The Swedish Twin Registry: a unique resource for clinical, epidemiological and genetic studies

Resilience and Risk for Alcohol Use Disorders: A Swedish Twin Study

BACKGROUND

Resilience has been shown to be protective against alcohol use disorders (AUDs), but the magnitude and nature of the relationship between these 2 phenotypes are not clear. The aim of this study was to examine the strength of this relationship and the degree to which it results from common genetic or common environmental influences.

METHODS

Resilience was assessed on a 9-point scale during a personal interview in 1,653,721 Swedish men aged 17 to 25 years. AUD was identified based on Swedish medical, legal, and pharmacy registries. The magnitude of the relationship between resilience and AUD was examined using logistic regression. The extent to which the relationship arises from common genetic or common environmental factors was examined using a bivariate Cholesky decomposition model.

RESULTS

The 5 single items that comprised the resilience assessment (social maturity, interest, psychological energy, home environment, and emotional control) all reduced risk for subsequent AUD, with social maturity showing the strongest effect. The linear effect by logistic regression showed that a 1-point increase on the resilience scale was associated with a 29% decrease in odds of AUD. The Cholesky decomposition model demonstrated that the resilience-AUD relationship was largely attributable to overlapping genetic and shared environmental factors (57 and 36%, respectively).

CONCLUSIONS

Resilience is strongly associated with a reduction in risk for AUD. This relationship appears to be the result of overlapping genetic and shared environmental influences that impact resilience and risk of AUD, rather than a directly causal relationship.

Heritability of non-HLA genetics in coeliac disease: a population-based study in 107 000 twins

BACKGROUND AND OBJECTIVE

Almost 100% individuals with coeliac disease (CD) are carriers of the human leucocyte antigen (HLA) DQ2/DQ8 alleles. Earlier studies have, however, failed to consider the HLA system when estimating heritability in CD, thus violating an underlying assumption of heritability analysis. We examined the heritability of CD in a large population-based sample of twins, considering HLA.

DESIGN

In a population-representative sample of 107 912 twins, we identified individuals with CD (equal to villous atrophy) through biopsy reports from all Swedish pathology departments. We calculated concordance rates and tetrachoric correlations for monozygotic (MZ) and dizygotic (DZ) twin pairs. Further, we estimated heritability of CD, first strictly from observed data, and then the non-HLA heritability, representing the heritability of all genetic factors except the HLA locus, using an approach that circumvent the violation of underlying assumptions.

RESULTS

We identified 513 twins with a diagnosis of CD (prevalence 0.48%). Concordance rates were higher in MZ pairs (0.49) than in DZ pairs (0.10), as were tetrachoric correlations (0.89 in MZ vs 0.51 in DZ pairs). The heritability of CD was 75% (95% CI 55% to 96%). The non-HLA heritability was slightly attenuated, 68% (95% CI 40% to 96%), with shared (17%) and non-shared (15%) environmental factors explaining the remaining variability of CD.

CONCLUSIONS

CD is characterised by a high heritability, but our study also suggests that non-shared environmental factors may be of importance to CD development. HLA seems to have only moderate impact on heritability estimates.

The Concordance and Heritability of Type 2 Diabetes in 34,166 Twin Pairs From International Twin Registers: The Discordant Twin (DISCOTWIN) Consortium

Twin pairs discordant for disease may help elucidate the epigenetic mechanisms and causal environmental factors in disease development and progression. To obtain the numbers of pairs, especially monozygotic (MZ) twin pairs, necessary for in-depth studies while also allowing for replication, twin studies worldwide need to pool their resources. The Discordant Twin (DISCOTWIN) consortium was established for this goal. Here, we describe the DISCOTWIN Consortium and present an analysis of type 2 diabetes (T2D) data in nearly 35,000 twin pairs. Seven twin cohorts from Europe (Denmark, Finland, Norway, the Netherlands, Spain, Sweden, and the United Kingdom) and one from Australia investigated the rate of discordance for T2D in same-sex twin pairs aged 45 years and older. Data were available for 34,166 same-sex twin pairs, of which 13,970 were MZ, with T2D diagnosis based on self-reported diagnosis and medication use, fasting glucose and insulin measures, or medical records. The prevalence of T2D ranged from 2.6% to 12.3% across the cohorts depending on age, body mass index (BMI), and national diabetes prevalence. T2D discordance rate was lower for MZ (5.1%, range 2.9-11.2%) than for same-sex dizygotic (DZ) (8.0%, range 4.9-13.5%) pairs. Across DISCOTWIN, 720 discordant MZ pairs were identified. Except for the oldest of the Danish cohorts (mean age 79), heritability estimates based on contingency tables were moderate to high (0.47-0.77). From a meta-analysis of all data, the heritability was estimated at 72% (95% confidence interval 61-78%). This study demonstrated high T2D prevalence and high heritability for T2D liability across twin cohorts. Therefore, the number of discordant MZ pairs for T2D is limited. By combining national resources, the DISCOTWIN Consortium maximizes the number of discordant MZ pairs needed for in-depth genotyping, multi-omics, and phenotyping studies, which may provide unique insights into the pathways linking genes to the development of many diseases.

On the Relationship Between Domain-Specific Creative Achievement and Sexual Orientation in Swedish Twins

Despite the commonly held belief that homosexual males and females are more creative compared to heterosexuals, empirical studies on homosexuality and its relationship to creativity have been sparse, often with questionable methodology and very small sample sizes, reporting mixed findings. No study till date has explored the associations described above in a large population-based and genetically informative sample. Here, we examined such potential associations between sexual orientation and creative achievement in several different domains (music, writing, dance, visual arts, science, invention, and theater) using a large cohort of 4494 Swedish twins (of which 7.5 % were not exclusively heterosexual). Data were analyzed for the sexes separately as well as pooled. Results showed significant associations between sexual orientation and two of the creative domains-theater and writing-with non-heterosexuals being more creative in these domains. In all other domains, no significant differences were found between the non-heterosexual and heterosexual groups. Findings from co-twin control analyses suggested that the significant associations may not be causal in nature (i.e., homosexual orientation leads to higher creativity) but due to shared liability. However, we lacked power to differentiate between shared genetic and shared environmental influences. Results and potential implications are discussed critically.

Anorexia Nervosa, Major Depression, and Suicide Attempts: Shared Genetic Factors

The extent to which genetic and environmental factors influenced anorexia nervosa (AN), major depressive disorder (MDD), and suicide attempts (SA) were evaluated. Participants were 6,899 women from the Swedish Twin Study of Adults: Genes and Environment. A Cholesky decomposition assessed independent and overlapping genetic and environmental contributions to AN, MDD, and SA. Genetic factors accounted for a substantial amount of liability to all three traits; unique environmental factors accounted for most of the remaining liability. Shared genetic factors may underlie the coexpression of these traits. Results underscore the importance of assessing for signs of suicide among individuals with AN.

