A genome-wide linkage scan for diabetic retinopathy susceptibility genes in Mexican Americans with type 2 diabetes from Starr County, Texas

Polymorphisms in HIF-1a gene are not associated with diabetic retinopathy in China

BACKGROUND

It has been reported that vascular endothelial growth factor (VEGF) is a susceptibility gene for both type 2 diabetes mellitus (T2DM) and diabetic retinopathy (DR). In response to hypoxia, VEGF mRNA levels are increased, which is mainly mediated by the binding of hypoxia-inducible factor-1 (HIF-1) and hypoxia response element upstream of the transcriptional start site of VEGF. Therefore, HIF-1a is supposed to be involved in pathology of DR.

AIM

To investigate whether the polymorphisms in HIF-1a gene are associated with DR.

METHODS

Two hundred and ninety-nine type 2 diabetic patients (128 males and 171 females) and 144 healthy volunteers were recruited. Mean age was 56.04 ± 21.05 years. According to the results of fundus fluorescein angiography and examination of ophthalmoscopy, patients were divided into two groups, DNR group (diabetes without retinopathy) and DR group (diabetes with retinopathy). There are 150 cases in DNR group and 149 cases in DR group. Two single nucleotide polymorphisms (SNP) of the HIF-1a gene were tested using matrix-assisted laser desorption/Ionization time of flight mass spectrometry. The frequency of genotypes and alleles, and odds ratio were measured.

RESULTS

The mean age of the cases with diabetes was 55.84 ± 3.66 years, the mean age of the cases with DR was 55.97 ± 4.66 years and that of controls was 56.32 ± 4.70 years. Two variations were found in 76 patients. Rs11549465 is the change of C-T base, rs11549467 is the change of G-A base. The rs11549467 G/A genotype was 5.33% in diabetes and 6.04% in DR patients, respectively. The rs11549465 C/T genotype was 10% and 12.75% in patients with diabetes and DR. The rs11549467 A allele frequencies and rs11549465 T frequencies was similar to that of controls. Paired SNP linkage disequilibrium analysis indicated that rs11549467 was in linkage disequilibrium with rs11549465. Haplotype association analysis denoted that the haplotype association exhibited similar distribution in the patients compared to the normal controls.

CONCLUSION

This study suggests that there is no relationship between the genetic variations of HIF1a and diabetes or DR.

Genetics of Diabetic Retinopathy, a Leading Cause of Irreversible Blindness in the Industrialized World

Diabetic retinopathy (DR) is a chronic complication of diabetes and a leading cause of blindness in the industrialized world. Traditional risk factors, such as glycemic control and duration of diabetes, are unable to explain why some individuals remain protected while others progress to a more severe form of the disease. Differences are also observed in DR heritability as well as the response to anti-vascular endothelial growth factor (VEGF) treatment. This review discusses various aspects of genetics in DR to shed light on DR pathogenesis and treatment. First, we discuss the global burden of DR followed by a discussion on disease pathogenesis as well as the role genetics plays in the prevalence and progression of DR. Subsequently, we provide a review of studies related to DR’s genetic contribution, such as candidate gene studies, linkage studies, and genome-wide association studies (GWAS) as well as other clinical and meta-analysis studies that have identified putative candidate genes. With the advent of newer cutting-edge technologies, identifying the genetic components in DR has played an important role in understanding DR incidence, progression, and response to treatment, thereby developing newer therapeutic targets and therapies.

Genotypes and Phenotypes: A Search for Influential Genes in Diabetic Retinopathy

Although gene–environment interactions are known to play an important role in the inheritance of complex traits, it is still unknown how a genotype and the environmental factors result in an observable phenotype. Understanding this complex interaction in the pathogenesis of diabetic retinopathy (DR) remains a big challenge as DR appears to be a disease with heterogenous phenotypes with multifactorial influence. In this review, we examine the natural history and risk factors related to DR, emphasizing distinct clinical phenotypes and their natural course in retinopathy. Although there is strong evidence that duration of diabetes and metabolic factors play a key role in the pathogenesis of DR, accumulating new clinical studies reveal that this disease can develop independently of duration of diabetes and metabolic dysfunction. More recently, studies have emphasized the role of genetic factors in DR. However, linkage analyses, candidate gene studies, and genome-wide association studies (GWAS) have not produced any statistically significant results. Our recently initiated genomics study, the Diabetic Retinopathy Genomics (DRGen) Study, aims to examine the contribution of rare and common variants in the development DR, and how they can contribute to clinical phenotype, rate of progression, and response to available therapies. Our preliminary findings reveal a novel set of genetic variants associated with proangiogenic and inflammatory pathways that may contribute to DR pathogenesis. Further investigation of these variants is necessary and may lead to development of novel biomarkers and new therapeutic targets in DR.

Do Genomic Factors Play a Role in Diabetic Retinopathy?

Although there is strong clinical evidence that the control of blood glucose, blood pressure, and lipid level can prevent and slow down the progression of diabetic retinopathy (DR) as shown by landmark clinical trials, it has been shown that these factors only account for 10% of the risk for developing this disease. This suggests that other factors, such as genetics, may play a role in the development and progression of DR. Clinical evidence shows that some diabetics, despite the long duration of their diabetes (25 years or more) do not show any sign of DR or show minimal non-proliferative diabetic retinopathy (NPDR). Similarly, not all diabetics develop proliferative diabetic retinopathy (PDR). So far, linkage analysis, candidate gene studies, and genome-wide association studies (GWAS) have not produced any statistically significant results. We recently initiated a genomics study, the Diabetic Retinopathy Genetics (DRGen) Study, to examine the contribution of rare and common variants in the development of different phenotypes of DR, as well as their responsiveness to anti-VEGF treatment in diabetic macular edema (DME). Our preliminary findings reveal a novel set of genetic variants involved in the angiogenesis and inflammatory pathways that contribute to DR progression or protection. Further investigation of variants can help to develop novel biomarkers and lead to new therapeutic targets in DR.

