Postoperative atrial fibrillation (PoAF) after cardiac surgery is
common and associated with increased morbidity and mortality. Increased
sympathetic activation after surgery contributes to PoAF, and beta-blockers
are the first-line recommendation for its prevention. We examined the
hypothesis that common functional genetic variants in the
β1-adrenoreceptor, the mediator of cardiac sympathetic
activation and drug target of beta-blockers, are associated with the risk
for PoAF and with the protective effect of beta-blockers.
In a prospective cohort study, we studied 947 adult European
Americans who underwent cardiac surgery at Vanderbilt University between
1999-2005. We genotyped two variants in the
β1-adrenoreceptor, rs1801253 (Arg389Gly) and rs1801252
(Ser49Gly), and used logistic regression to examine the association between
genotypes and PoAF occurring within 14 days after surgery, before and after
adjustment for demographic and clinical covariates.
PoAF occurred in 239 patients (25.2%) and was associated with
rs1801253 genotype (adjusted P=0.008), with Gly389Gly having an odds ratio
of 2.63 (95% confidence interval, 1.42 to 4.89) for PoAF compared to the
common Arg389Arg (P=0.002). In a predefined subgroup analysis, this
association appeared to be stronger among patients without beta-blocker
prophylaxis (adjusted OR=7.00; 95% CI, 1.82 to 26.96; P=0.005) compared to
patients with beta-blocker prophylaxis, among whom the association between
rs1801253 genotype and PoAF was not statistically significant (adjusted
The Gly389 variant in the β1-adrenoreceptor is
associated with PoAF, and this association appears to be modulated by
beta-blocker therapy. Future studies of the association of other adrenergic
pathway genes with PoAF will be of interest.
Sympathetic activation inhibits insulin secretion through activation of pancreatic α2A-adrenoreceptors (α2AARs). A common genetic α2AAR variant (rs553668) is associated with impaired insulin secretion. α2AR-agonists would be expected to decrease insulin secretion, but their effects on glucose homeostasis in humans are poorly characterized. We examined the hypothesis that the selective α2AR-agonist dexmedetomidine (DEX) decreases plasma insulin and increases plasma glucose in humans, and that these effects are modified by genetic α2AAR variants.
Healthy, fasting white (n=31) and black (n=33) subjects aged 18-45 years received 3 sequential infusions of placebo (normal saline) at 30 minute intervals followed by 3 infusions of DEX (0.1, 0.15, and 0.15 mcg/kg). Plasma insulin and glucose concentrations were measured at baseline, after placebo, and after DEX. We genotyped ADRA2A rs553668 and rs2484516, which characterize haplotypes 4 and 4b, respectively.
DEX decreased fasting insulin concentrations by 37%, from a median value after placebo of 7.9 (interquartile range, 6.0 to 12.6) μU/mL to 4.9 (3.5 to 7.9) μU/mL (P <0.001). Plasma glucose concentrations increased from 76±6 mg/dL to 79±7 mg/dL; P<0.001). The rs2484516 variant allele was associated with higher baseline insulin concentrations before (P=0.001) and after adjustment for potential confounders (P=0.014), and a greater decrease in insulin after DEX (P=0.016) that was no longer significant after adjustment for baseline concentrations and other confounders (P=0.58).
Low-dose DEX decreased plasma insulin and mildly increased plasma glucose concentrations in healthy fasting subjects. ADRA2A genetic variation may affect baseline insulin concentrations and thus the insulin decrease after DEX.
