Cardiovascular risk factors in a French Canadian population: resolution of genetic and familial environmental effects on blood pressure using twins, adoptees, and extensive information on environmental correlates

Mitochondrial tRNA Mutations Associated With Essential Hypertension: From Molecular Genetics to Function

Essential hypertension (EH) is one of the most common cardiovascular diseases worldwide, entailing a high level of morbidity. EH is a multifactorial disease influenced by both genetic and environmental factors, including mitochondrial DNA (mtDNA) genotype. Previous studies identified mtDNA mutations that are associated with maternally inherited hypertension, including tRNA^Ile m.4263A>G, m.4291T>C, m.4295A>G, tRNA^Met m.4435A>G, tRNA^Ala m.5655A>G, and tRNA^Met/tRNA^Gln m.4401A>G, et al. These mtDNA mutations alter tRNA structure, thereby leading to metabolic disorders. Metabolic defects associated with mitochondrial tRNAs affect protein synthesis, cause oxidative phosphorylation defects, reduced ATP synthesis, and increase production of reactive oxygen species. In this review we discuss known mutations of tRNA genes encoded by mtDNA and the potential mechanisms by which these mutations may contribute to hypertension.

Mitochondrial genome mutations in 13 subunits of respiratory chain complexes in Chinese Han and Mongolian hypertensive individuals

Mitochondrial DNA (mtDNA) mutations are associated with cardiovascular disease, including hypertension (HTN). Here we performed a genetic and molecular analysis of 13 mtDNA-encoded subunits of respiratory chain complexes in 100 Chinese Han and 80 Mongolian HTN cases, and 100 Han and 42 Mongolian normotension subjects. The total cholesterol of the Mongolian normotensive subjects was higher than that of the Han normotensive group (p < .05). Sequence analysis identified 636 point mutations in the 13 mtDNA-encoded subunits in the Han and Mongolian hypertensive individuals, including 66 in NADH dehydrogenase subunit 1(ND1), 62 in ND2, 71 in COI, 29 in COII, 17 in ATP8, one in ATP6/8, 49 in ATP6, 27 in COIII, 27 in ND3, 14 in ND4L, 74 in ND4, 97 in ND5, 24 in ND6, and 78 in CYTB. Eight of these point mutations were present at significantly different frequencies in Han and Mongolian hypertensive individuals. Thirty-one point mutations were present only in Mongolian hypertensive individuals, while 73 were present only in Han hypertensive individuals. The relation between point mutations in 13 mtDNA-encoded subunits of respiratory chain complexes and HTN is worth to further research in future; however, the functional effects of these mutations require elucidation.

New Developments in the Genetics of Hypertension: What Should Clinicians Know?

It has long been known that genetic factors play a major role in determining an individual's propensity to hypertension. In recent years, there has been major progress towards realizing the goal of identifying the specific genetic factors that lead to alterations in blood pressure. Of particular note, new genes regulating renal sodium handling and aldosterone regulation have been discovered via the study of rare Mendelian disorders. Similarly, a number of large genome-wide association studies have been completed, which have added to our understanding as well. Here, recent progress in the genetics of hypertension will be reviewed, with an emphasis towards highlighting specific areas where clinical practice has already or will soon be affected.

Mitochondrial tRNA mutations in Chinese hypertensive individuals

PURPOSE

Hypertension is a very important risk factor for cardiac vascular disease. The previous studies showed that mitochondrial DNA mutations are associated with cardiovascular disease, including hypertension.

METHODS

In this study we did systematical analysis on the total 22 mitochondrial tRNAs and the clinical, genetic and molecular changes of 140 Chinese hypertension and 124 controls.

RESULTS

This analysis identified 22 nucleotide changes among 15 different tRNA genes. There are 15 mutations with CI (Conservation index) larger than 75%. Of these, there are 26 patients with CI larger than 75% in the HTN group, higher than the 6 subjects in the control group (P=0.00). The tRNA(Phe) G586A, tRNA(Lys) G8313A and tRNA(His) G12147A mutations create highly conservative base-pairings on the D-stem, tRNA(Lys) G8342A on the T-stem, tRNA(Phe) T616C, tRNA(Ala) T5628C, tRNA(Tyr) G5856A and tRNA(Thr) A15924G on the AC stem, tRNA(Leu(CUN)) G12300A on the AC loop, tRNA(Met) C4467T, tRNA(Trp) T5578C, tRNA(Lys) A8296G, tRNA(Arg) T10463C and tRNA(Thr) C15891T on ACC stem, and tRNA(Ser(UCN)) C7492T on D-A junction, while the other tRNA variants were polymorphisms. The pedigrees of PLAH78 carrying the T5578C, PLAH84 carrying the C4467T, PLAH60 carrying the T5628C and PLAH118 carrying the C7492T mutation exhibited maternal transmission of essential hypertension. Sequence analysis of their mitochondrial genomes revealed the presence of T5578C, C4467T, T5628C or C7492T mutations but the absence of other functionally significant mutations in all matrilineal relatives of these families.

