Nedal H. Arar

Genome-wide scans for diabetic nephropathy and albuminuria in multiethnic populations: The Family Investigation of Nephropathy and Diabetes (FIND)

The Family Investigation of Nephropathy and Diabetes (FIND) was initiated to map genes underlying susceptibility to diabetic nephropathy. A total of 11 centers participated under a single collection protocol to recruit large numbers of diabetic sibling pairs concordant and discordant for diabetic nephropathy. We report the findings from the first-phase genetic analyses in 1,227 participants from 378 pedigrees of European-American, African-American, Mexican-American, and American Indian descent recruited from eight centers. Model-free linkage analyses, using a dichotomous definition for diabetic nephropathy in 397 sibling pairs, as well as the quantitative trait urinary albumin-to-creatinine ratio (ACR), were performed using the Haseman-Elston linkage test on 404 microsatellite markers. The strongest evidence of linkage to the diabetic nephropathy trait was on chromosomes 7q21.3, 10p15.3, 14q23.1, and 18q22.3. In ACR (883 diabetic sibling pairs), the strongest linkage signals were on chromosomes 2q14.1, 7q21.1, and 15q26.3. These results confirm regions of linkage to diabetic nephropathy on chromosomes 7q, 10p, and 18q from prior reports, making it important that genes underlying these peaks be evaluated for their contribution to nephropathy susceptibility. Large family collections consisting of multiple members with diabetes and advanced nephropathy are likely to accelerate the identification of genes causing diabetic nephropathy, a life-threatening complication of diabetes.

Heritability of the Severity of Diabetic Retinopathy: The FIND-Eye Study

PURPOSE Diabetic retinopathy (DR) and diabetic nephropathy (DN) are serious microvascular complications of diabetes mellitus. Correlations between severity of DR and DN and computed heritability estimates for DR were determined in a large, multiethnic sample of diabetic families. The hypothesis was that (1) the severity of DR correlates with the presence and severity of nephropathy in individuals with diabetes mellitus, and (2) the severity of DR is under significant familial influence in members of multiplex diabetic families. METHODS The Family Investigation of Nephropathy and Diabetes (FIND) was designed to evaluate the genetic basis of DN in American Indians, European Americans, African Americans, and Mexican Americans. FIND enrolled probands with advanced DN, along with their diabetic siblings who were concordant and discordant for nephropathy. These diabetic family members were invited to participate in the FIND-Eye study to determine whether inherited factors underlie susceptibility to DR and its severity. FIND-Eye participants underwent eye examinations and had fundus photographs taken. The severity of DR was graded by using the Early Treatment Diabetic Retinopathy Study Classification (ETDRS). Sib-sib correlations were calculated with the SAGE 5.0 program FCOR, to estimate heritability of retinopathy severity. RESULTS This report summarizes the results for the first 2368 diabetic subjects from 767 families enrolled in FIND-Eye; nearly 50% were Mexican American, the largest single ethnicity within FIND. The overall prevalence of DR was high; 33.4% had proliferative DR; 7.5%, 22.8%, and 9.5% had severe, moderate, and mild nonproliferative DR, respectively; 26.6% had no DR. The severity of DR was significantly associated with severity of DN, both by phenotypic category and by increasing serum creatinine concentration (chi(2) = 658.14, df = 20; P < 0.0001). The sib-sib correlation for DR severity was 0.1358 in the total sample and 0.1224 when limited to the Mexican-American sample. Broad sense heritabilities for DR were 27% overall and 24% in Mexican-American families. The polygenic heritability of liability for proliferative DR approximated 25% in this FIND-Eye sample. CONCLUSIONS These data confirm that the severity of DR parallels the presence and severity of nephropathy in individuals with diabetes mellitus. The severity of DR in members of multiplex diabetic families appears to have a significant familial connection.

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 × 10−3), 7q36.1 (P = 2.1 × 10−4), 8q13.3 (P = 4.6 × 10−4), and 18q23.3 (P = 2.7 × 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.