African Americans have increased hemodynamic responses to both physiologic and pharmacologic adrenergic stimulation compared to Caucasians, and this may contribute to the greater prevalence of hypertension in this ethnic group. A small study suggested enhanced α1-adrenoreceptor-mediated arterial vasoconstriction in the forearm vasculature of African Americans compared to Caucasians, but it is unknown whether this reflects a generalized vascular phenomenon. The objective of this study was to examine the hypothesis that there are ethnic differences in venous α1-adrenoreceptor responsiveness. Using a linear variable differential transformer, we measured local dorsal hand vein responses to increasing doses of the selective α1-adrenoreceptor agonist, phenylephrine, in 106 subjects (64 Caucasians and 42 African Americans). There was wide interindividual variability in responses to phenylephrine. The dose that produced 50% of maximal constriction (ED50) ranged from 11 to 5442 ng/min, and maximal venoconstriction (Emax) ranged from 13.5% to 100%. African Americans (geometric mean ED50=172 ng/min; 95% CI, 115 to 256 ng/min) were more sensitive to phenylephrine than Caucasians (310 ng/min; 95% CI, 222 to 434 ng/min; unadjusted P=0.026; adjusted P=0.003). Median Emax was slightly higher in African Americans (89%; IQR, 82% to 98%) compared to Caucasians (85%; IQR, 75% to 95%; P=0.07). Taken together with previous findings in arterial vessels, our results suggest a generalized increased sensitivity to α1-adrenoreceptor-mediated vasoconstriction in African Americans. Increased vascular α-adrenoreceptor sensitivity could predispose to hypertension, and future studies addressing this mechanism’s contribution to ethnic differences in the prevalence of hypertension will be of interest.
Alpha-1 adrenergic receptor; vasoconstriction; phenylephrine; ethnicity; hypertension
α2-Adrenoceptors (α2-AR) mediate both constriction and dilatation of blood vessels. There is substantial inter-individual variability in dorsal hand vein (DHV) constriction responses to α2-AR agonist activation. Genetic factors appear to contribute significantly to this variation. The present study was designed to identify genetic factors contributing to the inter-individual variability in α2-AR-mediated vascular constriction induced by the selective α2-AR agonist dexmedetomidine.
DHV constriction responses to local infusion of dexmedetomidine were assessed by measuring changes in vein diameter with a linear variable differential transformer. The outcome variable was log-transformed dexmedetomidine ED50 for constriction. A genome-wide association study (GWAS) of 433,378 single nucleotide polymorphisms (SNPs) was performed for the sensitivity of DHV responses in 64 healthy Finnish subjects. 20 SNPs were selected based on the GWAS results and their associations with the ED50 of dexmedetomidine were tested in an independent North American study population of 68 healthy individuals.
In both study populations (GWAS and replication samples), the SNP rs9922316 in the gene for protein kinase C type β was consistently associated with dexmedetomidine ED50 for dorsal hand vein constriction (unadjusted p = 0.00016 for the combined population).
Genetic variation in protein kinase C type β may contribute to the inter-individual variation in dorsal hand vein constriction responses to α2-AR activation by the agonist dexmedetomidine.
receptors, adrenergic, alpha; dorsal hand vein; GWAS; candidate genes; dexmedetomidine
There is a large interindividual variability in dexmedetomidine dose requirements for sedation of patients in intensive care units (ICU). Cytochrome P450 2A6 (CYP2A6) mediates an important route of dexmedetomidine metabolism, and genetic variation in CYP2A6 affects the clearance of other substrate drugs. We examined whether CYP2A6 genotypes affect dexmedetomidine disposition.
In 43 critically ill ICU patients receiving dexmedetomidine infusions adjusted to achieve the desired level of sedation, we determined a median of 5 plasma dexmedetomidine concentrations each. Forty subjects were genotyped for five common CYP2A6 alleles and grouped into normal (n=33), intermediate (n=5), and slow metabolizers (n=2).
Using a Bayesian hierarchical nonlinear mixture model, estimated dexmedetomidine clearance was 49.1 L/hr (posterior mean; 95% credible interval, 41.4 to 57.6 L/hr). There were no significant differences in dexmedetomidine clearance among normal, intermediate, and slow CYP2A6 metabolizer groups.
Genetic variation in CYP2A6 is not an important determinant of dexmedetomidine clearance in ICU patients.
CYP2A6; Dexmedetomidine; Pharmacogenetics; Bayesian Modeling
To examine the hypothesis that genetic variation in enzymes and transporters associated with synthesis, storage, release, and metabolism of catecholamines contributes to the interindividual variability in plasma catecholamine concentrations at rest and after exercise.
We measured plasma NE and epinephrine concentrations at rest and after a standardized exercise protocol in 165 healthy subjects (60% Caucasian, 40% African-American) and examined 29 functional or common variants in 14 genes involved in synthesis, transport, or metabolism of catecholamines. We examined the relationship among genotypes and NE concentrations at rest, and the increase after exercise (ΔNE), by multiple linear regression with adjustment for covariates (age, race, sex, BMI, fitness, and, for Δ NE, resting NE). As a secondary outcome, we performed similar analyses for epinephrine concentrations.
