Heritability of renal function in hypertensive families of African descent in the Seychelles (Indian Ocean)

Monogenic and polygenic concepts in chronic kidney disease (CKD)

Kidney function is strongly influenced by genetic factors with both monogenic and polygenic factors contributing to kidney function. Monogenic disorders with primarily autosomal dominant inheritance patterns account for 10% of adult and 50% of paediatric kidney diseases. However, kidney function is also a complex trait with polygenic architecture, where genetic factors interact with environment and lifestyle factors. Family studies suggest that kidney function has significant heritability at 35-69%, capturing complexities of the genome with shared environmental factors. Genome-wide association studies estimate the single nucleotide polymorphism-based heritability of kidney function between 7.1 and 20.3%. These heritability estimates, measuring the extent to which genetic variation contributes to CKD risk, indicate a strong genetic contribution. Polygenic Risk Scores have recently been developed for chronic kidney disease and kidney function, and validated in large populations. Polygenic Risk Scores show correlation with kidney function but lack the specificity to predict individual-level changes in kidney function. Certain kidney diseases, such as membranous nephropathy and IgA nephropathy that have significant genetic components, may benefit most from polygenic risk scores for improved risk stratification. Genetic studies of kidney function also provide a potential avenue for the development of more targeted therapies and interventions. Understanding the development and validation of genomic scores is required to guide their implementation and identify the most appropriate potential implications in clinical practice. In this review, we provide an overview of the heritability of kidney function traits in population studies, explore both monogenic and polygenic concepts in kidney disease, with a focus on recently developed polygenic risk scores in kidney function and chronic kidney disease, and review specific diseases which are most amenable to incorporation of genomic scores.

The Heritability of Kidney Function using an Older Australian Twin Population

Introduction

Twin studies are unique population models which estimate observed rather than inferred genetic components of complex traits. Nonmonogenic chronic kidney disease (CKD) is a complex disease process with strong genetic and environmental influences, amenable to twin studies. We aimed to assess the heritability of CKD using twin analysis and modeling within Older Australian Twin Study (OATS) data.

Methods

OATS had 109 dizygotic (DZ) and 126 monozygotic (MZ) twin pairs with paired serum creatinine levels. Heritability of kidney function as estimated glomerular filtration rate (eGFR CKD Epidemiology Collaboration [CKD-EPI]) was modeled using the ACE model to estimate additive heritability (A), common (C), and unique (E) environmental factors. Intratwin pair analysis using mixed effects logistic regression allowed analysis of variation in eGFR from established CKD risk factors.

Results

The median age was 69.71 (interquartile range 78.4–83.0) years, with 65% female, and a mean CKD-EPI of 82.8 ml/min (SD 6.7). The unadjusted ACE model determined kidney function to be 33% genetically determined (A), 18% shared genetic-environmental (C), and 49% because of unique environment (E). This remained unchanged when adjusted for age, hypertension, and sex. Hypertension was associated with eGFR; however, intertwin variance in hypertension did not explain variance in eGFR. Two or more hypertension medications were associated with decreased eGFR (P = 0.009).

Conclusion

This study estimates observed heritability at 33%, notably higher than inferred heritability in genome-wide association study (GWAS) (7.1%–18%). Epigenetics and other genomic phenomena may explain this heritability gap. Difference in antihypertension medications explains part of unique environmental exposures, though discordance in hypertension and diabetes does not.

Medical records-based chronic kidney disease phenotype for clinical care and "big data" observational and genetic studies

Chronic Kidney Disease (CKD) represents a slowly progressive disorder that is typically silent until late stages, but early intervention can significantly delay its progression. We designed a portable and scalable electronic CKD phenotype to facilitate early disease recognition and empower large-scale observational and genetic studies of kidney traits. The algorithm uses a combination of rule-based and machine-learning methods to automatically place patients on the staging grid of albuminuria by glomerular filtration rate ("A-by-G" grid). We manually validated the algorithm by 451 chart reviews across three medical systems, demonstrating overall positive predictive value of 95% for CKD cases and 97% for healthy controls. Independent case-control validation using 2350 patient records demonstrated diagnostic specificity of 97% and sensitivity of 87%. Application of the phenotype to 1.3 million patients demonstrated that over 80% of CKD cases are undetected using ICD codes alone. We also demonstrated several large-scale applications of the phenotype, including identifying stage-specific kidney disease comorbidities, in silico estimation of kidney trait heritability in thousands of pedigrees reconstructed from medical records, and biobank-based multicenter genome-wide and phenome-wide association studies.

Familial Aggregation of CKD and Heritability of Kidney Biomarkers in the General Population: The Lifelines Cohort Study

RATIONALE & OBJECTIVE

Chronic kidney disease (CKD) has a heritable component. We aimed to quantify familial aggregation of CKD in the general population and assess the extent to which kidney traits could be explained by genetic and environmental factors.

STUDY DESIGN

Cross-sectional 3-generation family study.

SETTING & PARTICIPANTS

Data were collected at entry into the Lifelines Cohort Study from a sample of the general population of the northern Netherlands, composed predominantly of individuals of European ancestry.

EXPOSURE

Family history of CKD.

OUTCOMES

The primary outcome was CKD, defined as estimated glomerular filtration rate (eGFR)<60mL/min/1.73m², where GFR was estimated using the CKD Epidemiology Collaboration creatinine equation. Among a subsample for which urinary albumin concentration was available (n=59,943), urinary albumin excretion was expressed as the rate of urinary albumin excretion (UAE) per 24 hours or urinary albumin-creatinine ratio (UACR).

ANALYTICAL APPROACH

Familial aggregation of CKD was assessed by calculating the recurrence risk ratio (RRR), using adapted Cox proportional hazards models. Heritability of continuous kidney-related traits was estimated using linear mixed models and defined as the ratio of the additive genetic variance to total phenotypic variance. All models were adjusted for age, sex, and known risk factors for kidney disease.

RESULTS

Among 155,911 participants with available eGFR data, the prevalence of CKD was 1.19% (1,862 cases per 155,911). The risk of CKD in those with an affected first-degree relative was 3 timeshigher than the risk in the total sample (RRR, 3.04 [95% CI, 2.26-4.09). In those with an affected spouse, risk of CKD was also higher (RRR, 1.56 [95% CI, 1.20-1.96]), indicative of shared environmental factors and/or assortative mating. Heritability estimates of eGFR, UAE, and UACR were 44%, 20%, and 18%, respectively. For serum urea, creatinine, and uric acid, estimates were 31%, 37%, and 48%, respectively, whereas estimates for serum electrolytes ranged from 22% to 28%.

LIMITATIONS

Use of estimated rather than measured GFR. UAE data only available in a subsample.

CONCLUSIONS

In this large population-based family study, a positive family history was strongly associated with increased risk of CKD. We observed moderate to high heritability of kidney traits and related biomarkers. These results indicate an important role of genetic factors in CKD risk.

Loci associated with genomic damage levels in chronic kidney disease patients and controls

Chronic kidney disease (CKD) is a multifactorial disorder with an important genetic component, and several studies have demonstrated potential associations with allelic variants. In addition, CKD patients are also characterized by high levels of genomic damage. Nevertheless, no studies have established relationships between DNA damage, or genomic instability present in CKD patients, and gene polymorphisms. To fill in this gap, the potential role of polymorphisms in genes involved in base excision repair (OGG1, rs1052133; MUTYH, rs3219489; XRCC1, rs25487), nucleotide excision repair (ERCC2/XPD, rs1799793, rs171140, rs13181; ERCC4, rs3136166); phase II metabolism (GSTP1, rs749174; GSTO1, rs2164624; GSTO2, rs156697), and antioxidant enzymes (SOD1, rs17880135, rs1041740, rs202446; SOD2, rs4880; CAT, rs1001179; GPX1, rs17080528; GPX3, rs870406: GPX4, rs713041) were inquired. In addition, some genes involved in CKD (AGT, rs5050; GLO1, rs386572987; SHROOM3, rs17319721) were also evaluated. The genomic damage, the genomic instability, and oxidative damage were evaluated by using the micronucleus and the comet assay in 589 donors (415 CKD patients and 174 controls). Our results showed significant associations between genomic damage and genes directly involved in DNA repair pathways (XRCC1, and ERCC2), and with genes encoding for antioxidant enzymes (SOD1 and GPX1). GSTO2, as a gene involved in phase II metabolism, and MUTYH showed also an association with genomic instability. Interestingly, the three genes associated with CKD (AGT, GLO1, and SHROOM3) showed associations with both the high levels of oxidatively damaged DNA and genomic instability. These results support our view that genomic instability can be considered a biomarker of the CKD status.

Loss of Histone H3 K79 Methyltransferase Dot1l Facilitates Kidney Fibrosis by Upregulating Endothelin 1 through Histone Deacetylase 2

BACKGROUND

The progression rate of CKD varies substantially among patients. The genetic and epigenetic contributions that modify how individual patients respond to kidney injury are largely unknown. Emerging evidence has suggested that histone H3 K79 methyltransferase Dot1l has an antifibrotic effect by repressing Edn1, which encodes endothelin 1 in the connecting tubule/collecting duct.

