Challenges and novel approaches in the epidemiological study of early life influences on later disease

Methodological approaches, challenges, and opportunities in the application of Mendelian randomisation to lifecourse epidemiology: A systematic literature review

Diseases diagnosed in adulthood may have antecedents throughout (including prenatal) life. Gaining a better understanding of how exposures at different stages in the lifecourse influence health outcomes is key to elucidating the potential benefits of disease prevention strategies. Mendelian randomisation (MR) is increasingly used to estimate causal effects of exposures across the lifecourse on later life outcomes. This systematic literature review explores MR methods used to perform lifecourse investigations and reviews previous work that has utilised MR to elucidate the effects of factors acting at different stages of the lifecourse. We conducted searches in PubMed, Embase, Medline and MedRXiv databases. Thirteen methodological studies were identified. Four studies focused on the impact of time-varying exposures in the interpretation of "standard" MR techniques, five presented methods for repeat measures of the same exposure, and four described methodological approaches to handling multigenerational exposures. A further 127 studies presented the results of an applied research question. Over half of these estimated effects in a single generation and were largely confined to the exploration of questions regarding body composition. The remaining mostly estimated maternal effects. There is a growing body of research focused on the development and application of MR methods to address lifecourse research questions. The underlying assumptions require careful consideration and the interpretation of results rely on select conditions. Whilst we do not advocate for a particular strategy, we encourage practitioners to make informed decisions on how to approach a research question in this field with a solid understanding of the limitations present and how these may be affected by the research question, modelling approach, instrument selection, and data availability.

Selection bias as an explanation for the observed protective association of childhood adiposity with breast cancer

OBJECTIVES

Recalled childhood adiposity is inversely associated with breast cancer observationally, including in Mendelian randomization (MR) studies. Breast cancer studies recruited in adulthood only include survivors of childhood adiposity and breast cancer or a competing risk. We assessed recalled childhood adiposity on participant reported sibling and maternal breast cancer to ensure ascertainment of nonsurvivors.

STUDY DESIGN AND SETTING

We obtained independent strong genetic predictors of recalled childhood adiposity for women and their associations with participant reported own, sibling and maternal breast cancer from UK Biobank genome wide association studies.

RESULTS

Recalled childhood adiposity in women was inversely associated with own breast cancer using Mendelian randomization inverse variance weighting (odds ratio (OR) 0.66, 95% confidence interval (CI) 0.52-0.84) but less clearly related to participant reported sibling (OR 0.89, 95% CI 0.69-1.14) or maternal breast cancer (OR 0.84, 95% CI 0.67-1.05).

CONCLUSION

Weaker inverse associations of recalled childhood adiposity with breast cancer with more comprehensive ascertainment of cases before recruitment suggests the inverse association of recalled childhood adiposity with breast cancer could be partly selection bias from preferential selection of survivors. Greater consideration of survival bias in public health relevant causal inferences would be helpful.

Parental cannabis and tobacco use during pregnancy and childhood hair cortisol concentrations

BACKGROUND

Fetal exposure to cannabis and tobacco during pregnancy leads to adverse fetal and childhood outcomes. We hypothesized that fetal exposure to cannabis and tobacco have persistent programming effects on hypothalamic pituitary adrenal (HPA) axis functioning in childhood. Therefore, we examined the associations of parental cannabis and tobacco use during pregnancy with childhood hair cortisol and cortisone concentrations at 6 years, as biomarkers of long-term HPA-axis functioning.

METHOD

In a population-based prospective birth cohort among 2577 mothers and their children, information of parental cannabis and tobacco use was collected by questionnaires, and maternal urine samples were additionally analyzed to detect cannabis metabolite concentrations. Cortisol and cortisone were measured in hair samples at 6 years. Linear regression analysis with adjustment for several confounders was used to test our hypothesis.

RESULTS

As compared to non-exposed children, offspring exposed to cannabis during pregnancy (in combination with tobacco) had higher childhood cortisol concentrations (log-10 transformed difference 0.16, 95 % Confidence Interval 0.04 to 0.28). This association was not mediated by birth weight. No differences in cortisone concentrations among cannabis-exposed children were observed. Maternal tobacco use during pregnancy was not associated with childhood cortisol or cortisone concentrations. Further, paternal cannabis or tobacco use was not associated with childhood cortisol or cortisone concentrations.

