University of Washington Twin Registry: Construction and Characteristics of a Community-Based Twin Registry

Does neighbourhood deprivation influence low back pain and arthritis: An empirical study using multilevel twin design

OBJECTIVE

Neighbourhood deprivation has been found to be associated with many health conditions, but its association with low back pain (LBP) and arthritis is unclear. This study aimed to examine the association between neighbourhood deprivation with LBP and arthritis, and its potential interaction with individual socioeconomic status (SES) on these outcomes.

METHODS

Monozygotic (MZ) twins from the Washington State Twin Registry were used to control for genetic and common environmental factors that could otherwise confound the purported relationship. Multilevel models were employed to examine the association between neighbourhood deprivation as well as individual-level SES with LBP/arthritis, adjusting for age, sex, body mass index (BMI) and residence rurality.

RESULTS

There were 6,380 individuals in the LBP sample and 2,030 individuals in the arthritis sample. Neighbourhood deprivation was not associated with LBP (P = 0.26) or arthritis (P = 0.61), and neither was its interaction with individual-level SES. People without a bachelor's degree were more likely to report LBP (OR 1.44, 95% CI 1.26-1.65) or both LBP and arthritis (OR 1.67, 95% CI 1.14-2.45) than those with a bachelor's degree, but not for arthritis alone (P = 0.17). Household income was not significantly associated with LBP (P = 0.16) or arthritis (p = 0.23) independent of age, sex, and BMI.

CONCLUSION

Our study did not find significant associations between neighbourhood deprivation and the presence of LBP or arthritis. More research using multilevel modelling to investigate neighbourhood effects on LBP and arthritis is recommended.

Quasi-causal associations between chronotype and post-traumatic stress disorder symptoms: A twin study

OBJECTIVE

The evening ("night owl") chronotype is associated with greater severity and lifetime prevalence of post-traumatic stress disorder (PTSD) symptoms compared to morning or intermediate chronotypes. This twin study investigated the gene-environment relationships between chronotype, recent PTSD symptoms, and lifetime intrusive symptoms.

METHODS

We used the reduced Horne-Östberg Morningness-Eveningness Questionnaire (rMEQ) to assess chronotype in a sample of 3777 same-sex adult twin pairs raised together (70.4% monozygotic, 29.6% dizygotic) in the community-based Washington State Twin Registry. PTSD symptoms were reported on the Impact of Events Scale (IES) and a single item for lifetime experience of intrusive symptoms after a stressful or traumatic event.

RESULTS

Genetic influences accounted for 50% of chronotype variance, 30% of IES score variance, and 14% of lifetime intrusive symptom variance. Bivariate twin models showed a phenotypic association (b_p) between evening chronotype and more severe PTSD symptoms (b_p = -0.16, SE = 0.02, p < .001) that remained significant even after adjusting for shared genetic and environmental influences (b_p = -0.10, SE = 0.04, p = .009), as well as age, sex, and self-reported sleep duration (b_p = -0.11, SE = 0.04, p = .004). An association was found between evening chronotype and lifetime intrusive symptoms (b_p = -0.11, SE = 0.03, p < .001) that was no longer significant after adjusting for shared genetic and environmental influences (b_p = 0.04, SE = 0.06, p = .558).

CONCLUSIONS

Our results suggest a "quasi-causal" relationship between evening chronotype and PTSD symptoms that is not purely attributable to genetic or shared environmental factors. Evening chronotype may increase vulnerability to pathologic stress responses in the setting of circadian misalignment, providing potential avenues of prevention and treatment using chronobiological strategies.

The pillars of health: influence of multiple lifestyle behaviors on body mass index and depressive symptoms in adult twins

Background

Guidelines promoting healthy lifestyles are cornerstones of chronic disease prevention and treatment. The purpose of this study is to investigate independent and joint associations of five key health behaviors with health outcomes (body mass index (BMI kg/m²) and depressive symptoms) in adult twins.

Methods

We included 6,048 twin pairs from a community-based registry. Five key health behaviors were: (1) ≥ 8 h of sleep per night, (2) ≥ 5 servings of fruits and vegetables daily, (3) ≤ 2 h sedentary time per day, (4) ≥ 150 min of moderate-to-vigorous physical activity (MVPA) per week, and (5) no smoking. We analyzed phenotypic associations between behaviors and outcomes; whether phenotypic associations were confounded by additive genetic and shared environmental factors within twin pairs (“quasi-causal” associations); and which behaviors, considered simultaneously, had the largest associations with outcomes.

Results

We found negative phenotypic associations between number of behaviors achieved with BMI and depressive symptoms score (ps < 0.05). Associations remained significant, though attenuated, when controlling for genetic and shared environmental factors, and demographics, for depressive symptoms score but not BMI (p < 0.05). Quantitative variable importance measures derived from regression tree models showed sedentary time and MVPA were the most important variables in partitioning twins with different BMI, and smoking and sedentary time for partitioning twins with different depressive symptoms score.

Conclusions

Achievement of commonly endorsed health behaviors is associated with lower BMI (especially sedentary and MVPA targets) and depressive symptoms score (especially sedentary and smoking targets). This provides further support of health behavior promotion to improve health outcomes.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12889-022-13901-7.

Changes in physical activity levels and mental health during COVID-19: Prospective findings among adult twin pairs

BACKGROUND

Physical distancing and other COVID-19 pandemic mitigation strategies have negatively impacted physical activity (PA) levels and mental health in cross-sectional studies. The purpose of this study was to investigate associations between changes in PA and mental health outcomes during the COVID-19 pandemic, following implementation of mitigation strategies, in a sample of adult twins.

METHODS

This was a prospective study of 3,057 adult twins from the Washington State Twin Registry. Study participants completed online surveys in 2020, at baseline (March 26 -April 5), and three follow-up waves (W1: April 20 -May 3; W2: Jul 16 -Aug 2; W3: Sept 16 -Oct 1). Physical activity was operationalized as self-reported moderate-to-vigorous PA (MVPA) and neighborhood walking (minutes/week), and mental health outcomes, operationalized as self-reported anxiety and perceived stress were assessed in the three waves of follow-up. Latent growth curve models (LGCMs) were used to assess changes in PA and mental health outcomes over time. Parallel LGCMs were used to estimate the cross-sectional, parallel, and prospective associations between PA and mental health over time. All models took into within-pair correlations and adjusted for age, sex, and race.

RESULTS

Individuals' amount of MVPA and walking decreased over time, whereas levels of anxiety remained stable, and stress increased slightly. Cross-sectional associations observed between both PA predictors and mental health outcomes were weak. After taking into account cross-sectional associations between PA and mental health outcomes, changes in PA over time were not associated with changes in mental health outcomes over time.

CONCLUSIONS

Over a time period aligned with COVID-19 mitigation strategies and social restrictions, changes in physical activity was not associated with changes in anxiety or stress levels in the current sample. Nonetheless, the average decline in PA over time is worrisome. Public health resources should continue to promote PA as a means to improve physical health during the pandemic.

Differences in Stress and Anxiety Among Women With and Without Children in the Household During the Early Months of the COVID-19 Pandemic

The COVID-19 pandemic and resulting lockdowns have had a disproportionate impact on parents of children under 18, particularly women. Mandatory school closures and loss of childcare resulted in parents balancing work, teaching, and childcare needs. A number of studies have examined changes in mental health of parents, but to date no studies have compared the differences in stress and anxiety levels between women with and without children in the United States. Adult women from the Washington State Twin Registry (WSTR) (N = 1,014, pair N = 529) and mothers of twin children enrolled in the WSTR (N = 147) completed an online survey examining several health-related behaviors and outcomes and their self-reported changes due to COVID-19. We conducted two studies to examine the impact of children on stress and anxiety levels among women. In study 1, we assessed whether women living in households with children under the age of 18 have higher levels of stress and anxiety than those without children in their household. We found that perceived stress levels did not differ between women with and without children in the household, but anxiety levels were higher among women living with children than those without. In study 2, we assessed whether the correlation between children in the household and stress/anxiety is accounted for by non-random genetic and environmental selection effects, causal processes, or both using a sample of adult female twins. We found that the presence of children in the household was associated with higher levels of stress and anxiety. However, this association is confounded by genetic and shared environmental factors. Our findings highlight the need to provide supporting resources to women living with children in the household during and after the COVID-19 pandemic.

