Healthy Twin Study Research Articles

Blood Triglycerides Levels and Dietary Carbohydrate Indices in Healthy Koreans.

Objectives:Previous studies have obtained conflicting findings regarding possible associations between indices measuring carbohydrate intake and dyslipidemia, which is an established risk factor of coronary heart disease. In the present study, we examined cross-sectional associations between carbohydrate indices, including the dietary glycemic index (GI), glycemic load (GL), total amount of carbohydrates, and the percentage of energy from carbohydrates, and a range of blood lipid parameters.Methods:This study included 1530 participants (554 men and 976 women) from 246 families within the Healthy Twin Study. We analyzed the associations using a generalized linear mixed model to control for familial relationships.Results:Levels of the Apo B were inversely associated with dietary GI, GL, and the amount of carbohydrate intake for men, but these relationships were not significant when fat-adjusted values of the carbohydrate indices were used. Triglyceride levels were positively associated with dietary GI and GL in women, and this pattern was more notable in overweight participants (body mass index [BMI] ≥25 kg/m2). However, total, low-density lipoprotein and high-density lipoprotein cholesterol levels were not significantly related with carbohydrate intake overall.Conclusions:Of the blood lipid parameters we investigated, only triglyceride levels were positively related with dietary carbohydrate indices among women participants in the Healthy Twin Study, with an interactive role observed for BMI. However, these associations were not observed in men, suggesting that the association between blood lipid levels and carbohydrate intake depends on the type of lipid, specific carbohydrate indices, gender, and BMI.

Multiple susceptibility loci at chromosome 11q23.3 are associated with plasma triglyceride in East Asians

Genetic studies of plasma TG levels have identified associations with multiple candidate loci on chromosome11q23.3, which harbors a number of genes, including BUD13, ZNF259, and APOA5-A4-C3-A1. This study aimed to examine whether these multiple candidate genes on the 11q23.3 regions exert independent effects on TG levels or whether their effects are confounded by linkage disequilibrium (LD). We performed a genome-wide association study and consequent fine-mapping analyses on TG levels in two Korean population-based cohorts: the Korea Association Resource study (n = 8,223) and the Healthy Twin study (n = 1,735). A total of 301 loci reached genome-wide significance level in pooled analysis, including 10 SNPs with weak LD (r(2) < 0.06) clustered on 11q23.3: ApoA5 (rs651821, rs2075291); ZNF259 (rs964184, rs603446); BUD13 (rs11216126); Apoa4 (rs7396851); SIK3 (rs12292858); PCSK7 (rs199890178); PAFAH1B2 (rs12420127), and SIDT2 (rs2269399). When the inter-dependence between alleles was examined using conditional models, five loci on BUD13, ZNF259, and ApoA5 showed possible independent associations. A haplotype analysis using five SNPs revealed both hyper- and hypotriglyceridemic haplotypes, which are relatively common in Koreans (haplotype frequency 0.08-0.22). Our findings suggest the presence of multiple functional loci on 11q23.3, which might exert their effects on plasma TG level independently or through complex interactions between functional loci.

Aging Trajectories in Different Body Systems Share Common Environmental Etiology: The Healthy Aging Twin Study (HATS).

Little is known about the extent to which aging trajectories of different body systems share common sources of variance. We here present a large twin study investigating the trajectories of change in five systems: cardiovascular, respiratory, skeletal, morphometric, and metabolic. Longitudinal clinical data were collected on 3,508 female twins in the TwinsUK registry (complete pairs:740 monozygotic (MZ), 986 dizygotic (DZ), mean age at entry 48.9 ± 10.4, range 18-75 years; mean follow-up 10.2 ± 2.8 years, range 4-17.8 years). Panel data on multiple age-related variables were used to estimate biological ages for each individual at each time point, in linear mixed effects models. A weighted average approach was used to combine variables within predefined body system groups. Aging trajectories for each system in each individual were then constructed using linear modeling. Multivariate structural equation modeling of these aging trajectories showed low genetic effects (heritability), ranging from 2% in metabolic aging to 22% in cardiovascular aging. However, we found a significant effect of shared environmental factors on the variations in aging trajectories in cardiovascular (54%), skeletal (34%), morphometric (53%), and metabolic systems (53%). The remainder was due to environmental factors unique to each individual plus error. Multivariate Cholesky decomposition showed that among aging trajectories for various body systems there were significant and substantial correlations between the unique environmental latent factors as well as shared environmental factors. However, there was no evidence for a single common factor for aging. This study, the first of its kind in aging, suggests that diverse organ systems share non-genetic sources of variance for aging trajectories. Confirmatory studies are needed using population-based twin cohorts and alternative methods of handling missing data.

