Sodium-lithium countertransport and probability of hypertension in Caucasians 47 to 89 years old

Association between SLC20A1 and sodium-lithium countertransport

BACKGROUND

Sodium-lithium countertransport (SLC) is a premorbidity marker of essential hypertension. Evidence from linkage analysis and kinetic studies in humans have suggested that SLC20A1 variations may affect SLC activity as measured in erythrocytes and leukocytes. SLC20A1 encodes a sodium-dependent phosphate cotransporter, and is widely expressed on the mammalian plasma membrane. In this study, we investigated the relationship between SLC20A1 and SLC activity.

METHODS

By means of gene expression profiling, we studied the expressions of SLC20A1 in individuals with high SLC activity as compared to those with low SLC activity. In order to investigate the allelic association of SLC20A1 with SLC, we genotyped six tag single-nucleotide polymorphisms (tSNPs) in SLC20A1 in subjects from the Rochester Family Heart Study (RFHS) involving 1,815 individuals from 252 pedigrees of mixed European ancestry. The genetic association of SLC20A1 with SLC was assessed by Sequential Oligogenic Linkage Analysis Routines (SOLAR).

RESULTS

Expression levels of SLC20A1 were higher in individuals with high SLC activity than in those with low SLC activity. Four SNPs in SLC20A1 were associated with SLC activity after adjusting for age, sex, body mass index, triglycerides, and antihypertensive drug treatment (P ≤ 0.05). The strongest evidence of association was in respect of rs4849091 (P = 0.001), and this association remained significant even after correction for multiple tests.

CONCLUSIONS

Allelic variations in SLC20A1 were associated with the levels of SLC activity, thereby supporting the hypothesis that SLC20A1 is involved in determining SLC activity.

Association between NEDD4L gene and sodium lithium countertransport

BACKGROUND

Sodium lithium countertransport (SLC) is an intermediate phenotype of essential hypertension. The aim of this study was to identify candidate genes for SLC by a strategy of combining gene expression profiling and linkage analysis, and to examine the association between the candidate gene and SLC as well as hypertension.

METHODS

In order to identify SLC-related genes, the top 1% of the genes that were differentially expressed between high- and low-SLC groups in a gene expression microarray were compared with published SLC/hypertension gene linkage maps so as to identify regions of overlap. The association between the genetic variation in the candidate gene and the SLC and blood pressure phenotypes were further assessed in the Rochester Family Heart Study (RFHS) involving 1,815 individuals of European ancestry, belonging to 252 pedigrees.

RESULTS

Based on gene expression profiling and evidence from genome-wide linkage analysis, and in the light of its potential biochemical function, E3 ubiquitin-protein ligase NEDD4-like (NEDD4L) was identified as being both a positional and a functional candidate gene for SLC. The difference in expressions of NEDD4L between the high- and low-SLC groups was confirmed by reverse transcription-PCR (RT-PCR) analysis. After adjusting for age, sex, and body mass index (BMI), four single-nucleotide polymorphisms (SNPs) in NEDD4L were found to be associated with SLC (P ≤ 0.05). Further, haplotype analysis revealed that, after correction for multiple testing, one of the haplotypes (H2) was still significantly (P = 0.006) associated with SLC.

CONCLUSIONS

The strategy of combining gene expression profiling and linkage analysis successfully guided us in identifying NEDD4L as a candidate gene involved in regulating SLC activity. Variations in the NEDD4L gene are associated with SLC activity.

Association of SLC34A2 variation and sodium-lithium countertransport activity in humans and baboons

BACKGROUND

Sodium-lithium countertransport (SLC) activity, an intermediate phenotype of essential hypertension, has been linked to a region of baboon chromosome 5, homologous to human chromosome 4p. Human SLC34A2, located at chromosome 4p15.1-p15.3, is a positional candidate gene for SLC. The specific aim of this study was to identify genetic variants of the SLC34A2 gene in both baboon and human, and to examine the relationship of these polymorphisms with SLC activity and blood pressure.

METHODS

Single-nucleotide polymorphism (SNP) was identified by sequencing the SLC34A2 gene in 24 baboon founders and 94 unrelated individuals. All tag SNPs in SLC34A2 were genotyped in 1,856 individuals from 252 pedigrees of mixed European ancestry. Three SNPs in baboon were genotyped in 634 baboons comprising 11 pedigrees.

