G proteins and hypertension: an alternative candidate gene approach

G-Protein β3-Subunit Gene C825T Polymorphism and Cardiovascular Risk: An Updated Review

Hypertension is a common disorder of multifactorial origin that constitutes a major risk factor for cardiovascular events such as stroke and myocardial infarction. The subunits of the heterotrimeric G proteins are attractive candidate gene products for susceptibility to hypertension, obesity and insulin resistance syndrome. A polymorphism (825C/T) in exon 10 of the GNB3 gene, encoding for the Gβ3 subunit, has been described. The 825T allele is associated with alternative splicing of the gene and formation of a truncated but functionally active β3 subunit. Many studies have investigated whether carriers of the 825T allele are at increased risk for hypertension, obesity, insulin-resistance and left ventricular hypertrophy with apparently conflicting results. The present review demonstrates that GNB3 825T allele is a useful genetic marker for better defining the risk profile of hypertensive patients, as it is associated with increased risk of stroke and myocardial infarction in longitudinal studies in Caucasians.

The D153del mutation in GNB3 gene causes tissue specific signalling patterns and an abnormal renal morphology in Rge chickens

BACKGROUND

The GNB3 gene is expressed in cone but not rod photoreceptors of vertebrates, where it acts as the β transducin subunit in the colour visual transduction process. A naturally occurring mutation 'D153del' in the GNB3 gene causes the recessively inherited blinding phenotype retinopathy globe enlarged (rge) disease in chickens. GNB3 is however also expressed in most other vertebrate tissues suggesting that the D153del mutation may exert pathological effects that outlie from eye.

PRINCIPAL FINDINGS

Recombinant studies in COS-7 cells that were transfected with normal and mutant recombinant GNB3 constructs and subjected to cycloheximide chase showed that the mutant GNB3d protein had a much shorter half life compared to normal GNB3. GNB3 codes for the Gβ3 protein subunit that, together with different Gγ and Gα subunits, activates and regulates phosphorylation cascades in different tissues. As expected, the relative levels of cGMP and cAMP secondary messengers and their activated kinases such as MAPK, AKT and GRK2 were also found to be altered significantly in a tissue specific manner in rge chickens. Histochemical analysis on kidney tissue sections, from rge homozygous affected chickens, showed the chickens had enlargement of the glomerular capsule, causing glomerulomegaly and tubulointerstitial inflammation whereas other tissues (brain, heart, liver, pancreas) were unaffected.

SIGNIFICANCE

These findings confirm that the D153del mutation in GNB3 gene targets GNB3 protein to early degradation. Lack of GNB3 signalling causes reduced phosphorylation activity of ERK2 and AKT leading to severe pathological phenotypes such as blindness and renal abnormalities in rge chickens.

G-protein beta-3 subunit C825T polymorphism, sodium and arterial blood pressure: a community-based study of Japanese men and women

Epidemiological evidence on gene-environment effects of the G-protein beta-3 subunit C825T polymorphisms and sodium on blood pressure in the free-living general population is limited. We examined the associations between the C825T polymorphism and blood pressure levels, stratified by the sodium variables estimated by 24-h urinary sodium excretion and a dietary questionnaire, among 1,471 men and women aged 30-74 from a community in Japan. Our a priori hypothesis was that individuals with the 825T allele have elevated blood pressure among subjects with a high sodium intake. Among the whole group, the systolic blood pressure level was +2.2 mmHg (p = 0.10) higher in TT than CC genotype individuals after adjustment for sex, age, antihypertensive medication use, body mass index, and alcohol consumption. This difference was more evident among individuals with low sodium excretion (+4.5 mmHg, p = 0.01), low present sodium intake (+3.2 mmHg, p = 0.11), and low past sodium intake (+4.8 mmHg, p = 0.02). No associations were observed among those with high sodium variables. Our results indicate that the G-protein beta-3 subunit C825T polymorphism is associated with higher systolic blood pressure levels in a large free-living Japanese population, and more specifically in women with a low sodium intake. This finding helps to explain part of the discrepancy between the previously reported genetic association among different ethnic groups.