Genetic effects on serum testosterone and sex hormone-binding globulin in men: a Korean twin and family study.

We conducted a community-based cross-sectional study to evaluate the role of genetics in determining the individual difference in total testosterone and sex hormone-binding globulin levels. Study participants comprised 730 Korean men consisting of 142 pairs of monozygotic twins, 191 pairs of siblings, and 259 father-offspring pairs from 270 families who participated in the Healthy Twin study. Serum concentration of total testosterone and sex hormone-binding globulin were measured by chemiluminescence immunoassay, and free testosterone and bioavailable testosterone were calculated using Vermeulen's method. Quantitative genetic analysis based on a variance decomposition model showed that the heritability of total testosterone, free testosterone, bioavailable testosterone, and sex hormone-binding globulin were 0.56, 0.45, 0.44, and 0.69, respectively after accounting for age and body mass index. Proportions of variance explained by age and body mass index varied across different traits, from 8% for total testosterone to 31% for sex hormone-binding globulin. Bivariate analysis showed a high degree of additive genetic correlation (ρG = 0.67) and a moderate degree of individual-specific environmental correlation (ρE = 0.42) between total testosterone and sex hormone-binding globulin. The findings confirmed the important role of genetics in determining the individually different levels of testosterone and sex hormone-binding globulin during adulthood in Korean men as found in non-Asian populations, which may suggest that common biologic control for determining testosterone level directly or indirectly through binding protein are largely shared among different populations.

Family-Based Association Study of Pulmonary Function in a Population in Northeast Asia.

The spirometric measurement of pulmonary function by measuring the forced expiratory volume in one second (FEV1) is a heritable trait that reflects the physiological condition of the lung and airways. Genome-wide linkage and association studies have identified a number of genes and genetic loci associated with pulmonary function. However, limited numbers of studies have been reported for Asian populations. In this study, we aimed to investigate genetic evidence of pulmonary function in a population in northeast Asia. We conducted a family-based association test with 706 GENDISCAN study participants from 72 Mongolian families to determine candidate genetic determinants of pulmonary function. For the replication, we chose seven candidate single nucleotide polymorphisms (SNPs) from the 5 loci, and tested 1062 SNPs for association with FEV1 from 2,729 subjects of the Korea Healthy Twin study. We identified TMEM132C as a potential candidate gene at 12q24.3, which is a previously reported locus of asthma and spirometric indices. We also found two adjacent candidate genes (UNC93A and TTLL2) in the 6q27 region, which has been previously identified as a pulmonary function locus in the Framingham cohort study. Our findings suggest that novel candidate genes (TMEM132C, UNC93A and TTLL2) in two different regions are associated with pulmonary function in a population in northeast Asia.

The Heritability of Corneal and Ocular Higher-Order Aberrations in Koreans: The Healthy Twin Study