METHODS

To determine if deletion of the Dot1l gene is a genetic and epigenetic risk factor through regulating Edn1, we studied four groups of mice: wild-type mice, connecting tubule/collecting duct-specific Dot1l conditional knockout mice (Dot1l^AC ), Dot1l and Edn1 double-knockout mice (DE^AC ), and Edn1 connecting tubule/collecting duct-specific conditional knockout mice (Edn1^AC ), under three experimental conditions (streptozotocin-induced diabetes, during normal aging, and after unilateral ureteral obstruction). We used several approaches (colocalization, glutathione S-transferase pulldown, coimmunoprecipitation, yeast two-hybrid, gel shift, and chromatin immunoprecipitation assays) to identify and confirm interaction of Dot1a (the major Dot1l splicing variant in the mouse kidney) with histone deacetylase 2 (HDAC2), as well as the function of the Dot1a-HDAC2 complex in regulating Edn1 transcription.

RESULTS

In each case, Dot1l^AC mice developed more pronounced kidney fibrosis and kidney malfunction compared with wild-type mice. These Dot1l^AC phenotypes were ameliorated in the double-knockout DE^AC mice. The interaction between Dot1a and HDAC2 prevents the Dot1a-HDAC2 complex from association with DNA, providing a counterbalancing mechanism governing Edn1 transcription by modulating H3 K79 dimethylation and H3 acetylation at the Edn1 promoter.

CONCLUSIONS

Our study confirms Dot1l to be a genetic and epigenetic modifier of kidney fibrosis, reveals a new mechanism regulating Edn1 transcription by Dot1a and HDAC2, and reinforces endothelin 1 as a therapeutic target of kidney fibrosis.

Genetics and ESKD Disparities in African Americans

African Americans have a 2- to 4-fold greater incidence of end-stage kidney disease (ESKD) than whites, which has long raised the possibility of a genetic cause for this disparity. Recent advances in genetic studies have shown a causal association of polymorphisms at the apolipoprotein L1 gene (APOL1) with the markedly increased risk for the nondiabetic component of the overall disparity in ESKD in African Americans. Although APOL1-associated kidney disease is thought to account for a substantial proportion of ESKD in African Americans, not all the increased risk for ESKD is accounted for, and a complete cataloging of disparities in genetic causes of ESKD eludes our current understanding of genetic-associated kidney disease. Genetic testing aids the screening, diagnosis, prognosis, and treatment of diseases with a genetic basis. Widespread use of genetic testing in clinical practice is limited by the small number of actionable genetic variants, limited health literacy of providers and patients, and underlying complex ethical, legal, and social issues. This perspective reviews racial and ethnic differences associated with genetic diseases and the development of ESKD in African Americans and discusses potential uncertainties associated with our current understanding of penetrance of genetically linked kidney disease and population-attributable risk percent.

Evaluation of a genetic risk score based on creatinine-estimated glomerular filtration rate and its association with kidney outcomes

Background

Meta-analysis of cross-sectional genome-wide association studies (GWAS) on creatinine-estimated glomerular filtration rate (eGFRcrea) identified 53 single-nucleotide polymorphisms (SNPs). These SNP effects can be aggregated into a genetic risk score (GRS) for chronic kidney disease (CKD). To assess its clinical utility, we examined associations with creatinine-estimated kidney outcomes, both cross-sectionally and longitudinally. Additionally, we examined associations with cystatin C-estimated kidney outcomes to verify that a GRS based on eGFRcrea SNPs represents the genetics underlying kidney function.

Methods

In the community-based Prevention of REnal and Vascular ENdstage Disease (PREVEND) study, we assessed eGFRcrea and eGFRcysc at baseline and four follow-up examinations. The GRS comprised 53 SNPs for eGFRcrea weighted for reported effect-sizes. We adjusted for baseline demographics and renal risk factors.

Results

We included 3649 subjects (median age 49 years, 52% male, median follow-up 11 years, n = 85 baseline CKD, n = 154 incident CKD). At baseline, a higher GRS associated with lower eGFRcrea {adjusted B [95% confidence interval (CI)] = -2.05 (-2.45 to - 1.65) mL/min/1.73 m2, P < 0.001} and higher CKD prevalence [adjusted odds ratio (95% CI) = 1.41 (1.12-1.77), P = 0.002]. During follow-up, a higher GRS associated with higher CKD incidence [adjusted hazard ratio (95% CI) = 1.28 (1.09-1.50), P = 0.004], but no longer significantly after adjustment for baseline eGFR. No significant association with eGFRcrea decline was found. Associations with cystatin C-estimated outcomes were similar.

Conclusions

The GRS robustly associated with baseline CKD and eGFR, independent of known risk factors. Associations with incident CKD were likely due to low baseline eGFR, not accelerated eGFR decline. The GRS for eGFRcrea likely represents the genetics underlying kidney function, not creatinine metabolism or underlying aetiologies. To improve the clinical utility of GWAS results for CKD, these need to specifically address eGFR decline and CKD incidence.

Sex and Body Mass Index Modify the Association Between Leptin and Sodium Excretion: A Cross-sectional Study in an African Population

BACKGROUND

Renal sodium handling could be a potential mediator linking adipokines to hypertension. The aim of the study was to assess the relationship of leptin with urinary sodium excretion and proximal sodium reabsorption in humans.

METHODS

This cross-sectional study was conducted on participants of hypertensive families from the Seychelles Island. A split urine (daytime and nighttime) collection and plasma leptin were measured. Endogenous lithium clearance was used to assess proximal sodium reabsorption. Mixed multiple linear regression tests adjusted for confounding factors were used.

RESULTS

Three hundred and sixty-five participants (57% women) were included in this analysis. Leptin and adiponectin were higher in women (P < 0.001). Leptin was associated positively with daytime (coefficient [c]: 0.16, standard deviation (SD): 0.03, P < 0.001), nighttime urinary sodium excretion (c: 0.17, SD: 0.04), P < 0.01), daytime lithium clearance (c: 0.40, SD: 0.08, P < 0.001), and nighttime lithium clearance (c: 0.39, SD: 0.10, P < 0.001) after adjusting for sex. The association was lost or mitigated only when BMI was introduced in the model. When BMI was categorized in normal vs. overweight participant, leptin was associated with daytime and nighttime sodium excretion rates (c: 0.14, SD: 0.05, P = 0.011 and c: 0.22, SD: 0.07, P = 0.002, respectively) only in overweight participants.

CONCLUSION

Leptin is associated positively with daytime and nighttime sodium excretion and lithium clearance suggesting a natriuretic rather than a sodium retaining effect of leptin. Sex and body mass index (BMI) are major confounders in this association. These results highlight the importance of sex and obesity in our understanding of the relationships between leptin, blood pressure, and renal sodium handling.

Identification of 13 novel susceptibility loci for early-onset myocardial infarction, hypertension, or chronic kidney disease

Early-onset cardiovascular and renal diseases have a strong genetic component. In the present study, exome-wide association studies (EWASs) were performed to identify genetic variants that confer susceptibility to early-onset myocardial infarction (MI), hypertension, or chronic kidney disease (CKD) in Japanese individuals. A total of 8,093 individuals aged ≤65 years was enrolled in the study. The EWASs for MI, hypertension, and CKD were performed in 6,926 subjects (1,152 cases, 5,774 controls), 8,080 subjects (3,444 cases, 4,636 controls), and 2,556 subjects (1,051 cases, 1,505 controls), respectively. Genotyping of single nucleotide polymorphisms (SNPs) was performed with Illumina Human Exome-12 DNA Analysis BeadChip or Infinium Exome-24 BeadChip arrays. The associations of allele frequencies for 31,245, 31,276, or 31,514 SNPs that passed quality control to MI, hypertension, and CKD, respectively, was examined with Fisher's exact test. Bonferroni's correction for statistical significance of association was applied to compensate for multiple comparisons of genotypes with MI, hypertension, or CKD. The EWASs of allele frequencies revealed that 25, 11, and 11 SNPs were significantly associated with MI (P<1.60×10⁻⁶), hypertension (P<1.60×10⁻⁶), or CKD (P<1.59×10⁻⁶), respectively. Multivariable logistic regression analysis with adjustment for covariates showed that all 25, 11, and 11 SNPs were significantly associated with MI (P<0.0005), hypertension (P<0.0011), or CKD (P<0.0011), respectively. On examination of the results from previous genome-wide association studies and linkage disequilibrium of the identified SNPs, 11 loci (TMOD4, COL6A3, ADGRL3-CXCL8-MARCH1, OR52E4, TCHP-GIT2, CCDC63, 12q24.1, OAS3, PLCB2-VPS33B, GOSR2, ZNF77), six loci (MOB3C-TMOD4, COL6A3, COL6A5, CXCL8-MARCH1, NFKBIL1-6p21.3-NCR3, PLCB2-VPS33B), and seven loci (MOB3C-TMOD4, COL6A3, COL6A5, ADGRL3-CXCL8-MARCH1, MUC17, PLCB2-VPS33B, ZNF77) were identified as novel loci significantly associated with MI, hypertension, and CKD, respectively. Furthermore, six genes (TMOD4, COL6A3, CXCL8, MARCH1, PLCB2, VPS33B) were significantly associated with MI, hypertension and CKD; two genes (ADGRL3, ZNF77) with MI and CKD; and two genes (COL6A5, MOB3C) with hypertension and CKD. Therefore, 13 novel loci (MOB3C-TMOD4, COL6A3, ADGRL3-CXCL8-MARCH1, OR52E4, TCHP- GIT2, CCDC63, 12q24.1, OAS3, PLCB2-VPS33B, ZNF77, COL6A5, NFKBIL1-NCR3, MUC17) were identified that confer susceptibility to early-onset MI, hypertension, or CKD. The determination of genotypes for the SNPs at these loci may provide informative for assessment of the genetic risk for MI, hypertension, or CKD.

