Relation of age-related cataract with obesity and obesity genes in an Asian population

Application of mendelian randomization in ocular diseases: a review

Ocular disorders can significantly lower patients' quality of life and impose an economic burden on families and society. However, for the majority of these diseases, their prevalence and mechanisms are yet unknown, making prevention, management, and therapy challenging. Although connections between exposure factors and diseases can be drawn through observational research, it is challenging to rule out the interference of confounding variables and reverse causation. Mendelian Randomization (MR), a method of research that combines genetics and epidemiology, has its advantage to solve this problem and thus has been extensively utilized in the etiological study of ophthalmic diseases. This paper reviews the implementation of MR in the research of ocular diseases and provides approaches for the investigation of related mechanisms as well as the intervention strategies.

Incidence of ocular pathology following bariatric surgery for with morbid obesity across a large United States National Database

BACKGROUND/OBJECTIVES

Bariatric surgery, as indicated for treatment of morbid obesity, has been studied in association with short term effects on ocular pathology. However, effects of surgery on postoperative disease incidence is largely unknown.

SUBJECTS/METHODS

In this retrospective cohort study, the TriNetX United States Collaborative Network national database, was queried for patients with an ICD-10 code for morbid obesity and a procedural code for bariatric surgery. Patients were propensity score matched across baseline demographics at the time of surgery and compared to those presenting with an ICD10 code for morbid obesity with no records of a procedural code for bariatric surgery, identifying 42,408 patients per cohort. New diagnoses or procedural codes found after the surgical index date for diabetic retinopathy, age-related macular degeneration, glaucoma, low vision, and blindness along with pertinent treatment metrics were monitored.

RESULTS

Bariatric surgery was found to be associated with reduced future risk of diabetic retinopathy (RR: 0.283; 95% CI: 0.252-0.319), macular edema (RR: 0.224; 95% CI: 0.170-0.297), vitreous hemorrhage (RR: 0.459; 95% CI: 0.323-0.653), ocular hypertension (RR: 0.387; 95% CI: 0.387-0.487), glaucoma (RR: 0.360; 95% CI: 0.326-0.399), use of ocular pressure lowering medications (RR: 0.565; 95% CI: 0.496-0.644), age-related macular degeneration (RR: 0.628; 95% CI: 0.447-0.882), cataract surgery (RR: 0.524; 95% CI: 0.448-0.612), and low vision and blindness (RR: 0.328; 95% CI: 0.294-0.365) compared to patients not surgically managed.

CONCLUSIONS

The present analysis comprising a large US cohort of patients suggests that bariatric surgery is associated with a decreased risk of future ocular morbidity and mortality.

Causal Associations of Glaucoma and Age-Related Macular Degeneration with Cataract: A Bidirectional Two-Sample Mendelian Randomisation Study

Common age-related eye disorders include glaucoma, cataract, and age-related macular degeneration (AMD); however, little is known about their relationship with age. This study investigated the potential causal relationship between glaucoma and AMD with cataract using genetic data from multi-ethnic populations. Single-nucleotide polymorphisms (SNPs) associated with exposure to cataract were selected as instrumental variables (IVs) from genome-wide association studies using meta-analysis data from BioBank Japan and UK Biobank. A bidirectional two-sample Mendelian randomisation (MR) study was conducted to assess the causal estimates using inverse variance weighted, MR-Egger, and MR pleiotropy residual sum and outlier tests. SNPs with (p < 5.0 × 10-8) were selected as IVs for cataract, primary open-angle glaucoma, and AMD. We found no causal effects of cataract on glaucoma or AMD (all p > 0.05). Furthermore, there were no causal effects of AMD on cataract (odds ratio [OR] = 1.02, p = 0.400). However, glaucoma had a substantial causal effect on cataract (OR = 1.14, p = 0.020). Our study found no evidence for a causal relationship of cataract on glaucoma or AMD and a casual effect of AMD on cataract. Nonetheless, glaucoma demonstrates a causal link with cataract formation, indicating the need for future investigations of age-related eye diseases.