RESULTS

In human, one SNP (rs12501856) was found to be significantly associated with SLC individually, though it did not pass multiple testing correction; however, haplotype 2 containing allele C of SNP rs12501856 showed strong evidence of association with SLC (P = 0.0037) after multiple comparison adjustment. This haplotype was also marginally associated with diastolic blood pressure and systolic blood pressure. This finding was confirmed in baboons, where a highly significant association was detected between SLC and baboon SNP Asn136Asn (P = 0.0001). However, the associated SNP did not account for the linkage signal on baboon chromosome 5.

CONCLUSIONS

Consistent results in two different species imply that SLC34A2 is associated with SLC activity and blood pressure.

Multivariate linkage analysis of blood pressure and body mass index

Multivariate linkage analyses of correlated traits provide greater statistical power to identify genetic loci with effects too small to be detected in single trait analyses. We conducted genomewide multivariate analyses of systolic BP, diastolic BP, and body mass index (BMI) in 1,848 non-Hispanic white subjects (968 females, 880 males) from 279 multigenerational pedigrees from Rochester, Minnesota. Blood pressure was measured by random zero sphygmomanometer; body mass index was calculated from measurements of height and weight; and genotypes were measured at 520 microsatellite marker loci distributed across the 22 autosomes. Univariate linkage analyses demonstrated tentative evidence of linkage (defined by univariate LOD scores of 1.30-1.99) for diastolic BP on chromosome 18 and for BMI on chromosomes 3, 10, and 18. Bivariate linkage analyses showed tentative evidence of linkage (defined by bivariate LOD scores of 2.06-2.86) for systolic and diastolic BP on chromosome 14 and for either measure of BP and BMI on chromosomes 2, 3, 10, and 18; and suggestive evidence of linkage (defined by bivariate LOD scores of 2.87-3.99) for either measure of BP and BMI on chromosomes 10 and chromosome 15. Trivariate linkage analyses of systolic and diastolic BP and BMI provided evidence of a region influencing all three traits on chromosome 10, where the trivariate LOD score rose to a maximum value of 4.09 (at 144 cM, P=0.0007), and possibly on chromosome 2, where it rose to a maximum value of 2.80 (at 77 cM, P=0.0075). For genomewide linkage analyses to succeed in localizing genes influencing BP, it may be advantageous to exploit the greater statistical power of multivariate linkage analyses to identify loci with pleiotropic effects on correlated traits.

Ambulatory blood pressure and coronary artery calcification in middle-aged and younger adults

BACKGROUND

The objective of this study was to investigate relationships between blood pressure (BP) determined by ambulatory monitoring and coronary artery calcification (CAC) determined by electron beam computed tomography (EBCT) in middle-aged and younger adults without symptoms of coronary artery disease.

METHODS

Measures of office and ambulatory BP were analyzed in 298 asymptomatic adults (134 women and 164 men) from the white population of Rochester, MN, who were 20 to 60 years old (mean +/- SD, 40 +/- 9 years). For the ambulatory BP measurements, the active period of the day was defined as the daytime, out-of-bed hours and the inactive period as the nighttime, in-bed hours. Logistic regression was used to assess whether ambulatory measures of BP influenced the probability of having CAC detected by EBCT.

RESULTS

After adjusting for sex, age, and office measures of BP, ambulatory diastolic BP during the active and inactive periods were each statistically significant additional predictors of the probability of having CAC. Similarly, after adjusting for sex, age, and ambulatory systolic BP, ambulatory diastolic BPs during each period were also statistically significant additional predictors of the probability of having CAC. In contrast, measures of ambulatory systolic BP, pulse pressure, and diurnal dipping of BP levels from the active to the inactive period did not make statistically significant additional contributions to the probability of having CAC.

CONCLUSION

These findings emphasize the role that the hemodynamic stress of diastolic BP may play in the early development of atherosclerotic coronary artery disease.

A genealogical study of essential hypertension with and without obesity in French Canadians

OBJECTIVES

To investigate genetic homogeneity in a set of hypertensive families and in subsets chosen for high and low prevalence of obesity; and to compare fasting insulin and lipids, ion transport, and water homeostasis in the obese and lean families.

RESEARCH METHODS AND PROCEDURES

The study was carried out in a relative population isolate of the Saguenay/Lac St. Jean region in Canada. Genetic homogeneity was evaluated with the mean coeffigcients of kinship (phi) and inbreeding (F) computed with ascending genealogies. Serum insulin and lipids were measured after overnight fasting. Total body water was estimated with bioelectrical impedance. Sodium-lithium countertransport and sodium-potassium co-transport were determined in freshly isolated erythrocytes.