To estimate the genetic influences on corneal and ocular higher-order aberrations (HoAs) in the Korean population. This was a prospective, family-based twin cohort study. A total of 1272 adult twins and their family members, who were part of the Korean Healthy Twin Study from 2007-2011, were included. Corneal and ocular HoAs were measured. The genetic influences on HoAs were investigated by using variance-component methods after adjusting for age and sex. Narrow-sense heritability was calculated. Intraclass correlation coefficients (ICCs) were used to calculate degrees of resemblance among different types of family relationships. A total of 269 monozygotic (MZ) twin pairs (including 176 MZ twin pairs and 93 orphan twins), 50 dizygotic (DZ) twin pairs (including 38 DZ twin pairs and 12 orphan twins), and 739 adult first-degree relatives of twins in 358 families were included. For more than half of corneal HoAs, the narrow-sense heritability estimates were not significant. Horizontal coma was highly heritable among corneal HoAs. The ICCs of horizontal coma from MZ twin pairs, pooled first-degree pairs, and spouse pairs were 0.41, 0.05, and 0.00, respectively. Among ocular HoAs, the estimated narrow-sense heritability of SA was 0.71 with the highest estimates. The ICCs of spherical aberration (SA) from MZ twins, pooled first-degree pairs, and spouse pairs were 0.76, 0.25, and 0.06, respectively. Both corneal and ocular HoAs demonstrated smaller heritability. Corneal HoAs showed low heritability, suggesting individual environmental factors explain most of the variance of these HoAs in the Korean population. Ocular HoAs were moderately heritable.

Adiponectin Levels and Longitudinal Changes in Metabolic Syndrome: The Healthy Twin Study.

We investigated the association of plasma adiponectin levels with longitudinal changes in metabolic syndrome and the metabolic syndrome-related traits [insulin and homeostasis model assessment of insulin resistance (HOMA-IR)], as well as their genetic and environmental correlations. A total of 1030 Koreans (380 men and 650 women; 44.0 ± 12.7 years old) without diabetes of the Healthy Twin Study visited at baseline (2005-2010) and returned for a follow-up examination 3.7 ± 1.2 years later. Baseline plasma adiponectin, metabolic syndrome components [waist circumference (WC), glucose, blood pressure, high-density lipoprotein cholesterol (HDL-C), and triglycerides (TGs)] and metabolic syndrome-related traits were measured at baseline and follow-up. After adjusting for age, sex, smoking, alcohol consumption, physical activity, caloric intake, education level, body mass index (BMI), family history of diabetes, and changes in BMI, 1 standard deviation increment in baseline adiponectin levels was associated with 38-63% lower odds of incident and persistent metabolic syndrome. After additionally adjusting for the baseline levels of each trait, baseline adiponectin levels were inversely associated with WC, blood pressure, insulin, HOMA-IR, and TGs values at follow-up. After adjusting for age, sex, and baseline values of each trait or sum of metabolic syndrome components, baseline adiponectin levels exhibited significantly inverse genetic and environmental correlations with insulin, HOMA-IR, and HDL-C values and the sum of metabolic syndrome components at follow-up. High adiponectin levels reduce the risk of developing metabolic syndrome and having persistent metabolic syndrome and increase of metabolic syndrome-related traits over time. These associations may be explained by pleiotropic genetic mechanisms.

Longitudinal relationships of metabolic syndrome and obesity with kidney function: Healthy Twin Study.

To examine cross-sectional and longitudinal relationships, including genetic and environmental correlations, of metabolic syndrome (MetS) and obesity with kidney function. Subjects were 3,437 Korean adults of the Healthy Twin Study for cross-sectional relationships and 1,881 participants for longitudinal relationships (follow-up interval 3.7 ± 1.4 years). Obesity (body mass index ≥ 25 vs. <25 kg/m(2)), MetS, and chronic kidney disease [CKD; estimated glomerular filtration rate (eGFR) (<60 mL/min/1.73 m(2) using the modification of diet in renal disease study equation)] were categorized at baseline and follow-up. The prevalence and incidence of chronic kidney disease were 2.5 and 3.3 %, respectively. Compared to individuals without obesity and MetS, prevalent CKD was associated with MetS regardless of weight status [adjusted odds ratio (AOR) 4.19 for those with MetS but without obesity; AOR 4.63 for those with MetS and obesity]. Incident CKD was associated with obesity regardless of baseline metabolic status (AOR 2.03 for those with obesity but without MetS; AOR 2.85 for those with obesity and MetS). MetS at follow-up was associated with incident CKD regardless of baseline MetS (AOR 2.42-2.52). Sex-adjusted bivariate analyses show inverse environmental correlations of the number of MetS components and BMI at baseline, with eGFR at baseline and follow-up (ρ E, -0.26 to -0.42, P < 0.001). MetS predicts prevalent CKD regardless of obesity, and obesity predicts incident CKD regardless of baseline MetS. Incident CKD is also associated with MetS at follow-up regardless of baseline MetS. These associations appear to be explained by shared environmental factors.

