Changing genetic architecture of body mass index from infancy to early adulthood: an individual based pooled analysis of 25 twin cohorts

Background

Body mass index (BMI) shows strong continuity over childhood and adolescence and high childhood BMI is the strongest predictor of adult obesity. Genetic factors strongly contribute to this continuity, but it is still poorly known how their contribution changes over childhood and adolescence. Thus, we used the genetic twin design to estimate the genetic correlations of BMI from infancy to adulthood and compared them to the genetic correlations of height.

Methods

We pooled individual level data from 25 longitudinal twin cohorts including 38,530 complete twin pairs and having 283,766 longitudinal height and weight measures. The data were analyzed using Cholesky decomposition offering genetic and environmental correlations of BMI and height between all age combinations from 1 to 19 years of age.

Results

The genetic correlations of BMI and height were stronger than the trait correlations. For BMI, we found that genetic correlations decreased as the age between the assessments increased, a trend that was especially visible from early to middle childhood. In contrast, for height, the genetic correlations were strong between all ages. Age-to-age correlations between environmental factors shared by co-twins were found for BMI in early childhood but disappeared altogether by middle childhood. For height, shared environmental correlations persisted from infancy to adulthood.

Conclusions

Our results suggest that the genes affecting BMI change over childhood and adolescence leading to decreasing age-to-age genetic correlations. This change is especially visible from early to middle childhood indicating that new genetic factors start to affect BMI in middle childhood. Identifying mediating pathways of these genetic factors can open possibilities for interventions, especially for those children with high genetic predisposition to adult obesity.

Concordance and comorbidities among monozygotic twins with tic disorders

Gilles de la Tourette Syndrome (GTS) is a multifactorial neurodevelopmental disorder characterized by tics and multiple comorbidities. The pathophysiology is not yet fully understood, but both environmental and genetic risk factors seem to be involved. Twin studies provide important knowledge on genetic factors. We assessed the concordance of GTS and chronic tic disorders (CTD) in monozygotic (MZ) twins, and examined tic severity, symptoms of obsessive-compulsive disorder (OCD), attention deficit/hyperactivity disorder and autism spectrum disorder. Twin pairs, where at least one twin was diagnosed with any tic disorder, were identified through Danish Twin Registry, Psychiatric Central Registry, Danish National Patient Registry and National Tourette Clinic, Copenhagen University Hospital, Herlev. Zygosity was tested with single-nucleotide polymorphism (SNP) genotyping and clinical assessment was done with validated tools. 14 MZ twin pairs were included: five were discordant. Seven twin pairs were concordant for GTS, and for two pairs one twin had GTS and the other CTD. Among the twins with CTD or GTS, 50% had at least one comorbidity, which is higher than in background populations. The GTS + OCD-phenotype was significantly more frequent among GTS-concordant than among discordant twins. No statistically significant differences were found between the GTS-concordant and discordant twin pairs regarding tic severity or comorbidities. Thorough clinical assessment and SNP-based genotyping are important when conducting clinical twin studies. We found high concordance of GTS and CTD, which supports the notion that both disorders have common genetic risk factors. Further studies with larger cohorts including dizygotic twins are warranted for more conclusive results.

Mechanisms underlying familial aggregation of exceptional health and survival: A three-generation cohort study

The familial resemblance in length of adult life is very modest. Studies of parent-offspring and twins suggest that exceptional health and survival have a stronger genetic component than lifespan generally. To shed light on the underlying mechanisms, we collected information on Danish long-lived siblings (born 1886-1938) from 659 families, their 5379 offspring (born 1917-1982), and 10,398 grandchildren (born 1950-2010) and matched background population controls through the Danish 1916 Census, the Civil Registration System, the National Patient Register, and the Register of Causes of Death. Comparison with the background, population revealed consistently lower occurrence of almost all disease groups and causes of death in the offspring and the grandchildren. The expected incidence of hospitalization for mental and behavioral disorders was reduced by half in the offspring (hazard ratio 0.53, 95% confidence interval 0.45-0.62) and by one-third in the grandchildren (0.69, 0.61-0.78), while the numbers for tobacco-related cancer were 0.60 (0.51-0.70) and 0.71 (0.48-1.05), respectively. Within-family analyses showed a general, as opposed to specific, lowering of disease risk. Early parenthood and divorce were markedly less frequent in the longevity-enriched families, while economic and educational differences were small to moderate. The longevity-enriched families in this study have a general health advantage spanning three generations. The particularly low occurrence of mental and behavioral disorders and tobacco-related cancers together with indicators of family stability and only modest socioeconomic advantage implicate behavior as a key mechanism underlying familial aggregation of exceptional health and survival.

Advanced Parental Age at Conception and Sex Affects Mitochondrial DNA Copy Number in Human and Fruit Flies

Aging is a multifactorial trait caused by early as well as late-life circumstances. A society trend that parents deliberately delay having children is of concern to health professionals, for example as advanced parental age at conception increases disease risk profiles in offspring. We here aim to study if advanced parental age at conception affects mitochondrial DNA content, a cross-species biomarker of general health, in adult human twin offspring and in a model organism. We find no deteriorated mitochondrial DNA content at advanced parental age at conception, but human mitochondrial DNA content was higher in females than males, and the difference was twofold higher at advanced maternal age at conception. Similar parental age effects and sex-specific differences in mitochondrial DNA content were found in Drosophila melanogaster. In addition, parental longevity in humans associates with both mitochondrial DNA content and parental age at conception; thus, we carefully propose that a poorer disease risk profile from advanced parental age at conception might be surpassed by superior effects of parental successful late-life reproduction that associate with parental longevity.

Twin Family Registries Worldwide: An Important Resource for Scientific Research

Much progress has been made in twin research since our last special issue on twin registries (Hur, Y.-M., & Craig, J. M. (2013). Twin Research and Human Genetics, 16, 1-12.). This special issue provides an update on the state of twin family registries around the world. This issue includes 61 papers on twin family registries from 25 countries, of which 3 describe consortia based on collaborations of several twin family registries. The articles included in this issue discuss the establishment and maintenance of twin registries, recruitment strategies, methods of zygosity assessment, research aims and major findings from twin family cohorts, as well as other important topics related to twin studies. The papers amount to approximately 1.3 million monozygotic, dizygotic twins and higher order multiples and their family members who participate in twin studies around the world. Nine new twin family registries have been established across the world since our last issue, which demonstrates that twin registers are increasingly important in studies of the determinants and correlates of complex traits from disease susceptibility to healthy development.

The risk of asthma is increased among women with polycystic ovary syndrome: a twin study

Background

Recent registry studies have demonstrated a higher prevalence of asthma among women with polycystic ovary syndrome (PCOS). We aimed to assess the association and heritability of PCOS and asthma in a Danish twin cohort.

Methods

Data for 32 382 female twins from the Danish Twin Registry were included. Twins with PCOS were identified by searching the Danish National Patient Registry for International Classification of Diseases-10 code E28.2. Asthma was diagnosed by questionnaires.

Results

103 (0.3%) women had a PCOS diagnosis. The risk of asthma was increased among women with PCOS compared with women without (18% versus 9%, respectively; OR 2.11 (95% CI 1.13-3.96); p=0.02). After adjustment for age, body mass index, alcohol consumption and smoking status, the risk of asthma was still increased, but was no longer statistically significant (OR 1.54 (95% CI 0.75-3.17); p=0.24). Variance components analysis showed that shared environmental factors explained 49% (95% CI 24-68%) and unique environmental factors explained 51% (95% CI 32-76%) of the susceptibility to PCOS. For asthma, 44% (95% CI 28-61%) of the variance was explained by genetic factors, whereas 25% (95% CI 11-38%) was ascribable to shared environmental factors and 31% (95% CI 26-36%) to unique environmental factors.