CONCLUSIONS

Our findings suggest that maternal cannabis use, combined with tobacco, during pregnancy is associated with alterations in offspring HPA-axis functioning. Further studies need to replicate these findings, and assess the causality and long-term consequences of these associations.

Preconception and prenatal cannabis use and the risk of behavioural and emotional problems in the offspring; a multi-informant prospective longitudinal study

BACKGROUND

Studies of the long-term consequences of maternal cannabis use on child development beyond the neonatal period are sparse. In the current study, we use a multi-information approach to assess the association of prenatal cannabis exposure and child behavioural and emotional functioning. To explore the possible causal nature of the association, we investigated whether maternal tobacco and paternal cannabis use during pregnancy were also associated with child problems.

METHODS

The study population included children of a population-based birth cohort in The Netherlands (n = 5903). Information on parental cannabis use was collected using questionnaires; urine of mothers was analysed for the presence of cannabis metabolites. Child behavioural and emotional problems at approximately 7-10 years were measured using validated teacher-, child- and mother-reports.

RESULTS

Our findings show associations of maternal cannabis use during pregnancy with offspring externalising problems (B = 0.53; 95% CI: 0.29-0.77), but not with internalising problems (B = -0.10; 95% CI: -0.31-0.11). However, maternal cannabis use before pregnancy was also associated with offspring externalising problems (B = 0.27; 95% CI: 0.02-0.52). Further, cannabis use by the father was associated with child externalising problems (B = 0.36; 95% CI: 0.22-0.49) but not internalising problems.

CONCLUSIONS

Prenatal exposure to maternal cannabis use is specifically associated with offspring behavioural problems, but not emotional problems. This association is probably not due to an effect of intrauterine cannabis exposure on fetal development, because both maternal and paternal cannabis exposure during pregnancy were related to offspring externalising problems. Our findings suggest that the association can be explained through residual confounding, most likely through shared genetic vulnerabilities for parental cannabis use and offspring behavioural problems.

Genetic studies of gestational duration and preterm birth

The fine control of birth timing is important to human survival and evolution. A key challenge in studying the mechanisms underlying the regulation of human birth timing is that human parturition is a unique to human event — animal models provide only limited information. The duration of gestation or the risk of preterm birth is a complex human trait under genetic control from both maternal and fetal genomes. Genomic discoveries through genome-wide association (GWA) studies would implicate relevant genes and pathways. Similar to other complex human traits, gestational duration is likely to be influenced by numerous genetic variants of small effect size. The detection of these small-effect genetic variants requires very large sample sizes. In addition, several practical and analytical challenges, in particular the involvement of both maternal and fetal genomes, further complicate the genetic studies of gestational duration and other pregnancy phenotypes. Despite these challenges, large-scale GWA studies have already identified several genomic loci associated with gestational duration or the risk of preterm birth. These genomic discoveries have revealed novel insights about the biology of human birth timing. Expanding genomic discoveries in larger datasets by more refined analytical approaches, together with the functional analysis of the identified genomic loci, will collectively elucidate the biological processes underlying the control of human birth timing.

Early Life Exposures and Adult Cancer Risk

Very little is known about the influence of early life exposures on adult cancer risk. The purpose of this narrative review was to summarize the epidemiologic evidence relating early life tobacco use, obesity, diet, and physical activity to adult cancer risk; describe relevant theoretical frameworks and methodological strategies for studying early life exposures; and discuss policies and research initiatives focused on early life. Our findings suggest that in utero exposures may indirectly influence cancer risk by modifying biological pathways associated with carcinogenesis; however, more research is needed to firmly establish these associations. Initiation of exposures during childhood and adolescence may impact cancer risk by increasing duration and lifetime exposure to carcinogens and/or by acting during critical developmental periods. To expand the evidence base, we encourage the use of life course frameworks, causal inference methods such as Mendelian randomization, and statistical approaches such as group-based trajectory modeling in future studies. Further, we emphasize the need for objective exposure biomarkers and valid surrogate endpoints to reduce misclassification. With the exception of tobacco use, there is insufficient evidence to support the development of new cancer prevention policies; however, we highlight existing policies that may reduce the burden of these modifiable risk factors in early life.

