Hong-Guang Xie

Identification of functionally variant MDR1 alleles among European Americans and African Americans

MDR1 (P‐glycoprotein) is an important factor in the disposition of many drugs, and the involved processes often exhibit considerable interindividual variability that may be genetically determined. Single‐strand conformational polymorphism analysis and direct sequencing of exonic MDR1 deoxyribonucleic acid from 37 healthy European American and 23 healthy African American subjects identified 10 single nucleotide polymorphisms (SNPs), including 6 nonsynonymous variants, occurring in various allelic combinations. Population frequencies of the 15 identified alleles varied according to racial background. Two synonymous SNPs (C1236T in exon 12 and C3435T in exon 26) and a nonsynonymous SNP (G2677T, Ala893Ser) in exon 21 were found to be linked (MDR1ast;2) and occurred in 62% of European Americans and 13% of African Americans. In vitro expression of MDR1 encoding Ala893 (MDR1ast;1) or a site‐directed Ser893 mutation (MDR1ast;2) indicated enhanced efflux of digoxin by cells expressing the MDR1‐Ser893 variant. In vivo functional relevance of this SNP was assessed with the known P‐glycoprotein drug substrate fexofenadine as a probe of the transporters activity. In humans, MDR1ast;1 and MDR1ast;2 variants were associated with differences in fexofenadine levels, consistent with the in vitro data, with the area under the plasma level–time curve being almost 40% greater in the *1/*1 genotype compared with the *2/*2 and the *1/*2 heterozygotes having an intermediate value, suggesting enhanced in vivo P‐glycoprotein activity among subjects with the MDR1ast;2 allele. Thus allelic variation in MDR1 is more common than previously recognized and involves multiple SNPs whose allelic frequencies vary between populations, and some of these SNPs are associated with altered P‐glycoprotein function.

CYP2C9 allelic variants: ethnic distribution and functional significance

Cytochrome P-450 (CYP) 2C9 CYP2C9 is a polymorphically expressed enzyme responsible for the metabolism of several clinically important drugs, some with a low therapeutic index. This review summarizes the structure-function relationship of the CYP2C9 promoter and coding regions, known polymorphisms, the functional significance of various CYP2C9 alleles in vitro and in vivo, and their population frequencies. In addition, possible molecular mechanisms underlying ethnic variability in the metabolism of CYP2C9 substrate drugs are discussed.

A common β1-adrenergic receptor polymorphism (Arg389Gly) affects blood pressure response to β-blockade

A common polymorphism of the β1‐adrenergic receptor Arg389Gly markedly affects function in vitro, but little is known about its in vivo significance.

Arg389Gly β1-adrenoceptor polymorphism varies in frequency among different ethnic groups but does not alter response in vivo

There are marked interethnic differences in beta 1-adrenoceptor-mediated responsiveness, with sensitivity decreased in African-Americans and increased in Chinese compared with Caucasians. Therefore, the frequency of a common naturally occurring polymorphism of the human beta 1-adrenoceptor gene (Arg389Gly), which has functional importance in vitro, was determined in 194 African-Americans, 316 Caucasian-Americans, 221 Hispanic-Americans and 142 Chinese. African-Americans were found to have a significantly lower frequency of the Arg389 allele than the other three ethnic groups (all P < 0.01). In the populations studied, the order of the distribution of the Arg389 allele was: Chinese (74%) > Caucasians (72%) > Hispanics (67%) > African-Americans (58%). To determine the functional significance of the Arg389Gly beta 1-adrenoceptor polymorphism, in-vivo heart rate responses to exercise were compared in healthy subjects homozygous for the Arg (n = 9) and Gly (n = 8) alleles. Heart rate response to exercise was not affected by genotype (P = 0.4). Although ethnic differences in the frequency of the beta 1-adrenoceptor Arg389Gly polymorphism exist, the polymorphism does not appear to have functional significance in healthy subjects and therefore may not contribute to ethnic differences in response to drugs acting through the beta 1-adrenoceptor.

St John's wort-associated drug interactions: Short-term inhibition and long-term induction?

t f ( e t i t s a p i c F S In this issue of the Journal, Rengelshausen et al eport an interesting observation that short-term adminstration of St John’s wort (SJW) resulted in a marked ncrease in voriconazole exposure (22% higher than ontrol during the initial 10 hours) whereas long-term xposure to SJW was associated with a significant eduction in voriconazole’s bioavailability (59% lower han control on day 15) as a result of increased oral learance. The mechanisms underling the ability of JW to both inhibit and induce metabolism of a drug re of interest. SJW, an extract made from the flowers of the perenial plant Hypericum perforatum L (Fig 1), is widely sed as an over-the-counter herbal antidepressant and ccounts for a significant portion of the

Alpha1A-adrenergic receptor polymorphism: association with ethnicity but not essential hypertension.

4 billion a year pent on alternative medicines in the United States. ecent in vitro and focused clinical studies have shown hat clinically important drug interactions associated ith the use of SJW can be explained by altered drug etabolism or drug transport. Importantly, a large numer of clinically relevant SJW-drug interactions, includng numerous case reports and clinical studies, have een reported (Table I). These types of studies and eports of interactions have led to the Food and Drug dministration to issue a warning to the medical comunity about the use of SJW. SJW is a natural product of more than 1 onstituent. Commercially available SJW prepara-

Beta-1-adrenoceptor genetic variants and ethnicity independently affect response to beta-blockade.

