Tissue-specific nonheritable influences drive endometrial immune system variation

Although human twin studies have revealed the combined contribution of heritable and environmental factors in shaping immune system variability in blood, the contribution of these factors to immune system variability in tissues remains unexplored. The human uterus undergoes constant regeneration and is exposed to distinct environmental factors. To assess uterine immune system variation, we performed a system-level analysis of endometrial and peripheral blood immune cells in monozygotic twins. Although most immune cell phenotypes in peripheral blood showed high genetic heritability, more variation was found in endometrial immune cells, indicating a stronger influence by environmental factors. Cytomegalovirus infection was identified to influence peripheral blood immune cell variability but had limited effect on endometrial immune cells. Instead, hormonal contraception shaped the local endometrial milieu and immune cell composition with minor influence on the systemic immune system. These results highlight that the magnitude of human immune system variation and factors influencing it can be tissue specific.

Surgically treated degenerative disk disease in twins

PURPOSE

Previous studies have suggested that genetic factors are important in the development of degenerative disk disease (DDD). However, the concordance rates for the phenotypes requiring surgery are unknown. The purpose of this study was to determine the concordance rates for DDD requiring surgery by studying monozygotic (MZ) and dizygotic (DZ) twin pairs.

METHODS

Patients, aged between 18 and 85 years, operated for DDD between 1996 and 2022 were identified in the national Swedish spine register (Swespine) and matched with the Swedish twin registry (STR) to identify MZ and DZ twins. Pairwise and probandwise concordance rates were calculated.

RESULTS

We identified 11,207 patients, 53% women, operated for DDD. By matching the Swespine patients with the STR, we identified 121 twin pairs (37 MZ and 84 DZ) where one or both twins were surgically treated for DDD. The total twin incidence for operated DDD was 1.1%. For DDD requiring surgery, we found no concordant MZ pair and no concordant DZ pair where both twins were operated for DDD. When we evaluated pairs where at least one twin was operated for DDD, we found two concordant MZ pairs (the co-twins were operated for spinal stenosis) and two  concordant DZ pairs (one co-twin operated for spinal stenosis and one (co-twin operated for disk herniation).

CONCLUSIONS

Our findings suggest that genetic factors are probably not a major etiologic component in most cases of DDD requiring surgery. The findings of this study can be used for counseling patients about the risk for requiring DDD surgery.

Can flow proneness be protective against mental and cardiovascular health problems? A genetically informed prospective cohort study

Flow is a phenomenon where one experiences optimal challenge, marked by an intense, effortless, and rewarding concentration on a task. Past research shows that flow proneness is associated with good mental and cardiovascular health. However, this research has been primarily cross-sectional, based on self-report data, and has not controlled for potential confounding effects of neuroticism. In a large, longitudinal twin sample (N = 9361), we used nationwide patient registry data to test whether flow proneness predicted registry-based diagnoses of depression, anxiety, schizophrenia, bipolar disorder, stress-related disorders, or cardiovascular diseases. We used survival analyses taking time to diagnosis into account to test if (a) there is a relationship between flow proneness and health diagnoses over time, (b) neuroticism confounds this relationship, and (c) the relationship remains present within discordant monozygotic twin pairs (N = 952), thereby controlling for genetic and shared environmental confounding. Individuals with higher flow proneness had a decreased risk of receiving diagnoses for depression (16%; CI [14%, 18%]), anxiety (16%; CI [13%, 18%]), schizophrenia (15%; CI [4%, 25%]), bipolar (12%; CI [6%, 18%]), stress-related (9%; CI [9%, 12%]), and cardiovascular disorders (4%; CI [1%, 8%]). When controlling for neuroticism, higher flow proneness still decreased the risk of depression (6%; CI [3%, 9%]) and anxiety diagnoses (5%; CI [1%, 8%]). Monozygotic twins who experienced more flow than their co-twin had a lower risk for depression (16%; CI [5%, 26%]) and anxiety (13%; CI [1%, 24%]), though only the association with depression remained significant when also controlling for neuroticism (13%; CI [1%, 24%]). Findings are in line with a causal protective role of flow experiences on depression and potentially anxiety and highlight that neuroticism and familial factors are notable confounding factors in observed associations between flow proneness and health outcomes.

Dementia and mortality in older adults: A twin study

INTRODUCTION

Dementia predicts increased mortality. We used case-control and co-twin control models to investigate genetic and shared environmental influences on this association.

METHODS

Case-control design, including 987 twins with dementia and 2938 age- and sex-matched controls in the Swedish Twin Registry. Co-twin control design, including 90 monozygotic (MZ) and 288 dizygotic (DZ) twin pairs discordant for dementia. To test for genetic and environmental confounding, differences were examined in mortality risk between twins with dementia and their matched or co-twin controls.

RESULTS

Twins with dementia showed greater mortality risk than age- and sex-matched controls (HR = 2.02 [1.86, 2.18]). Mortality risk is significantly elevated but attenuated substantially in discordant twin pairs, for example, comparing MZ twins with dementia to their co-twin controls (HR = 1.48 [1.08, 2.04]).

DISCUSSION

Findings suggest that genetic factors partially confound the association between dementia and mortality and provide an alternative hypothesis to increased mortality due to dementia itself. Highlights We studied dementia and mortality in twin pairs discordant for dementia. People without dementia outlived people with dementia. Identical twins with dementia and their co-twin controls had similar survival time. Findings suggest genotype may explain the link between dementia and mortality.

Genetic and environmental impact on mandibular growth in mono- and dizygotic twins during adolescence: A retrospective cohort study

INTRODUCTION

This study aimed to discover the genetic and environmental factors that contribute to the mandibular development of untreated monozygotic and dizygotic twins.

MATERIAL AND METHODS

The sample, taken from the Forsyth Moorrees Twin Study, included 52 untreated monozygotic twins (36 male, 16 female) and 46 untreated dizygotic twins (23 male, 23 female). At the ages of 12 and 17, lateral cephalograms were collected and traced to assess total mandibular length, mandibular ramus length, mandibular corpus length, gonial angle, SNB, and bony chin prominence. The genetic and environmental components of variation were assessed using multilevel mixed-effects structural equation modelling.

RESULTS

At 12 years of age, high additive genetic influences were observed for total mandibular length (74%), gonial angle (76%), SNB (41%), and bony chin prominence (64%), whereas strong dominant genetic components were observed for corpus length (72%), and mandibular ramus length was under unique environment influence (54%). At 17 years of age, only total mandibular length (45%), ramus length (53%), gonial angle (76%), and bony chin prominence (68%) were under strong additive genetic control, while the remainder were under strong dominant genetic control.

CONCLUSIONS

Although monozygotic and dizygotic twins share at least a portion of their DNA, additive, dominant, or environmental components were discovered during adolescence. Nonetheless, by the age of 17, the majority of the mandibular traits are under either additive or dominant genetic impact.

Twin studies as an innovative approach to address research questions in cancer care within primary care settings

This paper proposes the utilisation of twin studies as a novel and powerful methodological approach to investigate critical research questions pertaining to cancer prevention, screening, diagnosis, treatment and survivorship within primary care contexts. The inherent genetic similarity between monozygotic (MZ) (identical) twins provides a unique opportunity to disentangle genetic and environmental influences on cancer-related outcomes. MZ twins share virtually identical genetic makeup, offering a unique opportunity to discern the relative contributions of genetic and environmental factors to cancer-related outcomes. In contrast, dizygotic (DZ) twins, also known as fraternal twins, develop from two separate eggs fertilised by two different sperm and share on average 50% of their genetic material, the same level of genetic similarity found in non-twin siblings. Comparisons between MZ and DZ twins enable researchers to disentangle hereditary factors from shared environmental influences. This methodology has the potential to advance our understanding of the multifaceted interplay between genetic predisposition, lifestyle factors and healthcare interventions in the context of cancer care. This paper outlines the rationale, design considerations and potential applications of twin studies in primary care-based cancer research.

Genetic and Environmental Causes of Variation in an Automated Breast Cancer Risk Factor Based on Mammographic Textures

BACKGROUND

Cirrus is an automated risk predictor for breast cancer that comprises texture-based mammographic features and is mostly independent of mammographic density. We investigated genetic and environmental variance of variation in Cirrus.

METHODS

We measured Cirrus for 3,195 breast cancer-free participants, including 527 pairs of monozygotic (MZ) twins, 271 pairs of dizygotic (DZ) twins, and 1,599 siblings of twins. Multivariate normal models were used to estimate the variance and familial correlations of age-adjusted Cirrus as a function of age. The classic twin model was expanded to allow the shared environment effects to differ by zygosity. The SNP-based heritability was estimated for a subset of 2,356 participants.