CONCLUSIONS

These tRNAs mutations, associated with altered structures of tRNAs and mitochondrial dysfunction, may contribute to the hypertension in Chinese population. A lot of work still should be done for the mechanism and functional effect of the mtDNA mutation on hypertension.

Genetic contribution to the variance of blood pressure and heart rate: a systematic review and meta-regression of twin studies

The genetic contribution of blood pressure and heart rate (HR) varied widely between studies. Demographic factors such as ethnicity, age and/or sex might explain some of the heterogeneity. We performed a systematic review focusing on four phenotypes: systolic blood pressure (SBP), diastolic blood pressure (DBP), HR and pulse pressure (PP). Meta-regression was conducted to analyze potential factors in relation to SBP and DBP heritability. A total of 10,613 independent twins that came from 17 studies were included in the analysis. The weighted mean value of heritability for SBP and DBP was 0.54 (95% CIs: 0.48-0.60) and 0.49 (95% CIs: 0.42-0.56). Comparatively, three studies of HR and four studies of PP heritability were limited for the heterogeneity test. Meta-regression showed that, on average, SBP heritability with additive genes/unique environment (AE) model tend to have a higher heritability than additive genes/shared environment/unique environment (ACE) model (coefficient = 0.0947, p = .0142). A similar result was found for DBP as well. No other factors such as sex, age, ethnicity, publication year were significantly associated with heritability variance. Our study shows heritability estimates based on twin studies of both SBP and DBP are around 50%, using an AE rather than an ACE model; the variance due to C ended up in A, suggesting that the AE model may overestimate heritability if a small contribution of shared environment exists.

Systematic analysis of the clinical and biochemical characteristics of maternally inherited hypertension in Chinese Han families associated with mitochondrial

BACKGROUND

Mitochondrial DNA mutations may be associated with cardiovascular disease, including the common cardiac vascular disease, hypertension.

METHODS

In this study we performed segregation analysis and systematically evaluated the entire mitochondrial genome in nine maternally inherited hypertension probands from Chinese Han families. We also performed clinical, genetic and molecular characterization of 74 maternally inherited members from these families and 216 healthy controls.

RESULTS

In the maternally inherited members, 12 had coronary heart disease (CHD), six had cerebrovascular disease, five had diabetes, nine had hyperlipidemia and three had renal disease. Laboratory tests showed that the sodium and potassium levels in blood of the maternally inherited members were higher than those of the control group (P < 0.01), while no differences were observed in fasting blood glucose (FBG), total cholesterol (TC), triglyceride, low density lipoprotein cholesterol (LDL-c) and creatinine levels (P > 0.05). The high density lipoprotein cholesterol (HDL-c) level of the maternally inherited members was lower than that of the control group (P = 0.04). The whole mitochondrial DNA sequence analysis revealed a total of 172 base changes, including 17 in ribosomal RNA (rRNA) genes, four in transfer RNA (tRNA) genes, and 22 amino acid substitutions. The remainder were synonymous changes or were located in non-coding regions. We identified seven amino acid changes in the nine maternally inherited hypertension families, including four mutations in ATPase6 and three in Cytb. More interestingly, tRNA(Ser(UCN)) 7492 T > C was absent in controls and was present in <1% of 2704 mtDNAs, indicating potential functional significance.

CONCLUSIONS

This study showed that mutations in mtDNA may contribute to the pathogenesis of hypertension in these Chinese Han families. In the near future, identification of additional mtDNA mutations may indicate further candidate genes for hypertension.

The molecular basis of blood pressure variation

Advances in genetic mapping and sequencing techniques have led to substantial progress in the study of rare monogenic (Mendelian) forms of abnormal blood pressure. Many disease-defining pathways for hypertension have been identified in the past two decades. Perturbations in renal salt handling appear to be a common mechanism underlying these rare syndromes of hypertension. Excess activation at various points in the mineralocorticoid signaling pathway and malfunctioning of the autonomic (specifically sympathetic) nervous system have both been implicated in inducing hypertension, while complementary studies examining low blood pressure phenotypes have identified novel pathways exclusively linked to renal salt wasting in either the thick ascending limb or the distal nephron. The genetic defects and the physiological and cellular pathways affected in these various disorders are reviewed here. Importantly, studies have suggested that genetic variation affecting these same genes and pathways may play an important role in explaining the variation of blood pressure levels in the general population. The investigation of rare syndromes of human blood pressure variation has important implications for improving the diagnosis and treatment of hypertension.