An Underlying Common Factor, Influenced by Genetics and Unique Environment, Explains the Covariation Between Major Depressive Disorder, Generalized Anxiety Disorder, and Burnout: A Swedish Twin Study

Depression and anxiety are highly comorbid due to shared genetic risk factors, but less is known about whether burnout shares these risk factors. We aimed to examine whether the covariation between major depressive disorder (MDD), generalized anxiety disorder (GAD), and burnout is explained by common genetic and/or environmental factors. This cross-sectional study included 25,378 Swedish twins responding to a survey in 2005-2006. Structural equation models were used to analyze whether the trait variances and covariances were due to additive genetics, non-additive genetics, shared environment, and unique environment. Univariate analyses tested sex limitation models and multivariate analysis tested Cholesky, independent pathway, and common pathway models. The phenotypic correlations were 0.71 (0.69-0.74) between MDD and GAD, 0.58 (0.56-0.60) between MDD and burnout, and 0.53 (0.50-0.56) between GAD and burnout. Heritabilities were 45% for MDD, 49% for GAD, and 38% for burnout; no statistically significant sex differences were found. A common pathway model was chosen as the final model. The common factor was influenced by genetics (58%) and unique environment (42%), and explained 77% of the variation in MDD, 69% in GAD, and 44% in burnout. GAD and burnout had additive genetic factors unique to the phenotypes (11% each), while MDD did not. Unique environment explained 23% of the variability in MDD, 20% in GAD, and 45% in burnout. In conclusion, the covariation was explained by an underlying common factor, largely influenced by genetics. Burnout was to a large degree influenced by unique environmental factors not shared with MDD and GAD.

Grip Strength and Cognitive Abilities: Associations in Old Age

OBJECTIVES

Both physical functioning and cognitive abilities are important for well-being, not least in old age. Grip strength is often considered an indicator of general vitality and, as such, may predict cognitive functioning. Few longitudinal studies have examined the relationship between grip strength and cognition, especially where specific cognitive abilities have been targeted.

METHOD

Participants (n = 708, age range: 40-86 years at baseline) came from the population-based longitudinal Swedish Adoption/Twin Study of Aging. We used a longitudinal follow-up of 6 waves during 20 years. For the analyses, we used latent growth modeling, where latent growth trajectories were fitted to the cognitive traits (verbal ability, spatial ability, processing speed, and memory) or to the grip strength values and each, respectively, treated as time-varying covariates of the other trait.

RESULTS

Results supported a longitudinal influence of grip strength on changes in cognitive function. Grip strength performance was associated with change in the 4 cognitive abilities after age 65 years.

DISCUSSION

A rather stable connection was found between grip strength and cognitive abilities starting around 65 years of age. The starting period suggests that the association may be due to lifestyle changes, such as retirement, or to acceleration of the aging processes.

Validation of abridged mini-mental state examination scales using population-based data from Sweden and USA

The objective of this study is to validate two abridged versions of the mini-mental state examination (MMSE): one intended for use in face-to-face interviews, and the other developed for telephonic interviews, using data from Sweden and the US to validate the abridged scales against dementia diagnoses as well as to compare their performance to that of the full MMSE scale. The abridged versions were based on eight domains from the original MMSE scale. The domains included in the MMSE-SF were registration, orientation, delayed recall, attention, and visual spatial ability. In the MMSE-SF-C, the visual spatial ability item was excluded, and instead, one additional orientation item was added. There were 794 participants from the Swedish HARMONY study [mean age 81.8 (4.8); the proportion of cognitively impaired was 51 %] and 576 participants from the US ADAMS study [mean age 83.2 (5.7); the proportion of cognitively impaired was 65 %] where it was possible to compare abridged MMSE scales to dementia diagnoses and to the full MMSE scale. We estimated the sensitivity and specificity levels of the abridged tests, using clinical diagnoses as reference. Analyses with both the HARMONY and the ADAMS data indicated comparable levels of sensitivity and specificity in detecting cognitive impairment for the two abridged scales relative to the full MMSE. Receiver operating characteristic curves indicated that the two abridged scales corresponded well to those of the full MMSE. The two abridged tests have adequate validity and correspond well with the full MMSE. The abridged versions could therefore be alternatives to consider in larger population studies where interview length is restricted, and the respondent burden is high.

Secular Slowing of Auditory Simple Reaction Time in Sweden (1959-1985)

There are indications that simple reaction time might have slowed in Western populations, based on both cohort- and multi-study comparisons. A possible limitation of the latter method in particular is measurement error stemming from methods variance, which results from the fact that instruments and experimental conditions change over time and between studies. We therefore set out to measure the simple auditory reaction time (SRT) of 7,081 individuals (2,997 males and 4,084 females) born in Sweden 1959-1985 (subjects were aged between 27 and 54 years at time of measurement). Depending on age cut-offs and adjustment for aging related slowing of SRT, the data indicate that SRT has increased by between 3 and 16 ms in the 27 birth years covered in the present sample. This slowing is unlikely to be explained by attrition, which was evaluated by comparing the general intelligence × birth-year interactions and standard deviations for both male participants and dropouts, utilizing military conscript cognitive ability data. The present result is consistent with previous studies employing alternative methods, and may indicate the operation of several synergistic factors, such as recent micro-evolutionary trends favoring lower g in Sweden and the effects of industrially produced neurotoxic substances on peripheral nerve conduction velocity.

Nonparametric estimation of the rediscovery rate

Validation studies have been used to increase the reliability of the statistical conclusions for scientific discoveries; such studies improve the reproducibility of the findings and reduce the possibility of false positives. Here, one of the important roles of statistics is to quantify reproducibility rigorously. Two concepts were recently defined for this purpose: (i) rediscovery rate (RDR), which is the expected proportion of statistically significant findings in a study that can be replicated in the validation study and (ii) false discovery rate in the validation study (vFDR). In this paper, we aim to develop a nonparametric approach to estimate the RDR and vFDR and show an explicit link between the RDR and the FDR. Among other things, the link explains why reproducing statistically significant results even with low FDR level may be difficult. Two metabolomics datasets are considered to illustrate the application of the RDR and vFDR concepts in high-throughput data analysis. Copyright © 2016 John Wiley & Sons, Ltd.

Cognitive endophenotypes inform genome-wide expression profiling in schizophrenia

OBJECTIVE

We performed a whole-genome expression study to clarify the nature of the biological processes mediating between inherited genetic variations and cognitive dysfunction in schizophrenia.

METHOD

Gene expression was assayed from peripheral blood mononuclear cells using Illumina Human WG6 v3.0 chips in twins discordant for schizophrenia or bipolar disorder and control twins. After quality control, expression levels of 18,559 genes were screened for association with the California Verbal Learning Test (CVLT) performance, and any memory-related probes were then evaluated for variation by diagnostic status in the discovery sample (N = 190), and in an independent replication sample (N = 73). Heritability of gene expression using the twin design was also assessed.

RESULTS

After Bonferroni correction (p < 2.69 × 10-6), CVLT performance was significantly related to expression levels for 76 genes, 43 of which were differentially expressed in schizophrenia patients, with comparable effect sizes in the same direction in the replication sample. For 41 of these 43 transcripts, expression levels were heritable. Nearly all identified genes contain common or de novo mutations associated with schizophrenia in prior studies.

CONCLUSION

Genes increasing risk for schizophrenia appear to do so in part via effects on signaling cascades influencing memory. The genes implicated in these processes are enriched for those related to RNA processing and DNA replication and include genes influencing G-protein coupled signal transduction, cytokine signaling, and oligodendrocyte function.