Association between the GNB3 Polymorphism and Blood Pressure in Young Korean Men

PURPOSE

The purpose of the current study was to investigate the associations between the GNB3 C825T polymorphism, body fatness and fitness, and blood pressure in a sample of young Korean men.

METHODS

A total of 282 apparently healthy Korean men (nonobese N = 152, obese N = 130) aged 19-33 yr participated in the study. Body fatness and blood pressure were measured by using standardized protocols. The GNB3 C825T was genotyped by using polymerase chain reaction with restriction fragment length polymorphism.

RESULTS

Univariate analyses of variance and Bonferroni post hoc tests showed that in the obese group only, TT homozygotes had a lower VO2max (P = 0.015 and 0.043, respectively) but higher resting systolic (P = 0.025 and < 0.001, respectively) and mean arterial blood pressures (P = 0.049 and 0.002, respectively) than CC homozygotes or CT heterozygotes. We further investigated possible interactions between group and genotype and found a significant interaction in SBP only (P = 0.037) between the group and genotype, even when controlling for age, BMI, and VO2max as potential confounding factors. Stepwise regression analyses showed that body mass index and GNB3 genotype were two independent predictors for the variations in systolic blood pressure and heart rate up to by 14.3 and 14.5%, respectively. In addition, waist-to-hip ratio explained an individual variation in mean arterial pressure (MAP) up to by 11.2%.

CONCLUSION

The current findings of the study suggest that increased body fatness along with low cardiorespiratory fitness may magnify the genetic susceptibility of the GNB3 825T allele to elevated blood pressure in this study population.

T393C polymorphism of GNAS1 associated with the autonomic nervous system in young, healthy Japanese subjects

1. T393C polymorphism of the gene encoding the Gs-protein alpha-subunit (GNAS1) has been reported recently to be associated with hypertension in which dysfunctions of the autonomic nervous system (ANS) are closely involved. In the present study, the association of this polymorphism with ANS activity was investigated in young, healthy Japanese males. 2. Four hundred and one subjects were genotyped for the T393C polymorphism of GNAS1 by polymerase chain reaction-restriction fragment length polymorphism. Autonomic nervous system activity during supine rest and when standing was assessed in 137 subjects by electrocardiogram R-R interval power spectral analysis. 3. One hundred and fifty-four subjects (38.4%) were homozygous for the T allele (TT), 188 (46.9%) were heterozygous (TC) and 59 (14.7%) were homozygous for the C allele (CC). There were no significant differences as to genotype among the clinical characteristics investigated. In power spectral analysis of heart rate variability, the high-frequency component and parasympathetic nervous system (PNS) index during supine rest were significantly lower in TT and TC carriers than in CC carriers. Furthermore, the increase in heart rate and the responsiveness of sympathetic nervous system index and PNS index to postural change from supine rest to standing were significantly lower in TT and TC carriers than in CC carriers. 4. These observations suggest that the GNAS1 T393C polymorphism is associated with ANS activity in youth, so that it may be useful as a genetic marker for future pathogenesis of hypertension. Follow-up studies are necessary to clarify the prevalence rates of hypertension among 393T allele carriers in the present study.

G protein β3 subunit C825T polymorphism in primary IgA nephropathy

BACKGROUND

The T allele of the G protein beta3 subunit (GNB3) C825T polymorphism has been associated with increased signal transduction, increased activity of the kidney Na+/H+ exchanger, and also with late-onset essential hypertension. Hypertension is a strong independent risk factor for progression in IgA nephropathy (IgAN).

METHODS

We have studied this polymorphism in a regularly followed cohort of 299 biopsy-proven incident cases of IgAN, collected from 1989 to 1999 [208 males (70%)] and compared the genotypes and alleles distributions to 303 local Caucasian controls matched for the male predominance (214 males). The technique used was a polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) with BseDI as restriction enzyme and specific primers, followed by gel electrophoresis.