Heritability of blood pressure increases during mental stress

We studied the influence of mental stress on the contributions of genes and environment to individual variation in systolic (SBP) and diastolic (DBP) blood pressure by structural equation modelling in 320 adolescent male and female twins. Blood pressure data were collected during rest and during a reaction time and a mental arithmetic task. Univariate analyses of SBP and DBP showed familial aggregation for blood pressure. A genetic explanation for this resemblance was most likely, although during rest conditions a model that attributed familial resemblance to shared environmental factors, also fitted the data. There was no evidence for sex differences in heritabilities. Multivariate analyses showed significant heterogeneity between sexes for the intercorrelations of the blood pressure data measured under different rest and task conditions. Multivariate genetic analyses were therefore carried out separately in males and females. For SBP and DBP in females and for SBP in males an increase in heritabilities was seen for blood pressure measured during stress, as compared to rest measurements. The influence of shared environ-mental factors decreased during stress. For DBP in males no significant contributions of shared environment were found. The multivariate analyses indicated that the same genetic and environmental influences are expressed during rest and stress conditions.

Angiotensin II type 1 receptor polymorphisms and susceptibility to hypertension: a HuGE review

The angiotensin II type 1 receptor (AGTR1) plays an integral role in blood pressure control, and is implicated in the pathogenesis of hypertension. Polymorphisms within this gene have been extensively studied in association with hypertension; however, findings are conflicting. To clarify these data, we conducted a systematic review of association studies of AGTR1 polymorphisms and hypertension, and performed a meta-analysis of the rs5186 variant. Results show that the currently available literature is too heterogeneous to draw meaningful conclusions. The definition of hypertension and gender composition of individual studies helps to explain this heterogeneity. Although the structure and splicing pattern of AGTR1 would suggest a likely effect of polymorphisms within the promoter region on gene function, few studies have been conducted thus far. In conclusion, there is insufficient evidence that polymorphisms in the AGTR1 gene are risk factors for hypertension. However, most studies are inadequately powered, and larger well-designed studies of haplotypes are warranted.

Genetics of arterial hypertension and hypotension

Human hypertension affects affects more than 20% of the adult population in industrialized countries, and it is implicated in millions of deaths worldwide each year from stroke, heart failure and ischemic heart disease. Available evidence suggests a major genetic impact on blood pressure regulation. Studies in monogenic hypertension revealed that renal salt and volume regulation systems are predominantly involved in the genesis of these disorders. Mutations here affect the synthesis of mineralocorticoids, the function of the mineralocorticoid receptor, epithelial sodium channels and their regulation by a new class of kinases, termed WNK kinases. It has been learned from monogenic hypotension that almost all ion transporters involved in the renal uptake of Na(+) have a major impact on blood pressure regulation. For essential hypertension as a complex disease, many candidate genes have been analysed. These include components of the renin-angiotensin-aldosterone system, adducin, beta-adrenoceptors, G protein subunits, regulators of G protein signalling (RGS) proteins, Rho kinases and G protein receptor kinases. At present, the individual impact of common polymorphisms in these genes on the observed blood pressure variation, on risk for stroke and as predictors of antihypertensive responses remains small and clinically irrelevant. Nevertheless, these studies have greatly augmented our knowledge on the regulation of renal functions, cellular signal transduction and the integration of both. Together, this provides the basis for the identification of novel drug targets and, hopefully, innovative antihypertensive drugs.

Association of alpha1A adrenergic receptor gene variants on chromosome 8p21 with human stage 2 hypertension

OBJECTIVE AND DESIGN

We previously reported a significant linkage between human chromosome 8p22 with essential hypertension and systolic blood pressure levels. On the basis of this, we used an efficient age, sex and area-matched case-control scheme to test the association of the polymorphisms in the human alpha1A adrenergic receptor (ADRA1A) gene, located on chromosome 8p21-p11.2, with essential hypertension in a northern Han Chinese population.

METHODS

Seven polymorphisms were identified by direct sequencing of genomic DNA derived from 48 randomly recruited hypertensive and 48 healthy subjects. They were also examined for association with essential hypertension in 480 stage 2 hypertensive individuals and their individually matched controls.

RESULTS

We observed significantly higher frequencies of the 347Arg allele and 2547G alleles in the cases compared with their controls (P = 0.04 and 0.007, respectively). McNemar's test revealed that carriers of 2547G alleles were at a greater risk of essential hypertension with an odds ratio of 3.00 [95% confidence interval (CI) 1.23-8.35]. We then performed a conditional logistic regression to adjust the effects of conventional risk factors, revealing an odds ratio of 2.84 for carriers of the 2547G allele (95% CI 1.15-6.99). With the haplotypic probabilities estimated using PHASE software, we performed haplotype trend regression analysis, showing a significant association between haplotype 7 and essential hypertension (P = 0.02), after adjustment for conventional risk factors.

CONCLUSIONS

Our findings suggest that the genetic variations in the ADRA1A gene are significantly associated with essential hypertension, and may play an important role in the development of essential hypertension in this Chinese population.

Molecular genetics of human hypertension

EH (essential hypertension) is a major public health problem in many countries due to its high prevalence and its association with coronary heart disease, stroke, renal disease, peripheral vascular disease and other disorders. Epidemiological studies have demonstrated that EH is heritable. Owing to the fact that blood pressure is controlled by cardiac output and total peripheral resistance, many molecular pathways are believed to be involved in the disease. In this review, recent genetic studies investigating the molecular basis of EH, including different molecular pathways, will be highlighted.