There was large interindividual variability in resting NE (mean, 204±102 pg/mL; range, 39 to 616 pg/mL) and ΔNE (mean, 256±206 pg/mL; range, −97 to 953 pg/mL). Resting NE was significantly associated with variants in 4 genes: CYB561 (P<0.001), VMAT2 (P=0.016), CHGA (P=0.039), and PNMT (P=0.038). ΔNE after exercise was associated with 3 variants in PNMT (P=0.041) and COMT (P=0.033 and 0.035), and resting and exercise epinephrine concentrations with 2 variants each.
The findings of this exploratory study suggest that variation in catecholamine pathway genes contributes to the interindividual variability in plasma NE and epinephrine concentrations at rest and after exercise.
Sympathetic Nervous System; Catecholamines; Exercise; Genetics; Polymorphism
The purpose of this study is to develop a statistical methodology to handle a large proportion of artifactual outliers in a population pharmacokinetic (PK) modeling. The motivating PK data were obtained from a population PK study to examine associations between PK parameters such as clearance of dexmedetomidine and cytochrome P450 2A6 phenotypes. The blood samples were sparsely sampled from patients in intensive care units (ICUs) while different doses of dexmedetomidine were continuously infused. Conventional population PK analysis of these data revealed several challenges and intricacies. Especially, there was strong evidence that some plasma drug concentrations were artifactually high and likely contaminated with the infused drug due to blood sampling processes that are sometimes unavoidable in an ICU setting. If not addressed, or if arbitrarily excluded, these outlying values could lead to biased estimates of PK parameters and miss important relationships between PK parameters and covariates due to increased variability. We propose a novel population PK model, a Bayesian hierarchical nonlinear mixture model, to accommodate the artifactual outliers using a finite mixture as the residual error model. Our results showed that the proposed model handles the outliers well. We also conducted simulation studies with a varying proportion of the outliers. These simulation results showed that the proposed model can accommodate the outliers well so that the estimated PK parameters are less biased.
finite mixture; outlier; nonlinear mixed effect model; pharmacogenetics; pharmacokinetics; NONMEM
α2A-Adrenoceptors (α2A-ARs) have important roles in sympathetic cardiovascular regulation. Variants of ADRA2A affect gene transcription and expression and are associated with insulin release and risk for type 2 diabetes. We examined whether ADRA2A variants are also associated with cardiovascular responses to the selective α2-AR-agonist, dexmedetomidine.
Methods and Results
73 healthy subjects participated in a placebo-controlled single-blind study. After 3 infusions of placebo, subjects received 3 incremental infusions of dexmedetomidine (cumulative dose, 0.4 mcg/kg). Primary outcomes were changes in systolic blood pressure (SBP) and plasma norepinephrine concentrations, measured as difference of the area-under-the-curve during placebo and dexmedetomidine infusions (ΔAUC). We used multiple linear regression analysis to examine the associations between 9 ADRA2A tagging variants and 5 inferred haplotypes and ΔAUC after adjustment for covariates. Homozygous carriers of rs553668, and the corresponding haplotype 4, previously associated with increased α2A-AR expression, had a 2.2-fold greater decrease in AUCSBP after dexmedetomidine (adjusted P=0.006); similarly, the maximum decrease in SBP was 24.7±8.1 mmHg compared to 13.6±5.9 mmHg in carriers of the wildtype allele (P=0.007). Carriers of haplotype 3, previously associated with reduced α2A-AR expression, had a 44% smaller decrease in AUCSBP (P=0.013). Haplotype information significantly improved the model predicting the decrease in SBP (P<0.001). There were similar but non-significant trends for diastolic blood pressure and heart rate. Genotypes were not significantly associated with norepinephrine responses.
Common ADRA2A variants are associated with the hypotensive response to dexmedetomidine. Effects of specific variants/haplotypes in vivo are compatible with their known effects on gene expression in vitro.
receptors; adrenergic; alpha; genetic polymorphism; pharmacogenetics; receptor; variability in drug response
The presynaptic norepinephrine (NE) transporter (NET) mediates synaptic clearance and recycling of NE. NET-deficient transgenic mice have elevated blood pressure, heart rate, and catecholamine concentrations. However, the in vivo effects of common NET variants on cardiovascular regulation at rest and during exercise are unknown.
