The Gly(972)Arg variant of human IRS1 gene is associated with variation in glomerular filtration rate likely through impaired insulin receptor signaling

Whole Exome Sequencing Reveals Rare Variants in Genes Associated with Metabolic Disorders in Women with PCOS

Background

Polycystic ovary syndrome (PCOS) is a complex genetic trait, the pathogenesis of which is governed by an interplay of genetic and epigenetic factors. However, the aetiology of PCOS is not fully understood.

Aims

The objective of this study was to investigate the genetic causes of PCOS by identifying rare variants in genes implicated in its pathophysiology.

Settings and Design

This was a hospital-based observational study.

Materials and Methods

We used whole-exome sequencing for 52 PCOS women to identify the rare variants in genes related to PCOS pathogenesis. Subsequently, we analysed these variants using in silico prediction software to determine their functional effects. We then assessed the relationship between these variants and the clinical outcomes of the patients.

Statistical Analysis Used

Student's t-test and Fisher's exact test were used to compare clinical parameters and frequency differences amongst PCOS patients with and without variants.

Results

A total of four rare exonic variants in obesity- and hyperinsulinaemia-related genes including UCP1 (p.Thr227Ile), UCP2 (p.Arg88Cys), IRS1 (p.Ser892Gly) and GHRL (p.Leu72Met) were identified in eight patients. Significant differences were observed between the patients carrying variants and those without variants. PCOS patients with identified variants exhibited significantly higher average body mass index and fasting insulin levels of PCOS subjects with identified variants compared to those without variants (P < 0.05). Additionally, there were significant differences in the variant frequencies of four variants when compared to the population database (P < 0.05).

Conclusion

This study shows a prevalence of rare variants in obesity and hyperinsulinaemia-related genes in a cohort of PCOS women, thereby underscoring the impact of the identified rare variants on the development of obesity and associated metabolic derangements in PCOS women.

Insulin Receptor Substrate 1 Gly972Arg (rs1801278) Polymorphism Is Associated with Obesity and Insulin Resistance in Kashmiri Women with Polycystic Ovary Syndrome

Background: Polycystic ovary syndrome (PCOS) is commonly associated with metabolic abnormalities such as hyperinsulinemia, insulin resistance and obesity. The genetic variants of genes regulating insulin action, expression and regulation are suggested as possible factors involved in development and severity of clinical manifestations in PCOS. Aim: We investigated whether IRS-1Gly972Arg (rs1801278) polymorphism is associated with increased risk of PCOS in Kashmiri women. The correlation of various clinical, metabolic and hormonal markers with rs1801278 single nucleotide polymorphism was analyzed. The genotypic−phenotypic association of clinical manifestations of PCOS with the tested genetic variant was also assessed. Results: There were no significant differences in allele frequency (OR = 0.87, CI = 0.59−1.29, χ2 = 0.456, p = 0.499) or genotypic distribution (χ2 = 3.73, p = 0.15) between PCOS women and controls. No significant association was also found in the dominant (OR = 1.63, χ2 = 0.377, p = 0.53), recessive (OR = 0.79, χ2 = 1.01, p = 0.31) or heterozygote vs. homozygote (OR = 1.34, χ2 = 1.53, p = 0.22) genotype model analysis. The genotype−phenotype correlation analysis showed that the Arg allele was significantly associated with increased central adiposity markers hip circumference (p = 0.012), and body adiposity index BAI (p = 0.002) in the recessive model in PCOS women. The two-hour glucose (p = 0.04) and insulin resistance marker HOMA (p = 0.44) were significantly higher in Arg allele carriers. The androgen excess markers dehydroepiandrosterone sulfate DHEAS (p = 0.02), Ferriman−Gallwey score (p = 0.012), prevalence of acne, alopecia and hirsutism (all p < 0.01) were significantly elevated in the wild-type GG genotype. Conclusions:IRS-1Gly972Arg genetic variant does not increase the risk of PCOS in Kashmiri women. However, this polymorphism is associated with clinical manifestations of insulin resistance, obesity and hyperandrogenism, suggesting its possible role in variable phenotypic manifestations of PCOS.