A National Swedish Twin-Sibling Study of Alcohol Use Disorders

The relationship between the genetic and environmental risk factors for alcohol use disorders (AUD) detected in Swedish medical, pharmacy, and criminal registries has not been hitherto examined. Prior twin studies have varied with regard to the detection of shared environmental effects and sex differences in the etiology of AUD. In this report, structural equation modeling in OpenMx was applied to (1) the three types of alcohol registration in a population-based sample of male-male twins and reared-together full and half siblings (total 208,810 pairs), and (2) AUD, as a single diagnosis, in male-male, female-female, and opposite-sex (OS) twins and reared-together full and half siblings (total 787,916 pairs). An independent pathway model fit best to the three forms of registration and indicated that between 70% and 92% of the genetic and 63% and 98% of the shared environmental effects were shared in common with the remainder unique to each form of AUD registration. Criminal registration had the largest proportion of unique genetic and environmental factors. The best fit model for AUD estimated the heritability to be 22% and 57%, respectively, in females and males. Both shared (12% vs. 6%) and special twin environment (29% vs. 2%) were substantially more important in females versus males. In conclusion, AUD ascertained from medical, pharmacy, and criminal Swedish registries largely share the same genetic and environmental risk factors. Large sex differences in the etiology of AUD were seen in this sample, with substantially stronger familial environmental and weaker genetic effects in females versus males.

Contribution of non-genetic factors to dopamine and serotonin receptor availability in the adult human brain

The dopamine (DA) and serotonin (5-HT) neurotransmission systems are of fundamental importance for normal brain function and serve as targets for treatment of major neuropsychiatric disorders. Despite central interest for these neurotransmission systems in psychiatry research, little is known about the regulation of receptor and transporter density levels. This lack of knowledge obscures interpretation of differences in protein availability reported in psychiatric patients. In this study, we used positron emission tomography (PET) in a twin design to estimate the relative contribution of genetic and environmental factors, respectively, on dopaminergic and serotonergic markers in the living human brain. Eleven monozygotic and 10 dizygotic healthy male twin pairs were examined with PET and [(11)C]raclopride binding to the D2- and D3-dopamine receptor and [(11)C]WAY100635 binding to the serotonin 5-HT1A receptor. Heritability, shared environmental effects and individual-specific non-shared effects were estimated for regional D2/3 and 5-HT1A receptor availability in projection areas. We found a major contribution of genetic factors (0.67) on individual variability in striatal D2/3 receptor binding and a major contribution of environmental factors (pairwise shared and unique individual; 0.70-0.75) on neocortical 5-HT1A receptor binding. Our findings indicate that individual variation in neuroreceptor availability in the adult brain is the end point of a nature-nurture interplay, and call for increased efforts to identify not only the genetic but also the environmental factors that influence neurotransmission in health and disease.

Association Between Sleep Characteristics and Incident Dementia Accounting for Baseline Cognitive Status: A Prospective Population-Based Study

BACKGROUND

Although research has shown that sleep disorders are prevalent among people with dementia, the temporal relationship is unclear. We investigated whether atypical sleep characteristics were associated with incident dementia while accounting for baseline cognitive functioning.

METHODS

Screening Across the Lifespan Twin (SALT) study participants were 11,247 individuals from the Swedish Twin Registry who were at least 65 years at baseline (1998-2002). Sleep and baseline cognitive functioning were assessed via the SALT telephone screening interview. Data on dementia diagnoses came from national health registers. Cox regression was performed to estimate hazard ratios for dementia.

RESULTS

After 17 years of follow-up, 1,850 dementia cases were identified. Short (≤6 hours) and extended (>9 hours) time in bed (TIB) compared to the middle reference group (hazard ratio = 1.40, 95% confidence interval = 1.06-1.85; hazard ratio = 1.11, 95% confidence interval = 1.00-1.24, respectively) and rising at 8:00 AM or later compared to earlier rising (hazard ratio = 1.12, 95% confidence interval = 1.01-1.24) were associated with higher dementia incidence. Bedtime, sleep quality, restorative sleep, and heavy snoring were not significant predictors. Findings stratified by baseline cognitive status indicated that the association between short TIB and dementia remained in those cognitively intact at the start.

CONCLUSIONS

Short and extended TIB and delayed rising among older adults predicted increased dementia incidence in the following 17 years. The pattern of findings suggests that extended TIB and late rising represent prodromal features whereas short TIB appeared to be a risk factor for dementia.

Twin Studies: A Unique Epidemiological Tool

Twin studies are a special type of epidemiological studies designed to measure the contribution of genetics as opposed to the environment, to a given trait. Despite the facts that the classical twin studies are still being guided by assumptions made back in the 1920s and that the inherent limitation lies in the study design itself, the results suggested by earlier twin studies have often been confirmed by molecular genetic studies later. Use of twin registries and various innovative yet complex software packages such as the (SAS) and their extensions (e.g., SAS PROC GENMOD and SAS PROC PHREG) has increased the potential of this epidemiological tool toward contributing significantly to the field of genetics and other life sciences.

A longitudinal twin study on Tojikomori and depressive symptoms in Japanese elderly

BACKGROUND

Genetic factors associated with Tojikomori (being housebound without any limitations in physical functioning or mental problems) are unclear. We performed a twin study to help clarify the causes of Tojikomori and investigate the association between Tojikomori and depressive symptoms.

METHODS

Participants were members of the Osaka University Aged Twin Registry. A total of 157 twin pairs (314 individuals) who provided data in 2008 and 2012 were included. Depressive symptoms were measured by the Geriatric Depression Scale short version (GDS). Social Activities Scores (SAS) were measured with questions assessing the frequency of going out and contact with others. Activities of daily living were assessed with the Barthel index. Structural equation modelling analysis was used to evaluate the relative importance of genes and environments for the phenotypes measured.

RESULTS

In the first analysis, for SAS (2012), 25% of the variance was explained by additive genetic effects, and 75% was explained by unique environmental (including error) effects. For the GDS (2012), 28% of the variance was explained by additive genetic effects, and 72% was explained by unique environmental (including error) effects. In the second analysis, 25% of the total variance in SAS (2012) was explained by additive genetic factors, which influenced GDS scores (2008 and 2012). However, only 10% of the total variance in GDS scores (2012) was explained by additive genetic factors that influenced SAS (2008 and 2012).

CONCLUSIONS

Tojikomori was influenced by genetic factors in the elderly Japanese subjects. The association between Tojikomori and depressive symptoms was explained by genetic and environmental factors that are common to both phenotypes. Depressive symptoms were a cause of future Tojikomori and were affected by additive genetic factors. Genetic factors of Tojikomori were not a major cause of future depressive symptoms.

Genetic overlap between type 2 diabetes and depression in Swedish and Danish twin registries

A bidirectional association between type 2 diabetes (T2DM) and depression has been consistently reported. Depression is associated with worse biomedical outcomes and increased mortality. The mechanisms underlying the association of T2DM with depression remain unclear. One possible question we can address is the extent to which the co-occurrence of diabetes and depression is due to correlated genetic and/or environmental risk factors. In this study, we performed structural equation model fitting to population-level data from the Swedish (n=68 606) and Danish (n=95 403) twin registries. The primary outcomes were clinical diagnosis of T2DM and depression using national hospital discharge registries. The phenotypic correlation between T2DM and depression is modest in both samples. In the Swedish sample, unique environmental effects explain a greater proportion of the covariance in males, whereas the association is primarily attributed to genetic effects in females. In the Danish sample, genetic effects account for the majority of the covariance in both males and females. Qualitative genetic sex differences are observed in both samples. We believe this is the first study to demonstrate significant genetic overlap between T2DM and depression.