We studied cardiovascular responses and plasma catecholamine concentrations at rest and during bicycle exercise at increasing workloads (25, 50 and 75 W) in 145 healthy subjects. We used multiple linear regressions to analyze the effect of common, purportedly functional polymorphisms in NET (rs2242446 and rs28386840) on cardiovascular measures.
44% and 58.9% of subjects carried at least one variant allele for NET T-182C and A-3081T, respectively. Systolic blood pressure (SBP) during exercise and SBP area-under-the-curve were higher in carriers of variant NET alleles (P=0.003 and 0.009 for T-182C and A-3081T, respectively) and NET haplotype -182C/-081T compared to -82T/-3081A (all P<0.01). Diastolic blood pressure (DBP) during exercise was also higher at lower, but not at higher exercise stages in carriers of NET -182C (P<0.01) and -081T (P< 0.05). NET genotypes were not associated with catecholamine concentrations or heart rate.
Common genetic NET variants (-182C and -081T) are associated with greater blood pressure response to exercise in humans.
orepinephrine transporter; Blood pressure; Exercise; Polymorphism
By guiding initial warfarin dose, pharmacogenetic (PGx) algorithms may improve the safety of warfarin initiation. However, once INR response is known, the contribution of PGx to dose refinements is uncertain. This study sought to develop and validate clinical and PGx dosing algorithms for warfarin dose refinement on days 6–11 after therapy initiation.
Materials and Methods
An international sample of 2,022 patients at 13 medical centers on 3 continents provided clinical, INR, and genetic data at treatment days 6–11 to predict therapeutic warfarin dose. Independent derivation and retrospective validation samples were composed by randomly dividing the population (80%/20%). Prior warfarin doses were weighted by their expected effect on S-warfarin concentrations using an exponential-decay pharmacokinetic model. The INR divided by that “effective” dose constituted a treatment response index.
Treatment response index, age, amiodarone, body surface area, warfarin indication, and target INR were associated with dose in the derivation sample. A clinical algorithm based on these factors was remarkably accurate: in the retrospective validation cohort its R2 was 61.2% and median absolute error (MAE) was 5.0 mg/week. Accuracy and safety was confirmed in a prospective cohort (N=43). CYP2C9 variants and VKORC1-1639 G→A were significant dose predictors in both the derivation and validation samples. In the retrospective validation cohort, the PGx algorithm had: R2= 69.1% (P<0.05 vs. clinical algorithm), MAE= 4.7 mg/week.
A pharmacogenetic warfarin dose-refinement algorithm based on clinical, INR, and genetic factors can explain at least 69.1% of therapeutic warfarin dose variability after about one week of therapy.
warfarin; VKORC1; CYP2C9; pharmacogenetic
Vascular α1- and α2-adrenergic receptors (ARs) mediate vasoconstriction and are major determinants of peripheral vascular tone. There is wide variability in vasoconstrictor sensitivity to α1- and α2AR-agonists among individuals. In previous studies this variability was not explained by identified α1- and α2-AR genetic variants. Thus, we hypothesized that adrenergic vasoconstrictor sensitivity is determined by shared constrictor mechanisms downstream of the individual receptors and that α1- and α2-AR-mediated vasoconstrictor sensitivity would therefore be correlated.
Dorsal hand vein responses to increasing doses of the α1-AR agonist phenylephrine (12 ng/min –12,000 ng/min) and the α2-AR agonist dexmedetomidine (0.01 ng/min – 100 ng/min) were measured in healthy subjects using a linear variable differential transformer. From individual dose-response curves we calculated the dose of phenylephrine and dexmedetomidine that produced 50% (ED50) of maximum venoconstriction (Emax) for each subject. We examined the correlation between phenylephrine and dexmedetomidine ED50 and Emax before and after adjustment for covariates (age, gender, ethnicity, BMI, blood pressure, heart rate, and baseline plasma norepinephrine concentrations).