Genetics of Diabetic Retinopathy

PURPOSE OF REVIEW

The goal of this paper is to review the latest findings in understanding the genetics of diabetic retinopathy. We highlight recent literature using a variety of molecular genetic techniques to identify variants which contribute to genetic susceptibility for diabetic retinopathy.

RECENT FINDINGS

New genome-wide association study (GWAS) and whole-exome sequencing approaches have been utilized to identify both common and rare variants associated with diabetic retinopathy. While variants have been identified in isolated studies, no variants have been replicated across multiple studies. The identification of genetic factors associated with diabetic retinopathy remains elusive. This is due to the multifactorial nature of the disease, small sample sizes for GWAS, and difficulty in controlling covariates of the disease. Larger populations as well as utilization of new sequencing and data analysis techniques may lead to new insights into genetic factors associated with diabetic retinopathy in the future.

Understanding the growing epidemic of type 2 diabetes in the Hispanic population living in the United States

The prevalence and incidence of type 2 diabetes (T2D) among the Hispanic population in the United States are higher than the national average. This is partly due to sociocultural factors, such as lower income and decreased access to education and health care, as well as a genetic susceptibility to obesity and higher insulin resistance. This review focuses on understanding the Hispanic population living in the United States from a multidisciplinary approach and underlines the importance of cultural, social, and biological factors in determining the increased risk of T2D in this population. An overview of the acute and chronic complications of T2D upon this population is included, which is of paramount importance to understand the toll that diabetes has upon this population, the health system, and society as a whole. Specific interventions directed to the Hispanic populations are needed to prevent and alleviate some of the burdens of T2D. Different prevention strategies based on medications, lifestyle modifications, and educational programmes are discussed herein. Diabetes self-management education (DSME) is a critical element of care of all people with diabetes and is considered necessary to improve patient outcomes. To be more effective, programmes should take into consideration cultural factors that influence the development and progression of diabetes. These interventions aim to enhance long-term effects by reducing the incidence, morbidity, and mortality of T2D in the Hispanic population of the United States.

A genome-wide association study suggests new evidence for an association of the NADPH Oxidase 4 (NOX4) gene with severe diabetic retinopathy in type 2 diabetes

Purpose

Diabetic retinopathy is the most common eye complication in patients with diabetes. The purpose of this study is to identify genetic factors contributing to severe diabetic retinopathy.

Methods

A genome‐wide association approach was applied. In the Genetics of Diabetes Audit and Research in Tayside Scotland (GoDARTS) datasets, cases of severe diabetic retinopathy were defined as type 2 diabetic patients who were ever graded as having severe background retinopathy (Level R3) or proliferative retinopathy (Level R4) in at least one eye according to the Scottish Diabetic Retinopathy Grading Scheme or who were once treated by laser photocoagulation. Controls were diabetic individuals whose longitudinal retinopathy screening records were either normal (Level R0) or only with mild background retinopathy (Level R1) in both eyes. Significant Single Nucleotide Polymorphisms (SNPs) were taken forward for meta‐analysis using multiple Caucasian cohorts.

Results

Five hundred and sixty cases of type 2 diabetes with severe diabetic retinopathy and 4,106 controls were identified in the GoDARTS cohort. We revealed that rs3913535 in the NADPH Oxidase 4 (NOX4) gene reached a p value of 4.05 × 10⁻⁹. Two nearby SNPs, rs10765219 and rs11018670 also showed promising p values (p values = 7.41 × 10⁻⁸ and 1.23 × 10⁻⁸, respectively). In the meta‐analysis using multiple Caucasian cohorts (excluding GoDARTS), rs10765219 and rs11018670 showed associations for diabetic retinopathy (p = 0.003 and 0.007, respectively), while the p value of rs3913535 was not significant (p = 0.429).

Conclusion

This genome‐wide association study of severe diabetic retinopathy suggests new evidence for the involvement of the NOX4 gene.

Multilocus genetic risk score for diabetic retinopathy in the Han Chinese population of Taiwan

The aim of this study is to explore the effect of genetic variation on diabetic retinopathy (DR) risk in a Taiwanese population. The logistic regression model was used to evaluate the relationship between DR status and risk factors, including the conventional parameters and genetic risk score (GRS). Candidate single nucleotide polymorphisms (SNPs) in GRS were selected based on previous reports with a combined P < 10^-4 (genome-wide association) and P < 0.05 (meta-analysis). In total, 58 SNPs in 44 susceptibility loci were selected, and four were used to calculate GRS. After adjustment for age, systolic blood pressure, diabetes duration, and HbA1c, the DR risk was 4.95 times higher for patients in the top GRS third tile than for those in the bottom third tile (95% CI = 2.99-8.18; P < 0.001). The addition of genetic information improved DR prediction, increasing the area under the curve (AUC) from 0.72 to 0.77 (P = 0.0024) and improving the sensitivity of the model such that 40 more subjects were reclassified into DR status. The developed multivariate logistic regression model combining conventional risk factors and the multilocus GRS can predict DR, thus enabling timely treatment to reduce blindness in T2D patients.

Genetic Variants in CPA6 and PRPF31 Are Associated With Variation in Response to Metformin in Individuals With Type 2 Diabetes

Metformin is the first-line treatment for type 2 diabetes (T2D). Although widely prescribed, the glucose-lowering mechanism for metformin is incompletely understood. Here, we used a genome-wide association approach in a diverse group of individuals with T2D from the Action to Control Cardiovascular Risk in Diabetes (ACCORD) clinical trial to identify common and rare variants associated with HbA_1c response to metformin treatment and followed up these findings in four replication cohorts. Common variants in PRPF31 and CPA6 were associated with worse and better metformin response, respectively (P < 5 × 10^-6), and meta-analysis in independent cohorts displayed similar associations with metformin response (P = 1.2 × 10^-8 and P = 0.005, respectively). Previous studies have shown that PRPF31(+/-) knockout mice have increased total body fat (P = 1.78 × 10^-6) and increased fasted circulating glucose (P = 5.73 × 10^-6). Furthermore, rare variants in STAT3 associated with worse metformin response (q <0.1). STAT3 is a ubiquitously expressed pleiotropic transcriptional activator that participates in the regulation of metabolism and feeding behavior. Here, we provide novel evidence for associations of common and rare variants in PRPF31, CPA6, and STAT3 with metformin response that may provide insight into mechanisms important for metformin efficacy in T2D.