Alpha-adducin gene polymorphism is associated with essential hypertension in Chinese: a case-control and family-based study

OBJECTIVE

A polymorphism at position 460(G <-- W) of the alpha-adducin gene was found to be associated with essential hypertension in some but not all studies. The aim of the present study was to further investigate the association of the alpha-adducin 460W allele with essential hypertension in Chinese population.

METHODS

Individuals from a population-based sample (n = 748) and 95 nuclear families and 47 discordant sibships were studied by questionnaire as well as by physical examination and biochemical analyses. The alpha-adducin gene G460W polymorphism was determined by polymerase chain reaction and restriction enzyme digestion. Chi-square test, one-way analysis of variance, logistic regression, linear regression, haplotype-based haplotype relative risk and transmission/disequilibrium test analyses were used to determine the association between the alpha-adducin G460W polymorphism and essential hypertension.

RESULTS

In the case-control study, the prevalence of hypertension was higher in individuals with the WW genotype (40.0%) as compared with those with the GW and GG genotype (31.7%) (chi2 = 4.768, P = 0.029, odds ratio = 1.43). Adjusted for the conventional risk factors of hypertension, alpha-adducin polymorphism still plays an independent role on systolic blood pressure. We confirmed the results of our case-control study by observing a significant preferential transmission of the 460W allele of the alpha-adducin to the affected subjects in another northern Chinese population (for haplotype-based haplotype relative risk, chi2 = 6.24, P = 0.01; and for the transmission/disequilibrium test, chi2 = 4.69, P = 0.03).

CONCLUSIONS

The present findings show a positive association between the alpha-adducin G460W polymorphism and essential hypertension in a northern Chinese population. This evidence indicates that the alpha-adducin gene may be a susceptible gene to essential hypertension.

Health Promotion and Cardiovascular Health in Adult Monozygotic Twins

PURPOSE

To describe relationships among cardiovascular health indicators and health-promoting behaviors in adult monozygotic (MZ) twins.

DESIGN

The descriptive, correlational design with a convenience sample of adult twins (77 twin pairs, or N = 154) obtained during a public conference held in the Midwestern United States.

METHODS

Bivariate correlational analyses with twins as their own controls and with each twin pair. This double-entry approach allowed for the creation of a symmetrical scatterplot of twin characteristics and accounted for the genuine effects of outliers. Relationships were examined among cardiovascular health indicators (systolic and diastolic blood pressures, body mass indices, serum lipids) and health-promoting behaviors.

FINDINGS

Thirteen intermeasure correlations for cardiovascular indicators were significant, and 74% of the variance in the cardiovascular health indicators was shared within each twin pair, thus indicating evidence of common behavioral and genetic influences.

CONCLUSIONS

Both behavioral and genetic factors contributed to cardiovascular health in adult MZ twins who shared close familial and environmental ties. This finding is consistent with previous literature identifying behavioral and genetic effects on the development of cardiovascular disease.

[Genetics of obesity and metabolic complications in the Quebec Family Study]

Considerable progress has been accomplished over the past 10 years in the understanding of the genetic basis of obesity and its related metabolic complications. It is now well established that obesity aggregates within families and that genes are partly responsible for this familial aggregation. The number of genes potentially involved in obesity continues to grow. This review summarizes the evidence accumulated so for regarding the contribution of genetic factors in obesity and the number of gene and loci linked to obesity in the Quebec Family Study.

Sex differences in heritability of sensitization to Blomia tropicalis in asthma using regression of offspring on midparent (ROMP) methods

A genetic basis for asthma- and atopy-related quantitative traits, such as allergen-specific immunoglobulin E (IgE) levels, has been suggested by the observed familial aggregation of these traits in temperate climates. Less information is available for tropical climates, where different allergens may predominate. Sensitivity to the mite Blomia tropicalis is related to asthma in tropical climates, but heritability of B. tropicalis sensitivity and the impact of age, sex, and other environmental covariates on heritability have not been widely explored. Total and specific IgE levels were measured by immunochemiluminescent assay in 481 members of 29 Barbadian families (comprised of 340 parent-offspring trios or pairs) ascertained through two asthmatic siblings. Trait heritability was estimated using regression of offspring on mid-parent (ROMP) and pairwise correlation analysis of unadjusted IgE levels and on residual values after adjustment for covariates. Heritability of IgE levels to the major antigen of B. tropicalis (Blo t M) estimated by ROMP in 180 complete parent-offspring trios was 0.56. Heritability was consistently greater for male offspring than for female offspring. Similar sex-specific patterns were observed for specific IgE to Dermatophagoides pteronyssinus and total IgE levels and were relatively unaffected by adjustment for covariates. Pairwise correlational analyses of specific and total IgE levels showed similar results. Moderate heritability of Blo t M IgE levels was detected in these Barbadian families and was greater for sons than daughters. Adjustment for covariates had minimal impact. This suggests that future investigations of genetic determinants of IgE levels should include approaches that allow for potential sex differences in their expression.

