The Gly(972)Arg Variant of Human Insulin Receptor Substrate 1 Gene Is Associated With Variation in Glomerular Filtration Rate Likely Through Impaired Insulin Receptor Signaling Farook Thameem, 1 Sobha Puppala, 2 Jennifer Schneider, 2 Basant Bhandari, 1 Rector Arya, 3 Nedal H. Arar, 1,4 Tetyana L. Vasylyeva, 1 Vidya S. Farook, 2 Sharon Fowler, 5 Laura Almasy, 2 John Blangero, 2 Ravindranath Duggirala, 2 and Hanna E. Abboud 1,4 The objective of this study is to identify and characterize the genetic variants related to the glomerular filtration rate (GFR) linkage on 2q37. Of the positional candidate genes, we selected insulin receptor substrate 1 (IRS1) and resequenced its 2-kb promoter region and exons for sequence variants in 32 subjects. A total of 11 single nucleotide polymorphisms (SNPs) were iden- tified. To comprehensively cover the 59-kb-long intron-1, eight additional tagging SNPs were selected from the HapMap. All the 19 SNPs were genotyped by TaqMan Assay in the entire data set (N = 670; 39 families). Association analyses between the SNPs and GFR and type 2 diabetes–related traits were performed using the measured genotype approach. Of the SNPs examined for association, only the Gly(972)Arg variant of IRS1 exhibited a sig- nificant association with GFR (P = 0.0006) and serum triglycer- ides levels (P = 0.003), after accounting for trait-specific covariate effects. Carriers of Arg972 had significantly decreased GFR values. Gly(972)Arg contributed to 26% of the linkage signal on 2q. Expression of IRS1 mutant Arg972 in human mesangial cells significantly reduced the insulin-stimulated phosphoryla- tion of IRS1 and Akt kinase. Taken together, the data provide the first evidence that genetic variation in IRS1 may influence variation in GFR probably through impaired insulin receptor signaling. D iabetic nephropathy (DN) is a serious micro- vascular complication of both type 1 and type 2 diabetes (T2DM). Elevated urinary albumin ex- cretion and decreased GFR are risk factors for cardiovascular morbidity and mortality and progression to end-stage renal failure in individuals with diabetes. End- stage renal disease secondary to DN accounts for ;50% of cases in most developed countries. The pathogenesis of DN is multifactorial. Familial clustering of DN and related traits such as albuminuria and glomerular filtration rate (GFR) indicate that genes play a major role in its suscep- tibility (1). Despite the recent efforts to identify and characterize DN susceptibility genes, the genes involved in susceptibility to the development and or progression of DN and in particular GFR have yet to be identified. Equations that estimate GFR facilitate the diagnosis, evaluation, and management of chronic kidney disease. GFR varies between individuals and is influenced by ge- netic and environmental factors and their interactions. Both genome-wide linkage and association analytical appro- aches have been used to localize the susceptibility genes/ variants that influence variation in GFR (2–11). However, the functional significance of the finding from such studies is yet to be explored. To localize genes contributing to variation in GFR, we recently performed a genome-wide linkage screen using GFR data (N = 453) from the Mexican American participants of the San Antonio Family Diabetes/ Gallbladder Study (SAFDGS) and identified the strongest linkage of GFR to occur on chromosome 2q35–37 near the markers D2S1363-D2S427 (LOD = 3.8; LOD c = 3.3) after incorporating genotype by environment (T2DM) interac- tion effects (5). Identification of linkage of GFR on 2q is detailed in the Supplementary Data. We hypothesize that there is a gene(s) located on chromosome 2q35–37 that may be related to the linkage signal identified. Of the positional candidate genes in the critical interval, IRS1 gene is located 200 kb telomeric to the marker D2S1363. When phosphorylated, IRS1 acts as a docking protein for multiple Src homology-2 (SH2)–containing pro- teins such as phosphatidylinositol-3-kinase (PI3K), growth factor receptor–binding protein-2, and tyrosine phospha- tase SHP2. These proteins in turn further activate down- stream effectors that mediate the biological effects of insulin in several tissues including kidney (12,13). Mice deficient in IRS1 by targeted disruption of its gene display insulin resistance and impaired glucose tolerance (14), and mice heterozygous for defects in genes for both IRS1 and the insulin receptor (15) or IRS1 and glucokinase (16) develop overt diabetes. Genetic variation in IRS1, partic- ularly Gly(972)Arg (rs1801278) variant, plays a contribu- tory role in insulin resistance and diabetes susceptibility (17) probably through impaired insulin receptor signaling (18–20). Genetic variants (rs2943641; rs2943650) located 500 kb upstream of IRS1 were recently reported to be associated with insulin resistance, hyperinsulinemia, dys- lipidemia, and T2DM (21,22). Given the functional significance of IRS1 together with our finding that it is a positional candidate gene in the From the 1 Division of Nephrology, The University of Texas Health Science Center, San Antonio, Texas; the 2 Department of Genetics, Texas Biomedical Research Institute, San Antonio, Texas; the 3 Department of Pediatrics, The University of Texas Health Science Center, San Antonio, Texas; the 4 South Texas Veterans Healthcare System, San Antonio, Texas; and the 5 Division of Clinical Epidemiology, Department of Medicine, The University of Texas Health Science Center, San Antonio, Texas. Corresponding author: Farook Thameem, thameem@uthscsa.edu. Received 9 August 2011 and accepted 20 March 2012. DOI: 10.2337/db11-1078 This article contains Supplementary Data online at http://diabetes .diabetesjournals.org/lookup/suppl/doi:10.2337/db11-1078/-/DC1. Ó 2012 by the American Diabetes Association. Readers may use this article as long as the work is properly cited, the use is educational and not for profit, and the work is not altered. See http://creativecommons.org/licenses/by -nc-nd/3.0/ for details. diabetes.diabetesjournals.org DIABETES 1 ORIGINAL ARTICLE Diabetes Publish Ahead of Print, published online May 22, 2012