Aetiology of teenage childbearing: reasons for familial effects.

The aims of the present study were to evaluate the contribution of the genetic and environmental factors to the risk of teenage childbearing, and to study whether life style, socio-economic conditions, and personality traits could explain possible familial effects. We linked two population-based registers: the Swedish Twin Register and the Swedish Medical Birth Register. The study covers female twin pairs born between 1953 and 1958, having their first infant before the age of 30 years (n = 1885). In order to separate familial effects from other environmental influences, and genetic effects from shared environmental effects, only complete twin pairs with known zygosity were included, in all 260 monozygotic and 370 dizygotic twin pairs. We used quantitative genetic analyses to evaluate the importance of genetic and environmental effects for liability to teenage childbearing. Logistic regression analyses were used to estimate the effects of life style, socio-economic situation, and personality on the probability of teenage childbearing, and to study whether psychosocial factors could explain possible familial effects. Fifty-nine percent (0-76%) of the variance in being a teenage mother was attributable to heritable factors; 0% (0-49%) was due to shared environmental factors; and 41% (23-67%) was explained by non-shared environmental factors. Thus, the data were consistent with the hypothesis that the familial aggregation of teenage childbearing is completely explained by genetic factors, although the alternative hypothesis that familial aggregation is entirely explained by shared environmental factors cannot be ruled out. Significant effects of smoking habits, housing conditions, and educational level were found in relation to liability to teenage childbearing. However, the familial effects on risk of teenage childbearing were not mediated through similarities in life style and socio-economic factors. When studying risk factors for teenage childbearing, it is recommended to include life style and socio-economic variables as well as information about family history of teenage childbearing. Twin Research (2000) 3, 23-27.     

The validity and heritability of self-report osteoarthritis in an Australian older twin sample.

In order to investigate the genetic and environmental antecedents of osteoarthritis (OA), self-report measures of joint pain, stiffness and swelling were obtained from a population-based sample of 1242 twin pairs over 50 years of age. In order to provide validation for these self-report measures, a subsample of 118 twin pairs were examined according to the American College of Rheumatology clinical and radiographic criteria for the classification of osteoarthritis. A variety of statistical methods were employed to identify the model derived from self-report variables which would provide optimal prediction of these standardised assessments, and structural equation modelling was used to determine the relative influences of genetic and environmental influences on the development of osteoarthritis. Significant genetic effects were found to contribute to osteoarthritis of the hands, hips and knees in women, with heritability estimates ranging from 30-46% depending on the site. In addition, the additive genetic effects contributing to osteoarthritis in various parts of the body were confirmed to be the same. Statistically significant familial aggregation of osteoarthritis in men was also observed, but it was not possible to determine whether this was due to genetic or shared environmental effects.     

Individual differences in adolescent religiosity in Finland: familial effects are modified by sex and region of residence.

Data from 16-year-old Finnish twin pairs were used to estimate familial effects on religiosity and the modification of those effects by sex and residential region. The sample of 2265 twin boys and 2521 twin girls formed 779 monozygotic and 1614 dizygotic pairs, 785 of the same sex and 829 of opposite sex. We compared religiosity scores of twins living in more rural and traditional northern Finland with those living in the more urban and secular southern region. Girls had higher religiosity scores than did boys, and twins living in northern Finland had higher religiosity scores than those resident in southern Finland. Correlations for monozygotic twins were slightly higher than those for dizygotic twins, and covariance modeling found modest heritability of religiosity [11% (95% CI 0-24) for girls; 22% (95% CI 6-38) for boys], and substantial shared environmental effects [60% (95% CI 49-69) and 45% (95% CI 31-57)] among girls and boys, respectively. The correlation between shared environmental effects in boys and girls was estimated to be 0.84 (95% CI 0.73-0.99). In analyses distinguishing region of residence, girls living in southern Finland were found to have significantly higher unshared environmental effects than girls in northern Finland, while boys living in the urban south appeared to have lower shared environmental effects, and higher additive genetic effects, than boys living in the rural north.     

Otitis media: genetic factors and sex differences.