The Iranian Newborn Multiples Registry (IRNMR): a registry protocol

Purpose: Over the last decades, several twin/multiples registries have been developed worldwide, mostly concentrated in Europe and high-income countries (HICs). In Iran, we lack accurate nationwide epidemiological and biobank data on twins. We established the Iranian Newborn Multiples Registry (IRNMR) to evaluate the role of genetics and environmental factors in the variation of phenotypes among newborn monozygotic (MZ) and dizygotic (DZ) twin pairs. IRNMR is a multicenter hospital-based registry. Materials and methods: In the pilot phase, we collected epidemiological data from multiples born in Imam Khomeini Hospital complex and Aban Hospital located in Tehran, the capital of Iran, with a population exceeding 8 million, Allameh Bohlool Gonabadi Hospital, Gonabad, Razavi Khorasan, and Shahid Sadoughi Hospital, Yazd, Iran. Results: The IRNMR has recruited 457 sets of newborn twins and multiples so far. We hold follow-up sessions by mother and child health professionals to monitor multiples' growth, development, diseases, and mortality. Conclusions: We successfully developed a newborn multiples registry in Iran. This registry will create an invaluable database to study the relative influence of genes and environmental factors on various chronic diseases, growth, development, and behavioral disorders. We intend to collaborate with other centers to develop a large multicenter nationwide multiple birth registry and biobank in Iran.

Genetic and environmental influences on posttraumatic stress disorder symptoms and disinhibited eating behaviors

Posttraumatic stress disorder (PTSD) and eating disorders (ED) frequently co-occur, but the mechanisms underlying this association remain unclear. EDs are characterized by features of maladaptive eating behaviors including disinhibited eating and cognitive dietary restraint. Identifying the genetic overlap between PTSD symptoms and maladaptive eating behaviors may elucidate biological mechanisms and potential treatment targets. A community sample of 400 same-sex twins (102 monozygotic and 98 dizygotic pairs) completed the PTSD Checklist-Civilian (PCL-C) for PTSD symptoms and the Three-Factor Eating Questionnaire-Reduced (TFEQ-R18) for eating behaviors (uncontrolled eating, emotional eating, and cognitive dietary restraint). We used biometric modeling to examine the genetic and environmental relationships between PCL-C and TFEQ-R18 total and subscales scores. Heritability was estimated at 48% for PTSD symptoms and 45% for eating behavior overall. Bivariate models revealed a significant genetic correlation between PTSD symptoms and eating behavior overall (r_g =.34; CI:.07,.58) and Uncontrolled Eating (r_g =.53; CI:.24,.84), and a significant environmental correlation between PTSD symptoms and Emotional Eating (r_e =.30; CI:.12,.45). These findings suggest the influence of common etiology. Future research and clinical efforts should focus on developing integrated treatments.

Is COVID-19 Keeping us Up at Night? Stress, Anxiety, and Sleep Among Adult Twins

In response to the COVID-19 pandemic, a variety of social distancing measures to mitigate the virus outbreak have been implemented. These measures may have unintended consequences on individuals' well-being, such as increased stress, anxiety, and sleep disruptions. We investigated the extent to which individuals' mental health status is associated with perceived changes in sleep amount and sleep quality among a sample of adult twin pairs (N = 909 pairs; 77% MZ, 23% DZ), less than a month after the outbreak was declared a pandemic by the World Health Organization. About half of participants reported no change in sleep amount (50.1%) or sleep quality (55.6%). Approximately one-third of the participants had increased amount of sleep (29.8%), and 32.9% reported a decrease in sleep quality. We found that stress and anxiety levels were associated with sleep reduction (ORs = 2.36 and 3.12 for stress and anxiety, respectively) and poorer sleep quality (ORs = 2.45 and 3.73 for stress and anxiety, respectively), even after taking into account between-family confounds. A much smaller association was observed between levels of stress and anxiety and increased sleep amount (ORs = 1.42 and 1.60 for stress and anxiety, respectively) and sleep quality (OR = 1.21 and 1.29 for stress and anxiety, respectively), which was no longer significant after controlling for between-family confounds. Our results demonstrate that stress and anxiety associated with the COVID-19 pandemic and social distancing measures may be linked to reduced sleep amount and quality.

Fear and depression linked to COVID-19 exposure A study of adult twins during the COVID-19 pandemic

Millions of people have been impacted by the novel coronavirus (COVID-19) pandemic worldwide. High infection rates and death tolls, combined with social distancing measures, may have unintended psychological consequences on individuals. The goal of this study was to examine the interrelations between COVID-19 exposure, fear of COVID-19, and depression among a community-based sample of adult twins. We further explored whether fear of COVID-19 mediated the association between COVID-19 exposure and depression. 732 same-sex adult twin pairs (78.1% MZ, 21.9% DZ) completed an online survey examining their feelings in May 2020. About one-fifth of the participants reported having any COVID-19 exposure. Most participants (>80%) were somewhat concerned about themselves or their household members being infected by COVID-19. The average depression level was relatively low (M = 0.9 out of 6). We found that COVID-19 exposure was related to increased fears of COVID-19 and depression, and that depressive feelings increased with fear of COVID-19. The correlation between COVID-19 exposure and depression was partially mediated by fear of COVID-19. However, these associations were confounded by familial influences. As society navigates through the pandemic, it is essential to implement public health strategies to help individuals cope with the concerns and fears about COVID-19.

Association between low back pain and body mass index in adult twins: an analysis of monozygotic and dizygotic twins of the Washington State Twin Registry

BACKGROUND CONTEXT

Low back pain (LBP) is a common and significant cause of disability worldwide, however; questions about cause still remain.

PURPOSE

To investigate the association between LBP, body mass index (BMI), and moderate to vigorous physical activity (MVPA) in a twin sample.

STUDY DESIGN

Cross sectional study of monozygotic (MZ) and dizygotic (DZ) twins from the Washington State Twin Registry.

PATIENT SAMPLE

Monozygotic and dizygotic twins from the Washington State Twin Registry.

OUTCOME MEASURES

Self-report measures: Low back pain, body mass index, duration and intensity of exercise.

METHODS

The sample included 5,183 same-sex pairs (69% MZ). The outcome was self-reported diagnosis of LBP from a health care provider. A phenotypic model tested the association between BMI and LBP without including genetic or shared environmental confounds. We then re-estimated the association using a quasi-causal model which controls for those confounds. Finally, we used a mediation model to investigate if the association between LBP and MVPA was mediated by BMI.

RESULTS

In the phenotypic regression of LBP on BMI, there was a ~4-fold increase in the odds of having LBP with every one-unit increase in BMI (odds ratio [OR] =3.83; 95% confidence interval =3.28, 4.46). However, quasi-causal regression of LBP on BMI was reduced to zero (OR =0.95; 95% confidence interval =0.60, 1.49). A significant genetic background to BMI and LBP was present (b_A =1.66; p<.001), even after controlling for confounders. In another analysis there was a significant direct effect between MVPA and LBP (b_p=-0.092, standard error [SE] =0.017, p<.001). In mediation analysis, the effect of MVPA on LBP was partially mediated through MVPA effects on BMI ( [Formula: see text] =-0.043, SE=0.003, p<.001) and BMI effects on LBP ( [Formula: see text] =1.281, SE=0.079, OR=3.6, p<.001), however shared environmental factors confounded this relationship.

CONCLUSIONS

BMI was not associated with LBP, despite sharing a phenotypic association, but they may share a genetic influence. The effect of MVPA on LBP is, in part, mediated by BMI but shared environment confounds this relationship.

LEVEL OF EVIDENCE

Level 3.

Stress, Anxiety, and Change in Alcohol Use During the COVID-19 Pandemic: Findings Among Adult Twin Pairs

The novel coronavirus (COVID-19) has impacted the lives of people worldwide since being declared a pandemic on March 11, 2020. Social restrictions aimed at flattening the curve may be associated with an increase in stress and anxiety, which may increase the use of alcohol as a coping mechanism. The objective of this study was to examine if stress and anxiety were associated with changes in alcohol use in a sample of adult twins. Twins allowed us to control for genetic and shared environmental factors that would confound the alcohol - mental health relationship. Twins (N = 3,971; 909 same-sex pairs) from the Washington State Twin Registry (WSTR) completed an online survey examining several health-related behaviors and outcomes and their self-reported changes due to COVID-19. About 14% of the respondents reported an increase in alcohol use. We found an association between both stress and anxiety and increased alcohol use, where twins with higher levels of stress and anxiety were more likely to report an increase in alcohol consumption. The associations were small and confounded by between-family factors and demographic characteristics. However, there was no significant difference in stress or anxiety levels between non-drinkers and those who reported no change in alcohol use. Our findings suggest that individuals' mental health may be associated with changes in alcohol use during the COVID-19 pandemic.