Genome-Wide Scan for Estimated GFR in Multi-Ethnic Diabetic Populations: The Family Investigation of Nephropathy and Diabetes

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 × 10−3), 7q36.1 (P = 2.1 × 10−4), 8q13.3 (P = 4.6 × 10−4), and 18q23.3 (P = 2.7 × 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.

Genome Scan for Determinants of Serum Uric Acid Variability

Elevated serum uric acid level is associated with obesity, insulin resistance, diabetes, nephropathy, and hypertension. Epidemiologic studies suggest that serum uric acid levels are heritable. We sought to identify chromosomal regions harboring quantitative trait loci that influence serum uric acid in Mexican Americans using data from 644 participants in the San Antonio Family Heart Study. Serum uric acid was found to exhibit significant heritability (0.42) in this population (P = 2 x 10(-7)) after accounting for covariate effects. In addition, genetic correlations between serum uric acid and other cardiovascular risk factors, such as body mass index, waist circumference, systolic BP, and pulse pressure, were identified, suggesting that the genes associated with uric acid level are also associated with these phenotypes. Multipoint linkage analysis identified quantitative trait loci with measurable effects on serum uric acid variability. The highest multipoint logarithm of odds score of 3.3 was found at 133 cM on chromosome 6q22-23, a region that also contains genes that seem to influence familial IgA nephropathy, obesity, BP, insulin resistance, and type 2 diabetes. Given the relationship between uric acid level and these conditions, future studies should investigate potential candidate susceptibility genes found in this region.

Genomewide Linkage Scan for Diabetic Renal Failure and Albuminuria: The FIND Study

Background: Diabetic nephropathy (DN) is a leading cause of mortality and morbidity in patients with type 1 and type 2 diabetes. The multicenter FIND consortium aims to identify genes for DN and its associated quantitative traits, e.g. the urine albumin:creatinine ratio (ACR). Herein, the results of whole-genome linkage analysis and a sparse association scan for ACR and a dichotomous DN phenotype are reported in diabetic individuals. Methods: A genomewide scan comprising more than 5,500 autosomal single nucleotide polymorphism markers (average spacing of 0.6 cM) was performed on 1,235 nuclear and extended pedigrees (3,972 diabetic participants) ascertained for DN from African-American (AA), American-Indian (AI), European-American (EA) and Mexican-American (MA) populations. Results: Strong evidence for linkage to DN was detected on chromosome 6p (p = 8.0 × 10–5, LOD = 3.09) in EA families as well as suggestive evidence for linkage to chromosome 7p in AI families. Regions on chromosomes 3p in AA, 7q in EA, 16q in AA and 22q in MA displayed suggestive evidence of linkage for urine ACR. The linkage peak on chromosome 22q overlaps the MYH9/APOL1 gene region, previously implicated in AA diabetic and nondiabetic nephropathies. Conclusion: These results strengthen the evidence for previously identified genomic regions and implicate several novel loci potentially involved in the pathogenesis of DN.

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.

Endothelial Nitric Oxide Synthase (eNOS) gene polymorphisms and their association with Type 2 diabetes related traits in Mexican Americans

Genetic variants of the endothelial nitric oxide synthase (eNOS) gene such as T-786C, Glu298Asp and 27bp-VNTR have been examined for their association with type 2 diabetes (T2DM)-related traits in various populations but not in Mexican Americans. However, the results from such studies have been controversial. This study investigated whether these three polymorphisms are associated with T2DM and its related traits in Mexican Americans, a population at high risk for T2DM and its complications. The study participants (n=670; 39 families) were genotyped for the three polymorphisms using polymerase chain reaction followed by restriction fragment length polymorphism assay. Association analyses between these polymorphisms and T2DM and its related phenotypes were carried out using a measured genotype approach as implemented in the computer program SOLAR. Of the variants examined, only the 27bp-VNTR variant exhibited significant association with high-density lipoprotein cholesterol (HDL-C) (p=0.04) and diastolic blood pressure (DBP) levels (p=0.02) after accounting for trait-specific covariates. The carriers of the rare allele (27bp-VNTR-4a) were found to have decreased HDL-C and increased DBP levels. In conclusion, of the genetic polymorphisms examined at the eNOS locus, only 27bp-VNTR appears to be a minor contributor to the variation in T2DM-related traits in Mexican Americans.