Conclusion

The risk of asthma is twice as high among female twins with PCOS. The individual susceptibility to PCOS is mainly due to environmental factors and not genetics.

A Cohort Comparison of Lifespan After Age 100 in Denmark and Sweden: Are Only the Oldest Getting Older?

Although Denmark and Sweden have close cultural and historical ties, lifespans for Danes have generally been lower than those of Swedes. Recent improvements in Danish mortality after a period of stagnation have led to the suspicion that there may be positive trends at the very high ages at death within that population and that these trends could be quite different from those observed in Sweden. Although the mean ages at death for Danish and Swedish centenarians have been relatively constant at about 102 years for the cohorts born 1870-1904, the oldest-old in Denmark have been getting older, but no evidence has suggested any increase in lifespan for Swedes. Using quantile regression, we show that Danish centenarian lifespans in the 90th percentile have been lengthening, with those in 94th percentile (6 % longest-lived individuals) having a trend that is statistically significant at the 5 % level. We demonstrate that the increase observed is not due to the increasing sizes of birth cohorts and thus must be due to improving survival among this select top tier. We postulate that this super-select group in Denmark is best able to take advantage of the factors driving mortality reduction, whereas the majority of centenarians are not.

Myeloproliferative Neoplasms in Danish Twins

OBJECTIVE

Myeloproliferative neoplasms (MPNs) are a heterogeneous group of diseases characterized by clonal hyperproliferation of immature and mature cells of the myeloid lineage. Genetic differences have been proposed to play a role in the development of MPNs. Monozygotic twin pairs with MPNs have been reported in a few case reports, but the MPN concordance pattern in twins remains unknown.

METHOD

All twin pairs born in the period 1900-2010 were identified in the nationwide Danish Twin Registry. Only pairs with both twins alive on January 1, 1977, and those born thereafter were included to allow identification in the Danish National Patient Registry.

RESULTS

A total of 158 twin pairs were registered with an MPN diagnosis: 36 monozygotic, 104 dizygotic, and 18 pairs with unknown zygosity. MPNs were diagnosed in both twins in 4 pairs. The probandwise concordance rates for monozygotic twin pairs were higher than for dizygotic twin pairs (15 vs. 0%; p = 0.016).

CONCLUSION

An estimated concordance rate of 15% (95% CI 0.059-0.31) is modest, but given the rarity of MPNs this finding is clinically relevant and provides further support for the role of genetic predisposition in the development of MPNs.

Heritability of Schizophrenia and Schizophrenia Spectrum Based on the Nationwide Danish Twin Register

BACKGROUND

Twin studies have provided evidence that both genetic and environmental factors contribute to schizophrenia (SZ) risk. Heritability estimates of SZ in twin samples have varied methodologically. This study provides updated heritability estimates based on nationwide twin data and an improved statistical methodology.

METHODS

Combining two nationwide registers, the Danish Twin Register and the Danish Psychiatric Research Register, we identified a sample of twins born between 1951 and 2000 (N = 31,524 twin pairs). Twins were followed until June 1, 2011. Liability threshold models adjusting for censoring with inverse probability weighting were used to estimate probandwise concordance rates and heritability of the diagnoses of SZ and SZ spectrum disorders.

RESULTS

The probandwise concordance rate of SZ is 33% in monozygotic twins and 7% in dizygotic twins. We estimated the heritability of SZ to be 79%. When expanding illness outcome to include SZ spectrum disorders, the heritability estimate was almost similar (73%).

CONCLUSIONS

The key strength of this study is the application of a novel statistical method accounting for censoring in the follow-up period to a nationwide twin sample. The estimated 79% heritability of SZ is congruent with previous reports and indicates a substantial genetic risk. The high genetic risk also applies to a broader phenotype of SZ spectrum disorders. The low concordance rate of 33% in monozygotic twins demonstrates that illness vulnerability is not solely indicated by genetic factors.

The healthy donor effect impacts self-reported physical and mental health - results from the Danish Blood Donor Study (DBDS)

AIMS

This study aimed at quantifying the healthy donor effect by comparing self-perceived mental and physical health between blood donors and non-donors.

BACKGROUND

In theory, the selection process known as the healthy donor effect should result in better self-perceived, health-related quality of life in donors than in non-donors.

METHODS

The Short Form-12 data from the Danish Twin Registry (DTR) was compared with the data from the Danish Blood Donor Study (DBDS). Data on age, sex and smoking status were included in the analyses. The multivariable linear regression analysis was stratified by sex and age group intervals. Outcome variables were the mental component score (MCS) and the physical component score (PCS).

RESULTS

A total of 28 982 and 36 913 participants from the DTR and the DBDS, respectively, were included in this study. Younger donors had higher MCS than non-donors, whereas MCS was only marginally high in older donors compared with non-donors. In contrast, PCS was almost similar for both young donors and non-donors. With the increase in age, non-donors had lower PCS than donors.

CONCLUSIONS

Two selection patterns were revealed. Among young individuals, better self-perceived mental health was associated with a blood donor. With the increase in age, better self-perceived physical health was associated with blood donation.

Heritability of the Number of Teeth in Middle-Aged and Older Danish Twins

Tooth loss is a common health concern in older adults. We aimed to estimate the relative contributions of genetic and environmental factors to the variation in the number of teeth in middle-aged and older populations using a population-based cohort of Danish twins. The study included 5,269 Danish middle-aged or older twins who provided data on the number of teeth at baseline by structured interviews. The data were analyzed using univariate liability threshold modeling, stratified by sex and age, to estimate familial risk of tooth loss as well as estimates of heritability. In the whole cohorts, 23% of participants were edentate and 53% had retained 20 or more teeth. A statistical model including additive genetic factors and environmental factors partly shared by co-twins and partly unique to each individual twin gave the best statistical fit for the number of teeth in both age categories as well as in men and women. Overall, additive genetic factors explained 36% (95% confidence interval [CI]: 23% to 49%), common environmental factors 20% (95% CI: 9% to 31%), and unique environmental factors 44% (95% CI: 40% to 48%) of the total variation of the number of teeth. This study indicates that a substantial part of the variation in tooth loss is explained by genetic as well as environmental factors shared by co-twins. Our results implied that family background importantly affects tooth loss in both the middle-aged and the older populations. Family history is thus an important factor to take into account in dental health care.

Familial Risk and Heritability of Colorectal Cancer in the Nordic Twin Study of Cancer

BACKGROUND & AIMS

We analyzed data from twins to determine how much the familial risk of colorectal cancer can be attributed to genetic factors vs environment. We also examined whether heritability is distinct for colon vs rectal cancer, given evidence of distinct etiologies.

METHODS

Our data set included 39,990 monozygotic and 61,443 same-sex dizygotic twins from the Nordic Twin Study of Cancer. We compared each cancer's risk in twins of affected co-twins relative to the cohort risk (familial risk ratio [FRR]). We then estimated the proportion of variation in risk that could be attributed to genetic factors (heritability).