RESULTS

There was no evidence that the variance or familial correlations depended on age. The familial correlations were 0.52 (SE, 0.03) for MZ pairs and 0.16(SE, 0.03) for DZ and non-twin sister pairs combined. Shared environmental factors specific to MZ pairs accounted for 20% of the variance. Additive genetic factors accounted for 32% (SE = 5%) of the variance, consistent with the SNP-based heritability of 36% (SE = 16%).

CONCLUSION

Cirrus is substantially familial due to genetic factors and an influence of shared environmental factors that was evident for MZ twin pairs only. The latter could be due to nongenetic factors operating in utero or in early life that are shared by MZ twins.

IMPACT

Early-life factors, shared more by MZ pairs than DZ/non-twin sister pairs, could play a role in the variation in Cirrus, consistent with early life being recognized as a critical window of vulnerability to breast carcinogens.

Gene-environment interplays between family chaos and emotional problems among Nigerian adolescents: A twin study

Gene-environment correlations and interactions for the relationship between emotional problems (EP) and family environment in adolescents in low- to middle-income countries (LMIC) have been rarely investigated. In total, 3207 adolescent twins aged 12-18 (Mean = 14.6 ± 1.73) years attending public schools in Lagos State in Nigeria completed measures of EP and Family Chaos (FC). Model-fitting analyses suggested that genetic and non-shared environmental influences on EP were 21% and 71%, respectively, and the corresponding estimates were 23% and 71% for FC. Shared environmental influences were not significant (8% and 6% respectively). Phenotypic correlation between EP and FC was .30 (95% CI = .27-.34), which was significantly influenced by genetic (A - 49%, 95% CI: 0.01-0.97) and non-shared environmental factors (E - 32%, 95% CI: 0.10-0.54). Shared environmental influences were not significant (C - 19%, 95% CI: -0.13 to 0.50). Moderation effects were significant whereby as FC increased, A on EP decreased (βA = -0.07, 95% CI: -0.12 to -0.02) while E increased (βE = 0.06, 95% CI: 0.03-0.09). Our findings indicate that genetic and non-shared environmental risk factors may mediate the relationship between EP and FC, and that as FC increases, protective genetic influences on EP may be attenuated, whereas environmental influences may become stronger in adolescents in LMIC.

Broad and narrow environmental and genetic sources of personality differences: An extended twin family study

OBJECTIVE

Several personality theories distinguish between rather genetically rooted, universal dispositional traits (DTs) and rather environmentally shaped, more contextualized characteristic adaptations (CAs). However, no study so far has compared different measures of theoretically postulated DTs and CAs regarding their environmental and genetic components while considering differences in measurement abstraction and reliability. This study aims to bridge this gap by testing the assumed differences in the sensitivity to environmental influences based on representative sets of DTs (Big Five and HEXACO domains and facets) and CAs (goals, interests, value priorities, religiousness, and self-schemas).

METHOD

Using intra-class correlations and running extended twin family and spouses-of-twins model analyses, we analyzed a large data set (N = 1967) encompassing 636 twin pairs, 787 parent-offspring dyads, and 325 spouses/partners.

RESULTS

Findings consistently support lower environmentality of DTs compared to CAs. On average, more than half of reliable variance in DTs was genetic, whereas the reverse was found for CAs. Larger environmental components in CAs were primarily attributable to larger individual-specific effects (beyond error of measurement) and factors shared by spouses.

CONCLUSIONS

Findings are discussed against the background of the definitional distinction between DTs and CAs and the value of extended twin family data.

New insights into the (epi)genetics of twinning

Spontaneous dizygotic (DZ) twins, i.e. twins conceived without the use of ARTs, run in families and their prevalence varies widely around the globe. In contrast, monozygotic (MZ) twins occur at a constant rate across time and geographical regions and, with some rare exceptions, do not cluster in families. The leading hypothesis for MZ twins, which arise when a zygote splits during preimplantation stages of development, is random occurrence. We have found the first series of genes underlying the liability of being the mother of DZ twins and have shown that being an MZ twin is strongly associated with a stable DNA methylation signature in child and adult somatic tissues. Because identical twins keep this molecular signature across the lifespan, this discovery opens up completely new possibilities for the retrospective diagnosis of whether a person is an MZ twin whose co-twin may have vanished in the early stages of pregnancy. Here, we summarize the gene finding results for mothers of DZ twins based on genetic association studies followed by meta-analysis, and further present the striking epigenetic results for MZ twins.

Educational Influences on Late-Life Health: Genetic Propensity and Attained Education

OBJECTIVES

The educational gradient in late-life health is well established. Despite this, there are still ambiguities concerning the role of underlying confounding by genetic influences and gene-environment (GE) interplay. Here, we investigate the role of educational factors (attained and genetic propensities) on health and mortality in late life using genetic propensity for educational attainment (as measured by a genome-wide polygenic score, PGSEdu) and attained education.

METHODS

By utilizing genetically informative twin data from the Swedish Twin Registry (n = 14,570), we investigated influences of the educational measures, familial confounding as well as the possible presence of passive GE correlation on both objective and subjective indicators of late-life health, that is, the Frailty Index, Multimorbidity, Self-rated health, cardiovascular disease, and all-cause mortality.

RESULTS

Using between-within models to adjust for shared familial factors, we found that the relationship between educational level and health and mortality later in life persisted despite controlling for familial confounding. PGSEdu and attained education both uniquely predicted late-life health and mortality, even when mutually adjusted. Between-within models of PGSEdu on the health outcomes in dizygotic twins showed weak evidence for passive GE correlation (prGE) in the education-health relationship.

DISCUSSION

Both genetic propensity to education and attained education are (partly) independently associated with health in late life. These results lend further support for a causal education-health relationship but also raise the importance of genetic contributions and GE interplay.

The Colombo Twin and Singleton Study (COTASS): Piloting the Feasibility of Collecting Nutritional Data and Extension of the Sample to Include Children of Twins

Nutrition and diet are key modifiable risk factors for the rising burden of non-communicable diseases like cardio-vascular diseases and diabetes in low- and middle- income countries (LMICs). The nutritional transition in dietary behaviours in LMICs has most likely contributed to this problem. Although traditionally assumed to be environmental, dietary choices are also genetically influenced. Twin study designs can be used to investigate the relative influence of genes and environment on nutrition intake, eating behaviours and associated psychological health. The overall aim of this project is to: provide proof-of-concept for the feasibility of using dietary (biomarker) data within the Children-of-Twin design in nutrition studies, develop laboratory skills and statistical genetic skills and establish a Sri Lankan-specific food composition database. Currently, a pilot study is being conducted with 304 individuals (38 Monozygotic twin pairs, 38 Dizygotic twin pairs and their male or female adult offspring). Questionnaire data on nutritional intake, eating behaviours, psychological well-being, physical health, and bio-specimens are being collected. A Sri Lankan-specific food composition database was developed, training sessions on macro and micro element analysis in biological samples and statistical genetics skills development were conducted and Community Engagement and Involvement programs were carried out in two districts of Sri Lanka.

Self-reported sleep bruxism in 1990 and 2011 in a nationwide twin cohort: Evidence of trait persistence and genetic liability

BACKGROUND

Due to different assessment modes employed, a clear picture of the prevalence of sleep bruxism across time cannot be formed. Moreover, studies on the persistent or fluctuating nature of sleep bruxism have yielded divergent and even contradictory results. The aim of the present study was to evaluate in a nationwide twin cohort whether self-reported sleep bruxism was correlated longitudinally, pairwise and cross-twin over a 20-year period.

OBJECTIVES

Self-reported bruxism was assessed in 1990 and 2011 by mailed questionnaires in the Finnish Twin Cohort study of same-sex twins born 1945-1957.

METHODS

We assessed the phenotypic stability over time for all participating individuals (n = 4992). Among zygosity verified pairs (n = 516 MZ and n = 837 DZ), we estimated the cross-sectional zygosity correlations and the zygosity-specific cross-twin cross-time correlations.

RESULTS

Reported bruxism appeared rather persistent over time without significant difference regarding zygosity. The overall phenotypic longitudinal correlation was 0.540 and somewhat higher in men (0.596) than in women (0.507). Pairwise trait correlations in 1990 and 2011 were higher in MZ than in DZ pairs. The cross-twin cross-time correlations were higher in MZ twins than in DZ twins, but less than the cross-sectional MZ and DZ pairwise correlations.

CONCLUSIONS

The higher correlation of reported sleep bruxism in the cross-twin cross-time analyses in MZ than in DZ pairs implies a genetic background for bruxism persistence. Also, bruxism over time in individual twins appears to be fairly persistent and somewhat higher in men than women.

Mental, physical, and social well-being and quality of life in healthy young adult twin pairs discordant and concordant for body mass index

OBJECTIVE

The relationship between obesity and mental health is complex and is moderated by the level of obesity, age, sex, and social and genetic factors. In the current study, we used a unique co-twin control design, with twin pairs discordant for body mass index (BMI), to control for shared genetic and environmental effects between obesity and several dimensions of mental health.

