Erythrocyte sodium-lithium countertransport and blood pressure: a genome-wide linkage study

QTL mapping of rat blood pressure loci on RNO1 within a homologous region linked to human hypertension on HSA15

Fine-mapping of regions linked to the inheritance of hypertension is accomplished by genetic dissection of blood pressure quantitative trait loci (BP QTLs) in rats. The goal of the current study was to further fine-map two genomic regions on rat chromosome 1 with opposing blood pressure effects (BP QTL1b1 and BP QTL1b1a), the homologous region of which on human chromosome 15 harbors BP QTLs. Two new substrains were constructed and studied from the previously reported BP QTL1b1, one having significantly lower systolic BP by 17 mmHg than that of the salt-sensitive (S) rat (P = 0.007). The new limits of BP QTL1b1 were between 134.09 Mb and 135.40 Mb with a 43% improvement from the previous 2.31 Mb to the current 1.31 Mb interval containing 4 protein-coding genes (Rgma, Chd2, Fam174b, and St8sia2), 2 predicted miRNAs, and 4 lncRNAs. One new substrain was constructed and studied from the previously reported BPQTL1b1a having a significantly higher systolic BP by 22 mmHg (P = 0.006) than that of the S rat. The new limits of BPQTL1b1a were between 133.53 Mb and 134.52 Mb with a 32% improvement from the previous1.45 Mb to the current 990.21 Kb interval containing 1 protein-coding gene, Mctp2, and a lncRNA. The congenic segments of these two BP QTLs overlapped between 134.09 Mb and 134.52 Mb. No exonic variants were detected in any of the genes. These findings reiterate complexity of genetic regulation of BP within QTL regions, where elements beyond protein-coding sequences could be factors in controlling BP.

Alterations of membrane protein expression in red blood cells of Alzheimer's disease patients

Preventive measures, prognosis, or selected therapy in multifactorial maladies, including Alzheimer's disease (AD), require the application of a wide range of diagnostic assays. There is a large unmet need for relatively simple, blood-based biomarkers in this regard. We have recently developed a rapid and reliable flow cytometry and antibody-based method for the quantitative measurement of various red blood cell (RBC) membrane proteins from a drop of blood. Here, we document that the RBC expression of certain membrane proteins, especially that of the GLUT1 transporter and the insulin receptor (INSR), is significantly higher in AD patients than in age-matched healthy subjects. The observed differences may reflect long-term metabolic alterations relevant in the development of AD. These findings may pave the way for a diagnostic application of RBC membrane proteins as relatively stable and easily accessible personalized biomarkers in AD.

Mutation within the hinge region of the transcription factor Nr2f2 attenuates salt-sensitive hypertension

Genome-wide association studies (GWAS) have prioritized a transcription factor, nuclear receptor 2 family 2 (NR2F2), as being associated with essential hypertension in humans. Here we provide evidence that validates this association and indicates that Nr2f2 is a genetic determinant of blood pressure (BP). Using the zinc-finger nuclease technology, the generation of a targeted Nr2f2-edited rat model is reported. The resulting gene-edited rats have a 15 bp deletion in exon 2 leading to a five-amino-acid deletion in the hinge region of the mutant Nr2f2 protein. Both systolic and diastolic blood pressures of the Nr2f2(mutant) rats are significantly lower than controls. Because the hinge region of Nr2f2 is required for interaction with Friend of Gata2 (Fog2), protein-protein interaction is examined. Interaction of Nr2f2(mutant) protein with Fog2 is greater than that with the wild-type Nr2f2, indicating that the extent of interaction between these two transcription factors critically influences BP.