In 62 subjects (36 males, 34 African American, 28 Caucasians) the median ED50 for dexmedetomidine was 1.32 ng/min (IQR, 0.45–5.37 ng/min), and for phenylephrine 177.8 ng/min (IQR, 40.7– 436.5 ng/min). The Emax for phenylephrine was 90.8% (82.2–99.6%) and for dexmedetomidine 80.0% (64.7–95.2%). There was no correlation between individual sensitivities (ED50) to phenylephrine and dexmedetomidine, before and after adjustment for covariates (p>0.30).
Phenylephrine and dexmedetomidine both produce strong venoconstriction in the dorsal hand vein; however, there is no significant correlation between vascular sensitivity to an α1-AR and α2-AR agonist. These findings suggest independent regulation of vascular α1- and α2-AR-mediated responses.
α1 adrenoceptors; α2 adrenoceptors; vasoconstriction
The considerable variability in the warfarin dose–response relationship between individuals, is explained mainly by genetic variation in its major metabolic (CYP2C9) and target (VKORC1) enzymes. Despite the predominance of pharmacogenetics, environmental factors also affect the pharmacokinetics and pharmacodynamics of warfarin, and are often overlooked. Among these factors, dietary and supplemental vitamin K consumption is a controllable contributor to within-, and between-patient variability of warfarin sensitivity. In this commentary we review the current role of vitamin K in warfarin anticoagulation therapy, with emphasis on the following:
The effect of dietary and supplemental vitamin K on warfarin anticoagulation, beyond the impact of genetic variability in CYP2C9 and VKORC1. We deal separately with the effects of vitamin K on warfarin dose requirements during the induction of therapy, as opposed to its effect on stability of anticoagulation control during maintenance therapy.The role of vitamin K supplementation in warfarin treated patients with vitamin K deficiency as well as in patients with unstable warfarin anticoagulation, andThe role of therapeutic vitamin K in cases of warfarin over-anticoagulation.
genotype; pharmacogenetics; vitamin K; warfarin
The mechanisms underlying interindividual variability in pain perception and cognitive responses are undefined but highly heritable. α2C- and α2A-adrenergic receptors regulate noradrenergic activity and are important mediators of pain perception and analgesia. We hypothesized that common genetic variants in these genes, particularly the ADRA2C del322–325 deletion variant, affect pain perception or cognitive responses.
We studied 73 healthy subjects (37 Caucasians and 36 African-Americans) aged 25.4±4.6 years. Pain response to a cold pressor test was measured using a 10 cm visual analog scale and again on the next day, after 3 infusions of the selective α2-agonist dexmedetomidine. Standardized cognitive tests were administered at baseline and after each infusion. The contribution of ADRA2C deletion genotype, dexmedetomidine concentration, and other covariates to pain perception and cognitive responses was determined using multiple linear regression models. Secondary analysis examined the effects of ADRA2A and other ADRA2C variants on pain perception.
ADRA2C Del homozygotes had higher pain scores in response to cold at baseline (6.3±1.8 cm) and after dexmedetomidine (5.6±2.2 cm) than insertion allele carriers (4.6±2.1 cm [baseline] and 3.8±1.9 cm [after dexmedetomidine]; adjusted P-values=0.019 and 0.004, respectively). Cognitive responses were unrelated to ADRA2C Ins/Del genotype. None of the other ADRA2A and ADRA2C variants was significantly related to cold pain sensitivity before dexmedetomidine; after dexmedetomidine, ADRA2A rs1800038 was marginally associated (P=0.03).
The common ADRA2C del322–325 variant affected pain perception before and after dexmedetomidine but did not affect other cognitive responses, suggesting that it contributes to interindividual variability in pain perception.
α2C-adrenoceptor; α2A-adrenoceptor; pain; genetics; cognition
Vascular alpha2B adrenergic receptors (α2B-ARs) mediate vasoconstriction and contribute to peripheral regulation of vascular tone. In vitro, a common 301-303 deletion in the α2B-AR gene, ADRA2B, results in loss of α2B-AR desensitization. We examined the hypothesis that ADRA2B del301-303 or other common ADRA2B variants alter vascular desensitization in vivo.