Mendelian randomization analyses in ocular disease: a powerful approach to causal inference with human genetic data

Ophthalmic epidemiology is concerned with the prevalence, distribution and other factors relating to human eye disease. While observational studies cannot avoid confounding factors from interventions, human eye composition and structure are unique, thus, eye disease pathogenesis, which greatly impairs quality of life and visual health, remains to be fully explored. Notwithstanding, inheritance has had a vital role in ophthalmic disease. Mendelian randomization (MR) is an emerging method that uses genetic variations as instrumental variables (IVs) to avoid confounders and reverse causality issues; it reveals causal relationships between exposure and a range of eyes disorders. Thus far, many MR studies have identified potentially causal associations between lifestyles or biological exposures and eye diseases, thus providing opportunities for further mechanistic research, and interventional development. However, MR results/data must be interpreted based on comprehensive evidence, whereas MR applications in ophthalmic epidemiology have some limitations worth exploring. Here, we review key principles, assumptions and MR methods, summarise contemporary evidence from MR studies on eye disease and provide new ideas uncovering aetiology in ophthalmology.

Identification of the interactions between specific genetic polymorphisms and nutrient intake associated with general and abdominal obesity in middle-aged adults

BACKGROUND & AIMS

Comprehensive understanding of gene-diet interactions is necessary to establish proper dietary guidelines to prevent and manage general and abdominal obesity. We investigated the role of genetic variants and their interactions with general and abdominal obesity-associated nutrients using a largescale genome-wide association study of Korean adults.

METHODS

A total of 50,808 participants from a Korean genome and epidemiology study were included. Dietary intake was assessed using a food frequency questionnaire. Obesity was defined as a body mass index ≥25 kg/m². Abdominal obesity (AO) was defined as waist circumference ≥90 cm and 80 cm in males and females, respectively. Dietary nutrient intake was classified based on Korean Dietary Reference Intakes (DRIs). Odds ratios and 95% confidence intervals were calculated after adjusting for age, sex, exercise, smoking, alcohol drinking, total energy consumption, PC1, and PC2.

RESULTS

Among the individuals consuming fat (%) above DRI, carriers of Ca binding protein 39 (CAB39)- rs6722579 minor allele (A) have a higher risk of AO than those not carrying the SNP (odds ration [OR] = 3.73, p-value = 2.05e-07; interaction p-value = 1.80e-07). Among the individuals consuming vitamin C above DRI, carriers of carboxypeptidase Q (CPQ)- rs59465035 minor allele (T) have a lower risk of AO than those without that SNP (OR = 0.89, p-value = 1.44e-08; interaction p-value = 9.50e-06). The genetic association with obesity was stronger among individuals with a genetic variant rs4130113 near GHR gene region in those consume folate above DRI and with a genetic variant rs5760920 near CRYBB2 gene region in those consume vitamin B2 above DRI.

CONCLUSION

Our study results suggested that interactions of specific polymorphisms at loci and certain nutrients may influence obesity and abdominal obesity.

Genetically Mediated Lipid Metabolism and Risk of Insulin Resistance: Insights from Mendelian Randomization Studies

Dysregulated lipid metabolism, characterized by higher levels of circulating triglycerides, higher levels of small, low density lipoprotein, and accumulation of intracellular lipids, is linked to insulin resistance and related complications such as type 2 diabetes mellitus (T2DM) and cardiovascular diseases (CVD). Considering that various metabolic, genetic, and environmental factors are involved in the development of T2DM and CVD, the causalities of these diseases are often confounded. In recent years, Mendelian randomization (MR) studies coupling genetic data in population studies have revealed new insights into the risk factors influencing the development of CVD and T2DM. This review briefly conceptualizes MR and summarizes the genetic traits related to lipid metabolism by evaluating their effects on the indicators of insulin resistance based on the results of recent MR studies. The data from the MR study cases referred to in this review indicate that the causal associations between lipid status and insulin resistance in MR studies are not conclusive. Furthermore, available data on Asian ethnicities, including Korean, are very limited. More genome-wide association studies and MR studies on Asian populations should be conducted to identify Asian- or Korean-specific lipid traits in the development of insulin resistance and T2DM. The present review discusses certain studies that investigated genetic variants related to nutrient intake that can modify lipid metabolism outcomes. Up-to-date inferences on the causal association between lipids and insulin resistance using MR should be interpreted with caution because of several limitations, including pleiotropic effects and lack of information on genotype and ethnicity.

