Twins and fetal programming of blood pressure. Questioning the role of genes and maternal nutrition

The Foetal Origins of Adult Disease: Interpreting the Evidence From Twin Studies

Twin studies have a contribution to make to the debate concerning the foetal origins of adult disease. Twins are growth retarded compared to singletons and experience post-natal catch-up growth. However, there is no evidence that twins are at increased risk of cardiovascular disease. Studying whether discordance in size at birth within monozygotic twin pairs is predictive of discordance in later life disease should help resolve whether the association between size at birth and later disease is due to common genetic factors. Results from studies of blood pressure in childhood and adult life looking at these within twin effects are far from conclusive. There are, however, methodological problems in the interpretation of these results, not least of which is the relatively small numbers of twin pairs studied. Studies exploring the effect of zygosity and chorion type on later disease provide may provide a useful extension of the research agenda. In summary, twin studies to date have raised more questions about the foetal origins hypothesis than they have resolved.

Irritable bowel syndrome in twins: genes and environment

BACKGROUND AND AIMS

Both environmental and genetic factors may contribute to irritable bowel syndrome (IBS). Nutrition in fetal life, an early environmental factor, seems to influence the development of chronic diseases later in life, such as coronary heart disease, hypertension, and non-insulin diabetes. This population based twin study evaluated the association between intrauterine growth, measured by weight and gestational age, and IBS. Structural equation analyses were conducted to analyse genetic and environmental sources of variation in liability to IBS.

METHODS

A postal questionnaire was sent to 12 700 Norwegian twins born between 1967 and 1979. The questionnaire included a checklist of 31 illnesses and symptoms, including IBS. The influence of birth weight on developing IBS was tested in four weight groups. Disease discordant monozygotic (MZ) pairs were analysed to test the association between intrauterine growth and IBS.

RESULTS

Concordance for IBS was significantly greater (p = 0.011) in monozygotic (22.4%) than in dizygotic (9.1%) twins. The heritability of IBS was estimated to be 48.4% among females. Birth weight below 1500 g (adjusted odds ratio 2.4 (95% confidence interval 1.1, 5.3)) contributed significantly to the development of IBS, which appeared 7.7 years earlier than in higher weight groups. In the MZ group with birth weights lower than 2500 g, twins with IBS were significantly lighter than twins without disease (190.6 g; p = 0.02).

CONCLUSION

The present study demonstrates that restricted fetal growth has a significant influence on the development of IBS later in life. Weight below 1500 g influences age at onset. Genetic contribution appears to be important for IBS among females.

Wall shear stress and related hemodynamic parameters in the fetal descending aorta derived from color Doppler velocity profiles

This paper presents a methodology for estimating the wall shear stress in the fetal descending aorta from color Doppler velocity profiles obtained during the second half of pregnancy. The Womersley model was applied to determine the wall shear stress and related hemodynamic parameters. Our analysis indicates that the aortic diameter can be modeled as a function of the gestational age in weeks as: Diameter (mm) = 0.17.ga + 0.15 (R2 = 0.64, p < 0.001). The aortic volume flow showed a log linear gestational age-related increase that fit the model: F (mL/min) = e(0.08.ga + 3.49) (R2 = 0.61, p < 0.001). The Womersley number increased linearly with gestational age from 3.3 to 6.2 (p < 0.001) and the pressure gradient decreased linearly from 2.68 to 1.16 mPa/mm (p = 0.003) during the second half of pregnancy; the mean wall shear stress for the study group was 2.2 Pa (SD = 0.59) and was independent of gestational age. This study suggests that the size of the fetal aorta adapts to flow demands and maintains constant mean wall shear stress.

Studies of twins: what can they tell us about the fetal origins of adult disease?

There has been much interest in evidence that people with lower birthweight have higher risk of adult cardiovascular disease, but the causal pathways underlying such observations are uncertain. Study of twins offers an opportunity to shed light on the underlying causal pathways, in particular by investigating the role of 'shared' factors vs. factors affecting each individual fetus. This involves comparing results of within-cohort vs. within-pair analyses. Twins share many factors during gestation but birthweight discordance (difference in birthweight within a twin pair) cannot be determined by these shared factors and must relate to factors affecting growth of each individual fetus. If associations seen in a cohort of twins remain in within-pair analyses, then factors specific to each individual must be involved in the underlying causal pathways. Conversely, if the relationships disappear or substantially diminish in within-pair analyses, then factors common to the pair must be involved. Comparison of findings in monozygotic vs. dizygotic twins may provide insights into the role of genetic factors, although issues related to chorionicity need to be taken into account. We tabulate published data and conclude that differences in methodology and analyses preclude informative meta-analysis, and that analysis of pooled data would provide more useful information.

Is birth weight related to later glucose and insulin metabolism?--A systematic review

AIM

To determine the relationship of birth weight to later glucose and insulin metabolism.

METHODS

Systematic review of the published literature. Data sources were Medline and Embase. Included studies were papers reporting the relationship of birth weight with a measure of glucose or insulin metabolism after 1 year of age, including the prevalence of Type 2 diabetes mellitus (DM). Three reviewers abstracted information from each paper according to specified criteria.