Hypertension in black people: study of specific genotypes and phenotypes will provide a greater understanding of interindividual and interethnic variability in blood pressure regulation than studies based on race

Hypertension is more frequent and more severe in some Black populations. Although many studies have focused on hypertension in black people in an attempt to understand the genetic and environmental factors that regulate blood pressure, this approach has not been productive. Study of the relationship between specific phenotypes and genotypes, both within and across ethnic groups, is more likely to advance our understanding of the regulation of blood pressure than studies focused on race and blood pressure.

NHLBI family blood pressure program: methodology and recruitment in the HyperGEN network. Hypertension genetic epidemiology network

PURPOSE

Hypertension is a common precursor of serious disorders including stroke, myocardial infarction, congestive heart failure, and renal failure in whites and to a greater extent in African Americans. Large genetic-epidemiological studies of hypertension are needed to gain information that will improve future methods for diagnosis, treatment, and prevention of hypertension, a major contributor to cardiovascular morbidity and mortality.

METHODS

We report successful implementation of a new structure of research collaboration involving four NHLBI "Networks," coordinated under the Family Blood Pressure Program. The Hypertension Genetic Epidemiology Network (HyperGEN) involves scientists from six universities and the NHLBI who seek to identify and characterize genes promoting hypertension. Blood samples and clinical data were projected to be collected from a sample of 2244 hypertensive siblings diagnosed before age 60 from 960 sibships (half African-American) with two or more affected persons. Nonparametric sibship linkage analysis of over one million genotype determinations (20 candidate loci and 387 anonymous marker loci) was projected to have sufficient power for detecting genetic loci promoting hypertension. For loci showing evidence for linkage in this study and for loci reported linked or associated with hypertension by other groups, genotypes are compared in hypertensive cases versus population-based controls to identify or confirm genetic variants associated with hypertension. For some of these genetic variants associated with hypertension, detailed physiological and biochemical characterization of untreated adult offspring carriers versus non-carriers may help elucidate the pathophysiological mechanisms that promote hypertension.

RESULTS

The projected sample size of 2244 hypertensive participants was surpassed, as 2407 hypertensive individuals (1262 African-Americans and 1145 whites) from 917 sibships were examined. Detailed consent forms were designed to offer participants several options for DNA testing; 94% of participants gave permission for DNA testing now or in the future for any confidential medical research, with only 6% requesting restrictions for tests performed on their DNA. Since this is a family study, participants also are asked to list all first degree relatives (along with names, addresses, and phone numbers) and to indicate for each relative whether they were willing to allow study staff to make a contact. Seventy percent gave permission to contact some relatives; about 30% gave permission to contact all first degree relatives; and less than 1% asked that no relatives be contacted. Successes after the first four years of this study include: 1) productive collaboration of eight centers from six different locations; 2) early achievement of recruitment goals for study participants including African-Americans; 3) an encouraging rate of consent for DNA testing (including future testing) and relative contacting; 4) completed analyses of genetic linkage and association for several candidate gene markers and polymorphisms; 5) completed genotyping of random markers for over half of the full sample; and 6) early sharing of results among the four Family Blood Pressure Program networks for candidate and genome search analyses.

CONCLUSIONS

Experience after four years of this five-year program (1995-2000) suggests that the newly initiated NHLBI Network Program mechanism is fulfilling many of the expectations for which it was designed. It may serve as a paradigm for future genetic research that can benefit from large sample sizes, frequent sharing of ideas among laboratories, and prompt independent confirmation of early findings, which are required in the search for common genes with relatively small effects such as those that predispose to human hypertension.

Positional genomic analysis identifies the beta(2)-adrenergic receptor gene as a susceptibility locus for human hypertension

BACKGROUND

-After genome-wide linkage analyses of blood pressure levels, we resequenced 5 positional candidate genes in a linkage region on chromosome 5 and genotyped selected variants in several family samples from Rochester, Minn.

METHODS AND RESULTS

In a sample of 55 pedigrees containing >/=1 sibling-pair(s) discordant for systolic blood pressure, polymorphisms within the beta(2)-adrenergic receptor gene (Arg16Gly, P=0.009) and the glutathione peroxidase 3 gene (-302G-->A, P=0.037; -623A-->C, P=0.013) were significantly related to blood pressure levels. In a second sample of 298 nuclear families (n=1283 individuals), the Arg16Gly polymorphism was significantly associated with diastolic blood pressure in family-based analyses (P=0.016) and with both diastolic (P=0.009) and mean arterial blood pressure (P=0.038) in analyses of the parental generation only. Neither polymorphism in the glutathione peroxidase 3 gene was associated with blood pressure levels in this sample. An additional 291 families (n=1240 individuals) were added to the nuclear family sample, and the Gln27Glu polymorphism in the beta(2)-adrenergic receptor gene was significantly associated with both systolic (P=0.034) and mean arterial blood pressure (P=0.035) in the parental generation of the combined 589 families. The frequencies of both the Gly16 and Glu27 alleles were higher in hypertensives than in normotensives (0.649 versus 0.604 and 0.490 versus 0.429, respectively), and the odds ratio for the occurrence of hypertension was 1.80 (95% confidence interval, 1.08 to 3.00; P=0. 023) for the Glu27 allele.