RESULTS

F and phi were increased in hypertensive families compared with families selected at random. F and phi were further increased within the subsets of obese and lean families. In addition, fasting insulin, total body water, sodium-lithium countertransport, and sodium-potassium co-transport were higher in the obese than in the lean families. The two subsets of families did not differ by fasting lipids.

DISCUSSION

In the Saguenay/Lac St. Jean population, the degree of genetic homogeneity was increased in families selected for hypertension, and it was further increased in subsets of hypertensive families with high and low prevalence of obesity. This suggests that hypertension in lean and obese individuals may represent, at least in part, separate genetic entities. Some of the extra genes shared in common within the subsets may contribute to their differences in body weight, insulin sensitivity, ion transport, and water homeostasis.

Correlates of family history of coronary artery disease in children

The atherosclerotic process begins in childhood but, in general, does not reach the clinical horizon until after the fifth decade of life, at which point the best opportunities for prevention and intervention have been lost. In order to identify children with a high risk of developing coronary artery disease (CAD), risk factors measured in children that are the most informative indicators of future risk must be identified. Using a novel analytical strategy that incorporates a continuum of information about context dependency, we investigated whether there were significant differences in intermediate biochemical and physiological traits between children (189 females and 188 males, ages 5-20.5 years) with and without a strong family history of clinically-defined CAD at three levels of context dependency (coarse grain, medium grain, and fine grain). In the coarse-grained analysis we tested for differences in mean levels of nine intermediate traits (lipids, apolipoproteins, blood pressure traits) and indices of external and internal environmental context (age, body mass index, smoking status). Female children with a strong family history had higher average levels for total cholesterol, triglyceride, Apo B, and systolic blood pressure and were on average older and weighed more than female children with a weak family history of CAD. Male children with a strong family history of CAD had higher average levels of triglycerides and were on average older than male children with a weak family history. In the medium-grained analysis we investigated whether the regression relationships between each intermediate trait and each measure of environmental context was significantly different between children with and without a strong family history of CAD. Our results indicate that children with a strong family history of CAD have a significantly different relationship between their intermediate traits and environmental contexts than children with a weak family history. In the fine-grained analysis, we stratified the sample into age, BMI, and smoking subgroups and tested for mean differences in the intermediate traits between children with and without a strong family history. For seven of the nine intermediate traits we found evidence of significant mean differences between children with and without a strong family history of CAD in particular age and BMI subgroups in nonsmokers that were not expected given the results from separate age-dependent or BMI-dependent marginal analyses. From these analyses, we conclude that the inferences about intermediate biochemical and physiological trait associations with family history of CAD depend on where on the coarse-grain to fine-grain continuum of context dependency the analysis is performed. In many cases, inferences at one level of investigation are different than the inferences made at a coarser or finer level. This study documents the complexity of the associations between intermediate traits and risk of CAD and raises the question of how many models are needed to maximize disease prediction and where these models should fall on the coarse- to fine-grain continuum.

Sodium-lithium countertransport: physiology and function

Current opinions on the relationships between erythrocyte sodium-lithium countertransport kinetics and primary hypertension, hyperlipidaemia and diabetic nephropathy are reviewed. Problems associated with the assay are analysed. Some possible mechanisms that could modify the kinetics of ion exchange are examined. The question of what catalyses sodium-lithium countertransport is discussed, but not answered. Some models are put forward showing how a study of sodium-lithium countertransport kinetics could further our understanding of important disease processes.

Insulin, sodium-lithium countertransport, and microalbuminuria in hypertensive patients

Both microalbuminuria (>0.290 nmol/min [20 microg/min]) and high sodium-lithium countertransport (SLC) in diabetic or hypertensive humans are predictive of overt nephropathy and more aggressive cardiovascular complications, perhaps induced by insulin resistance. To analyze the relationships between microalbuminuria, SLC, microalbuminuria, and insulin in essential hypertension, we studied 90 hypertensive white patients, 25 of whom had microalbuminuria and 32 of whom were healthy. When urine sampling was completed for albuminuria determination, SLC was measured; all patients then underwent standard (75 g) oral glucose load to measure basal (0 minutes) and 2-hour glucose and insulin serum levels. Glucose-insulin ratio was used as insulin sensitivity index (ISI). In both hypertensive patients with normal microalbuminuria and those with pathological microalbuminuria, plasma insulin at 120 minutes was significantly higher than in control subjects. When the patients with pathological microalbuminuria were divided into thirds on the basis of their microalbuminuria, in the lower third, we found statistically significant less fasting insulin and higher basal ISI. SLC was higher in hypertensives than normotensives and, among hypertensives, higher in the subgroup with elevated microalbuminuria. In hypertensives, we found a weak but significant correlation between SLC and microalbuminuria, independent of insulin or ISI. The prevalence of high value of SLC (> or =0.383 mmol x L-1 x h-1) was significantly lower in hypertensives with normal rather than abnormal urinary albumin excretion. Our results indicate that in nondiabetic hypertensive whites, higher microalbuminuria is accompanied by signs of insulin resistance; moreover, a link exists between SLC and microalbuminuria, both predictive of aggressive complications of hypertension.