RESULTS

The TT, CT, and CC genotype frequencies were 13.7%, 45.8%, and 40.5% in IgAN, respectively, versus 7.6%, 47.2%, and 45.2% in controls, respectively (chi(2)= 6.16; P= 0.05). The excess of TT patients versus non-TT was significant in IgAN versus controls (chi(2)= 5.94; P= 0.015). The T allele frequency was 0.366 in IgAN versus 0.312 in controls (chi(2)= 3.97; P= 0.05). This data indicated that this polymorphism had a significant but mild influence on the occurrence/initiation of IgAN (RR = 1.81; 95% CI 1.07-3.07). In contrast, we could not demonstrate any significant and sustained difference in the clinical presentation and evolution of the homozygous TT patients compared to non-TT patients (CC + CT) despite a mean and median follow-up about 10 years. The progression to arterial hypertension or to chronic renal failure or to end-stage renal failure (ESRF) was not significantly different. In addition, multivariate Cox regression analysis excluded a significant independent role of C825T polymorphism on progression.

CONCLUSION

The C825T GNB3 polymorphism had a mild influence on occurrence/initiation of IgAN, but played no significant role in the progression of the disease.

G-protein beta3-subunit gene 825T allele and hypertension: a longitudinal study in young grade I hypertensives

The 825T allele of the GNB3 gene has been associated with essential hypertension and obesity in cross-sectional studies. We have therefore planned a longitudinal cohort study to assess whether the GNB3 825T allele is predictive of blood pressure increase in young subjects with grade I hypertension. We genotyped at the GNB3 825 locus 461 participants of the Hypertension and Ambulatory Recording Venetia Study (HARVEST) study (age, 18 to 45 years) at low cardiovascular risk, according to 1999 ISH/WHO criteria. The study end point was eligibility for antihypertensive medication, that is, progression to grade II hypertension during the first year of observation or office systolic blood pressure > or =150 mm Hg and/or office diastolic blood pressure > or =95 mm Hg in two later consecutive visits during follow-up. At baseline, there was no statistically significant difference among genotypes with respect to body mass index, blood pressure, and heart rate. During follow-up (mean, 4.7 years), 113 (51.1%) patients with CC genotype and 145 (60.4%) patients with TT/TC genotype reached the end point. According to survival analysis, the patients carrying the 825T allele had an increased risk of reaching the blood pressure end point (CI, 1.108 to 1.843; P=0.006). In young patients with grade I hypertension, the 825T allele is associated with increased risk of progression to more severe hypertension requiring antihypertensive therapy. The GNB3 825T allele may be considered a genetic marker of predisposition for hypertension.

Epithelial sodium channel and the control of sodium balance: interaction between genetic and environmental factors

The epithelial sodium channel (ENaC) expressed in aldosterone-responsive epithelial cells of the kidney and colon plays a critical role in the control of sodium balance, blood volume, and blood pressure. In lung, ENaC has a distinct role in controlling the ionic composition of the air-liquid interface and thus the rate of mucociliary transport. Loss-of-function mutations in ENaC cause a severe salt-wasting syndrome in human pseudohypoaldosteronism type 1 (PHA-1). Gain-of-function mutations in ENaC beta and gamma subunits cause pseudoaldosteronism (Liddle's syndrome), a severe form of salt-sensitive hypertension. This review discusses genetically defined forms of a salt sensitivity and salt resistance in human monogenic diseases and in animal models mimicking PHA-1 or Liddle's syndrome. The complex interaction between genetic factors (ENaC mutations) and the risk factor (salt intake) can now be studied experimentally. The role of single-nucleotide polymorphisms (SNPs) in determining salt sensitivity or salt resistance in general populations is one of the main challenges of the post-genomic era.

Effect of the (C825T) Gbeta(3) polymorphism on adrenoceptor-mediated lipolysis in human fat cells