Diabetic Microvascular Disease: An Endocrine Society Scientific Statement

Both type 1 and type 2 diabetes adversely affect the microvasculature in multiple organs. Our understanding of the genesis of this injury and of potential interventions to prevent, limit, or reverse injury/dysfunction is continuously evolving. This statement reviews biochemical/cellular pathways involved in facilitating and abrogating microvascular injury. The statement summarizes the types of injury/dysfunction that occur in the three classical diabetes microvascular target tissues, the eye, the kidney, and the peripheral nervous system; the statement also reviews information on the effects of diabetes and insulin resistance on the microvasculature of skin, brain, adipose tissue, and cardiac and skeletal muscle. Despite extensive and intensive research, it is disappointing that microvascular complications of diabetes continue to compromise the quantity and quality of life for patients with diabetes. Hopefully, by understanding and building on current research findings, we will discover new approaches for prevention and treatment that will be effective for future generations.

Diabetic retinopathy: Breaking the barrier

Diabetic retinopathy (DR) remains a major complication of diabetes and a leading cause of blindness among adults worldwide. DR is a progressive disease affecting both type I and type II diabetic patients at any stage of the disease, and targets the retinal microvasculature. DR results from multiple biochemical, molecular and pathophysiological changes to the retinal vasculature, which affect both microcirculatory functions and ultimately photoreceptor function. Several neural, endothelial, and support cell (e.g., pericyte) mechanisms are altered in a pathological fashion in the hyperglycemic environment during diabetes that can disturb important cell surface components in the vasculature producing the features of progressive DR pathophysiology. These include loss of the glycocalyx, blood-retinal barrier dysfunction, increased expression of inflammatory cell markers and adhesion of blood leukocytes and platelets. Included in this review is a discussion of modifications that occur at or near the surface of the retinal vascular endothelial cells, and the consequences of these alterations on the integrity of the retina.

Gene therapy for diabetic retinopathy: Are we ready to make the leap from bench to bedside?

Diabetic retinopathy (DR), a chronic and progressive complication of diabetes mellitus, is a sight-threatening disease characterized in the early stages by neuronal and vascular dysfunction in the retina, and later by neovascularization that further damages vision. A major contributor to the pathology is excess production of vascular endothelial growth factor (VEGF), a growth factor that induces formation of new blood vessels and increases permeability of existing vessels. Despite the recent availability of effective treatments for the disease, including laser photocoagulation and therapeutic VEGF antibodies, DR remains a significant cause of vision loss worldwide. Existing anti-VEGF agents, though generally effective, are limited by their short therapeutic half-lives, necessitating frequent intravitreal injections and the risk of attendant adverse events. Management of DR with gene therapies has been proposed for several years, and pre-clinical studies have yielded enticing findings. Gene therapy holds several advantages over conventional treatments for DR, such as a longer duration of therapeutic effect, simpler administration, the ability to intervene at an earlier stage of the disease, and potentially fewer side-effects. In this review, we summarize the current understanding of the pathophysiology of DR and provide an overview of research into DR gene therapies. We also examine current barriers to the clinical application of gene therapy for DR and evaluate future prospects for this approach.

Genetic components in diabetic retinopathy

Diabetic retinopathy (DR) is a serious complication of diabetes, which is fast reaching epidemic proportions worldwide. While tight glycemic control remains the standard of care for preventing the progression of DR, better insights into DR etiology require understanding its genetic basis, which in turn may assist in the design of novel treatments. During the last decade, genomic medicine is increasingly being applied to common multifactorial diseases such as diabetes and age-related macular degeneration. The contribution of genetics to the initiation and progression of DR has been recognized for some time, but the involvement of specific genes and genetic variants remains elusive. Several investigations are currently underway for identifying DR susceptibility loci through linkage studies, candidate gene approaches, and genome-wide association studies. Advent of next generation sequencing and high throughput genomic technologies, development of novel bioinformatics tools and collaborations among research teams should facilitate such investigations. Here, we review the current state of genetic studies in DR and discuss reported findings in the context of biochemical, cell biological and therapeutic advances. We propose the development of a consortium in India for genetic studies with large cohorts of patients and controls from limited geographical areas to stratify the impact of the environment. Uniform guidelines should be established for clinical phenotyping and data collection. These studies would permit identification of genetic loci for DR susceptibility in the Indian population and should be valuable for better diagnosis and prognosis, and for clinical management of this blinding disease.

Diabetic Retinopathy Clinical Practice Guidelines: Customized for Iranian Population

PURPOSE

To customize clinical practice guidelines (CPGs) for management of diabetic retinopathy (DR) in the Iranian population.

METHODS

Three DR CPGs (The Royal College of Ophthalmologists 2013, American Academy of Ophthalmology [Preferred Practice Pattern 2012], and Australian Diabetes Society 2008) were selected from the literature using the AGREE tool. Clinical questions were designed and summarized into four tables by the customization team. The components of the clinical questions along with pertinent recommendations extracted from the above-mentioned CPGs; details of the supporting articles and their levels of evidence; clinical recommendations considering clinical benefits, cost and side effects; and revised recommendations based on customization capability (applicability, acceptability, external validity) were recorded in 4 tables, respectively. Customized recommendations were sent to the faculty members of all universities across the country to score the recommendations from 1 to 9.

RESULTS

Agreed recommendations were accepted as the final recommendations while the non-agreed ones were approved after revision. Eventually, 29 customized recommendations under three major categories consisting of screening, diagnosis and treatment of DR were developed along with their sources and levels of evidence.

CONCLUSION

This customized CPGs for management of DR can be used to standardize the referral pathway, diagnosis and treatment of patients with diabetic retinopathy.