Cystatin C Predicts Incident Cardiovascular Disease in Twins

BACKGROUND

Cystatin C is associated with both renal function and atherosclerotic cardiovascular disease (ASCVD). We have previously shown a genetic correlation between cystatin C and prevalent ASCVD. The objective of this article is to study whether variation in cystatin C or creatinine predicts incident ASCVD when controlled for genetic factors.

METHODS AND RESULTS

The predictive value of cystatin C and creatinine for incident ASCVD was studied in 11 402 Swedish twins, free of CVD at baseline, in an adjusted Cox-regression model during a median follow-up of 71 months. Twin pairs discordant for incident stroke, myocardial infarction and ASCVD during follow-up were identified and within-pair comparisons regarding cystatin C and creatinine levels were performed. We also investigated whether contact frequency and degree of shared environment influences were associated with similarity in cystatin C levels. In univariate analysis, cystatin C predicted incident ASCVD hazard ratio 1.57, 95% CI 1.47-1.67. When adjusted for traditional Framingham risk factors as covariates, cystatin C remained a predictor of incident stroke hazard ratio 1.45, 95% CI (1.25-1.70), ASCVD hazard ratio 1.26, 95% CI (1.13-1.41), and myocardial infarction hazard ratio 1.16, 95% CI (1.01-1.33). In twins discordant for incident stroke, cystatin C at baseline was higher in the twin who experienced a stroke compared to the healthy co-twin (1.11±0.3 mg/L versus 1.06±0.3 mg/L), whereas creatinine was lower in the twin who developed CVD compared to their healthy co-twins (76.1±16.9 μmol/L versus 79.4±20.3 μmol/L).

CONCLUSIONS

Variation in cystatin C relates to incident ASCVD and to stroke when adjusted for genetic confounding. In identical twins, cystatin C may be a sensitive marker of early hypertensive end-organ damage and small-vessel disease, whereas creatinine level may reflect nutritional status. The findings in disease-discordant monozygotic twins indicate that unique, possibly preventable, environmental factors are important.

High Job Demands, Job Strain, and Iso-Strain are Risk Factors for Sick Leave due to Mental Disorders: A Prospective Swedish Twin Study With a 5-Year Follow-Up

OBJECTIVE

To investigate whether psychosocial work environment and health behaviors are risk factors for sick leave due to mental disorders, and whether familial confounding (genetics and shared environment) explains the associations.

METHODS

Respondents (n = 11,729), given to complete a questionnaire in 2004 to 2006, were followed up approximately 5 years for sick leave spells due to mental disorders, using national registry data. Data were analyzed using logistic regression, and conditional logistic regression for twin pairs discordant for sick leave (cotwin control).

RESULTS

High job demands, job strain, and iso-strain were independent risk factors for sick leave due to mental disorders. Familial factors seem to be of importance in the associations between job support, smoking, a combination of unhealthy behaviors and sick leave.

CONCLUSIONS

Improving the psychosocial work environment may be effective in preventing sick leave due to mental disorders.

Concordance in Anti-OmpC and Anti-I2 Indicate the Influence of Genetic Predisposition: Results of a European Study of Twins with Crohn's Disease

BACKGROUND AND AIMS

An adaptive immunological response to microbial antigens has been observed in Crohn's disease (CD). Intriguingly, this serological response precedes the diagnosis in some patients and has also been observed in healthy relatives. We aimed to determine whether genetic factors are implicated in this response in a CD twin cohort.

METHODS

In total, 82 twin pairs (Leuven n = 13, Maastricht n = 8, Örebro n = 61) took part: 81 pairs with CD (concordant monozygotic n = 16, discordant monozygotic n = 22, concordant dizygotic n = 3, discordant dizygotic n = 40) and 1 monozygotic pair with both CD and ulcerative colitis. Serology for Pseudomonas fluorescens-related protein (anti-I2), Escherichia coli outer membrane porin C (anti-OmpC), CBir1flagellin (anti-CBir1) and antibodies to oligomannan (anti-Saccharomyces cerevisiae antibody [ASCA]) was determined by standardized enzyme-linked immunoassay.

RESULTS

All markers were more often present in CD twins than in their healthy twin siblings. Using the intraclass correlation coefficient (ICC), agreements in concentrations of anti-OmpC and anti-I2 were observed in discordant monozygotic but not in discordant dizygotic twin pairs with CD (anti-OmpC, ICC 0.80 and -0.02, respectively) and (anti-I2, ICC 0.56 and 0.05, respectively). In contrast, no agreements were found in anti-CBir, immunoglobulin (Ig) G ASCA and ASCA IgA.

CONCLUSIONS

We show that anti-I2 and anti-CBir1 statuses have specificity for CD and confirm previous reported specificities for anti-OmpC and ASCA. Based on quantitative analyses and observed ICCs, genetics seems to predispose to the anti-OmpC and anti-I2 response but less to ASCA and anti-CBir1 responses.

How well do ACPA discriminate and predict RA in the general population: a study based on 12 590 population-representative Swedish twins

Objective

Anti-citrullinated protein antibodies (ACPA) are highly specific for rheumatoid arthritis (RA), but the diagnostic accuracy of ACPA in the general population has not been thoroughly assessed. We aimed to assess the diagnostic accuracy of ACPA for RA in the general population and to further characterise the citrullinated peptide recognition pattern.

Methods

Serum samples from a large population-representative twin cohort consisting of 12 590 individuals were analysed for the presence of ACPA using anti-CCP2 ELISA. All ACPA-positive samples were further tested on ELISAs for four peptide-specific ACPA. RA cases were identified by linkage to the Swedish National Patient Register at inclusion and after a median follow-up of 37 months (IQR 31–49).

Results

350 out of 12 590 individuals had a positive anti-CCP2 test, measuring ACPA. Of these, 103 had an RA diagnosis at the time of blood donation and inclusion. During a median follow-up of 3 years, an additional 21 of the remaining 247 ACPA-positive individuals developed RA. Overall, a positive anti-CCP2 test had a positive predictive value of 29% for prevalent RA at inclusion (negative predictive value of 99.6%). High titres (>3× cut-off) of anti-CCP2 increased the positive predictive value to 48% (negative predictive value of 99.5%). ACPA-positive individuals without RA had lower anti-CCP2 titres and fewer peptide-specific ACPA than ACPA-positive patients with RA and higher C reactive protein levels than ACPA-negative individuals without RA.

Conclusion

Presence of ACPA and especially high titres of anti-CCP2 have a high diagnostic accuracy for an RA diagnosis in a population setting.

Genetic and environmental components to self-induced vomiting

OBJECTIVE

We examined the association between the genetic and environmental factors contributing to the liability to having ever engaged in self-induced vomiting (SIV initiation) and the genetic and environmental factors contributing to regular SIV behaviors (weekly or daily) for weight control.

METHOD

SIV was assessed in 3,942 women from monozygotic twin pairs and 2,790 women from same-sex dizygotic twin pairs, aged 20-47, from the Swedish Twin study of Adults: Genes and Environment. A causal-contingent-common pathway model assessed the extent to which genetic and environmental factors that influence initiation of SIV also influence regular SIV behaviors.