Association of IRS1 Gly972Arg and IRS2 Gly1057Asp polymorphisms with gastric cancer in Turkish subjects

Insulin receptor substrate (IRS) proteins are cytoplasmic adaptors that transmit the signal from the IR and insulin-like growth factor-1 receptor to effector proteins. Overexpression of IRS proteins has been indicated to be linked to cancer development. In addition to their expression profiles, studies have indicated that polymorphisms of IRS1 and IRS2 are also associated with the susceptibility to numerous cancer types. IRS1 Gly972Arg and IRS2 Gly1057Asp are the common variants of these genes. The present study aimed to determine the association of IRS1 Gly972Arg and IRS2 Gly1057Asp polymorphisms with gastric cancer development. The study included 100 patients with gastric cancer and 100 controls. Single-nucleotide polymorphisms were detected by real-time PCR using Taqman assays. The results suggested that in individuals with the IRS1 Gly/Arg genotype, the odds of having gastric cancer was increased by 7.891-fold (95% CI: 3.251-19.154, P<0.001) and in individuals with the IRS1 Arg/Arg genotype, it was increased by 22.716-fold (95% CI: 6.311-81.761, P<0.001) compared with those with the IRS1 Gly/Gly genotype. Although the IRS2 Gly1057Asp genotype analysis suggested that subjects with the Asp/Asp genotype had a 2,311-fold increased odds of having gastric cancer compared to those with the Gly/Gly genotype, the result was not statistically significant (95% CI: 0.800-6.678, P=0.122). The combined effects of the IRS1 and IRS2 variants on gastric cancer were also determined. The results suggested that individuals with Gly/Arg+Gly/Asp and Gly/Arg+Asp/Asp genotypes had a higher odds of having gastric cancer compared to individuals of the Gly/Gly+Gly/Gly genotype (P=0.001 and P=0.027, respectively). In conclusion, the present results suggested that the IRS1 Gly972Arg and IRS2 Gly1057Asp variations may be associated with an increased susceptibility to develop gastric cancer. Further studies with larger sample sizes are required to support the present results and to explore the use of these variations as a biomarker for gastric cancer.

A Missense Mutation in IRS1 is Associated with the Development of Early-Onset Type 2 Diabetes

There could be an overlap of monogenic diabetes and early-onset type 2 diabetes mellitus. Precise diagnosis of early-onset diabetes has proven valuable for understanding the mechanism of diabetes and selecting optimal therapy. The majority of maturity onset diabetes of the young (MODY) pathogenic genes in China is still unknown. In this study, a family with suspected MODY was enrolled. Whole-exome sequencing (WES) was used to analyze the variants of the proband. Variants were filtered according to their frequency, location, functional consequences, and bioinformatics software. Candidate pathogenic variants were validated by Sanger sequencing and tested for cosegregation in other members of the family and nonrelated healthy controls. KEGG (Kyoto Encyclopedia of Genes and Genomes) and PPI (protein-protein interaction) analysis were conducted using the DAVID (Database for Annotation, Visualization, and Integrated Discovery) and the STRING online analysis tools for the candidate pathogenic gene. A total of 123291 variants including 105344 SNPs and 17947 InDels were found in WES. A likely pathogenic rare missense heterozygous mutation in diabetes genes (c.2137C > T, p.His713Tyr in IRS1) was identified, which was a cosegregate in this family and not in nonrelated healthy controls. The position of the mutation in the aminoacid sequence of the gene is highly conserved among the species. 2 significantly enriched KEGG pathways were identified including bta04930, type II diabetes mellitus (GCK, INS, PDX1, ABCC8, and IRS1), and bta04910, insulin signaling pathway (GCK, INS, and IRS1). PPI analysis displayed that IRS1 interacts with 3 known pathogenic proteins including INS, KCNJ11, and GCK. We conclude that WES could be an initial option for genetic testing in patients with early-onset diabetes. IRS1 p.His713Tyr is implicated as a possible pathogenic mutation in monogenic diabetes, which might require further validation, and the precise molecular mechanism underlying the influence of IRS1 p.His713Tyr on the development of diabetes remains to be determined in the further prospective studies.