METHODS

We studied 74 monozygotic (MZ) twin pairs, of whom 36 were BMI-discordant (intra-pair difference in BMI ≥ 3 kg/m2), and 77 dizygotic (DZ) twin pairs (46 BMI-discordant). We assessed subjective health, especially mental health and mental well-being (depression, anxiety, self-esteem, health-related quality of life, life satisfaction, and social well-being) through questionnaires.

RESULTS

Heavier MZ co-twins from BMI-discordant pairs had poorer general health (58.8±3.0 vs. 72.4±3.8, P = 0.001, FDR = 0.017 on a scale from 0 to 100 where higher scores indicate more positive results), physical functioning (90.3±1.1 vs. 95.5±2.2, P = 0.024, FDR = 0.122), energy levels (55.6±3.4 vs. 66.6±3.3, P = 0.013, FDR = 0.109), and emotional well-being (65.9±3.2 vs. 75.4±2.9, P = 0.031, FDR = 0.122), as well as a tendency for depressive symptoms (8.4±1.3 vs. 5.6±0.9, P = 0.071, FDR = 0.166) compared to their leaner co-twins. Heavier DZ co-twins had poorer total physical well-being (91.6±1.9 vs. 95.6±1.0, P = 0.035, FDR = 0.356) and more depressive symptoms (4.3±0.9 vs. 2.4±0.5, P = 0.016, FDR = 0.345 on a scale from 0 to 63 where lower scores indicate fewer depressive symptoms) than their leaner co-twins. Association analyses, using all twin pairs, confirmed that higher BMI within pairs linked to general health, physical functioning and depressive symptoms. No association was found between BMI and anxiety, self-esteem, life satisfaction, or social well-being.

CONCLUSIONS

In conclusion, this study underscores the notable association between elevated BMI and physical well-being and to a lesser extent between elevated BMI and depressive symptoms, while revealing no discernible connections with anxiety, self-esteem, life satisfaction, or social well-being.

Polygenic risk of obesity and BMI trajectories over 36 years: A longitudinal study of adult Finnish twins

OBJECTIVE

This study investigated 36-year BMI trajectories in twins whose BMI in young adulthood was below, within, or above their genetically predicted BMI, with a focus on twin pairs with large intrapair BMI differences (within-pair ΔBMI ≥ 3 kg/m2 ).

METHODS

Together, 3227 like-sexed twin pairs (34% monozygotic) were examined at age ~30 years in 1975 and followed up in 1981, 1990, and 2011. An individual's observed BMI in 1975 was considered within (±2.0), below (<-2.0), or above (>+2.0) genetically predicted BMI, measured by a polygenic risk score of 996,919 single nucleotide polymorphisms.

RESULTS

In monozygotic and dizygotic twin pairs with large intrapair BMI differences, the co-twin with a higher observed BMI in 1975 deviated above predicted BMI more frequently (~2/3) than the co-twin with a lower BMI deviated below prediction (~1/3). Individuals below, within, and above prediction in 1975 reached, respectively, normal weight, overweight, and obesity by 2011, with a mean BMI increase of 4.5 (95% CI: 4.3-4.8).

CONCLUSIONS

Categorizing BMI as below, within, or above polygenic risk score-predicted BMI helps identifying individuals who have been resistant or susceptible to weight gain. This may provide new insights into determinants and consequences of obesity.

Inference on the Genetic Architecture of Breast Cancer Risk

BACKGROUND

What are the major determinants of women's breast cancer risk? Rare mutations such as those in the BRCA1/2 genes, polygenic scores of common alleles identified by genome-wide association studies, or nongenetic factors?

METHODS

The population-based Nordic Twin Study of Cancer, with 3,933 breast cancer cases among 21,054 monozygotic (MZ) and 30,939 dizygotic (DZ) female twin pairs, provides three key clues to this question: (i) the average lifetime risk, approximately 8%, does not differ by twin zygosity; (ii) the mean time interval between diagnoses when both twins develop disease (i.e., disease concordance) also does not differ by zygosity; but, (iii) conditioning on one twin having developed disease, the incidence rate in the co-twin is approximately 1% per year if the pair is MZ and 0.5% per year if DZ.

RESULTS

Assuming that nongenetic risk factors are shared similarly between twins regardless of zygosity, we can draw two conclusions from (i) to (iii).

CONCLUSIONS

First, (i) and (iii) imply that the chief determinant of risk is in the germline DNA, because the conditional incidence rate is several-fold higher than the average risk (8% lifetime) in MZ twins but only half as much in DZ twins. Second, the seeming inconsistency between the two-fold conditional incidence rate (iii) and the equality of the mean inter-twin disease intervals in disease concordance (ii) can be resolved if the risk factors in the germline DNA are rare variants, not common variants.

IMPACT

This paper details simple deductive reasoning for these conclusions and draws a critical inference regarding breast cancer etiology. See related In the Spotlight, p. 1477.

A population-based twin study of the symptomatic diagnostic criteria for major depression that occur within versus outside of major depressive episodes

BACKGROUND

Are genetic risk factors for current depressive symptoms good proxies for genetic risk factors for syndromal major depression (MD)?

METHODS

In over 9000 twins from the population-based Virginia Adult Twin Study of Psychiatric and Substance Use Disorders, the occurrence of all nine DSM symptomatic criteria for MD in the last year was assessed at personal interview and then grouped by their temporal co-occurrence. The DSM criteria which occurred outside (OUT) v. inside of (IN) MD episodes were then separated. We calculated tetrachoric correlations for OUT and IN depressive criteria in monozygotic (MZ) and dizygotic (DZ) pairs and fitted univariate and bivariate ACE twin models using OpenMx.

RESULTS

The mean twin correlations (±95% CIs) for IN depressive criteria were substantially higher than for OUT depressive criteria in both MZ [+0.35 (0.32-0.38) v. 0.20 (0.17-0.24)] and DZ pairs [0.20 (0.17-0.24) v. 0.10 (0.04-0.16]. The mean IN-OUT cross-correlation in MZ and DZ pairs was modest [+0.15 (0.07-0.24) and +0.07 (0.03-0.12)]. The mean heritability estimates for the nine In v. Out depressive criteria was 0.31 (0.22-0.41) and 0.15 (0.08-0.21), in MZ and DZ pairs, respectively. The mean genetic correlation between the nine IN and OUT depressive criteria was +0.07 (-0.07 to 0.21).

CONCLUSIONS

Depressive criteria occurring outside depressive episodes are less heritable than those occurring within. These two ways criteria can manifest are not closely genetically related. Current depressive symptoms - most of which are occurring outside of depressive episodes - are not, for genetic studies, good proxies for MD.

Worldwide Well-Being: Simulated Twins Reveal Genetic and (Hidden) Environmental Influences

What are the major sources of worldwide variability in subjective well-being (SWB)? Twin and family studies of SWB have found substantial heritability and strong effects from unique environments but virtually no effects from shared environments. However, extant findings are not necessarily valid at the global level. Prior studies have examined within-countries variability but did not take into account mean differences across nations. In this article, we aim to estimate the effects of genetic factors, individual environmental exposures, and shared environments for the global population. We combine a set of knowns from national well-being studies (means and standard deviations) and behavioral-genetic studies (heritability) to model a scenario of twin studies across 157 countries. For each country, we simulate data for a set of twin pairs and pool the data into a global sample. We find a worldwide heritability of 31% to 32% for SWB. Individual environmental factors explain 46% to 52% of the variance (including measurement error), and shared environments account for 16% to 23% of the global variance in SWB. Worldwide, well-being is somewhat less heritable than within nations. In contrast to previous within-countries studies, we find a notable effect of shared environments. This effect is not limited to within families but operates at a national level.

Comorbidity and Characteristics of Autism Spectrum Disorders and ADHD in Children: A Clinically-Based Study

OBJECTIVE

We examined the characteristics and heritability of Autism Spectrum Disorder (ASD) and ADHD through a twin study.

METHOD

Our sample included 44 twins, with at least one twin diagnosed with ASD. Among the participants, 30 had ASD, and 18 of them also had coexisting ADHD.

RESULTS

We observed higher concordance rates for ASD in monozygotic twins compared to dizygotic twins (67% vs. 25%), indicating a genetic influence on ASD. Inattentive symptoms of ADHD were more prevalent in monozygotic twins. The ASD + ADHD group exhibited significantly higher Social Responsiveness Scale scores, indicating greater social difficulties compared to the ASD and typical development groups. Twin analyses revealed that shared genetic factors accounted for 72.25% of the variance in both ASD and ADHD symptoms.

CONCLUSIONS

Our findings suggest that the comorbidity of ASD and ADHD may indicate increased severity and can be explained by shared genetic factors underlying both conditions.