Perceived change in physical activity levels and mental health during COVID-19: Findings among adult twin pairs

Background

Physical distancing and other COVID-19 pandemic mitigation strategies may have unintended consequences on a number of health behaviors and health outcomes. The purpose of this study was to investigate the association between perceived change in physical activity or exercise and mental health outcomes over the short-term in response to COVID-19 mitigation strategies in a sample of adult twins.

Methods

This was a cross-sectional study of 3,971 identical and same-sex fraternal adult twins (909 pairs, 77% identical) from the community-based Washington State Twin Registry. Participants in this study completed an online survey examining the impact of COVID-19 mitigation on a number of health-related behaviors and outcomes, administered between March 26 and April 5, 2020. In the present study, the exposure was perceived change in physical activity or exercise. The outcomes were levels of perceived anxiety and stress.

Results

More twin pairs reported a decrease in physical activity levels (42%) than those reporting no change (31%) or increased physical activity levels (27%). A perceived decrease in physical activity or exercise was associated with higher stress and anxiety levels. However, the physical activity–stress relationship was confounded by genetic and shared environmental factors. On the other hand, the physical activity–anxiety relationship held after controlling for genetic and shared environmental factors, although it was no longer significant after further controlling for age and sex, with older twins more likely to report lower levels of anxiety and females more likely to report higher levels of anxiety.

Conclusions

Strategies to mitigate the COVID-19 pandemic may be impacting physical activity and mental health, with those experiencing a decrease in physical activity also having higher levels of stress and anxiety. These relationships are confounded by genetic and shared environmental factors, in the case of stress, and age and sex, in the case of anxiety.

Sleep duration and post-traumatic stress disorder symptoms: a twin study

STUDY OBJECTIVES

Long and short sleep duration are associated with greater risk of developing post-traumatic stress disorder (PTSD); however, it is unknown how genetic and environmental influences affect this relationship. Thus, we investigated the association between sleep duration and PTSD symptoms using twin models.

METHODS

Data were obtained from 1865 monozygotic and 758 dizygotic twin pairs enrolled in the community-based Washington State Twin Registry. PTSD symptoms were assessed using the Impact of Events Scale (IES). A classical twin model decomposed the variances of sleep duration and IES score into additive genetic, shared environmental, and unique environmental components. We used correlated factor models to examine the moderation of variance components of sleep duration and IES.

RESULTS

Shorter and longer sleep duration were associated with higher IES scores with a quadratic association (p < 0.001). The heritability of sleep duration was 36%, and IES 31%. Variance in sleep duration attributable to shared (b1C1 = 2.91, 95% CI = 1.40 to 4.43; p < 0.001) and unique (b1E1 = 0.18, 95% CI = 0.10 to 0.27; p < 0.001) environment was moderated by IES score. Similarly, but to a lesser extent, variance in IES attributable to additive genetics (b1A2 = -0.23, 95% CI = -0.45 to 0.00; p = 0.048) was moderated by sleep duration.

CONCLUSIONS

Greater PTSD symptom severity was associated with short and long sleep duration. Increasing PTSD symptoms increased variability in sleep duration primarily via shared environmental factors, whereas decreasing sleep duration increased variability in PTSD symptoms primarily via additive genetic factors. This suggests childhood experiences affect variability of sleep duration and genetic factors affect the variability of PTSD symptoms in trauma-exposed individuals.

Associations between changes in resilient coping and posttraumatic stress disorder symptoms

The pervasive, damaging nature of posttraumatic stress disorder (PTSD) presents enormous clinical challenges. Understanding the relationship between patients' perceptions of PTSD symptoms and resilient coping strategies may prompt investigation of clinical interventions that improve adaptive, resilient coping skills. In this study, we examined whether changes in resilient coping were related to changes over time in the PTSD symptoms of intrusion and avoidance. A secondary analysis was conducted using longitudinal data from the community-based Washington State Twin Registry. Participants completed the four-item Brief Resilient Coping Scale (BRCS) and the Avoidance and Intrusion subscales of the Impact of Events Scale (IES) at two points in time that were at least 2 years apart. To limit analyses to participants reporting PTSD symptoms at baseline, an initial value of at least 1.0 on either Avoidance (n = 1,337) or Intrusion (n = 1,206) was required for inclusion in the sample. Using linear regression, we assessed associations of change in BRCS with a change in IES scores, controlling for the respective initial scores on each measure. Controlling for initial BRCS and IES-Intrusion values, we observed a small, statistically significant association between change in BRCS and change in IES-Intrusion scores (b* = -0.07; p = .003). There was no statistically significant association between change in BRCS and change in IES-Avoidance (b* < 0.01; p = .869). In this large, longitudinal sample, increases in resilient coping were related to decreases in intrusive thoughts over time. Because coping patterns can be taught, these results warrant further investigations into adaptive coping patterns associated with diminishing PTSD symptoms.

Neighborhood Deprivation Moderates Shared and Unique Environmental Influences on Hazardous Drinking: Findings from a Cross-Sectional Co-Twin Study

Background: There has been increased interest in the interplay of genetic and environmental factors in the development of problematic alcohol use, including socioeconomic conditions of the neighborhood. Using a co-twin design, we examined the extent to which contributions of genetic, shared environmental, and unique environmental influences on hazardous drinking differed according to levels of neighborhood socioeconomic deprivation. Method: Data came from 1,521 monozygotic (MZ) and 609 dizygotic (DZ) twin pairs surveyed in Washington State. A measure of neighborhood deprivation was created based on census-tract-level variables and the Alcohol Use Disorders Identification Test 3-item instrument was used to assess level of hazardous drinking. We tested a series of nested structural equation models to examine associations among hazardous drinking, neighborhood deprivation, and the variance components (genetic [A], shared [C] and unique environmental [E] influences) of these two constructs, testing for both main effects and moderation by neighborhood deprivation. Results: Neighborhood deprivation was significantly associated with increased hazardous drinking, after accounting for A and C variance common to both phenotypes. Adjusting for within-pair differences in income and education, neighborhood deprivation moderated the magnitude of variance components of hazardous drinking, with the variance attributable to shared environment and non-shared environment increasing in more deprived neighborhoods. Conclusions: Findings point to amplification of early childhood as well as unique adulthood environmental risk on hazardous drinking in areas of greater deprivation.

Heritability of Type 2 Diabetes in the Washington State Twin Registry

Approximately 12% of U.S. adults have type 2 diabetes (T2D). Diagnosed T2D is caused by a combination of genetic and environmental factors including age and lifestyle. In adults 45 years and older, the Discordant Twin (DISCOTWIN) consortium of twin registries from Europe and Australia showed a moderate-to-high contribution of genetic factors of T2D with a pooled heritability of 72%. The purpose of this study was to investigate the contributions of genetic and environmental factors of T2D in twins 45 years and older in a U.S. twin cohort (Washington State Twin Registry, WSTR) and compare the estimates to the DISCOTWIN consortium. We also compared these estimates with twins under the age of 45. Data were obtained from 2692 monozygotic (MZ) and same-sex dizygotic (DZ) twin pairs over 45 and 4217 twin pairs under 45 who responded to the question 'Has a doctor ever diagnosed you with (type 2) diabetes?' Twin similarity was analyzed using both tetrachoric correlations and structural equation modeling. Overall, 9.4% of MZ and 14.7% of DZ twins over the age of 45 were discordant for T2D in the WSTR, compared to 5.1% of MZ and 8% of DZ twins in the DISCOTWIN consortium. Unlike the DISCOTWIN consortium in which heritability was 72%, heritability was only 52% in the WSTR. In twins under the age of 45, heritability did not contribute to the variance in T2D. In a U.S. sample of adult twins, environmental factors appear to be increasingly important in the development of T2D.

The Washington State Twin Registry: 2019 Update

It has been over 5 years since the last special issue of Twin Research and Human Genetics on 'Twin Registries Worldwide: An Important Resource for Scientific Research' was published. Much progress has been made in the broad field of twin research since that time, and the current special issue is a follow-up to update the scientific community about twin registries around the globe. The present article builds upon our 2013 Registry description by summarizing current information on the Washington State Twin Registry (WSTR), including history and construction methods, member characteristics, available data, and major research goals. We also provide a section with brief summaries of recently completed studies and discuss the future research directions of the WSTR. The Registry has grown in terms of size and scope since 2013; highlights include recruitment of youth pairs under 18 years of age, extensive geocoding work to develop environmental exposures that can be linked to survey and administrative health data such as death records, and expansion of a biobank with specimens collected for genotyping, DNA methylation, and microbiome based-studies.