Prenatal Exposure to Fever and Infections and Academic Performance: A Multilevel Analysis

Prenatal exposure to fever and infections has been linked to various neurodevelopmental disorders, but it is not yet known whether more subtle effects on neurodevelopment may exist as well. Therefore, we aimed to investigate whether these early-life exposures were associated with academic performance in childhood and early adolescence. Children and mothers who were enrolled in the Danish National Birth Cohort during 1996-2002 were included in this study. Information on fever and infections common in pregnancy was prospectively collected in 2 pregnancy interviews and linked with assessments of academic performance from the 2010-2013 Danish National Tests. Hierarchical multilevel linear regression of 216,350 assessments made in 71,850 children born to 67,528 mothers revealed no differences in academic performance among the children according to prenatal exposure to fever (odds ratio (OR) = 1.01, 95% confidence interval (CI): 1.00, 1.03), any infection (OR = 1.00, 95% CI: 0.99, 1.01), genitourinary infection (OR = 1.01, 95% CI: 0.99, 1.02), prolonged cough (OR = 1.00, 95% CI: 0.99, 1.02), or diarrhea (OR = 0.99, 95% CI: 0.97, 1.00). The findings were supported in different types of academic assessments, with different timings of exposure, and in sibling comparisons. This large population-based study suggested that prenatal exposure to fever and common infections does not affect the child's basic school performance.

Drosophila melanogaster: An emerging model of transgenerational effects of maternal obesity

The prevalence of obesity in the world is endemic with one rapidly growing health concern being maternal obesity. Obesity during pregnancy increases the risk of gestational diabetes, miscarriage, and preeclampsia, while rendering offspring susceptible to developmental anomalies and long-term metabolic complications including type 2 diabetes and cardiovascular disease. Several studies in humans and rodents demonstrate a correlation between the risks of maternal overnutrition and factors such as epigenetics, mitochondrial dysfunction, insulin resistance, ER stress, and immune system disruption. At present, the molecular mechanisms connecting these factors to maternal obesity are unknown. This review focuses on the use of Drosophila melanogaster to study human metabolic diseases, including obesity, and its emerging use to elucidate the mechanisms of maternal overnutrition and the impact on offspring.

The early origins of obesity and insulin resistance: timing, programming and mechanisms

Maternal obesity is associated with an increased risk of developing gestational diabetes mellitus and it also results in an increased risk of giving birth to a large baby with increased fat mass. Furthermore, it is also contributes to an increased risk of obesity and insulin resistance in the offspring in childhood, adolescence and adult life. It has been proposed that exposure to maternal obesity may therefore result in an 'intergenerational cycle' of obesity and insulin resistance. There is significant interest in whether exposure to maternal obesity around the time of conception alone contributes directly to poor metabolic outcomes in the offspring and whether dieting in the obese mother before pregnancy or around the time of conception has metabolic benefits for the offspring. This review focusses on experimental and clinical studies that have investigated the specific impact of exposure to maternal obesity during the periconceptional period alone or extending beyond conception on adipogenesis, lipogenesis and on insulin signalling pathways in the fat, liver and muscle of the offspring. Findings from these studies highlight the need for a better evidence base for the development of dietary interventions in obese women before pregnancy and around the time of conception to maximize the metabolic benefits and minimize the metabolic costs for the next generation.

Perspectives on the design and methodology of periconceptional nutrient supplementation trials

Periconceptional supplementation could extend the period over which maternal and fetal nutrition is improved, but there are many challenges facing early-life intervention studies. Periconceptional trials differ from pregnancy supplementation trials, not only because of the very early or pre-gestational timing of nutrient exposure but also because they generate subsidiary information on participants who remain non-pregnant. The methodological challenges are more complex although, if well designed, they provide opportunities to evaluate concurrent hypotheses related to the health of non-pregnant women, especially nulliparous adolescents. This review examines the framework of published and ongoing randomised trial designs. Four cohorts typically arise from the periconceptional trial design--two of which are non-pregnant and two are pregnant--and this structure provides assessment options related to pre-pregnant, maternal, pregnancy and fetal outcomes. Conceptually the initial decision for single or micronutrient intervention is central--as is the choice of dosage and content--in order to establish a comparative framework across trials, improve standardisation, and facilitate interpretation of mechanistic hypotheses. Other trial features considered in the review include: measurement options for baseline and outcome assessments; adherence to long-term supplementation; sample size considerations in relation to duration of nutrient supplementation; cohort size for non-pregnant and pregnant cohorts as the latter is influenced by parity selection; integrating qualitative studies and data management issues. Emphasis is given to low resource settings where high infection rates and the possibility of nutrient-infection interactions may require appropriate safety monitoring. The focus is on pragmatic issues that may help investigators planning a periconceptional trial.