RESULTS

From earliest registration in 1943 through 2010, there were 1861 individuals diagnosed with colon cancer and 1268 diagnosed with rectal cancer. Monozygotic twins of affected co-twins had an FRR for colorectal cancer of 3.1 (95% confidence interval [CI], 2.4-3.8) relative to the cohort risk. Dizygotic twins of affected co-twins had an FRR for colorectal cancer of 2.2 (95% CI, 1.7-2.7). We estimated that 40% (95% CI, 33%-48%) of the variation in colorectal cancer risk could be attributed to genetic factors; unique environment only accounted for the remaining liability. For colon cancer, the FRR was 3.3 (95% CI, 2.1-4.5) for monozygotic twins and 2.6 (95% CI, 1.7-3.5) for dizygotic twins. For rectal cancer, comparable estimates were 3.3 (95% CI, 1.5-5.1) for monozygotic twins and 2.6 (95% CI, 1.2-4.0) for dizygotic twins. Heritability estimates for colon and rectal cancer were 16% (95% CI, 0-46%) and 15% (95% CI, 0-50%), common environment estimates were 15% (95% CI, 0-38%) and 11% (95% CI, 0-38%), and unique environment estimates were 68% (95% CI, 57%-79%) and 75% (95% CI, 61%-88%), respectively.

CONCLUSIONS

Interindividual genetic differences could account for 40% of the variation in susceptibility to colorectal cancer; risk for colon and rectal cancers might have less of a genetic component than risk for colorectal cancer. Siblings, and particularly monozygotic co-twins, of individuals with colon or rectal cancer should consider personalized screening.

Differences in genetic and environmental variation in adult BMI by sex, age, time period, and region: an individual-based pooled analysis of 40 twin cohorts

Background: Genes and the environment contribute to variation in adult body mass index [BMI (in kg/m²)], but factors modifying these variance components are poorly understood.Objective: We analyzed genetic and environmental variation in BMI between men and women from young adulthood to old age from the 1940s to the 2000s and between cultural-geographic regions representing high (North America and Australia), moderate (Europe), and low (East Asia) prevalence of obesity.Design: We used genetic structural equation modeling to analyze BMI in twins ≥20 y of age from 40 cohorts representing 20 countries (140,379 complete twin pairs).Results: The heritability of BMI decreased from 0.77 (95% CI: 0.77, 0.78) and 0.75 (95% CI: 0.74, 0.75) in men and women 20-29 y of age to 0.57 (95% CI: 0.54, 0.60) and 0.59 (95% CI: 0.53, 0.65) in men 70-79 y of age and women 80 y of age, respectively. The relative influence of unique environmental factors correspondingly increased. Differences in the sets of genes affecting BMI in men and women increased from 20-29 to 60-69 y of age. Mean BMI and variances in BMI increased from the 1940s to the 2000s and were greatest in North America and Australia, followed by Europe and East Asia. However, heritability estimates were largely similar over measurement years and between regions. There was no evidence of environmental factors shared by co-twins affecting BMI.Conclusions: The heritability of BMI decreased and differences in the sets of genes affecting BMI in men and women increased from young adulthood to old age. The heritability of BMI was largely similar between cultural-geographic regions and measurement years, despite large differences in mean BMI and variances in BMI. Our results show a strong influence of genetic factors on BMI, especially in early adulthood, regardless of the obesity level in the population.

Association of Psoriasis With the Risk for Type 2 Diabetes Mellitus and Obesity

IMPORTANCE

Psoriasis has been shown to be associated with overweight and type 2 diabetes mellitus. The genetic association is unclear.

OBJECTIVE

To examine the association among psoriasis, type 2 diabetes mellitus, and body mass index (BMI) (calculated as weight in kilograms divided by height in meters squared) in twins.

DESIGN, SETTING, AND PARTICIPANTS

This cross-sectional, population-based twin study included 34 781 Danish twins, 20 to 71 years of age. Data from a questionnaire on psoriasis was validated against hospital discharge diagnoses of psoriasis and compared with hospital discharge diagnoses of type 2 diabetes mellitus and self-reported BMI. Data were collected in the spring of 2002. Data were analyzed from January 1 to October 31, 2014.

MAIN OUTCOMES AND MEASURES

Crude and adjusted odds ratios (ORs) were calculated for psoriasis in relation to type 2 diabetes mellitus, increasing BMI, and obesity in the whole population of twins and in 449 psoriasis-discordant twins. Variance component analysis was used to measure genetic and nongenetic effects on the associations.

RESULTS

Among the 34 781 questionnaire respondents, 33 588 with complete data were included in the study (15 443 men [46.0%]; 18 145 women [54.0%]; mean [SD] age, 44.5 [7.6] years). After multivariable adjustment, a significant association was found between psoriasis and type 2 diabetes mellitus (odds ratio [OR], 1.53; 95% CI, 1.03-2.27; P = .04) and between psoriasis and increasing BMI (OR, 1.81; 95% CI, 1.28-2.55; P = .001 in individuals with a BMI>35.0). Among psoriasis-discordant twin pairs, the association between psoriasis and obesity was diluted in monozygotic twins (OR, 1.43; 95% CI, 0.50-4.07; P = .50) relative to dizygotic twins (OR, 2.13; 95% CI, 1.03-4.39; P = .04). Variance decomposition showed that additive genetic factors accounted for 68% (95% CI, 60%-75%) of the variance in the susceptibility to psoriasis, for 73% (95% CI, 58%-83%) of the variance in susceptibility to type 2 diabetes mellitus, and for 74% (95% CI, 72%-76%) of the variance in BMI. The genetic correlation between psoriasis and type 2 diabetes mellitus was 0.13 (-0.06 to 0.31; P = .17); between psoriasis and BMI, 0.12 (0.08 to 0.19; P < .001). The environmental correlation between psoriasis and type 2 diabetes mellitus was 0.10 (-0.71 to 0.17; P = .63); between psoriasis and BMI, -0.05 (-0.14 to 0.04; P = .44).

CONCLUSIONS AND RELEVANCE

This study determines the contribution of genetic and environmental factors to the interaction between obesity, type 2 diabetes mellitus, and psoriasis. Psoriasis, type 2 diabetes mellitus, and obesity are also strongly associated in adults after taking key confounding factors, such as sex, age, and smoking, into account. Results indicate a common genetic etiology for psoriasis and obesity.

Does the sex of one's co-twin affect height and BMI in adulthood? A study of dizygotic adult twins from 31 cohorts

Background

The comparison of traits in twins from opposite-sex (OS) and same-sex (SS) dizygotic twin pairs is considered a proxy measure of prenatal hormone exposure. To examine possible prenatal hormonal influences on anthropometric traits, we compared mean height, body mass index (BMI), and the prevalence of being overweight or obese between men and women from OS and SS dizygotic twin pairs.

Methods

The data were derived from the COllaborative project of Development of Anthropometrical measures in Twins (CODATwins) database, and included 68,494 SS and 53,808 OS dizygotic twin individuals above the age of 20 years from 31 twin cohorts representing 19 countries. Zygosity was determined by questionnaires or DNA genotyping depending on the study. Multiple regression and logistic regression models adjusted for cohort, age, and birth year with the twin type as a predictor were carried out to compare height and BMI in twins from OS pairs with those from SS pairs and to calculate the adjusted odds ratios and 95% confidence intervals for being overweight or obese.