A Twin Study of Genetic Influences on Nephrolithiasis in Women and Men

BACKGROUND

Nephrolithiasis is a complex phenotype influenced by both genetic and environmental factors. Previously we found a genetic component to stone disease using a sample of male twin pairs. We now report on the genetic contribution to stones in a sample of female and male twin pairs.

METHODS

We conducted a classic twin study of kidney stones using the Washington State Twin Registry. Data were collected by questionnaire to obtain self-reported history of kidney stones. Univariate structural equation modeling was used to determine the relative contributions of additive genetics, common environment, and unique environment.

RESULTS

There were 7053 same-sex pairs with kidney stone data. The mean age of the sample was 39 years, similar in women and men. The prevalence of stones was 4.9% of women and 6.2% of men. We found significant contributions from genetics and the unique environment (P < 0.05 for both) for the risk for stone disease in women and men. There was no significant contribution of the common environment for either sex. After adjusting for age, heritability was 46% (95% confidence interval 0.36-0.56) in women and 57% (0.46-0.68) in men, which was significantly different (P < 0.05).

CONCLUSIONS

Nephrolithiasis in women has a heritable component less than that we again demonstrate in men. This finding may in part explain why more stone formers are men than women. Women twins demonstrated a greater effect of the unique environment on stone prevalence. The specific environmental risk factors that account for this effect are not currently known.

Plantar pressures in identical and non-identical twins

Identifying environmental risk factors for musculoskeletal disorders is challenging due to the number of potential confounders. Twins are of particular interest for researchers interested in studying these types of problems due to their inherent control for the influence of genetic factors. In twin studies, this population can allow environmental risk factors to be more easily identified, and this type of study design may allow the role of biomechanics in injury and disease to be further explored. At present, it is unclear if foot function displays more similarity between certain types of twins. In this study, we hypothesized that the plantar pressures of monozygotic (identical) twins would be more similar between pairs than dizygotic (non-identical) twins. We measured static and dynamic plantar pressures from five pairs of each twin type. Statistical parametric modeling was used to compare pressure distributions at the sensor level. For >80% of stance phase, the pixel level analysis indicated that monozygotic twins had less variation in plantar pressure between pairs. The average z-statistic across the entire trial was 0.88 for the monozygotic group and 1.13 for the dizygotic group. In this study we provide evidence of greater similarity of plantar pressures in monozygotic twin pairs compared to dizygotic twins. This finding supports the use of co-twin studies investigating potentially modifiable environmental and biomechanical risk factors for musculoskeletal conditions that affect the foot and ankle.

Study Design and Rationale for the Mood and Methylation Study: A Platform for Multi-Omics Investigation of Depression in Twins

Major depression is a complex disorder with no single, direct causal mechanism. Morbidity has been linked to genetic processes, developmental history, and unique environmental exposures. Epigenetic mechanisms, especially DNA methylation, are also likely important factors in the pathogenesis of major depressive disorder (MDD). A community-based twin sample has many advantages for epigenetic studies, given the shared genetic and developmental histories of same-sex twin pairs. This article describes the rationale and study design for the Mood and Methylation Study in which 133 twin pairs (101 monozygotic and 32 dizygotic), both discordant and concordant for lifetime history of MDD, were evaluated on a large number of variables related to MDD. The twins also provided blood samples for an epigenome-wide association study of differentially methylated regions (DMR) relevant to MDD. Although MDD is typically considered a disorder of the central nervous system, it is unfeasible to obtain a large sample of brain tissues. However, epigenetic variation is not limited to the affected tissue but can also be detected in peripheral blood leukocytes. Thus, this study focused on monocytes for the major analyses. Additional plans for the study include gene expression analysis from the same set of twins using RNA-seq and validation of significant DMRs in postmortem brain tissues from a separate sample. Moreover, sufficient samples have been collected to perform future ‘multi-omic’ analyses, including metabolome, microbiome, and transcriptome. Our long-term goal is to understand how epigenomic and other ‘omic’ factors can be manipulated for diagnostic, preventive, and therapeutic purposes for MDD and its related conditions.

Childhood Trauma, DNA Methylation of Stress-Related Genes, and Depression: Findings From Two Monozygotic Twin Studies

OBJECTIVE

DNA methylation has been associated with both early life stress and depression. This study examined the combined association of DNA methylation at multiple CpG probes in five stress-related genes with depressive symptoms and tested whether these genes methylation mediated the association between childhood trauma and depression in two monozygotic (MZ) twin studies.

METHODS

The current analysis comprised 119 MZ twin pairs (84 male pairs [mean = 55 years] and 35 female pairs [mean = 36 years]). Peripheral blood DNA methylation of five stress-related genes (BDNF, NR3C1, SLC6A4, MAOA, and MAOB) was quantified by bisulfite pyrosequencing or 450K BeadChip. We applied generalized Poisson linear-mixed models to examine the association between each single CpG methylation and depressive symptoms. The joint associations of multiple CpGs in a single gene or all five stress-related genes as a pathway were tested by weighted truncated product method. Mediation analysis was conducted to test the potential mediating effect of stress gene methylation on the relationship between childhood trauma and depressive symptoms.

RESULTS

Multiple CpG probes showed nominal individual associations, but very few survived multiple testing. Gene-based or gene-set approach, however, revealed significant joint associations of DNA methylation in all five stress-related genes with depressive symptoms in both studies. Moreover, two CpG probes in the BDNF and NR3C1 mediated approximately 20% of the association between childhood trauma and depressive symptoms.

CONCLUSIONS

DNA methylation at multiple CpG sites are jointly associated with depressive symptoms and partly mediates the association between childhood trauma and depression. Our results highlight the importance of testing the combined effects of multiple CpG loci on complex traits and may unravel a molecular mechanism through which adverse early life experiences are biologically embedded.

Differential models of twin correlations in skew for body-mass index (BMI)

Background

Body Mass Index (BMI), like most human phenotypes, is substantially heritable. However, BMI is not normally distributed; the skew appears to be structural, and increases as a function of age. Moreover, twin correlations for BMI commonly violate the assumptions of the most common variety of the classical twin model, with the MZ twin correlation greater than twice the DZ correlation. This study aimed to decompose twin correlations for BMI using more general skew-t distributions.

Methods

Same sex MZ and DZ twin pairs (N = 7,086) from the community-based Washington State Twin Registry were included. We used latent profile analysis (LPA) to decompose twin correlations for BMI into multiple mixture distributions. LPA was performed using the default normal mixture distribution and the skew-t mixture distribution. Similar analyses were performed for height as a comparison. Our analyses are then replicated in an independent dataset.

Results

A two-class solution under the skew-t mixture distribution fits the BMI distribution for both genders. The first class consists of a relatively normally distributed, highly heritable BMI with a mean in the normal range. The second class is a positively skewed BMI in the overweight and obese range, with lower twin correlations. In contrast, height is normally distributed, highly heritable, and is well-fit by a single latent class. Results in the replication dataset were highly similar.

Conclusions

Our findings suggest that two distinct processes underlie the skew of the BMI distribution. The contrast between height and weight is in accord with subjective psychological experience: both are under obvious genetic influence, but BMI is also subject to behavioral control, whereas height is not.

Dexamethasone-suppressed Salivary Cortisol and Pain Sensitivity in Female Twins

OBJECTIVES

Hypothalamic-pituitary-adrenal (HPA) axis dysregulation is associated with chronic pain. Studying pain sensitivity and the HPA axis could elucidate the role of stress in chronic pain development, which might be influenced by familial factors, including genes.

METHODS

Associations between pain sensitivity and salivary cortisol and familial confounding in these associations were examined in 88 female, community-based twin pairs (75% monozygotic, mean age 29 y). Cortisol was assessed after 0.25 mg dexamethasone (DEX), recovery from 0.25 mg DEX, and after 0.5 mg DEX. Cold pressor task (CPT) pain ratings were obtained at threshold and at tolerance. Conditioned pain modulation (CPM) was examined using thermal heat as the testing stimulus and hot water as the conditioning stimulus. Generalized estimating equation models were used and adjusted for baseline pain rating, age, and other relevant covariates.

RESULTS

After controlling for baseline cortisol, greater cortisol suppression following DEX administration and lower recovery cortisol levels were associated with higher pain ratings at tolerance during the CPT (Bs=-2.42 to -17.82; Ps=0.031 to<0.001) as well as with reduced CPM (Bs=-0.92 to -1.68; Ps=0.003 to 0.046). Interestingly, familial confounding was evident in the CPT and CPM during recovery from DEX administration, but not immediately following DEX administration.