Assessing the Causal Relationship of Maternal Height on Birth Size and Gestational Age at Birth: A Mendelian Randomization Analysis

BACKGROUND

Observational epidemiological studies indicate that maternal height is associated with gestational age at birth and fetal growth measures (i.e., shorter mothers deliver infants at earlier gestational ages with lower birth weight and birth length). Different mechanisms have been postulated to explain these associations. This study aimed to investigate the casual relationships behind the strong association of maternal height with fetal growth measures (i.e., birth length and birth weight) and gestational age by a Mendelian randomization approach.

METHODS AND FINDINGS

We conducted a Mendelian randomization analysis using phenotype and genome-wide single nucleotide polymorphism (SNP) data of 3,485 mother/infant pairs from birth cohorts collected from three Nordic countries (Finland, Denmark, and Norway). We constructed a genetic score based on 697 SNPs known to be associated with adult height to index maternal height. To avoid confounding due to genetic sharing between mother and infant, we inferred parental transmission of the height-associated SNPs and utilized the haplotype genetic score derived from nontransmitted alleles as a valid genetic instrument for maternal height. In observational analysis, maternal height was significantly associated with birth length (p = 6.31 × 10-9), birth weight (p = 2.19 × 10-15), and gestational age (p = 1.51 × 10-7). Our parental-specific haplotype score association analysis revealed that birth length and birth weight were significantly associated with the maternal transmitted haplotype score as well as the paternal transmitted haplotype score. Their association with the maternal nontransmitted haplotype score was far less significant, indicating a major fetal genetic influence on these fetal growth measures. In contrast, gestational age was significantly associated with the nontransmitted haplotype score (p = 0.0424) and demonstrated a significant (p = 0.0234) causal effect of every 1 cm increase in maternal height resulting in ~0.4 more gestational d. Limitations of this study include potential influences in causal inference by biological pleiotropy, assortative mating, and the nonrandom sampling of study subjects.

CONCLUSIONS

Our results demonstrate that the observed association between maternal height and fetal growth measures (i.e., birth length and birth weight) is mainly defined by fetal genetics. In contrast, the association between maternal height and gestational age is more likely to be causal. In addition, our approach that utilizes the genetic score derived from the nontransmitted maternal haplotype as a genetic instrument is a novel extension to the Mendelian randomization methodology in casual inference between parental phenotype (or exposure) and outcomes in offspring.

Can 'early programming' be partly explained by smoking? Results from a prospective, population-based cohort study

BACKGROUND

Numerous studies have focused the association between low birthweight and later disease. Our objective was to study the association between birthweight and later adult smoking and thereby explore a possible mechanism for the association between low birthweight and later adult disease.

METHODS

We studied associations between birthweight of women (n=247704) born in 1967-1995 and smoking habits at the end of their pregnancy 13-42 years later in a prospective, population-based cohort study from The Medical Birth Registry of Norway. Similarly, the association between birthweight of men (n=194393) and smoking habits of their partners were assessed. Finally, we studied the relation between smoking habits of the participating women and the cause specific death of their mothers (n=222808).

RESULTS

Twenty per cent of women with birthweight less than 2000 g were adult daily smokers compared with 11% with birthweight 4000-4499 g [relative risk=1.8, 95% confidence interval 1.4, 2.2]. Similarly, we found an association between men's birthweight and their partners smoking habits. Mothers of smoking women had doubled risk of dying from lung cancer and from cardiovascular disease compared with mothers of non-smoking women.

CONCLUSIONS

Being born with low birthweight is associated with smoking in adulthood. Associations of adult smoking with partners' birthweight and mothers' smoking-related causes of death suggest a shared smoking environment, and may account for some of the established association between birthweight and later cardiovascular disease.

Approaches for drawing causal inferences from epidemiological birth cohorts: a review

Large-scale population-based birth cohorts, which recruit women during pregnancy or at birth and follow up their offspring through infancy and into childhood and adolescence, provide the opportunity to monitor and model early life exposures in relation to developmental characteristics and later life outcomes. However, due to confounding and other limitations, identification of causal risk factors has proved challenging and published findings are often not reproducible. A suite of methods has been developed in recent years to minimise problems afflicting observational epidemiology, to strengthen causal inference and to provide greater insights into modifiable intra-uterine and early life risk factors. The aim of this review is to describe these causal inference methods and to suggest how they may be applied in the context of birth cohorts and extended along with the development of birth cohort consortia and expansion of "omic" technologies.