Epigenome-wide association studies identify DNA methylation associated with kidney function

Chronic kidney disease (CKD) is defined by reduced estimated glomerular filtration rate (eGFR). Previous genetic studies have implicated regulatory mechanisms contributing to CKD. Here we present epigenome-wide association studies of eGFR and CKD using whole-blood DNA methylation of 2264 ARIC Study and 2595 Framingham Heart Study participants to identify epigenetic signatures of kidney function. Of 19 CpG sites significantly associated (P < 1e-07) with eGFR/CKD and replicated, five also associate with renal fibrosis in biopsies from CKD patients and show concordant DNA methylation changes in kidney cortex. Lead CpGs at PTPN6/PHB2, ANKRD11, and TNRC18 map to active enhancers in kidney cortex. At PTPN6/PHB2 cg19942083, methylation in kidney cortex associates with lower renal PTPN6 expression, higher eGFR, and less renal fibrosis. The regions containing the 243 eGFR-associated (P < 1e-05) CpGs are significantly enriched for transcription factor binding sites of EBF1, EP300, and CEBPB (P < 5e-6). Our findings highlight kidney function associated epigenetic variation.

Genome-wide association studies of kidney function show enrichment of associated genetic variants in regulatory regions. Here, the authors perform epigenome-wide association studies of kidney function and disease, identifying 19 CpG sites significantly associated with these.

Serum Creatine Kinase-MB Isoenzyme Activity among Subjects with Uncomplicated Essential Hypertension: Any Sex Differences

Hypertension (high blood pressure) is a major health challenge and more women than men are affected by the condition. Complications as a result of this condition often lead to disabilities and premature death. The objective of this study was to evaluate creatine kinase-MB (CK-MB) activity in uncomplicated hypertension and to know whether sex differences exist in the activity of the enzyme. Serum creatine kinase-MB isoenzyme activity, troponin I, and lipid profile were evaluated in 140 male and 100 female Nigerians with hypertension. The control group was comprised of 100 (50 males and 50 females) normotensive subjects. Measured parameters were assayed using Selectra Pros chemistry analyzer. The means were compared between males and females using Students't-test. The mean CK-MB activity of the female hypertensive subjects was significantly higher (p < 0.001) than the males. Similarly, the mean cardiac troponin I (cTnI) of the female hypertensive subjects was significantly higher (p < 0.001) than the males. Conversely, the mean CK-MB activity of the female normotensive subjects was significantly lower (p < 0.001) than the male counterparts. There was no difference in the levels of cTnI between male and female normotensive subjects. Serum CK-MB activity was higher in female than male hypertensive subjects. In the light of these results, cardiac markers should be routinely done in the evaluation of hypertensive subjects and sex-specific consideration may be recognized in the management of these patients.

Genome-Wide Association of Copy Number Polymorphisms and Kidney Function

Genome-wide association studies (GWAS) using single nucleotide polymorphisms (SNPs) have identified more than 50 loci associated with estimated glomerular filtration rate (eGFR), a measure of kidney function. However, significant SNPs account for a small proportion of eGFR variability. Other forms of genetic variation have not been comprehensively evaluated for association with eGFR. In this study, we assess whether changes in germline DNA copy number are associated with GFR estimated from serum creatinine, eGFRcrea. We used hidden Markov models (HMMs) to identify copy number polymorphic regions (CNPs) from high-throughput SNP arrays for 2,514 African (AA) and 8,645 European ancestry (EA) participants in the Atherosclerosis Risk in Communities (ARIC) study. Separately for the EA and AA cohorts, we used Bayesian Gaussian mixture models to estimate copy number at regions identified by the HMM or previously reported in the HapMap Project. We identified 312 and 464 autosomal CNPs among individuals of EA and AA, respectively. Multivariate models adjusted for SNP-derived covariates of population structure identified one CNP in the EA cohort near genome-wide statistical significance (Bonferroni-adjusted p = 0.067) located on chromosome 5 (876-880kb). Overall, our findings suggest a limited role of CNPs in explaining eGFR variability.

Estimating Heritability from Nuclear Family and Pedigree Data

Heritability is a measure of familial resemblance. Estimating the heritability of a trait could be one of the first steps in the gene mapping process. This chapter describes how to estimate heritability for quantitative traits from nuclear and pedigree data using the ASSOC program in the Statistical Analysis in Genetic Epidemiology (S.A.G.E.) software package. Estimating heritability rests on the assumption that the total phenotypic variance of a quantitative trait can be partitioned into independent genetic and environmental components. In turn, the genetic variance can be divided into an additive (polygenic) genetic variance, a dominance variance (nonlinear interaction effects between alleles at the same locus) and an epistatic variance (interaction effects between alleles at different loci). The last two are often assumed to be zero. The additive genetic variance represents the average effects of individual alleles on the phenotype and reflects transmissible resemblance between relatives. Heritability in the narrow sense (h ² ) refers to the ratio of the additive genetic variance to the total phenotypic variance. Heritability is a dimensionless population-specific parameter. ASSOC estimates association parameters (regression coefficients) and variance components from family data. ASSOC uses a linear regression model in which the total residual variance is partitioned, after regressing on covariates, into the sum of random components such as an additive polygenic component, a random sibship component, random nuclear family components, a random marital component, and an individual-specific random component. Assortative mating, nonrandom ascertainment of families, and failure to account for key confounding factors may bias heritability estimates.

Dietary sodium and potassium intakes: Data from urban and rural areas

OBJECTIVES

Hypertension is highly prevalent in West African populations, but little data is available on salt and potassium intake in these populations. We assumed in this study that sodium and potassium intake might be high and low, respectively, in the Beninese population in view of the emerging nutritional transition. The aim of this study was to estimate dietary sodium and potassium intakes based on 24-h urine collections.

METHODS

We selected 420 individuals (ages 25-64 y), representative of the population, from urban and rural areas in Benin. Urine was collected over 24 h, and sodium, potassium, and creatinine were quantified. Blood pressure was measured on the left arm using a validated electronic oscillometric monitor.

RESULTS

Adequate data were available for 354 participants. Mean dietary intake of sodium and potassium were 4.4 ± 2.1 and 1.8 ± 0.9 g/24 h, respectively. High intake of sodium was associated with urban area, age <44 y, administrative occupation, higher income, body mass index (BMI) ≥25 kg/m², and a large waist circumference. High potassium intake was associated with male sex, administrative occupation, BMI ≥25 kg/m², and large waist circumference. Sodium intake was associated with high systolic and diastolic blood pressures. In multivariate analysis, only age <44 y and, marginally, BMI ≥25 kg/m² were associated with high sodium intake, whereas male sex and a BMI ≥25 kg/m² were associated with high potassium intake.

CONCLUSION

Large proportions of the population had sodium intake higher, and potassium intake lower, than dietary recommendations. These results suggest that interventions to reduce salt consumption and promote potassium-rich foods, including fruits and vegetables, are needed in Benin.

Cystatin C Predicts Incident Cardiovascular Disease in Twins

BACKGROUND

Cystatin C is associated with both renal function and atherosclerotic cardiovascular disease (ASCVD). We have previously shown a genetic correlation between cystatin C and prevalent ASCVD. The objective of this article is to study whether variation in cystatin C or creatinine predicts incident ASCVD when controlled for genetic factors.