RESULTS

In the best-fit model, genetic and individual-specific environmental factors influenced liability to SIV initiation. The genetic factors influencing regular SIV behaviors were the same as the genetic factors influencing SIV initiation. Additional individual-specific environmental factors that were unrelated to SIV initiation influenced regular SIV behaviors.

DISCUSSION

Our findings provide evidence that the underlying liabilities for SIV initiation and regular SIV lie on the same continuum given the degree of overlap in risk between SIV initiation and regular SIV behaviors. Further, the lack of specific genetic factors and the importance of individual-specific environmental factors for regular SIV behaviors highlight the significance of environmental factors in the etiology of eating disorder symptomatology and the non-deterministic nature of genetic factors. Finally, our results suggest that when it comes to preventing individuals from developing regular SIV behavior, intervening at an environmental level is warranted.

Genetic overlap between impulsivity and alcohol dependence: a large-scale national twin study

BACKGROUND

Alcohol dependence is associated with increased levels of impulsivity, but the genetic and environmental underpinnings of this overlap remain unclear. The purpose of the current study was to investigate the degree to which genetic and environmental factors contribute to the overlap between alcohol dependence and impulsivity.

METHOD

Univariate and bivariate twin model fitting was conducted for alcohol dependence and impulsivity in a national sample of 16 819 twins born in Sweden from 1959 to 1985.

RESULTS

The heritability estimate for alcohol dependence was 44% [95% confidence interval (CI) 31-57%] for males and 62% (95% CI 52-72%) for females. For impulsivity, the heritability was 33% (95% CI 30-36%) in males and females. The bivariate twin analysis indicated a statistically significant genetic correlation between alcohol dependence and impulsivity of 0.40 (95% CI 0.23-0.58) in males and 0.20 (95% CI 0.07-0.33) in females. The phenotypic correlation between alcohol dependence and impulsivity was 0.20 and 0.17 for males and females, respectively, and the bivariate heritability was 80% (95% CI 47-117%) for males and 53% (95% CI 19-86%) for females. The remaining variance in all models was accounted for by non-shared environmental factors.

CONCLUSIONS

The association between alcohol dependence and impulsivity can be partially accounted for by shared genetic factors. The genetic correlation was greater in men compared with women, which may indicate different pathways to the development of alcohol dependence between sexes. The observed genetic overlap has clinical implications regarding treatment and prevention, and partially explains the substantial co-morbidity between alcohol dependence and psychiatric disorders characterized by impulsive behaviour.

Individual Differences in Personality Masculinity-Femininity: Examining the Effects of Genes, Environment, and Prenatal Hormone Transfer

Males and females score differently on some personality traits, but the underlying etiology of these differences is not well understood. This study examined genetic, environmental, and prenatal hormonal influences on individual differences in personality masculinity-femininity (M-F). We used Big-Five personality inventory data of 9,520 Swedish twins (aged 27 to 54) to create a bipolar M-F personality scale. Using biometrical twin modeling, we estimated the influence of genetic and environmental factors on individual differences in a M-F personality score. Furthermore, we tested whether prenatal hormone transfer may influence individuals' M-F scores by comparing the scores of twins with a same-sex versus those with an opposite-sex co-twin. On average, males scored 1.09 standard deviations higher than females on the created M-F scale. Around a third of the variation in M-F personality score was attributable to genetic factors, while family environmental factors had no influence. Males and females from opposite-sex pairs scored significantly more masculine (both approximately 0.1 SD) than those from same-sex pairs. In conclusion, genetic influences explain part of the individual differences in personality M-F, and hormone transfer from the male to the female twin during pregnancy may increase the level of masculinization in females. Additional well-powered studies are needed to clarify this association and determine the underlying mechanisms in both sexes.

Applying the demand-control-support model on burnout in managers and non-managers

Purpose

– The purpose of this paper is to study the demand-control-support (DCS) model on burnout in male and female managers and non-managers, taking into account genetic and shared family environmental factors, contributing to the understanding of mechanisms of how and when work stress is related to burnout.

Design/methodology/approach

– A total of 5,510 individuals in complete same-sex twin pairs from the Swedish Twin Registry were included in the analyses. Co-twin control analyses were performed using linear mixed modeling, comparing between-pairs and within-pair effects, stratified by zygosity and sex.

Findings

– Managers scored higher on demands and control in their work than non-managers, and female managers seem to be particularly at risk for burnout facing more demands which are not reduced by a higher control as in their male counterparts. Co-twin analyses showed that associations between control and burnout as well as between demands and burnout seem to be affected by shared family environmental factors in male non-managers but not in male managers in which instead the associations between social support and burnout seem to be influenced by shared family environment.

Practical implications

– Taken together, the study offers knowledge that shared environment as well as sex and managerial status are important factors to consider in how DCS is associated to exhaustion.

Originality/value

– Using twin data with possibilities to control for genetics, shared environment, sex and age, this study offers unique insight into the DCS research, which focusses primarily on the workplace environment rather than individual factors.

Somatic Comorbidity in Women with Overactive Bladder Syndrome

PURPOSE

We explore the influence of co-occurring somatic illnesses on prevalent overactive bladder in women of premenopausal age.

MATERIALS AND METHODS

Data for the present study were derived from a nationwide survey on complex diseases among all twins in the Swedish Twin Registry born 1959 to 1985. The present study was limited to female twins participating in the survey (12,850). Generalized estimating equations were used to estimate odds ratios with 95% CIs. Environmental and genetic influences were assessed in co-twin control analysis.

RESULTS

Generalized estimating equations analysis showed a significant association between overactive bladder and migraine (OR 1.34, 95% CI 1.15-1.57), fibromyalgia (1.83, 1.54-2.18), chronic fatigue (1.81, 1.49-2.19) and eating disorders (1.56, 1.24-1.96). There was also a significant association with allergic disorders including asthma (1.24, 1.01-1.52) and eczema (1.22, 1.04-1.43). Among reproductive disorders, urinary tract infections (1.60, 1.40-1.84), dysmenorrhea (1.53, 1.33-1.76) and pelvic pain (1.60, 1.31-1.94) showed the strongest association with overactive bladder. Results from co-twin control analysis indicated that the significant associations observed in generalized estimating equations analysis were influenced by environmental and genetic factors without a common pathway model.

CONCLUSIONS

Our results suggest a multifactorial and complex pathogenesis of overactive bladder in which associations between various somatic illnesses and overactive bladder may be affected by environmental and genetic factors.

Pregnancy hypertensive disease and risk of dementia and cardiovascular disease in women aged 65 years or older: a cohort study

OBJECTIVE

The primary aim was to study pregnancy hypertensive disease and subsequent risk of dementia. The second aim was to study if the increased risks of cardiovascular disease (CVD) and stroke after pregnancy hypertensive disease persist in an elderly population.

DESIGN

Cohort study.

SETTING

Sweden.

POPULATION OR SAMPLE

3232 women 65 years or older (mean 71 years) at inclusion.

METHODS

Cox proportional hazards regression analyses were used to calculate risks of dementia, CVD and/or stroke for women exposed to pregnancy hypertensive disease. Exposure data were collected from an interview at inclusion during the years 1998-2002. Outcome data were collected from the National Patient Register and Cause of Death Register from the year of inclusion until the end of 2010. Age at inclusion was set as a time-dependent variable, and adjustments were made for body mass index, education and smoking.