Novel Genetic Actors of Diabetes-Associated Microvascular Complications: Retinopathy, Kidney Disease and Neuropathy

Both type 1 and type 2 diabetes mellitus can lead to the common microvascular complications of diabetic retinopathy, kidney disease, and neuropathy. Diabetic patients do not universally develop these complications. Long duration of diabetes and poor glycemic control explain a lot of the variability in the development of microvascular complications, but not all. Genetic factors account for some of the remaining variability because of the heritability and familial clustering of these complications. There have been a large number of investigations, including linkage studies, candidate gene studies, and genome-wide association studies, all of which have sought to identify the specific variants that increase susceptibility. For retinopathy, several genome-wide association studies have been performed in small or midsize samples, but no reproducible loci across the studies have been identified. For diabetic kidney disease, genome-wide association studies in larger samples have been performed, and loci for this complication are beginning to emerge. However, validation of the existing discoveries, and further novel discoveries in larger samples is ongoing. The amount of genetic research into diabetic neuropathy has been very limited, and much is dedicated to the understanding of genetic risk factors only. Collaborations that pool samples and aim to detect phenotype classifications more precisely are promising avenues for a better explanation of the genetics of diabetic microvascular complications.

High prevalence of diabetic retinopathy and lack of association with integrin α2 gene polymorphisms in patients with type 2 diabetes from Northeastern Mexico

Diabetic retinopathy (DR) is one of the primary causes of blindness in the working age population and is characterized by angiogenesis in the retina. Platelets have been suggested to be involved in the pathogenesis of diabetic microvascular complications. The integrin receptor for collagen/laminin, α2β1, mediates platelet primary adhesion to subendothelial tissues, which is an essential first step in thrombus formation. The gene encoding the α2 subunit of α2β1 integrin has ≥8 polymorphisms, including a BglII/NdeI restriction fragment length polymorphism. To explore the prevalence of DR in a population from Northeastern Mexico, unrelated, hospitalized patients who had received a diagnosis of type 2 diabetes mellitus (DM2) at least 10 years previously were recruited (n=177). DR was diagnosed in a masked manner by independent ophthalmologists using fundus images captured using a non-mydriatic retinal camera. A total of 121 patients with DM2 (68%) had some degree of DR development (DR patients), and 56 patients with DM2 (32%) did not exhibit any sign of DR (No-DR patients). The results showed that after 15 years of DM2 progression, there is an increased risk of DR (P=0.0497; odds ratio, 1.993). In addition, insulin therapy and family history of DM2 were significantly associated with DR. In order to detect a possible association between DR and BglII/NdeI α2 gene polymorphisms, a comparative cross-sectional study between DR and No-DR patients was conducted. The α2 gene was genotyped by polymerase chain reaction-restriction fragment length polymorphism assay. Statistical analysis revealed no association between BglII/NdeI genotypes and the development of DR in this group of patients. In conclusion, the present data indicate a high prevalence of DR in the Mexican population and suggest that the damage in DR is due to other factors, such as the duration of the DM2, and is not linked to BglII/NdeI α2 gene polymorphisms.

Global DNA methylation as a possible biomarker for diabetic retinopathy

BACKGROUND

We evaluated whether global levels of DNA methylation status were associated with retinopathy as well as providing a predictive role of DNA methylation in developing retinopathy in a case-control study of 168 patients with type 2 diabetes.

METHODS

The 5-methylcytosine content was assessed by reversed-phase high-pressure liquid chromatography of peripheral blood leukocytes to determine an individual's global DNA methylation status in the two groups, either with or without retinopathy.

RESULTS

The global DNA methylation levels were significantly higher in diabetic retinopathy patients compared with those in non-retinopathy patients (4.90 ± 0.12 vs. 4.22 ± 0.13, respectively; p = 0.001). There was a significant increasing trend in global DNA methylation levels in terms of progressing retinopathy (without retinopathy, 4.22 ± 0.13; non-proliferative diabetic retinopathy, 4.62 ± 0.17; proliferative diabetic retinopathy, 5.07 ± 0.21) (p = 0.006). Additionally, global DNA methylation independent of retinopathy risk factors, which include dyslipidaemia, hypertension, hyperglycaemia and duration of diabetes, was a predictive factor for retinopathy (OR = 1.53, p = 0.015).

CONCLUSIONS

Global DNA methylation is modulated during or possibly before the primary stage of diabetes. This observation verifies the metabolic memory effect of hyperglycaemia in early stage of an aetiological process that leads to type 2 diabetes and its associated complications.

Human genetics of diabetic retinopathy

There is evidence demonstrating that genetic factors contribute to the risk of diabetic retinopathy (DR). Genetics variants, structural variants (copy number variation, CNV) and epigenetic changes play important roles in the development of DR. Genetic linkage and association studies have uncovered a number of genetic loci and common genetic variants susceptibility to DR. CNV and interactions of gene by environment have also been detected by association analysis. Apart from nucleus genome, mitochondrial DNA plays critical roles in regulation of development of DR. Epigenetic studies have indicated epigenetic changes in chromatin affecting gene transcription in response to environmental stimuli, which provided a large body of evidence of regulating development of diabetes mellitus. Identification of genetic variants and epigenetic changes contributed to risk or protection of DR will benefit uncovering the complex mechanism underlying DR. This review focused on the current knowledge of the genetic and epigenetic basis of DR.

A genome-wide association study for diabetic retinopathy in a Japanese population: potential association with a long intergenic non-coding RNA

Elucidation of the genetic susceptibility factors for diabetic retinopathy (DR) is important to gain insight into the pathogenesis of DR, and may help to define genetic risk factors for this condition. In the present study, we conducted a three-stage genome-wide association study (GWAS) to identify DR susceptibility loci in Japanese patients, which comprised a total of 837 type 2 diabetes patients with DR (cases) and 1,149 without DR (controls). From the stage 1 genome-wide scan of 446 subjects (205 cases and 241 controls) on 614,216 SNPs, 249 SNPs were selected for the stage 2 replication in 623 subjects (335 cases and 288 controls). Eight SNPs were further followed up in a stage 3 study of 297 cases and 620 controls. The top signal from the present association analysis was rs9362054 in an intron of RP1-90L14.1 showing borderline genome-wide significance (P_met = 1.4×10⁻⁷, meta-analysis of stage 1 and stage 2, allele model). RP1-90L14.1 is a long intergenic non-coding RNA (lincRNA) adjacent to KIAA1009/QN1/CEP162 gene; CEP162 plays a critical role in ciliary transition zone formation before ciliogenesis. The present study raises the possibility that the dysregulation of ciliary-associated genes plays a role in susceptibility to DR.