A multivariate twin study of the genetic association between present moment attention and subjective wellbeing

Considerable evidence supports the role of present-moment attention, a central feature of mindfulness, in subjective wellbeing maintenance and enhancement. Yet it is not clear why such a relation exists. This study examined the genetic and environmental contributions of present-moment attention to subjective wellbeing. Consistent with the "generalist genes hypothesis" and prior evidence, we hypothesized that presence and subjective wellbeing would show a substantial genetic correlation and smaller environmental correlation. Using a large epidemiological sample of healthy 16-year-old twins in the United Kingdom (N = 1136 monozygotic (MZ) and dizygotic (DZ) twin pairs), genetic overlap was found between presence and the cognitive component of subjective wellbeing (life satisfaction), and to a lesser extent, the affective component of subjective wellbeing (operationalized as happiness). The non-shared environmental overlap between these constructs was substantial. This study provides the first evidence known to us showing that present-centered attention, a primary component of mindfulness, has both genetic and environmental overlap with subjective wellbeing. The findings have implications for understanding mechanisms by which presence is associated with positive emotions and life satisfaction, and suggest, pending additional research, that mindfulness-based interventions to enhance wellbeing may be best suited to those with a genetic propensity toward mindful presence.

The contribution of genetics to dental caries, oral habits and occlusal traits in Turkish twins: A comparative study

OBJECTIVE

The aims of this study were to investigate and compare the prevalence of dental caries and the frequency of oral habits, molar relationships and occlusal traits between children of multiple births, and singletons, and to determine the relative contributions of genetics and environmental factors to these parameters by using twin study design.

METHODS

The study group consisted of 345 multiple births (34 monozygotic and 122 dizygotic twin pairs, 11 sets of triplets) and 345 singletons between the ages of 2 and 17. The prevalence of dental caries, and the frequency of tooth brushing, the children's oral habits, molar relationships, and occlusal traits were recorded.

RESULTS

The percentage of children who brushed their teeth more than twice daily was statistically significantly higher in multiple births than in singletons. Higher correlation coefficients were found in dental caries index, except for decayed, filled (df) (2-5 age group) and filled (f) (6-11 age group), in the monozygotic twin pairs compared to those in the dizygotic twin pairs. In children between the ages of 6 and 11 years, mouth breathing, bruxism, lip biting, and pencil biting were higher in singletons than in children of multiple births. There were statistically significant differences between children of multiple births and singletons, with increased overjet in the 2-5 year age group being observed.

CONCLUSION

When analyzing these parameters, environmental factors must also be investigated. Due to the low incidence of twin births, longitudinal follow-up studies with more twin pairs are necessary to determine whether these results are generalizable.

Reduced brain connectivity along the autism spectrum controlled for familial confounding by co-twin design

Previous studies on brain connectivity correlates of autism have often focused on selective connections and yielded inconsistent results. By applying global fiber tracking and utilizing a within-twin pair design, we aimed to contribute to a more unbiased picture of white matter connectivity in association with clinical autism and autistic traits. Eighty-seven twin pairs (n = 174; 55% monozygotic; 24 with clinical autism) underwent diffusion tensor imaging. Linear regressions assessed within-twin pair associations between structural brain connectivity of anatomically defined brain regions and both clinical autism and autistic traits. These were explicitly adjusted for IQ, other neurodevelopmental/psychiatric conditions and multiple testing, and implicitly for biological sex, age, and all genetic and environmental factors shared by twins. Both clinical autism and autistic traits were associated with reductions in structural connectivity. Twins fulfilling diagnostic criteria for clinical autism had decreased brainstem-cuneus connectivity compared to their co-twins without clinical autism. Further, twins with higher autistic traits had decreased connectivity of the left hippocampus with the left fusiform and parahippocampal areas. These associations were also significant in dizygotic twins alone. Reduced brainstem-cuneus connectivity might point towards alterations in low-level visual processing in clinical autism while higher autistic traits seemed to be more associated with reduced connectivity in networks involving the hippocampus and the fusiform gyrus, crucial especially for processing of faces and other (higher order) visual processing. The observed associations were likely influenced by both genes and environment.

Stressful life events increase the risk of major depressive episodes: A population-based twin study

BACKGROUND

Previous studies have found that stressful life events (SLEs) are associated with an increased risk of adult depression. However, many studies are observational in nature and limited by methodological issues, such as potential confounding by genetic factors. Genetically informative research, such as the co-twin control design, can strengthen causal inference in observational studies. Discrete-time survival analysis has several benefits and multilevel survival analysis can incorporate frailty terms (random effects) to estimate the components of the biometric model. In the current study, we investigated associations between SLEs and depression risk in a population-based twin sample (N = 2299).

METHODS

A co-twin control design was used to investigate the influence of the occurrence of SLEs on depression risk. The co-twin control design involves comparing patterns of associations in the full sample and within dizygotic (DZ) and monozygotic twins (MZ). Associations were modelled using discrete-time survival analysis with biometric frailty terms. Data from two time points were used in the analyses. Mean age at Wave 1 was 28 years and mean age at Wave 2 was 38 years.

RESULTS

SLE occurrence was associated with increased depression risk. Co-twin control analyses indicated that this association was at least in part due to the causal influence of SLE exposure on depression risk for event occurrence across all SLEs and for violent SLEs. A minor proportion of the total genetic risk of depression reflected genetic effects related to SLEs.

CONCLUSIONS

The results support previous research in implicating SLEs as important risk factors with probable causal influence on depression risk.

Genetic and Environmental Correlation Analysis of Serum Creatinine Levels in Chinese Twins

Almost all creatinine is excreted by the kidney in individuals. Serum creatinine concentration, a widely used renal function index in clinical practice, can be affected by both genetic and environmental factors, as evidenced by current research exploring the relationship between these factors and kidney function. However, few studies have explored the heritability of serum creatinine in Asian populations. Therefore, we explored the genetic and environmental factors that affect the serum creatinine level in Asian populations. Participants in this study came from the Qingdao Twin Registry in China, and 374 pairs of twins were included, of which 139 pairs were dizygotic twins, whose ages ranged from 40 to 80 years old, and the serum creatinine level ranged from 10 to 126 μmol/L. Structural equation models were constructed using Mx software to calculate heritability, with adjusted covariates being age, sex, and body mass index. The results of heritability analysis showed that ACE was the best fit model. Serum creatinine level is influenced by genetic and environmental factors. The result of heritability was 35.44%, and the influence of shared environmental factors accounted for 52.13%. This study provided the relevant basis for future research on genetic and environmental factors affecting serum creatinine levels in Asian populations.

Twin Research and Human Genetics - News, Views & Comments: Memoirs and Autobiographies by Twins: Research Angles and Human Interest/Twin Research Reviews: Fetal Reduction in Twin Pregnancy; Twins' Personality and Military Service; Growth Restriction in Twins; Advances in Conjoined Twin Separation/Media Reports: Update on Scientist Who Performed Gene Editing on Twins; Twins From 33-Year-Old Embryos; Twins' Outcomes From Dietary Differences; Fraternal Twins With the World's Largest Height Difference; Twin Home Experts Conquer Rat Infestation

Twins' memoirs and autobiographies both enlighten and entertain. These works, often overlooked by researchers, may suggest new avenues for investigation, such as nonshared environmental events that propel twins in different directions. Of course, MZ twins' generally parallel experiences and DZ twins generally criss-crossing paths are the bases of fascinating life stories. The following sections examined recent research on fetal reduction in twin pregnancy, twins' personality and military service, growth restriction in twins, and advances in conjoined twin separation. This article closes with reports of a scientist who performed gene editing on twins, a twin conception from 33-year-old embryos, twins' physical outcomes from dietary differences, fraternal twins with the world's largest height difference and the Twin Home Experts who conquer rat infestation in New York.

Pseudohypoaldosteronism type 1b in fraternal twins of a Chinese family: report of two cases and literature review

Here, we report the clinical observations of two Chinese fraternal twins who presented with severe dehydration, poor feeding, and absence of stimuli responses within a few days of birth. Trio clinical exome sequencing of the family identified compound heterozygous intronic variants (c.1439+1G>C and c.875+1G>A ) in SCNN1A gene in these two patients. Sanger sequencing results showed that the c.1439+1G>C variant was inherited from the mother, and c.875+1G>A from the father, rarely reported in pseudohypoaldosteronism type 1 with sodium epithelial channel destruction (PHA1b) patients. Case 2 received timely symptomatic treatment and management after obtaining these results, which improved the clinical crisis. Our results suggest that the compound heterozygous splicing variants in SCNN1A were responsible for PHA1b in these Chinese fraternal twins. This finding extends the knowledge of the variant spectrum in PHA1b patients and highlights the application of exome sequencing in critically ill newborns. Finally, we discuss supportive case management, particularly in maintaining blood potassium concentration.

