Subrata D. Nath

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.

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.

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.

Genetics of variation in HOMA-IR and cardiovascular risk factors in Mexican-Americans

Insulin resistance is a major biochemical defect underlying the pathogenesis of cardiovascular disease (CVD). Mexican-Americans are known to have an unfavorable cardiovascular profile. Thus, the aim of this study was to investigate the genetic effect on variation in HOMA-IR and to evaluate its genetic correlations with other phenotypes related to risk of CVD in Mexican-Americans. The homeostatic model assessment method (HOMA-IR) is one of several approaches that are used to measure insulin resistance and was used here to generate a quantitative phenotype for genetic analysis. For 644 adults who had participated in the San Antonio Family Heart Study (SAFHS), estimates of genetic contribution were computed using a variance components method implemented in SOLAR. Traits that exhibited significant heritabilities were body mass index (BMI) (h2 = 0.43), waist circumference (h2 = 0.48), systolic blood pressure (h2 = 0.30), diastolic blood pressure (h2 = 0.21), pulse pressure (h2 = 0.32), triglycerides (h2 = 0.51), LDL cholesterol (h2 = 0.31), HDL cholesterol (h2 = 0.24), C-reactive protein (h2 = 0.17), and HOMA-IR (h2 = 0.33). A genome-wide scan for HOMA-IR revealed significant evidence of linkage on chromosome 12q24 (close to PAH (phenylalanine hydroxylase), LOD = 3.01, p < 0.001). Bivariate analyses demonstrated significant genetic correlations (p < 0.05) of HOMA-IR with BMI (ρG = 0.36), waist circumference (ρG = 0.47), pulse pressure (ρG = 0.39), and HDL cholesterol (ρG = -0.18). Identification of significant linkage for HOMA-IR on chromosome 12q replicates previous family-based studies reporting linkage of phenotypes associated with type 2 diabetes in the same chromosomal region. Significant genetic correlations between HOMA-IR and phenotypes related to CVD risk factors suggest that a common set of gene(s) influence the regulation of these phenotypes.

Assessing Overweight and Cardiovascular Risks among College Students.

Abstract Although studies regarding health issues and the obesity epidemic have increased in recent years, few of these studies target college-aged students. The primary purpose of this study was to evaluate the differences in race/ethnicity with respect to prevalence of overweight/obesity (defined by body mass index or BMI) among college students attending an urban university. In addition, the demographic characteristics and cardiovascular risks between the overweight and obese group (n=138) were compared to the underweight and normal weight group (n=349). The study included 487 college students under 40 years of age who identified their origin as white (non-Hispanic), black (non-Hispanic), or Hispanic. There were 32.65% white, 33.26% Hispanic, and 34.09% black. The mean and median ages were 21 and 19 years, respectively. The overall prevalence of overweight/obesity was 28.11%, with 23.91% (white), 34.06% (Hispanic), and 42.03% (black). Age-, gender-, and race-/ethnicity-adjusted cardiovascular risk levels (blood pressure, pulse pressure, blood glucose and lipid profiles) significantly varied between two groups. The findings suggest that screening cardiovascular risks among a college-aged population is warranted. Our study further indicates the need for weight management and risk reduction of overweight-related chronic diseases on campus.

Relation of Low Glomerular Filtration Rate to Metabolic Disorders in Individuals without Diabetes and with Normoalbuminuria

BACKGROUND AND OBJECTIVES Microalbuminuria increases cardiovascular risk and is considered a metabolic disorder. Low glomerular filtration rate is also associated with increased cardiovascular risk, but the relation of low glomerular filtration rate to metabolic disorders is not well understood. DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS Designed as a cross-sectional, epidemiologic study, the Insulin Resistance Atherosclerosis Study was conducted in four centers: San Antonio (Texas), San Luis Valley (Colorado), and Oakland and Los Angeles (California). The Modification of Diet in Renal Disease equation was used to classify individuals without diabetes and with normoalbuminuria (n = 856; age 40 to 69 yr) by the presence or absence of low glomerular filtration rate (<60 ml/min per 1.73 m(2)). A direct marker of insulin resistance, the insulin sensitivity index, was measured by the frequently sampled intravenous glucose tolerance test. RESULTS Low glomerular filtration rate was related to hypertension and the metabolic syndrome. Low glomerular filtration rate was associated with fasting insulin concentration and insulin sensitivity index. Low glomerular filtration rate was also associated with insulin concentration after adjustment for potential determinants of glomerular filtration rate but was not associated with insulin sensitivity index. CONCLUSIONS Low glomerular filtration rate is associated with increased insulin concentration in individuals without diabetes and with normoalbuminuria. Longitudinal analyses are needed to determine whether insulin concentration (insulin resistance) precedes the deterioration of renal function.