Results

OS females were, on average, 0.31 cm (95% confidence interval (CI) 0.20, 0.41) taller than SS females. OS males were also, on average, taller than SS males, but this difference was only 0.14 cm (95% CI 0.02, 0.27). Mean BMI and the prevalence of overweight or obesity did not differ between males and females from SS and OS twin pairs. The statistically significant differences between OS and SS twins for height were small and appeared to reflect our large sample size rather than meaningful differences of public health relevance.

Conclusions

We found no evidence to support the hypothesis that prenatal hormonal exposure or postnatal socialization (i.e., having grown up with a twin of the opposite sex) has a major impact on height and BMI in adulthood.

Electronic supplementary material

The online version of this article (doi:10.1186/s13293-017-0134-x) contains supplementary material, which is available to authorized users.

Incidence of Chronic Persistent Rheumatoid Arthritis and the Impact of Smoking: A Historical Twin Cohort Study

OBJECTIVE

To assess the incidence of chronic persistent rheumatoid arthritis (RA) in a population-based cohort of twins and to determine the impact of smoking.

METHODS

In a historical cohort study on twins born in 1920 to 1982, we identified 157 cases of RA among 45,280 responders (response rate 80%). Information on smoking was obtained by questionnaire and interview. A mixed-effects Poisson regression model was used to estimate incidence rate ratios with age, sex, smoking duration, and smoking intensity as covariates. We used the SplitLexis procedure in the Epi R package to study a possible effect of period or cohort in addition to age on the variation of the incidence.

RESULTS

The annual incidence of chronic persistent RA was 18.8 per 100,000 person-years, ages 15-73 years (females 25.2, males 12.0), increasing with age to a maximum at age 60 years in females and age 70 years in males. The incidence rate ratio among ever-smoking patients was 1.96 (95% confidence interval [95% CI] 1.43-3.76), 1.93 (95% CI 1.00-3.7) after 30 pack-years, and 1.034 (P < 0.001) per year of smoking, implying a doubling of risk after 20 years regardless of sex and smoking intensity. We did not detect significant period or cohort effects.

CONCLUSION

The incidence of chronic persistent RA is lower than the incidence figures reported in inception cohorts. Smoking duration, but not intensity, doubled the risk of RA after 20 years of smoking in both sexes.

Is an Early Age at Illness Onset in Schizophrenia Associated With Increased Genetic Susceptibility? Analysis of Data From the Nationwide Danish Twin Register

Background

Early age at illness onset has been viewed as an important liability marker for schizophrenia, which may be associated with an increased genetic vulnerability. A twin approach can be valuable, because it allows for the investigation of specific illness markers in individuals with a shared genetic background.

Methods

We linked nationwide registers to identify a cohort of twin pairs born in Denmark from 1951 to 2000 (N = 31,524 pairs), where one or both twins had a diagnosis in the schizophrenia spectrum. We defined two groups consisting of; N = 788 twin pairs (affected with schizophrenia spectrum) and a subsample of N = 448 (affected with schizophrenia). Survival analysis was applied to investigate the effect of age at illness onset.

Findings

We found that early age at illness onset compared to later onset in the first diagnosed twin can be considered a major risk factor for developing schizophrenia in the second twin. Additionally, we found that the stronger genetic component in MZ twins compared to DZ twins is manifested in the proximity of assigned diagnosis within pairs.

Discussion

Early onset schizophrenia could be linked to a more severe genetic predisposition, indicating that age might be perceived as a clinical marker for genetic vulnerability for the illness.

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Early age at schizophrenia onset in one twin increases risk of illness in the second twin 4.7 times compared to a later onset.

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A stronger genetic predisposition may be needed to affect early schizophrenia onset in females compared to males.

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Genetic factors seem to play an important role in the proximity of assigned diagnosis within twin pairs.

The main aim of this study is to examine if early age at schizophrenia onset can be viewed as a clinical marker for increased genetic vulnerability in the illness. By linking the Danish Twin Register to other nationwide health registers we obtain accurate data regarding psychiatric diagnostic outcome in a complete twin population (> 31,000 twin pairs). Our study shows how an early age at illness onset in the first diagnosed twin in a pair is a major risk factor for developing schizophrenia in the second twin, underlining the importance of genetic factors in illness vulnerability.

Genetic and environmental influences on adult human height across birth cohorts from 1886 to 1994

Human height variation is determined by genetic and environmental factors, but it remains unclear whether their influences differ across birth-year cohorts. We conducted an individual-based pooled analysis of 40 twin cohorts including 143,390 complete twin pairs born 1886–1994. Although genetic variance showed a generally increasing trend across the birth-year cohorts, heritability estimates (0.69-0.84 in men and 0.53-0.78 in women) did not present any clear pattern of secular changes. Comparing geographic-cultural regions (Europe, North America and Australia, and East Asia), total height variance was greatest in North America and Australia and lowest in East Asia, but no clear pattern in the heritability estimates across the birth-year cohorts emerged. Our findings do not support the hypothesis that heritability of height is lower in populations with low living standards than in affluent populations, nor that heritability of height will increase within a population as living standards improve.

DOI:http://dx.doi.org/10.7554/eLife.20320.001

Early-life mortality risks in opposite-sex and same-sex twins: a Danish cohort study of the twin testosterone transfer hypothesis

PURPOSE

To investigate the twin testosterone transfer (TTT) hypothesis by comparing early-life mortality risks of opposite-sex (OS) and same-sex (SS) twins during the first 15 years of life.

METHODS

We performed a population-based cohort study to compare mortality in OS and SS twins. We included 68,629 live-born Danish twins from 1973 to 2009 identified through the Danish Twin Registry and performed piecewise stratified Cox regression and log-binomial regression.

RESULTS

Among 1933 deaths, we found significantly higher mortality for twin boys than for twin girls. For both sexes, OS twins had lower mortality than SS twins; the difference persisted for the first year of life for boys and for the first week of life for girls.

CONCLUSIONS

Although the mortality risk for OS boys was in the expected direction according to the TTT hypothesis, the results for OS girls pointed in the opposite direction, providing no clear evidence for the TTT hypothesis.

Genetic analysis of cause of death in a mixture model of bivariate lifetime data

A mixture model in multivariate survival analysis is presented, whereby heterogeneity among subjects creates divergent paths for the individual’s risk of experiencing an event (i.e., disease), as well as for the associated length of survival. Dependence among competing risks is included and rendered testable. This method is an extension of the bivariate correlated gamma-frailty model. It is applied to a data set on Danish twins, for whom cause-specific mortality is known. The use of multivariate data solves the identifiability problem which is inherent in the competing risk model of univariate lifetimes. We analyse the influence of genetic and environmental factors on frailty. Using a sample of 1470 monozygotic (MZ) and 2730 dizygotic (DZ) female twin pairs, we apply five genetic models to the associated mortality data, focusing particularly on death from coronary heart disease (CHD). Using the best fitting model, the inheritance risk of death from CHD was 0.39 (standard error 0.13). The results from this model are compared with the results from earlier analysis that used the restricted model, where the independence of competing risks was assumed. Comparing both cases, it turns out, that heritability of frailty on mortality due to CHD change substantially. Despite the inclusion of dependence, analysis confirms the significant genetic component to an individual’s risk of mortality from CHD. Whether dependence or independence is assumed, the best model for analysis with regard to CHD mortality risks is a model assuming that additive factors are responsible for heritability in susceptibility to CHD. The paper ends with a discussion of limitations and possible further extensions to the model presented.