Although genetic factors are recognised as major contributors to otitis media, the presence of sex differences in heritability needs clarification. The aim of this study was to estimate the relative contribution of genetic and environmental effects in otitis media liability with particular focus on sex differences. Data from a cohort of Norwegian twins born between 1967 and 1979 with repeated measures on recurrent childhood otitis media were analysed. Altogether the sample included 4247 twin pairs. The tetrachoric correlations for monozygotic twins were .71 and .65 for males and females respectively. In dizygotic twins the correlations were .35 and .25 for males and females, respectively, and was.34 in opposite sexed pairs. The contribution of genetic and environmental effects was analyzed using structural equation modeling. The best fitting model showed that additive genetic effects explained 72% and 61% of the variance in males and females, respectively. The remaining variance was attributed to individual environmental effects. A model specifying equal heritability estimates for males and females yielded an almost equivalent fit. We found substantial genetic effects for liability to otitis media. There is no evidence that different sets of genes influence liability in males and females, but there may be sex differences in the relative importance of genetic effects.     

Genetic influences on osteoarthritis in women: a twin study.

OBJECTIVES--To assess the relative contribution of genetic and environmental factors to common forms of osteoarthritis of the hands and knees. DESIGN--Classic twin study with unselected twins who were screened radiologically for osteoarthritis. SUBJECTS--130 identical and 120 non-identical female twins aged 48-70 recruited from a London based twin register and through a national media campaign. MAIN OUTCOME MEASURES--Similarity in identical compared with non-identical twin pairs for radiographic changes at the interphalangeal and first carpometacarpal joints of the hands and the tibiofemoral joint and patellofemoral joint of the knee expressed as intraclass correlations. RESULTS--The intraclass correlations of radiographic osteophytes and narrowing at most sites and the presence of Heberden''s nodes and knee pain were higher in the identical pairs. The intraclass correlation of the total radiographic osteoarthritis score in identical pairs (rMZ) was 0.64 (SE 0.05) compared with 0.38 (0.08) in non-identical pairs. The proportion of genetic variance of total osteoarthritis score (osteophytes and narrowing) with modelling techniques was estimated at 0.54 (95% confidence interval 0.43 to 0.65) and ranged from 0.39 to 0.65 for different sites and features (p < 0.001) after adjustment for age and weight. CONCLUSIONS--These results demonstrate for the first time a clear genetic effect for radiographic osteoarthritis of the hand and knee in women, with a genetic influence ranging from 39-65%, independent of known environmental or demographic confounders. The results of this study should lead to further work on isolating the gene or genes involved in the pathogenesis of the common disabling disease.     

Heritability of maximal isometric muscle strength in older female twins.

The purpose of the present study was to examine genetic and environmental effects on maximal isometric handgrip, knee extension, and ankle plantar flexion strength. In addition, we wanted to investigate whether the strength of these three muscle groups shares a genetic component or whether the genetic effect is specific for each muscle group. Muscle strength was measured as part of the Finnish Twin Study on Aging in 97 monozygotic (MZ) and 102 dizygotic (DZ) female twin pairs, aged 63-76 yr. The MZ and DZ individuals did not differ from each other in age, body height, weight, or self-related health. The age-adjusted pairwise (intraclass) correlations of the MZ and DZ twins were, respectively, 0.462 and 0.242 in knee extension, 0.435 and 0.345 in handgrip, and 0.512 and 0.435 in ankle plantar flexion strength. The multivariate genetic analysis showed that handgrip and knee extension strength shared a genetic component, which accounted for 14% (95% confidence interval: 4-28%) of the variance in handgrip strength and 31% (95% confidence interval: 18-45%) in knee extension strength. The influence of genetic effects on ankle plantar flexion strength was minor and not significant. Furthermore, these three muscle groups had a nongenetic familial effect in common and nonshared environmental effects in common. The results suggested that muscle strength is under a genetic regulation, but also environmental effects have a significant role in explaining the variability in the muscle strength.     

Genetic influences on premature parturition in an Australian twin sample.