The combined role of allelic variants of IRS-1 and IRS-2 genes in susceptibility to type2 diabetes in the Punjabi Pakistani subjects

BACKGROUND

Diabetes mellitus is a multifactorial disorder characterized by a high level of glucose in the blood. Both genetic and environmental factors interact to cause diabetes. Insulin receptor substrate (IRS) proteins have a significant part in insulin signaling pathways. We aimed to investigate the relationship of type 2 diabetes with a Gly972Arg (G972R) variant of the IRS-1 gene and Gly1057Asp (G1057D) polymorphism of IRS-2 gene in the population of Punjab, Pakistan.

METHODS

We collected 926 samples, 500 healthy controls (fasting blood sugar < 99 mg/dL, random blood sugar < 126 mg/dL) and 426 cases with diabetes (fasting blood sugar > 99 mg/dL, random blood sugar > 126 mg/dL). Several anthropometric measurements were measured. Statistical analysis was performed by using SPSS to determine the allele group/genotype frequency of the selected variants in the study population.

RESULTS

The genotyping results of G972R by RLFP-PCR showed the allelic frequency of G = 0.68 and R = 0.32 in controls while G = 0.71 and R = 0.29 in the cases. The minor R allele had a slightly higher frequency in the cases than the controls (OR = 0.86, CI 0.706-1.052, p = 0.17). The genotyping results of G1057D showed allelic frequency G = 0.74 and D = 0.26 in the controls while G = 0.961 and D = 0.29 in the cases. The minor D allele appeared to be a risk allele for this SNP although the difference in the allele frequencies was not statistically significant (OR = 1.55, CI 0.961-1.41, p = 0.108). The combined genotype analysis showed that the difference in the allele and genotype frequencies reached statistical difference between the cases and the controls as well as the odds ratio substantially increased when the R allele (G972R) was present together with D allele (G1057D) in any combination. When the association of single variants with the lipid traits was observed, only D allele (G1057D) showed significant association with TG, HDL and LDL, however when the analysis was repeated for combined genotypes using general linear model, many more significant associations between the genotype where D allele and R allele are together, were seen with many lipid traits.

CONCLUSION

In conclusion, the single nucleotide polymorphisms with low-modest effect size may not affect the phenotype individually but when in combination, the effect becomes stronger and more visible, therefore, for the SNP association studies, the more the number of SNPs included in the analysis, the more meaningful the results.

The Evolving Importance of Insulin Signaling in Podocyte Health and Disease

Diabetic kidney disease (DKD) is the leading cause of end-stage renal disease worldwide, occuring in approximately one-third of diabetic patients. One of the earliest hallmarks of DKD is albuminuria, often occurring following disruptions to the glomerular filtration barrier. Podocytes are highly specialized cells with a central role in filtration barrier maintenance; hence, podocyte dysfunction is a major cause of albuminuria in many settings, including DKD. Numerous studies over the last decade have highlighted the importance of intact podocyte insulin responses in the maintenance of podocyte function. This review summarizes our current perspectives on podocyte insulin signaling, highlighting evidence to support the notion that dysregulated podocyte insulin responses contribute toward podocyte damage, particularly during the pathogenesis of DKD.