METHODS AND RESULTS

The predictive value of cystatin C and creatinine for incident ASCVD was studied in 11 402 Swedish twins, free of CVD at baseline, in an adjusted Cox-regression model during a median follow-up of 71 months. Twin pairs discordant for incident stroke, myocardial infarction and ASCVD during follow-up were identified and within-pair comparisons regarding cystatin C and creatinine levels were performed. We also investigated whether contact frequency and degree of shared environment influences were associated with similarity in cystatin C levels. In univariate analysis, cystatin C predicted incident ASCVD hazard ratio 1.57, 95% CI 1.47-1.67. When adjusted for traditional Framingham risk factors as covariates, cystatin C remained a predictor of incident stroke hazard ratio 1.45, 95% CI (1.25-1.70), ASCVD hazard ratio 1.26, 95% CI (1.13-1.41), and myocardial infarction hazard ratio 1.16, 95% CI (1.01-1.33). In twins discordant for incident stroke, cystatin C at baseline was higher in the twin who experienced a stroke compared to the healthy co-twin (1.11±0.3 mg/L versus 1.06±0.3 mg/L), whereas creatinine was lower in the twin who developed CVD compared to their healthy co-twins (76.1±16.9 μmol/L versus 79.4±20.3 μmol/L).

CONCLUSIONS

Variation in cystatin C relates to incident ASCVD and to stroke when adjusted for genetic confounding. In identical twins, cystatin C may be a sensitive marker of early hypertensive end-organ damage and small-vessel disease, whereas creatinine level may reflect nutritional status. The findings in disease-discordant monozygotic twins indicate that unique, possibly preventable, environmental factors are important.

Heritability of ambulatory and office blood pressure in the Swiss population

BACKGROUND

Blood pressure (BP) is known to aggregate in families. Yet, heritability estimates are population-specific and no Swiss data have been published so far. We estimated the heritability of ambulatory and office BP in a Swiss population-based sample.

METHODS

The Swiss Kidney Project on Genes in Hypertension is a population-based family study focusing on BP genetics. Office and ambulatory BP were measured in 1009 individuals from 271 nuclear families. Heritability was estimated for SBP, DBP, and pulse pressure using a maximum likelihood method implanted in the Statistical Analysis in Genetic Epidemiology software.

RESULTS

The 518 women and 491 men included in this analysis had a mean (±SD) age of 48.3 (±17.4) and 47.3 (±17.7) years, and a mean BMI of 23.8 (±4.2) and 25.9 (±4.1) kg/m, respectively. Narrow-sense heritability estimates (±standard error) for ambulatory SBP, DBP, and pulse pressure were 0.37 ± 0.07, 0.26 ± 0.07, and 0.29 ± 0.07 for 24-h BP; 0.39 ± 0.07, 0.28 ± 0.07, and 0.27 ± 0.07 for day BP; and 0.25 ± 0.07, 0.20 ± 0.07, and 0.30 ± 0.07 for night BP, respectively (all P < 0.001). Heritability estimates for office SBP, DBP, and pulse pressure were 0.21 ± 0.08, 0.25 ± 0.08, and 0.18 ± 0.07 (all P < 0.01).

CONCLUSIONS

We found significant heritability estimates for both ambulatory and office BP in this Swiss population-based study. Our findings justify the ongoing search for the genetic determinants of BP.

Glycosylation Profile of IgG in Moderate Kidney Dysfunction

Glycans constitute the most abundant and diverse form of the post-translational modifications, and animal studies have suggested the involvement of IgG glycosylation in mechanisms of renal damage. Here, we explored the associations between IgG glycans and renal function in 3274 individuals from the TwinsUK registry. We analyzed the correlation between renal function measured as eGFR and 76 N-glycan traits using linear regressions adjusted for covariates and multiple testing in the larger population. We replicated our results in 31 monozygotic twin pairs discordant for renal function. Results from both analyses were then meta-analyzed. Fourteen glycan traits were associated with renal function in the discovery sample (P<6.5×10(-4)) and remained significant after validation. Those glycan traits belong to three main glycosylation features: galactosylation, sialylation, and level of bisecting N-acetylglucosamine of the IgG glycans. These results show the role of IgG glycosylation in kidney function and provide novel insight into the pathophysiology of CKD and potential diagnostic and therapeutic targets.

Copeptin is associated with kidney length, renal function, and prevalence of simple cysts in a population-based study

Arginine vasopressin (AVP) has a key role in osmoregulation by facilitating water transport in the collecting duct. Recent evidence suggests that AVP may have additional effects on renal function and favor cyst growth in polycystic kidney disease. Whether AVP also affects kidney structure in the general population is unknown. We analyzed the association of copeptin, an established surrogate for AVP, with parameters of renal function and morphology in a multicentric population-based cohort. Participants from families of European ancestry were randomly selected in three Swiss cities. We used linear multilevel regression analysis to explore the association of copeptin with renal function parameters as well as kidney length and the presence of simple renal cysts assessed by ultrasound examination. Copeptin levels were log-transformed. The 529 women and 481 men had median copeptin levels of 3.0 and 5.2 pmol/L, respectively (P<0.001). In multivariable analyses, the copeptin level was associated inversely with eGFR (β=-2.1; 95% confidence interval [95% CI], -3.3 to -0.8; P=0.002) and kidney length (β=-1.2; 95% CI, -1.9 to -0.4; P=0.003) but positively with 24-hour urinary albumin excretion (β=0.11; 95% CI, 0.01 to 0.20; P=0.03) and urine osmolality (β=0.08; 95% CI, 0.05 to 0.10; P<0.001). A positive association was found between the copeptin level and the presence of renal cysts (odds ratio, 1.6; 95% CI, 1.1 to 2.4; P=0.02). These results suggest that AVP has a pleiotropic role in renal function and may favor the development of simple renal cysts.

Evaluation of neurotrophic tyrosine receptor kinase 2 (NTRK2) as a positional candidate gene for variation in estimated glomerular filtration rate (eGFR) in Mexican American participants of San Antonio Family Heart study

Background

The estimated glomerular filtration rate (eGFR) is a well-known measure of kidney function and is commonly used for the diagnosis and management of patients with chronic kidney disease. The inter-individual variation in eGFR has significant genetic component. However, the identification of underlying genetic susceptibility variants has been challenging. In an attempt to identify and characterize susceptibility genetic variant(s) we previously identified the strongest evidence for linkage of eGFR occurring on chromosome 9q21 in the Mexican American participants of San Antonio Family Heart Study (SAFHS). The objective of the present study was to examine whether the common genetic variants in Neurotrophic Tyrosine Receptor Kinase 2 (NTRK2), a positional candidate gene on 9q21, contribute to variation in eGFR.

Results

Twelve tagging single nucleotide polymorphisms (SNPs) across the NTRK2 gene region were selected (r2 ≥ 0.80, minor allele frequency of ≥ 0.05) from the Hapmap database. SNPs were genotyped by TaqMan assay in the 848 Mexican American subjects participated in the SAFHS. Association analysis between the genotypes and eGFR (estimated by the Modification of Diet in Renal Disease equation) were performed by measured genotype approach as implemented in the program SOLAR. Of the 12 common genetic variants examined, the rs1036915 (located in 3′UTR) and rs1187274 (located in intron-14), present in perfect linkage disequilibrium, exhibited an association (P = 0.017) with eGFR after accounting for the effects of age, sex, diabetes, diabetes duration, systolic blood pressure and blood pressure medication. The carriers of minor allele of rs1036915 (G; 38%) had increased eGFR (104 ± 25 ml/min/1.73 m²) in comparison to the carriers of major allele A (98 ± 25 ml/min/1.73 m²).

Conclusion

Together, our results suggest for the first time that the genetic variants in NTRK2 may regulate eGFR.

Measuring GFR: a systematic review

BACKGROUND

No comprehensive systematic review of the accuracy of glomerular filtration rate (GFR) measurement methods using renal inulin clearance as reference has been published.

STUDY DESIGN

Systematic review with meta-analysis of cross-sectional diagnostic studies.

SETTING & POPULATION

Published original studies and systematic reviews in any population.

SELECTION CRITERIA FOR STUDIES

Index and reference measurements conducted within 48 hours; at least 15 participants studied; GFR markers measured in plasma or urine; plasma clearance calculation algorithm verified in another study; tubular secretion of creatinine had not been blocked by medicines.

INDEX TESTS

Endogenous creatinine clearance; renal or plasma clearance of chromium 51-labeled ethylenediaminetetraacetic acid (51Cr-EDTA), diethylenetriaminepentaacetic acid (DTPA), iohexol, and iothalamate; and plasma clearance of inulin.

REFERENCE TEST

Renal inulin clearance measured under continuous inulin infusion and urine collection.

RESULTS

Mean bias <10%, median bias <5%, the proportion of errors in the index measurements that did not exceed 30% (P30) ≥80%, and P10 ≥50% were set as requirements for sufficient accuracy. Based on the GRADE (Grading of Recommendations Assessment, Development and Evaluation) approach, the quality of evidence across studies was rated for each index method. Renal clearance of iothalamate measured GFR with sufficient accuracy (strong evidence). Renal and plasma clearance of 51Cr-EDTA and plasma clearance of iohexol were sufficiently accurate to measure GFR (moderately strong evidence). Renal clearance of DTPA, renal clearance of iohexol, and plasma clearance of inulin had sufficient accuracy (limited evidence). Endogenous creatinine clearance was an inaccurate method (strong evidence), as was plasma clearance of DTPA (limited evidence). The evidence to determine the accuracy of plasma iothalamate clearance was insufficient. With the exception of plasma clearance of inulin, only renal clearance methods had P30 >90%.