Cataracts Risk Factors and Comparison of Blood Glucose Levels in Diabetic and Non-Diabetic Patients towards the Occurrence of Cataracts

BACKGROUND:

Cataracts are a multifactorial systemic disease that causes opacity of the optical lens. One aetiology of cataracts is chronic hyperglycemia, usually caused by uncontrolled diabetes mellitus.

AIM:

The objective of this study is to identify risk factors of cataracts and to analyse if there is a significant difference in blood glucose values between diabetic patients with cataracts and non-diabetic patients with cataracts.

METHODS:

This was an analytical case-control study with a sample size of 140 patients that were obtained via consecutive sampling of medical records.

RESULTS:

This study found that age, high body mass index and hypertension were the dominant risk factors of cataracts. The mean value of blood glucose levels in diabetic patients with cataracts is 195.58 ± 63.9 and 109.7 ± 26.4 in non-diabetic patients with cataracts. There was a significant difference between the blood glucose values of diabetic patients with cataracts and non-diabetic patients with cataracts (p < 0.001). The dominant risk factors of cataracts were old age, high body mass index and hypertension. The majority of hyperglycemic patients belong in the group of diabetic patients with cataracts.

CONCLUSION:

Chronic hyperglycemia can increase a patient’s risk of cataracts.

Evaluating the associations between obesity and age-related cataract: a Mendelian randomization study

BACKGROUND

The obesity-cataract association has been inconsistently reported. The fat mass and obesity-related (FTO) single-nucleotide polymorphism (SNP) rs9939609 is a major SNP associated with obesity and has been used as an instrumental variable for obesity in a Mendelian randomization (MR) approach. An interaction between the FTO SNP and macronutrient intake for obesity was suggested previously.

OBJECTIVE

The aim of this study was to assess the associations between obesity and cataract, using FTO SNP rs9939609 as an instrumental variable in an MR approach, and explore interactions of this SNP with macronutrient intake in relation to risk of cataract in a population-based cohort.

METHODS

The Blue Mountains Eye Study (BMES) is a longitudinal population-based study of common eye disease. Of 3654 baseline participants of the BMES (1992-1994), 2334 (75.8% of survivors) and 1952 (76.7% of survivors) were followed 5 and 10 y later. During the 5-y follow-up, 1174 new participants were examined. Cumulative cataract was defined as the presence of cortical, nuclear, or posterior subcapsular (PSC) cataract at any visit, following the Wisconsin Cataract Grading System. Imputed dosage of the FTO SNP rs9939609 was used. Quintiles of macronutrient intake (carbohydrates, protein, fats) were derived from an FFQ. ORs and 95% CIs were estimated using multivariable-adjusted logistic regression models.

RESULTS

After multivariable adjustment, there were no associations between BMI and any cataract types in MR models using rs9939609 as an instrumental variable. However, an interaction between rs9939609 and protein intake for PSC cataract risk was suggested (P = 0.03). In analyses stratified by quintiles of protein intake, each minor allele of rs9939609 was associated with increased odds of PSC (OR: 2.14; 95% CI: 1.27, 3.60) in the lowest quintile subgroup only.

CONCLUSIONS

Obesity was not causally associated with age-related cataract. However, among persons in the lowest quintile of protein intake, obesity may be associated with PSC cataract.

Association of OGG1 and MTHFR polymorphisms with age-related cataract: A systematic review and meta-analysis

PURPOSE

To discern and confirm genetic biomarkers that help identify populations at high risk for age-related cataract (ARC).

METHODS

A literature search was performed in the PubMed, Web of Science and China National Knowledge Internet databases for genetic association studies published before June 26, 2016 regarding ARC susceptibility. All genetic polymorphisms reported were systematically reviewed, followed by extraction of candidate genes/loci with sufficient genotype data in ≥3 studies for the meta-analysis. A random/fixed-effects model was used to calculate the pooled odds ratios and 95% confidence intervals to evaluate the associations considering multiple genetic models. Sensitivity analysis was also performed.