We measured sensitivity to a highly selective α2-AR agonist, dexmedetomidine, (0.01–1000 ng/min) in the dorsal hand vein in 41 healthy subjects. To induce desensitization a dose of dexmedetomidine that resulted in submaximal constriction was infused for 180 minutes and dorsal hand vein responses measured. Desensitization was defined as the ratio between the area-under-the-effect curve for each individual’s response, and the hypothetical area-under-the-effect curve assuming that the initial response had been maintained for 180 minutes (ratio below 1 reflecting desensitization). The relationship between six ADRA2B variants (1 promoter, 3 coding, and 2 in the 3′ UTR) with an allele frequency > 5% and desensitization was determined.
Forty-one subjects (22 men, 21 whites, age 18-45 years) were studied. The ADRA2B 301-303 deletion allele (ins/del and del/del, n=18) was associated with resistance to desensitization [1.01 (IQR 0.90-1.06)] compared to ins/ins homozygous subjects (n=23) [0.91 (IQR 0.73-0.99)], p=0.026. In addition, the −98 GG, 1182 CC, and 1776 CC genotypes were associated with significantly less desensitization than GC or CC, and CA or AA genotypes, respectively.
Common ADRA2B variants contribute to the interindividual variability in vascular desensitization to an α2-AR agonist in vivo.
Adrenergic receptor agonists; Alpha 2B adrenergic receptor; Genetic variability; Human; Receptor, adrenergic alpha; Vasoconstriction; Receptor desensitization
Cold-induced vasoconstriction is mediated in part by selective enhancement of local α2C–adrenoceptor (α2C-AR) activity. A common insertion-deletion variant in the α2C-AR gene (ADRA2C del322-325) results in an approximately 85% reduction of agonist-mediated function in vitro. We tested the hypothesis that individuals with the ADRA2C del322-325 variant have attenuated vasoconstriction in response to cold.
Cutaneous digital blood flow (flux) was measured by laser Doppler flowmetry in a controlled environment at room temperature and during two cycles of graduated local heat and cold exposure in 31 subjects. Temperature-response curves were analyzed to estimate the following measures: Emin (minimal flux during cooling), and ET50 and ET90 (the local temperature at which flux decreased by 50% and 90% respectively).
We found no significant genotypic differences in Emin (24.3±19.5, 30.0±20.5, and 21.5±25.9 AU for ins/ins, ins/del, and del/del genotypes, respectively; P=0.48), ET50 (25.5±6.0, 25.1±6.7, and 25.1±7.1 °C; P=0.99), or ET90 (20.5±4.7, 22.1±4.0, and 20.8±6.7 °C; P=0.77) in either the first or second heating and cooling cycle (cycle 1 values presented).
The ADRA2C del322-325 variant did not affect vascular sensitivity to local cold exposure.
adrenergic receptor; microcirculation; ADRA2C; cold; laser Doppler flowmetry
A common, functionally significant polymorphism in GRK5 (Gln41Leu) encodes a gain-of-function enzyme that enhances desensitization of the β1-adrenergic receptor. GRK5 Leu41 has been postulated to confer endogenous ‘genetic β-blockade’ and contribute to an attenuated response to β-blockers in black subjects. The effects of this GRK5 variant on sensitivity to a β-blocker have not been studied in humans. We hypothesized that the GRK5 Gln41Leu variant contributes to interindividual variability in response to β-blockade and to the ethnic difference in sensitivity between black and Caucasian individuals.
Materials & methods
We measured the heart rate at rest and during a graded incremental exercise in 154 healthy subjects (85 white and 69 black) before and after an oral administration of 25 mg atenolol. We determined the genotypes of GRK5 (Gln41Leu), β1-adrenergic receptor (ADRB1 Ser49Gly and Arg389Gly) genotypes and plasma atenolol concentrations. The effects of genotype and covariates on sensitivity to atenolol, measured as the reduction in exercise-induced tachycardia, were determined using multiple regression analyses.
The minor allele frequency of GRK5 Leu41 was 32.6% in blacks and 0% in whites. Black individuals were less sensitive to atenolol than white individuals (p ≤ 0.011) but this was not explained by the GRK5 genotype. The GRK5 genotype had no effect on resting heart rate before (p = 0.61) and after adjustment for age, sex, ethnicity, atenolol concentrations, BMI and ADRB1 genotypes (p = 0.81). The decrease in heart rate after atenolol administration did not differ significantly according to the GRK5 genotype at rest or after exercise, before (all p > 0.14) and after statistical adjustment for covariates (all p > 0.17).