Epidemiology and genetics of hypodontia and microdontia: a study of twin families.

To identify genetic and environmental factors contributing to hypodontia and microdontia by using Korean twin family data. A total of 1267 individuals (525 men and 742 women; 180 monozygotic twins [MZ] and 43 dizygotic twins [DZ] from 282 families) underwent an oral examination as part of the Healthy Twin Study in Korea. Dental anomalies classified as hypodontia or microdontia were diagnosed using radiographs and clinical examinations. In order to estimate genetic contributions to dental anomalies, we estimated the pairwise concordance rate (PCR), recurrence risk ratio (RRR), and heritability (h2). The prevalence of hypodontia and microdontia was 3.55% and 3.00%, respectively. MZ had the highest PCR and RRR (13.0-15.3). The PCR and RRR values for both anomalies were much higher for DZ (5.0-11.9) than for siblings (1.4-2.6), despite the fact that DZ pairs and sibling pairs share 50% genetic identity. Further genetic analysis revealed both an additive genetic effect (0.38 when hypodontia and microdontia were pooled) and a strong "twin effect" (0.52 when hypodontia and microdontia were pooled). This twin-based study revealed that the formation of dental anomalies is affected by both genetic and environmental factors, and that the impact of these factors varies according to the specific dental anomaly.

Genetic and environmental relationships of metabolic and weight phenotypes to metabolic syndrome and diabetes: the healthy twin study.

We aimed to examine the relationships, including genetic and environmental correlations, between metabolic and weight phenotypes and factors related to diabetes and metabolic syndrome. Participants of the Healthy Twin Study without diabetes (n=2687; 895 monozygotic and 204 dizygotic twins, and 1588 nontwin family members; mean age, 42.5±13.1 years) were stratified according to body mass index (BMI) (<25 vs. ≥25 kg/m(2)) and metabolic syndrome categories at baseline. The metabolic traits, namely diabetes and metabolic syndrome, metabolic syndrome components, glycated hemoglobin (HbA1c) level, and homeostasis model assessment of insulin resistance (HOMA-IR), were assessed after 2.5±2.1 years. In a multivariate-adjusted model, those who had metabolic syndrome or overweight phenotypes at baseline were more likely to have higher HbA1C and HOMA-IR levels and abnormal metabolic syndrome components at follow-up as compared to the metabolically healthy normal weight subgroup. The incidence of diabetes was 4.4-fold higher in the metabolically unhealthy but normal weight individuals and 3.3-fold higher in the metabolically unhealthy and overweight individuals as compared with the metabolically healthy normal weight individuals. The heritability of the metabolic syndrome/weight phenotypes was 0.40±0.03. Significant genetic and environmental correlations were observed between the metabolic syndrome/weight phenotypes at baseline and the metabolic traits at follow-up, except for incident diabetes, which only had a significant common genetic sharing with the baseline phenotypes. The genetic and environmental relationships between the metabolic and weight phenotypes at baseline and the metabolic traits at follow-up suggest pleiotropic genetic mechanisms and the crucial role of lifestyle and behavioral factors.

Genetic and environmental relationships between change in weight and insulin resistance: the Healthy Twin Study.