We investigated possible genetic influences on women's liability to preterm birth, using data from a large sample of Australian female twin pairs. In a 1988-90 questionnaire survey, both members of 905 parous twin pairs (579 monozygotic and 326 dizygotic) reported on whether deliveries had been more than two weeks preterm. Tetrachoric twin pair correlations for first birth were rMZ = 0.20+/-0.11 and rDZ = -0.03+/-0.14, and for any birth were rMZ = 0.30+/-0.08 and rDZ = 0.03+/-0.11. Best-fitting models to data contained only additive genetic influences and individual environmental effects. Heritability was 17% for preterm delivery in first pregnancy, and 27% for preterm delivery in any pregnancy. In the former case, however, we could not reject a model without genetic influences. Although our data did not allow for differentiation of the varying aetiologies of premature parturition, results from this exploratory analysis suggest that further investigation of genetic influences on specific reasons for preterm birth is warranted.     

The genetic liability to disability retirement: a 30-year follow-up study of 24,000 Finnish twins

Background

No previous studies on the effect of genetic factors on the liability to disability retirement have been carried out. The main aim of this study was to investigate the contribution of genetic factors on disability retirement due to the most common medical causes, including depressive disorders.

Methods

The study sample consisted of 24 043 participants (49.7% women) consisting of 11 186 complete same-sex twin pairs including 3519 monozygotic (MZ) and 7667dizygotic (DZ) pairs. Information on retirement events during 1.1.1975–31.12.2004, including disability pensions (DPs) with diagnoses, was obtained from the Finnish nationwide official pension registers. Correlations in liability for MZ and DZ twins and discrete time correlated frailty model were used to investigate the genetic liability to age at disability retirement.

Results

The 30 year cumulative incidence of disability retirement was 20%. Under the best fitting genetic models, the heritability estimate for DPs due to any medical cause was 0.36 (95% CI 0.32–0.40), due to musculoskeletal disorders 0.37 (0.30–0.43), cardiovascular diseases 0.48 (0.39–0.57), mental disorders 0.42 (0.35–0.49) and all other reasons 0.24 (0.17–0.31). The effect of genetic factors decreased with increasing age of retirement. For DP due to depressive disorders, 28% of the variance was explained by environmental factors shared by family members (95% CI 21–36) and 58% of the variance by the age interval specific environmental factors (95% CI 44–71).

Conclusions

A moderate genetic contribution to the variation of disability retirement due to any medical cause was found. The genetic effects appeared to be stronger at younger ages of disability retirement suggesting the increasing influence of environmental factors not shared with family members with increasing age. Familial aggregation in DPs due to depressive disorders was best explained by the common environmental factors and genetic factors were not needed to account for the pattern of familial aggregation.     

Genetic influences in self-reported symptoms of obstructive sleep apnoea and restless legs: a twin study.

Sleep disorders, such as obstructive sleep apnoea (OSA) and restless legs syndrome (RLS), are very common. The relative importance of genetic and nongenetic (environmental) influences on the symptomatology of these conditions has not been well studied. This study uses the twin design to examine this by evaluating OSA and RLS symptoms in monozygotic (MZ) and dizygotic (DZ) twins. Six thousand six hundred unselected female twin pairs, identified from a national volunteer twin register, were asked to complete a medical questionnaire. This questionnaire included questions on OSA and RLS symptoms, as well as questions on subject demographics, past medical history, smoking history and menopausal status. Responses were obtained from 4503 individuals (68% response rate). A total of 1937 twin pairs were evaluable: 933 MZ pairs (mean [range] age 51 [20-76] years) and 1004 DZ pairs (age 51 [20-80] years). Concordance rates were higher for MZ than DZ twins for OSA and RLS symptoms. Multifactorial liability threshold modeling suggests that additive genetic effects combined with unique environmental factors provide the best model for OSA and RLS symptoms. Heritability was estimated to be 52% (95% confidence interval 36% to 68%) for disruptive snoring, 48% (37% to 58%) for daytime sleepiness, 54% (44% to 63%) for restless legs, and 60% (51% to 69%) for legs jerking. These estimates dropped only slightly after adjustment for potential confounding influences on the symptoms of snoring and daytime sleepiness. These results suggest a substantial genetic contribution to the symptomatology of OSA and RLS. More research is needed to identify the genes responsible, and may ultimately lead to new therapies.     

A twin study of the etiology of prolonged fatigue and immune activation.