Birth order and myopia

PURPOSE

An association between birth order and reduced unaided vision (a surrogate for myopia) has been observed previously. We examined the association between birth order and myopia directly in four subject groups.

METHODS

Subject groups were participants in (1) the Avon Longitudinal Study of Parents and Children (ALSPAC; UK; age 15 years; N = 4401), (2) the Singapore Cohort Study of Risk Factors for Myopia (SCORM; Singapore; age 13 years; N = 1959), (3) the Raine Eye Health Study (REHS; Australia; age 20 years; N = 1344), and (4) Israeli Defense Force Pre-recruitment Candidates (IDFC; Israel; age 16-22 years; N = 888,277). The main outcome was odds ratios (OR) for myopia in first-born versus non-first-born individuals after adjusting for potential risk factors.

RESULTS

The prevalence of myopia was numerically higher in first-born versus non-first-born individuals in all study groups, but the strength of evidence varied widely. Adjusted ORs (95% confidence intervals, CIs) were: ALSPAC, 1.31 (1.05-1.64); SCORM, 1.25 (0.89-1.77); REHS, 1.18 (0.90-1.55); and IDFC, 1.04 (1.03-1.06). In the large IDFC sample, the effect size was greater (a) for the first-born versus fourth- or higher-born comparison than for the first-born versus second/third-born comparison (p < 0.001) and (b) with increasing myopia severity (p < 0.001).

CONCLUSIONS

Across all studies, the increased risk of myopia in first-born individuals was low (OR < 1.3). Indeed, only the studies with >4000 participants provided strong statistical support for the association. The available evidence suggested the relationship was independent of established risk factors such as time outdoors/reading, and thus may arise through a different causal mechanism.

Effect of prenatal alcohol exposure on childhood academic outcomes: contrasting maternal and paternal associations in the ALSPAC study

BACKGROUND

The impact of low-to-moderate levels of alcohol consumption during pregnancy on child cognitive outcomes has been of recent concern. This study has tested the hypothesis that low-to-moderate maternal alcohol use in pregnancy is associated with lower school test scores at age 11 in the offspring via intrauterine mechanisms.

METHODS

We used data from the Avon Longitudinal Study of Parents and Children (ALSPAC), a birth cohort study based in the South West of England. Analyses were conducted on 7062 participants who had complete data on: maternal and paternal patterns of alcohol use in the first trimester and at 18 weeks' gestation, child's academic outcomes measured at age 11, gender, maternal age, parity, marital status, ethnicity, household crowding, home ownership status and parental education. We contrasted the association of mother's alcohol consumption during pregnancy with child's National Curriculum Key Stage 2 (KS2) test scores with the association for father's alcohol consumption (during the time the mother was pregnant) with child's National Curriculum Key Stage 2 (KS2) test scores. We used multivariate linear regression to estimate mean differences and 95% confidence intervals [CI] in KS2 scores across the exposure categories and computed f statistics to compare maternal and paternal associations.

FINDINGS AND CONCLUSIONS

Drinking up to 1 unit of alcohol a day during pregnancy was not associated with lower test scores. However, frequent prenatal consumption of 4 units (equivalent to 32 grams of alcohol) on each single drinking occasion was associated with reduced educational attainment [Mean change in offspring KS2 score was -0.68 (-1.03, -0.33) for maternal alcohol categories compared to 0.27 (0.07, 0.46) for paternal alcohol categories]. Frequent consumption of 4 units of alcohol during pregnancy may adversely affect childhood academic outcomes via intrauterine mechanisms.

Maternal adiposity--a determinant of perinatal and offspring outcomes?

Experimental and animal data suggest that maternal obesity during pregnancy adversely affects offspring health in the short-term and the long-term. Whether these effects occur in humans and influence population health is less clear. This Review explores evidence from intervention studies and observational studies that have used designs (such as family-based comparisons and Mendelian randomization) that might help improve understanding of the causal effects of maternal obesity in humans. Collectively, human studies provide evidence that maternal overweight and obesity is causally related to pregnancy complications, increased offspring weight and adiposity at birth, and the difficulties associated with delivery of large-for-gestational-age infants. The underlying mechanisms for these effects probably involve maternal and fetal dysregulation of glucose, insulin, lipid and amino acid metabolism. Some evidence exists that extreme maternal obesity (BMI ≥40 kg/m(2)) is causally related to a long-term increase in offspring adiposity, but further exploration of this relationship is needed. High gestational weight gain may result in a long-term increase in offspring adiposity if women are already overweight or have obesity at the start of pregnancy. To date, little high-quality human evidence exists that any of these effects are mediated by epigenetic mechanisms, but approaches to appropriately test this possibility are being developed.