LIMITATIONS

The included studies were few and most were old and small, which may limit generalizability. Requirements for sufficient accuracy may depend on clinical setting.

CONCLUSIONS

At least moderately strong evidence suggests that renal clearance of 51Cr-EDTA or iothalamate and plasma clearance of 51Cr-EDTA or iohexol are sufficiently accurate methods to measure GFR.

Recent advances in hypertension in sub-Saharan Africa

BACKGROUND

Hypertension was once considered rare in sub-Saharan Africa (SSA), but currently it has become a widespread problem with immense socioeconomic importance. The purpose of this review is to summarise new information on hypertension in SSA that has been published since the last major review in 2008.

METHODS AND RESULTS

A literature search was performed in Pubmed, Embase, WHO Global Cardiovascular Infobase, African Journal On-Line, and African Index Medicus using the following search criteria: hypertension, high blood pressure, and Africa/SSA. Epidemiological surveys that used the WHO STEPS approach or similar methods were also included. The overall prevalence of hypertension in SSA was estimated at 16.2% (95% CI 14.2% to 20.3%) with an estimated number of hypertensive individuals to be 74.7 million. The prevalence of hypertension varies widely from country to country. It is projected that the number of affected individuals will increase by 68% (125.5 million) by 2025. Mass migration of rural Africans to urban areas and rapid changes in lifestyle and risk factors account for the rising prevalence of hypertension.

CONCLUSIONS

Proactive public health interventions at a population level need to be introduced to control the growing hypertension epidemic, and there needs to be a major improvement in access to hypertensive care for the individual. There is an important need for better epidemiological data and hypertension related outcome trials in SSA.

A family-based association study after genome-wide linkage analysis identified two genetic loci for renal function in a Mongolian population

The estimated glomerular filtration rate is a well-known measure of renal function and is widely used to follow the course of disease. Although there have been several investigations establishing the genetic background contributing to renal function, Asian populations have rarely been used in these genome-wide studies. Here, we aimed to find candidate genetic determinants of renal function in 1007 individuals from 73 extended families of Mongolian origin. Linkage analysis found two suggestive regions near 9q21 (logarithm of odds (LOD) 2.82) and 15q15 (LOD 2.70). The subsequent family-based association study found 2 and 10 significant single-nucleotide polymorphisms (SNPs) in each region, respectively. The strongest SNPs on chromosome 9 and 15 were rs17400257 and rs1153831 with P-values of 7.21 × 10(-9) and 2.47 × 10(-11), respectively. Genes located near these SNPs are considered candidates for determining renal function and include FRMD3, GATM, and SPATA5L1. Thus, we identified possible loci that determine renal function in an isolated Asian population. Consistent with previous reports, our study found genes linked and associated with renal function in other populations.

Integration of genome-wide association studies with biological knowledge identifies six novel genes related to kidney function

In conducting genome-wide association studies (GWAS), analytical approaches leveraging biological information may further understanding of the pathophysiology of clinical traits. To discover novel associations with estimated glomerular filtration rate (eGFR), a measure of kidney function, we developed a strategy for integrating prior biological knowledge into the existing GWAS data for eGFR from the CKDGen Consortium. Our strategy focuses on single nucleotide polymorphism (SNPs) in genes that are connected by functional evidence, determined by literature mining and gene ontology (GO) hierarchies, to genes near previously validated eGFR associations. It then requires association thresholds consistent with multiple testing, and finally evaluates novel candidates by independent replication. Among the samples of European ancestry, we identified a genome-wide significant SNP in FBXL20 (P = 5.6 × 10(-9)) in meta-analysis of all available data, and additional SNPs at the INHBC, LRP2, PLEKHA1, SLC3A2 and SLC7A6 genes meeting multiple-testing corrected significance for replication and overall P-values of 4.5 × 10(-4)-2.2 × 10(-7). Neither the novel PLEKHA1 nor FBXL20 associations, both further supported by association with eGFR among African Americans and with transcript abundance, would have been implicated by eGFR candidate gene approaches. LRP2, encoding the megalin receptor, was identified through connection with the previously known eGFR gene DAB2 and extends understanding of the megalin system in kidney function. These findings highlight integration of existing genome-wide association data with independent biological knowledge to uncover novel candidate eGFR associations, including candidates lacking known connections to kidney-specific pathways. The strategy may also be applicable to other clinical phenotypes, although more testing will be needed to assess its potential for discovery in general.

Estimating heritability from nuclear family and pedigree data

Heritability is a measure of familial resemblance. Estimating the heritability of a trait represents one of the first steps in the gene mapping process. This chapter describes how to estimate heritability for quantitative traits from nuclear and pedigree data using the ASSOC program in the Statistical Analysis for Genetic Epidemiology (S.A.G.E.) software package. Estimating heritability rests on the assumption that the total phenotypic variance of a quantitative trait can be partitioned into independent genetic and environmental components. In turn, the genetic variance can be divided into an additive (polygenic) genetic variance, a dominance variance (nonlinear interaction effects between alleles at the same locus), and an epistatic variance (interaction effects between alleles at different loci). The last two are often assumed to be zero. The additive genetic variance represents the average effects of individual alleles on the phenotype and reflects transmissible resemblance between relatives. Heritability in the narrow sense (h (2)) refers to the ratio of the additive genetic variance to the total phenotypic variance. Heritability is a dimensionless population-specific parameter. ASSOC estimates association parameters (regression coefficients) and variance components from family data. ASSOC uses a linear regression model in which the total residual variance is partitioned, after regressing on covariates, into the sum of a random additive polygenic component, a random sibship component, random nuclear family components, a random marital component, and an individual-specific random component. Assortative mating, nonrandom ascertainment of families and failure to account for key confounding factors may bias heritability estimates.

Predicting diabetic nephropathy using a multifactorial genetic model

Aims

The tendency to develop diabetic nephropathy is, in part, genetically determined, however this genetic risk is largely undefined. In this proof-of-concept study, we tested the hypothesis that combined analysis of multiple genetic variants can improve prediction.

Methods

Based on previous reports, we selected 27 SNPs in 15 genes from metabolic pathways involved in the pathogenesis of diabetic nephropathy and genotyped them in 1274 Ashkenazi or Sephardic Jewish patients with Type 1 or Type 2 diabetes of >10 years duration. A logistic regression model was built using a backward selection algorithm and SNPs nominally associated with nephropathy in our population. The model was validated by using random “training” (75%) and “test” (25%) subgroups of the original population and by applying the model to an independent dataset of 848 Ashkenazi patients.

Results

The logistic model based on 5 SNPs in 5 genes (HSPG2, NOS3, ADIPOR2, AGER, and CCL5) and 5 conventional variables (age, sex, ethnicity, diabetes type and duration), and allowing for all possible two-way interactions, predicted nephropathy in our initial population (C-statistic = 0.672) better than a model based on conventional variables only (C = 0.569). In the independent replication dataset, although the C-statistic of the genetic model decreased (0.576), it remained highly associated with diabetic nephropathy (χ² = 17.79, p<0.0001). In the replication dataset, the model based on conventional variables only was not associated with nephropathy (χ² = 3.2673, p = 0.07).

Conclusion

In this proof-of-concept study, we developed and validated a genetic model in the Ashkenazi/Sephardic population predicting nephropathy more effectively than a similarly constructed non-genetic model. Further testing is required to determine if this modeling approach, using an optimally selected panel of genetic markers, can provide clinically useful prediction and if generic models can be developed for use across multiple ethnic groups or if population-specific models are required.

Fetal, developmental, and parental influences on cystatin C in childhood: the Uppsala Family Study

BACKGROUND

The aim was to identify determinants (biomedical and social characteristics of children and their parents) of cystatin C levels in healthy children drawn from a population sample.

STUDY DESIGN

Cross-sectional study.

SETTING & PARTICIPANTS

425 pairs of consecutive full siblings born 1987-1995 in Uppsala were identified using the Swedish Medical Birth Registry and invited with their parents for examination in 2000-2001.

OUTCOME

Serum cystatin C level was log-transformed and analyzed using random-effects models.

MEASUREMENTS

The examination in parents and children consisted of a nonfasting blood sample, anthropometry, and questionnaires about lifestyle and socioeconomic position. Tanner stage was used for assessment of pubertal status.