RESULTS

A total of 144 polymorphisms in 36 genes were reported in the 61 previous genetic association studies. Thereby, three polymorphisms of two genes (8-oxoguanine DNA glycosylase-1 [OGG1]; methylenetetrahydrofolate reductase NADPH [MTHFR]) in eight studies were included in the meta-analysis. Regarding the OGG1-rs1052133, the GG (OR = 1.925; 95%CI, 1.181-3.136; p = 0.009) and CG (OR = 1.384; 95%CI, 1.171-1.636; p<0.001) genotypes indicated higher risk of ARC. For the MTHFR gene, the CC+TT genotype of rs1801133 might be protective (OR, 0.838; 95%CI, 0.710-0.989; p = 0.036), whereas the AA+CC genotype of rs1801131 indicated increased risk for the mixed subtype (OR = 1.517; 95%CI, 1.113-2.067; p = 0.008).

CONCLUSIONS

Polymorphisms of OGG1 and MTHFR genes are associated with ARC susceptibility and may help identify populations at high risk for ARC.

Causal inference in obesity research

Obesity is a risk factor for a plethora of severe morbidities and premature death. Most supporting evidence comes from observational studies that are prone to chance, bias and confounding. Even data on the protective effects of weight loss from randomized controlled trials will be susceptible to confounding and bias if treatment assignment cannot be masked, which is usually the case with lifestyle and surgical interventions. Thus, whilst obesity is widely considered the major modifiable risk factor for many chronic diseases, its causes and consequences are often difficult to determine. Addressing this is important, as the prevention and treatment of any disease requires that interventions focus on causal risk factors. Disease prediction, although not dependent on knowing the causes, is nevertheless enhanced by such knowledge. Here, we provide an overview of some of the barriers to causal inference in obesity research and discuss analytical approaches, such as Mendelian randomization, that can help to overcome these obstacles. In a systematic review of the literature in this field, we found: (i) probable causal relationships between adiposity and bone health/disease, cancers (colorectal, lung and kidney cancers), cardiometabolic traits (blood pressure, fasting insulin, inflammatory markers and lipids), uric acid concentrations, coronary heart disease and venous thrombosis (in the presence of pulmonary embolism), (ii) possible causal relationships between adiposity and gray matter volume, depression and common mental disorders, oesophageal cancer, macroalbuminuria, end-stage renal disease, diabetic kidney disease, nuclear cataract and gall stone disease, and (iii) no evidence for causal relationships between adiposity and Alzheimer's disease, pancreatic cancer, venous thrombosis (in the absence of pulmonary embolism), liver function and periodontitis.

Selecting instruments for Mendelian randomization in the wake of genome-wide association studies

Mendelian randomization (MR) studies typically assess the pathogenic relevance of environmental exposures or disease biomarkers, using genetic variants that instrument these exposures. The approach is gaining popularity-our systematic review reveals a greater than 10-fold increase in MR studies published between 2004 and 2015. When the MR paradigm was first proposed, few biomarker- or exposure-related genetic variants were known, most having been identified by candidate gene studies. However, genome-wide association studies (GWAS) are now providing a rich source of potential instruments for MR analysis. Many early reviews covering the concept, applications and analytical aspects of the MR technique preceded the surge in GWAS, and thus the question of how best to select instruments for MR studies from the now extensive pool of available variants has received insufficient attention. Here we focus on the most common category of MR studies-those concerning disease biomarkers. We consider how the selection of instruments for MR analysis from GWAS requires consideration of: the assumptions underlying the MR approach; the biology of the biomarker; the genome-wide distribution, frequency and effect size of biomarker-associated variants (the genetic architecture); and the specificity of the genetic associations. Based on this, we develop guidance that may help investigators to plan and readers interpret MR studies.

Childbearing May Increase the Risk of Nondiabetic Cataract in Chinese Women's Old Age