Familial intergenerational and maternal aggregation patterns in nutrient intakes in the Lifeways Cross-Generation Cohort Study

OBJECTIVE

The current study prospectively examines the intra-uterine hypothesis by comparing maternal, paternal and grandparental lineage influences on children’s diet and also maternal–child aggregation patterns during pregnancy and early childhood.

DESIGN

Prenatal dietary information was available for expectant mothers, fathers and up to four grandparents through a detailed validated semi-quantitative FFQ. At 6-year follow-up, when children averaged 5 years of age, dietary information was re-collected for mothers and a subset of maternal grandmothers using the same FFQ. Child’s FFQ version was used for children. Anthropometric and sociodemographic variables were also collected.

SETTINGS

Three-generation familial cohort representative of the contemporary Irish national population.

SUBJECTS

Children aged 5 years (n 567) and their parents and grandparents.

RESULTS

Associations for energy, macronutrient and fibre intakes were compared using Pearson’s correlations, intra-class correlations (ICC) and linear regression models, adjusted for energy and potential confounders. Significant, moderatestrength positive correlations were observed for nutrient intakes in children’s nuclear families (ICC (range)50?22–0?28). The father–child associations (r (range)5 0?13–0?20) were weaker than the mother–child associations (r (range)50?14–0?33). In general, associations were stronger for maternal postnatal intake–child intake than for maternal prenatal intake–child intake, except for percentage of energy from fat (adjusted b50?16, 95% CI 0?05, 0?26; P50?004), which was stronger for maternal prenatal intake, specifically in non-breast-fed children (adjusted b50?28, 95% CI 0?12, 0?44; P50?001). Among all grandparents, correlations were significant only for maternal grandmother–mother pairs (r (range)50?10–0?36). Significant positive ICC were observed for nutrient intakes of maternal grandmother–mother–child triads (ICC (range)50?12–0?27), not found in paternal lines.

CONCLUSIONS

These findings suggest that maternal-environment programming influences dietary intake.

Prenatal famine and genetic variation are independently and additively associated with DNA methylation at regulatory loci within IGF2/H19

Both the early environment and genetic variation may affect DNA methylation, which is one of the major molecular marks of the epigenome. The combined effect of these factors on a well-defined locus has not been studied to date. We evaluated the association of periconceptional exposure to the Dutch Famine of 1944-45, as an example of an early environmental exposure, and single nucleotide polymorphisms covering the genetic variation (tagging SNPs) with DNA methylation at the imprinted IGF2/H19 region, a model for an epigenetically regulated genomic region. DNA methylation was measured at five differentially methylated regions (DMRs) that regulate the imprinted status of the IGF2/H19 region. Small but consistent differences in DNA methylation were observed comparing 60 individuals with periconceptional famine exposure with unexposed same-sex siblings at all IGF2 DMRs (P(BH)<0.05 after adjustment for multiple testing), but not at the H19 DMR. IGF2 DMR0 methylation was associated with IGF2 SNP rs2239681 (P(BH) = 0.027) and INS promoter methylation with INS SNPs, including rs689, which tags the INS VNTR, suggesting a mechanism for the reported effect of the VNTR on INS expression (P(BH) = 3.4 × 10(-3)). Prenatal famine and genetic variation showed similar associations with IGF2/H19 methylation and their contributions were additive. They were small in absolute terms (<3%), but on average 0.5 standard deviations relative to the variation in the population. Our analyses suggest that environmental and genetic factors could have independent and additive similarly sized effects on DNA methylation at the same regulatory site.

When things are not as they appear: assessing the adequacy of cluster randomization when outcome events are rare at baseline

The present study randomly assigned 15 Bahamian elementary schools to one of three intervention conditions. To assess the adequacy of cluster randomization, we examined two concerns identified by the local research team: inequality of gender distribution and environmental risk among groups. Baseline significant differences in risk and protective behaviors were minimal. There were significantly more males in the intervention group. Males had higher rates of risk behavior at all assessments. Poor school performance was also higher among the intervention condition and was significantly associated with increased rates of many but not all risk behaviors. Prior to adjusting for gender and school performance, several risk behaviors appeared to be higher after intervention among intervention youth. Adjusting for gender and school performance eradicated the group differences in risk behavior rates. Results demonstrate the importance of adequate randomization where outcomes of interest are rare events at baseline or differ by gender and there is an unequal gender distribution and the importance of the local research team's knowledge of potential inequalities in environmental risk (i.e., school performance). Not considering such individual differences could impact the integrity of trial outcomes.

