Corroboration of Dahl S Q276L alpha1Na,K-ATPase protein sequence: impact on affinities for ligands and on E1 conformation

A functional 12T-insertion polymorphism in the ATP1A1 promoter confers decreased susceptibility to hypertension in a male Sardinian population

Identification of susceptibility genes for essential hypertension in humans has been a challenge due to its multifactorial pathogenesis complicated by gene-gene and gene-environment interactions, developmental programing and sex specific differences. These concurrent features make identification of causal hypertension susceptibility genes with a single approach difficult, thus requiring multiple lines of evidence involving genetic, biochemical and biological experimentation to establish causal functional mutations. Here we report experimental evidence encompassing genetic, biochemical and in vivo modeling that altogether support ATP1A1 as a hypertension susceptibility gene in males in Sardinia, Italy. ATP1A1 encodes the α1Na,K-ATPase isoform, the sole sodium pump in vascular endothelial and renal tubular epithelial cells. DNA-sequencing detected a 12-nucleotide long thymidine (12T) insertion(ins)/deletion(del) polymorphism within a poly-T sequence (38T vs 26T) in the ATP1A1 5’-regulatory region associated with hypertension in a male Sardinian population. The 12T-insertion allele confers decreased susceptibility to hypertension (P = 0.035; OR = 0.50 [0.28–0.93]) accounting for 12.1 mmHg decrease in systolic BP (P = 0.02) and 6.6 mmHg in diastolic BP (P = 0.046). The ATP1A1 promoter containing the 12T-insertion exhibited decreased transcriptional activity in in vitro reporter-assay systems, indicating decreased α1Na,K-ATPase expression with the 12T-insertion, compared with the 12T-deletion ATP1A1 promoter. To test the effects of decreased α1Na,K-ATPase expression on blood pressure, we measured blood pressure by radiotelemetry in three month-old, highly inbred heterozygous knockout ATP1A1+/− male mice with resultant 58% reduction in ATP1A1 protein levels. Male ATP1A1+/− mice showed significantly lower blood pressure (P < 0.03) than age-matched male wild-type littermate controls. Concordantly, lower ATP1A1 expression is expected to lower Na-reabsorption in the kidney thereby decreasing sodium-associated risk for hypertension and sodium-induced endothelial stiffness and dysfunction. Altogether, data support ATP1A1 as a hypertension susceptibility gene in a male Sardinian population, and mandate further investigation of its involvement in hypertension in the general population.

Leptin and the Regulation of Renal Sodium Handling and Renal Na-Transporting ATPases: Role in the Pathogenesis of Arterial Hypertension

Leptin, an adipose tissue hormone which regulates food intake, is also involved in the pathogenesis of arterial hypertension. Plasma leptin concentration is increased in obese individuals. Chronic leptin administration or transgenic overexpression increases blood pressure in experimental animals, and some studies indicate that plasma leptin is elevated in hypertensive subjects independently of body weight. Leptin has a dose- and time-dependent effect on urinary sodium excretion. High doses of leptin increase Na(+) excretion in the short run; partially by decreasing renal Na(+),K(+)-ATPase (sodium pump) activity. This effect is mediated by phosphatidylinositol 3-kinase (PI3K) and is impaired in animals with dietary-induced obesity. In contrast to acute, chronic elevation of plasma leptin to the level observed in patients with the metabolic syndrome impairs renal Na(+) excretion, which is associated with the increase in renal Na(+),K(+)-ATPase activity. This effect results from oxidative stress-induced deficiency of nitric oxide and/or transactivation of epidermal growth factor receptor and subsequent stimulation of extracellular signal-regulated kinases. Ameliorating "renal leptin resistance" or reducing leptin level and/or leptin signaling in states of chronic hyperleptinemia may be a novel strategy for the treatment of arterial hypertension associated with the metabolic syndrome.

The Atp1a1 gene from inbred Dahl salt sensitive rats does not contain the A1079T missense transversion

The existence of the A1079T transversion in the alpha1 isoform of the Na(+), K(+)-ATPase (Atp1a1) gene in Dahl salt-sensitive rat (SS/Jr) strain, discovered by Herrera and Ruiz-Opazo and proposed to underlay hypertension sensitivity, represents one of the most controversial topics in hypertension research. As our research group did not have any previous connection to any party in this dispute nor to hypertension-related research, we were asked (J Hypertens. 2006;24:2312-2313) to definitively adjudge the existence of the A1079T transversion. Hence, different state-of-the art SNP detection technologies that depend on a variety of mechanisms and enzymes to detect the transversion in genomic DNA as well as cDNA derived from different tissues were used. Although it was possible to readily detect other silent polymorphisms between SS and SR strains in the Atp1a1 gene by all methods used, no evidence for the existence of the A1079T transversion in SS/Jr rats was found.

The central role of the brain in salt-sensitive hypertension

PURPOSE OF REVIEW

To integrate recent studies showing that abnormal Na transport in the central nervous system plays a pivotal role in genetic models of salt-sensitive hypertension.