CONCLUSIONS

The results of this study provide support for further detailed investigations of the mechanistic pathways by which variations in the beta(2)-adrenergic receptor gene may influence blood pressure levels.

The role of beta(2)-adrenergic receptor variation in human hypertension

Hypertension results from a complex and diverse array of metabolic and physiologic processes that interact with environmental factors to ultimately determine blood pressure levels and disease. Consequently, the identification of genes related to hypertension is complicated by the heterogeneity of its etiology and the likelihood that several genes with moderate effects, possibly acting in a context-dependent manner, influence blood pressure and the occurrence of hypertension. A number of studies have recently implicated variation within the beta(2)-adrenergic receptor in blood pressure regulation and the development of hypertension. The role of these findings is reviewed here, and their possible clinical implications in human hypertension.

Maximum-likelihood generalized heritability estimate for blood pressure in Nigerian families

Elevated blood pressure (BP) is more common in relatives of hypertensives than in relatives of normotensives, indicating familial resemblance of the BP phenotypes. Most published studies have been conducted in westernized societies. To assess the ability to generalize these estimates, we examined familial patterns of BP in a population-based sample of 510 nuclear families, including 1552 individuals (320 fathers, 370 mothers, 475 sons, and 387 daughters) from Ibadan, Nigeria. The prevalence of obesity in this community is low (body mass index: fathers, 21.6; mothers, 23.6; sons, 19.2; and daughters=21.0 kg/m2). The BP phenotype used in all analyses was created from the best regression model by standardizing the age-adjusted systolic blood pressure (SBP) and diastolic blood pressure (DBP) to 0 mean and unit variance. Heritability was estimated by use of the computer program SEGPATH from the most parsimonious model of "no spouse and neither gender nor generation difference" as 45% for SBP and 43% for DBP. The lack of a significant spouse correlation is consistent with little or no influence of the common familial environment. However, the heritability estimate of <50% for both SBP and DBPs reinforces the importance of the nonshared environmental effect.

Genetic and environmental influences on cardiovascular disease risk factors in adolescents

BACKGROUND

Although there is consensus that prevention of cardiovascular diseases is a worthwhile activity and that these efforts should begin in childhood, some controversies remain about the efficacy and timing of preventive efforts.

OBJECTIVE

To differentiate the cardiovascular risk factors that have a potential to respond to environmental and lifestyle modification.

METHODS

The sample consisted of 56 monozygotic and 29 same-sex dyzogotic twin pairs, equally distributed by gender with a mean age of 12.62 years. Systolic and diastolic blood pressure, triceps skinfold thickness, body mass, and fasting blood specimens for lipid profiles were collected during home visits. Teachers rated the subjects' Type A behaviors using the Matthews Youth Test for Health.

RESULTS

Statistically significant estimates of genetic variance were obtained for cholesterol, triglycerides, high-density lipoprotein cholesterol, systolic and diastolic blood pressure, and body mass index. Compared with the previous phase of this longitudinal study, higher estimates of genetic variance were observed for components of the lipid profile and blood pressure and lower estimates were observed for Type A behavior variables.

CONCLUSIONS

Overall, the genetic influence on risk factors was moderate, leading to the conclusion that the potential to modify risk profiles during the transition from childhood to adolescence is substantial. Attitudes, behaviors, and environmental inducements that establish and maintain healthy lifestyles over long period should be the focus of interventions and further research.

Genetics and behavioral medicine: risk factors for cardiovascular disease

This is the second in a series of three articles addressing the intersection of interests in behavioral genetics and behavioral medicine. In this article, we use risk factors for cardiovascular disease as a prototypical trait for which behavioral genetic approaches provide powerful tools for understanding how risk factors, behavior, and health outcomes are related. The approach synthesizes a number of methods and areas of interest in an attempt to arrive at a comprehensive, whole-organism understanding of health-related risk factors and their response to behavioral interventions.