Early origins of child obesity: bridging disciplines and phases of development -- September 30--October 1, 2010

This report summarizes a conference: “Early Origins of Child Obesity: Bridging Disciplines and Phases of Development”, held in Chicago on September 30–October 1, 2010. The conference was funded in part by the National Institutes of Health and the Williams Heart Foundation, to achieve the conference objective: forging a next-step research agenda related to the early origins of childhood obesity. This research agenda was to include working with an array of factors (from genetic determinants to societal ones) along a continuum from prenatal life to age 7, with an emphasis on how the developing child deals with the challenges presented by his/her environment (prenatal, parental, nutritional, etc.). The conference offered a unique opportunity to facilitate communication and planning of future work among a variety of researchers whose work separately addresses different periods in early life. Over the span of two days, speakers addressed existing, critical research topics within each of the most-studied age ranges. On the final day, workshops fostered the discussion needed to identify the highest priority research topics related to linking varied early factor domains. These are presented for use in planning future research and research funding.

Metabolic insights from extreme human insulin resistance phenotypes

As well as improving diagnostic and clinical outcomes for affected patients, understanding the genetic basis of rare human metabolic disorders has resulted in several fundamental biological insights. In some cases understanding extreme phenotypes has also informed thinking about more prevalent metabolic diseases. Insulin resistance underpins the twin epidemics of obesity and type 2 diabetes as well as accounting for many of the metabolic problems encompassed by the term metabolic syndrome. This review provides a brief update on current understanding of human severe insulin resistance syndromes, before highlighting recent insights provided by studies in these rare syndromes into the molecular pathogenesis of elements of the metabolic syndrome.

Fetal and infant origins of asthma

Previous studies have suggested that asthma, like other common diseases, has at least part of its origin early in life. Low birth weight has been shown to be associated with increased risks of asthma, chronic obstructive airway disease, and impaired lung function in adults, and increased risks of respiratory symptoms in early childhood. The developmental plasticity hypothesis suggests that the associations between low birth weight and diseases in later life are explained by adaptation mechanisms in fetal life and infancy in response to various adverse exposures. Various pathways leading from adverse fetal and infant exposures to growth adaptations and respiratory health outcomes have been studied, including fetal and early infant growth patterns, maternal smoking and diet, children's diet, respiratory tract infections and acetaminophen use, and genetic susceptibility. Still, the specific adverse exposures in fetal and early postnatal life leading to respiratory disease in adult life are not yet fully understood. Current studies suggest that both environmental and genetic factors in various periods of life, and their epigenetic mechanisms may underlie the complex associations of low birth weight with respiratory disease in later life. New well-designed epidemiological studies are needed to identify the specific underlying mechanisms. This review is focused on specific adverse fetal and infant growth patterns and exposures, genetic susceptibility, possible respiratory adaptations and perspectives for new studies.

Drawing causal inferences in epidemiologic studies of early life influences

Observational studies can describe associations between early life exposures and subsequent outcomes in human populations. It is challenging to draw causal inferences from these associations because exposures often occur many years before the outcome and are related to other early life exposures. An approach is required that combines traditional epidemiologic and statistical principles with the use of novel and sophisticated analytic methods. To minimize the bias in longitudinal studies of early origins, researchers need to do all they can to reduce losses to follow-up and to describe individuals who are lost to follow-up. To reduce the role of chance, researchers should concentrate on effect sizes and the strength of the evidence to support these effect sizes, and they should be cautious in their interpretation of subgroup analyses. More complex analytic approaches can and should be used to handle missing data and repeated measurements. Addressing the issue of confounding is not straightforward. Statistical adjustment for the confounders measured in a study may help, but a lack of attenuation does not guarantee that the association is not confounded. Ecologic studies, observational studies in populations with different confounding structures, and the follow-up of randomized trials (where these exist) can be informative. Genetic and nongenetic instrumental variable approaches (eg, Mendelian randomization) may also provide causal insights. These approaches to confounding often require the comparison of data from different populations or a combination of studies to ensure adequate power to provide robust estimates of the causal effect.