RECENT FINDINGS

Na transport-regulating mechanisms classically considered to reflect renal control of the blood pressure, i.e. aldosterone-mineralocorticoid receptors-epithelial sodium channels-Na/K-ATPase, have now been demonstrated to be present in the central nervous system contributing to regulation of cerebrospinal fluid [Na] by the choroid plexus and to neuronal responsiveness to cerebrospinal fluid/brain [Na]. Dysfunction of either or both can activate central nervous system pathways involving 'ouabain' and angiotensin type 1 receptor stimulation. The latter causes sympathetic hyperactivity and adrenal release of marinobufagenin - a digitalis-like inhibitor of the alpha1 Na/K-ATPase isoform - both contributing to hypertension on high salt intake. Conversely, specific central nervous system blockade of mineralocorticoid receptors or epithelial sodium channels prevents the development of hypertension on high salt intake, irrespective of the presence of a 'salt-sensitive kidney'. Variants in the coding regions of some of the genes involved in Na transport have been identified, but sodium sensitivity may be mainly determined by abnormal regulation of expression, pointing to primary abnormalities in regulation of transcription.

SUMMARY

Looking beyond the kidney is providing new insights into mechanisms contributing to salt-sensitive hypertension, which will help to dissect the genetic factors involved and to discover novel strategies to prevent and treat salt-sensitive hypertension.

Association of ATP1A1 and dear single-nucleotide polymorphism haplotypes with essential hypertension: sex-specific and haplotype-specific effects

Essential hypertension remains a major risk factor for cardiovascular and cerebrovascular diseases. As a complex multifactorial disease, elucidation of susceptibility loci remains elusive. ATP1A1 and Dear are candidate genes for 2 closely linked rat chromosome-2 blood pressure quantitative trait loci. Because corresponding human syntenic regions are on different chromosomes, investigation of ATP1A1 (chromosome [chr]-1p21) and Dear (chr-4q31.3) facilitates genetic analyses of each blood pressure quantitative trait locus in human hypertension. Here we report the association of human ATP1A1 (P<0.000005) and Dear (P<0.03) with hypertension in a relatively isolated, case/control hypertension cohort from northern Sardinia by single-nucleotide polymorphism haplotype analysis. Sex-specific haplotype analyses detected stronger association of both loci with hypertension in males than in females. Haplotype trend-regression analyses support ATP1A1 and Dear as independent susceptibility loci and reveal haplotype-specific association with hypertension and normotension, thus delineating haplotype-specific subsets of hypertension. Although investigation in other cohorts needs to be performed to determine genetic effects in other populations, haplotype subtyping already allows systematic stratification of susceptibility and, hence, clinical heterogeneity, a prerequisite for unraveling the polygenic etiology and polygene-environment interactions in essential hypertension. As hypertension susceptibility genes, coexpression of ATP1A1 and Dear in both renal tubular cells and vascular endothelium suggest a cellular pathogenic scaffold for polygenic mechanisms of hypertension, as well as the hypothesis that ATP1A1 and/or Dear could contribute to the known renal and vascular endothelial dysfunction associated with essential (polygenic) hypertension.

Sex-specific QTLs and interacting loci underlie salt-sensitive hypertension and target organ complications in Dahl S/jrHS hypertensive rats

Sex-specific differences in polygenic (essential) hypertension are commonly attributed to the role of sex steroid hormone-receptor systems attenuating sex-common disease mechanisms in premenopausal women. However, emerging observations indicate sex-specific genetic susceptibility in various traits, thus requiring systematic study. Here we report a comparative analysis of independent total genome scans for salt-sensitive hypertension susceptibility quantitative trait loci (QTLs) in male and female F2 [Dahl R/jrHS x S/jrHS] intercross rats exposed to high-salt (8% NaCl) rat diets. Hypertension was phenotyped with three quantitative traits: blood pressure (BP) elevation associated with increased hypertensive renal disease [glomerular injury score (GIS)] and increased cardiac mass [relative heart weight (RHW)] obtained 8-12 wk after high-salt challenge; 24-h nonstress, telemetric BP measurements were used. Although sex-common QTLs were detected for BP [chromosome (chr) 1-144.3 Mbp; chr 1-208.8 Mbp], GIS (chr 1-208.8 Mbp), and cardiac mass (chr 5-150.3 Mbp), most QTLs across the three phenotypes studied are gender specific as follows: female QTLs for BP (chr 2-106.7 Mbp, chr 2-181.7 Mbp, chr 5-113.9 Mbp, chr 5-146.7 Mbp, chr 12-12.8 Mbp), GIS (chr 15-59.6 Mbp), and RHW (chr 2-31.5 Mbp, chr 5-154.7 Mbp, chr 5-110.9 Mbp); male QTLs for BP (chr 2-196.7 Mbp, chr 11-48.0 Mbp, chr 20-35.7 Mbp), GIS (chr 6-3.3 Mbp, chr 20-40.7 Mbp), and RHW (chr 6-3.3 Mbp, chr 20-40.7 Mbp). Furthermore, interacting loci with significant linkage were detected only in female F2 intercross rats for BP and hypertensive renal disease. Comparative analyses revealed concordance of BP QTL peaks with previously reported rat model and human hypertension susceptibility genes and with BP QTLs in previous Dahl S-derived F2 intercross studies and also suggest strain-specific genetic modifiers of sex-specific determinants. Altogether, the data provide key experimental bases for sex-specific investigation of mechanisms and intervention and prevention strategies for polygenic hypertension in humans.