Persistent Insulin Resistance in Podocytes Caused by Epigenetic Changes of SHP-1 in Diabetes

Poor glycemic control profoundly affects protein expression and the cell signaling action that contributes to glycemic memory and irreversible progression of diabetic nephropathy (DN). We demonstrate that SHP-1 is elevated in podocytes of diabetic mice, causing insulin unresponsiveness and DN. Thus, sustained SHP-1 expression caused by hyperglycemia despite systemic glucose normalization could contribute to the glycemic memory effect in DN. Microalbuminuria, glomerular filtration rate, mesangial cell expansion, and collagen type IV and transforming growth factor-β expression were significantly increased in diabetic Ins2^+/C96Y mice compared with nondiabetic Ins2^+/+ mice and remained elevated despite glucose normalization with insulin implants. A persistent increase of SHP-1 expression in podocytes despite normalization of systemic glucose levels was associated with sustained inhibition of the insulin signaling pathways. In cultured podocytes, high glucose levels increased mRNA, protein expression, and phosphatase activity of SHP-1, which remained elevated despite glucose concentration returning to normal, causing persistent insulin receptor-β inhibition. Histone posttranslational modification analysis showed that the promoter region of SHP-1 was enriched with H3K4me1 and H3K9/14ac in diabetic glomeruli and podocytes, which remained elevated despite glucose level normalization. Hyperglycemia induces SHP-1 promoter epigenetic modifications, causing its persistent expression and activity and leading to insulin resistance, podocyte dysfunction, and DN.

IRS proteins and diabetic complications

IRS proteins are cellular adaptor molecules that mediate many of the key metabolic actions of insulin. When tyrosine is phosphorylated by the activated insulin receptor, IRS proteins recruit downstream effectors, such as phosphoinositide 3-kinase and mitogen-activated protein kinase, in order to elicit cellular responses such as glucose uptake, lipid metabolism and cell proliferation. There are two main IRS proteins in humans (IRS1 and IRS2), both of which are widely expressed. Given their central role in the insulin signalling pathway, it is not surprising that male mice lacking Irs1 or Irs2 present with elevated blood glucose or type 2 diabetes, respectively. For reasons yet to be identified, female Irs2 (-/-) mice do not develop type 2 diabetes. A number of organs are affected by complications of diabetes; macrovascular complications include stroke and coronary artery disease, while nephropathy, neuropathy and retinopathy fall into the category of microvascular complications. Given the serious consequences of these complications on patient morbidity and mortality, it is essential to identify the molecular pathogenesis underlying diabetic complications, with a view to improving therapeutic intervention and patient outcomes. A number of recently published papers have converged on the hypothesis that the loss of insulin signalling and IRS proteins is instrumental to the development and/or progression of diabetic complications. This review will summarise some highlights from the published work in which this hypothesis is discussed.

The impact of insulin resistance on the kidney and vasculature

Insulin resistance is a systemic disorder that affects many organs and insulin-regulated pathways. The disorder is characterized by a reduced action of insulin despite increased insulin concentrations (hyperinsulinaemia). The effects of insulin on the kidney and vasculature differ in part from the effects on classical insulin target organs. Insulin causes vasodilation by enhancing endothelial nitric oxide production through activation of the phosphatidylinositol 3-kinase pathway. In insulin-resistant states, this pathway is impaired and the mitogen-activated protein kinase pathway stimulates vasoconstriction. The action of insulin on perivascular fat tissue and the subsequent effects on the vascular wall are not fully understood, but the hepatokine fetuin-A, which is released by fatty liver, might promote the proinflammatory effects of perivascular fat. The strong association of salt-sensitive arterial hypertension with insulin resistance indicates an involvement of the kidney in the insulin resistance syndrome. The insulin receptor is expressed on renal tubular cells and podocytes and insulin signalling has important roles in podocyte viability and tubular function. Renal sodium transport is preserved in insulin resistance and contributes to the salt-sensitivity of blood pressure in hyperinsulinaemia. Therapeutically, renal and vascular insulin resistance can be improved by an integrated holistic approach aimed at restoring overall insulin sensitivity and improving insulin signalling.

Diabetic kidney disease: a role for advanced glycation end-product receptor 1 (AGE-R1)?