Genetic and environmental effects on body mass index from infancy to the onset of adulthood: an individual-based pooled analysis of 45 twin cohorts participating in the COllaborative project of Development of Anthropometrical measures in Twins (CODATwins) study

BACKGROUND

Both genetic and environmental factors are known to affect body mass index (BMI), but detailed understanding of how their effects differ during childhood and adolescence is lacking.

OBJECTIVES

We analyzed the genetic and environmental contributions to BMI variation from infancy to early adulthood and the ways they differ by sex and geographic regions representing high (North America and Australia), moderate (Europe), and low levels (East Asia) of obesogenic environments.

DESIGN

Data were available for 87,782 complete twin pairs from 0.5 to 19.5 y of age from 45 cohorts. Analyses were based on 383,092 BMI measurements. Variation in BMI was decomposed into genetic and environmental components through genetic structural equation modeling.

RESULTS

The variance of BMI increased from 5 y of age along with increasing mean BMI. The proportion of BMI variation explained by additive genetic factors was lowest at 4 y of age in boys (a(2) = 0.42) and girls (a(2) = 0.41) and then generally increased to 0.75 in both sexes at 19 y of age. This was because of a stronger influence of environmental factors shared by co-twins in midchildhood. After 15 y of age, the effect of shared environment was not observed. The sex-specific expression of genetic factors was seen in infancy but was most prominent at 13 y of age and older. The variance of BMI was highest in North America and Australia and lowest in East Asia, but the relative proportion of genetic variation to total variation remained roughly similar across different regions.

CONCLUSIONS

Environmental factors shared by co-twins affect BMI in childhood, but little evidence for their contribution was found in late adolescence. Our results suggest that genetic factors play a major role in the variation of BMI in adolescence among populations of different ethnicities exposed to different environmental factors related to obesity.

Risk for multiple sclerosis in dizygotic and monozygotic twins

We investigated the risks of twins for multiple sclerosis (MS). Our data are linked registers of all Danish twins and of all Danes born between 1920 and 1970 in whom MS was diagnosed before 1997. We compared differences in the risks for MS by Cox regression and standardized incidence ratios. Our analyses suggest that dizygotic twins have an approximately 60% lower risk for MS than monozygotic twins and a 20% lower risk than singletons. Monozygotic twins appear to have a somewhat higher risk for MS than singletons albeit not statistically significant. We offer no biological explanation for our findings, but suggest that either sharing fetal life with a genotypically different individual is beneficial for the immune system or that there is a linkage between the genes that influence dizygotic twinning and other genes that protect against MS.

Concordance for multiple sclerosis in Danish twins: an update of a nationwide study

The occurrence of multiple sclerosis (MS) in twins has not previously been studied in complete nationwide data sets. The existence of almost complete MS and twin registries in Denmark ensures that essentially unbiased samples of MS cases among twins can be obtained. In this population-based study, virtually all Danish MS cases among twins born before 1983 with onset of MS after 1948 and diagnosis before 1 January 1997 were identified. Of 13 286 MS cases, 178 were twins and, of these 164 twin pairs were discordant and seven were concordant. We found significantly higher proband-wise concordance among monozygotic twins than dizygotic twins, with estimated proband-wise concordances of 24% (95% confidence interval (CI): 5-39%) for monozygotic and 3% (95% CI: 0-8%) for dizygotic twins. Thus, a monozygotic twin whose co-twin has MS has a 24% risk of developing the disease, while the corresponding risk for a dizygotic twin is only 3%. Our results largely confirm previously published concordance estimates and indicate that genetic factors are of importance in susceptibility to MS.

Genetic and environmental influences on height from infancy to early adulthood: An individual-based pooled analysis of 45 twin cohorts

Height variation is known to be determined by both genetic and environmental factors, but a systematic description of how their influences differ by sex, age and global regions is lacking. We conducted an individual-based pooled analysis of 45 twin cohorts from 20 countries, including 180,520 paired measurements at ages 1-19 years. The proportion of height variation explained by shared environmental factors was greatest in early childhood, but these effects remained present until early adulthood. Accordingly, the relative genetic contribution increased with age and was greatest in adolescence (up to 0.83 in boys and 0.76 in girls). Comparing geographic-cultural regions (Europe, North-America and Australia, and East-Asia), genetic variance was greatest in North-America and Australia and lowest in East-Asia, but the relative proportion of genetic variation was roughly similar across these regions. Our findings provide further insights into height variation during childhood and adolescence in populations representing different ethnicities and exposed to different environments.

Sex differences in risk and heritability estimates on primary knee osteoarthritis leading to total knee arthroplasty: a nationwide population based follow up study in Danish twins

BACKGROUND

Symptomatic knee osteoarthritis is a highly age and sex associated complex disease. Little is known about the causes behind this age and sex associated increase, or if genetic and environmental factors impacts differently by gender. Our study examined the risk and heritability of primary knee osteoarthritis leading to total knee arthroplasty and whether these differences were attributable to sex and age differences in heritability.

METHODS

All twins of known zygosity from The Danish Twin Register alive in 1997 were examined in a nationwide population based follow-up study collecting information on all twins recorded in The Danish Knee Arthroplasty from 1997 to follow-up in 2010. Our main outcomes were the cumulative incidence, probandwise concordance rates, heritability, within pair correlations in monozygotic and dizygotic twin pairs and the genetic and environmental influence estimated in models taking into account that individuals may not have had a total knee arthroplasty at follow up.

RESULTS

92,748 twins were eligible for analyses and 576 twins had a record of primary knee osteoarthritis in The Danish Knee Arthroplasty Register at follow-up comprising 358 female and 218 male twin cases. The risk increased particular after the age of 50 years displaying significant sex differences in the elderly. In the sex stratified analyses a discrete genetic component was found in females, but in males no genetic component could be detected. In both genders common and unique environmental factors were highly significant. In the sex-adjusted analysis an additive genetic component of 18 % (0; 62), a shared environmental component of 61 % (25; 97) and an individual environmental component of 21 % (6; 36) accounted for the variation in liability to primary total knee arthroplasty.

CONCLUSION

The risk of primary total knee arthroplasty increases significantly after the age of 50 years, in particular in females, displaying significant sex differences in the elderly. After sex-adjustment 82 % of the variation in liability to primary total knee arthroplasty was attributable to common and unique environmental factors; the remaining 18 % of this variation was attributable to additive genetic factors indicating a pivotal impact of environmental factors on this disease.

Prayer and meditation among Danish first time mothers-a questionnaire study

Background

Mothers’ existential dimensions in the transition to motherhood have not been described thoroughly. They might experience disruption and new perspectives in existential ways and this may especially be the case in preterm birth. The aim of this study was twofold. First we investigated the existential dimension of motherhood transition in a secularized context, through practices of prayer and meditation. Second we described the relationship between time of birth (term/preterm) and the prayer/meditation practices of the mothers.

Methods

Data were gathered from a nationwide questionnaire survey among first time mothers conducted during the summer 2011. All Danish women who gave birth before the 32^nd pregnancy week (n = 255), and double the number of mothers who gave birth at full term (n = 658) in 2010 were included (total n = 913). The questionnaire consisted of 46 overall items categorized in seven sections, which independently cover important aspects of existential meaning-making related to becoming a mother. The respondent rate was 57 % (n = 517).