DISCUSSION

These findings contribute to understanding possible mechanisms underlying chronic pain by demonstrating that HPA axis response to negative feedback is related to pain sensitivity.

Cross-sectional association between soda consumption and body mass index in a community-based sample of twins

BACKGROUND

Consumption of sugar-sweetened beverages, such as soda, have been shown to play an important role in weight gain. Although soda consumption has been associated with body mass index (BMI) in many studies, it has been difficult to ascertain a true causal relationship between soda consumption and BMI for two reasons. First, findings have been based largely on observational and cross-sectional studies, with much less evidence from randomized controlled trials. Second, the reported relationships may be confounded by genetic and shared environmental factors that affect both soda consumption and BMI. In the present study, we used the twin design to better understand the relationship between soda consumption and BMI by accounting for measured and unmeasured confounds in non-experimental data. Associations from genetically informed tests in twins are considered "quasi-causal," suggesting that our confidence in the causal underpinning of the association between soda consumption and BMI has been strengthened. We hypothesized that the association between soda consumption and BMI would be significant both between and within twins.

METHODS

This was a cross sectional study of 5787 same sex adult twin pairs (18-97 years, 66% female) from the community based Washington State Twin Registry. Structural equation modeling (SEM) was employed to investigate associations between soda consumption and BMI in the population (the phenotypic association between exposure and outcome among all twins treated as individuals) and within pairs of identical and fraternal twins (the quasi-causal association controlling for between pair genetic and environmental confounds).

RESULTS

Among all twins, there was a significant phenotypic association between soda consumption and BMI that held when controlling for age, sex, race, annual household income, and education level (P < 0.05). In the quasi-causal model, however, the effect of soda consumption on BMI was greatly reduced and no longer significant, with a large genetic confound in both men and women (P < 0.05).

CONCLUSION

Among a large group of adult twin pairs, increased soda consumption was associated with increased BMI; however, the observed association was mediated by a genetic background common to both.

Neighborhood walkability moderates the association between low back pain and physical activity: A co-twin control study

The aim of this study was to investigate whether neighborhood walkability moderates the association between low back pain (LBP) and physical activity (PA), using a co-twin design to control for genetics and shared environmental factors. A cross-sectional analysis was performed on 10,228 twins from the Washington State Twin Registry with available data on LBP from recruitment surveys between 2009 and 2013. LBP within the past 3months was our exposure variable. Our outcome variables were sufficient moderate or vigorous-intensity PA (MVPA, defined as at least 75min of vigorous-intensity PA, or 150min of moderate-intensity PA per week), and walking (≥150min per week). Neighborhood walkability, estimated using the commercially available Walk Score®, was our moderator variable. After controlling for the influence of genetics and shared environment, individuals reporting LBP were significantly less likely to engage in sufficient MVPA if they lived in a neighborhood with high walkability (OR=0.59, 95%CI: 0.36-0.96). There was no association between LBP and sufficient MVPA for individuals living in a neighborhood with low walkability (OR=1.27, 95%CI: 0.93-1.72), demonstrating that walkability is a significant moderator of the association between LBP and PA (interaction p=0.013). These findings were similar for the association between LBP and walking (high walkability OR=0.42, 95%CI: 0.22-0.78; low walkability OR=0.71, 95%CI: 0.46-1.12), although the interaction was not significant (p=0.700). Neighborhood walkability moderates the association between LBP and PA. Our results highlight the importance of targeting interventions promoting PA towards individuals with LBP living in a neighborhood with good walkable access to amenities.

A Twin Study on Perceived Stress, Depressive Symptoms, and Marriage

Marriage is associated with reductions in both perceived stress and depressive symptoms, two constructs found to be influenced by common genetic effects. A study of sibling twins was used to test whether marriage decreases the proportion of variance in depressive symptoms accounted for by genetic and environmental effects underlying perceived stress. The sample consisted of 1,612 male and female twin pairs from the University of Washington Twin Registry. The stress-buffering role of marriage was tested relative to two unmarried groups: the never married and the divorced. Multivariate twin models showed that marriage reduced genetic effects of perceived stress on depressive symptoms but did not reduce environmental effects. The findings suggest a potential marital trade-off for women: access to a spouse may decrease genetic effects of perceived stress on depressive symptoms, although marital and family demands may increase environmental effects of perceived stress on depressive symptoms.

Neighborhood deprivation and depression in adult twins: genetics and gene×environment interaction

BACKGROUND

Depression is a significant problem and it is vital to understand its underlying causes and related policy implications. Neighborhood characteristics are implicated in depression but the nature of this association is unclear. Unobserved or unmeasured factors may confound the relationship. This study addresses confounding in a twin study investigating neighborhood-level effects on depression controlling for genetics, common environment, and gene×environment (G × E) interactions.

METHOD

Data on neighborhood deprivation and depression were gathered from 3155 monozygotic twin pairs and 1275 dizygotic pairs (65.7% female) between 2006 and 2013. The variance for both depression and neighborhood deprivation was decomposed into three components: additive genetic variance (A); shared environmental variance (C); and non-shared environmental variance (E). Depression was then regressed on neighborhood deprivation to test the direct association and whether that association was confounded. We also tested for a G × E interaction in which the heritability of depression was modified by the level of neighborhood deprivation.

RESULTS

Depression and neighborhood deprivation showed evidence of significant A (21.8% and 15.9%, respectively) and C (13.9% and 32.7%, respectively) variance. Depression increased with increasing neighborhood deprivation across all twins (p = 0.009), but this regression was not significant after controlling for A and C variance common to both phenotypes (p = 0.615). The G × E model showed genetic influences on depression increasing with increasing neighborhood deprivation (p < 0.001).

CONCLUSIONS

Neighborhood deprivation is an important contributor to depression via increasing the genetic risk. Modifiable pathways that link neighborhoods to depression have been proposed and should serve as targets for intervention and research.

Genetic and Environmental Influences on Sleep, Pain, and Depression Symptoms in a Community Sample of Twins

OBJECTIVE

We used quantitative genetic methods to evaluate whether sleep quality, pain, and depression symptoms share a common genetic diathesis, to estimate the genetic and environmental sources of covariance among these symptoms, and to test for possible causal relationships.

METHODS

A community sample of 400 twins from the University of Washington Twin Registry completed standardized self-report questionnaires. We used biometric modeling to assess genetic and environmental contribution to the association between sleep quality measured by the Pittsburgh Sleep Quality Index, pain measured by the Brief Pain Inventory, and depression symptoms measured by the Brief Symptom Inventory. Trivariate Cholesky structural equation models were used to decompose correlations among the phenotypes.

RESULTS

Heritability was estimated at 37% (95% confidence interval = 20%-51%) for sleep quality, 25% (9%-41%) for pain, and 39% (22%-53%) for depression. Nonshared environmental influences accounted for the remaining variance. The genetic correlation between sleep quality and pain had an rg value of .69 (95% confidence interval [CI] = 0.33-0.97), rg value of .56 (95% CI = 0.55-0.98) between pain and depression, and rg value of .61 (95% CI = 0.44-0.88) between depression and sleep quality. Nonshared environmental overlap was present between pain and sleep quality as well as depression and sleep quality.

CONCLUSIONS

The link between sleep quality, pain, and depression was primarily explained by shared genetic influences. The genetic factors influencing sleep quality and pain were highly correlated even when accounting for depression. Findings support the hypothesis of a genetic link between depression and pain as well as potential causality for the association of sleep quality with pain and depression.

Is marriage a buzzkill? A twin study of marital status and alcohol consumption

Married adults have consistently been found to drink less than their single or divorced counterparts. This correlation may not be causal, however, as people nonrandomly "select" into marriage and into alcohol use. The current study uses a sample of 2,425 same-sex twin pairs (1,703 MZ; 722 DZ) to control for genetic and shared environmental selection, thereby eliminating a great many third variable, alternative explanations to the hypothesis that marriage causes less drinking. Married twins were compared with their single, divorced, and cohabiting cotwins on drinking frequency and quantity. Married cotwins consumed fewer alcoholic beverages than their single or divorced cotwins, and drank less frequently than their single cotwins. Alcohol use patterns did not differ among married and cohabiting twins. These findings provide strong evidence that intimate relationships cause a decline in alcohol consumption. (PsycINFO Database Record

Genetic and Environmental Overlap Between Childhood Maltreatment and Adult Physical Health

Past research demonstrates a phenotypic relationship between childhood maltreatment and adult health problems. Explanations of this association usually point to either: (a) a direct causal link, whereby exposure to early stress disrupts biological functioning during sensitive periods of development; or (b) an indirect effect operating through socioeconomic attainment, poor health behaviors, or some other pathway leading from childhood to adulthood. The current study examined whether the association between childhood maltreatment and adult health reflects genetic or environmental mediation. Using a large sample of adult American twins, we separately estimated univariate biometric models of child maltreatment and adult physical health, followed by a bivariate biometric model to estimate genetic and environmental correlations between the two variables. We found that a summary count of chronic health conditions shared non-trivial genetic overlap with childhood maltreatment. Our results have implications for understanding the relationship between maltreatment and health as one of active interplay rather than a simple cause and effect model that views maltreatment as an exogenous shock.