RESULTS

In age-, height-, and body mass index-adjusted analyses, cystatin C level increased by 2.6% (95% CI, 0.3%-4.8%) higher in Tanner stage 2 vs 1 girls, and 1.6% (95%CI, 0.2%-3.1%) lower in boys than girls. For every 10% increase in maternal cystatin C level, offspring cystatin C level increased by 3.0% (95% CI, 2.2%-3.8%); the equivalent effect for paternal cystatin C level was 2.1% (95% CI, 1.3%-2.9%). Lower maternal education was associated with a 2.4% (95% CI, 0.3%-4.6%) higher cystatin C level in their offspring.

LIMITATIONS

Cross-sectional study design, missing cystatin C values for subset of parents, lack of urinary measurements, no gold-standard measurement of glomerular filtration rate.

CONCLUSIONS

There are intergenerational associations of cystatin C level in families in line with previous reports of heritability of kidney disease. Lower maternal education is associated with higher cystatin C levels in their children. Further studies of healthy children are needed to explore the biological mechanisms for these findings. If cystatin C is measured, these studies will need to record pubertal stages.

Genome-wide association studies in nephrology research

Kidney diseases constitute a serious public health burden worldwide, with substantial associated morbidity and mortality. The role of a genetic contribution to kidney disease is supported by heritability studies of kidney function measures, the presence of monogenic diseases with renal manifestations, and familial aggregation studies of complex kidney diseases, such as chronic kidney disease. Because complex diseases arise from the combination of multiple genetic and environmental risk factors, the identification of underlying genetic susceptibility variants has been challenging. Recently, genome-wide association studies have emerged as a method to conduct searches for such susceptibility variants. They have successfully identified genomic loci that contain variants associated with kidney diseases and measures of kidney function. For example, common variants in the UMOD and PRKAG2 genes are associated with risk of chronic kidney disease; variants in CLDN14 with risk of kidney stone disease; and variants in or near SHROOM3, STC1, LASS2, GCKR, NAT8/ALMS1, TFDP2, DAB2, SLC34A1, VEGFA, FAM122A/PIP5K1B, ATXN2, DACH1, UBE2Q2/FBXO22, and SLC7A9, with differences in glomerular filtration rate. The purpose of this review is to provide an overview of the genome-wide association study method as it relates to nephrology research and summarize recent findings in the field. Results from genome-wide association studies of renal phenotypes represent a first step toward improving our knowledge about underlying mechanisms of kidney function and disease and ultimately may aid in the improved treatment and prevention of kidney diseases.

Heritability of measures of kidney disease among Zuni Indians: the Zuni Kidney Project

BACKGROUND

The long-term goal of the GKDZI (Genetics of Kidney Disease in Zuni Indians) Study is to identify genes, environmental factors, and genetic-environmental interactions that modulate susceptibility to renal disease and intermediate phenotypes.

STUDY DESIGN

A community-based participatory research approach was used to recruit family members of individuals with kidney disease.

SETTING & PARTICIPANTS

The study was conducted in the Zuni Indians, a small endogamous tribe located in rural New Mexico. We recruited members of extended families, ascertained through a proband with kidney disease and at least 1 sibling with kidney disease. 821 participants were recruited, comprising 7,702 relative pairs. PREDICTOR OUTCOMES & MEASUREMENTS: Urine albumin-creatinine ratio (UACR) and hematuria were determined in 3 urine samples and expressed as a true ratio. Glomerular filtration rate (GFR) was estimated using the Modification of Diet in Renal Disease (MDRD) Study equation modified for American Indians. Probands were considered to have kidney disease if UACR was >or=0.2 in 2 or more of 3 spot urine samples or estimated GFR was decreased according to the CRIC (Chronic Renal Insufficiency Cohort) Study criteria.

RESULTS

Kidney disease was identified in 192 participants (23.4%). There were significant heritabilities for estimated GFR, UACR, serum creatinine, serum urea nitrogen, and uric acid and a variety of phenotypes related to obesity, diabetes, and cardiovascular disease. There were significant genetic correlations of some kidney-related phenotypes with these other phenotypes.

LIMITATIONS

Limitations include absence of renal biopsy, possible misclassification bias, lack of direct GFR measurements, and failure to include all possible environmental interactions.

CONCLUSIONS

Many phenotypes related to kidney disease showed significant heritabilities in Zuni Indians, and there were significant genetic correlations with phenotypes related to obesity, diabetes, and cardiovascular disease. The study design serves as a paradigm for the conduct of research in relatively isolated, endogamous, underserved populations.

Marked association between obesity and glomerular hyperfiltration: a cross-sectional study in an African population

BACKGROUND

Obesity and African American ethnicity are established independent risk factors for the development of chronic kidney disease. No data exist about the association between obesity and renal hemodynamics in the African region.

STUDY DESIGN

Cross-sectional study.

SETTING & PARTICIPANTS

301 nondiabetic participants (97 lean, 108 overweight, and 96 obese) of African descent with a positive family history of hypertension from the Seychelles islands.

PREDICTOR

Body mass index (BMI).

OUTCOMES

Glomerular hyperfiltration, glomerular filtration rate (GFR), effective renal plasma flow (ERPF), and filtration fraction.

MEASUREMENTS

GFR and ERPF were measured using inulin and para-aminohippurate clearances, respectively. Participants' baseline demographics, laboratory data, and blood pressure were measured using standard techniques.

RESULTS

The prevalence of glomerular hyperfiltration (defined as GFR >or=140 mL/min) increased across BMI categories (7.2%, 14.8%, and 27.1% for lean, overweight, and obese participants, respectively; P < 0.001). Higher BMI was associated with higher median GFR (99, 110, and 117 mL/min for lean, overweight, and obese participants, respectively; P < 0.001), ERPF (424, 462, and 477 mL/min, respectively; P = 0.01), and filtration fraction (0.23, 0.24, and 0.25; P < 0.001). Multivariate analyses adjusting for age, sex, blood pressure, fasting glucose level, and urinary sodium excretion and accounting for familial correlations confirmed the associations between high BMI (>25 kg/m(2)) and increased GFR, ERPF, and filtration fraction. No association between BMI categories and GFR was found with adjustment for body surface area.

LIMITATIONS

Participants had a positive family history of hypertension.

CONCLUSION

Overweight and obesity are associated with increased GFR, ERPF, and filtration fraction and a high prevalence of glomerular hyperfiltration in nondiabetic individuals of African descent. The absence of associations between BMI categories and GFR indexed for body surface area raises questions regarding the appropriateness of indexing GFR for body surface area in overweight populations.

Glomerular hyperfiltration and increased proximal sodium reabsorption in subjects with type 2 diabetes or impaired fasting glucose in a population of the African region

BACKGROUND. Glomerular hyperfiltration (GHF) is a well-recognized early renal alteration in diabetic patients. As the prevalence of GHF is largely unknown in populations in the African region with respect to normal fasting glucose (NFG), impaired fasting glucose (IFG) and type 2 diabetes [diabetes mellitus (DM)], we conducted a cross-sectional study in the Seychelles islands among families including at least one member with hypertension. METHODS. The glomerular filtration rate (GFR), effective renal plasma flow (ERPF) and proximal tubular sodium reabsorption were measured using inulin, p-aminohippurate (PAH) and endogenous lithium clearance, respectively. Twenty-four-hour urine was collected on the preceding day. RESULTS. Of the 363 participants (mean age 44.7 years), 6.6% had IFG, 9.9% had DM and 63.3% had hypertension. The prevalence of GHF, defined as a GFR >140 ml/min, was 17.2%, 29.2% and 52.8% in NFG, IFG and DM, respectively (P trend <0.001). Compared to NFG, the adjusted odds ratio for GHF was 1.99 [95% confidence interval (CI) 0.73-5.44] for IFG and 5.88 (2.39-14.45) for DM. Lithium clearance and fractional excretion of lithium were lower in DM and IFG than NFG (P < 0.001). CONCLUSION. In this population of African descent, subjects with impaired fasting glucose or type 2 diabetes had a high prevalence of GHF and enhanced proximal sodium reabsorption. These findings provide further insight on the elevated incidence of nephropathy reported among African diabetic individuals.

Association of APOE polymorphism with chronic kidney disease in a nationally representative sample: a Third National Health and Nutrition Examination Survey (NHANES III) Genetic Study

Background

Apolipoprotein E polymorphisms (APOE) have been associated with lowered glomerular filtration rate (GFR) and chronic kidney disease (CKD) with e2 allele conferring risk and e4 providing protection. However, few data are available in non-European ethnic groups or in a population-based cohort.

Methods

The authors analyzed 5,583 individuals from the Third National Health and Nutrition Examination Survey (NHANES III) to determine association with estimated GFR by the Modification of Diet in Renal Disease (MDRD) equation and low-GFR cases. Low-GFR cases were defined as GFR <75 ml/min/1.73 m²; additionally, GFR was analyzed continuously.

