Associations between birth size and later height from infancy through adulthood: An individual based pooled analysis of 28 twin cohorts participating in the CODATwins project

BACKGROUND

There is evidence that birth size is positively associated with height in later life, but it remains unclear whether this is explained by genetic factors or the intrauterine environment.

AIM

To analyze the associations of birth weight, length and ponderal index with height from infancy through adulthood within mono- and dizygotic twin pairs, which provides insights into the role of genetic and environmental individual-specific factors.

METHODS

This study is based on the data from 28 twin cohorts in 17 countries. The pooled data included 41,852 complete twin pairs (55% monozygotic and 45% same-sex dizygotic) with information on birth weight and a total of 112,409 paired height measurements at ages ranging from 1 to 69 years. Birth length was available for 19,881 complete twin pairs, with a total of 72,692 paired height measurements. The association between birth size and later height was analyzed at both the individual and within-pair level by linear regression analyses.

RESULTS

Within twin pairs, regression coefficients showed that a 1-kg increase in birth weight and a 1-cm increase in birth length were associated with 1.14-4.25 cm and 0.18-0.90 cm taller height, respectively. The magnitude of the associations was generally greater within dizygotic than within monozygotic twin pairs, and this difference between zygosities was more pronounced for birth length.

CONCLUSION

Both genetic and individual-specific environmental factors play a role in the association between birth size and later height from infancy to adulthood, with a larger role for genetics in the association with birth length than with birth weight.

Placental telomere length decreases with gestational age and is influenced by parity: a study of third trimester live-born twins

BACKGROUND

In contrast to the postnatal period, little is known about telomere length (TL) during prenatal life. The decrease in placental TL remains unknown, although intra uterine growth retardation and preeclampsia are associated with shorter placental TL. The aim of this study is to assess the decrease of placental TL during the third trimester of gestation and to explore the role of potential "growth influencing factors".

METHODS

The study sample consisted of 329 live-born twins from the East Flanders Prospective Twin Survey. TL was determined using a multiplex quantitative PCR method. Gestational age, sex, birth order, placental characteristics, parity, maternal and paternal age, diabetes, hypertension, smoking, alcohol use, and socio economic status (SES) were considered "growth influencing factors". Bivariable multilevel regression analysis with "growth influencing factors" was performed.

RESULTS

Placental TL ranged from 4.3 kbp to 84.4 kbp with a median of 10.8 kbp. Ln(TL) decreased in a linear fashion with an estimated TL decreasing from 13.98 kbp at 28 weeks to 10.56 kbp at 42 weeks. The regression coefficient of gestational age became smaller if considered together with SES (b = -0.017; p = 0.08) or diabetes (b = -0.018; p = 0.07) and bigger if considered together with parity (b = -0.022; p = 0.02), indicating that part of the association between gestational age and telomere length is explained by these three confounding factors.

CONCLUSION

Placental TL decreases during the third trimester of gestation of live-born twins with approximately 25% indicating that telomere shortening may play a role in aging of the placenta.

Is PPARα intron 7 G/C polymorphism associated with muscle strength characteristics in nonathletic young men?

Peroxisome proliferator-activated receptor alpha (PPARα), a ligand-dependent transcription factor, regulates fatty acid metabolism in heart and skeletal muscle. The intron 7 G/C polymorphism (rs4253778) has been associated with athletic performance. The rare C-allele was predominant in power athletes, whereas the G-allele was more frequent in endurance athletes. In the present study, we investigated the association between this polymorphism and strength characteristics in nonathletic, healthy young adults (n = 500; age 24.2 ± 4.4 years). Knee torque was measured during concentric knee flexion and extension movements at 60°/s, 120°/s, and 240°/s during 3, 25, and 5 repetitions, respectively. Also, resistance to muscle fatigue (i.e. work last 20% repetitions/work first 20% repetitions *100) was calculated. Differences in knee strength phenotypes between GG homozygous individuals and C-allele carriers were analyzed. The polymorphism did not influence the ability to produce isometric or dynamic knee flexor or extensor peak torque during static or dynamic conditions in this population (0.23 < P < 0.95). Similar results were found for the endurance ratio, a measure for resistance to muscle fatigue. In conclusion, the PPARα intron 7 G/C polymorphism does not seem to influence strength characteristics in a nonathletic population.