The alpha1-adrenergic receptor (alpha1-AR) mediates vasoconstriction and plays an important role in the regulation of vascular tone. Increased alpha1-AR-mediated vasoconstrictor sensitivity, increased vascular reactivity to stress, and an increased prevalence of hypertension occur in African-Americans. The human alpha1A-AR is the predominant alpha1-AR subtype in vascular smooth muscle. The potential relevance of alpha1A-AR genetic variation to ethnic differences in vascular response and to the pathogenesis of hypertension prompted us to determine the frequency distribution of a recently identified polymorphism (Arg492 to Cys) in the alpha1A-AR in normotensive and hypertensive black and white American individuals. Polymerase chain reaction-based PstI restriction fragment length polymorphisms in the human alpha1A-AR gene were determined in 231 African-American and 282 Caucasian individuals, both with and without hypertension. There were marked differences in the genotypic and allelic distributions of the Arg492 to Cys alpha1A-AR polymorphism between African-American and Caucasian individuals (Cys492/Cys492 genotype, normotensive: 7.6% versus 30.1%; hypertensive: 7.1% versus 26.2%; Cys492 allele, normotensive: 29.5% versus 53.8%; hypertensive: 28.8% versus 55.2%; blacks versus whites, P < 0.0001). The frequency of the variant Cys492 allele was similar in normotensive and hypertensive individuals, both in African-Americans (29.5% versus 28.8%) and Caucasians (53.8% versus 55.2%). There were no significant intergenotypic differences in blood pressure (all P > 0.05). The data indicate that this polymorphism is not associated with essential hypertension in black or white Americans, but that the frequency of the alpha1A-AR Arg492 allele occurs significantly more commonly in African-Americans than in Caucasians. The potential role of the Arg492 to Cys alpha1A-AR polymorphism in ethnic differences in vascular alpha1-adrenergic response requires further investigation.

In-vivo effects of Glu298Asp endothelial nitric oxide synthase polymorphism.

Objectives Black patients may be less responsive to &bgr;-blockers than whites. Genetic variants in the &bgr;1-adrenergic receptor (&bgr;1-AR) associated with lesser response to &bgr;-blockers are more common in blacks than in whites. The purpose of this study was to determine whether ethnic differences in response to &bgr;-blockade can be explained by differing distributions of functional genetic variants in the &bgr;1-AR. Methods We measured sensitivity to &bgr;-blockade by the attenuation of exercise-induced tachycardia in 165 patients (92 whites), who performed a graded bicycle exercise test before and 2.5 h after oral atenolol (25 mg). We determined heart rate at rest and at three exercise levels from continuous ECG recordings and calculated the area under the curve. We also measured plasma atenolol concentrations and determined genotypes for variants of the &bgr;1-AR (Ser49Gly, Arg389Gly) and &agr;2C-AR (del322-325). The effects of ethnicity, genotype, and other covariates on the heart rate reduction after atenolol were estimated in multiple regression analyses. Results Atenolol resulted in a greater reduction in exercise heart rate in whites than in blacks (P=0.006). &bgr;1-AR Arg389 (P=0.003), but not the &agr;2C-AR 322-325 insertion allele (P=0.31), was independently associated with a greater reduction in heart rate area under the curve. Ethnic differences in sensitivity to atenolol remained significant (P=0.006) after adjustment for &bgr;1-AR and &agr;2C-AR genotypes. Conclusion Ethnic differences in sensitivity to the &bgr;1-blocker atenolol persist even after accounting for different distributions of functional genetic &bgr;1-AR variants, suggesting that additional, as yet unidentified factors contribute to such ethnic differences.

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.

Endothelial nitric oxide synthase catalyses the formation of the vasodilator nitric oxide, a major regulator of vascular tone. The Asp298 polymorphism of the nitric oxide synthase gene is associated with altered function and expression of the enzyme in vitro and myocardial infarction and coronary artery spasm in vivo. We examined the effect of the Glu298Asp polymorphism on: (1) local vascular responses to phenylephrine, acetylcholine, glyceryl trinitrate and prostaglandin E1 in the dorsal hand vein; (2) changes in forearm blood flow during mental stress, a measure of nitric oxide-mediated effect on resistance vessels; (3) excretion of urinary nitrite/nitrate as a measure of total body nitric oxide production; and (4) F2-isoprostane metabolite, a measure of oxidative stress, in healthy Glu298 (n = 12) and Asp298 (n = 13) homozygotes. There were no significant differences in acetylcholine dose responses (P = 0.29) in Glu298 and Asp298 homozygotes. Responses to glyceryl trinitrate, prostaglandin E1 and the alpha-adrenergic agonist phenylephrine did not differ by genotype. Forearm blood flow was similar at rest and increased significantly (from 7.5 ml/min/100 ml to 12.2 ml/min/100 ml; P = 0.003), but similarly (P = 0.2), during mental stress in both genotypes. Asp298 homozygotes excreted significantly less nitrate/nitrite than Glu298 homozygotes (nitrate + nitrite/creatinine ratio 0.05 +/- 0.01 vs. 0.09 +/- 0.01, respectively; P < 0.005). Urinary F2-isoprostane metabolite excretion did not differ (Glu298, 2.04 +/- 0.25 ng/mg creatinine; Asp298, 1.85 +/- 0.37 ng/mg creatinine; P = 0.7). We conclude that in healthy volunteers the Glu298Asp polymorphism affects endogenous nitric oxide production without affecting nitric oxide-mediated vascular responses. This polymorphism may only have clinical significance in the presence of endothelial dysfunction.

Variations in the α2A-adrenergic receptor gene and their functional effects

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.