Diabetic patients are postulated to be in a perpetual state of oxidative stress and inflammation at sites where chronic complications occur. The accumulation of AGEs derived from both endogenous and exogenous sources (such as the diet) have been implicated in the development and progression of diabetic complications, particularly nephropathy. There has been some interest in investigating the potential for reducing the AGE burden in chronic disease, through the action of AGE "clearance" receptors, such as the advanced glycation end-product receptor 1 (AGE-R1). Reducing the burden of AGEs has been linked to attenuation of inflammation, slower progression of diabetic complications (in particular vascular and renal complications) and has been shown to extend lifespan. To date, however, there have been no direct investigations into whether AGE-R1 has any role in modulating normal kidney function, or specifically during the development and progression of diabetes. This mini-review will focus on the recent advances in knowledge around the mechanistic function of AGE-R1 and the implications of this for the pathogenesis of diabetic kidney disease.

Genetic Variants in Toll-Like Receptor 4 Gene and Their Association Analysis with Estimated Glomerular Filtration Rate in Mexican American Families

BACKGROUND/AIM

Toll-like receptor 4 (TLR4) is one of the regulators of the innate immune response. Genetic variations in TLR4 have been associated with inflammatory diseases, including type 2 diabetes. However, to our knowledge, there are no reports on the role of variations in TLR4 in chronic kidney disease susceptibility. The objective of this study is to determine whether the genetic variants in TLR4 are associated with the estimated glomerular filtration rate (eGFR), a measure of renal function.

METHODS

To evaluate the association between TLR4 variants and eGFR, we used data obtained from 434 Mexican American participants from the San Antonio Family Diabetes/Gallbladder Study. GFR was estimated using the Modification of Diet in Renal Disease formula. The Asp(299)Gly (rs4986790) and Thr(399)Ile (rs4986791) variants of TLR4 were genotyped using the TaqMan assay. Association analyses between genotypes and eGFR were performed using the measured genotype approach.

RESULTS

Of the two genetic markers examined for association, only the Asp(299)Gly variant of TLR4 exhibited a nominally significant association with eGFR (p = 0.025) after accounting for the covariate effects of age and sex terms, diabetes, duration of diabetes, systolic blood pressure, body mass index, and antihypertensive treatment. Carriers of Gly299 had significantly decreased eGFR values. Although, the Thr(399)Ile variant failed to exhibit a statistically significant association with eGFR, the carriers of Ile399, however, showed a trend towards decrease in eGFR.

CONCLUSION

We show for the first time that Asp(299)Gly variants of TLR4 are associated with decrease in renal function in Mexican Americans.

Lack of association between the insulin receptor substrates-1 Gly972Arg polymorphism and type-2 diabetes mellitus among Saudis from Eastern Saudi Arabia

Objectives:

To investigate the association between the insulin receptor substrate-1 (IRS1) Gly972Arg polymorphism and type-2 diabetes mellitus (T2DM) among Saudis from Eastern Saudi Arabia.

Methods:

This study was conducted between May and December 2014 at King Fahad Hospital of the University, Al-Khobar, Kingdom of Saudi Arabia. In a case-control study design, a total of 143 subjects (age range: 35-73 years) comprising 74 healthy controls and 69 patients with T2DM were examined. Blood samples were collected from subjects and subjected to genomic DNA extraction and chemical analysis. The IRS1 Gly972Arg polymorphism was then genotyped using the standard polymerase chain reaction-restriction fragment length polymorphism technique.

Results:

Eight out of 74 (10.8%) of the control group carried at least one copy of the mutated allele. The frequency (8.7%) of the IRS1 variant was also found in the diabetic group. Logistic regression analysis showed an adjusted odds ratio of 1.04, 95% confidence interval 0.28 - 3.95, and a p-value of 0.94.

Conclusion:

We failed to find any association between the IRS1 Gly972Arg polymorphism and T2DM.