Genetic polymorphisms in OGG1 and their association with angiomyolipoma, a benign kidney tumor in patients with tuberous sclerosis.

The enzyme 8-oxoguanine glycosylase 1 (OGG1) repairs 8-oxo-2- deoxyguanosine residue (8-oxodG) an oxidatively damaged promutagenic base. Genetic variations in OGG1 gene have been shown to modulate DNA repair capacity and are related risk of tumor development. However, epidemiologic findings have been inconsistent. The purpose of this study is to determine whether genetic variants in OGG1 play a role in susceptibility to angiomyolipoma, a benign kidney tumor associated with tuberous sclerosis complex (TSC) patients. To identify genetic variations, all 7 exons of the OGG1 gene were amplified by PCR and sequenced in 22 TSC patients with angiomyolipoma (cases) and 18 controls. By direct sequencing, we identified four missense mutations in OGG1: Arg(45)Gln, Ala(85)Ser, Arg(229)Gln, and Ser(326)Cys. Genotypic association with angiomyolipoma was performed using the measured genotype approach as implemented in the variance component analytical tools. Association analysis showed the presence of significant association (p=0.01) only between the Ser(326)Cys polymorphism of OGG1 and angiomyolipoma. We also assessed the presence of oxidative DNA damage in kidney section from normal healthy subjects, normal kidney tissue from TSC patients and kidney angiomyolipoma tissue from TSC patients by immunostaining for 8-oxodG. 8-OxodG staining was highly abundant in kidney angiomyolipoma tissue from TSC patients compared to weak staining in uninvolved tissue from the same TSC patients or normal kidney from healthy subjects. Taken together, our findings suggest that Ser(326)Cys variant of OGG1 may confer risk for development of angiomyolipomas by increasing oxidative DNA damage.

Genome-wide scans for microalbuminuria in Mexican Americans: The San Antonio Family Heart Study

Purpose: Microalbuminuria, defined as urine albumin-to-creatinine ratio of 0.03 to 0.299 mg/mg, is a major risk factor for cardiovascular disease. Several genetic epidemiological studies have established that microalbuminuria clusters in families, suggesting a genetic predisposition.Method: We estimated heritability of microalbuminuria and performed a genome-wide linkage analysis to identify chromosomal regions influencing urine albumin-to-creatinine ratio in 486 Mexican Americans from 26 multiplex families.Results: Significant heritability was demonstrated for urine albumin-to-creatinine ratio (h2 = 24%, P < 0.003) after accounting for age, sex, body mass index, triglycerides, and hypertension. Genome scan revealed significant evidence of linkage of urine albumin-to-creatinine ratio to a region on chromosome 20q12 (LOD score of 3.5, P < 0.001) near marker D20S481. This region also exhibited a LOD score of 2.8 with diabetes status as a covariate and 3.0 with hypertension status as a covariate suggesting that the effect of this locus on urine albumin-to-creatinine ratio is largely independent of diabetes and hypertension.Conclusion: Findings indicate that there is a gene or genes located on human chromosome 20q12 that may have functional relevance to albumin excretion in Mexican Americans. Identifying and understanding the role of the genes that determine albumin excretion would lead to the development of novel therapeutic strategies targeted at high-risk individuals in whom intensive preventive measures may be most beneficial.

Evaluation of polymorphisms in paraoxonase 2 (PON2) gene and their association with cardiovascular-renal disease risk in Mexican Americans

Background/Aims: Genetic polymorphisms in the paraoxonase 2 (PON2) gene are thought to alter its activity and contribute to the development of cardiovascular and renal disease risk. The purpose of this study is to determine whether the Arg148Gly, Cys311Ser and rs12794795 polymorphisms of PON2 examined previously by others, are associated with type 2 diabetes (T2DM), and subclinical measures of cardiovascular and renal disease risk in Mexican Americans. Methods: Study participants (n = 848; 21 families) were genotyped for the three polymorphisms by TaqMan assay. Association between the genotypic and phenotypic data was performed by measured genotype approach as implemented in the variance component analytical tools. Results: The Arg148Gly variant was found to be monomorphic in our dataset. Of the phenotypes examined for association, the A/C variant located in intron-1 (rs12794795) exhibited statistically significant association only with diastolic blood pressure (p = 0.018) after accounting for the trait-specific covariate effects. The Cys311Ser variant failed to show statistically significant association with any of the phenotypes examined. Conclusion: In conclusion, the variants examined at the PON2 locus in Mexican Americans do not appear to be a major contributor to T2DM, cardiovascular or renal disease risk, although they exhibited a small effect on the blood pressure values.