Alterations in Connectivity on Functional Magnetic Resonance Imaging with Provocation of Lower Urinary Tract Symptoms: A MAPP Research Network Feasibility Study of Urological Chronic Pelvic Pain Syndromes

PURPOSE

Urological chronic pelvic pain syndromes have refractory bladder or pelvic pain as the dominant symptom. This has been attributed to changes in the central nervous system caused by a chronic barrage of noxious stimuli. We developed what is to our knowledge a novel challenge protocol that induced bladder distention in study participants to reproduce pain and urinary symptoms. We tested to see whether it could discriminate between persons with urological chronic pelvic pain syndrome-like symptoms and asymptomatic controls.

MATERIALS AND METHODS

We recruited 10 female twin pairs who were discordant for urological chronic pelvic pain syndrome-like symptoms. Before scanning each twin urinated to completion and then consumed 500 cc water. Each twin was scanned with our resting state functional magnetic resonance imaging protocol immediately and approximately 50 minutes after consumption. Time series were extracted from the right and left periaqueductal gray, and the right and left amygdala subregions. We performed the repeated measures 2-sample t-test to assess differences in connectivity between symptomatic and asymptomatic twins before and after bladder distention.

RESULTS

Group by condition interaction effects were found from the periaqueductal gray to the right cerebellum VIIIa, the amygdala, the right premotor cortex/supplementary motor area and the insular cortex, and between the amygdala and the frontal pole/medial orbital frontal cortex, the hypothalamus, the insular cortex, the thalamus and the anterior cingulate cortex.

CONCLUSIONS

These findings demonstrate that our noninvasive bladder distention protocol can detect differences in the processing of urinary sensation between twins discordant for lower urinary tract pain.

Sleep Duration and Area-Level Deprivation in Twins

STUDY OBJECTIVES

We used quantitative genetic models to assess whether area-level deprivation as indicated by the Singh Index predicts shorter sleep duration and modifies its underlying genetic and environmental contributions.

METHODS

Participants were 4,218 adult twin pairs (2,377 monozygotic and 1,841 dizygotic) from the University of Washington Twin Registry. Participants self-reported habitual sleep duration. The Singh Index was determined by linking geocoding addresses to 17 indicators at the census-tract level using data from Census of Washington State and Census Tract Cartographic Boundary Files from 2000 and 2010. Data were analyzed using univariate and bivariate genetic decomposition and quantitative genetic interaction models that assessed A (additive genetics), C (common environment), and E (unique environment) main effects of the Singh Index on sleep duration and allowed the magnitude of residual ACE variance components in sleep duration to vary with the Index.

RESULTS

The sample had a mean age of 38.2 y (standard deviation [SD] = 18), and was predominantly female (62%) and Caucasian (91%). Mean sleep duration was 7.38 h (SD = 1.20) and the mean Singh Index score was 0.00 (SD = 0.89). The heritability of sleep duration was 39% and the Singh Index was 12%. The uncontrolled phenotypic regression of sleep duration on the Singh Index showed a significant negative relationship between area-level deprivation and sleep length (b = -0.080, P < 0.001). Every 1 SD in Singh Index was associated with a ∼4.5 min change in sleep duration. For the quasi-causal bivariate model, there was a significant main effect of E (b(0E) = -0.063; standard error [SE] = 0.30; P < 0.05). Residual variance components unique to sleep duration were significant for both A (b(0Au) = 0.734; SE = 0.020; P < 0.001) and E (b(0Eu) = 0.934; SE = 0.013; P < 0.001).

CONCLUSIONS

Area-level deprivation has a quasi-causal association with sleep duration, with greater deprivation being related to shorter sleep. As area-level deprivation increases, unique genetic and nonshared environmental residual variance in sleep duration increases.

Heritability of pain catastrophizing and associations with experimental pain outcomes: a twin study

This study used a twin paradigm to examine genetic and environmental contributions to pain catastrophizing and the observed association between pain catastrophizing and cold-pressor task (CPT) outcomes. Male and female monozygotic (n = 206) and dizygotic twins (n = 194) from the University of Washington Twin Registry completed a measure of pain catastrophizing and performed a CPT challenge. As expected, pain catastrophizing emerged as a significant predictor of several CPT outcomes, including cold-pressor Immersion Tolerance, Pain Tolerance, and Delayed Pain Rating. The heritability estimate for pain catastrophizing was found to be 37% with the remaining 63% of variance attributable to unique environmental influence. Additionally, the observed associations between pain catastrophizing and CPT outcomes were not found attributable to shared genetics or environmental exposure, which suggests a direct relationship between catastrophizing and experimental pain outcomes. This study is the first to examine the heritability of pain catastrophizing and potential processes by which pain catastrophizing is related to experimental pain response.

Associations Between Fast-Food Consumption and Body Mass Index: A Cross-Sectional Study in Adult Twins

Obesity is a substantial health problem in the United States, and is associated with many chronic diseases. Previous studies have linked poor dietary habits to obesity. This cross-sectional study aimed to identify the association between body mass index (BMI) and fast-food consumption among 669 same-sex adult twin pairs residing in the Puget Sound region around Seattle, Washington. We calculated twin-pair correlations for BMI and fast-food consumption. We next regressed BMI on fast-food consumption using generalized estimating equations (GEE), and finally estimated the within-pair difference in BMI associated with a difference in fast-food consumption, which controls for all potential genetic and environment characteristics shared between twins within a pair. Twin-pair correlations for fast-food consumption were similar for identical (monozygotic; MZ) and fraternal (dizygotic; DZ) twins, but were substantially higher in MZ than DZ twins for BMI. In the unadjusted GEE model, greater fast-food consumption was associated with larger BMI. For twin pairs overall, and for MZ twins, there was no association between within-pair differences in fast-food consumption and BMI in any model. In contrast, there was a significant association between within-pair differences in fast-food consumption and BMI among DZ twins, suggesting that genetic factors play a role in the observed association. Thus, although variance in fast-food consumption itself is largely driven by environmental factors, the overall association between this specific eating behavior and BMI is largely due to genetic factors.

Behavioral and Environmental Modification of the Genetic Influence on Body Mass Index: A Twin Study

Body mass index (BMI) has a strong genetic basis, with a heritability around 0.75, but is also influenced by numerous behavioral and environmental factors. Aspects of the built environment (e.g., environmental walkability) are hypothesized to influence obesity by directly affecting BMI, by facilitating or inhibiting behaviors such as physical activity that are related to BMI, or by suppressing genetic tendencies toward higher BMI. The present study investigated relative influences of physical activity and walkability on variance in BMI using 5079 same-sex adult twin pairs (70 % monozygotic, 65 % female). High activity and walkability levels independently suppressed genetic variance in BMI. Estimating their effects simultaneously, however, suggested that the walkability effect was mediated by activity. The suppressive effect of activity on variance in BMI was present even with a tendency for low-BMI individuals to select into environments that require higher activity levels. Overall, our results point to community- or macro-level interventions that facilitate individual-level behaviors as a plausible approach to addressing the obesity epidemic among US adults.

Access to green space, physical activity and mental health: a twin study

BACKGROUND

Increasing global urbanisation has resulted in a greater proportion of the world's population becoming exposed to risk factors unique to urban areas, and understanding these effects on public health is essential. The aim of this study was to examine the association between access to green space and mental health among adult twin pairs.

METHODS

We used a multilevel random intercept model of same-sex twin pairs (4338 individuals) from the community-based University of Washington Twin Registry to analyse the association between access to green space, as measured by the Normalised Difference Vegetation Index and self-reported depression, stress, and anxiety. The main parameter of interest was the within-pair effect for identical (monozygotic, MZ) twins because it was not subject to confounding by genetic or shared childhood environment factors. Models were adjusted for income, physical activity, neighbourhood deprivation and population density.

