism.

Genetic variation in paraoxonase-1 and paraoxonase-2 is associated with variation in plasma lipoproteins in Alberta Hutterites

In a sample taken from the genetically isolated Alberta Hutterites, we previously found that PON1 variation was associated with variation in plasma lipoprotein traits, including LDL and HDL cholesterol. With the recent cloning of the PON1-related gene PON2, we undertook studies of the association between genetic variation in PON2 and variation in plasma quantitative traits variation in a sample of 745 Alberta Hutterites. We found novel genetic associations between PON2 variation and variation in fasting plasma concentrations of total cholesterol and apolipoprotein AI. We confirmed our previously observed significant associations in this study sample between PON1 genetic variation and variation in plasma apo B-related traits, such as LDL, non-HDL and HDL cholesterol and apo B itself. Furthermore, there was almost complete linkage disequilibrium between PON2 alleles G148 and C311. We found no association between PON2 variation and plasma glucose or insulin. Taken together, our results suggest that common genetic variation on chromosome 7q21.3-22.1 in both PON1 and PON2 that affects the amino acid sequence of the respective gene products is associated with significant variation in intermediate traits in plasma lipoprotein metabolism.

Polymorphisms at the angiotensinogen (AGT) and angiotensin II type 1 receptor (AT1R) loci and normal blood pressure

The M235T polymorphism at the angiotensinogen (AGT) locus and the A1166C polymorphism at the angiotensin II type 1 receptor (AT1R) locus have been reported to be associated with hypertension in several populations. We examined these polymorphisms in three samples of healthy Norwegians with respect to normal blood pressure (BP) levels. None of the genotypes defined by the polymorphisms or their combinations were associated with systolic (S) BP (SBP) or diastolic (D) BP (DBP) level. However, there was a trend in all three series that individuals carrying the C allele of the A1166C polymorphism at the AT1R locus (homozygotes as well as heterozygotes) had higher SBP, than AA homozygous individuals. The observation did not reach statistical significance in any of the series. When examining these two polymorphisms with respect to possible variability gene effects on BP in two series of monozygote (MZ) twin pairs, no such effect was detected. We could not detect any interaction between the loci studied with respect to BP level or variability. Thus, neither the AGT locus nor AT1R locus, separately analysed or together, seem to have variability gene effects or definite level gene effects on normal BP.

Analyses of mutations in the human renal kallikrein (hKLK1) gene and their possible relevance to blood pressure regulation and risk of myocardial infarction

The kallikrein-kinin system is involved in the maintenance of blood pressure (BP), and studies have shown an inverse correlation between BP and urinary kallikrein levels. These and other effects, make the human tissue kallikrein (hKLK1) gene a candidate gene with respect to BP regulation as well as risk of myocardial infarction (MI). By analysis for single-stranded conformation polymorphisms (SSCPs), patterns consistent with four different variants of the gene were detected and further characterized by DNA sequencing. Three of the variants have not been described before. Two of the polymorphisms changed the codon for an amino acid. Methods based on the polymerase chain reaction (PCR) were developed to analyze these polymorphisms at the hKLK1 locus. We found no evidence of association between any genotype in the polymorphisms and normal BP level in two series of healthy, unrelated individuals. In a third series, diastolic BP exhibited a weak association with genotypes in three of the four polymorphisms. Since no such association was detected in the other two series, we conclude that no effect on normal BP level is exerted by variants in the hKLK1 as expressed in these polymorphisms. In two series of monozygotic (MZ) twin pairs, there were no differences between genotypes in within-pair difference in systolic BP or diastolic BP. Finally, no differences in allele frequencies or genotype frequencies in the four polymorphisms at the hKLK1 locus were found between a series of young MI survivors and a series of controls. Thus, genes in the four polymorphisms at the hKLK1 locus detected by SSCP and DNA sequencing did not exhibit associations with MI, and had neither "level gene" nor "variability gene" effects on normal blood pressure.

Genetic variants of the renin-angiotensin system and ambulatory blood pressure in essential hypertension

OBJECTIVE

To examine whether the angiotensinogen M235T and angiotensin converting enzyme insertion/deletion (I/D) variants are related to the severity of hypertension in patients with established essential hypertension.

DESIGN

A cross-sectional study.

SETTING

The hypertension clinic of the Benjamin Franklin University Hospital, Free University of Berlin.

PARTICIPANTS

Three hundred and forty-three consecutive Caucasian patients who presented with treated or untreated (n = 115) hypertension were enrolled into the study. Twenty-two patients were excluded from analysis because they had secondary hypertension.

MAIN OUTCOME MEASURES

Angiotensinogen M235T and angiotensin-converting enzyme I/D genotypes, 24 h ambulatory blood pressure values, the number of antihypertensive medications administered and left ventricular dimensions assessed by two-dimensional echocardiography.

RESULTS

Neither the angiotensinogen nor the angiotensin converting enzyme genotype was related significantly to the average ambulatory blood pressure and left ventricular dimensions in hypertensives. Furthermore, neither the number of antihypertensive medications administered to treated patients nor blood pressure levels in untreated patients (n = 115) differed significantly between the genotypic groups.

CONCLUSIONS

These results do not support the hypothesis that the studied molecular variants of the renin-angiotensin system may represent clinically useful markers of the severity of hypertension in Caucasians with established essential hypertension.