RESULTS

Forty-eight papers fulfilled the criteria for inclusion, mostly of adults in developed countries. Most studies reported an inverse relationship between birth weight and fasting plasma glucose concentrations (15 of 25 papers), fasting plasma insulin concentrations (20 of 26), plasma glucose concentrations 2 h after a glucose load (20 of 25), the prevalence of Type 2 DM (13 of 16), measures of insulin resistance (17 of 22), and measures of insulin secretion (16 of 24). The predominance of these inverse relationships and the demonstration in a minority of studies of other directions of the relationships could not generally be explained by differences between studies in the sex, age, or current size of the subjects. However, the relationship of birth weight with insulin secretion was inconsistent in studies of adults.

CONCLUSIONS

The published literature shows that, generally, people who were light at birth have an adverse profile of later glucose and insulin metabolism. This is related to higher insulin resistance, but the relationship to insulin secretion in adults is less clear.

Unravelling the fetal origins hypothesis: is there really an inverse association between birthweight and subsequent blood pressure?

BACKGROUND

The association between birthweight and subsequent blood pressure levels has been considered to provide some of the strongest, and most consistent, support for the "fetal origins" hypothesis of adult disease. It had been estimated that a 1 kg higher birthweight is typically associated with a 2-4 mm Hg lower systolic blood pressure.

METHODS

55 studies that had reported regression coefficients of systolic blood pressure on birthweight (with 48 further studies that reported only the direction of this association), and seven such studies within twin pairs, were identified. Each study was weighted according to the inverse of the variance of the regression coefficient (ie, "statistical size"), and combined using a "fixed effects" approach.

FINDINGS

Among the 55 studies that reported regression coefficients, there was a clear trend (p<0.0001) towards weaker associations in the larger studies: -1.9 mm Hg/kg in those with less than about 1000 participants; -1.5 mm Hg/kg with about 1000-3000 participants; and -0.6 mm Hg/kg with more than 3000 participants. By contrast with the inverse associations reported in 52 of these 55 studies, only 25 of the 48 studies that did not report regression coefficients found an inverse association (p<0.0001 for heterogeneity). Almost all of these regression coefficients had been adjusted for current weight (whereas few were adjusted for potential confounding factors), and removal of this adjustment in the larger studies reduced the estimated association to -0.4 mm Hg/kg. For studies within monozygotic twin pairs, the combined estimate was -0.6 mm Hg/kg with adjustment for current weight, and was also reduced without this adjustment.

INTERPRETATION

Claims of a strong inverse association between birthweight and subsequent blood pressure may chiefly reflect the impact of random error, selective emphasis of particular results, and inappropriate adjustment for current weight and for confounding factors. These findings suggest that birthweight is of little relevance to blood pressure levels in later life.

Fetal growth and systolic blood pressure in young adulthood: the Swedish Young Male Twins Study

The objective was to test the fetal origins hypothesis by examining the association between fetal growth and systolic blood pressure in a large cohort of adult male twins of known zygosity. This cohort study is based on a record linkage of routinely collected data from the Swedish Medical Birth Registry, the Swedish Military Conscription Registry and a mailed questionnaire. It comprises 886 pairs of same-sex male twins born in Sweden in 1973-79 for whom data were available on size at birth and systolic blood pressure, height and weight at age 17-19 years. Based on self-reported similarity of appearance in childhood, 384 twin pairs were classified as monozygotic (MZ), 269 as dizygotic (DZ) and 233 as of uncertain zygosity (XZ). Differences in systolic blood pressure within and between twin pairs were analysed in relation to birthweight and birthweight-for-gestational-age z-score using random effects linear models. Within twin pairs, a 1 kg difference in birthweight was associated with a difference of -1.30 mmHg in systolic blood pressure [95% confidence interval -4.15, +1.54] for MZ twins and +0.14 [-3.49, +3.76] for DZ twins; for all twins combined it was -0.21 [-2.13, +1.71]. Between twin pairs, a 1 kg difference in birthweight was associated with a difference of -2.68 mmHg in systolic blood pressure [-4.95, -0.42] for MZ twins and +0.28 [-2.35, +2.91] for DZ twins; for all twins combined -1.68 [-3.15, -0.22]. All these estimates included adjustment for age, year and conscription centre of examination, gestational age and height and weight at conscription. These results provide little support for the fetal origins hypothesis. The estimates of the within-pair effect in MZ twins and their wide confidence intervals (including zero) cannot exclude the existence of common genetic mechanisms, but are also not inconsistent with an in utero programming effect. This study highlights some methodological problems with twin studies.