Results

In univariate analysis, the e4 allele was negatively associated with low-GFR cases in non-Hispanic whites, odds ratio (OR): 0.76, 95% confidence interval (CI): 0.60, 0.97. In whites, there was a significant association between increasing APOE score (indicating greater number of e2 alleles) and higher prevalence of low-GFR cases (OR: 1.21, 95%CI: 1.01, 1.45). Analysis of continuous GFR in whites found the e4 allele was associated with higher levels of continuous GFR (β-coefficient: 2.57 ml/min/1.73 m², 95%CI: 0.005, 5.14); in non-Hispanic blacks the e2 allele was associated with lower levels of continuous GFR (β-coefficient: -3.73 ml/min/1.73 m², 95%CI: -6.61, -0.84). APOE e2 and e4 alleles were rare and not associated with low-GFR cases or continuous GFR in Mexican Americans.

Conclusion

In conclusion, the authors observed a weak association between the APOE e4 allele and low-GFR cases and continuous GFR in non-Hispanic whites, and the APOE e2 allele and continuous GFR in non-Hispanic blacks, but found no association with either measure of kidney function in Mexican Americans. Larger studies including multiethnic groups are needed to determine the significance of this association.

Association between white-coat effect and blunted dipping of nocturnal blood pressure

BACKGROUND

In this study, we assessed whether the white-coat effect (difference between office and daytime blood pressure (BP)) is associated with nondipping (absence of BP decrease at night).

METHODS

Data were available in 371 individuals of African descent from 74 families selected from a population-based hypertension register in the Seychelles Islands and in 295 Caucasian individuals randomly selected from a population-based study in Switzerland. We used standard multiple linear regression in the Swiss data and generalized estimating equations to account for familial correlations in the Seychelles data.

RESULTS

The prevalence of systolic and diastolic nondipping (<10% nocturnal BP decrease) and white-coat hypertension (WCH) was respectively 51, 46, and 4% in blacks and 33, 37, and 7% in whites. When white coat effect and nocturnal dipping were taken as continuous variables (mm Hg), systolic (SBP) and diastolic BP (DBP) dipping were associated inversely and independently with white-coat effect (P < 0.05) in both populations. Analogously, the difference between office and daytime heart rate was inversely associated with the difference between daytime and night-time heart rate in the two populations. These results did not change after adjustment for potential confounders.

CONCLUSIONS

The white-coat effect is associated with BP nondipping. The similar associations between office-daytime values and daytime-night-time values for both BP and heart rate suggest that the sympathetic nervous system might play a role. Our findings also further stress the interest, for clinicians, of assessing the presence of a white-coat effect as a means to further identify patients at increased cardiovascular risk and guide treatment accordingly.

Multiple loci associated with indices of renal function and chronic kidney disease

Chronic kidney disease (CKD) has a heritable component and is an important global public health problem because of its high prevalence and morbidity. We conducted genome-wide association studies (GWAS) to identify susceptibility loci for glomerular filtration rate, estimated by serum creatinine (eGFRcrea) and cystatin C (eGFRcys), and CKD (eGFRcrea < 60 ml/min/1.73 m(2)) in European-ancestry participants of four population-based cohorts (ARIC, CHS, FHS, RS; n = 19,877; 2,388 CKD cases), and tested for replication in 21,466 participants (1,932 CKD cases). We identified significant SNP associations (P < 5 × 10(-8)) with CKD at the UMOD locus, with eGFRcrea at UMOD, SHROOM3 and GATM-SPATA5L1, and with eGFRcys at CST and STC1. UMOD encodes the most common protein in human urine, Tamm-Horsfall protein, and rare mutations in UMOD cause mendelian forms of kidney disease. Our findings provide new insights into CKD pathogenesis and underscore the importance of common genetic variants influencing renal function and disease.

Determinants of kidney function in Swedish families: role of heritable factors

The aim of this study was to evaluate the determinants of kidney function and the role of heritable factors in a sample of 249 siblings free from known cardiovascular disease and without antihypertensive drugs belonging to 110 families. Four different measures and estimates of kidney function were considered. Blood pressure was recorded during 24 h by ambulatory blood pressure monitoring. Heritability was estimated with and without adjustment for significant covariates.In multivariate analysis, in addition to age, sex, BMI, HDL-cholesterol, 24-h systolic and mean blood pressure, systolic nocturnal blood pressure dipping resulted independently related to serum creatinine, estimated Cockcroft-Gault-creatinine clearance and estimated by the modification of diet in renal disease-glomerular filtration rate. After full adjustment, the heritability values were 51% for the measured creatinine clearance (P < 0.01), 58% for the estimated Cockcroft-Gault-creatinine clearance (P < 0.001), 40% for the estimated by the modification of diet in renal disease-glomerular filtration rate (P < 0.001), but 8% (P = 0.34) for serum creatinine.Our data confirm that kidney function is partially under genetic control and that genetic variants of importance for this trait could be mapped. The association of the circadian rhythm of blood pressure with kidney function in this sample deserves further investigation.

Genome-wide association study for renal traits in the Framingham Heart and Atherosclerosis Risk in Communities Studies

BACKGROUND

The Framingham Heart Study (FHS) recently obtained initial results from the first genome-wide association scan for renal traits. The study of 70,987 single nucleotide polymorphisms (SNPs) in 1,010 FHS participants provides a list of SNPs showing the strongest associations with renal traits which need to be verified in independent study samples.

METHODS

Sixteen SNPs were selected for replication based on the most promising associations with chronic kidney disease (CKD), estimated glomerular filtration rate (eGFR), and serum cystatin C in FHS. These SNPs were genotyped in 15,747 participants of the Atherosclerosis in Communities (ARIC) Study and evaluated for association using multivariable adjusted regression analyses. Primary outcomes in ARIC were CKD and eGFR. Secondary prospective analyses were conducted for association with kidney disease progression using multivariable adjusted Cox proportional hazards regression. The definition of the outcomes, all covariates, and the use of an additive genetic model was consistent with the original analyses in FHS.

RESULTS

The intronic SNP rs6495446 in the gene MTHFS was significantly associated with CKD among white ARIC participants at visit 4: the odds ratio per each C allele was 1.24 (95% CI 1.09-1.41, p = 0.001). Borderline significant associations of rs6495446 were observed with CKD at study visit 1 (p = 0.024), eGFR at study visits 1 (p = 0.073) and 4 (lower mean eGFR per C allele by 0.6 ml/min/1.73 m2, p = 0.043) and kidney disease progression (hazard ratio 1.13 per each C allele, 95% CI 1.00-1.26, p = 0.041). Another SNP, rs3779748 in EYA1, was significantly associated with CKD at ARIC visit 1 (odds ratio per each T allele 1.22, p = 0.01), but only with eGFR and cystatin C in FHS.

CONCLUSION

This genome-wide association study provides unbiased information implicating MTHFS as a candidate gene for kidney disease. Our findings highlight the importance of replication to identify common SNPs associated with renal traits.

Association of ABCB1 genetic variants with renal function in Africans and in Caucasians

BACKGROUND

The P-glycoprotein, encoded by the ABCB1 gene, is expressed in human endothelial and mesangial cells, which contribute to control renal plasma flow and glomerular filtration rate. We investigated the association of ABCB1 variants with renal function in African and Caucasian subjects.

METHODS

In Africans (290 subjects from 62 pedigrees), we genotyped the 2677G>T and 3435 C>T ABCB1 polymorphisms. Glomerular filtration rate (GFR) was measured using inulin clearance and effective renal plasma flow (ERPF) using para-aminohippurate clearance. In Caucasians (5382 unrelated subjects), we analyzed 30 SNPs located within and around ABCB1, using data from the Affymetrix 500 K chip. GFR was estimated using the simplified Modification of the Diet in Renal Disease (MDRD) and Cockcroft-Gault equations.

RESULTS

In Africans, compared to the reference genotype (GG or CC), each copy of the 2677T and 3435T allele was associated, respectively, with: GFR higher by 10.6 +/- 2.9 (P < 0.001) and 4.4 +/- 2.3 (P = 0.06) mL/min; ERPF higher by 47.5 +/- 11.6 (P < 0.001) and 28.1 +/- 10.5 (P = 0.007) mL/min; and renal resistances lower by 0.016 +/- 0.004 (P < 0.001) and 0.011 +/- 0.004 (P = 0.004) mm Hg/mL/min. In Caucasians, we identified 3 polymorphisms in the ABCB1 gene that were strongly associated with all estimates of GFR (smallest P value = 0.0006, overall P = 0.014 after multiple testing correction).

CONCLUSION

Variants of the ABCB1 gene were associated with renal function in both Africans and Caucasians and may therefore confer susceptibility to nephropathy in humans. If confirmed in other studies, these results point toward a new candidate gene for nephropathy in humans.