RESULTS

When treating twins as individuals and not as members of a twin pair, green space was significantly inversely associated with each mental health outcome. The association with depression remained significant in the within-pair MZ univariate and adjusted models; however, there was no within-pair MZ effect for stress or anxiety among the models adjusted for income and physical activity.

CONCLUSIONS

These results suggest that greater access to green space is associated with less depression, but provide less evidence for effects on stress or anxiety. Understanding the mechanisms linking neighbourhood characteristics to mental health has important public health implications. Future studies should combine twin designs and longitudinal data to strengthen causal inference.

Quasi-causal associations of physical activity and neighborhood walkability with body mass index: a twin study

OBJECTIVE

Physical activity, neighborhood walkability, and body mass index (BMI, kg/m(2)) associations were tested using quasi-experimental twin methods. We hypothesized that physical activity and walkability were independently associated with BMI within twin pairs, controlling for genetic and environmental background shared between them.

METHODS

Data were from 6376 (64% female; 58% identical) same-sex pairs, University of Washington Twin Registry, 2008-2013. Neighborhood walking, moderate-to-vigorous physical activity (MVPA), and BMI were self-reported. Residential address was used to calculate walkability. Phenotypic (non-genetically informed) and biometric (genetically informed) regression was employed, controlling for age, sex, and race.

RESULTS

Walking and MVPA were associated with BMI in phenotypic analyses; associations were attenuated but significant in biometric analyses (Ps<0.05). Walkability was not associated with BMI, however, was associated with walking (but not MVPA) in both phenotypic and biometric analyses (Ps<0.05), with no attenuation accounting for shared genetic and environmental background.

CONCLUSIONS

The association between activity and BMI is largely due to shared genetic and environmental factors, but a significant causal relationship remains accounting for shared background. Although walkability is not associated with BMI, it is associated with neighborhood walking (but not MVPA) accounting for shared background, suggesting a causal relationship between them.

Clinical and evoked pain, personality traits, and emotional states: can familial confounding explain the associations?

OBJECTIVES

Pain is a complex phenomenon influenced by context and person-specific factors. Affective dimensions of pain involve both enduring personality traits and fleeting emotional states. We examined how personality traits and emotional states are linked with clinical and evoked pain in a twin sample.

METHODS

99 female twin pairs were evaluated for clinical and evoked pain using the McGill Pain Questionnaire (MPQ) and dolorimetry, and completed the 120-item International Personality Item Pool (IPIP), the Positive and Negative Affect Scale (PANAS), and ratings of stress and mood. Using a co-twin control design we examined a) the relationship of personality traits and emotional states with clinical and evoked pain and b) whether genetics and common environment (i.e. familial factors) may account for the associations.

RESULTS

Neuroticism was associated with the sensory component of the MPQ; this relationship was not confounded by familial factors. None of the emotional state measures was associated with the MPQ. PANAS negative affect was associated with lower evoked pressure pain threshold and tolerance; these associations were confounded by familial factors. There were no associations between IPIP traits and evoked pain.

CONCLUSIONS

A relationship exists between neuroticism and clinical pain that is not confounded by familial factors. There is no similar relationship between negative emotional states and clinical pain. In contrast, the relationship between negative emotional states and evoked pain is strong while the relationship with enduring personality traits is weak. The relationship between negative emotional states and evoked pain appears to be non-causal and due to familial factors.

Salivary cortisol and cold pain sensitivity in female twins

BACKGROUND

There is a dearth of knowledge about the link between cortisol and pain sensitivity.

PURPOSE

We examined the association of salivary cortisol with indices of cold pain sensitivity in 198 female twins and explored the role of familial confounding.

METHODS

Three-day saliva samples were collected for cortisol levels and a cold pressor test was used to collect pain ratings and time to threshold and tolerance. Linear regression modeling with generalized estimating equations examined the overall and within-pair associations.

RESULTS

Lower diurnal variation of cortisol was associated with higher pain ratings at threshold (p = 0.02) and tolerance (p < 0.01). The relationship of diurnal variation with pain ratings at threshold and tolerance was minimally influenced by familial factors (i.e., genetics and common environment).

CONCLUSIONS

Understanding the genetic and non-genetic mechanisms underlying the link between HPA axis dysregulation and pain sensitivity may help to prevent chronic pain development and maintenance.

Does caregiving cause psychological distress? The case for familial and genetic vulnerabilities in female twins

BACKGROUND

Informal caregiving can be deleterious to mental health, but research results are inconsistent and may reflect an interaction between caregiving and vulnerability to stress.

METHODS

We examined psychological distress among 1,228 female caregiving and non-caregiving twins. By examining monozygotic and dizygotic twin pairs discordant for caregiving, we assessed the extent to which distress is directly related to caregiving or confounded by common genes and environmental exposures.

RESULTS

Caregiving was associated with distress as measured by mental health functioning, anxiety, perceived stress, and depression. The overall association between caregiving and distress was confounded by common genes and environment for mental health functioning, anxiety, and depression. Common environment also confounded the association of caregiving and perceived stress.

CONCLUSIONS

Vulnerability to distress is a factor in predicting caregivers' psychosocial functioning. Additional research is needed to explicate the mechanisms by which common genes and environment increase the risk of distress among informal caregivers.

Use of an electronic medical record to create the marshfield clinic twin/multiple birth cohort

Population-based genetic analyses, such as the Genome-Wide Association Study (GWAS), have proven powerful for describing the genetic complexities of common disease in epidemiologic research. However, the significant challenges faced by population-based study designs have resulted in revitalization of family-based approaches, including twin studies. Twin studies are unique in their ability to ascertain both heritable and environmental contributions to human disease. Several regional and national twin registries have been constructed using a variety of methods to identify potential twins. A significant challenge in constructing these large twin registries includes the substantial resources required to recruit participants, collect phenotypic data, and update the registries as time progresses. Here we describe the use of the Marshfield Clinic electronic medical record (EMR) to identify a cohort of 19,226 patients enriched for twins or multiples. This cohort defines the Marshfield Clinic Twin/Multiple Birth Cohort (MCTC). An EMR system provides both a mechanism to identify potential twins and a source of detailed phenotypic data in near real time without the need for patient contact outside standard medical care. To demonstrate that the MCTC can be used for genetic-based epidemiologic research, concordance rates for muscular dystrophy (MD) and fragile-X syndrome-two highly heritable diseases-were assessed. Observations indicate that both MD and fragile-X syndrome are highly correlated among affected twins in the MCTC (P ≅ 3.7 × 10(-6) and 1.1 × 10(-4) , respectively). These findings suggest that EMR systems may not only be an effective resource for predicting families of twins, but can also be rapidly applied to epidemiologic research.

A twin study of differences in the response of plasma ghrelin to a milkshake preload in restrained eaters

BACKGROUND

Genetic, physiological, and psychological factors can affect food intake, but twin studies can distinguish inherited from environmental contributors. We examined the influence of attempted cognitive control of eating ("restrained eating") on levels of appetite-regulating hormones.

METHODS

Sixteen female, monozygotic twin pairs, discordant for Restraint Scale score (i.e., one twin a restrained eater with score>15 whereas the co-twin was unrestrained), were selected from the University of Washington Twin Registry. Serial plasma ghrelin concentrations were monitored during meals and a preload study paradigm involving intake of a milkshake followed by an ad libitum ice cream "taste test."

RESULTS

Body weight, body mass index, resting energy expenditure, and fasting leptin levels were very similar between restrained and unrestrained twins. In a preload study, twins ate similar amounts of ice cream shortly after drinking identical milkshakes (mean±SD; restrained 239±158 vs. unrestrained 228±132kcal; P=0.83). However, ghrelin concentrations during the preload study were significantly higher (P=0.03) in restrained twins than in their unrestrained co-twins. Regardless of restraint status, ghrelin levels prior to the preload study were prospectively and positively associated with ice cream intake (P=0.001).

CONCLUSIONS

Compared to their unrestrained co-twins, restrained twins had higher endogenous ghrelin levels during a preload study, but ate similar amounts. This finding is consistent with exertion of cognitive control relative to the state of physiologic appetite stimulation. Moreover, these findings in twins suggest that higher ghrelin levels result from restrained eating behavior and not from genetic predisposition.

Age at dieting onset, body mass index, and dieting practices. A twin study

OBJECTIVE

Using a twin study design, we sought to determine whether an early age at dieting onset is a risk factor for higher adult body mass index (BMI) or use of risky dieting practices, independent of genetic and familial factors.

METHOD

Female twins ages 18-60 years (N=950) from the University of Washington Twin Registry completed 2 surveys an average of 3 years apart. Analyses of individual twins and within-twin pairs tested associations of self-reported age at dieting onset with (1) adult BMI at baseline, (2) change in BMI between the two surveys and (3) risky dieting behaviors at baseline.