Gene-by-Environment Interaction Effects of Social Adversity on Externalizing Behavior in ABCD Youth

This study tested whether multiple domains of social adversity, including neighborhood opportunity/deprivation and life stress, moderate genetic (A), common environmental (C), and unique environmental (E) influences on externalizing behaviors in 760 same-sex twin pairs (332 monozygotic; 428 dizygotic) ages 10-11 from the ABCD Study. Proportion of C influences on externalizing behavior increased at higher neighborhood adversity (lower overall opportunity). A decreased and C and E increased at lower levels of educational opportunity. A increased at lower health-environment and social-economic opportunity levels. For life stress, A decreased and E increased with number of experienced events. Results for educational opportunity and stressful life experiences suggest a bioecological gene-environment interaction pattern such that environmental influences predominate at higher levels of adversity, whereas limited access to healthcare, housing, and employment stability may potentiate genetic liability for externalizing behavior via a diathesis-stress mechanism. More detailed operationalization of social adversity in gene-environment interaction studies is needed.

The sleep homeostatic response to sleep deprivation in humans is heritable

STUDY OBJECTIVES

Following sleep deprivation, increases in delta power have historically been used to index increases in sleep pressure. Research in mice has demonstrated that the homeostatic delta power response to sleep deprivation is heritable. Whether this is true in humans is unknown. In the present study, we used delta power and ORP, a novel measure of sleep depth, to investigate the effects of acute sleep deprivation on sleep depth and to assess the heritability of sleep homeostasis in humans.

METHODS

ORP and delta power were examined during baseline and recovery sleep following 38 h of sleep deprivation in 57 monozygotic and 38 dizygotic same-sex twin pairs. Two complementary methods were used to estimate the trait heritability of sleep homeostasis.

RESULTS

During recovery sleep, ORP was lower and delta power was higher than at baseline, indicating deeper sleep. However, at the end of the recovery night, delta power reached baseline levels but ORP demonstrated incomplete recovery. Both ORP and delta power showed a broad sense heritability of sleep homeostasis following sleep deprivation. The classical approach demonstrated an h2 estimate of 0.43 for ORP and 0.73 for delta power. Mixed-effect multilevel models showed that the proportion of variance attributable to additive genetic transmission was 0.499 (95% CI = 0.316-0.682; p < .0001) for ORP and 0.565 (95% CI = 0.403-0.726; p < .0001 for delta power.

CONCLUSIONS

These results demonstrate that the homeostatic response to sleep deprivation is a heritable trait in humans and confirm ORP as a robust measure of sleep depth.

Genetic and Environmental Factors Underlying Parallel Changes in Body Mass Index and Alcohol Consumption: A 36-Year Longitudinal Study of Adult Twins

INTRODUCTION

While the genetic and environmental underpinnings of body weight and alcohol use are fairly well-known, determinants of simultaneous changes in these traits are still poorly known. We sought to quantify the environmental and genetic components underlying parallel changes in weight and alcohol consumption and to investigate potential covariation between them.

METHODS

The analysis comprised 4,461 adult participants (58% women) from the Finnish Twin Cohort with four measures of alcohol consumption and body mass index (BMI) over a 36-year follow-up. Trajectories of each trait were described by growth factors, defined as intercepts (i.e., baseline) and slopes (i.e., change over follow-up), using latent growth curve modeling. Growth values were used for male (190 monozygotic pairs, 293 dizygotic pairs) and female (316 monozygotic pairs, 487 dizygotic pairs) same-sex complete twin pairs in multivariate twin modeling. The variances and covariances of growth factors were then decomposed into genetic and environmental components.

RESULTS

The baseline heritabilities were similar in men (BMI: h2 = 79% [95% confidence interval: 74, 83]; alcohol consumption: h2 = 49% [32, 67]) and women (h2 = 77% [73, 81]; h2 = 45% [29, 61]). Heritabilities of BMI change were similar in men (h2 = 52% [42, 61]) and women (h2 = 57% [50, 63]), but the heritability of change in alcohol consumption was significantly higher (p = 0.03) in men (h2 = 45% [34, 54]) than in women (h2 = 31% [22, 38]). Significant additive genetic correlations between BMI at baseline and change in alcohol consumption were observed in both men (rA = -0.17 [-0.29, -0.04]) and women (rA = -0.18 [-0.31, -0.06]). Non-shared environmental factors affecting changes in alcohol consumption and BMI were correlated in men (rE = 0.18 [0.06, 0.30]). Among women, non-shared environmental factors affecting baseline alcohol consumption and the change in BMI were inversely correlated (rE = -0.11 [-0.20, -0.01]).

CONCLUSIONS

Based on genetic correlations, genetic variation underlying BMI may affect changes in alcohol consumption. Independent of genetic effects, change in BMI correlates with change in alcohol consumption in men, suggesting direct effects between them.

The relative and interactive impact of multiple risk factors in schizophrenia spectrum disorders: a combined register-based and clinical twin study

BACKGROUND

Research has yielded evidence for genetic and environmental factors influencing the risk of schizophrenia. Numerous environmental factors have been identified; however, the individual effects are small. The additive and interactive effects of multiple risk factors are not well elucidated. Twin pairs discordant for schizophrenia offer a unique opportunity to identify factors that differ between patients and unaffected co-twins, who are perfectly matched for age, sex and genetic background.

METHODS

Register data were combined with clinical data for 216 twins including monozygotic (MZ) and dizygotic (DZ) proband pairs (one or both twins having a schizophrenia spectrum diagnosis) and MZ/DZ healthy control (HC) pairs. Logistic regression models were applied to predict (1) illness vulnerability (being a proband v. HC pair) and (2) illness status (being the patient v. unaffected co-twin). Risk factors included: A polygenic risk score (PRS) for schizophrenia, birth complications, birth weight, Apgar scores, paternal age, maternal smoking, season of birth, parental socioeconomic status, urbanicity, childhood trauma, estimated premorbid intelligence and cannabis.

RESULTS

The PRS [odds ratio (OR) 1.6 (1.1-2.3)], childhood trauma [OR 4.5 (2.3-8.8)], and regular cannabis use [OR 8.3 (2.1-32.7)] independently predicted illness vulnerability as did an interaction between childhood trauma and cannabis use [OR 0.17 (0.03-0.9)]. Only regular cannabis use predicted having a schizophrenia spectrum diagnosis between patients and unaffected co-twins [OR 3.3 (1.1-10.4)].

CONCLUSION

The findings suggest that several risk factors contribute to increasing schizophrenia spectrum vulnerability. Moreover, cannabis, a potentially completely avoidable environmental risk factor, seems to play a substantial role in schizophrenia pathology.

Evidence for distinct genetic and environmental influences on fear acquisition and extinction

BACKGROUND

Anxiety disorders are highly prevalent with an early age of onset. Understanding the aetiology of disorder emergence and recovery is important for establishing preventative measures and optimising treatment. Experimental approaches can serve as a useful model for disorder and recovery relevant processes. One such model is fear conditioning. We conducted a remote fear conditioning paradigm in monozygotic and dizygotic twins to determine the degree and extent of overlap between genetic and environmental influences on fear acquisition and extinction.

METHODS

In total, 1937 twins aged 22-25 years, including 538 complete pairs from the Twins Early Development Study took part in a fear conditioning experiment delivered remotely via the Fear Learning and Anxiety Response (FLARe) smartphone app. In the fear acquisition phase, participants were exposed to two neutral shape stimuli, one of which was repeatedly paired with a loud aversive noise, while the other was never paired with anything aversive. In the extinction phase, the shapes were repeatedly presented again, this time without the aversive noise. Outcomes were participant ratings of how much they expected the aversive noise to occur when they saw either shape, throughout each phase.

RESULTS

Twin analyses indicated a significant contribution of genetic effects to the initial acquisition and consolidation of fear, and the extinction of fear (15, 30 and 15%, respectively) with the remainder of variance due to the non-shared environment. Multivariate analyses revealed that the development of fear and fear extinction show moderate genetic overlap (genetic correlations 0.4-0.5).

CONCLUSIONS

Fear acquisition and extinction are heritable, and share some, but not all of the same genetic influences.

What is driving the relationship between height and cognition? Evidence from the Twins Early Development Study

Taller children tend to have better cognitive ability, and the relationship between height and cognition has been proposed as an explanation for the height-wage labor market premium. Height-cognition associations may arise due to social factors that favor taller individuals or be driven by "common factors" that are correlated with height and cognition. Indeed, there is now evidence of a genetic correlation between height and cognition that provides specific evidence for this concern. We examine whether genetic factors explain the relationship by estimating associations between childhood height and cognition in the Twins Early Development Study. We find that height is associated with better cognition even after controlling for genetic and environmental factors shared by twins. The association between height and cognition within fraternal twin pairs is also robust to controlling for individual genetic predictors of height and cognition. These results suggest that genetic factors are not solely responsible for driving the relationship between height and cognition.