Results

Moments of praying or meditation 6–18 months post partum were reported by 65 %, and mothers who responded affirmatively, practiced prayer (n = 286) more than meditation (n = 89), p < 0,001. We did not observe differences in affirmative responses to prayer or meditation between mothers of full term or preterm born children, not even after controlling for perinatal or post partum loss, mode of birth, age, status of cohabiting or education.

Conclusions

In this explorative study we found specific practices of existential meaning-making through prayer and/or meditation among first time mothers, living in a very secularized context. Yet we know only little about character or importance of these practices among mothers, and hardly anything about existential meaning-making among new fathers. Hence the implications of meaning-making practices related to other dimensions of health are difficult to address in a qualified way in care for new mothers and families.

Cancer and aging: Epidemiology and methodological challenges

Epidemiological cancer data shed light on key questions within basic science, clinical medicine and public health. For decades, Denmark has had linkable health registers that contain individual level data on the entire population with virtually complete follow-up. This has enabled high quality studies of cancer epidemiology and minimized the challenges often faced in many countries, such as uncertain identification of the study base, age misreporting, and low validity of the cancer diagnoses. However, methodological challenges still remain to be addressed, especially in cancer epidemiology studies among the elderly and the oldest-old. For example, a characteristic pattern for many cancer types is that the incidence increases up to a maximum at about ages 75-90 years and is then followed by a decline or a leveling off at the oldest ages. It has been suggested that the oldest individuals may be asymptomatic, or even insusceptible to cancer. An alternative interpretation is that this pattern is an artifact due to lower diagnostic intensity among the elderly and oldest-old caused by higher levels of co-morbidities in this age group. Currently, the available cancer epidemiology data are not able to provide clear evidence for any of these hypotheses.

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.

Genetic Factors Explain Variation in the Age at Onset of Psoriasis: A Population-based Twin Study

The aim of this study was to determine the age at onset of psoriasis in a population-based twin sample. Questionnaire-data in 10,725 twin pairs, 20-71 years of age, from the Danish Twin Registry, was collected, and analysed using survival regression analysis. Median age at onset was 25 and 28 years among women and men, respectively. The correlation between the ages was 0.84 (bootstrap standard error?=?0.044) in monozygotic twin pairs and 0.60 (0.051) in dizygotic twin pairs, permutation p?=?0.001. Age at onset of psoriasis in the index twin did not predict risk of psoriasis in the co-twin, hazard ratio (per year of later onset =?1.01 (0.99-1.03), p?=?0.434. In conclusion, these data support that the age at onset of psoriasis is, in part, an inherited property. Our results do not support that early-onset psoriasis is more genetically determined.

The Heritability of Breast Cancer among Women in the Nordic Twin Study of Cancer

BACKGROUND

Family history is an established risk factor for breast cancer. Although some important genetic factors have been identified, the extent to which familial risk can be attributed to genetic factors versus common environment remains unclear.

METHODS

We estimated the familial concordance and heritability of breast cancer among 21,054 monozygotic and 30,939 dizygotic female twin pairs from the Nordic Twin Study of Cancer, the largest twin study of cancer in the world. We accounted for left-censoring, right-censoring, as well as the competing risk of death.

RESULTS

From 1943 through 2010, 3,933 twins were diagnosed with breast cancer. The cumulative lifetime incidence of breast cancer taking competing risk of death into account was 8.1% for both zygosities, although the cumulative risk for twins whose co-twins had breast cancer was 28% among monozygotic and 20% among dizygotic twins. The heritability of liability to breast cancer was 31% [95% confidence interval (CI), 10%-51%] and the common environmental component was 16% (95% CI, 10%-32%). For premenopausal breast cancer these estimates were 27% and 12%, respectively, and for postmenopausal breast cancer 22% and 16%, respectively. The relative contributions of genetic and environmental factors were constant between ages 50 and 96. Our results are compatible with the Peto-Mack hypothesis.

CONCLUSION

Our findings indicate that familial factors explain almost half of the variation in liability to develop breast cancer, and results were similar for pre- and postmenopausal breast cancer

IMPACT

We estimate heritability of breast cancer, taking until now ignored sources of bias into account.

Probability and heritability estimates on primary osteoarthritis of the hip leading to total hip arthroplasty: a nationwide population based follow-up study in Danish twins

INTRODUCTION

Primary hip osteoarthritis, radiographic as well as symptomatic, is highly associated with increasing age in both genders. However, little is known about the mechanisms behind this, in particular if this increase is caused by genetic factors. This study examined the risk and heritability of primary osteoarthritis of the hip leading to a total hip arthroplasty, and if this heritability increased with increasing age.

METHODS

In a nationwide population-based follow-up study 118,788 twins from the Danish Twin Register and 90,007 individuals from the Danish Hip Arthroplasty Register for the period 1995 to 2010 were examined. Our main outcomes were the cumulative incidence, proband-wise concordance and heritability on age, within-pair correlations in monozygotic and dizygotic twin pairs, and the genetic and environmental influence estimated in models taking into account that individuals may not have had a total hip arthroplasty at the time of follow-up.

RESULTS

There were 94,063 twins eligible for analyses, comprising 835 cases of 36 concordant and 763 discordant twin pairs. The probability increased particularly from 50 years of age. After sex and age adjustment a significant additive genetic component of 47% (12:79), a shared environmental component of 21% (2:76) and a unique environment component of 32% (21:41) accounted for the variation in population liability to total hip arthroplasty. The sex-adjusted proband-wise concordance and heritability on age indicated an increasing age-associated genetic influence onwards from 60 years of age.

CONCLUSION

The cumulative incidence in primary hip osteoarthritis leading to total hip arthroplasty increases in particular after the age of 50 years in both genders. Family factors of genes and shared environment are highly significant and account for 68% of the variation in the population liability to total hip arthroplasty; however, the genetic influence increases significantly from 60 years of age onwards.

A Danish Twin Study of Schizophrenia Liability: Investigation from Interviewed Twins for Genetic Links to Affective Psychoses and for Cross-Cohort Comparisons

We studied schizophrenia liability in a Danish population-based sample of 44 twin pairs (13 MZ, 31 DZ, SS plus OS) in order to replicate previous twin study findings using contemporary diagnostic criteria, to examine genetic liability shared between schizophrenia and other disorders, and to explore whether variance in schizophrenia liability attributable to environmental factors may have decreased with successive cohorts exposed to improvements in public health. ICD-10 diagnoses were determined by clinical interview. Although the best-fitting, most parsimonious biometric model of schizophrenia liability specified variance attributable to additive genetic and non-shared environmental factors, this model did not differ significantly from a model that also included non-additive genetic factors, consistent with recent interview-based twin studies. Schizophrenia showed strong genetic links to other psychotic disorders but much less so for the broader category of psychiatric disorders in general. We also observed a marginally significant decline in schizophrenia variance attributable to environmental factors over successive Western European cohorts, consistent perhaps with improvements in diagnosis and in prenatal and perinatal care and with a secular decline in the prevalence of schizophrenia in that region.