Evidence-based clinical practice guidelines for nonalcoholic fatty liver disease/nonalcoholic steatohepatitis

Nonalcoholic fatty liver disease (NAFLD) is currently the most common cause of chronic liver disease in industrialized countries worldwide, and has become a serious public health issue not only in Western countries but also in many Asian countries including Japan. Within the wide spectrum of NAFLD, nonalcoholic steatohepatitis (NASH) is a progressive form of disease, which often develops into liver cirrhosis and increases the risk of hepatocellular carcinoma. In turn, a large proportion of NAFLD/NASH is the liver manifestation of metabolic syndrome, suggesting that NAFLD/NASH plays a key role in the pathogenesis of systemic atherosclerotic diseases. Currently, a definite diagnosis of NASH requires liver biopsy, though various noninvasive measures are under development. The mainstays of prevention and treatment of NAFLD/NASH include dietary restriction and exercise; however, pharmacological approaches are often necessary. Currently, vitamin E and thiazolidinedione derivatives are the most evidence-based therapeutic options, although the clinical evidence for long-term efficacy and safety is limited. This practice guideline for NAFLD/NASH, established by the Japanese Society of Gastroenterology in cooperation with The Japan Society of Hepatology, covers lines of clinical evidence reported internationally in the period starting from 1983 to January 2012, and each clinical question was evaluated using the GRADE system. Based on the primary release of the full version in Japanese, this English summary provides the core essentials of this clinical practice guideline comprising the definition, diagnosis, and current therapeutic recommendations for NAFLD/NASH in Japan.

Evidence-based clinical practice guidelines for nonalcoholic fatty liver disease/nonalcoholic steatohepatitis

Nonalcoholic fatty liver disease (NAFLD) is currently the most common cause of chronic liver disease in industrialized countries worldwide, and has become a serious public health issue not only in Western countries but also in many Asian countries including Japan. Within the wide spectrum of NAFLD, non-alcoholic steatohepatitis (NASH) is a progressive form of disease, which often develops into liver cirrhosis and increases the risk of hepatocellular carcinoma. In turn, a large proportion of NAFLD/NASH is the liver manifestation of metabolic syndrome, suggesting that NAFLD/NASH plays a key role in the pathogenesis of systemic atherosclerotic diseases. Currently, a definite diagnosis of NASH requires liver biopsy, though various non-invasive measures are under development. The mainstays of prevention and treatment of NAFLD/NASH include dietary restriction and exercise; however, pharmacological approaches are often necessary. Currently, vitamin E and thiazolidinedione derivatives are the most evidence-based therapeutic options, although the clinical evidence for long-term efficacy and safety is limited. This practice guideline for NAFLD/NASH, established by the Japanese Society of Gastroenterology in cooperation with The Japan Society of Hepatology, covers lines of clinical evidence reported internationally in the period starting from 1983 through January 2012, and each clinical question was evaluated using the GRADE system. Based on the primary release of the full version in Japanese, this English summary provides the core essentials of this clinical practice guideline comprising the definition, diagnosis, and current therapeutic recommendations for NAFLD/NASH in Japan.

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.

Associations of Haplotypes Upstream of IRS1 with Insulin Resistance, Type 2 Diabetes, Dyslipidemia, Preclinical Atherosclerosis, and Skeletal Muscle LOC646736 mRNA Levels

The genomic region ~500 kb upstream of IRS1 has been implicated in insulin resistance, type 2 diabetes, adverse lipid profile, and cardiovascular risk. To gain further insight into this chromosomal region, we typed four SNPs in a cross-sectional cohort and subjects with type 2 diabetes recruited from the same geographic region. From 16 possible haplotypes, 6 haplotypes with frequencies >0.01 were observed. We identified one haplotype that was protective against insulin resistance (determined by HOMA-IR and fasting plasma insulin levels), type 2 diabetes, an adverse lipid profile, increased C-reactive protein, and asymptomatic atherosclerotic disease (assessed by intima media thickness of the common carotid arteries). BMI and total adipose tissue mass as well as visceral and subcutaneous adipose tissue mass did not differ between the reference and protective haplotypes. In 92 subjects, we observed an association of the protective haplotype with higher skeletal muscle mRNA levels of LOC646736, which is located in the same haplotype block as the informative SNPs and is mainly expressed in skeletal muscle, but only at very low levels in liver or adipose tissues. These data suggest a role for LOC646736 in human insulin resistance and warrant further studies on the functional effects of this locus.

