Since 2006, the advent of increasingly larger genome‐wide association studies and their meta‐analyses have led to numerous, replicated findings of genetic polymorphisms associated with many diseases ...and traits. Early studies suggested that the identified loci generally accounted for a small fraction of the genetic variance estimated from twin and family studies. This led to the concept of ‘missing heritability’. Here, the progress in accounting for a greater proportion of the variance is reviewed. In particular, gene–environment interactions can, for some traits and in certain circumstances, explain part of this missing heritability.
The relationship between smoking and suicide remains controversial.
A total of 16 282 twin pairs born before 1958 in Finland and alive in 1974 were queried with detailed health and smoking ...questionnaires in 1975 and 1981, with response rates of 89% and 84%. Smoking status and dose, marital, employment, and socio-economic status, and indicators of psychiatric and somatic illness were assessed at both time points. Emergent psychiatric and medical illness and vital status, including suicide determined by forensic autopsy, were evaluated over 35-year follow-up through government registries. The association between smoking and suicide was determined in competing risks hazard models. In twin pairs discordant for smoking and suicide, the prospective association between smoking and suicide was determined using a matched case-control design.
Smokers had a higher cumulative suicide incidence than former or never smokers. Heavy smokers had significantly higher suicide risk hazard ratio (HR) 3.47, 95% confidence interval (CI) 2.31-5.22 than light smokers (HR 2.30, 95% CI 1.61-3.23) (p = 0.017). Compared with never smokers, smokers, but not former smokers, had increased suicide risk (HR 2.56, 95% CI 1.43-4.59), adjusting for depressive symptoms, alcohol and sedative-hypnotic use, and excluding those who developed serious somatic or psychiatric illness. In twin pairs discordant for smoking and suicide, suicide was more likely in smokers odds ratio (OR) 6.0, 95% CI 2.06-23.8.
Adults who smoked tobacco were more likely to die by suicide, with a large, dose-dependent effect. This effect remained after consideration of many known predictors of suicide and shared familial effects, consistent with the hypothesis that exposure to tobacco smoke increases the risk of suicide.
Because sustained physical activity is important for a healthy life, this paper examined whether a greater diversity of sport activities during adolescence predicts higher levels of leisure‐time ...physical activity (LTPA) in adulthood. From sport activity participation reported by 17‐year‐old twins, we formed five groups: 1, 2, 3, 4, and 5+ different sport activities. At follow‐up in their mid‐thirties, twins were divided into four activity classes based on LTPA, including active commuting. Multinomial regression analyses, adjusted for several confounders, were conducted separately for male (N=1288) and female (N=1770) participants. Further, conditional logistic regression analysis included 23 twin pairs discordant for both diversity of sport activities in adolescence and LTPA in adulthood. The diversity of leisure‐time sport activities in adolescence had a significant positive association with adulthood LTPA among females. Membership in the most active adult quartile, compared to the least active quartile, was predicted by participation in 2, 3, 4, and 5+ sport activities in adolescence with odds ratios: 1.52 (P=.11), 1.86 (P=.02), 1.29 (P=.39), and 3.12 (P=5.4e‐05), respectively. Within‐pair analyses, limited by the small sample of twins discordant for both adolescent activities and adult outcomes, did not replicate the association. A greater diversity of leisure‐time sport activities in adolescence predicts higher levels of LTPA in adulthood in females, but the causal nature of this association remains unresolved.
In this systematic review, we aimed to collect together all previous twin and adoption studies on childhood and adolescent obesity up to the age of 18 years. Using several sources, we identified nine ...twin and five adoption studies; all of these studies had used relative weight as an indicator of obesity. Except the two twin studies from the Korean population, all studies represented Caucasian populations. In a meta-analysis of these twin studies, we found that genetic factors had a strong effect on the variation of body mass index (BMI) at all ages. The common environmental factors showed a substantial effect in mid-childhood, but this effect disappeared at adolescence. Adoption studies supported the role of family environment in childhood obesity as correlations were found between adoptees and adoptive parents; however, correlations were substantially stronger between parents and their biological offspring, further supporting the importance of genetic factors. In the future, more studies implementing genetic and environmental measures into twin models are needed as they allow estimation of the proportion of total genetic variation explained by candidate genes and analyses of gene-environment interactions. More studies of genetic architecture in non-Caucasian populations, of gene-environment interactions, and of body composition and body fat distribution are needed.
Little is known about epigenetic alterations associated with subcutaneous adipose tissue (SAT) in obesity. Our aim was to study genome-wide DNA methylation and gene expression differences in SAT in ...monozygotic (MZ) twin pairs who are discordant for body mass index (BMI). This design completely matches lean and obese groups for genetic background, age, gender and shared environment.