Multiple blood pressure loci with opposing blood pressure effects on rat chromosome 1 in a homologous region linked to hypertension on human chromosome 15

Genetic dissection of blood pressure (BP) quantitative trait loci (QTLs) in rats has facilitated the fine-mapping of regions linked to the inheritance of hypertension. The goal of the current study was to further fine-map one such genomic region on rat chromosome 1 (BPQTL1b1), the homologous region of which on human chromosome 15 harbors BP QTLs, as reported by four independent studies. Of the six substrains constructed and studied, the systolic BP of two of the congenic strains were significantly lower by 36 and 27 mm Hg than that of the salt-sensitive (S) rat (P < 0.0001, P = 0.0003, respectively). The congenic segments of these two strains overlapped between 135.12 and 138.78 Mb and contained eight genes and two predicted miRNAs. None of the annotations had variants within expressed sequences. These data taken together with the previous localization resolved QTL1b1 with a 70% improvement from the original 7.39 Mb to the current 2.247 Mb interval. Furthermore, the systolic BP of one of the congenic substrains was significantly higher by 20 mm Hg (P < 0.0001) than the BP of the S rat. The limits of this newly identified QTL with a BP increasing effect (QTL1b1a) were between 134.12 and 135.76 Mb, spanning 1.64 Mb, containing two protein-coding genes, Mctp2 and Rgma, and a predicted miRNA. There were four synonymous variants within Mctp2. These data provide evidence for two independent BP QTLs with opposing BP effects within the previously identified BP QTL1b1 region. Additionally, these findings illustrate the complexity underlying the genetic mechanisms of BP regulation, wherein inherited elements beyond protein-coding sequences or known regulatory regions could be operational.

Cell surface membrane proteins as personalized biomarkers: where we stand and where we are headed

Personalized medicine requires the development of a wide array of biomarker diagnostic assays, reflecting individual variations and thus allowing tailored therapeutic interventions. Membrane proteins comprise approximately 30% of total human proteins; they play a key role in various physiological functions and pathological conditions, although, currently, only a limited number of membrane proteins are applied as biomarkers. In many normal tissues, cell surface membrane proteins are not easily accessible for diagnostic sampling, and tumor-derived membrane preparations - while serving as potential tumor biomarkers - may not reflect physiological protein expression. In addition to post-translational modifications, which may include glycosylation, phosphorylation and lipid modifications, the trafficking of membrane proteins is also regulated. Moreover, a tight cellular quality control monitors membrane protein maturation, and continuous removal and reinsertion, involving special signaling systems, occurs in many cases. However, cell surface membrane proteins already serve as valuable prognostic and predicative biomarkers, for example, in hematological and immunological diseases, by the determination of the cluster of differentiation markers. In this review, we demonstrate the relevance of cell surface membrane biomarkers in various diseases and call attention to the potential application of red blood cell (erythrocyte) membrane proteins in this regard. Surprisingly, red blood cells express hundreds of membrane proteins, which seem to reflect a general genetic and regulatory background, and may serve as relatively stable and easily accessible personalized membrane biomarkers. Quantitative membrane protein detection in red blood cells by flow cytometry may bring a breakthrough in this regard.

Sodium-lithium countertransport activity in healthy, dyslipidemic, and hypertensive individuals

The aim of our study was to investigate the role of dyslipidemia on red blood cell sodium-lithium countertransport activity in healthy and hypertensive individuals. A total of 128 Caucasian individuals, aged 20 to 60 years old, were divided into 4 groups: dyslipidemic/ hypertensive, dyslipidemic/normotensive, normolipidemic/hypertensive, and normolipidemic/ normotensive (controls). Sodium-lithium countertransport activity was determined based on the Canessa et al method. Sodium-lithium countertransport activity was significantly higher in all patient groups compared with controls (P < .001) and similar in the 3 patient groups. Sodium-lithium countertransport activity was significantly and positively associated with triglyceride levels (P < .001), body mass index (P < .001), total cholesterol levels (P = .001), and systolic (P = .001) and diastolic blood pressure (P = .001). In multivariate regression analysis, triglycerides made the largest contribution to sodium-lithium countertransport variation among the variables tested (R(2) = 0.273). Our results suggest that dyslipidemia affects sodium-lithium countertransport activity independently of essential hypertension and even to a greater extent than hypertension.