Health and function assessments in two adjacent Danish birth cohorts of centenarians: Impact of design and methodology

Using the results from measures of functional ability, cognitive and physical performance from two adjacent birth cohorts of 100-year-old adults, we aimed to elucidate the possible impact of difference in participation rates, design, and interviewer mode. Participants were birth cohort members born in 1910 (DK-1910) and 1911-12 (DK-1911). Both surveys used the same assessment instruments, but the design was different, and data collection was carried out by trained survey agency interviewers in DK-1910 and trained nurses in DK-1911. Participation rate in DK-1911 (49.8 % (251/504)) was lower than in DK-1910 (66.9 % (273/408)) (p < 0.001). The proportion of interviews with the participant answering alone or mainly alone was significantly higher in DK-1911 (77 %) than in DK-1910 (56 %), and the proportion living in nursing home was significantly lower (44 vs. 54 %, respectively). Higher proportions of DK-1911 independently performed all activities of daily living (ADL) compared to DK-1910, but only significantly for toileting, bathing, and feeding (all p < 0.01). Mini-mental state examination (MMSE) score was higher in DK-1911 than in DK-1910 (23.5 vs. 21.0; p < 0.001). Handgrip strength, gait speed, and chair stand were almost similar. DK-1911 participants had significantly better one-year survival than DK-1911 non-participants and DK-1910 participants and non-participants (p = 0.001). These results suggest that lower participation rate entails selection towards healthier participants in terms of ADL and cognitive functioning. Caution is warranted when comparing studies of centenarians with different participation rates, design, and interviewer mode, and further studies of these methodological issues are required.

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.

Smoking and risk for psoriasis: a population-based twin study

BACKGROUND

Smoking is a potential risk factor for psoriasis. Both psoriasis and smoking habits are partly explained by genetic factors. However, twin studies investigating the association between these traits are limited.

METHODS

Questionnaire-based data on smoking habits and psoriasis were collected for 34,781 twins, aged 20-71 years, from the Danish Twin Registry. A co-twin control analysis was performed on 1700 twin pairs discordant for lifetime history of smoking. Genetic and environmental correlations between smoking and psoriasis were estimated using classical twin modeling.

RESULTS

After multivariable adjustment, age group (50-71 vs. 20-49 years) and childhood exposure to environmental tobacco smoke (ETS) were significantly associated with psoriasis in the whole population (odds ratio [OR] 1.15, 95% confidence interval [CI] 1.02-1.29 [P = 0.021] and OR 1.28, 95% CI 1.10-1.49 [P = 0.002], respectively). Risk for psoriasis increased substantially (OR 2.18, 95% CI 1.82-2.61; P < 0.001) for smokers with a history of >5 pack-years, even after adjusting for age, sex, and childhood ETS. Among twin pairs discordant for smoking, risk for psoriasis in the ever-smoking twin was lower among monozygotic twins (OR 1.23, 95% CI 0.59-2.56; P = 0.578) than among same-sex dizygotic twins (OR 2.21, 95% CI 1.36-3.58; P = 0.001). Genetic factors explained 20% (14-25%; P < 0.001) of the correlation between psoriasis and smoking, whereas non-shared environmental factors explained 8% (0-22%; P = 0.504).

CONCLUSIONS

Tobacco consumption and childhood ETS are significantly associated with psoriasis. Results indicate shared genetic factors for smoking and psoriasis.

Low tobacco-related cancer incidence in offspring of long-lived siblings: a comparison with Danish national cancer registry data

PURPOSE

Familial clustering of longevity is well documented and includes both genetic and other familial factors, but the specific underlying mechanisms are largely unknown. We examined whether low incidence of specific cancers is a mechanism for familial clustering of longevity.

METHODS

The study population of individuals from longevity-enriched families consisted of 3267 offspring from 610 Danish long-lived families defined by two siblings attaining an age of 90 years or more. The offspring of the long-lived siblings were followed from 1968 to 2009. Using high-quality registry data, observed numbers of cancers were compared with expected numbers based on gender-, calendar period-, and age-specific incidence rates in the general population.

RESULTS

During the 41-year-follow-up period, a total of 423 cancers occurred in 397 individuals. The standardized incidence ratios (95% confidence interval) for offspring of long-lived individuals were 0.78 (0.70-0.86) for overall cancer; 0.66 (0.56-0.77) for tobacco-related cancer; 0.34 (0.22-0.51) for lung cancer; 0.88 (0.71-1.10) for breast cancer; 0.91 (0.62-1.34) for colon cancer.

CONCLUSIONS

The low incidence of tobacco-related cancers in long-lived families compared with non-tobacco-related cancers suggests that health behavior plays a central role in lower early cancer incidence in offspring of long-lived siblings in Denmark.

The impact of ventilation tubes in otitis media on the risk of cholesteatoma on a national level

OBJECTIVE

To estimate the impact of treatment with middle ear ventilation tube insertion (VTI) in children with otitis media (OM) on the risk of cholesteatoma on a national level.

METHODS

Data were obtained from the Danish National Patient Register, the National Health Service Register and Statistics Denmark. Cumulative incidence proportions were estimated by the Kaplan-Meier method and hazard ratios with Cox regression analysis. The first surgically treated middle ear cholesteatoma in a child (STMEC1) was considered an event.

RESULTS

A total of 217,206 children, born after December 31, 1996, who had VTI from January 1, 1997 to August 31, 2011 were identified. Of these, 374 subsequently had a STMEC1. A corresponding 36,981 children without any VTI were identified for comparison using a random 5% sample of the Danish population. Of these, 5 had a STMEC1. The cumulative incidence proportion with STMEC1 at 12 years of age for children with 0, 1, 2, 3, and ≥4 VTI's was 0.04% (95% confidence interval 0.02-0.12%), 0.21% (0.18-0.26%), 0.35% (0.28-0.43%), 0.40% (0.30-0.54%), and 0.55% (0.44-0.70%), respectively. In the regression model each additional year of age before the first VTI increased the risk of STMEC1 by 54% (47-63%), while each additional year between two successive tube insertions increased the risk by 28% (15-43%).

CONCLUSION

We found that prolonged OM requiring multiple VTIs was associated with an increased risk of STMEC1. Early age at first VTI and short time between two VTIs was associated with a lower risk of STMEC1. This may be the result of reduced time with negative middle ear pressure and OM. However, these findings may be susceptible to selection bias, as age at first VTI and time between VTIs, as well as the outcome variable, STMEC1, may all depend on the underlying indication for VTI. In short the present study suggests that treatment with VTI in children with OM reduces the risk of STMEC1 on a population level. However, for the individual child the absolute risk reduction is very small, and the decision of treatment with VTI must always rely on the symptoms and clinical findings in the individual child.

Familial risk and heritability of genitourinary cancers in the Nordic Twin Cohorts

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Background: Twin studies estimate familial risk and heritability by leveraging the unique genetic relatedness of twins. We analyzed data from the Nordic Twin Study to investigate the genetic underpinnings of risk of genitourinary cancers. Methods: The cohort included 202,868 monozygotic (MZ) and same-sex dizygotic (DZ) twins from nationwide registries with follow-up via cancer registries. We examined cancers of the prostate, bladder, kidney and testes, and estimated familial risk, risk of a specific cancer in a twin, given that his co-twin was diagnosed with the same cancer, and heritability, the proportion of variation in cancer risk due to genetic differences. Statistical models used time-to-event analyses and accounted for censoring and competing risk of death. Results: During 32 years of follow-up, 5,041 cases of genitourinary cancer were diagnosed. The cumulative incidences in the cohort overall were 10.5% for prostate, 2.2% for bladder, 0.8% for kidney and 0.5% for testes. The strongest familial effects were found for testicular cancer. The familial risks of testicular cancer were 6% for DZ and 14% for MZ twins, i.e. a 12-fold and 28-fold higher risk for DZ and MZ twins, respectively, when their twin was also diagnosed with testicular cancer. Familial risk was also elevated for prostate, bladder, and kidney cancer and showed higher estimates among MZ than DZ twins. Statistically significant estimates of heritability were observed for prostate (57%) and kidney (38%) cancer, while testicular cancer showed evidence of both genetic (37%) and shared environmental (24%) effects. Conclusions: Familial risk estimates provide evidence of significant excess risks for individuals whose siblings develop genitourinary cancers. The higher concordance in MZ vs. DZ twins translated to strong heritability estimates for these malignancies. Results from this study have the potential to be directly translated to clinical practice and genetic counseling, and provide a context for the findings from genome wide association studies.