Cellular sodium membrane transport and cardiovascular risk factors in non-insulin-dependent diabetes mellitus

Association have been described between cardiovascular risk factors and abnormalities of both sodium-lithium countertransport (SLC) and sodium-hydrogen ion exchange in subjects with insulin-dependent diabetes mellitus. The data in subjects with non-insulin-dependent diabetes mellitus (NIDDM) are few and more conflicting. This investigation examines erythrocyte SLC rates and platelet sodium-hydrogen ion-exchange kinetics and their relationship to cardiovascular risk factors in 45 nondiabetic and 35 NIDDM white men. The two groups did not differ significantly in erythrocyte SLC or platelet buffering capacity, sodium-hydrogen ion-exchange maximal rate (Vmax), or Km for extracellular sodium. Within the whole group, controlling for the presence of diabetes, SLC correlated weakly with triglyceride concentration (r = .23, P = .05), but not with urinary albumin excretion rate (AER), systolic or diastolic blood pressure, body mass index (BMI), or concentrations of glucose, insulin, or total or high-density lipoprotein (HDL) cholesterol. Platelet sodium-hydrogen exchange was not significantly related to any cardiovascular risk factor studied. In conclusion, (1)SLC activity was not increased in NIDDM subjects; (2) SLC rates correlated weakly with serum triglyceride concentrations; (3) platelet sodium-hydrogen exchange Vmax and K(m) for extracellular sodium and platelet buffering capacity did not differ between diabetic and nondiabetic groups; and (4) there was no significant relationship between platelet Na+/H(+)-exchange kinetics and any of the cardiovascular risk factors studied.

Probability of having hypertension: effects of sex, history of hypertension in parents, and other risk factors

The objective of the present study was to determine whether paternal and maternal history of hypertension contributes to the probability of an individual having hypertension before and after other measured traits, including sex, are considered. A cross-sectional sample of 217 men and 196 women was selected from the general Caucasion population of Rochester, Minnesota without respect to the hypertension status of subjects and their parents. Logistic regression analyses indicated that when no other information was considered, paternal history of hypertension contributed to the probability of having hypertension in men (chi 2 = 4.14, df = 1, p = 0.042) and in women (chi 2 = 4.12, df = 1, p = 0.042). The odds ratio associated with paternal history of hypertension was 2.80 in men (95% confidence interval [CI] = 1.0-8.0); the odds ratio was 4.11 in women (95% CI = 0.9-19.3). Maternal history of hypertension provided a marginally significant contribution to the prediction of probability of having hypertension in men (chi 2 = 3.86, df = 1, p = 0.049 in men), and less so in women (chi 2 = 3.31, df = 1, p = 0.068). The odds ratios associated with maternal history of hypertension were 2.85 in men (95% CI = 0.9-8.8) and 3.60 in women (95% CI = 0.8-16.9). A stepwise selection algorithm was used to select other predictors of hypertension in men and women. Other predictors of hypertension identified in men were age and sodium-lithium countertransport level. After these other predictors were considered, paternal but not maternal history of hypertension contributed to the probability of having hypertension in men. In a model that contained these other predictors, the partial odds ratio associated with paternal history of hypertension was 3.38 (95% CI = 1.1 = 10.1). Other predictors identified in women were age, apolipoprotein (apo) B, and apo B squared. After these other predictors were considered, neither paternal nor maternal history of hypertension made a statistically significant contribution to the probability of having hypertension in women. These results suggest that evaluation of the hypertension risk of an individual depends on the sex of the individual, the sexes of the individual's hypertensive parents, and the values of other measured risk factor traits.

Atrial natriuretic peptide and blood pressure in a population-based sample

OBJECTIVE

To assess the relationship of plasma atrial natriuretic peptide (ANP) levels with blood pressure levels and the occurrence of hypertension in a large population-based sample.