A genome-wide search for linkage of estimated glomerular filtration rate (eGFR) in the Family Investigation of Nephropathy and Diabetes (FIND)

Objective

Estimated glomerular filtration rate (eGFR), a measure of kidney function, is heritable, suggesting that genes influence renal function. Genes that influence eGFR have been identified through genome-wide association studies. However, family-based linkage approaches may identify loci that explain a larger proportion of the heritability. This study used genome-wide linkage and association scans to identify quantitative trait loci (QTL) that influence eGFR.

Methods

Genome-wide linkage and sparse association scans of eGFR were performed in families ascertained by probands with advanced diabetic nephropathy (DN) from the multi-ethnic Family Investigation of Nephropathy and Diabetes (FIND) study. This study included 954 African Americans (AA), 781 American Indians (AI), 614 European Americans (EA) and 1,611 Mexican Americans (MA). A total of 3,960 FIND participants were genotyped for 6,000 single nucleotide polymorphisms (SNPs) using the Illumina Linkage IVb panel. GFR was estimated by the Modification of Diet in Renal Disease (MDRD) formula.

Results

The non-parametric linkage analysis, accounting for the effects of diabetes duration and BMI, identified the strongest evidence for linkage of eGFR on chromosome 20q11 (log of the odds [LOD] = 3.34; P = 4.4×10⁻⁵) in MA and chromosome 15q12 (LOD = 2.84; P = 1.5×10⁻⁴) in EA. In all subjects, the strongest linkage signal for eGFR was detected on chromosome 10p12 (P = 5.5×10⁻⁴) at 44 cM near marker rs1339048. A subsequent association scan in both ancestry-specific groups and the entire population identified several SNPs significantly associated with eGFR across the genome.

Conclusion

The present study describes the localization of QTL influencing eGFR on 20q11 in MA, 15q21 in EA and 10p12 in the combined ethnic groups participating in the FIND study. Identification of causal genes/variants influencing eGFR, within these linkage and association loci, will open new avenues for functional analyses and development of novel diagnostic markers for DN.

The IRS1 G972R polymorphism and glomerular filtration rate in patients with type 2 diabetes of European ancestry

BACKGROUND

In Mexican Americans, the IRS1 G972R polymorphism (rs1801278) has been associated to such a marked reduction in glomerular filtration rate (GFR) (i.e. β = -8.3 mL/min/1.73 m(2)) to be considered a major determinant of kidney function.

METHODS

This was a cross-sectional study to investigate whether a similarly strong effect can also be observed among individuals of European ancestry. We investigated a total of 3973 White patients with type 2 diabetes. Standardized serum creatinine was measured by the modified kinetic Jaffè reaction and estimated GFR (eGFR) calculated by the modification diet renal disease (MDRD) formula; rs1801278 was genotyped by TaqMan assay.

RESULTS

No significant association was observed, with R972 carriers showing only a modestly, not significant, lower eGFR level as compared with other subjects (β = -1.82 mL/min/1.73 m(2), P = 0.086).

CONCLUSIONS

Our data indicate that IRS1 G972R is not a strong determinant of GFR in diabetic patients of European ancestry as in Mexican Americans. Since we had 100% power to detect the previously reported association, the risk our finding is a false negative one is minimal.