MAIN OUTCOME MEASURES

Dementia, CVD, stroke.

RESULTS

During the years of follow-up, 7.6% of the women exposed to pregnancy hypertensive disease received a diagnosis of dementia, compared with 7.4% among unexposed women (HR 1.19; 95% CI 0.79 to 1.73). The corresponding rates for CVD were 22.9% for exposed women and 19.0% for unexposed women (HR 1.29; 95% CI 1.02 to 1.61), and for stroke 13.4% for exposed women and 10.7% for unexposed women (HR 1.36; 95% CI 1.00 to 1.81).

CONCLUSIONS

There was no increased risk of dementia after self-reported pregnancy hypertensive disease in our cohort. We found that the previously reported increased risk of CVD and stroke after pregnancy hypertensive disease persists in an older population.

Anxiety trajectories in the second half of life: Genetic and environmental contributions over age

Clinically significant anxiety symptoms are prevalent among the elderly, yet knowledge about the longitudinal course of anxiety symptoms in later life remains scarce. The goals of this study were to (a) characterize age trajectories of state anxiety symptoms in the second half of life, and (b) estimate genetic and environmental contributions to individual differences in the age trajectory of state anxiety. This study was based on data from 1,482 participants in the Swedish Adoption/Twin Study of Aging who were aged 50 and older at their first occasion (512 complete twin pairs, 458 singletons) and had up to 6 measurement occasions spanning 11 years. Consistent with life span developmental theories of age-related emotional change, anxiety symptom levels declined during the transition from midlife to the mid-60s, followed by a mild increase that gradually plateaued in the 80s. There were substantial individual differences in the age trajectory of anxiety. After accounting for effects of sex, cohort, mode of testing, and proximity to death, this longitudinal variation was partitioned into biometric sources. Nonshared environmental variance was highest in the late 60s and declined thereafter, whereas genetic variance increased at an accelerated pace from approximately age 60 onward. There was no evidence for effects of rearing or other shared environment on anxiety symptoms in later life. These findings highlight how the etiology of anxiety symptoms changes from midlife to old age.

Impact of asthma medication and familial factors on the association between childhood asthma and attention-deficit/hyperactivity disorder: a combined twin- and register-based study: Epidemiology of Allergic Disease

BACKGROUND

Asthma and attention-deficit/hyperactivity disorder (ADHD) are prevalent in childhood and may cause functional impairment and stress in families. Previous research supports an association between asthma and ADHD in children, but several aspects of this relationship are unclear.

OBJECTIVE

Our aim was to study whether the association between asthma and ADHD is restricted to either the inattentive or the hyperactive/impulsive symptoms of ADHD, to explore the impact of asthma severity and asthma medication and the contribution of shared genetic and environmental risk factors on the asthma-ADHD relationship.

METHODS

Data on asthma, ADHD, zygosity and possible confounders were collected from parental questionnaires at 9 or 12 years on 20 072 twins through the Swedish Twin Register, linked to the Swedish Medical Birth Register, the National Patient Register and the Prescribed Drug Register. The association between asthma and ADHD, the impact of asthma severity and medication, was assessed by generalized estimating equations. Cross-twin-cross-trait correlations (CTCT) were estimated to explore the relative importance of genes and environment for the association.

RESULTS

Asthmatic children had a higher risk of also having ADHD [odds ratio (OR) 1.53, 95% confidence interval (CI): 1.16-2.02]. The association was not restricted to either of the two dimensions of ADHD. The magnitude of the association increased with asthma severity (OR 2.84, 95% CI: 1.86-4.35) for ≥ 4 asthma attacks in the last 12 months and was not affected by asthma treatment. The CTCTs possibly indicate that the genetic component in overlap of the disorders is weak.

CONCLUSIONS AND CLINICAL RELEVANCE

Childhood asthma, especially severe asthma, is associated with ADHD. Asthma medication seems not to increase the risk of ADHD. Clinicians should be aware of the potential of ADHD in asthma. Optimal asthma care needs to be integrated with effective evaluation and treatment of ADHD in children with co-existing disorders.

Familial Risk and Heritability of Cancer Among Twins in Nordic Countries

IMPORTANCE

Estimates of familial cancer risk from population-based studies are essential components of cancer risk prediction.

OBJECTIVE

To estimate familial risk and heritability of cancer types in a large twin cohort.

DESIGN, SETTING, AND PARTICIPANTS

Prospective study of 80,309 monozygotic and 123,382 same-sex dizygotic twin individuals (N = 203,691) within the population-based registers of Denmark, Finland, Norway, and Sweden. Twins were followed up a median of 32 years between 1943 and 2010. There were 50,990 individuals who died of any cause, and 3804 who emigrated and were lost to follow-up.

EXPOSURES

Shared environmental and heritable risk factors among pairs of twins.

MAIN OUTCOMES AND MEASURES

The main outcome was incident cancer. Time-to-event analyses were used to estimate familial risk (risk of cancer in an individual given a twin's development of cancer) and heritability (proportion of variance in cancer risk due to interindividual genetic differences) with follow-up via cancer registries. Statistical models adjusted for age and follow-up time, and accounted for censoring and competing risk of death.

RESULTS

A total of 27,156 incident cancers were diagnosed in 23,980 individuals, translating to a cumulative incidence of 32%. Cancer was diagnosed in both twins among 1383 monozygotic (2766 individuals) and 1933 dizygotic (2866 individuals) pairs. Of these, 38% of monozygotic and 26% of dizygotic pairs were diagnosed with the same cancer type. There was an excess cancer risk in twins whose co-twin was diagnosed with cancer, with estimated cumulative risks that were an absolute 5% (95% CI, 4%-6%) higher in dizygotic (37%; 95% CI, 36%-38%) and an absolute 14% (95% CI, 12%-16%) higher in monozygotic twins (46%; 95% CI, 44%-48%) whose twin also developed cancer compared with the cumulative risk in the overall cohort (32%). For most cancer types, there were significant familial risks and the cumulative risks were higher in monozygotic than dizygotic twins. Heritability of cancer overall was 33% (95% CI, 30%-37%). Significant heritability was observed for the cancer types of skin melanoma (58%; 95% CI, 43%-73%), prostate (57%; 95% CI, 51%-63%), nonmelanoma skin (43%; 95% CI, 26%-59%), ovary (39%; 95% CI, 23%-55%), kidney (38%; 95% CI, 21%-55%), breast (31%; 95% CI, 11%-51%), and corpus uteri (27%; 95% CI, 11%-43%).

CONCLUSIONS AND RELEVANCE

In this long-term follow-up study among Nordic twins, there was significant excess familial risk for cancer overall and for specific types of cancer, including prostate, melanoma, breast, ovary, and uterus. This information about hereditary risks of cancers may be helpful in patient education and cancer risk counseling.