Genetics of diabetes complications

Chronic hyperglycemia and duration of diabetes are the major risk factors associated with development of micro- and macrovascular complications of diabetes. Although it is believed that hyperglycemia induces damage to the particular cell subtypes, e.g., mesangial cells in the renal glomerulus, capillary endothelial cells in the retina, and neurons and Schwann cells in peripheral nerves, the exact mechanisms underlying these damaging defects are not yet well understood. Clustering of micro- and macrovascular complications in families of patients with diabetes suggests a strong genetic susceptibility. However, until now only a handful number of genetic variants were reported to be associated with either nephropathy (ACE, ELMO1, FRMD3, and AKR1B1) or retinopathy (VEGF, AKR1B1, and EPO), and only a few studies were carried out for genetic susceptibility to cardiovascular diseases (ADIPOQ, GLUL) in patients with diabetes. It is, therefore, obvious that the accumulation of more data from larger studies and better phenotypically characterized cohorts is needed to facilitate genetic discoveries and unravel novel insights into the pathogenesis of diabetic complications.

Genetics of diabetic retinopathy

Diabetic retinopathy (DR) is a polygenic disorder. Twin studies and familial aggregation studies have documented clear familial clustering. Heritability has been estimated to be as high as 27 % for any DR and 52 % for proliferative diabetic retinopathy (PDR), an advanced form of the disease. Linkage analyses, candidate gene association studies and genome-wide association studies (GWAS) performed to date have not identified any widely reproducible risk loci for DR. Combined analysis of the data from multiple GWAS is emerging as an important next step to explain the unaccounted heritability. Key factors to future discovery of the genetic underpinnings of DR are precise DR ascertainment, a focus on the more heritable disease forms such as PDR, stringent selection of control participants with regards to duration of diabetes, and methods that allow combination of existing datasets from different ethnicities to achieve sufficient sample sizes to detect variants with modest effect sizes.

Private haplotypes can reveal local adaptation

BACKGROUND

Genome-wide scans for regions that demonstrate deviating patterns of genetic variation have become common approaches for finding genes targeted by selection. Several genomic patterns have been utilized for this purpose, including deviations in haplotype homozygosity, frequency spectra and genetic differentiation between populations.

RESULTS

We describe a novel approach based on the Maximum Frequency of Private Haplotypes--MFPH--to search for signals of recent population-specific selection. The MFPH statistic is straightforward to compute for phased SNP- and sequence-data. Using both simulated and empirical data, we show that MFPH can be a powerful statistic to detect recent population-specific selection, that it performs at the same level as other commonly used summary statistics (e.g. FST, iHS and XP-EHH), and that MFPH in some cases capture signals of selection that are missed by other statistics. For instance, in the Maasai, MFPH reveals a strong signal of selection in a region where other investigated statistics fail to pick up a clear signal that contains the genes DOCK3, MAPKAPK3 and CISH. This region has been suggested to affect height in many populations based on phenotype-genotype association studies. It has specifically been suggested to be targeted by selection in Pygmy groups, which are on the opposite end of the human height spectrum compared to the Maasai.

CONCLUSIONS

From the analysis of both simulated and publicly available empirical data, we show that MFPH represents a summary statistic that can provide further insight concerning population-specific adaptation.

Predictive factors of diabetic complications: a possible link between family history of diabetes and diabetic retinopathy

BACKGROUND

The aim of this study was assessment of predictive factors of diabetic retinopathy.

METHODS

A cross-sectional study was designed by recruiting 1228 type 2 diabetic patients from a diabetes referral clinic over a six-month period (from July to December, 2012). Diabetes risk factors, complications, laboratory results have been recorded.

RESULTS

Of the 1228 diabetic patients (54% women, mean age 58.48 ± 9.94 years), prevalence of diabetes retinopathy was 26.6%. There were significant associations between retinopathy and family history of diabetes (p = 0.04), hypertension (p = 0.0001), diabetic duration (p = 0.0001), poor glycemic control (p = 0.0001) and age of onset of diabetes (p = 0.0001). However, no significant associations were found between retinopathy with dyslipidemia and obesity. In logistic regression model, poor glycemic control (p = 0.014), hypertension (p = 0.0001), duration of diabetes (p = 0.0001) and family history of diabetes (p = 0.012) independently predicted retinopathy after adjustment for age and sex.

CONCLUSIONS

Diabetic complications are resulting from an interaction from genes and environmental factors. A family history of diabetes is pointing toward a possible genetic and epigenetic basis for diabetic retinopathy. Our findings suggest the role of epigenetic modifications and metabolic memory in diabetic retinopathy in subjects with family history of diabetes.

Genetics of diabetic retinopathy

Multiple studies have shown that genetic factors may play an important role in determining an individual's risk for the development of diabetic retinopathy (DR) and progression to proliferative DR. However, consistent and definitive genetic associations with DR across broad populations have been not been established. Numerous genes have been studied for their association with DR and the results of these investigations have most specifically pointed to three specific genes that are likely involved in DR development and progression. The gene coding for vascular endothelial growth factor, aldose reductase, and the receptor for advanced glycation end products have been extensively evaluated, and specific polymorphisms of these genes have been suggested to potentially increase the risk of DR development. In this paper, we have reviewed the published literature on the genetics of DR and the potential implications for DR development and progression.

Challenges in elucidating the genetics of diabetic retinopathy

IMPORTANCE

In the past decade, significant progress in genomic medicine and technologic developments has revolutionized our approach to common complex disorders in many areas of medicine, including ophthalmology. A disorder that still needs major genetic progress is diabetic retinopathy (DR), one of the leading causes of blindness in adults.

OBJECTIVE

To perform a literature review, present the current findings, and highlight some key challenges in DR genetics.