14We analyzed DNA methylome and gene expression from SAT, together with body composition (magnetic resonance imaging/spectroscopy) and glucose tolerance test, lipids and C-reactive protein from 26 rare BMI-discordant (intrapair difference in BMI ⩾3 kg m(-2)) MZ twin pairs identified from 10 birth cohorts of young adult Finnish twins.
We found 17 novel obesity-associated genes that were differentially methylated across the genome between heavy and lean co-twins. Nine of them were also differentially expressed. Pathway analyses indicated that dysregulation of SAT in obesity includes a paradoxical downregulation of lipo/adipogenesis and upregulation of inflammation and extracellular matrix remodeling. Furthermore, CpG sites whose methylation correlated with metabolically harmful fat depots (intra-abdominal and liver fat) also correlated with measures of insulin resistance, dyslipidemia and low-grade inflammation, thus suggesting that epigenetic alterations in SAT are associated with the development of unhealthy obesity.
This is the first study in BMI-discordant MZ twin pairs reporting genome-wide DNA methylation and expression profiles in SAT. We found a number of novel genes and pathways whose methylation and expression patterns differ within the twin pairs, suggesting that the pathological adaptation of SAT to obesity is, at least in part, epigenetically regulated.
Objective: Both obesity and exercise behavior are influenced by genetic and environmental factors. However, whether obesity and physical inactivity share the same genetic vs environmental etiology ...has rarely been studied. We therefore analyzed these complex relationships, and also examined whether physical activity modifies the degree of genetic influence on body mass index (BMI) and waist circumference (WC). Methods: The FinnTwin16 Study is a population-based, longitudinal study of five consecutive birth cohorts (1975-1979) of Finnish twins. Data on height, weight, WC and physical activity of 4343 subjects at the average age of 25 (range, 22-27 years) years were obtained by a questionnaire and self-measurement of WC. Quantitative genetic analyses based on linear structural equations were carried out by the Mx statistical package. The modifying effect of physical activity on genetic and environmental influences was analyzed using gene-environment interaction models. Results: The overall heritability estimates were 79% in males and 78% in females for BMI, 56 and 71% for WC and 55 and 54% for physical activity, respectively. There was an inverse relationship between physical activity and WC in males (r=-0.12) and females (r=-0.18), and between physical activity and BMI in females (r=-0.12). Physical activity significantly modified the heritability of BMI and WC, with a high level of physical activity decreasing the additive genetic component in BMI and WC. Conclusions: Physically active subjects were leaner than sedentary ones, and physical activity reduced the influence of genetic factors to develop high BMI and WC. This suggests that the individuals at greatest genetic risk for obesity would benefit the most from physical activity.
Twin studies have estimated the relative contribution of genes and the environment to variance in exercise behavior and it is known that parental education positively affects exercise levels. This ...study investigates the role of parental education as a potential modifier of variance in exercise behavior from age 7 to 18 years. The study is based on large datasets from the Netherlands Twin Register (NTR: N = 24 874 twins; surveys around the ages of 7, 10, 12, 14, 16 and 18 years) and two Finnish twin cohorts (FinnTwin12: N = 4399; 12, 14 and 17 years; FinnTwin16: N = 4648; 16, 17 and 18 years). Regular participation in moderate‐to‐vigorous exercise activities during leisure time was assessed by survey. Parental education was dichotomized (“both parents with a low education” vs “at least one parent with a high education”). The mean in exercise behavior tended to be higher and the variance tended to be lower in children of high educated parents. Evidence for gene‐by‐environment interaction was weak. To develop successful interventions that specifically target children of low educated parents, the mechanisms causing the mean and variance differences between the two groups should be better understood.
Decline in episodic memory performance usually causes the first clinical symptoms of Alzheimer's disease. At present, Alzheimer's disease can only be diagnosed at a very late stage when ...neurodegeneration and cognitive impairment is already irreversible. New early disease markers are needed for earlier and more efficient Alzheimer's disease intervention. To identify early disease markers, we implemented a genome-wide bisulphite sequencing method for the analysis of plasma cell-free DNA methylation profiles and compared differences associated with episodic memory performance in Finnish twin pairs. A noticeable amount of cell-free DNA was present in plasma, however, the amounts as well as the genomic coverage of these fragments varied substantially between individuals. We found no significant markers associated with episodic memory performance in the twins' plasma cell-free DNA methylation profiles. Furthermore, our results indicate that due to the low genomic coverage of cell-free DNA fragments and the variety in these fragments between individuals, the implemented genome-wide bisulphite sequencing method is not optimal for comparing cell-free DNA methylation differences between large groups of individuals.
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