Susceptibility gene search for nephropathy and related traits in Mexican-Americans

The rising global epidemic of diabetic nephropathy (DN) will likely lead to increase in the prevalence of cardiovascular morbidity and mortality posing a serious burden for public health care. Despite greater understanding of the etiology of diabetes and the development of novel treatment strategies to control blood glucose levels, the prevalence and incidence rate of DN is increasing especially in minority populations including Mexican-Americans. Mexican-Americans with type 2 diabetes (T2DM) are three times more likely to develop microalbuminuria, and four times more likely to develop clinical proteinuria compared to non-Hispanic whites. Furthermore, Mexican-Americans have a sixfold increased risk of developing renal failure secondary to T2DM compared to Caucasians. Prevention and better treatment of DN should be a high priority for both health-care organizations and society at large. Pathogenesis of DN is multi-factorial. Familial clustering of DN-related traits in MAs show that DN and related traits are heritable and that genes play a susceptibility role. While, there has been some progress in identifying genes which when mutated influence an individual's risk, major gene(s) responsible for DN are yet to be identified. Knowledge of the genetic causes of DN is essential for elucidation of its mechanisms, and for adequate classification, prognosis, and treatment. Self-identification and collaboration among researchers with suitable genomic and clinical data for meta-analyses in Mexican-Americans is critical for progress in replicating/identifying DN risk genes in this population. This paper reviews the approaches and recent efforts made to identify genetic variants contributing to risk for DN and related phenotypes in the Mexican-American population.

The insulin receptor and the kidney

PURPOSE OF REVIEW

In recent years, it has become clear that the insulin receptor is important in a variety of renal cell types. It is through this transmembrane receptor that insulin, and to a lesser extent insulin-like growth factor, hormones bind and can control important cellular functions. This review will summarize the advances in our understanding of the role of the insulin receptor and insulin signalling in the glomeruli and tubules of the kidney.

RECENT FINDINGS

The insulin receptor is important for podocyte function and when lost results in a number of features resembling diabetic nephropathy. Exciting recent data also highlight the importance of mammalian target of rapamycin in nutrient sensing and protein biosynthesis in the podocyte, which may also be regulated by the insulin receptor. The insulin receptor has also been shown to perform an important role in the distal regions of the renal tubules, regulating sodium excretion and blood pressure control here.

SUMMARY

The insulin receptor is crucial for renal function in glomeruli and tubules. When signalling is diminished here, as may occur in insulin-resistant states, it may be responsible for a number of important renal complications including albuminuric glomerular disease and hypertension.

Insulin receptor substrate-2 is expressed in kidney epithelium and up-regulated in diabetic nephropathy

Diabetic nephropathy (DN) is a progressive fibrotic condition that may lead to end-stage renal disease and kidney failure. Transforming growth factor-β1 and bone morphogenetic protein-7 (BMP7) have been shown to induce DN-like changes in the kidney and protect the kidney from such changes, respectively. Recent data identified insulin action at the level of the nephron as a crucial factor in the development and progression of DN. Insulin requires a family of insulin receptor substrate (IRS) proteins for its physiological effects, and many reports have highlighted the role of insulin and IRS proteins in kidney physiology and disease. Here, we observed IRS2 expression predominantly in the developing and adult kidney epithelium in mouse and human. BMP7 treatment of human kidney proximal tubule epithelial cells (HK-2 cells) increases IRS2 transcription. In addition, BMP7 treatment of HK-2 cells induces an electrophoretic shift in IRS2 migration on SDS/PAGE, and increased association with phosphatidylinositol-3-kinase, probably due to increased tyrosine/serine phosphorylation. In a cohort of DN patients with a range of chronic kidney disease severity, IRS2 mRNA levels were elevated approximately ninefold, with the majority of IRS2 staining evident in the kidney tubules in DN patients. These data show that IRS2 is expressed in the kidney epithelium and may play a role in the downstream protective events triggered by BMP7 in the kidney. The specific up-regulation of IRS2 in the kidney tubules of DN patients also indicates a novel role for IRS2 as a marker and/or mediator of human DN progression.

Insulin signalling to the kidney in health and disease

Ninety-one years ago insulin was discovered, which was one of the most important medical discoveries in the past century, transforming the lives of millions of diabetic patients. Initially insulin was considered only important for rapid control of blood glucose by its action on a restricted number of tissues; however, it has now become clear that this hormone controls an array of cellular processes in many different tissues. The present review will focus on the role of insulin in the kidney in health and disease.