DESIGN, SETTING, AND PARTICIPANTS

We performed a thorough literature review of the genetic factors for DR, including heritability scores, twin studies, family studies, candidate gene studies, linkage studies, and genome-wide association studies (GWASs).

MAIN OUTCOME MEASURES

Environmental and genetic factors for DR.

RESULTS

Although there is clear demonstration of a genetic contribution in the development and progression of DR, the identification of susceptibility loci through candidate gene approaches, linkage studies, and GWASs is still in its infancy. The greatest obstacles remain a lack of power because of small sample size of available studies and a lack of phenotype standardization.

CONCLUSIONS AND RELEVANCE

The field of DR genetics is still in its infancy and is a challenge because of the complexity of the disease. This review outlines some strategies and lessons for future investigation to improve our understanding of this complex genetic disorder.

Genetics in diabetic retinopathy: current concepts and new insights

There is emerging evidence which indicates the essential role of genetic factors in the development of diabetic retinopathy (DR). In this regard it should be highlighted that genetic factors account for 25-50% of the risk of developing DR. Therefore, the use of genetic analysis to identify those diabetic patients most prone to developing DR might be useful in designing a more individualized treatment. In this regard, there are three main research strategies: candidate gene studies, linkage studies and Genome-Wide Association Studies (GWAS). In the candidate gene approach, several genes encoding proteins closely related to DR development have been analyzed. The linkage studies analyze shared alleles among family members with DR under the assumption that these predispose to a more aggressive development of DR. Finally, Genome-Wide Association Studies (GWAS) are a new tool involving a massive evaluation of single nucleotide polymorphisms (SNP) in large samples. In this review the available information using these three methodologies is critically analyzed. A genetic approach in order to identify new candidates in the pathogenesis of DR would permit us to design more targeted therapeutic strategies in order to decrease this devastating complication of diabetes. Basic researchers, ophthalmologists, diabetologists and geneticists should work together in order to gain new insights into this issue.

Genome-wide association study in a Chinese population with diabetic retinopathy

Diabetic retinopathy (DR) is a leading cause of preventable blindness in adults. To identify genetic contributions in DR, we studied 2071 type 2 diabetics. We first conducted a genome-wide association study of 1007 individuals, comparing 570 subjects with ≥8 years duration without DR (controls) with 437 PDR (cases) in the Chinese discovery cohort. Cases and controls were similar for HbA1c, diabetes duration and body mass index. Association analysis with imputed data identified three novel loci: TBC1D4-COMMD6-UCHL3 (rs9565164, P = 1.3 × 10(-7)), LRP2-BBS5 (rs1399634, P = 2.0 × 10(-6)) and ARL4C-SH3BP4 (rs2380261, P = 2.1 × 10(-6)). Analysis of an independent cohort of 585 Hispanics diabetics with or without DR though did not confirm these signals. These genes are still of particular interest because they are involved in insulin regulation, inflammation, lipid signaling and apoptosis pathways, all of which are possibly involved with DR. Our finding nominates possible novel loci as potential DR susceptibility genes in the Chinese that are independent of the level of HbA1c and duration of diabetes and may provide insight into the pathophysiology of DR.

Association of genes on chromosome 6, GRIK2 , TMEM217 and TMEM63B (linked to MRPL14 ) with diabetic retinopathy

PURPOSE

Diabetic retinopathy (DR) is one of the most common complications of diabetes mellitus (DM). The susceptibility genes responsible for increasing the risk for DR in type 2 diabetes (T2D) were sought in this study.

METHODS

A case-control study was carried out, comprising 749 unrelated T2D individuals with (n = 174) and without (n = 575) DR. Genotypic distributions of single nucleotide polymorphisms (SNPs) were determined for subjects with and without DR.

RESULTS

Eight chromosome 6 SNPs, having the most significant differences, were delineated: rs10499298, rs10499299, rs17827966, rs1224329, rs1150790, rs713050, rs2518344 and rs487083; all were associated with genes TMEM217, MRPL14 and GRIK2. After adjusting for the duration of DM and levels of hemoglobin A(1c), the TT genotype of rs713050, and the AG + AA genotypes of rs2518344 and rs10499298, differed significantly between those with and without DR. Haplotype analysis revealed haplotype C-A-C, residing in rs10499299, rs10499298 and rs17827966, to have significant linkage disequilibrium.

CONCLUSIONS

We identified new loci on chromosome 6 associated to DR; all loci showed high levels of linkage disequilibrium.

Replication analysis for severe diabetic retinopathy

PURPOSE

The purpose of this study is to attempt to replicate the top single nucleotide polymorphism (SNP) associations from a previous genome-wide association study (GWAS) for the sight-threatening complications of diabetic retinopathy in an independent cohort of diabetic subjects from the Wisconsin Epidemiologic Study of Diabetic Retinopathy (WESDR).

METHODS

This study included 469 type 1 diabetic, Caucasian subjects from WESDR. Cases (n = 208) were defined by prior laser treatment for either proliferative diabetic retinopathy or diabetic macular edema. Controls (n = 261) were all other subjects in the cohort. Three hundred eighty-nine SNPs were tested for association using the Illumina GoldenGate custom array. A retinopathy-only subanalysis was conducted in 437 subjects by removing those with end-stage renal disease. Evaluation for association between cases and controls was conducted by using chi-square tests. A combined analysis incorporated the results from WESDR with the prior GWAS.

RESULTS

No associations were significant at a genome-wide level. The analysis did identify SNPs that can be pursued in future replication studies. The top association was at rs4865047, an intronic SNP, in the gene CEP135 (P value 2.06 × 10(-5)). The top association from the subanalysis was at rs1902491 (P value 2.81 × 10(-5)), a SNP that sits upstream of the gene NPY2R.

CONCLUSIONS

This study nominates several novel genetic loci that may be associated with severe diabetic retinopathy. In order to confirm these findings, replication and extension in additional cohorts will be necessary as susceptibility alleles for diabetic retinopathy appear to be of modest effect.