Associations Between Fetal Growth and Self-Perceived Health Throughout Adulthood: A Co-twin Control Study

The literature shows evidence for long-lasting effects of low birth weight (LBW) on many health outcomes, but little is known about effects on self-perceived health. Findings are mixed and studies are small, mostly focusing on LBW effects on health outcomes before adulthood. Further, as LBW and most health conditions including self-perceived health are partly heritable, associations between birth weight (BW) and adverse health outcomes may also be due to shared genetic as well as other (pre- and postnatal) unmeasured environmental influences. We explored LBW effects on self-perceived health in early and later adulthood using a very large and genetically informative sample of more than 50,000 Swedish twins. In addition, analyses within twin pairs (the co-twin control design) were used to examine potential associations between BW and the offspring's risk for poor self-perceived health independent of shared environmental or genetic factors, evidence which is critical for the understanding of underlying mechanisms. Results showed that lower BW was significantly associated with poorer self-perceived health during adulthood, although the effect size was small. Co-twin control analyses suggested that this increased risk may be due to shared underlying liability (environmental or genetic) rather than a direct effect of BW, but findings were not conclusive.

Psychological strength assessed in late adolescence and risk for criminal behavior: a Swedish prospective cohort and twin analysis

BACKGROUND

Certain personality traits predispose to criminal behavior (CB). We further clarify this relationship in a Swedish national sample.

METHOD

Psychological strength (PS) was assessed on a nine-point scale at personal interview in 1 653 721 Swedish men aged 18-20 years. We examined the association between PS and total, violent and recurrent CB over the lifetime (logistic regression), prospectively (Cox regression) and by bivariate Cholesky decomposition in 2507 monozygotic and 2244 dizygotic twin pairs (OpenMx).

RESULTS

Examining linear effects by logistic regression, PS was robustly associated with lifetime risk of total CB (per point, odds ratio = 0.74) and even more strongly associated with risk for violent (0.69) and recurrent CB (0.52). Prospective predictions of these three forms of CB by PS were similar, with hazard ratios of 0.80, 0.73 and 0.54, respectively. Twin modeling demonstrated that, for all three CB types, the association with PS arose almost entirely from familial effects. Common shared environment accounted for 72, 56 and 43% of the phenotypic correlation between PS and, respectively, total, violent and recurrent CB. Parallel figures for common genetic effects were for 24, 37 and 54%, respectively.

CONCLUSIONS

PS is strongly related to risk for total CB, and even more strongly for violent and, especially, recurrent CB. This association is probably not causal but rather results from shared familial risk factors that make an impact both on PS and risk for CB. PS has a stronger overall correlation with more severe criminal outcomes and a higher proportion of that correlation results from common genetic factors.

Genetic and Environmental Influences on Disability Pension Due To Mental Diagnoses: Limited Importance of Major Depression, Generalized Anxiety, and Chronic Fatigue

BACKGROUND

Previous research indicates that liability to disability pension (DP) due to mental diagnoses is moderately influenced by genetic factors. This study investigates whether genetic contributions to the liability to DP due to mood and neurotic diagnoses overlap with the genetic influences on major depression (MD), generalized anxiety disorder (GAD), or chronic fatigue (CF).

METHOD

A prospective cohort study including 9,985 female twins born in Sweden 1933-1958. The presence of MD, GAD, and CF was assessed by computer-assisted telephone interviews conducted in 1998-2002. Data on DP due to mood and neurotic diagnoses were obtained from nationwide registers for the years 1998-2010. Common genetic and environmental influences on the phenotypes were estimated by applying structural equation modeling.

RESULTS

The prevalence of MD/GAD was 30%, CF 8%, and DP due to mood and neurotic diagnoses 3% in 2010. Genetic effects on MD/GAD explained 31% of the total genetic variation in DP, whereas genetic contributions in common with CF were small and not significant. The majority of the total non-shared environmental variance in DP (85%) was explained by the factors that were unique to DP.

CONCLUSIONS

Large proportions of genetic and non-shared environmental influences in DP due to mood and neurotic diagnoses were not explained by the contributions from MD/GAD or CF. The results suggest that the process leading to DP is complex and influenced by factors other than those related to the disorder underlying DP.

Antemortem Prediction of Braak Stage

We examined the extent to which tauopathy distribution, as determined by Braak staging, might be predicted by various risk factors in older individuals. The Swedish Twin Registry provided extensive information on neuropsychological function, lifestyle, and cardiovascular risk factors of 128 patients for whom autopsy data including Braak staging were available. Logistic regression was used to develop a prognostic model that targeted discrimination between Braak stages 0 to II and III to VI. The analysis showed that Braak stages III to VI were significantly predicted by having 1 or more APOE ε4 alleles, older age, high total cholesterol, absence of diabetes and cardiovascular disease, and poorer scores on the Wechsler Adult Intelligence Score Information test, verbal fluency, and recognition memory but better verbal recall. The algorithm predicted Braak stages III to VI well (receiver-operating characteristic area under curve, 0.897; 95% confidence interval, 0.842-0.951). Using a cutoff of 50% risk or more, the sensitivity was 85%, the specificity was 70%, and the negative predictive value was 69%. This study demonstrates that tauopathy distribution can be accurately predicted using a combination of antemortem patient data. These results provide further insight into tauopathy development and AD-related disease mechanisms and suggest a prognostic model that predicts the spread of neurofibrillary tangles above the transentorhinal stage.

A Swedish Population-Based Multivariate Twin Study of Externalizing Disorders

In epidemiological and twin populations, prior interview studies have identified an externalizing spectrum of disorders. Could this be detected utilizing objective registry data? In 20,603 twin pairs from the Swedish Twin Registry, we obtained information from national medical, criminal and pharmacy records on drug abuse (DA), criminal behavior (CB) and alcohol use disorders (AUD). Multivariate twin modeling was performed with the OpenMx package. A common pathway model with quantitative but not qualitative sex effects fit best with twin resemblance for the latent liability to externalizing syndromes due to both genetic and shared environmental factors. Heritability of the liability was higher in females (76 vs. 62%) while shared environmental influences were considerably stronger in males (23 vs. 3%). In both sexes, this latent liability was most strongly indexed by DA and least by CB. All three syndromes had specific genetic influences (especially CB and AUD in males, and CB in females) and specific shared environmental effects (especially DA and CB in males, and AUD in females). For DA, CB and AUD in men, and DA and AUD in women, at least 75% of the genetic risk arose through the common factor. The best fit model assumed that genetic and environmental influences on these externalizing syndromes in males and females were the same. We identified, in registry data, a highly heritable externalizing spectrum. DA, CB and AUD share substantial genetic and modest to moderate shared environmental influences. The nature of the externalizing spectrum differed meaningfully between the sexes.

Heritability, family, school and academic achievement in adolescence

We demonstrate how genetically informed designs can be applied to administrative exam data to study academic achievement. ACE mixture latent class models have been used with Year 6 and 9 exam data for seven cohorts of Polish students which include 24,285 pairs of twins. Depending on a learning domain and classroom environment history, from 58% to 88% of variance in exam results is attributable to heritability, up to 34% to shared environment and from 8% to 15% depends on unique events in students' lives. Moreover, between 54% and 66% of variance in students' learning gains made between Years 6 and 9 is explained by heritability. The unique environment accounts for between 34% and 46% of that variance. However, we find no classroom effects on student progress made between Years 6 and 9. We situate this finding against the view that classroom peer groups and teachers matter for adolescent learning.