Backgrounds. Ocular changes may arise during pregnancy and after childbirth, but very few studies have reported the association between childbearing and cataract among older adults. Methods. 14,292 individuals aged 60+ years were recruited in Xiamen, China, in 2013. Physician-diagnosed cataract and diabetes status were assessed by a self-reported questionnaire. Childbearing status was measured by number of children (NOC). Structural equation modeling (SEM) analysis was conducted to examine the relationships among NOC, diabetes, and cataract. Gender-specific logistic models regressing nondiabetic cataract on NOC were performed by adjusting some covariates. Results. 14,119 participants had complete data, of whom 5.01% suffered from cataract, with higher prevalence in women than men (6.41% versus 3.51%). Estimates of SEM models for women suggested that both NOC and diabetes were risk factors for cataract and that no correlation existed between NOC and diabetes. Women who had one or more children faced roughly 2–4 times higher risk of nondiabetic cataract than their childless counterparts (OR [95% CI] = 3.88 [1.24, 17.71], 3.21 [1.04, 14.52], 4.32 [1.42, 19.44], 4.41 [1.46, 19.74], and 3.98 [1.28, 18.10] for having 1, 2, 3, 4-5, and 6 or more children, resp.). Conclusions. Childbearing may increase the risk of nondiabetic cataract in Chinese women's older age.

Mendelian randomization studies: a review of the approaches used and the quality of reporting

BACKGROUND

Mendelian randomization (MR) studies investigate the effect of genetic variation in levels of an exposure on an outcome, thereby using genetic variation as an instrumental variable (IV). We provide a meta-epidemiological overview of the methodological approaches used in MR studies, and evaluate the discussion of MR assumptions and reporting of statistical methods.

METHODS

We searched PubMed, Medline, Embase and Web of Science for MR studies up to December 2013. We assessed (i) the MR approach used; (ii) whether the plausibility of MR assumptions was discussed; and (iii) whether the statistical methods used were reported adequately.

RESULTS

Of 99 studies using data from one study population, 32 used genetic information as a proxy for the exposure without further estimation, 44 performed a formal IV analysis, 7 compared the observed with the expected genotype-outcome association, and 1 used both the latter two approaches. The 80 studies using data from multiple study populations used many different approaches to combine the data; 52 of these studies used some form of IV analysis; 44% of studies discussed the plausibility of all three MR assumptions in their study. Statistical methods used for IV analysis were insufficiently described in 14% of studies.

CONCLUSIONS

Most MR studies either use the genotype as a proxy for exposure without further estimation or perform an IV analysis. The discussion of underlying assumptions and reporting of statistical methods for IV analysis are frequently insufficient. Studies using data from multiple study populations are further complicated by the combination of data or estimates. We provide a checklist for the reporting of MR studies.

Overweight, obesity, and age-related cataract: a meta-analysis

PURPOSE

To examine the longitudinal association of overweight/obesity with age-related cataract.

METHODS

A systematic review of the literature was performed using PubMed and Embase from their inception until June 2013 for prospective data on body mass index categories identical or similar to the World Health Organization-recommended classifications of body weight and age-related cataract including nuclear, cortical, and posterior subcapsular (PSC) cataract. Meta-analyses were conducted using random-effects models with results reported as adjusted relative risks (RRs).

RESULTS

A total of 163,013 subjects aged 40 to 84 years from six prospective cohort studies were included in the meta-analysis. Obesity was associated with an increasing risk of nuclear cataract (pooled RR, 1.12; 95% confidence interval [CI], 1.02 to 1.25), cortical cataract (pooled RR, 1.34; 95% CI, 1.07 to 1.66), and PSC cataract (pooled RR, 1.52; 95% CI, 1.31 to 1.77). Overweight was only associated with an increasing risk of PSC cataract (pooled RR, 1.23; 95% CI, 1.09 to 1.40).

CONCLUSIONS

The longitudinal associations of obesity with incident age-related cataract are confirmed by the findings in this six-study meta-analysis. The association of obesity with PSC cataract is stronger than that with nuclear or cortical cataract. Randomized control trials are warranted to examine the effectiveness and cost-effectiveness of weight reduction in obese populations to decrease the risk of age-related cataract.

Association between cataract and the degree of obesity

PURPOSE

To investigate the association between the degree of obesity and cataract.

METHODS

We examined 3248 subjects (1421 men and 1827 women) aged 50 years and older who did not have a previous cataract operation. Data were derived from the fourth Korea National Health and Nutrition Examination Survey (2009). Cataract was evaluated by using Lens Opacities Classification System III. Body mass index was categorized into four groups (underweight, <18.5 kg/m(2); normal weight, 18.5 to 22.9 kg/m(2); overweight, 23.0 to 24.9 kg/m(2); and obese, ≥25.0 kg/m(2)). Association between the degree of obesity and cataract was evaluated using logistic regression analyses with adjustments of age, the total pack-years of cigarettes smoked, the amount of pure alcohol (g) consumed per day, daily time spent in vigorous physical activity, diabetes mellitus, sunlight exposure, education level, and income.