RESULTS

In analyses mimicking studies of unrelated individuals, an earlier age at dieting onset was associated with greater adult BMI (p=0.003), higher Restraint Scale scores (p<0.001), greater use of risky dieting behaviors (p=0.04) and more weight cycling episodes (p<0.001). In within-pair models that control for genetic and familial factors, the only significant association was between an earlier age at dieting onset and more weight cycling episodes (p=0.006).

DISCUSSION

Underlying genetic and familial factors may influence associations of early dieting with higher adult BMIs and risky dieting practices in women.

Adverse childhood experiences, health perception, and the role of shared familial factors in adult twins

To examine the relationship between adverse childhood experiences (ACE) and health perception in adulthood, and to explore the contribution of shared familial factors to these associations. Data were collected from 180 female twins (90 pairs) from the community-based University of Washington Twin Registry. Participants completed questionnaires including the modified ACE Questionnaire, Traumatic Life Events Questionnaire, McGill Pain Questionnaire-Short Form, and the SF-36. Mixed effects linear regression modeling investigated the effects of ACE on indices of health perception controlling for correlated twin data. Additional models examined the associations while controlling for the experience of physical and/or sexual abuse in childhood; within-twin pair models that inherently adjust for familial factors explored shared familial influences. After controlling for relevant demographic variables, more ACE was associated with worse perceptions of general health (p=.01) and vitality (p=.05) on the SF-36. After controlling for childhood physical and/or sexual abuse, the relationship between ACE and general health remained significant (p=.01) while vitality was no longer significant. None of the associations remained significant after accounting for the influence of familial factors. These results support previous findings on the negative link between ACE and perceived health in adulthood. The detrimental effects of ACE on vitality may be accounted for by the experience of childhood physical and/or sexual abuse. Shared familial factors might play a partial role in the relationship between ACE and health perception. Future research should further investigate the genetic and environmental mechanisms that may explain this relationship.

Current smoking as a predictor of chronic musculoskeletal pain in young adult twins

Chronic pain is common during adolescence and young adulthood and is associated with poor quality of life, depression, and functional disability. Recognizing that chronic pain has significant consequences, it is important to identify modifiable health behaviors that may place young adults at risk for chronic pain. This study examines associations between chronic musculoskeletal pain and smoking in young adult twins (n = 1,588, ages 18-30) participating in a statewide twin registry. Twins completed questionnaires assessing smoking, mood (anxiety, depressive symptoms, and stress), and chronic musculoskeletal pain. Analyses examined associations between chronic pain and smoking, particularly the role of genetics/shared familial factors and psychological symptoms. As predicted, results revealed a near-2-fold increased risk for chronic musculoskeletal pain in twins who currently smoked compared to nonsmokers, even when accounting for psychological factors. Results of within-pair analyses were only minimally attenuated, suggesting that associations between smoking and chronic musculoskeletal pain are better accounted for by nonshared factors than by shared familial factors/genetic effects. Future twin research is needed to identify what nonshared factors (eg, attitudes, direct effects of smoking on pain) contribute to these associations to further understand comorbidity. Longitudinal studies and recruitment of participants prior to smoking initiation and chronic pain onset will better identify causal associations.

PERSPECTIVE

This article describes associations between musculoskeletal pain and smoking in young adult twins, taking into account psychological symptoms. Findings highlight the importance of nonshared factors in associations between pain and smoking and the need to explore the roles of lifestyle, individual attitudes, and direct effects of smoking on pain.

Can familial factors account for the association of body mass index with poor mental health in men or women?

OBJECTIVE

This study examined if associations between body mass index (BMI) and mental and physical health were independent of genetic and familial factors.

METHOD

Data from 2831 twins (66% female) were used in an epidemiological co-twin control design with measures of BMI and mental and physical health outcomes. Generalized estimating equation regressions assessed relationships between BMI and health outcomes controlling for interdependency among twins and demographics. Within-pair regression analyses examined the association of BMI with health outcomes controlling for genetic and familial influences.

RESULTS

Adjusted analyses with individual twins found associations in women between BMI and perceived stress (P=.01) and depression (P=.002), and the link between BMI and depression (P=.03) was significant in men. All physical health outcomes were significantly related to BMI. Once genetic and familial factors were taken into account, mental health outcomes were no longer significantly associated with BMI. BMI in women remained related to ratings of physical health (P=.01) and body pain (P=.004), independent of genetic and familial influences.

CONCLUSION

These findings suggest that genetic and familial factors may account for the relationship between increased weight and poor mental health.

University of Washington Twin Registry: poised for the next generation of twin research

The University of Washington Twin Registry is a unique community-based registry of twin pairs who join specifically to participate in scientific research. It was founded in 2002 to serve as a resource for investigators throughout the scientific community. Current enrollment exceeds 7,200 pairs, and plans are in place to increase enrollment to 10,000 pairs by 2015. In addition to serving as a recruitment base for new research studies, the registry maintains extensive and continually expanding survey data on physical and mental health, as well as a biorepository that includes DNA from more than 8,800 individual twins. The registry is engaged in linking member data to birth records and to diagnostic and procedure variables for hospital-based care provided to members in Washington State. It also incorporates several innovative variables relevant to the built and social environments, which were created by geocoding twin addresses and linking the resulting coordinates to geospatial information systems databases. This combination of existing data and biospecimens, characterizing a group of twins who are willing to participate in research, is a valuable resource for the new wave of twin studies. These include 'omics', epigenetics, gene-by-environment interactions, and other novel methods to understand human health.

The comorbidity of self-reported chronic fatigue syndrome, post-traumatic stress disorder, and traumatic symptoms

BACKGROUND

Data from primary care and community samples suggest higher rates of post-traumatic stress disorder (PTSD) among individuals with chronic fatigue syndrome (CFS).

OBJECTIVE

This study investigated the co-occurrence of CFS, PTSD, and trauma symptoms and assessed the contribution of familial factors to the association of CFS with lifetime PTSD and current traumatic symptoms.

METHOD

Data on lifetime CFS and PTSD, as measured by self-report of a doctor's diagnosis of the disorder, and standardized questionnaire data on traumatic symptoms, using the Impact of Events Scale (IES), were obtained from 8544 female and male twins from the community-based University of Washington Twin Registry.

RESULTS

Lifetime prevalence of CFS was 2% and lifetime prevalence of PTSD was 4%. Participants who reported a history of PTSD were over eight times more likely to report a history of CFS. Participants with scores ≥ 26 on the IES were over four times more likely to report CFS than those who had scores ≤ 25. These associations were attenuated but remained significant after adjusting for familial factors through within-twin pair analyses.

CONCLUSION

These results support similar findings that a lifetime diagnosis of CFS is strongly associated with both lifetime PTSD and current traumatic symptoms, although familial factors, such as shared genetic and environmental contributions, played a limited role in the relationship between CFS, PTSD, and traumatic symptoms. These findings suggest that future research should investigate both the familial and the unique environmental factors that may give rise to both CFS and PTSD.

The Estimated Probability of Dizygotic Twins: A Comparison of Two Methods

This study presents a general model of two binary variables and applies it to twin sex pairing data from 21 twin data sources to estimate the frequency of dizygotic twins. The purpose of this study is to clarify the relationship between maximum likelihood and Weinberg's differential rule zygosity estimation methods. We explore the accuracy of these zygosity estimation measures in relation to twin ascertainment methods and the probability of a male. Twin sex pairing data from 21 twin data sources representing 15 countries was collected for use in this study. Maximum likelihood estimation of the probability of dizygotic twins is applied to describe the variation in the frequency of dizygotic twin births. The differences between maximum likelihood and Weinberg's differential rule zygosity estimation methods are presented as a function of twin data ascertainment method and the probability of a male. Maximum likelihood estimation of the probability of dizygotic twins ranges from 0.083 (95% approximate CI: 0.082, 0.085) to 0.750 (95% approximate CI: 0.749, 0.752) for voluntary ascertainment data sources and from 0.374 (95% approximate CI: 0.373, 0.375) to 0.987 (95% approximate CI: 0.959, 1.016) for active ascertainment data sources. In 17 of the 21 twin data sources differences of 0.01 or less occur between maximum likelihood and Weinberg zygosity estimation methods. The Weinberg and maximum likelihood estimates are negligibly different in most applications. Using the above general maximum likelihood estimate, the probability of a dizygotic twin is subject to substantial variation that is largely a function of twin data ascertainment method.