Risk factors to prolonged fatigue syndromes (PFS) are controversial. Pre-morbid and/or current psychiatric disturbance, and/or disturbed cell-mediated immunity (CMI), have been proposed as etiologic factors. Self-report measures of fatigue and psychologic distress and three in vitro measures of CMI were collected from 124 twin pairs. Crosstwin-crosstrait correlations were estimated for the complete monozygotic (MZ; 79 pairs) and dizygotic (DZ; 45 pairs) twin groups. Multivariate genetic and environmental models were fitted to explore the patterns of covariation between etiologic factors. For fatigue, the MZ correlation was more than double the DZ correlation (0.49 versus 0.16) indicating strong genetic control of familial aggregation. By contrast, for in vitro immune activation measures MZ and DZ correlations were similar (0.49-0.69 versus 0.42-0.53) indicating the etiologic role of shared environments. As small univariate associations were noted between prolonged fatigue and the in vitro immune measures (r = -0.07 to -0.12), multivariate models were fitted. Relevant etiologic factors included: a common genetic factor accounting for 48% of the variance in fatigue which also accounted for 4%, 6% and 8% reductions in immune activation; specific genetic factors for each of the in vitro immune measures; a shared environment factor influencing the three immune activation measures; and, most interestingly, unique environmental influences which increased fatigue but also increased markers of immune activation. PFS that are associated with in vitro measures of immune activation are most likely to be the consequence of current environmental rather than genetic factors. Such environmental factors could include physical agents such as infection and/or psychologic stress.     

Genetic liability to disability pension in women and men: a prospective population-based twin study

Background

Previous studies of risk factors for disability pension (DP) have mainly focused on psychosocial, or environmental, factors, while the relative importance of genetic effects has been less studied. Sex differences in biological mechanisms have not been investigated at all.

Methods

The study sample included 46,454 Swedish twins, consisting of 23,227 complete twin pairs, born 1928–1958, who were followed during 1993–2008. Data on DP, including diagnoses, were obtained from the National Social Insurance Agency. Within-pair similarity in liability to DP was assessed by calculating intraclass correlations. Genetic and environmental influences on liability to DP were estimated by applying discrete-time frailty modeling.

Results

During follow-up, 7,669 individuals were granted DP (18.8% women and 14.1% men). Intraclass correlations were generally higher in MZ pairs than DZ pairs, while DZ same-sexed pairs were more similar than opposite-sexed pairs. The best-fitting model indicated that genetic factors contributed 49% (95% CI: 39–59) to the variance in DP due to mental diagnoses, 35% (95% CI: 29–41) due to musculoskeletal diagnoses, and 27% (95% CI: 20–33) due to all other diagnoses. In both sexes, genetic effects common to all ages explained one-third, whereas age-specific factors almost two-thirds, of the total variance in liability to DP irrespective of diagnosis. Sex differences in liability to DP were indicated, in that partly different sets of genes were found to operate in women and men, even though the magnitude of genetic variance explained was equal for both sexes.

Conclusions

The findings of the study suggest that genetic effects are important for liability to DP due to different diagnoses. Moreover, genetic contributions to liability to DP tend to differ between women and men, even though the overall relative contribution of genetic influences does not differ by sex. Hence, the pathways leading to DP might differ between women and men.     

Genetic and environmental influences on migraine: a twin study across six countries.

Migraine is a common neurovascular brain disorder that is manifested in recurrent episodes of disabling headache. The aim of the present study was to compare the prevalence and heritability of migraine across six of the countries that participate in GenomEUtwin project including a total number of 29,717 twin pairs. Migraine was assessed by questionnaires that differed between most countries. It was most prevalent in Danish and Dutch females (32% and 34%, respectively), whereas the lowest prevalence was found in the younger and older Finnish cohorts (13% and 10%, respectively). The estimated genetic variance (heritability) was significant and the same between sexes in all countries. Heritability ranged from 34% to 57%, with lowest estimates in Australia, and highest estimates in the older cohort of Finland, the Netherlands, and Denmark. There was some indication that part of the genetic variance was non-additive, but this was significant in Sweden only. In addition to genetic factors, environmental effects that are non-shared between members of a twin pair contributed to the liability of migraine. After migraine definitions are homogenized among the participating countries, the GenomEUtwin project will provide a powerful resource to identify the genes involved in migraine.     