A common Gbeta(3) gene polymorphism (C825T) influences G protein receptor-mediated signal transduction. We investigated whether this polymorphism influences lipolysis in isolated subcutaneous fat cells from 114 healthy obese subjects. The Gbeta(3) protein content was markedly decreased in adipocytes of TT carriers, but the alternatively spliced short form of Gbeta(3) previously shown in platelets of 825T carriers was not detected. Fat cells of TT carriers showed a significant 10-fold decrease in the half-maximum effective concentration of agonists selective for lipolytic beta(1)- and beta(2)-adrenoceptors as well as for the antilipolytic alpha(2)A-adrenoceptor. In TT carriers, maximum beta-adrenoceptor agonist-stimulated lipolysis was decreased, but the maximum antilipolytic effect of alpha(2)-adrenoceptors was less marked. Norepinephrine induced adipocyte lipolysis and circulating fasting levels of free fatty acids and glycerol were reduced by half in TT carriers. The polymorphism did not influence the adipocyte content of alpha(2)A-adrenoceptors, beta(2)-adrenoceptors, Galpha(i), or Galpha(s). In conclusion, the C825T variant of Gbeta(3) influences lipolysis. Adipocytes of TT carriers have a lower Gbeta(3) protein content and a decreased function of native G(s)- as well as G(i)-coupled adrenoceptors, which reduces the lipolytic effect of catecholamines. These data differ from those obtained in other cell systems that have shown increased expression of an alternative spliced Gbeta(3) variant and enhanced G protein signaling in 825T carriers, indicating that the polymorphism has cell type-specific effects that may be of importance for type 2 diabetes and other insulin-resistant conditions.

Psychopharmacogenetics--a challenge for pharmacotherapy in psychiatry

Differences in response to treatment or the incidence of adverse drug effects are quite common in clinical psychopharmacotherapy. Although several factors may account for these discrepancies, there is increasing knowledge that genetic factors play a major role. The aim of pharmacogenetics, a new and rapidly growing field in research, is to elucidate the variability in drug response and metabolism due to hereditary differences. According to the hypotheses on the mechanisms of drug action, several mutations in genes coding for neurotransmitter receptors, degrading enzymes, transport proteins or enzymes of the drug metabolizing system (P-450 isoenzymes) have been identified and investigated in psychiatric disorders over the last years. Although some controversy exists among the results, many studies are supportive of the hypothesis that psychopharmacogenetics will be helpful in predicting an individual patient's drug response while minimising the rate of side effects.

Analysis of 825C/T polymorphism of G proteinbeta3 subunit in obese/diabetic Japanese

The GNB3 825C/T polymorphism, which is common worldwide, is associated with enhanced G-protein activation. The frequency of 825-T allele was increased with body mass index (BMI) and finally had a high frequency in relatively mild obese (BMI >27 kg/m(2)) subjects in some populations. In the present study, we investigated 208 severely obese [BMI >or=30 kg/m(2) (97th percentile)] Japanese subjects including 146 probands with diabetes. No increase in the 825-T allele frequency was observed in the 208 severely obese and even in a subgroup of the 55 most obese [BMI >or=35 kg/m(2) (99.7th percentile)] subjects compared with that in 150 controls (BMI <25 kg/m(2)) (0.48 and 0.48 vs 0.51, respectively). Also, the frequency was not increased in the 146 obese subjects with diabetes (0.48). We concluded that the 825-T allele is not associated with obesity or diabetes associated with obesity at least in the Japanese population.

Altered gene expression in cerebral capillaries of stroke-prone spontaneously hypertensive rats

Stroke-prone spontaneously hypertensive rats (SHRSP) are a well-characterized, genetic model for stroke. We showed earlier that the structure and function of the tight junctions in SHRSP blood-brain barrier endothelial cells is disturbed prior to stroke. To investigate the molecular events leading to endothelial dysfunction in SHRSP cerebral capillaries, we carried out suppression subtractive hybridization (SSH) in combination with a cDNA filter screening step. We identified two cDNA fragments that were upregulated in SHRSP, compared to stroke-resistant spontaneously hypertensive rats (SHR), and found open reading frames of 133 and 138 amino acids, respectively. These peptides did not match any known proteins in public databases. A third upregulated SHRSP cDNA fragment was identified as the rat sulfonylurea receptor 2B (SUR2B). We also isolated and cloned the cDNA of the rat homologue for the mouse G-protein signaling 5 (RGS5) regulator. This regulator was downregulated in SHRSP. We used in situ hybridization to show that rat RGS5 is expressed in the brain capillary endothelium and in the choroid plexus. Our findings may lead to the identification of new stroke-related genes.