A major gene for resting metabolic rate unassociated with body composition: results from the Québec Family Study

A major gene hypothesis for resting metabolic rate (RMR) was investigated using segregation analysis (POINTER) of data on families participating in Phase 2 of the Québec Family Study. Complete analyses were conducted on RMR adjusted for age, and also on RMR adjusted for age and other covariates, primarily fat mass (FM) and fat-free mass (FFM). Prior to adjustment for covariates, support for a major gene hypothesis was equivocal-i.e., there was evidence for either a major gene or a multifactorial component (i.e., polygenic and/or familial environment). The multifactorial model was preferred over the major gene model, although the latter did segregate according to Mendelian expectations. However, after the effects of FM and FFM were accounted for, a major gene effect was unambiguous and compelling. The putative locus accounted for 57% of the variance, affected 7% of the sample, and led to high values of RMR. The lack of a significant multifactorial effect suggested that the familial etiology of RMR adjusted for FM and FFM was likely to be entirely a function of the major locus. Comparing the RMR results from pre- and post-adjustment for FM and FFM suggests a plausible hypothesis. We know from earlier studies in this sample that there is a putative major gene for FM and a major non-Mendelian effect for FFM. The current study leads us to speculate that: (1) the gene(s) affecting body size and body composition also may have an effect on RMR, and further (2) removal of the effect of the major gene(s) for body size and composition allowed for detection of an additional major gene affecting only the RMR. Thus, RMR appears to be an oligogenic trait.

Familial correlations, cohabitation effects, and heritability for cardiovascular risk factors

Familial correlations in cardiovascular risk factors were investigated with use of data from a community-based sample of 1319 nuclear families involving 4178 adult persons collected in the Busselton Population Health Surveys over the period 1966 to 1981. The risk factors considered were systolic blood pressure, diastolic blood pressure, body mass index, triceps fatfold, and cholesterol. All risk factors showed positive familial correlations, with correlations generally being lower for spouses than for parent-offspring pairs or for siblings. Spouse correlations showed little variation with age, suggesting that observed correlations are primarily due to assortative mating and not to cohabitation. The parent-offspring correlations tended to decline with age of (adult) offspring; this observation suggests that the effect of a shared household environment during childhood and adolescence diminishes over time when living apart during adulthood. The sibling correlations decreased with age for blood pressure and serum cholesterol and increased with age for body mass index and triceps fatfold. The estimated heritabilities were 27% for systolic and diastolic blood pressure, 37% for serum cholesterol, 52% for body mass index, and 23% for triceps fatfold. These results confirm that substantial familial aggregation of cardiovascular risk factors occurs and that much of this aggregation has a genetic basis, although assortative mating (in spouses) and environmental influences (in offspring and siblings) are also present. The nuclear family should be considered as a point of intervention in cardiovascular disease prevention programs.

Genetic pleiotropy for resting metabolic rate with fat-free mass and fat mass: the Québec Family Study

Shared genetic and familial environmental causes for the associations among resting metabolic rate (RMR), fat-free mass (FFM), and fat mass (FM) were investigated in families participating in phase 2 of the Québec Family Study. A multivariate familial correlation model assessing the pattern of significant cross-trait correlations between family members (e.g., RMR in parents with FFM in offspring) was used to infer the etiology of the associations. For each of FM and FFM with RMR, significant sibling, parent-offspring, and intraindividual cross-trait correlations suggests the associations are familial. Furthermore, the lack of significant spouse cross-trait correlations suggests that the familial aggregation is primarily genetic. Bivariate heritability estimates suggest that as much as 45% to 50% of the shared variance between FFM and RMR may be genetic, and as much as 28% to 34% for FM and RMR. This study supports the notion that the gene(s) affecting each of FFM and FM also influence the RMR. Moreover, the lack of any familial associations between FFM and FM suggests that the effects of each body size component on RMR are independent, i.e., more than one genetic source on the RMR-body size association. The possibility that RMR is an oligogenic trait (i.e., more than one underlying genetic etiology) should be further investigated using more complex multivariate segregation methods until specific genes can be tested.

General purpose model and a computer program for combined segregation and path analysis (SEGPATH): automatically creating computer programs from symbolic language model specifications

A general purpose model and a flexible computer program, called SEGPATH, have been developed to assist in the creation and implementation of a variety of genetic epidemiological models. SEGPATH is a computer program which can be used to generate programs to implement linear models for pedigree data, based upon a flexible, model-specification syntax. SEGPATH models can perform segregation analysis, path analysis, or combined segregation and path analysis using any user-specified path model and can be structured to analyze any number of multivariate phenotypes, environmental indices, and/or measured covariate fixed effects (including measured genotypes). Population heterogeneity models, repeated-measures models, longitudinal models, auto-regressive models, developmental models, and gene-by-environment interaction models can all be created under SEGPATH. Pedigree structures can be defined to be arbitrarily complex, and the data analyzed with programs generated by SEGPATH can have any missing value structure, with entire individuals missing, or missing on one or more measurements. Corrections for ascertainment can be done on a vector of phenotypes and/or other measures. Because the model specification syntax is general, SEGPATH can also be used in non-genetic applications where there is a hierarchical structure, such as longitudinal, repeated-measures, time series, or nested models. A variety of applications are demonstrated.