A genome-wide search for linkage to chronic kidney disease in a community-based sample: the SAFHS

BACKGROUND

Chronic kidney disease (CKD) phenotypes such as albuminuria measured by urinary albumin creatinine ratio (ACR), elevated serum creatinine (SrCr) and/or decreased creatinine clearance (CrCl) and glomerular filtration rate (eGFR) are major risk factors for renal and cardiovascular diseases. Epidemiological studies have reported that CKD phenotypes cluster in families suggesting a genetic predisposition. However, studies reporting chromosomal regions influencing CKD are very limited. Therefore, the purpose of this study is to identify susceptible chromosomal regions for CKD phenotypes in Mexican American families enrolled in the San Antonio Family Heart Study (SAFHS).

METHODS

We used the variance components decomposition approach (implemented in the software package SOLAR) to perform linkage analysis on 848 participants from 26 families. A total of 417 microsatellite markers were genotyped at an average interval of 10 cM spanning 22 autosomal chromosomes.

RESULTS

All phenotypes were measured by standard procedures. Mean +/- SD values of ACR, SrCr, CrCl and eGFR were 0.06 +/- 0.38, 0.85 +/- 0.72 mg/dl, 129.85 +/- 50.37 ml/min and 99.18 +/- 25.69 ml/min/1.73 m(2) body surface area, respectively. All four CKD phenotypes exhibited significant heritabilities (P < 0.0001). A genome-wide scan showed linkage on chromosome 2p25 for SrCr, CrCl and eGFR. Significant linkage was also detected on chromosome 9q21 for eGFR [logarithm of the odds (LOD) score = 3.87, P = 0.00005] and SrCr (LOD score = 2.6, P = 0.00026). ACR revealed suggestive evidence for linkage to a region on chromosome 20q12 (LOD score = 2.93, P = 0.00020).

CONCLUSION

Findings indicate that chromosomal regions 2p25, 9q21 and 20q12 may have functional relevance to CKD phenotypes in Mexican Americans.

Nighttime blood pressure and nocturnal dipping are associated with daytime urinary sodium excretion in African subjects

Blood pressure (BP) follows a circadian rhythm, with 10% to 15% lower values during nighttime than during daytime. The absence of a nocturnal BP decrease (dipping) is associated with target organ damage, but the determinants of dipping are poorly understood. We assessed whether the nighttime BP and the dipping are associated with the circadian pattern of sodium excretion. Ambulatory BP and daytime and nighttime urinary electrolyte excretion were measured simultaneously in 325 individuals of African descent from 73 families. When divided into sex-specific tertiles of day:night ratios of urinary sodium excretion rate, subjects in tertile 1 (with the lowest ratio) were 6.5 years older and had a 9.8-mm Hg higher nighttime systolic BP (SBP) and a 23% lower SBP dipping (expressed in percentage of day value) compared with subjects in tertile 3 (P for trend <0.01). After adjustment for age, the SBP difference across tertiles decreased to 5.4 mm Hg (P=0.002), and the SBP dipping difference decreased to 17% (P=0.05). A similar trend across tertiles was found with diastolic BP. In multivariate analyses, daytime urinary sodium and potassium concentrations were independently associated with nighttime SBP and SBP dipping (P<0.05 for each). These data, based on a large number of subjects, suggest that the capacity to excrete sodium during daytime is a significant determinant of nocturnal BP and dipping. This observation may help us to understand the pathophysiology and clinical consequences of nighttime BP and to develop therapeutic strategies to normalize the dipping profile in hypertensive patients.

Genome-wide scan for estimated glomerular filtration rate in multi-ethnic diabetic populations: the Family Investigation of Nephropathy and Diabetes (FIND)

OBJECTIVE

Diabetic nephropathy, the most common cause of end-stage renal disease, aggregates in families and specific ethnic groups. Deconstructing diabetic nephropathy into intermediate, quantitative phenotypes may increase feasibility of detecting susceptibility loci by genetic screens. Glomerular filtration rate (GFR), which characterizes diabetic nephropathy, was employed as a quantitative trait in a preliminary whole-genome scan.

RESEARCH DESIGN AND METHODS

Estimated GFR (eGFR) was calculated for 882 diabetic sibpairs (mean age 57 years) of African-American (25.6% of total), American Indian (8.6%), European-American (14.2%), and Mexican-American (51.6%) descent enrolled in the initial phase of the Family Investigation of Nephropathy and Diabetes (FIND). A whole-genome scan was performed using 404 microsatellite markers (average spacing 9 cM) and model-free linkage analysis.

RESULTS

For all ethnicities combined, strong evidence for linkage was observed on chromosomes 1q43 (P = 3.6 x 10(-3)), 7q36.1 (P = 2.1 x 10(-4)), 8q13.3 (P = 4.6 x 10(-4)), and 18q23.3 (P = 2.7 x 10(-3)). Mexican-American families, who comprised the major ethnic subpopulation in FIND, contributed to linkage on chromosomes 1q43, 2p13.3, 7q36.1, 8q13.3, and 18q23.3, whereas African-American and American-Indian families displayed linkage peaks on chromosomes 11p15.1 and 15q22.3, respectively.

CONCLUSIONS

We have demonstrated multiple chromosomal regions linked to eGFR in a multi-ethnic collection of families ascertained by a proband with diabetic nephropathy. Identification of genetic variants within these loci that are responsible for the linkage signals could lead to predictive tests or novel therapies for subsets of patients at risk for diabetic nephropathy.

Estimates of genetic and environmental contribution to 43 quantitative traits support sharing of a homogeneous environment in an isolated population from South Tyrol, Italy

As part of the genomic health care program 'GenNova', we measured 43 quantitative traits in 1,136 subjects living in three isolated villages in South Tyrol (Italy), for which extended genealogical information was available. Thirty-seven of the studied traits had been previously investigated in other populations, while six of them are, to the best of our knowledge, studied here for the first time. For all 43 traits we estimated narrow-sense heritability, individual-specific environmental effects, and shared environmental effects. Estimates of narrow-sense heritability were in good agreement with previous findings. We found significant heritability for all traits; after correcting for multiple testing, all traits except serum concentration of glutamic oxaloacetic transaminase (GOT) and potassium still showed significant heritability. In contrast, the effect of living in the same sibship or village (the so-called sibship and household effects, respectively) was significant for a few traits only, and after correcting for multiple testing no trait showed significant shared environment effect. We suggest that the sharing of a highly similar environment by the subjects included in this study explains the low contribution of the household effects to the overall trait variation. This peculiarity should provide an advantage in gene-mapping projects by reducing environmental bias.

CYP3A5 and ABCB1 genes influence blood pressure and response to treatment, and their effect is modified by salt

The permeability-glycoprotein efflux-transporter encoded by the multidrug resistance 1 (ABCB1) gene and the cytochromes P450 3A4/5 encoded by the CYP3A4/5 genes are known to interact in the transport and metabolism of many drugs. Recent data have shown that the CYP3A5 genotypes influence blood pressure and that permeability-glycoprotein activity might influence the activity of the renin-angiotensin system. Hence, these 2 genes may contribute to blood pressure regulation in humans. We analyzed the association of variants of the ABCB1 and CYP3A5 genes with ambulatory blood pressure, plasma renin activity, plasma aldosterone, endogenous lithium clearance, and blood pressure response to treatment in 72 families (373 individuals; 55% women; mean age: 46 years) of East African descent. The ABCB1 and CYP3A5 genes interact with urinary sodium excretion in their effect on ambulatory blood pressure (daytime systolic: P=0.05; nighttime systolic and diastolic: P<0.01), suggesting a gene-gene-environment interaction. The combined action of these genes is also associated with postproximal tubular sodium reabsorption, plasma renin activity, plasma aldosterone, and with an altered blood pressure response to the angiotensin-converting enzyme inhibitor lisinopril (P<0.05). This is the first reported association of the ABCB1 gene with blood pressure in humans and demonstration that genes encoding for proteins metabolizing and transporting drugs and endogenous substrates contribute to blood pressure regulation.

Plasma aldosterone is independently associated with the metabolic syndrome

The aim of this study was to analyze the associations of plasma aldosterone and plasma renin activity with the metabolic syndrome and each of its components. We analyzed data from a family based study in the Seychelles made up of 356 participants (160 men and 196 women) from 69 families of African descent. In multivariable models, plasma aldosterone was associated positively (P < 0.05) with blood pressure in older individuals (interaction with age, P < 0.05) and with waist circumference in men (interaction with sex, P < 0.05) and negatively with high-density lipoprotein cholesterol, in particular in individuals with elevated urinary potassium excretion (interaction with urinary potassium, P < 0.05); plasma renin activity was significantly associated with triglycerides and fasting blood glucose. Plasma aldosterone, but not plasma renin activity, was associated with the metabolic syndrome per se, independently of the association with its separate components. The observation that plasma renin activity was associated with some components of the metabolic syndrome, whereas plasma aldosterone was associated with other components of the metabolic syndrome, suggests different underlying mechanisms. These findings reinforce previous observations suggesting that aldosterone is associated with several cardiovascular risk factors and also suggest that aldosterone might contribute to the increased cardiovascular disease risk in individuals of African descent with the metabolic syndrome.