Evidence for an association between a G-protein beta3-gene variant with depression and response to antidepressant treatment

Abnormal signal transduction pathways have been implicated in the pathogenesis of bipolar disorder and major depression. G-proteins are key elements of these pathways in the regulation of cellular responses by transmission of signals from receptors to effector proteins. In recent years several studies have reported altered levels and activities of G-protein alpha subunits in depressive patients. A recently identified polymorphism of a G-protein beta3 subunit (C825T) has been shown to be associated with increased signal transduction and ion transport activity. Therefore, we investigated whether this Gbeta3 polymorphism is associated with affective disorders or with the response to antidepressant treatment in 88 depressive patients (10 bipolar disorder, 78 major depression) compared with 68 schizophrenic patients and 111 healthy controls. We found a significantly higher frequency of the T allele in depressive patients than in healthy controls (genotype: chi2 = 9.571, df = 2, p = 0.008; alleles: p = 0.004, OR = 1.87, 95% CI 1.23-2.84; Fisher's exact test, two sided) and schizophrenic patients (genotype: chi2 = 8.037, df = 2, p = 0.018; alleles: p = 0.009, OR = 1.94, 95% CI 1.99-3.14; Fisher's exact test, two sided). We also found a statistical significant association between TT homozygosity and response to antidepressant treatment after four weeks (p = 0.01). The results of this study suggest that the investigated G-protein beta3 subunit seems to be a susceptibility factor for major depression and maybe even for bipolar disorder, but not for schizophrenia. Further, the presence of the T allele could be an indicator for treatment response.

[Genetic polymorphism of the G-protein beta3 subunit, obesity and essential hypertension]

Following a classical candidate gene approach we have detected a C825T polymorphism in the gene GNB3 which encodes the G beta 3 subunit of heterotrimeric G proteins. The 825T allele causes alternative splicing of the gene and the generation of a truncated but functionally active splice variant of G beta 3 which is referred to as G beta 3s. Thus, genotyping for the C825T polymorphism is predictive for the activation of certain G proteins in humans. The 825T allele is significantly associated with an increased risk for hypertension in Caucasians, most likely "low renin hypertension" and it accumulates significantly in individuals with a strong family history of hypertension. Highest frequencies of the 825T allele (up to 80%) are found in old ethnicities, e.g. black Africans, African Americans, bushmen, and Australian aborigines. This suggests that enhanced G protein activation represents a thrifty genotype which might have facilitated survival in our ancestors. Frequencies of the 825T allele are significant lower in Asians (approximately 40 to 50%) and Caucasians (30%). More recent studies show that young 825T allele carriers are predisposed for obesity and this association could be confirmed across different ethnicities including young Germans, as well as Chinese and black African individuals. Thus, genotyping at the GNB3 locus represents an ideal tool for preventive medicine in that individuals at risk for obesity and hypertension can be identified early and counteract their genetic predisposition through changes in lifestyle. In individuals with borderline hypertension genotyping can facilitate the decision for medical treatment as a positive test result confirms an inherited form of hypertension.

G-protein beta3 subunit gene splice variant and body fat distribution in Nunavut Inuit

The GNB3 825T allele encodes a product that has enhanced activation of heterotrimeric G proteins in vitro and could play a role in adipogenesis. We therefore evaluated the possibility that the GNB3 825T allele was associated with obesity in a sample of 213 healthy Canadian Inuit. We found that body weight, body mass index, waist girth, hip girth, subscapular skinfold thickness, and triceps skinfold thickness were significantly higher in subjects with the GNB3 825T/T genotype than in subjects with other genotypes. Furthermore, two anthropometric ratios, namely that of waist to hip circumference and that of subscapular to triceps skinfold thickness, were not significantly different across genotypes. This suggested that the increased deposition of fat in subjects with the GNB3 825T/T genotype was generalized and not localized to particular subregions. There was no association of this genetic variation with blood pressure. The GNB3 825T/T genotype accounted for between 1.6% and 3.3% of the total variation (< or =13% of attributable variation) of the obesity-related traits. The potential for a genetic marker of obesity creates opportunities for future studies in the Inuit, not just to confirm the associations, but also to examine prospectively the influence of interventions and possible relationships between GNB3 825T and longer term complications of obesity.