Genotype by Diabetes Interaction Effects on the Detection of Linkage of Glomerular Filtration Rate to a Region on Chromosome 2q in Mexican Americans

OBJECTIVE—Glomerular filtration rate (GFR) is used to assess the progression of renal disease. We performed linkage analysis to localize genes that influence GFR using estimated GFR data from the San Antonio Family Diabetes/Gallbladder Study. We also examined the effect of genotype by diabetes interaction (G × DM) on the detection of linkage to address whether genetic effects on GFR differ in diabetic and nondiabetic subjects. RESEARCH DESIGN AND METHODS—GFR (N = 453) was estimated using the recently recalculated Cockcroft-Gault (GFR-CGc) and the simplified Modification of Diet in Renal Disease (GFR-4VMDRD) formulae. Both estimates of GFR exhibited significant heritabilities, but only GFR-CGc showed significant G × DM interaction. We therefore performed multipoint linkage analyses on both GFR measures using models that did not include G × DM interaction effects (Model 1) and that included G × DM interaction effects (Model 2, in the case of GFR-CGc). RESULTS—The strongest evidence for linkage (Model 1) of both GFR-CGc (logarithm of odds [LOD] 2.9) and GFR-4VMDRD (LOD 2.6) occurred between markers D9S922 and D9S1120 on chromosome 9q. However, using Model 2, the strongest evidence for linkage of GFR-CGc on chromosome 2q was found near marker D2S427 (corrected LOD score [LODC] 3.3) compared with the LOD score of 2.7 based on Model 1. Potential linkages (LOD or LODC ≥1.2) were found only for GFR-CGc on chromosomes 3p, 3q, 4p, 8q, 11q, and 14q. CONCLUSIONS—We found a major locus on chromosome 2q that differentially influences GFR in diabetic and nondiabetic environments in the Mexican-American population.

Genetics of variation in serum uric acid and cardiovascular risk factors in Mexican Americans

BACKGROUND Elevated serum uric acid is associated with several cardiovascular disease (CVD) risk factors such as hypertension, inflammation, endothelial dysfunction, insulin resistance, dyslipidemia, and obesity. However, the role of uric acid as an independent risk factor for CVD is not yet clear. OBJECTIVE The aim of the study was to localize quantitative trait loci regulating variation in serum uric acid and also establish the relationship between serum uric acid and other CVD risk factors in Mexican Americans (n = 848; men = 310, women = 538) participating in the San Antonio Family Heart Study. METHODS Quantitative genetic analysis was conducted using variance components decomposition method, implemented in the software program SOLAR. RESULTS Mean +/- SD of serum uric acid was 5.35 +/- 1.38 mg/dl. Univariate genetic analysis showed serum uric acid and other CVD risk markers to be significantly heritable (P < 0.005). Bivariate analysis showed significant correlation of serum uric acid with body mass index, waist circumference, waist/hip ratio, total body fat, plasma insulin, serum triglycerides, high-density lipoprotein cholesterol, C-reactive protein, and granulocyte macrophage-colony stimulating factor (P < 0.05). A genome-wide scan for detecting quantitative trait loci regulating serum uric acid variation showed a significant logarithm of odds (LOD) score of 4.72 (empirical LOD score = 4.62; P < 0.00001) on chromosome 3p26. One LOD support interval contains 25 genes, of which an interesting candidate gene is chemokine receptor 2. SUMMARY There is a significant genetic component in the variation in serum uric acid and evidence of pleiotropy between serum uric acid and other cardiovascular risk factors.