Developing the evidence base for the new guidelines for reduction of cardiovascular risk for children by the National Heart, Lung, and Blood Institute

In 2006, a process was initiated to develop evidence-based paediatric guidelines directed towards physicians for reduction of cardiovascular risk. In contrast to prior consensus-based guidelines from the National Heart, Lung, and Blood Institute, this process was to be evidenced based. This manuscript describes the process undertaken to write the evidence-based guidelines from the National Heart, Lung, and Blood Institute, beginning with the search for evidence, then the process of review of the evidence, and finally the writing of the final document. This manuscript also provides some thoughts on how this process might be adapted in developing guidelines for caring for patients with congenital cardiac disease.

Mitochondrial dysfunction in patients with primary congenital insulin resistance

Mitochondrial dysfunction is associated with insulin resistance and type 2 diabetes. It has thus been suggested that primary and/or genetic abnormalities in mitochondrial function may lead to accumulation of toxic lipid species in muscle and elsewhere, impairing insulin action on glucose metabolism. Alternatively, however, defects in insulin signaling may be primary events that result in mitochondrial dysfunction, or there may be a bidirectional relationship between these phenomena. To investigate this, we examined mitochondrial function in patients with genetic defects in insulin receptor (INSR) signaling. We found that phosphocreatine recovery after exercise, a measure of skeletal muscle mitochondrial function in vivo, was significantly slowed in patients with INSR mutations compared with that in healthy age-, fitness-, and BMI-matched controls. These findings suggest that defective insulin signaling may promote mitochondrial dysfunction. Furthermore, consistent with previous studies of mouse models of mitochondrial dysfunction, basal and sleeping metabolic rates were both significantly increased in genetically insulin-resistant patients, perhaps because mitochondrial dysfunction necessitates increased nutrient oxidation in order to maintain cellular energy levels.

Familial associations of adiposity: findings from a cross-sectional study of 12,181 parental-offspring trios from Belarus

BACKGROUND

It is suggested that maternal adiposity has a stronger association with offspring adiposity than does paternal adiposity. Furthermore, a recent small study reported gender assortment in parental-offspring adiposity associations. We aimed to examine these associations in one of the largest studies to date using data from a low-middle income country that has recently undergone a major political and economic transition.

METHODS AND PRINCIPAL FINDINGS

In a cross-sectional study of 12,181 parental-offspring trios from Belarus (mean age (SD) of mothers 31.7 (4.9), fathers 34.1 (5.1) and children 6.6 (0.3) at time of assessment), we found positive graded associations of mother's and father's BMI with offspring adiposity. There was no evidence that these associations differed between mothers and fathers. For example, the odds ratio of offspring overweight or obesity (based on BMI) comparing obese and overweight mothers to normal weight mothers was 2.03 (95%CI 1.77, 2.31) in fully adjusted models; the equivalent result for father's overweight/obesity was 1.81 (1.58, 2.07). Equivalent results for offspring being in the top 10% waist circumference were 1.91 (1.67, 2.18) comparing obese/overweight to normal weight mothers and 1.72 (1.53, 1.95) comparing obese/overweight to normal weight fathers. Similarly, results for offspring being in the top 10% of percent fat mass were 1.58 (1.36, 1.84) and 1.76 (1.49, 2.07), for mother's and father's obese/overweight exposures respectively. There was no strong or consistent evidence of gender assortment--i.e. associations of maternal adiposity exposures with offspring outcomes were similar in magnitude for their daughters compared to equivalent associations in their sons and paternal associations were also similar in sons and daughters.

CONCLUSIONS/SIGNIFICANCE

These findings suggest that genetic and/or shared familial environment explain family clustering of adiposity. Interventions aimed at changing overall family lifestyle are likely to be important for population level obesity prevention.

Usefulness of Mendelian randomization in observational epidemiology

Mendelian randomization refers to the random allocation of alleles at the time of gamete formation. In observational epidemiology, this refers to the use of genetic variants to estimate a causal effect between a modifiable risk factor and an outcome of interest. In this review, we recall the principles of a "Mendelian randomization" approach in observational epidemiology, which is based on the technique of instrumental variables; we provide simulations and an example based on real data to demonstrate its implications; we present the results of a systematic search on original articles having used this approach; and we discuss some limitations of this approach in view of what has been found so far.