DESIGN

We performed a cross-sectional study of the relationship between ANP levels and blood pressure levels, diagnosis of hypertension, and family history of hypertension in Caucasians from Rochester, Minnesota.

MATERIAL AND METHODS

Plasma ANP and blood pressure levels were measured in 1,338 Caucasian subjects who were members of 301 three-generation families from the population of Rochester. Hypertension was defined as systolic blood pressure of 140 mm Hg or more or diastolic blood pressure of 90 mm Hg or more. Each subject in the parental generation was categorized as having zero, one, or two parents with hypertension. Analyses were done separately for each generation and gender stratum.

RESULTS

Within gender and generation strata, we noted no consistent pattern of positive or negative correlation of plasma ANP levels with systolic blood pressure, diastolic blood pressure, or heart rate. Within the grandparental generation, mean plasma ANP levels did not differ between those with normal blood pressure and those with hypertension. In the parental generation, mean plasma ANP levels did not differ between subjects with zero, one, or two parents with hypertension.

CONCLUSION

In Caucasians, interindividual differences in plasma ANP levels are not associated with interindividual differences in blood pressure levels, the diagnosis of hypertension, or family history of hypertension.

Variation at the M235T locus of the angiotensinogen gene and essential hypertension: a population-based case-control study from Rochester, Minnesota

A variant of the angiotensinogen gene, M235T, has been associated with essential hypertension in selected subjects from Paris, France and Salt Lake City, Utah. In the present report, we studied a population-based sample consisting of 104 subjects diagnosed with hypertension before age 60 and 195 matched normotensive individuals from Rochester, Minnesota. We determined whether there was a relationship between the M235T polymorphism of the angiotensinogen gene and the occurrence of essential hypertension using two methods. First, a contingency chi-square analysis was carried out to test for an association between the M235T polymorphism and hypertension status. Second, multivariable conditional logistic regression was used to determine whether variation at the M235T polymorphism was a significant predictor of the probability of having essential hypertension. We detected no statistically significant association between the M235T polymorphism and the occurrence of essential hypertension. In particular, the association was not significant in either gender or in a subset of severely hypertensive subjects requiring two or more anti-hypertensive medications. Furthermore, variation in the number of M235T alleles did not make a significant contribution to predicting the probability of having essential hypertension, either alone or in conjunction with other predictor variables. These results suggest that the contribution of variation in the angiotensinogen gene to the occurrence of essential hypertension is less than initially suspected, or may not be constant across populations.

Analysis of systems influencing renal hemodynamics and sodium excretion. I. Biochemical systems theory

In this article we present a new methodology--Biochemical Systems Theory and Analysis--as an alternative to traditional parametric statistical procedures for investigating differences between risk groups in a population. We review the systems theory and how it can be used to represent a model of processes influencing renal hemodynamics and sodium (Na+) excretion. We also discuss the potential for new measures of the biology of common diseases that can emerge from a synergism between systems theory and population-based statistical approaches.

Sodium-lithium countertransport genotype and the probability of hypertension in adults

The objective of the present study was to determine whether information about a biometrically inferred single gene with large effects on erythrocyte sodium-lithium countertransport is useful in predicting the probability of having hypertension. We used multivariate logistic regression to model the relationship between the probability of having hypertension and predictor traits in a sample of 382 unrelated adult women and 347 unrelated adult men from Rochester, Minn. First, we identified a set of demographic, biochemical, and physiological predictors. Second, we analyzed whether the relationship between the probability of having hypertension and the identified predictor traits was heterogeneous between the biometrically inferred single locus genotypes with large effects on sodium-lithium countertransport level. Third, if there was no heterogeneity, we assessed whether sodium-lithium countertransport genotypes made an additional contribution to predicting the probability of having hypertension after other predictors were considered. In women, the predictors of the probability of having hypertension were age, plasma apolipoprotein CIII, body mass index, and an interaction term involving age and body mass index. The relationship between the probability of having hypertension and the identified predictors was not heterogeneous between sodium-lithium countertransport genotypes, and genotype did not contribute to the prediction of the probability of having hypertension after the identified predictors were considered. In men, predictors of the probability of having hypertension were age, plasma levels of high-density lipoprotein cholesterol, apolipoproteins AI and CII, sodium-lithium countertransport level, and sodium-lithium countertransport genotype. The relationship between the probability of having hypertension and sodium-lithium countertransport level and age were heterogeneous between biometrically inferred sodium-lithium countertransport genotypes.(ABSTRACT TRUNCATED AT 250 WORDS)