The GRK5 Gln41Leu polymorphism does not affect sensitivity to the β1-adrenergic blocker, atenolol, during acute physiological adrenergic stimulation, nor does it contribute to the ethnic differences in sensitivity to atenolol among black and Caucasian individuals.
β-blockade; atenolol; ethnicity; GRK5; pharmacogenetics
The neurotransmitter acetylcholine (ACh) plays a critical role in brain circuits mediating motor control, attention, learning and memory. Cholinergic dysfunction is associated with multiple brain disorders including Alzheimer’s Disease, addiction, schizophrenia and Attention-Deficit Hyperactivity Disorder (ADHD). The presynaptic choline transporter (CHT, SLC5A7) is the major, rate-limiting determinant of ACh production in the brain and periphery and is consequently upregulated during tasks that require sustained attention. Given the contribution of central cholinergic circuits to the control of movement and attention, we hypothesized that functional CHT gene variants might impact risk for ADHD. We performed a case-control study, followed by family-based association tests on a separate cohort, of two purportedly functional CHT polymorphisms (coding variant Ile89Val (rs1013940) and a genomic SNP 3’ of the CHT gene (rs333229), affording both a replication sample and opportunities to reduce potential population stratification biases. Initial genotyping of pediatric ADHD subjects for two purportedly functional CHT alleles revealed a 2–3 fold elevation of the Val89 allele (n = 100; P = 0.02) relative to healthy controls, as well as a significant decrease of the 3’SNP minor allele in Caucasian male subjects (n = 60; P = 0.004). In family based association tests, we found significant overtransmission of the Val89 variant to children with a Combined subtype diagnosis (OR = 3.16; P = 0.01), with an increased Odds Ratio for a haplotype comprising both minor alleles. These studies show evidence of cholinergic deficits in ADHD, particularly for subjects with the Combined subtype, and, if replicated, may encourage further consideration of cholinergic agonist therapy in the disorder.
Electronic supplementary material
The online version of this article (doi:10.1007/s11689-009-9033-8) contains supplementary material, which is available to authorized users.
Choline; Transporter; SLC5A7; Attention-deficit hyperactivity disorder; Polymorphism
Black subjects may be less responsive to β-blockers than whites. Genetic variants in the β1-adrenergic receptor (β1-AR) associated with lesser response to β-blockers are more common in blacks than whites. The purpose of this study was to determine whether ethnic differences in response to β-blockade can be explained by differing distributions of functional genetic variants in the β1-AR.
We measured sensitivity to β-blockade by the attenuation of exercise-induced tachycardia in 165 subjects (92 whites) who performed a graded bicycle exercise test before and 2.75 hours after oral atenolol (25 mg). We determined heart rate at rest and at 3 exercise levels from continuous ECG recordings and calculated the area-under-the-curve (AUC). We also measured plasma atenolol concentrations and determined genotypes for variants of the β1-AR (Ser49Gly, Arg389Gly) and α2C-AR (del322–325). The effects of ethnicity, genotype, and other covariates on the heart rate reduction after atenolol were estimated in multiple regression analyses.
Atenolol resulted in a greater reduction in exercise heart rate in whites than blacks (P=0.006). β1-AR Arg389 (P=0.003), but not the α2C-AR 322–325 insertion allele (P=0.31), was independently associated with a greater reduction in heart rate AUC. Ethnic differences in sensitivity to atenolol remained significant (P=0.006) after adjustment for β1-AR and α2C-AR genotypes.
Ethnic differences in sensitivity to the β1-blocker atenolol persist even after accounting for different distributions of functional genetic β1-AR variants, suggesting that additional, as yet unidentified factors contribute to such ethnic differences.
Beta-adrenergic receptor; Genetics; Pharmacology; Populations
Cardiovascular mortality is increased in systemic lupus erythematosus (SLE). Increased plasma concentrations of N-terminal pro brain natriuretic peptide (NT-proBNP) are associated with cardiovascular morbidity and mortality in the general population. We examined the hypothesis that NT-proBNP concentrations are higher in patients with SLE, and are related to inflammation, augmentation index, coronary atherosclerosis, and cardiovascular risk factors.