Closely linked non-additive blood pressure quantitative trait loci

There is enough evidence through linkage and substitution mapping to indicate that rat chromosome 1 harbors multiple blood pressure (BP) quantitative trait loci (QTLs). Of these, BP QTL1b was previously reported from our laboratory using congenic strains derived by introgressing normotensive alleles from the LEW rat onto the genetic background of the hypertensive Dahl salt-sensitive (S) rat. The region spanned by QTL1b is quite large (20.92 Mb), thus requiring further mapping with improved resolution so as to facilitate systematic identification of the underlying genetic determinant(s). Using congenic strains containing the LEW rat chromosomal segments on the Dahl salt-sensitive (S) rat background, further iterations of congenic substrains were constructed and characterized. Collective data obtained from this new iteration of congenic substrains provided evidence for further fragmentation of QTL1b with improved resolution. At least two separate genetic determinants of blood pressure underlie QTL1b. These are within 7.40 Mb and 7.31 Mb and are known as the QTL1b1 region and the QTL1b2 region, respectively. A genetic interaction was detected between the two BP QTLs. Interestingly, five of the previously reported differentially expressed genes located within the newly mapped QTL1b1 region remained differentially expressed. The congenic strain S.LEW(D1Mco36-D1Mco101), which harbors the QTL1b1 region alone but not the QTL1b2 region, serves as a genetic tool for further dissection of the QTL1b1 region and validation of Nr2f2 as a positional candidate gene. Overall, this study represents an intermediary yet obligatory progression towards the identification of genetic elements controlling BP.

Regional association-based fine-mapping for sodium-lithium countertransport on chromosome 10

BACKGROUND

Increased erythrocyte sodium-lithium countertransport (SLC) has been observed in patients with essential hypertension. Consistent evidence of genetic linkage was shown for SLC on chromosome 10, and a region of interest was localized between 26 and 56 Mb.

METHODS

This study surveyed single nucleotide polymorphisms (SNPs) in 54 genes that reside in the region of interest, and investigated their association with SLC and blood pressure. These SNPs were genotyped in 1,133 non-Hispanic white individuals from 255 pedigrees comprising the second phase of the Rochester Family Heart Study. The variance-components-based genetics software package SOLAR was used for evaluating whether an SNP contributed to a significant fraction of the trait heritability.

RESULTS

Of the 77 SNPs surveyed in this study across the region of interest, four SNPs were associated with SLC (P < 0.04), five SNPs were associated with blood pressure (P < 0.04), and two SNPs in mannose-binding lectin 2 (MBL2) were associated with both phenotypes. In general, the pairwise linkage disequilibrium among the genotyped SNPs was low.

CONCLUSIONS

This fine-mapping survey of genetic variation in a linkage region of interest provides overall support for association-mapping for SLC on chromosome 10. Genes significantly associated with systolic blood pressure and/or SLC in these families will be prioritized for future studies.

Transcriptional profiling with a blood pressure QTL interval-specific oligonucleotide array

Although the evidence for a genetic predisposition to human essential hypertension is compelling, the genetic control of blood pressure (BP) is poorly understood. The Dahl salt-sensitive (S) rat is a model for studying the genetic component of BP. Using this model, we previously reported the identification of 16 different genomic regions that contain one or more BP quantitative trait loci (QTLs). The proximal region of rat chromosome 1 contains multiple BP QTLs. Of these, we have localized the BP QTL1b region to a 13.5-cM (20.92 Mb) region. Interestingly, five additional independent studies in rats and four independent studies in humans have reported genetic linkage for BP control by regions homologous to QTL1b. To view the overall renal transcriptional topography of the positional candidate genes for this QTL, we sought a comparative gene expression profiling between a congenic strain containing QTL1b and control S rats by employing 1) a saturated QTL1b interval-specific oligonucleotide array and 2) a whole genome cDNA microarray representing 20,465 unique genes that are positioned outside the QTL. Results indicated that 17 of the 231 positional candidate genes for this QTL are differentially expressed between the two strains tested. Surprisingly, >1,500 genes outside of QTL1b were differentially expressed between the two rat strains. Integrating the results from the two approaches revealed at least one complex network of transcriptional control initiated by the positional candidate Nr2f2. This network appears to account for the majority of gene expression differences occurring outside of the QTL interval. Further substitution mapping is currently underway to test the validity of each of these differentially expressed positional candidate genes. These results demonstrate the importance of using a saturated oligonucleotide array for identifying and prioritizing differentially expressed positional candidate genes of a BP QTL.