Comparison of the Concordance of Cardiometabolic Diseases and Physical and Laboratory Examination Findings between Monozygotic and Dizygotic Korean Adult Twins: A Cross-Sectional Study Using KoGES HTS Data

This study investigated the contribution of genetic and environmental factors to cardiometabolic diseases (CMDs) by comparing disease concordance in monozygotic and dizygotic twins. This cross-sectional study analyzed 1294 (1040 monozygotic and 254 dizygotic) twin pairs (>20 years) based on the Korean Genome and Epidemiology Study data (2005−2014). The odds ratios of disease concordance were calculated using binomial and multinomial logistic regression models. The occurrence of CMDs (hypertension, hyperlipidemia, type 2 diabetes, cerebral stroke, transient ischemic attack, and ischemic heart disease) and related physical and laboratory levels did not differ between the monozygotic and dizygotic twin groups. The odds for concordance of the presence/absence of CMDs and the likelihood of incident CMD within monozygotic twins were comparable to that of dizygotic twins. The absolute differences in hemoglobin A1c, insulin, low- and high-density lipoprotein cholesterol, total cholesterol, triglycerides, and systolic blood pressure were lower in monozygotic twins than in dizygotic twins. Absolute differences in fasting glucose and diastolic blood pressure did not differ between groups. Although baseline levels of several laboratory parameters related to CMD showed a strong likelihood of heritability in monozygotic twins, CMD phenotype appears to be largely affected by environmental factors.

Comparison of Concordance of Peptic Ulcer Disease, Non-Adenomatous Intestinal Polyp, and Gallstone Disease in Korean Monozygotic and Dizygotic Twins: A Cross-Sectional Study

Epidemiological studies have suggested the role of multiple genetic and environmental factors in the development of non-neoplastic gastrointestinal (GI) diseases; however, little information is available on these factors in the Korean population. Therefore, this cross-sectional study explored the effect of these factors by analyzing the concordance of several benign GI disorders in 525 monozygotic twins compared to that in 122 dizygotic twins aged >20 years from the Healthy Twin Study data of the Korean Genome and Epidemiology Study (2005-2014). Chi-square test, Wilcoxon rank-sum, and binomial and multinomial logistic regression models were used for statistical analysis. There was lack of concordance of gastric/duodenal ulcers and cholelithiasis/cholangitis between monozygotic twins compared to that in dizygotic twins, suggesting that environmental factors may mediate those concordant disease expressions in monozygotic twins. The concordance of intestinal polyps in monozygotic twins was 32% lower than that in dizygotic twins (p = 0.028), indicating that the effect of genetic factors on the risk for intestinal polyp development may be low. In conclusion, the lack or low concordance of several benign GI diseases between monozygotic and dizygotic twin groups suggests the relative importance of environmental factors, indicating that these are preventable diseases.

Concordance for Gender Dysphoria in Genetic Female Monozygotic (Identical) Triplets

The biopsychosocial etiology of gender dysphoria is poorly understood, but current thought suggests a complex interaction of genetic, hormonal, environmental, and differences in brain development and physiology. Twin studies have implicated a genetic role in the formation of gender identity. Congruence for gender dysphoria is more common among monozygotic twins compared to dizygotic twins. We present a case of monozygotic (identical) triplets who have each transitioned from female to male under the care of a university transgender health service. Each triplet experienced gender dysphoria from childhood and has undergone transitional endocrine care and various aspects of gender-affirming surgery. Although a pure genetic or biological component cannot be attributed as a cause of their gender dysphoria with absolute certainty since the triplets were raised together, this unusual case of gender dysphoria among a set of monozygotic triplets adds support for a heritable role in gender identity formation.

Heritability of Psychological Traits and Developmental Milestones in Infancy: A Systematic Review and Meta-analysis

IMPORTANCE

Although infancy is the most rapid period of postnatal growth and development, factors associated with variation in infant traits are not well understood.

OBJECTIVE

To synthesize the large twin study literature partitioning phenotypic variance in psychological traits and developmental milestones in infancy into estimates of heritability and shared and nonshared environment.

DATA SOURCES

PubMed, PsycINFO, and references of included publications were searched up to February 11, 2021.

STUDY SELECTION

Peer-reviewed publications using the classical twin design to study psychological traits and developmental milestones from birth to 2 years old were included.

DATA EXTRACTION AND SYNTHESIS

Data were extracted in line with Preferred Reporting Items for Systematic Reviews and Meta-Analyses and categorized using the International Classification of Functioning, Disability and Health: Children and Youth Version. Data were pooled in 3-level random effects models, incorporating within-cohort variance in outcome measurement and between-cohort variance. Data were analyzed from March 2021 through September 2021.

MAIN OUTCOMES AND MEASURES

The primary outcomes were monozygotic and dizygotic twin correlations. These were used to calculate genetic and shared and nonshared environment estimates.

RESULTS

Among 139 publications that were systematically retrieved, data were available on 79 044 twin pairs (31 053 monozygotic and 47 991 dizygotic pairs), 52 independent samples, and 21 countries. Meta-analyses were conducted on psychological traits and developmental milestones from 106 publications organized into 10 categories of functioning, disability, and health. Moderate to high genetic estimates for 8 categories were found, the highest of which was psychomotor functions (pooled h2, 0.59; 95% CI, 0.25-0.79; P < .001). Several categories of traits had substantial shared environment estimates, the highest being mental functions of language (pooled c2, 0.59; 95% CI, 0.24-0.86; P = .001). All examined categories of traits had moderate or high nonshared environment estimates, the highest of which were emotional functions (pooled e2, 0.42; 95% CI, 0.33-0.50; P < .001) and family relationships (pooled e2, 0.42; 95% CI, 0.30-0.55; P < .001).

CONCLUSIONS AND RELEVANCE

These findings may be an important source of information to guide future gene discovery research, public perspectives on nature and nurture, and clinical insights into the degree to which family history and environments may estimate major domains of infant functioning, disability, and health in psychological traits and developmental milestones.

Cancer-related reductions in survival: extent and duration evaluated using a large cohort study of twins, 1943-2011

BACKGROUND

The time during which there is an increased risk of death for cancer survivors was evaluated in a large twin study, which allows for matching on shared components such as age, genes, and socioeconomic factors in childhood.

METHODS

By use of data from Danish registers, time to death from initial cancer was studied prospectively in twins in two different settings. The twins were diagnosed with at least one cancer in the period 1943-2011. Setting I included 5,680 same-sex twin pairs aged six and over, while Setting II included 3,218 twin individuals from age 70 and over. The study provides comparisons within twin pairs and across birth cohorts, age at diagnoses, and time at diagnosis.

RESULTS

In 2001-2011, the five-year mortality risk for a twin surviving cancer after the age of 70 was twofold that of the co-twin, regardless of sex and zygosity, and it was 1.5-fold if the twin survived the initial nine months. After five to six years, the mortality risk corresponded to that of the co-twin. In previous decades, the excess hazard risk was considerably higher for both older and younger cohorts. There were no indications of change in relative survival across old birth cohorts.

CONCLUSIONS

This large twin study suggested that for a cancer-treatment survivor diagnosed at age 70 or later, the additional mortality risk was largely absent five years later, by which time the survival relative to the co-twin was 60%.

IMPACT

Elevated mortality risk after cancer is offset after five-six years.

Left Ventricular Adaptation to Exercise Training via Magnetic Resonance Imaging: Studies of Twin Responses to Understand Exercise THerapy

PURPOSE

Changes in left ventricular mass (LVM) and end-diastolic volume (EDV) in response to exercise training are important determinants of functional capacity in health and disease, but the impact of different exercise modalities remains unclear.

METHODS

Using a randomized crossover design we studied the impact of resistance (RES) and endurance (END) training using cardiac magnetic resonance imaging in previously untrained monozygotic (MZ) and dizygotic (DZ) twin pairs (n = 72; 22 MZ pairs, 14 DZ same-sex pairs; 26.1 ± 5.4 yr). Twins, as pairs, undertook 3 months of RES and 3 months of END training (order randomized), separated by a 3-month washout.

RESULTS

Group results revealed that END increased LVM (P < 0.001) and EDV (P = 0.007), whereas RES did not (P > 0.05). A higher proportion of individuals responded to END than RES for LVM (72% vs 38%, P < 0.001) and EDV (67% vs 40%, P = 0.003). Baseline cross-sectional intraclass correlations were higher for MZ than DZ twin pairs for all variables (e.g., LVM heritability = 0.42), but no significant correlations were apparent between pairs for change in any variable in response to either RES or END (P > 0.05).