No major role for X-inactivation in variations of intelligence and behavioral problems at middle childhood

Although members of monozygotic (MZ) twin pairs are identical in genomic sequence, epigenetic mechanisms may occasion difference in gene expression and, consequently, twin discordance in complex traits. Recent work suggests that the epigenetic process of X-inactivation in female individuals may impact on intelligence and child behavioral problems. The timing of X-inactivation has been linked to chorionic splitting in MZ twins. Dichorionic monozygotic (DC-MZ) twinning, unlike monochorionic monozygotic (MC-MZ) twinning, occurs prior to the time of X-inactivation in female organisms. Therefore, the hypothesis of a causal role of X-inactivation in intelligence and behavioral problems can be analyzed by modeling the statistical interaction between sex and chorion type for within-pair differences in these traits in MZ twins. In this study, the effect of X-inactivation on childhood behavioral problems, measured with the CBCL, was studied in a sample of 324 MZ twin pairs from the EFPTS and the effect of X-inactivation on IQ was studied in a sample of 272 twin pairs from the same twin survey. Information on chorion type, gestational age, and birth weight was additionally collated. No significant statistical interaction was found between sex and chorion type, indicating that X-inactivation is not likely involved in variations in intelligence or behavioral problems in middle childhood. Further studies are required to replicate these findings and may explore the role of X-inactivation at different ages or at the extreme scores in the spectrum of intelligence and behavioral problems or may focus on other epigenetic mechanisms.

Genome-wide linkage scan for maximum and length-dependent knee muscle strength in young men: significant evidence for linkage at chromosome 14q24.3

Background:

Maintenance of high muscular fitness is positively related to bone health, functionality in daily life and increasing insulin sensitivity, and negatively related to falls and fractures, morbidity and mortality. Heritability of muscle strength phenotypes ranges between 31% and 95%, but little is known about the identity of the genes underlying this complex trait. As a first attempt, this genome-wide linkage study aimed to identify chromosomal regions linked to muscle and bone cross-sectional area, isometric knee flexion and extension torque, and torque–length relationship for knee flexors and extensors.

Methods:

In total, 283 informative male siblings (17–36 years old), belonging to 105 families, were used to conduct a genome-wide SNP-based multipoint linkage analysis.

Results:

The strongest evidence for linkage was found for the torque–length relationship of the knee flexors at 14q24.3 (LOD  = 4.09; p<10⁻⁵). Suggestive evidence for linkage was found at 14q32.2 (LOD  = 3.00; P = 0.005) for muscle and bone cross-sectional area, at 2p24.2 (LOD  = 2.57; p = 0.01) for isometric knee torque at 30° flexion, at 1q21.3, 2p23.3 and 18q11.2 (LOD  = 2.33, 2.69 and 2.21; p<10⁻⁴ for all) for the torque–length relationship of the knee extensors and at 18p11.31 (LOD  = 2.39; p = 0.0004) for muscle-mass adjusted isometric knee extension torque.

Conclusions:

We conclude that many small contributing genes rather than a few important genes are involved in causing variation in different underlying phenotypes of muscle strength. Furthermore, some overlap in promising genomic regions were identified among different strength phenotypes.

Anthropometry, carbohydrate and lipid metabolism in the East Flanders Prospective Twin Survey: heritabilities

AIMS/HYPOTHESIS

We determined the genetic contribution of 18 anthropometric and metabolic risk factors of type 2 diabetes using a young healthy twin population.