Serum concentrations of NT-proBNP were measured in 113 patients with SLE and in 80 control subjects. Coronary calcification and augmentation index were measured by electron beam computed tomography and non-invasive pulse wave analysis, respectively.
Patients with SLE had higher concentrations of NT-proBNP [median 38.6 (IQR 2.5–126.9) pg/mL] than controls [11.7 (1.6–47.9) pg/mL] (P=0.002). Augmentation index was higher in patients with SLE [25.0% (20.5%–31.5%)] than controls [20.5% (12.0%–29.0%)], (P=0.04). In patients with SLE, NT-proBNP concentrations were associated with disease damage (rho=0.31, P<0.001) and duration (rho=0.21, P=0.02) but not with disease activity, CRP, ESR, TNF-α, IL-6, coronary calcium score, or augmentation index (P all ≥0.18).
Patients with SLE have increased concentrations of NT-proBNP but this is not explained by atherosclerotic burden, augmentation index, or inflammatory state.
systemic lupus erythematosus; atherosclerosis; NT-proBNP
Cardiovascular responses to stressors are regulated by sympathetic activity, increased in black Americans, and associated with future cardiovascular morbidity. Our aim was to determine whether two functional variants in genes regulating sympathetic activity, a deletion in the α2C-adrenergic receptor (ADRA2C del322-325) and a G-protein β3 subunit variant (GNB3 G825T), affect cardiovascular responses to physiologic stressors and contribute to their ethnic differences.
We measured heart rate and blood pressure responses to a cold pressor test (CPT) in 79 healthy subjects (40 blacks, 39 whites), aged 25.7 ± 5.3 years, and determined genotypes for the ADRA2C and GNB3 variants. We examined the response variables (increase in heart rate and blood pressure) in multiple linear regression analyses adjusting first for baseline measures, ethnicity, and other covariates, and then additionally for genotypes.
Black subjects had a greater heart rate response to CPT than whites (mean difference, 9.9 bpm; 95% confidence interval (CI), 4.1 to 15.6; P=0.001). Both the ADRA2C del/del (15.8 bpm; 95% CI, 8.0 to 23.7; P<0.001) and GNB3 T/T genotypes (6.8 bpm; 95% CI, 0.9 to 12.7; P=0.026) were associated with greater heart rate response. After adjusting for genotypes, the ethnic difference was abrogated (1.3 bpm; 95% CI, −5.4 to 8.0; P=0.70), suggesting that the genetic variants contributed substantially to ethnic differences.
Variation in genes that regulate sympathetic activity affects hemodynamic stress responses and contributes to their ethnic differences. This study elucidates how genetic factors may in part explain ethnic differences in cardiovascular regulation.
Cardiovascular physiology; Stress; Ethnic groups; Genetics; Receptors, Adrenergic, alpha-2; G-protein beta3 subunit
Genetic determinants of variability in response to β-blockers are poorly characterized. We defined changes in mRNA expression after a β-blocker to identify novel genes that could affect response and correlated these with inhibition of exercise-induced tachycardia, a measure of β-blocker sensitivity.
Nine subjects exercised before and after a single oral dose of 25mg atenolol and mRNA gene expression was measured using an Affymetrix GeneChip Human Gene 1.0 ST Array. The area under the heart rate-exercise intensity curve (AUC) was calculated for each subject; the difference between post- and pre-atenolol AUCs (Δ AUC), a measure of β-blocker response, was correlated with the fold-change in mRNA expression of the genes that changed more than 1.3-fold.
Fifty genes showed more than 1.3-fold increase in expression; 9 of these reached statistical significance (P < 0.05). Thirty-six genes had more than 1.3-fold decrease in expression after atenolol; 6 of these reached statistical significance (P < 0.05). Change in mRNA expression of FGFBP2 and Probeset ID 8118979 was significantly correlated with atenolol response (P = 0.03 and 0.02, respectively).
The expression of several genes not previously identified as part of the adrenergic signaling pathway changed in response to a single oral dose of atenolol. Variation in these genes could contribute to unexplained differences in response to β-blockers.
Atenolol; mRNA expression; Microarray