CONCLUSIONS

Our findings indicate that cardiac adaptation in response to exercise is modality-specific and that low responders to one mode of exercise can be high responders to an alternative. Heritability estimates based on cross-sectional data, which suggested a genetic contribution to LVM, do not accord with estimates based on training effects, which indicated limited genetic impact on adaptation in this 3-month study of exercise training. This study has implications for understanding the physiological and health impacts of typically used exercise modalities on cardiac adaptation in previously untrained individuals.

Shared etiology of type 1 diabetes and Hashimoto's thyroiditis: a population-based twin study

OBJECTIVE

Type 1 diabetes and Hashimoto's thyroiditis frequently cluster in individuals and in families, indicating shared origins. The objective of this study was to investigate familial co-aggregation of these diseases and to quantify shared genetic and environmental factors.

DESIGN

This study is a twin cohort study.

METHODS

National health registers were used to identify cases among 110 814 Swedish twins. Co-aggregation was calculated as risk ratios for type 1 diabetes among co-twins of individuals with Hashimoto's thyroiditis, and vice-versa. Variance explained by genetics (i.e. heritability), and the proportions thereof shared between the diseases, was estimated by contrasting associations in monozygotic and dizygotic twins using structural equation models.

RESULTS

Individuals with one disease were at a high risk for the other disease (adjusted risk ratio: 11.4 (95% CI: 8.5-15.3)). Co-aggregation was more common in monozygotic than in dizygotic pairs, with adjusted risk ratios of 7.0 (95% CI: 3.2-15.1) and 1.7 (95% CI: 0.7-4.1), respectively. Genetic effects shared across diseases accounted for 11% of the variance for type 1 diabetes and 9% of the variance for Hashimoto's thyroiditis, while environmental factors unique to individual twins, but shared across diseases, accounted for 10% of the variance for type 1 diabetes and 18% of the variance for Hashimoto's thyroiditis.

CONCLUSIONS

Both genes and environment unique to individual twins contribute to considerable etiologic overlap between type 1 diabetes and Hashimoto's thyroiditis. These findings add to the current knowledge on the mechanisms behind autoimmune disease clustering and could guide future research aimed at identifying pathophysiological mechanisms and intervention targets.

Heritability of specific cognitive functions and associations with schizophrenia spectrum disorders using CANTAB: a nation-wide twin study

BACKGROUND

Many cognitive functions are under strong genetic control and twin studies have demonstrated genetic overlap between some aspects of cognition and schizophrenia. How the genetic relationship between specific cognitive functions and schizophrenia is influenced by IQ is currently unknown.

METHODS

We applied selected tests from the Cambridge Neuropsychological Test Automated Battery (CANTAB) to examine the heritability of specific cognitive functions and associations with schizophrenia liability. Verbal and performance IQ were estimated using The Wechsler Adult Intelligence Scale-III and the Danish Adult Reading Test. In total, 214 twins including monozygotic (MZ = 32) and dizygotic (DZ = 22) pairs concordant or discordant for a schizophrenia spectrum disorder, and healthy control pairs (MZ = 29, DZ = 20) were recruited through the Danish national registers. Additionally, eight twins from affected pairs participated without their sibling.

RESULTS

Significant heritability was observed for planning/spatial span (h² = 25%), self-ordered spatial working memory (h² = 64%), sustained attention (h² = 56%), and movement time (h² = 47%), whereas only unique environmental factors contributed to set-shifting, reflection impulsivity, and thinking time. Schizophrenia liability was associated with planning/spatial span (r_ph = -0.34), self-ordered spatial working memory (r_ph = -0.24), sustained attention (r_ph = -0.23), and set-shifting (r_ph = -0.21). The association with planning/spatial span was not driven by either performance or verbal IQ. The remaining associations were shared with performance, but not verbal IQ.

CONCLUSIONS

This study provides further evidence that some cognitive functions are heritable and associated with schizophrenia, suggesting a partially shared genetic etiology. These functions may constitute endophenotypes for the disorder and provide a basis to explore genes common to cognition and schizophrenia.

Twin study reveals non-heritable immune perturbations in multiple sclerosis

Multiple sclerosis (MS) is a chronic inflammatory disorder of the central nervous system underpinned by partially understood genetic risk factors and environmental triggers and their undefined interactions1,2. Here we investigated the peripheral immune signatures of 61 monozygotic twin pairs discordant for MS to dissect the influence of genetic predisposition and environmental factors. Using complementary multimodal high-throughput and high-dimensional single-cell technologies in conjunction with data-driven computational tools, we identified an inflammatory shift in a monocyte cluster of twins with MS, coupled with the emergence of a population of IL-2 hyper-responsive transitional naive helper T cells as MS-related immune alterations. By integrating data on the immune profiles of healthy monozygotic and dizygotic twin pairs, we estimated the variance in CD25 expression by helper T cells displaying a naive phenotype to be largely driven by genetic and shared early environmental influences. Nonetheless, the expanding helper T cells of twins with MS, which were also elevated in non-twin patients with MS, emerged independent of the individual genetic makeup. These cells expressed central nervous system-homing receptors, exhibited a dysregulated CD25-IL-2 axis, and their proliferative capacity positively correlated with MS severity. Together, our matched-pair analysis of the extended twin approach allowed us to discern genetically and environmentally determined features of an MS-associated immune signature.

Genetic and environmental correlates of the nonlinear recovery of cognitive ability in Twins

Twins regularly score nearly a standard deviation below the population mean on standardized measures of cognitive development in infancy but recover to the population mean by early childhood, making rapid gains through the toddler years. To date, only polynomial growth models have been fit to model cognitive recovery across childhood, limiting the applicability of the growth parameters to later developmental periods. We fit a nonlinear asymptotic Gompertz growth model to prospective cognitive scores from 1,153 individual twins from 578 families (47.9% male, 91.5% White, 61.6% monozygotic) measured at 16 time points between 3 months and 15 years. Twins displayed a lower asymptote of 86.47 (.90 SD below the population mean) and gained on average 17.01 points, achieving an upper asymptote of 103.48. Growth was observed to be most rapid at 3.26 years, highlighting the importance of the toddler years in cognitive development. Biometric analyses revealed that shared environmental factors accounted for the majority of the variance in initial cognitive ability as well as asymptotic growth in cognitive ability. Gestational age and family socioeconomic status (SES) were robust predictors of cognitive growth. Results from the present study provide insight into the growth processes underlying the recovery of cognitive ability to the population mean for children evincing slight delays in their initial cognitive ability. In particular, findings highlight prenatal factors and family economic resources as important aspects of the environment in the recovery of cognitive ability. (PsycInfo Database Record (c) 2022 APA, all rights reserved).

Heritability of Dental Caries: Twin Studies

The study of twins is a powerful tool to infer the presence and amount of contribution of genetic variation to a particular trait or disease. The ability to compare identical or monozygotic twins with dizygotic twins permits the direct comparison of pairs of individuals that share 100% of their genomic DNA with pairs that share only 50%, with the assumption that these pairs are under the same environment. In the case of dental caries, the environment is same parents, under the same roof, with the same diet, oral hygiene habits, culture, and lifestyle. These data have consistently suggested that a relevant proportion of the variation of dental caries in populations is due to genetics.

Handedness in twins: meta-analyses

BACKGROUND

In the general population, 10.6% of people favor their left hand over the right for motor tasks. Previous research suggests higher prevalence of atypical (left-, mixed-, or non-right-) handedness in (i) twins compared to singletons, and in (ii) monozygotic compared to dizygotic twins. Moreover, (iii) studies have shown a higher rate of handedness concordance in monozygotic compared to dizygotic twins, in line with genetic factors playing a role for handedness.

METHODS

By means of a systematic review, we identified 59 studies from previous literature and performed three sets of random effects meta-analyses on (i) twin-to-singleton Odds Ratios (21 studies, n = 189,422 individuals) and (ii) monozygotic-to-dizygotic twin Odds Ratios (48 studies, n = 63,295 individuals), both times for prevalence of left-, mixed-, and non-right-handedness. For monozygotic and dizygotic twin pairs we compared (iii) handedness concordance Odds Ratios (44 studies, n = 36,217 twin pairs). We also tested for potential effects of moderating variables, such as sex, age, the method used to assess handedness, and the twins' zygosity.

RESULTS

We found (i) evidence for higher prevalence of left- (Odds Ratio = 1.40, 95% Confidence Interval = [1.26, 1.57]) and non-right- (Odds Ratio = 1.36, 95% Confidence Interval = [1.22, 1.52]), but not mixed-handedness (Odds Ratio = 1.08, 95% Confidence Interval = [0.52, 2.27]) among twins compared to singletons. We further showed a decrease in Odds Ratios in more recent studies (post-1975: Odds Ratio = 1.30, 95% Confidence Interval = [1.17, 1.45]) compared to earlier studies (pre-1975: Odds Ratio = 1.90, 95% Confidence Interval = [1.59-2.27]). While there was (ii) no difference between monozygotic and dizygotic twins regarding prevalence of left- (Odds Ratio = 0.98, 95% Confidence Interval = [0.89, 1.07]), mixed- (Odds Ratio = 0.96, 95% Confidence Interval = [0.46, 1.99]), or non-right-handedness (Odds Ratio = 1.01, 95% Confidence Interval = [0.91, 1.12]), we found that (iii) handedness concordance was elevated among monozygotic compared to dizygotic twin pairs (Odds Ratio = 1.11, 95% Confidence Interval = [1.06, 1.18]). By means of moderator analyses, we did not find evidence for effects of potentially confounding variables.