METHODS

Traits were measured in 240 monozygotic (MZ) and 138 dizygotic (DZ) twin pairs aged 18 to 34 years. Twins were recruited from the Belgian population-based East Flanders Prospective Twin Survey, which is characterised by its accurate zygosity determination and extensive collection of perinatal and placental data, including information on chorionicity. Heritability was estimated using structural equation modelling implemented in the Mx software package.

RESULTS

Intra-pair correlations of the anthropometric and metabolic characteristics did not differ between MZ monochorionic and MZ dichorionic pairs; consequently heritabilities were estimated using the classical twin approach. For body mass, BMI and fat mass, quantitative sex differences were observed; genetic variance explained 84, 85 and 81% of the total variation in men and 74, 75 and 70% in women, respectively. Heritability estimates of the waist-to-hip ratio, sum of four skinfold thicknesses and lean body mass were 70, 74 and 81%, respectively. The heritability estimates of fasting glucose, fasting insulin, homeostasis model assessment of insulin resistance and beta cell function, as well as insulin-like growth factor binding protein-1 levels were 67, 49, 48, 62 and 47%, in that order. Finally, for total cholesterol, LDL-cholesterol, HDL-cholesterol, total cholesterol:HDL-cholesterol ratio, triacylglycerol, NEFA and leptin levels, genetic factors explained 75, 78, 76, 79, 58, 37 and 53% of the total variation, respectively.

CONCLUSIONS/INTERPRETATION

Genetic factors explain the greater part of the variation in traits related to obesity, glucose intolerance/insulin resistance and dyslipidaemia.

Heritability of somatotype components: a multivariate analysis

OBJECTIVE

To study the genetic and environmental determination of variation in Heath-Carter somatotype (ST) components (endomorphy, mesomorphy and ectomorphy).

DESIGN

Multivariate path analysis on twin data.

SUBJECTS

Eight hundred and three members of 424 adult Flemish twin pairs (18-34 years of age).

RESULTS

The results indicate the significance of sex differences and the significance of the covariation between the three ST components. After age-regression, variation of the population in ST components and their covariation is explained by additive genetic sources of variance (A), shared (familial) environment (C) and unique environment (E). In men, additive genetic sources of variance explain 28.0% (CI 8.7-50.8%), 86.3% (71.6-90.2%) and 66.5% (37.4-85.1%) for endomorphy, mesomorphy and ectomorphy, respectively. For women, corresponding values are 32.3% (8.9-55.6%), 82.0% (67.7-87.7%) and 70.1% (48.9-81.8%). For all components in men and women, more than 70% of the total variation was explained by sources of variance shared between the three components, emphasising the importance of analysing the ST in a multivariate way.

CONCLUSIONS

The findings suggest that the high heritabilities for mesomorphy and ectomorphy reported in earlier twin studies in adolescence are maintained in adulthood. For endomorphy, which represents a relative measure of subcutaneous adipose tissue, however, the results suggest heritability may be considerably lower than most values reported in earlier studies on adolescent twins. The heritability is also lower than values reported for, for example, body mass index (BMI), which next to the weight of organs and adipose tissue also includes muscle and bone tissue. Considering the differences in heritability between musculoskeletal robustness (mesomorphy) and subcutaneous adipose tissue (endomorphy) it may be questioned whether studying the genetics of BMI will eventually lead to a better understanding of the genetics of fatness, obesity and overweight.

A battery of DNA effect biomarkers to evaluate environmental exposure of Flemish adolescents