Inheritance of human finger and palm dermatoglyphic characteristics]

The degree of genetic determination of 25 quantitative dermatoglyphic characteristics has been studied on family: twin material: 45 pairs of MZ and 75 single-sex DZ twins; and 53 single-sex "parent-child" pairs. Approximating formulae were used to estimate main components of phenotypic variance due to additive interaction of genetic factors, to non-linear effects (intralocus dominance) and to the effect of total-familiar and random environmental factors. All the finger dermatoglyphic characteristics studied had a high degree of genetic determination (G greater than 0,80), and for most of them the contribution into the large variance of intralocus dominance effects was comparable with that of additive gene interaction, included in the determination of these characters. There are some palm dermatoglyphic characteristics ("ad" distance "cd" comb counting, "bad", "adt" and "cda" angles), which degree of genetic determination is low (G less than 0,35). At least ten quantitative finger and palm dermatogliphic characteristics with a high degree of genetic determination can be used for special studies in frames of multidimentional genetical analysis, including determination of twin zygosity type. Earlier described "indices" (using twin data) of relative role of genetic and environment factors in the determination of populational variability of quantitative characters are considered. None of them is shown to be a reliable estimate of the coefficient of genetic character determination. The use of these indices in practical studies can result in wrong conclusions on the degree and the character of genetic determination of quantitative characters.     

Genetic and environmental influences on expression of recurrent headache as a function of the reporting age in twins.

To explore age-related mechanisms in the expression of recurrent headache, we evaluated whether genetic and environmental influences are a function of the reporting age using questionnaire information that was gathered in 1973 for 15- to 47-year-old Swedish twins (n = 12,606 twin pairs). Liability to mixed headache (mild migraine and tension-type headache) was explained by non-additive genetic influences (49%) in men aged from 15 to 30 years and additive genetic plus shared environmental influences (28%) in men aged from 31 to 47 years. In women, the explained proportion of variance, which was mainly due to additive genetic effects, ranged from 61% in adolescent twins to 12% in twins aged from 41 to 47 years, whereas individual specific environmental variance was significantly lower in twins aged from 15 to 20 years than in twins aged from 21 to 30 years. Liability to migrainous headache (more severe migraine) was explained by non-additive genetic influences in men, 32% in young men and 45% in old men, while total phenotypic variance was significantly lower in young men than in old men. In women, the explained proportion of variance ranged from 91% in the youngest age group to 37% in the oldest age group, with major contributions from non-additive effects in young and old women (15-20 years and 41-47 years, respectively) and additive genetic effects in intermediate age groups (21-40 years). While total variance showed a positive age trend, genetic variance tended to be stable across age groups, whereas individual specific environmental variance was significantly lower in adolescent women as compared to older women.     

Gene-environment interaction effects on behavioral variation and risk of complex disorders: the example of alcoholism and other psychiatric disorders.

There have been few replicated examples of genotype x environment interaction effects on behavioral variation or risk of psychiatric disorder. We review some of the factors that have made detection of genotype x environment interaction effects difficult, and show how genotype x shared environment interaction (GxSE) effects are commonly confounded with genetic parameters in data from twin pairs reared together. Historic data on twin pairs reared apart can in principle be used to estimate such GxSE effects, but have rarely been used for this purpose. We illustrate this using previously published data from the Swedish Adoption Twin Study of Aging (SATSA), which suggest that GxSE effects could account for as much as 25% of the total variance in risk of becoming a regular smoker. Since few separated twin pairs will be available for study in the future, we also consider methods for modifying variance components linkage analysis to allow for environmental interactions with linked loci.     

Quantitative genetic analysis of internalising and externalising problems in a large sample of 3-year-old twins.