CONCLUSION

We provide the largest and most comprehensive meta-analysis on handedness in twins. Although a raw, unadjusted analysis found a higher prevalence of left- and non-right-, but not mixed-handedness among twins compared to singletons, left-handedness was substantially more prevalent in earlier than in more recent studies. The single large, recent study which included birth weight, Apgar score and gestational age as covariates found no twin-singleton difference in handedness rate, but these covariates could not be included in the present meta-analysis. Together, the secular shift and the influence of covariates probably make it unsafe to conclude that twinning has a genuine relationship to handedness.

Financial strain moderates genetic influences on self-rated health: support for diathesis-stress model of gene-environment interplay

Data from the Interplay of Genes and Environment across Multiple Studies (IGEMS) consortium were used to examine predictions of different models of gene-by-environment interaction to understand how genetic variance in self-rated health (SRH) varies at different levels of financial strain. A total of 11,359 individuals from 10 twin studies in Australia, Sweden, and the United States contributed relevant data, including 2,074 monozygotic and 2,623 dizygotic twin pairs. Age ranged from 22 to 98 years, with a mean age of 61.05 (SD = 13.24). A factor model was used to create a harmonized measure of financial strain across studies and items. Twin analyses of genetic and environmental variance for SRH incorporating age, age², sex, and financial strain moderators indicated significant financial strain moderation of genetic influences on self-rated health. Moderation results did not differ across sex or country. Genetic variance for SRH increased as financial strain increased, matching the predictions of the diathesis-stress and social comparison models for components of variance. Under these models, environmental improvements would be expected to reduce genetically based health disparities.

Microbial Species that Initially Colonize the Human Gut at Birth or in Early Childhood Can Stay in Human Body for Lifetime

In recent years, many studies have described the composition and function of the human microbiome at different body sites and suggested a role for the microbiome in various diseases and health conditions. Some studies, using longitudinal samples, have also suggested how the microbiome changes over time due to disease, diet, development, travel, and other environmental factors. However, to date, no study has demonstrated whether the microorganisms established at birth or in early childhood, either transmitted from parents or obtained from the environment, can stay in the human body until adult or senior age. To directly answer this question is difficult, because microbiome samples at childhood and at later adulthood for the same individual will need to be compared and the field is not old enough to have allowed for that type of sample collection. Here, using a metagenomic approach, we analyzed 1004 gut microbiome samples from senior adults (65 ± 7.8 years) from the TwinsUK cohort. Our data indicate that many species in the human gut acquired in early childhood can stay for a lifetime until senior ages. We identified the rare genomic variants (single nucleotide variation and indels) for 27 prevalent species with enough sequencing coverage for confident genomic variant identification. We found that for some species, twin pairs, including both monozygotic (MZ) and dizygotic (DZ) twins, share significantly more rare variants than unrelated subject pairs. But no significant difference is found between MZ and DZ twin pairs. These observations strongly suggest that these species acquired in early childhood remained in these persons until senior adulthood.

Genetic and environmental influences on serum oxylipins, endocannabinoids, bile acids and steroids

Lipid bioactivity is a result of direct action and the action of lipid mediators including oxylipins, endocannabinoids, bile acids and steroids. Understanding the factors contributing to biological variation in lipid mediators may inform future approaches to understand and treat complex metabolic diseases. This research aims to determine the contribution of genetic and environmental influences on lipid mediators involved in the regulation of inflammation and energy metabolism. This study recruited 138 monozygotic (MZ) and dizygotic (DZ) twins aged 18-65 years and measured serum oxylipins, endocannabinoids, bile acids and steroids using liquid chromatography mass-spectrometry (LC-MS). In this classic twin design, the similarities and differences between MZ and DZ twins are modelled to estimate the contribution of genetic and environmental influences to variation in lipid mediators. Heritable lipid mediators included the 12-lipoxygenase products 12-hydroxyeicosatetraenoic acid [0.70 (95% CI: 0.12,0.82)], 12-hydroxyeicosatetraenoic acid [0.73 (95% CI: 0.30,0.83)] and 14‑hydroxy-docosahexaenoic acid [0.51 (95% CI: 0.07,0.71)], along with the endocannabinoid docosahexaenoy-lethanolamide [0.52 (95% CI: 0.15,0.72)]. For others such as 13-hydroxyoctadecatrienoic acid and lithocholic acid the contribution of environment to variation was stronger. With increased understanding of lipid mediator functions in health, it is important to understand the factors contributing to their variance. This study provides a comprehensive analysis of lipid mediators and extends pre-existing knowledge of the genetic and environmental influences on the human lipidome.

Disparity between central and peripheral refraction inheritance in twins

The last decades have witnessed a sudden increase in myopia incidence among youngsters that have been related to modern lifestyle along with the use of emerging technologies affecting visual exposure. Increasing exposures to known risk factors for myopia, such as time spent indoors, close-distance work, or low-light conditions are thought to be responsible for this public health issue. In most cases, development of myopia is secondary to a vitreous chamber enlargement, although the related mechanisms and the potential interaction between central and peripheral retinal area remain unclear. For a better understanding, we performed a classical twin study where objective refractive error along 70° of horizontal retinal arc was measured in 100 twin pairs of university students, 78% of which showed manifest myopia. We found the variance of shared environmental origin (range 0.34 to 0.67) explained most of the objective refractive error variance within central 42° of the retina (22° temporal to 19° nasal), whereas additive genetic variance (range 0.34 to 0.76) was predominant in the peripheral retinal areas measured. In this sample of millennial university students, with a large prevalence of myopia, environmental exposures were mostly responsible for inter-individual variation in the retinal horizontal area surrounding the macula, while their relative weight on phenotypic variance was gradually descending, and replaced by the variance of genetic origin, towards the retinal periphery.

Genetic and environmental components of vertical growth in mono- and dizygotic twins up to 15-18 years of age

OBJECTIVES

To determine the additive genetic and environmental contributions to the vertical growth of craniofacial structures.

MATERIALS AND METHODS

The sample consisted of 64 untreated monozygotic (44 male, 20 female) and 61 untreated dizygotic twins (32 male, 29 female). Lateral cephalograms taken at 15 and 18 years of age were traced to analyze the sella-nasion-nasal line angle (SN-NL), nasal line-mandibular line angle (ML-NL), sella-nasion-mandibular line angle (SN-ML), sella-nasion-sella-gnathion angle (Y-axis), posterior face height/anterior face height (PFH/AFH), and lower anterior face height/anterior face height (LAFH/AFH). The genetic and environmental components of variance were analyzed with structural equation modeling for multilevel mixed effects.

RESULTS

At 15 years of age, strong dominant genetic control was seen for NL-ML (81%), LAFH/AFH (73%), and Y-axis (57%), whereas strong additive genetic components were found for PFH/AFH (78%), SN-NL (58%), and SN-ML (57%). Unique environmental factors accounted for 18-42% of observed variance, with SN-NL being affected the most (42%). At 18 years of age, only LAFH/AFH (86%) was under strong dominant genetic control, whereas the remainder were under additive genetic influence. The sole exception was SN-NL, which changed from additive to unique environmental influence.

CONCLUSIONS

Either additive or dominant genetic components were found at 15 or 18 years of age for most vertical variables. Environmental factors accounted for about 10-40%, with SN-NL being mostly affected.

Genetic Influences on the Covariance and Genetic Correlations in a Bivariate Twin Model: An Application to Well-Being

The distinction between genetic influences on the covariance (or bivariate heritability) and genetic correlations in bivariate twin models is often not well-understood or only one is reported while the results show distinctive information about the relation between traits. We applied bivariate twin models in a large sample of adolescent twins, to disentangle the association between well-being (WB) and four complex traits (optimism, anxious-depressed symptoms (AD), aggressive behaviour (AGG), and educational achievement (EA)). Optimism and AD showed respectively a strong positive and negative phenotypic correlation with WB, the negative correlation of WB and AGG is lower and the correlation with EA is nearly zero. All four traits showed a large genetic contribution to the covariance with well-being. The genetic correlations of well-being with optimism and AD are strong and smaller for AGG and EA. We used the results of the models to explain what information is retrieved based on the bivariate heritability versus the genetic correlations and the (clinical) implications.