Genome-wide association study of diabetic retinopathy in a Taiwanese population

PURPOSE

Diabetic retinopathy (DR) is a microvascular complication of diabetes with a complex multifactorial pathogenesis. The aim of this study was to identify the susceptibility genes that increase the risk of DR in type 2 diabetes (T2D) and to further elucidate the underlying mechanism of DR pathogenesis.

DESIGN

A case-control study.

PARTICIPANTS

We included 749 unrelated individuals with T2D (174 with DR and 575 without DR) and 100 nondiabetic controls.

METHODS

We conducted a genome-wide association study using Illumina HumanHap550-Duo BeadChips.

MAIN OUTCOME MEASURES

Compared with the genotypic distribution of single nucleotide polymorphisms (SNPs) between subjects with DR and without DR.

RESULTS

Using statistical models, we selected a total of 12 SNPs with P-values <1 × 10(-6) that were associated with DR. After controlling for diabetes duration and hemoglobin A(1C), 9 of the 12 SNPs located on 5 chromosomal regions were found to be associated with DR. Five loci not previously associated with DR susceptibility were identified in and around the following genes: MYSM1 (Myb-like, SWIRM, and MPN domains 1) located on chromosome 1p (odds ratio [OR], 1.50; 95% confidence interval [CI], 1.03-2.20); PLXDC2 (plexin domain-containing 2) located on the chromosome 10p (OR, 1.67; 95% CI, 1.06-2.65); ARHGAP22 (Rho GTPase-activating protein 22) located on chromosome 10q (OR, 1.65; 95% CI, 1.05-2.60); and HS6ST3 (heparan sulfate 6-O-sulfotransferase 3) located on chromosome 13q (OR, 2.33; 95% CI, 1.13-4.77). The SNPs rs13163610 and rs17376456 located in the unknown gene on chromosome 5q were also associated with DR (OR, 3.63; 95% CI, 1.38-9.58).

CONCLUSIONS

We identified a genetic association for susceptibility to DR in 5 novel chromosomal regions and PLXDC2 and ARHGAP22, the latter 2 of which are genes implicated in endothelial cell angiogenesis and increased capillary permeability. These findings suggest unsuspected pathways in the pathogenesis of DR.

Association analysis of nine candidate gene polymorphisms in Indian patients with type 2 diabetic retinopathy

Background

Diabetic retinopathy (DR) is classically defined as a microvasculopathy that primarily affects the small blood vessels of the inner retina as a complication of diabetes mellitus (DM).It is a multifactorial disease with a strong genetic component. The aim of this study is to investigate the association of a set of nine candidate genes with the development of diabetic retinopathy in a South Indian cohort who have type 2 diabetes mellitus (T2DM).

Methods

Seven candidate genes (RAGE, PEDF, AKR1B1, EPO, HTRA1, ICAM and HFE) were chosen based on reported association with DR in the literature. Two more, CFH and ARMS2, were chosen based on their roles in biological pathways previously implicated in DR. Fourteen single nucleotide polymorphisms (SNPs) and one dinucleotide repeat polymorphism, previously reported to show association with DR or other related diseases, were genotyped in 345 DR and 356 diabetic patients without retinopathy (DNR). The genes which showed positive association in this screening set were tested further in additional sets of 100 DR and 90 DNR additional patients from the Aravind Eye Hospital. Those which showed association in the secondary screen were subjected to a combined analysis with the 100 DR and 100 DNR subjects previously recruited and genotyped through the Sankara Nethralaya Hospital, India. Genotypes were evaluated using a combination of direct sequencing, TaqMan SNP genotyping, RFLP analysis, and SNaPshot PCR assays. Chi-square and Fisher exact tests were used to analyze the genotype and allele frequencies.

Results

Among the nine loci (15 polymorphisms) screened, SNP rs2070600 (G82S) in the RAGE gene, showed significant association with DR (allelic P = 0.016, dominant model P = 0.012), compared to DNR. SNP rs2070600 further showed significant association with DR in the confirmation cohort (P = 0.035, dominant model P = 0.032). Combining the two cohorts gave an allelic P < 0.003 and dominant P = 0.0013). Combined analysis with the Sankara Nethralaya cohort gave an allelic P = 0.0003 and dominant P = 0.00011 with an OR = 0.49 (0.34 - 0.70) for the minor allele. In HTRA1, rs11200638 (G>A), showed marginal significance with DR (P = 0.055) while rs10490924 in LOC387715 gave a P = 0.07. No statistical significance was observed for SNPs in the other 7 genes studied.

Conclusions

This study confirms significant association of one polymorphism only (rs2070600 in RAGE) with DR in an Indian population which had T2DM.

Identification of Diabetic Retinopathy Genes through a Genome-Wide Association Study among Mexican-Americans from Starr County, Texas

To identify genetic loci for severe diabetic retinopathy, 286 Mexican-Americans with type 2 diabetes from Starr County, Texas, completed physical examinations including fundus photography for diabetic retinopathy grading. Individuals with moderate-to-severe non-proliferative and proliferative diabetic retinopathy were defined as cases. Direct genotyping was performed using the Affymetrix GeneChip Human Mapping 100 K Set, and SNPs passing quality control criteria were used to impute markers available in HapMap Phase III Mexican population (MXL) in Los Angeles, California. Two directly genotyped markers were associated with severe diabetic retinopathy at a P-value less than .0001: SNP rs2300782 (P = 6.04 × 10(-5)) mapped to an intron region of CAMK4 (calcium/calmodulin-dependent protein kinase IV) on chromosome 5, and SNP rs10519765 (P = 6.21 × 10(-5)) on chromosomal 15q13 in the FMN1 (formin 1) gene. Using well-imputed markers based on the HapMap III Mexican population, we identified an additional 32 SNPs located in 11 chromosomal regions with nominal association with severe diabetic retinopathy at P-value less than .0001. None of these markers were located in traditional candidate genes for diabetic retinopathy or diabetes itself. However, these signals implicate genes involved in inflammation, oxidative stress and cell adhesion for the development and progression of diabetic retinopathy.