G×E Interaction Influences Trajectories of Hand Grip Strength

Age-related decline in grip strength predicts later life disability, frailty, lower well-being and cognitive change. While grip strength is heritable, genetic influence on change in grip strength has been relatively ignored, with non-shared environmental influence identified as the primary contributor in a single longitudinal study. The extent to which gene-environment interplay, particularly gene-environment interactions, contributes to grip trajectories has yet to be examined. We considered longitudinal grip strength measurements in seven twin studies of aging in the Interplay of Genes and Environment across Multiple Studies consortium. Growth curve parameters were estimated for same-sex pairs, aged 34-99 (N = 10,681). Fisher's test for mixture distribution of within-monozygotic twin-pair differences (N = 1724) was performed on growth curve parameters. We observed significant gene-environment interaction on grip strength trajectories. Finally, we compared the variability of within-pair differences of growth curve parameters by APOE haplotypes. Though not statistically significant, the results suggested that APOE ɛ2ɛ2/ɛ2ɛ3 haplotypes might buffer environmental influences on grip strength trajectories.

Risk of Sex-Specific Cancers in Opposite-Sex and Same-Sex Twins in Denmark and Sweden

BACKGROUND

Increasing evidence shows that some cancers originate in utero. It is hypothesized that elevated exposure to some steroid hormones might increase cancer risk and that hormone transfer between twin fetuses could result in different prenatal exposure to testosterone.

METHODS

This large-scale prospective twin study compared opposite-sex (OS) and same-sex (SS) twins to test the impact of intrauterine exposures on cancer risk. On the basis of the Danish and Swedish twin and cancer registries, we calculated incidence rate ratios for OS and SS twins, whereas standardized incidence ratios (SIR) with 95% confidence intervals (CI) were calculated for OS/SS twins compared with the general population.

RESULTS

A total of 18,001 cancers were identified during 1943-2009. No significant differences were observed between OS and SS twins, neither for the sex-specific cancers nor for cancer at all sites. All-cause cancer was slightly reduced for OS and SS twins compared with the general population, significant for OS males (SIR, 0.95; 95% CI, 0.92-0.98) and for SS males and females (SIR, 0.97; 95% CI, 0.94-0.99).

CONCLUSIONS

Our data suggest that having a male co-twin-which may entail higher exposure to prenatal testosterone-does not increase the risk of sex-specific cancers in OS females. Furthermore, the study supports that twinning per se is not a risk factor of cancer.

IMPACT

Findings are reassuring, as they fail to provide evidence for the hypothesis that endocrine or other difference in the in utero milieu affects the risk of sex-specific cancers.

A Swedish national twin study of criminal behavior and its violent, white-collar and property subtypes

BACKGROUND

We sought to clarify the etiological contribution of genetic and environmental factors to total criminal behavior (CB) measured as criminal convictions in men and women, and to violent (VCB), white-collar (WCCB) and property criminal behavior (PCB) in men only.

METHOD

In 21 603 twin pairs from the Swedish Twin Registry, we obtained information on all criminal convictions from 1973 to 2011 from the Swedish Crime Register. Twin modeling was performed using the OpenMx package.

RESULTS

For all criminal convictions, heritability was estimated at around 45% in both sexes, with the shared environment accounting for 18% of the variance in liability in females and 27% in males. The correlation of these risk factors across sexes was estimated at +0.63. In men, the magnitudes of genetic and environmental influence were similar in the three criminal conviction subtypes. However, for violent and white-collar convictions, nearly half and one-third of the genetic effects were respectively unique to that criminal subtype. About half of the familial environmental effects were unique to property convictions.

CONCLUSIONS

The familial aggregation of officially recorded CB is substantial and results from both genetic and familial environmental factors. These factors are moderately correlated across the sexes suggesting that some genetic and environmental influences on criminal convictions are unique to men and to women. Violent criminal behavior and property crime are substantially influenced respectively by genetic and shared environmental risk factors unique to that criminal subtype.

Engagement in dance is associated with emotional competence in interplay with others

This study has explored the relation between dance achievement and alexithymia in a larger Swedish population sample (Swedish Twin Registry) with a study sample of 5431 individuals. Dance achievement (CAQ) was assessed in relation to Alexithymia (Toronto Alexithymia Scale, TAS-20) including the three subscales: Difficulty Identifying Feelings (DIF), Difficulty Describing Feelings (DDF), and Externally Oriented Thinking (EOT). The results show a significant negative association between the TAS subscale (EOT) and creative achievement in dance. A high EOT score corresponds to poor ability to communicate feelings to the environment. There was no consistent association between the other factors DIF and DDF and dance achievement. Dance activity and training seem to be involved in the body’s emotional interplay with others. Embodied cognition, emotional perception, and action are discussed as factors relevant to measuring the skill of a dancer.

Personality traits and the risk for Parkinson disease: a prospective study

In this study, we explored the association between the personality traits, neuroticism and introversion, and risk of Parkinson disease (PD). A population-based cohort study was conducted using questionnaire data from the Swedish Twin Registry for twins born 1926–1958 (n > 29,000). Personality traits were assessed in 1973 by a short form of Eysenck’s Personality Inventory. The cohort was followed from 1974 to 2012 through Swedish patient and cause of death registers for PD ascertainment. Cox proportional hazards regression was used to estimate subsequent risk of PD, adjusting for attained age, sex and smoking. A mediation analysis was performed to further explore the role of smoking in the relationship between personality trait and PD. Confounding by familial factors was explored using a within-pair analysis. During a mean follow-up time of 36.8 years, 197 incident PD cases were identified. Both neuroticism and introversion were associated with an increased risk of PD after adjustment. Smoking was a significant mediator in the relationship between personality traits and PD that partly accounted for the effect of introversion, whereas it acted as a suppressor for the effect of neuroticism on PD risk. In the within-pair analyses, associations for neuroticism and introversion were attenuated. In conclusion, our study provides evidence that neuroticism is associated with an increased risk of PD that is in part suppressed by smoking. There was a weak association between introversion and PD and this effect was at least partly mediated through smoking. The observed effects may partly be explained by familial factors shared by twins.

Association between major depression and type 2 diabetes in midlife: findings from the Screening Across the Lifespan Twin Study

OBJECTIVE

Cohort studies suggest that the relationship between major depression (MD) and Type 2 diabetes (T2DM) is bidirectional. However, this association may be confounded by shared genetic or environmental factors. The objective of this study was to use a twin design to investigate the association between MD and T2DM.

METHODS

Data come from the Screening Across the Lifespan Twin Study, a sample of monozygotic and dizygotic twins 40 years or older sampled from the Swedish Twin Registry (n = 37,043). MD was assessed by using the Composite International Diagnostic Inventory. Structural equation twin modeling and Cox proportional hazards modeling were used to assess the relationship between MD and T2DM.

RESULTS

Approximately 19% of respondents had a history of MD and 5% had a history of T2DM. MD was associated with 32% increased likelihood of T2DM (95% confidence interval = 1.00-1.80) among twins aged 40 to 55 years, even after accounting for genetic risk, but was not associated with T2DM among twins older than 55 years. T2DM was associated with 33% increased likelihood of MD (95% confidence interval = 1.02-1.72) among younger, but not older twins. Cholesky decomposition twin modeling indicated that common unique environmental factors contribute to the association between MD and T2DM.

CONCLUSIONS

Environmental factors that are unique to individuals (i.e., not shared within families) but common to both MD and T2DM contribute to their co-occurrence in midlife. However, we cannot exclude the possibility of bidirectional causation as an alternate explanation. It is likely that multiple processes are operating to effect the relation between psychiatric and medical conditions in midlife.