We aimed to investigate the association between weight change from 20 years of age and insulin resistance (IR), and genetic and environmental relationships between these traits. In 594 Korean twins and family members (209 men, 385 women, 44.0 ± 10.8 years old), the percentage of weight change was calculated using self-reported body weight at 20 years of age and currently measured bodyweight. IR traits were assessed using fasting plasma glucose and insulin, the homeostasis model assessment of IR index (HOMA-IR), and the quantitative insulin sensitivity check index (QUICKI). Linear mixed analysis was applied after adjusting for household, body mass index (BMI) at the age of 20 years, age, sex, alcohol, smoking, physical activity, and caloric intake. Heritabilities and genetic and environmental correlations were estimated after adjusting for covariates. In 55 monozygotic twin pairs discordant for HOMA-IR level by >0.3, a conditional logistic regression analysis was conducted regarding weight change. Increases in glucose, insulin, and HOMA-IR and a decrease in QUICKI were associated with a higher percentage of weight change (p < .05). Estimated heritabilities for IR traits were 0.401-0.606 (p < .001). In cross-trait relationships, environmental correlations were -0.43-0.42 (p < .05 for all IR), while genetic correlations were -0.27-0.27 (p < .05 for QUICKI, insulin, and HOMA-IR). In 55 pairs of monozygotic twins, the odds ratio (95% confidence interval) for having a higher level of HOMA-IR was 1.10 (1.03-1.17) with 1% increase in weight change since 20 years old, after adjusting for lifestyle-related factors. In conclusion, both genetic and environmental influences played significant roles in the positive association between weight change from 20 years of age and IR.

Genetic Influences on Hallux Valgus in Koreans: The Healthy Twin Study

Hallux valgus (HV) is a common foot deformity of multifactorial etiology, but knowledge about the relative importance of genetics and environments on HV has been limited. In order to estimate genetic influences on HV, 1,265 adults, including 175 monozygotic twin (MZ) pairs, 31 dizygotic twin (DZ) pairs, and 853 first-degree singleton family members of the twins were included from the Healthy Twin study, a population-based twin-family cohort in Korea. All participants underwent foot examination and weight-bearing radiographic assessment (anterior-posterior and lateral) in addition to a general health survey. Of the subjects, 208 (16.4%) were classified as HV (as HV angle >20°). The genetic influence on HV was estimated to be substantial; the heritability of HV was 0.51 (95% CI 0.42-0.59) and the heritability of HV angle was 0.47 (0.38-0.56), while contributions from shared environmental effects were negligible. These findings suggest that genetic factors play an important role in determining HV deformity.

Eating behaviors and weight over time in a prospective study: the Healthy Twin Study.

We examined the relationships of combined initial restrained and external/emotional eating with initial BMI and change in weight and these subscales over time. BMI and the Dutch Eating Behavior Questionnaire were twicemeasured in 1361 Korean twins and families (482 men, 879 women) over a period of 2.7±0.9 years. Subjects were classified by combination of initial sex-specific restrained and external (or emotional) eating tertiles. Linear mixed models were performed after adjusting for confounders at baseline (household, sibling relations, sex, age, education level, smoking, alcohol use, energy intake, physical activity, and medical history). In adjusted models, initial BMI increased with increasing tertiles of initial restrained eating across initial external/emotional eating tertiles. Weight was less likely to increase over time with increasing tertiles of initial restrained eating in the lowest external eating tertile and middle tertile of emotional eating at baseline. Subscale scores decreased over time with increasing tertiles of corresponding subscales at baseline. These findings suggest that high dietary restraint and external/emotional eating may indicate concurrent high BMI and attenuated weight gain and decreases in corresponding subscales over time.

Genome‐wide association studies identify locus on 6p21 influencing lung function in the Korean population

Loss of lung function is an important chronic obstructive pulmonary disease phenotype and decreased forced expiratory volume in 1 s (FEV₁) is an independent risk factor of morbidity and mortality. Genome-wide association studies (GWAS) identifying genetic variants underlying lung function have been performed mostly in Caucasian populations. In this study, we aimed to identify genetic variants influencing lung function in a Korean population. GWAS on lung function (FEV₁ and FEV₁/forced vital capacity (FVC) ratio) were performed in two cohort studies. A population-based cohort, the Korean Association Resource phase 3 (KARE3) (6223 subjects), served as a discovery set. The replication analysis was performed in a family-based cohort, the Healthy Twin Study (HTS; 2730 subjects). Dense single-nucleotide polymorphism array data from each study were imputed and used for genetic analysis. At the discovery phase, variants in 6p21 and 17q24 showed the strongest association with FEV₁/FVC ratio and FEV₁. Several variants in FAM13A on 4q22 locus exhibited positive association with FEV₁/FVC ratio. In the replication set, PPT2 in the 6p21 region showed significant association with lung function in the HTS, although the 4q22 locus and the 17q24 locus were not replicated. We identified that PPT2 on chromosome 6p21 is associated with loss of lung function in the Korean population.