Human genetics of diabetic retinopathy: current perspectives

Diabetic retinopathy (DR) is a most severe microvascular complication which, if left unchecked, can be sight-threatening. With the global prevalence of diabetes being relentlessly projected to rise to 438 million subjects by 2030, DR will undoubtedly pose a major public health concern. Efforts to unravel the human genetics of DR have been undertaken using the candidate gene and linkage approaches, while GWAS efforts are still lacking. Aside from evidence for a few genes including aldose reductase and vascular endothelial growth factor, the genetics of DR remain poorly elucidated. Nevertheless, the promise of impactful scientific discoveries may be realized if concerted and collaborative efforts are mounted to identify the genes for DR. Harnessing new genetic technologies and resources such as the upcoming 1000 Genomes Project will help advance this field of research, and potentially lead to a rich harvest of insights into the biological mechanisms underlying this debilitating complication.

Absence of diabetic retinopathy in a patient who has had diabetes mellitus for 69 years, and inadequate glycemic control: case presentation

The main risk factors for the development and progression of diabetic retinopathy (DR) are chronic hyperglycemia, disease duration and systemic blood pressure. So far chronic hyperglycemia is the strongest evidence concerning the risk of developing DR. However there are some patients with poor metabolic control who never develop this diabetic complication. We present a case of a 73-year-old woman with type 1 diabetes mellitus, diagnosed 69 years ago. The patient is 73 years old, with no evidence of DR, despite poor glycemic control and several risk factors for DR. This case suggests the presence of a possible protection factor, which could be genetic.

[Diabetic retinopathy risk factors]

Diabetic retinopathy (DR) occurs in about 95% of patients with type 1 diabetes mellitus (DM) and in 60% of type 2 DM patients and it is the main cause of legal blindness in adult people. The aim of this manuscript was to review the main risk factors for DR. The major environmental risk factors are hyperglycemia, high blood pressure levels, and long-term duration of DM. However, not all patients will not develop DR, suggesting the presence of a genetic predisposition to DR, especially for severe forms of DR. Special strategies has been used to evaluate the genetic role in DR. Family studies shown that there is a familial aggregation of DR. Candidates genes have been studied (RAGE; VEGF; PPAR-delta; ICAM-1; ECA; ENPP 1; eNOS) and positive or negative associations with DR were demonstrated. Some chromosomes were also associated to DR in selected populations. Finally, genetic expression studies reinforce the association of candidate genes, or participation of others genes, with the presence of DR. DR is a common complication of DM and, along with non-genetic or environmental risk factors, the identification of genes related to DR could result in more specific and efficient DR treatment.

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.

Heritability of proliferative diabetic retinopathy

OBJECTIVE

Diabetic nephropathy clusters in families, suggesting that genetic factors play a role in its pathogenesis. We investigated whether similar clustering exists for proliferative retinopathy in families with two or more siblings with type 1 diabetes.

RESEARCH DESIGN AND METHODS

The FinnDiane Study has characterized 20% (4,800 patients) of adults with type 1 diabetes in Finland. In 188 families, there were at least two siblings with type 1 diabetes. Ophthalmic records were obtained for 369 of 396 (93%) and fundus photographs for 251 of 369 (68%) patients. Retinopathy was graded based on photographs and/or repeated ophthalmic examinations using the Early Treatment of Diabetic Retinopathy grading scale.

RESULTS

Mean age at onset of diabetes was 14.3 +/- 10.2 years, and mean duration was 25.9 +/- 11.8 years. Proliferative retinopathy was found in 115 of 369 patients (31%). The familial risk of proliferative retinopathy was estimated in 168 of 188 sibships, adjusted for A1C, duration, and mean blood pressure. Proliferative retinopathy in the probands (48 of 168) was associated with an increased risk (odds ratio 2.76 [95% CI 1.25- 6.11], P = 0.01) of proliferative retinopathy in the siblings of probands (61 of 182). The heritability of proliferative retinopathy was h(2) = 0.52 +/- 0.31 (P < 0.05).

CONCLUSIONS

We found a familial clustering of proliferative retinopathy in patients with type 1 diabetes. The observation cannot be accounted for by conventional risk factors, suggesting a genetic component in the pathogenesis of proliferative retinopathy in type 1 diabetes.

Multiple superoxide dismutase 1/splicing factor serine alanine 15 variants are associated with the development and progression of diabetic nephropathy: the Diabetes Control and Complications Trial/Epidemiology of Diabetes Interventions and Complications Genetics study

BACKGROUND

Despite familial clustering of nephropathy and retinopathy severity in type 1 diabetes, few gene variants have been consistently associated with these outcomes.

RESEARCH DESIGN AND METHODS

We performed an individual-based genetic association study with time to renal and retinal outcomes in 1,362 white probands with type 1 diabetes from the Diabetes Control and Complications Trial/Epidemiology of Diabetes Interventions and Complications (DCCT/EDIC) study. Specifically, we genotyped 1,411 SNPs that capture common variations in 212 candidate genes for long-term complications and analyzed them for association with the time from DCCT baseline to event for renal and retinal outcomes using multivariate Cox proportion hazards models. To address multiple testing and assist interpretation of the results, false discovery rate q values were calculated separately for each outcome.

RESULTS

We observed association between rs17880135 in the 3' region of superoxide dismutase 1 (SOD1) and the incidence of both severe nephropathy (hazard ratio [HR] 2.62 [95% CI 1.64-4.18], P = 5.6 x 10(-5), q = 0.06) and persistent microalbuminuria (1.82 [1.29-2.57], P = 6.4 x 10(-4), q = 0.46). Sequencing and fine-mapping identified additional SOD1 variants, including rs202446, rs9974610, and rs204732, which were also associated (P < 10(-3)) with persistent microalbuminuria, whereas rs17880135 and rs17881180 were similarly associated with the development of severe nephropathy. Attempts to replicate the findings in three cross-sectional case-control studies produced equivocal results. We observed no striking differences between risk genotypes in serum SOD activity, serum SOD1 mass, or SOD1 mRNA expression in lymphoblastoid cell lines.

CONCLUSIONS

Multiple variations in SOD1 are significantly associated with persistent microalbuminuria and severe nephropathy in the DCCT/EDIC study.