For a quantitative genetic study of pre-school problem behaviours, we have collected data with the Child Behavior Checklist for 2 and 3-year-old children (CBCL 2/3). Questionnaires were completed by mothers of 3620 twin pairs: 633 monozygotic males, 581 dizygotic males, 695 monozygotic females, 519 dizygotic females and 1192 dizygotic opposite sex twin pairs. The genetic and environmental influences on the Externalising and Internalising Problem scales were estimated, simultaneously with sex differences and sibling interaction effects. Genetic factors explained most of the observed variance for both Externalising and Internalising Problems. Cooperative sibling interactions were found for Externalising Problems, indicating that twins reinforce each other's behaviour. Sex differences in genetic architecture were found for Externalising Problems. Genetic factors explained 75% of the variance in girls and 50% in boys. Shared environmental influences were only of importance in boys. For both problem scales, non-shared environmental factors accounted for 25 to 32% of the variance. The observed variances of Internalising Problems could be adequately explained by genetic and nonshared environmental factors, with genetic factors accounting for 68% of the variance.     

Genetic and Environmental Contributions to BMI in Adolescent and Young Adult Women

The objective of this study was to determine the genetic and environmental contributions to variation in BMI over time in European‐American (EA) and African‐American (AA) adolescent and young adult women. Self‐reported BMI (kg/m²) data from 2,816 EA (1,306 twin pairs, 56.5% monozygotic (MZ)) and 404 AA (178 twin pairs, 42.7% MZ) women at baseline (T1; median age 15 years) and 3,225 EA (1,511 twin pairs, 55.3% MZ) and 539 AA (252 pairs, 43.3% MZ) women at follow‐up (T2; median age 22 years) from a Midwestern US, population‐based twin registry were used to construct biometrical genetic models. For EA women, the majority of the variance in BMI was attributable to additive genetic effects at both time points (82% for each), with the remaining variance attributable to nonshared environment. Genetic and nonshared environment correlations between adolescent and young adult BMI were 0.87 and 0.23, respectively. Among AA women, nonadditive genetic effects comprised 68% of the variance at T1 and 73% at T2, and were highly correlated (r_D = 0.94). The proportions of variance attributable to nonshared environment at T1 (29%) and T2 (25%) were more modestly correlated (r_E = 0.31). The remaining variance in AA women could be attributed to additive genetic effects. Additive vs. nonadditive genetic effects contribute differentially to BMI in AA vs. EA adolescent and young adult women. Additional research is needed to better characterize the environmental and genetic factors related to BMI in persons of different races to aid understanding of the complex determinants of body weight in individuals.     

Glucocorticoid receptor binding in twin pairs is affected by shared environment but not by shared genes

We set out to determine whether glucocorticoid receptor activity is affected mainly by genetic or environmental factors. The affinity and capacity of the glucocorticoid receptor was measured using dexamethasone binding in whole leukocytes from 53 monozygotic and 48 dizygotic twin pairs. Receptor binding characteristics assayed from twin pairs on the same day were highly correlated within twin pairs irrespective of zygosity. Apparent Kd was negatively correlated with environmental temperature (R²=0.13, P<0.0001) but this did not confound the intra-pair correlation, suggesting a strong familial component independent of zygosity. Receptor binding parameters were not more closely correlated in monozygotic twins than dizygotic twin pairs indicating that there is no major genetic contribution to receptor binding and that environmental influences predominate. The close similarity in binding between twin pairs in adulthood raises the possibility that familial, non-genetic, factors such as shared early life environment may programme the glucocorticoid receptor.     

HLA-linked rheumatoid arthritis.

Twenty-eight pedigrees were ascertained through pairs of first-degree relatives diagnosed with rheumatoid arthritis (RA). RA was confirmed in 77 pedigree members including probands; the absence of disease was verified in an additional 261 pedigree members. Pedigree members were serologically typed for HLA. We used likelihood analysis to statistically characterize the HLA-linked RA susceptibility locus. The genetic model assumed tight linkage to HLA. The analysis supported the existence of an HLA-linked RA susceptibility locus, estimated the susceptibility allele frequency as 2.16%, and estimated the lifetime penetrance as 41% in male homozygotes and as 48% in female homozygotes. Inheritance was recessive in males and was nearly recessive in females. In addition, the analysis attributed 78% of the variance within genotypes to genetic or environmental effects shared by siblings. The genetic model inferred in this analysis is consistent with previous association, linkage, and familial aggregation studies of RA. The inferred HLA-linked RA susceptibility locus accounts for approximately one-half of familial RA, although it accounts for only approximately one-fifth of the RA in the population. Although other genes may account for the remaining familial RA, a large portion of RA cases may occur sporadically.