RESULTS

Compared with the normal-weight group, the overweight group had significantly lower risk of any type of cataract (odds ratio, 0.70; 95% confidence interval, 0.50 to 0.97) in men and (odds ratio, 0.70; 95% confidence interval, 0.51 to 0.97) in women in the multiple logistic regression analyses. We could not find any unusual lifestyles or metabolic risks for explaining this low cataract prevalence in the overweight groups. However, nutrient intakes (e.g., vitamin B2, niacin, vitamin C, and vitamin A) were highest in the overweight group.

CONCLUSIONS

The overweight group had significantly lower risk of cataract formation than the normal-weight group in Korean population.

Gender difference in the association of metabolic syndrome and its components with age-related cataract: the Korea National Health and Nutrition Examination Survey 2008-2010

Purpose

To explore the relationship of the metabolic syndrome (MetS) and its components with age-related cataract in a representative Korean population.

Methods

We analyzed the data from the Korea National Health and Nutrition Examination Surveys (2008–2010). A total of 11,076 adults (4,811 men and 6,265 women) aged 40 and over who completed ophthalmologic examination were evaluated. Cataract was defined as the presence of cortical, nuclear, anterior (sub)capsular or posterior subcapsular cataract, from slit-lamp examination or previous cataract surgery. MetS was defined according to the Joint Interim Statement proposed in 2009 from the International Diabetes Federation and the American Heart Association/National Heart, Lung, and Blood Institute.

Results

The prevalence of cataract and MetS in this population was 39.4% (37.1% for men and 41.6% for women) and 38.5% (37.6% for men and 39.4% for women), respectively. Cataract prevalence tended to increase with the number of MetS components in both genders (both P< 0.001). After being controlled for confounders, however, MetS was significantly associated with cataract only in women (adjusted odds ratio (aOR), 1.24; 95% confidence interval (CI), 1.02–1.50]. Reduced HDL cholesterol, elevated fasting glucose, and elevated triglycerides were also significantly associated with cataract in women (aOR, 95% CI; 1.27 (1.07–1.50), 1.23 (1.01–1.50), and 1.26 (1.04–1.52), respectively). In the subgroup analysis for cataract subtype, MetS and reduced HDL cholesterol were significantly associated with nuclear cataract in women (aOR, 95% CI; 1.25 (1.07–1.55) and 1.25 (1.03–1.52), respectively). However, such associations were not found in men.

Conclusions

Our results suggest that MetS and its components appear to be associated with age-related cataract only among Korean women, especially in nuclear cataract.

Cigarette smoking, body mass index associated with the risks of age-related cataract in male patients in northeast China

AIM

To determine the association between cigarettes smoking, body mass index (BMI) and the risk of age-related cataract (ARC) in middle-aged and elderly men in Northeast China.

METHODS

A hospital-based case control study was conducted. Cases (n =362) were men who had surgically treated ARC, 45-85 years old; controls frequency-matched (n =362) were men who had been admitted to the same hospital as cases for other diseases not related with eye diseases. Cases and controls were matched with 1:1. The cases and controls were interviewed during their hospital stay, using a structured interviewer-administrated questionnaire that included information on sociodemographic characteristics, socioeconomic, lifestyle habits (tobacco smoking and alcohol consumption, etc.), anthropometric measures, personal medical history, and family history of ARC in first-degree relatives, and simultaneously BMI was calculated. The odds ratios (OR) and 95% confidence intervals (CI) of ARC were estimated using multiple logistic regression models.