Worldwide ethnic distribution of the G protein beta3 subunit 825T allele and its association with obesity in Caucasian, Chinese, and Black African individuals

Recently, it was demonstrated that one allele (825T) of the gene encoding the G protein beta3 subunit (GNB3) is associated with hypertension in Germans. This study investigates a possible association with obesity in young male Germans, Chinese, and black South Africans with low, intermediate, and high 825T allele frequencies, respectively. In each of these three distinct cohorts, the 825T allele frequency was increased significantly in overweight (body mass index [BMI] > or =25 kg/m2) and obese individuals (BMI >27 kg/m2) compared to those with normal weight. The 825T allele frequencies in these three BMI groups were, respectively, 29.5, 39.3, and 47.7% in Germans, 46.8, 53.9, and 58.6% in Chinese, and 83.1, 87.7, and 90.9% in South Africans. In each of these three distinct groups, the 825T allele was significantly associated with obesity with odds ratios between 2 and 3. More urban than rural black Africans were overweight despite similar 825T allele frequencies in both populations, which underscores the role of both genetic and environmental factors. BP values in young male whites increased significantly with increasing BMI values but were independent of the C825T polymorphism, suggesting that hypertension associated with the 825T allele could be a consequence of obesity. Genotyping of 5254 individuals from 55 native population samples from Africa, the Americas, Europe, Asia, Australia, and New Guinea demonstrated highest 825T allele frequencies in black Africans (82%) and intermediate values in east Asians (47%). It is anticipated that high frequencies of the 825T allele in Africans and Asians may contribute to an obesity and hypertension epidemic if Westernization of lifestyles continues.

Genetic determinants of blood pressure regulation

Blood pressure homeostasis in humans reflects the coordinate interactions of cardiac output, peripheral vascular resistance, renal volume control, and CNS integration in response to short- and long-term environmental stimuli. Variations in mean arterial pressure within the population include a significant hereditary component. The clearest examples of this genetic contribution occur in rare forms of monogenic hypertension (glucocorticoid remediable aldosteronism, apparent mineralocoid excess, Liddle's syndrome) or hypotension (pseudohypoaldosteronism type I, Bartter's syndrome, Gitelman's syndrome). Primary hypertension, which comprises approximately 95% of hypertensives and is a major risk factor for coronary heart disease, stroke, and renal disease in the U.S., represents a multifactorial and polygenic disease with incremental contributions from genetic and environmental determinants. Efforts to date have identified several candidate genes involved in primary hypertension, including angiotensinogen (AGT), a vasoactive peptide; alpha-adducin, a protein that regulates sodium transport; and the G protein beta 3 subunit, a protein involved in intracellular signal transduction. Advances in knowledge and technology associated with the Human Genome Project, combined with continuing basic research on the physiologic and biochemical causes of hypertension, offer promise for improved diagnosis and therapy of this prevalent disease.

Molecular genetics of human hypertension

For the past decade, hypertension research has shifted strongly in the direction of molecular genetics. The success stories are the monogenic hypertensive syndromes. Classic linkage analyses has located the responsible genes for glucocorticoid-remediable aldosteronism, Liddle syndrome, and apparent mineralocorticoid excess. The genes have been cloned and their function elucidated. Other monogenic syndromes are currently being intensively studied. However, in the area of primary hypertension, the successes have relied on the candidate gene approach. Allelic variants in the genes for angiotensinogen, alpha-adducin, beta2-adrenergic receptor, the G-protein beta3-subunit and the T594M mutation in the beta-subunit of the epithelial sodium channel have been identified; however, the importance of these allelic variants to primary hypertension as a whole, is not yet clear. A variant in the angiotensin-converting enzyme gene could not, initially, be convincingly associated with hypertension, but more recent analyses suggest an influence of the deletion allele on blood pressure in men, but apparently not in women. In all likelihood we are dealing with many genes with small effects. Affected sibling pair linkage analyses will probably not be successful in identifying the loci of these genes. To find new genes, novel approaches will be necessary, including searching for quantitative trait loci linked to blood pressure in normotensive persons, haplotype sharing methodology in trios and family units, the use of better study designs, and the investigation of isolated populations. Finally, rethinking the phenotype 'hypertension' and its intermediates must also receive priority.