Statistical measures of foetal growth using linear mixed models applied to the foetal origins hypothesis

Statistical models of the relationship between the distribution of each of five foetal dimensions and gestational age are developed based on serial ultrasound biometric data from a prospective longitudinal study in Perth, Western Australia. Both the response variable and the gestational age timescale are transformed to establish an approximately linear relationship within subjects. This relationship is modelled using a linear mixed effects model that accounts for between-subject heterogeneity by incorporating subject specific random effects for both intercept and gradient. These models are used to motivate three measures of foetal growth: the conditional centile or z-score of a current measurement given an earlier value for the same measurement; the best linear unbiased predictor (BLUP) of the subject specific random effect gradient (which is shown to be invariant to transformations of location and scale), and the standardized residual at a given gestational age, which characterizes departures from the modelled growth trajectory. We illustrate how these three measures of growth might be applied to subsequent health outcomes in later life by relating growth in foetal abdominal circumference to blood pressure in children from the same cohort at six years of age. Foetuses whose summary measures indicate poor growth in abdominal circumference have higher blood pressure in early childhood, supporting the 'foetal origins' hypothesis that many chronic diseases of adulthood have their origins before birth.

Do genetic factors contribute to the association between birth weight and blood pressure?

OBJECTIVES

To evaluate whether genetic factors contribute to the association between low birth weight and increased blood pressure among adolescents.

DESIGN

Historical cohort study of twin pairs. It was evaluated whether (1) a negative association between birth weight and systolic blood pressure was found in the overall twin sample and (2) whether the intrapair difference in birth weight correlated with the intrapair difference in systolic blood pressure-thereby controlling for the effect of genetic factors (all in monozygotic and on average half in dizygotic pairs).

SETTING

The Minnesota Twin Family Study.

PARTICIPANTS

1311 pairs of adolescent twins.

MAIN RESULTS

A negative association between birth weight and systolic blood pressure was retrieved in the overall sample. The regression coefficient after controlling for current weight was -1.88 mm Hg/kg (SE 0.61), which corresponds to results from previous studies of singleton adolescents. The regression coefficient fell to -0.64 mm Hg/kg (SE 0.86) when the intrapair analyses were used. The largest reduction was observed among monozygotic twins: from -2.44 mm Hg/kg (SE 0.75) in the overall monozygotic twin sample to -1.06 mm Hg/kg (SE 1.14) in the analyses of the within monozygotic pair differences.

CONCLUSION

The association between low birth weight and increased blood pressure later in life is well established. "The fetal programming hypothesis" suggests that the association is caused by intrauterine malnutrition while a new hypothesis "the fetal insulin hypothesis" proposes that genetically determined insulin resistance also contributes significantly to the association. A recent twin study of middle aged twins showed no evidence for an influence of genetic factors while this larger study provides support for the fetal insulin hypothesis: the association between birth weight and blood pressure attenuated among adolescents when genetic factors were controlled. Together this suggests an important contribution of genetic factors to the association between fetal growth and systolic blood pressure in adolescence.

Birthweight, early environment, and genetics: a study of twins discordant for acute myocardial infarction

BACKGROUND

Epidemiological studies that used birthweight as a crude marker of fetal growth have suggested that low birthweight is associated with increased risk of coronary heart disease. Through investigation of this association within same-sexed twin pairs, confounding by genetic and early environmental factors can be greatly decreased. We undertook a case-control study in twins discordant for acute myocardial infarction (AMI).

METHODS

The case-control study was nested within the population-based Swedish Twin Registry and linked with the national cause-of-death and hospital-discharge registries. We manually retrieved birth records containing information on birth and maternal characteristics for 132 same-sexed twin pairs discordant for AMI and 118 individually matched control twin pairs.

FINDINGS

In comparisons between AMI cases and external matched control twins, cases had significantly lower birthweight (mean 2556 [SD 500] vs 2699 [530] g, p=0.04), birth length (47.1 [2.8] vs 47.9 [2.7] cm, p=0.04), and head circumference (33.0 [1.8] vs 33.5 [2.0] cm, p=0.03) than controls. In within-pair comparisons between AMI cases and healthy co-twins, no significant differences in birth measurements were found (birthweight 2458 [510] vs 2534 [530] g, p=0.73; birth length 47.1 [2.8] vs 47.2 [2.8] cm, p=0.91; head circumference 33.0 [1.7] vs 33.0 [1.8] cm, p=0.92).

INTERPRETATION

The lack of an association between birth characteristics and AMI within twin pairs suggests that previously reported associations may be influenced by genetic and early environmental factors, or possibly, by unmeasured maternal factors that operate independently of birthweight.

Angiogenesis and intrauterine growth restriction

Human placental development involves co-ordinated angiogenesis and trophoblast outgrowth that are compromised in intrauterine growth restriction. Adaptive angiogenesis in IUGR placental villi is a result of an imbalance in the orderly progression of the expression profile of vascular endothelial growth factor, placenta growth factor and angiopoietin during placental development. VEGF receptors and the angiopoietin receptor Tie-2 are expressed on trophoblast, and their activation leads to trophoblast proliferation, migration and production of nitric oxide. Thus, these vascular factors act as autocrine regulators of trophoblast behaviour in the development of the utero-/feto-placental circulation, an action independent of their well-established roles in vascular endothelium.