Model-fitting and linkage analysis of sodium–lithium countertransport

Increased sodium-lithium countertransport activity (SLC) associates with hypertension and is highly heritable, yet the underlying genes remain unknown. SLC, measured on 1113 and remeasured 2-3 years later on 675 adult members of 48 Utah pedigrees, was tested for candidate gene association, major locus inheritance, and linkage to genome scan markers using a bivariate model with genotype-specific effects of age, body mass index (BMI), and triglycerides level (TG). No effect of the alpha-adducin Gly460Trp polymorphism on SLC was found. In contrast, SLC increased with age in carriers of apolipoproteinE varepsilon2 (85 individuals; 8.7% of the sample) and decreased in noncarriers. Model-fitting analyses inferred two additional loci with genotype-specific responses to BMI and TG. Using the inferred model, lod scores >2 were obtained for D3S3038, D11S4464, and D10S677 for the BMI-responsive locus, and for D8S1048 for the TG-responsive locus.

Baseline Na–Li countertransport and risk of hypertension in children: a 10-year prospective study in Hanzhong children

Sodium-lithium countertransport (Na-Li CT) is associated with blood pressure (BP) and in many cross-sectional investigations and some longitudinal studies, essential hypertension has been proposed as a biochemical marker or predictor of hypertension risk in adults. The present study investigated prospectively whether baseline Na-Li CT rate was an index of increased risk of future development of hypertension in children. At baseline visit in 1987 of the Hanzhong Children Hypertension Study comprising 4000 school children aged 5-6 years old, 310 samples were randomly selected for measurement of baseline Na-Li CT rate; we made a 10-year follow-up of them in the same season in 1997. This cohort of children is the sample for analysis in the present report. Baseline Na-Li CT rate was positively correlated to systolic BP (SBP) both in baseline and follow-up (baseline, gamma=0.21, P<0.05; follow-up, gamma=0.32, P<0.01), and positively correlated to diastolic BP (DBP) (gamma=0.20, P<0.05) and body mass index (gamma=0.18, P<0.05) in follow-up examination. Longitudinal analysis of 10-year BP evolution, children in higher baseline Na-Li CT (ie, >260 micromoll RBC/h) had greater BP change than children in lower baseline Na-Li CT (ie, </=260 mumol/l RBC/h) (SBP, 15.8+/-12.9 vs 19.3+/-13.1, DBP 8.8+/-11.2 and 11.3+/-10.6, P<0.05). Multiple logistic regression analysis showed that children in the higher Na-Li CT (>260 mumol/l RBC/h) were associated with approximately 1.5 times greater risk of high BP) in comparison to placement in lower Na-Li CT (</=260 mumol/l RBC/h). Elevated baeline RBC Na-Li countertransport could be a risk predictor predisposing to the development of hypertension in children.

Dietary NaCl regulates renal aminopeptidase N: relevance to hypertension in the Dahl rat

Aminopeptidase N (APN) is an abundant metallohydrolase in the brush border of kidney proximal tubule cells that degrades angiotensin III (Ang III) to angiotensin IV (Ang IV) and, along with dipeptidylaminopeptidase, degrades Ang IV. We examined the impact of a high-salt diet on renal APN activity and transcript abundance in the Sprague-Dawley and Dahl salt-sensitive (SS/Jr) rat strains. APN transcript abundance and protein abundance were approximately 2-fold greater (P<0.05; n=6) in the kidneys of Sprague-Dawley and Lewis rats ingesting 8% versus 0.3% salt diets, suggesting that increased aminopeptidase activity may contribute to decreased renal sodium uptake during adaptation to a high-salt diet. In contrast, renal APN transcript abundance and activity were the same in Dahl SS/Jr rats ingesting 8.0% versus 0.3% salt diets. The APN gene was mapped, using a radiation-hybrid panel, to known quantitative loci on chromosome 1 for blood pressure in the Dahl SS/Jr rat. The results suggest that the APN gene is a good candidate for salt-sensitivity in the Dahl SS/Jr rat.