The present paper deals with the evaluation of a battery of genotoxicity biomarkers in healthy Flemish adolescents and their relation with common pollutants occurring in their life environment. DNA damage as reflected by the comet assay appeared to be most sensitive to ozone (partial r(2) = 0.102, p < 0.00001), and to a lesser extent to ortho-cresol (partial r(2) = 0.055; p = 0.001) and 1-hydroxy-pyrene (1-OH-pyrene, partial r(2) = 0.031; p = 0.013). 8-hydroxy-deoxyguanosine (8-OHdG) was only related to ortho-cresol (r(2) = 0.069; p < 0.007). Interestingly, the comet assay results and urinary 8-OHdG concentrations were positively correlated with a Pearson r = 0.21 (p = 0.003, N = 200). Logistic regression models revealed significant relations between chromatid breaks and 1-OH-pyrene (relative risk (RR): 1.58; p = 0.008), and t,t-muconic acid (RR: 1.71; p = 0.014). There was no correlation between micronucleus formation or occurrence of chromosomal or chromatid breaks on the one hand and comet or 8-OHdG results on the other hand. Thus, in this study the comet assay on whole blood samples and urine 8-OHdG measurements especially appeared sensitive biomarkers for assessing the genetic effects of environmental pollutants to which adolescents may be exposed.

Deconstructing the familiality of the emotive component of psychotic experiences in the general population

OBJECTIVE

Genetic and environmental influences on variation in distress associated with subclinical psychotic experiences were examined.

METHOD

A total of 289 twin pairs filled in the Community Assessment of Psychic Experiences, a self-report instrument assessing subclinical positive and negative psychotic experiences and associated distress (distresspos and distressneg). Using structural equation modelling, univariate and bivariate models were fitted.

RESULTS

Univariate model fitting showed genetic and non-shared environmental influences on both distresspos and distressneg. Bivariate model fitting showed that 52% of the correlation between the two phenotypes (r=0.46) was because of shared genes and that non-shared environmental factors accounted for 48% of the correlation.

CONCLUSION

Liability to psychosis not only refers to the development of psychosis per se, but also to the liability to develop dysfunctional emotional appraisals. The emotive component of psychosis liability involves genetic transmission of a general, non-symptom-specific distress factor that may be a target for molecular genetic research.

Quantitative trait loci for human muscle strength: linkage analysis of myostatin pathway genes

This study reports the results of a multipoint linkage study that aims to unravel the genetic basis of muscle strength and muscle mass in humans. Myostatin (GDF8) is known to be a strong inhibitor of muscle growth in animals. However, studies examining human myostatin polymorphisms are rare and are limited to the GDF8 gene itself. Here, the contribution to isometric and concentric knee strength of nine key proteins involved in the myostatin pathway is studied in a nonparametric multipoint linkage analysis by means of a variance components and regression method. A sample of 367 healthy young male siblings was phenotyped on an isokinetic dynamometer and genotyped for markers of the myostatin pathway genes. Three of the loci were found significantly linked with a quantitative trait locus (QTL) for knee muscle strength. First, D13S1303 showed replication of an explorative single-point linkage study with a maximum LOD score of 2.7 (P = 0.0002). Second, maximum LOD scores of 3.4 (P = 0.00004) and 3.3 (P = 0.00005) were observed for markers D12S1042 and D12S85, respectively, at 12q12-14. Finally, marker D12S78 showed an LOD score of 2.7 at 12q22-23. We conclude that several genes involved in the myostatin pathway, but not the myostatin gene itself, are important QTLs for human muscle strength. An additional set of valuable candidate genes that were not part of the myostatin pathway was found in the chromosome 12 and 13 genomic regions.

Association of tumour necrosis factor alpha variants with the CF pulmonary phenotype

BACKGROUND

The pulmonary phenotype in patients with cystic fibrosis (CF), even in those with the same CF transmembrane conductance regulator (CFTR) genotype, is variable and must therefore be influenced by secondary genetic factors as well as environmental factors. Possible candidate genes that modulate the CF lung phenotype may include proinflammatory cytokines. One such protein is tumour necrosis factor alpha (TNFalpha), a member of the immune system.

METHODS

Three polymorphic loci in the promoter (-851c/t, -308g/a, -238g/a) and one polymorphic locus in intron 1 (+691g ins/del) of the TNFalpha gene were typed by a single nucleotide primer extension assay in CF patients and healthy controls. Spirometric data and first age of infection with Pseudomonas aeruginosa were collected retrospectively from patients' medical records.