RESULTS

After adjusting for age and multiple potential confounders, higher BMI was associated with an increased risk of ARC. Cigarette smoking, years smoking or moderate cigarette smoking (1-29 cigarettes per day) had no relation with the risk of ARC (P>0.05), although patients smoking ≥30 cigarettes per day had an elevated risk of ARC as compared with the non-smokers (OR=1.55, 95% CI; 1.16-2.85, P=0.026). Higher BMI was associated with an increased risk of ARC. Both overweight and obesity was associated with an obviously increased risk for surgically ARC (OR=1.55, 95% CI: 1.02-1.98, P=0.015 and OR=1.71, 95% CI: 1.32-2.39, P=0.013 respectively) compared to normal BMI. Then participants were grouped into quartiles of BMI (Q1 to Q4), compared to controls in the lowest quartile, the OR for cases in the highest quartile of BMI was 1.54 (OR=1.54, 95% CI: 1.08-2.46, P=0.022). The results of univariate analysis showed cigarette smoking was not associated with ARC formation for men with lower or normal BMI (P>0.05). Compared to the non-smokers, for men of overweight or obesity, cigarette smoking was associated with a significantly increased risk for surgically ARC (OR=2.00, 95% CI: 1.49-6.65, P=0.003 and OR=1.66, 95% CI: 1.63-13.21, P=0.002 respectively). Similarly, smokers in the highest quartile of BMI had approximately 1.5 times the risk of ARC as non-smokers in the lowest quartile (OR=1.46, 95% CI: 1.06-5.29, P<0.001). Followed multivariate models revealed that the association had never changed.

CONCLUSION

Current cigarette smoking is positively related to ARC only among those who smoking 30 or more cigarettes per day. For men who are both overweight and obesity, cigarette smoking is associated with a significantly increased risk for ARC.

Common genetic link between metabolic syndrome components and senile cataract

Relationship between cataract and metabolic syndrome (MetS) is well established, but genetic link remains to be explored. D2S439 at 2q37 linked with QTL controlling visceral fat was investigated for its association with senile cataract. Two hundred and twenty-seven subjects including 119 cataract cases were genotyped for D2S439, tetra nucleotide repeat marker. Statistical tools assessed the association of marker's allele with anthropometric, clinical and oxidation stress parameters. Cases with longer allele ≥ (CTAT)(12) repeats, differed significantly from controls (0.77 vs. 0.58, p < 0.0001). Cases with at least one longer allele had higher waist circumference (50% vs. 15%, p = 0.0090), hyper-triglyceridemia (28% vs. 11%), hypo-HDL cholesterolemia (80% vs. 74%) and high diastolic blood pressure (37% vs. 26%) when compared to cases bearing the shorter allele. Cataract subjects with at least one longer allele had significantly raised lipid peroxidation levels (p = 0.0095) and showed an increased risk for cataract (OR = 5.86, CI(95%) = 1.49-23.11, p = 0.0114) after controlling for dependent variables. This exploratory study suggests that presence of even a single longer allele of D2S439 is associated with both cataract and MetS components in Asian Indians, unraveling the existence of a shared genetic locus.

Association between gap junction protein-alpha 8 polymorphisms and age-related cataract

GJA8 plays an important role in lens growth and transparency. Therefore, we hypothesized that two single nucleotide polymorphisms (SNPs) in GJA8 might be associated with age-related cataract. We investigated the SNPs rs1495960 and rs9437983 using polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) and DNA sequencing, in 96 age-related cataract patients, and 208 gender- and age-matched healthy controls. No significant differences between cases and controls were seen in genotype or allele distributions of rs1495960 (P > 0.05). The allele distribution of rs9437983 was different between cases and controls, but no difference was detected in its genotype distribution. Cataract patients had a significantly lower G-G haplotype frequency (4.9% vs. 15.5%, P = 0.0001), and a significantly higher G-A haplotype frequency (45.6% vs. 36.4%, P = 0.030) than controls. Limiting to nuclear cataract cases significantly increased the differences between cases and controls for G-G and G-A haplotypes. These results support that the GJA8 gene may be a novel susceptibility gene for age-related cataracts.

Usefulness of Mendelian randomization in observational epidemiology

Mendelian randomization refers to the random allocation of alleles at the time of gamete formation. In observational epidemiology, this refers to the use of genetic variants to estimate a causal effect between a modifiable risk factor and an outcome of interest. In this review, we recall the principles of a "Mendelian randomization" approach in observational epidemiology, which is based on the technique of instrumental variables; we provide simulations and an example based on real data to demonstrate its implications; we present the results of a systematic search on original articles having used this approach; and we discuss some limitations of this approach in view of what has been found so far.