Dietary restraint is non-genetically associated with change in body mass index: the Healthy Twin Study.

PurposeWe aimed to examine if past and more recent body mass index (BMI) changes are associated with eating behavior (EB) traits and whether these associations are due to non-genetic factors.Materials and MethodsIn 1321 Korean twins and family members, recent and past BMI change groups were defined using quartiles of BMI change between first and second visits over 2.4±0.9 years and BMI change between 20 years old and second visit, respectively. We applied linear mixed analysis for relationships of past or recent BMI change groups and each EB (restrained, external, and emotional EB using the Dutch Eating Behavior Questionnaire) assessed at second visit after adjusting for household effect and covariates (age, gender, education, medical history of diabetes, hypertension, and dyslipidemia, alcohol use, physical activity, smoking habit, and calorie intake). In monozygotic twin pairs, paired t-test for within-pair comparison and conditional logistic regression analysis were conducted regarding EB.ResultsGreater past BMI change was associated with higher restrained eating scores (P for trend=0.031), whereas greater recent BMI change was associated with higher external eating scores (P for trend=0.046). In co-twin-control analysis, twins with greater past BMI change were more likely to have higher restrained eating scores as compared with their co-twins with lower past BMI change (odds ratio 1.80; 95% confidence interval 1.13-2.87), whereas there were no associations between recent BMI change and external eating scores.ConclusionGreater BMI change since 20 years old is associated with higher dietary restraint, and non-genetic factors explain this relationship.

Genetic and environmental influences on sodium intake determined by using half-day urine samples: the Healthy Twin Study

Salt is essential in our diet, but excess intake is a well-established risk factor for hypertension. The presence and importance of genetic contributions to salt intake, however, are not well understood. The aim of this study was to examine whether a genetic predisposition and an environmental influence exist for sodium intake and salt habit. In a twin-family cohort, half-day urine samples from 1204 individuals (133 pairs of monozygotic twins, 29 pairs of dizygotic twins, and 880 singletons) were collected to assess 24-h sodium intakes. Daily total sodium intake, sodium density per calorie (Na-D), and salt habit questions were analyzed with adjustment for other epidemiologic characteristics. We calculated heritability (h2) and intraclass correlations to examine the genetic and shared environmental contributions to total sodium intake traits. The average sodium intake was 208.4 ± 107.0 mmol/d. Men had a higher absolute sodium intake (242.6 ± 117.4 mmol/d), but Na-D did not differ by sex. Moderate genetic influences existed (h2 = 0.31-0.34) for sodium intake and Na-D. We also found that sharing current residence rather than being a family member explained 22% of the variance in Na-D. Our findings suggest that both genetic predisposition and shared environment contribute to sodium intakes and salt habits alike.

Risk factors associated with human papillomavirus infection status in a Korean cohort

By surveying extensive epidemiological behavioural and sexual risk factors in a Korean twin cohort, risk factors for human papillomavirus (HPV) infection were investigated in South Korea. A total of 912 vaginal specimens were collected from the Healthy Twin Study, consisting of twins and their families. A range of epidemiological, behavioural, and sexual activity characteristics were evaluated using multivariate logistic regression analyses of family and twin relationships, adjusted to elucidate the risk factors for HPV infection. Of the various epidemiological characteristics, the possibility of extramarital affairs [odds ratio (OR) 2·48, 95% confidence interval (CI) 1·02-6·02] significantly increased the prevalence of HPV infection. Our multivariate regression analysis indicated that oral contraceptive use (OR 40·64, 95% CI 0·99-1670·7) and history of sexually transmitted disease (OR 2·56, 95% CI 0·93-7·10) were strongly associated with an increase in HPV infection. On the other hand, more frequent vaginal douching (OR 0·32, 95% CI 0·13-0·77) significantly decreased the prevalence of HPV infection. Our results suggested that HPV infection is associated with both biological and behavioural factors.