Genetic and environmental influences in Dupuytren's disease: a study of 30,330 Danish twin pairs

We aimed to assess the relative contribution of genes and environment in the aetiology of Dupuytren's disease by studying Danish twins born between 1870 and 2000. Twins with a diagnosis (n = 365) and the subgroup who also had an operation (n = 259) after 1977 were identified through linkage with a nationwide hospital registry among 30,330 monozygotic and same-sexed dizygotic twin pairs. Since monozygotic twins share all their genes and dizygotic twins share on average half of their genetic material, greater phenotypic similarity is expected in monozygotic than in dizygotic twins if a genetic component is involved. The number of concordant male twin pairs with Dupuytren's disease was 17 and 7 (monozygotic and dizygotic pairs, respectively), compared with 60 and 174 discordant monozygotic and dizygotic pairs, yielding probandwise concordance rates of 0.37 (95% confidence interval (CI): 0.26 to 0.50) and 0.07 (95% CI: 0.04 to 0.14), respectively. The heritability of Dupuytren's disease was approximately 80%. We conclude that genetic factors play a major role in the development of Dupuytren's disease.

[Nazi Terror against the Danish Medical Profession. The February 20, 1945 Murders in Odense]

On February 20, 1945, during the German occupation of Denmark, members of a notorious Nazi terror organization named the Petergroup murdered four young medical doctors at the city and regional hospital of Odense. On the 70th anniversary of the crime, a symposium was organized at the Odense University Hospital, and a monument revealed close to the site of the murders in commemoration of the four victims of the crime. The young physicians were not known to be connected with the Danish resistance, and they were shot without their murderers even knowing their identities in an attempt to revenge the growing resistance in Denmark's central, third largest city, and as a reprisal for several cases where the hospital had treated wounded resistance fighters, and prevented their being handed over to the German police. The article describes the terror action of February 20, 1945 and its perpetrators, as well as other Nazi attacks on members of the Danish medical profession. It lines out the strong protest voiced by the Danish central administration against the Odense hospital killings which were on the very same day seconded by further killings and a German campaign of blowing up important Odense buildings including two newspaper printing houses. Conclusively, the authors - by way of obituaries and material from relatives of the murdered - portray the four victims of the atrocity Christian Fabricius Møller, Jørgen Hvalkof, Henning Magnus Adelsteen Dalsgaard, and Henning Ørsberg.

Cholesteatoma risk in 8,593 orofacial cleft cases and 6,989 siblings: A nationwide study

OBJECTIVES/HYPOTHESIS

To estimate the risk of surgically treated middle ear cholesteatoma in individuals with a nonsyndromic orofacial cleft and in their siblings compared with the general population.

STUDY DESIGN

Historical cohort study.

METHODS

Using the unique civil registration number for linkage, data from three national registers were used for the Danish 1936-2009 birth cohorts. Hazard ratios (HRs) were estimated with Cox regression analyses using age as the underlying time variable. Individuals were followed from January 1, 1977 until time of surgically treated cholesteatoma, and censored at emigration, death, or end of follow-up (December 31, 2010).

RESULTS

A total of 8,593 individuals with nonsyndromic orofacial cleft and 6,989 siblings were identified, undergoing 201 and 21 first-time cholesteatoma surgeries, respectively. A 5% random sample of the Danish population comprising 249,708 persons without an orofacial cleft was created, and 175,724 siblings to these persons were identified. These controls underwent 485 and 332 first-time cholesteatoma surgeries, respectively. For individuals with cleft lip and palate the HR for cholesteatoma surgery was 14 (95% confidence interval [CI], 12-18) and for individuals with cleft palate the HR was 20 (95% CI, 16-24) when compared with the random sample. In siblings of individuals with cleft palate, the HR for cholesteatoma surgery was 2.1 (95% CI, 1.1-4.1) when compared with siblings of the random sample.

CONCLUSIONS

A 20-fold increase in the risk of cholesteatoma was found in individuals with cleft palate, whereas cleft lip did not pose a risk of cholesteatoma. Furthermore, the study indicates an increased risk of cholesteatoma in unaffected siblings of individuals with cleft palate.

LEVEL OF EVIDENCE

2b Laryngoscope, 125:1225-1229, 2015.

Metabolic effects of lifestyle intervention in obese pregnant women. Results from the randomized controlled trial 'Lifestyle in Pregnancy' (LiP)

AIMS

The Lifestyle in Pregnancy intervention in obese pregnant women resulted in significantly lower gestational weight gain compared with the control group, but without improvement in rates of clinical pregnancy complications. The impact of the lifestyle intervention on metabolic measurements in the study participants is now reported.

METHODS

The Lifestyle in Pregnancy study was a randomized controlled trial among 360 obese women (BMI 30-45 kg/m²) who were allocated in early pregnancy to lifestyle interventions with diet counselling and physical activities or to the control group. Fasting blood samples, including plasma glucose, insulin, lipid profile and capillary blood glucose during a 2-h oral glucose tolerance test were carried out three times throughout pregnancy. Insulin resistance was estimated with the homeostasis model assessment of insulin resistance.

RESULTS

Three hundred and four women (84%) were followed until delivery. Women in the intervention group had a significantly lower change in insulin resistance (HOMA-IR) from randomization to 28-30 weeks' gestation compared with control subjects (mean ± SD: 0.7 ± 1.3 vs. 1.0 ± 1.3, P = 0.02). Despite a significantly lower gestational weight gain in the intervention group, there was no difference between the groups with respect to total cholesterol, HDL, LDL or triglycerides.

CONCLUSIONS

Lifestyle intervention in obese pregnant women resulted in attenuation of the physiologic pregnancy-induced insulin resistance. Despite restricted gestational weight gain, there were no changes in glucose or lipid metabolism between the groups.

Religiousness and religious coping in a secular society: the gender perspective

Women are found to be more religious than men and more likely to use religious coping. Only few studies have explored religious gender differences in more secular societies. This population-based study comprised 3,000 Danish men and women (response rate 45 %) between 20 and 40 years of age. Information about demographics, religiousness and religious coping was obtained through a web-based questionnaire. We organized religiousness in the three dimensions: Cognition, Practice and Importance, and we assessed religious coping using the brief RCOPE questionnaire. We found substantial gender differences in both religiousness and religious coping. Nearly, 60 % of the women believed in some sort of spirit or in God compared to 40 % of the men. Generally, both men and women scored low on the RCOPE scale. However, for respondents reporting high levels of religiousness, the proportion of men who scored high in the RCOPE exceeded the proportion of women in using positive and especially negative coping strategies. Also, in a secular society, women are found to be more religious than men, but in a subset of the most religious respondents, men were more inclined to use religious coping. Further studies on religious coping in secular societies are required.