Inbreeding and congenital heart diseases in a north Indian population

The study was performed in six mohallahs (colonies) of Aligarh City (North India). All six mohallahs are predominantly inhabited by Qureshi (meat sellers, a highly endogamous group) Muslims. A total of 1721 infants and children up to the age of 6 years were examined to determine the incidence of congenital heart diseases (CHD) in relation to the degree of consanguinity of the parents. Around 43% of the subjects were the offspring of consanguineous marriages including second-cousin, first-cousin-once-removed and first-cousin. A higher percentage of CHD was found in the offspring of consanguineous marriages: about 3.37% out of 741 children as compared to 1.22% in 980 offspring of non-consanguineous marriages, whereas in the first-cousin offspring, the percentage of CHD rose to 4.41%. The differences were found to be statistically significant. The present study suggests a genetic influence and also casts doubt on the applicability of a polygenic threshold model to all forms of cardiac malformation.

Age-specific genetic effects for blood pressure

Correlations between relatives were determined for systolic and diastolic blood pressure. The correlations decrease as age differences between relatives increase in a Norwegian sample with 43,751 parent-offspring pairs, 19,140 pairs of siblings, and 169 pairs of twins. A simple biometric model specifying only age-specific genetic additive effects and environmental effects fitted well to correlations between cotwins, pairs of siblings, and parent-offspring dyads in subsets of relatives grouped by age differences. None of the environmental effects appeared to be due to environmental factors that are shared by family members. Models that excluded a parameter for the age-specific genetic influence did not fit the data. The results may partly explain what seems to be a discrepancy between relatively low parent-offspring correlations from previous nuclear family studies and high correlations from twin studies, especially in identical twins.

Assessment of adiposity in an Indian population: familial correlations

Familial correlations for five measures of adiposity were assessed using data from 473 nuclear families residing in the Chittoor district of Andhra Pradesh, India. Fat patterning, measured as the ratio of trunk to extremity subcutaneous fat, and the ratio of the subscapular to the sum of the subscapular and suprailiac skinfolds, and three measures of generalized fatness: body mass index, the sum of six skinfolds and the sum of three trunk skinfolds were analyzed. Maximum likelihood estimates of the familial correlations were obtained for each phenotype, after adjusting for the effects of (1) age within sex, and (2) current levels of energy intake and expenditure, and age within sex. Hypotheses regarding sex-specific, and generational differences in these correlations were assessed for each phenotype, under both adjustment schemes. The strength and pattern of the familial correlations for the three measures of generalized fatness were markedly influenced by the specific adjustments applied to the data. In contrast, the familial correlations for the fat patterning phenotypes were quite similar under the two adjustment schemes. Comparison of our results with previously published data indicates that the familial correlations for a subset of these adiposity measures may be heterogeneous across populations.

Genetic and environmental effects on blood pressure in a Norwegian sample

Systolic (SBP) and diastolic (DBP) blood pressures were measured in a health screening of the adult population in Nord-Trøndelag, Norway. Correlations were computed for 23,936 pairs of spouses, 43,586 pairs of parent and offspring, 19,151 pairs of siblings, 1,251 pairs of grandparents-grandchildren, 1,146 pairs of biological uncles/aunts-nephews/nieces (avuncular), 801 non-biological avuncular pairs, 169 pairs of same-sex twins, and smaller groups of other types of relationships. Spouse correlations of 0.08 and 0.09 were approximately constant or slightly decreasing with marital duration. The correlation values for SBP and DBP were approximately 0.16 for parents-offspring, 0.19 to 0.23 for same-sex siblings with similar values for DZ twins, 0.19 and 0.16 for opposite-sex siblings, 0.52 and 0.43 for MZ twins, and close to zero for most of the second-order relationships. Genetic additive variance was estimated at 0.29 and genetic dominance variance at 0.18 with the best model for SBP. The corresponding estimates from the best models for DBP were 0.29 or lower and 0.22 or lower, the sum not exceeding 0.35. There was evidence of a moderate effect of environmental factors shared by same-sex siblings and twins (for DBP), but no cultural transmission, and whether or not adult relatives live together does not affect familial resemblance for BP. The data did not permit a very precise resolution of the relative magnitude of genetic dominance and sibling effects. The correlation structure did not show sex-specific genetic effects.

Heritability of blood pressure in 7- to 12-year-old Chinese twins, with special reference to body size effects

Systolic and diastolic blood pressures and body-size indices such as body height, body weight, sitting height, chest circumference, skinfold thickness, and body mass index (BMI) were assessed in 110 pairs of like-sex Chinese twins (75 monozygotic and 35 dizygotic) aged 7-12 years. Significant correlations of blood pressure with body-size indices were found. Prior to adjusting for body-size effects, three twin methods yielded low heritability estimates for both systolic (0.32-0.41) and diastolic (0.32-0.51) pressures. Adjusting systolic pressure for body height and BMI via multiple regression nearly halved heritability estimates, but adjusting diastolic pressure for body height and skinfold thickness only changed the estimates slightly.