Heritability of Myopia and Ocular Biometrics in Koreans: The Healthy Twin Study

To estimate the heritabilities of myopia and ocular biometrics among different family types among a Korean population.We studied 1508 adults in the Healthy Twin Study. Spherical equivalent, axial length, anterior chamber depth, and corneal astigmatism were measured by refraction, corneal topography, and A-scan ultrasonography. To see the degree of resemblance among different types of family relationships, intraclass correlation coefficients (ICC) were calculated. Variance-component methods were applied to estimate the genetic contributions to eye phenotypes as heritability based on the maximum likelihood estimation. Narrow sense heritability was calculated as the proportion of the total phenotypic variance explained by additive genetic effects, and linear and nonlinear effects of age, sex, and interactions between age and sex were adjusted.A total of 240 monozygotic twin pairs, 45 dizygotic twin pairs, and 938 singleton adult family members who were first-degree relatives of twins in 345 families were included in the study. ICCs for spherical equivalent from monozygotic twins, pooled first-degree pairs, and spouse pairs were 0.83, 0.34, and 0.20, respectively. The ICCs of other ocular biometrics were also significantly higher in monozygotic twins compared with other relative pairs, with greater consistency and conformity. The estimated narrow sense heritability (95% confidence interval) was 0.78 (0.71-0.84) for spherical equivalent; 0.86 (0.82-0.90) for axial length; 0.83 (0.76-0.91) for anterior chamber depth; and 0.70 (0.63-0.77) for corneal astigmatism.The estimated heritability of spherical equivalent and ocular biometrics in the Korean population suggests the compelling evidence that all traits are highly heritable.

Socioeconomic Status and Number of Children Among Korean Women: The Healthy Twin Study

ObjectivesThis study aimed to evaluate whether the birth rate is associated with socioeconomic status in the women of the Republic of Korea, where the birth rate is rapidly decreasing.MethodsThis study included 732 females from the Healthy Twin Study, a family-twin cohort. The participants were classified into 3 socioeconomic groups according to their average income, education, and occupation. The association between socioeconomic status and number of children was assessed using gamma regression analysis with a generalized linear mixed model, adjusting for the age group, smoking/alcohol status, and family relationships.ResultsThe group with the highest education level had significantly fewer children compared with the group with the lowest education level (p=0.004). However, no significant associations were found according to household income level. The non-manual labor group had significantly fewer children compared with those working as homemakers (p=0.008).ConclusionsThis study aimed to explain the causal relationship between socioeconomic status and number of children. Associations between some socioeconomic status and number of children were found in Korea.

Combined Genome-Wide Linkage and Association Analyses of Fasting Glucose Level in Healthy Twins and Families of Korea

This study was undertaken to identify genetic polymorphisms that are associated with the risk of an elevated fasting glucose (FG) level using genome-wide analyses. We explored a quantitative trait locus (QTL) for FG level in a genome-wide study from a Korean twin-family cohort (the Healthy Twin Study) using a combined linkage and family-based association analysis approach. We investigated 1,754 individuals, which included 432 families and 219 pairs of monozygotic twins. Regions of chromosomes 2q23.3-2q31.1, 15q26.1-15q26.3, 16p12.1, and 20p13-20p12.2, were found to show evidence of linkage with FG level, and several markers in these regions were found to be significantly associated with FG level using family-based or general association tests. In particular, a single-nucleotide polymorphism (rs6138953) on the PTPRA gene in the 20p13 region (combined P = 1.8 × 10-6) was found to be associated with FG level, and the PRKCB1 gene (in 16p12.1) to be possibly associated with FG level. In conclusion, multiple regions of chromosomes 2q23.3-2q31.1, 15q26.1-15q26.3, 16p12.1, and 20p13-20p12.2 are associated with FG level in our Korean twin-family cohort. The combined approach of genome-wide linkage and family-based association analysis is useful to identify novel or known genetic regions concerning FG level in a family cohort study.