RESULTS

An association was found between the TNFalpha +691g ins/del polymorphic locus and severity of CF lung disease. Patients heterozygous for +691g ins and +691g del were more likely to have better pulmonary function (mean (SD) forced expiratory volume in 1 second (FEV1) 79.7 (12.8)% predicted) than patients homozygous for +691g ins (mean (SD) FEV1 67.5 (23.0)% predicted; p = 0.008, mean difference 12.2%, 95% CI 3.5 to 21.0). Also, patients heterozygous for +691g ins and +691g del were more likely to have an older first age of infection with P aeruginosa (mean (SD) 11.4 (6.0) years) than patients homozygous for +691g ins (mean (SD) 8.3 (4.6) years; p = 0.018, mean difference 3.1 years, 95% CI 0.5 to 5.6). An association was also found with the -851c/t polymorphic locus. In the group of patients with more severe FEV1% predicted, a higher proportion of patients were homozygous for the -851c allele than in the other group of patients (p = 0.04, likelihood ratio chi2, odds ratio = 2.4).

CONCLUSION

TNFalpha polymorphisms are associated with the severity of CF lung disease in Czech and Belgian patients with CF.

Epithelioid granulomas, pattern recognition receptors, and phenotypes of Crohn's disease

INTRODUCTION

Crohn's disease is a chronic inflammatory disorder of the gut. It is assumed that a defective interaction between the bacterial flora of the gut and the innate immune system plays a key role in the pathogenesis of the disease. This may lead to specific histological lesions. The epithelioid granuloma is particularly interesting in this regard as it is also observed in several bacterial infections of the gut.

AIMS AND METHODS

We hypothesised that genetic or environmental factors with a known influence on inflammation or immunity would lead to an increased prevalence of granulomas. Therefore, surgical specimens from 161 patients were evaluated for the presence of granulomas. Patients were genotyped for the three single nucleotide polymorphisms in caspase recruitment domain 15 (CARD15)/NOD2 associated with CD and for Asp299Gly in Toll-like receptor 4 (TLR4).

RESULTS

The overall prevalence of granulomas was 68.9%. We did not find a significant correlation between granulomas and TLR4 or CARD15 variants. The frequency of granulomas increased with more distal disease (63% small bowel, 72% right colon, 88% left colon, 90% rectum; p=0.01). Granulomas were more frequent in younger patients (odds ratio 0.95 (95% confidence interval 0.92-0.98) p=0.007).

CONCLUSION

In this study of 161 well documented CD patients, we found no significant association between CARD15 and TLR4 variants and granulomas. This finding seems to refute our initial hypothesis. However, it may be that additional factors are needed for granuloma development. Granulomas may develop only when specific bacterial components are present. Therefore, future research on granuloma pathogenesis should be orientated towards detection and identification of bacterial components in these lesions.

Tumour necrosis factor-alpha receptor 1 and 2 polymorphisms in inflammatory bowel disease and their association with response to infliximab

BACKGROUND

The role of tumour necrosis factor-alpha in the pathogenesis of inflammatory bowel disorders is well-known and is underscored by the effectiveness of antitumour necrosis factor-alpha treatment. Tumour necrosis factor-alpha exerts its effect by binding TNFR1 and TNFR2, which genes map to inflammatory bowel disorders susceptibility loci.

AIMS AND METHODS

Since TNFR1 and TNFR2 are good candidate genes for inflammatory bowel disorders, we studied the functional TNFR2T587G and the TNFR1A36G mutation in 344 Crohn's disease and 152 ulcerative colitis patients and investigated the relation with disease phenotypes. An association with response to infliximab was evaluated in 166 Crohn's disease patients.

RESULTS

The TNFR2 587G allele was more frequent in ulcerative colitis compared with controls (P = 0.03). Both single nucleotide polymorphisms were negatively associated with smoking at diagnosis in Crohn's disease (TNFR1A36G odds ratio: 0.614, 95% confidence interval: 0.452, 0.99 and TNFR2T587G odds ratio: 0.572, 95% confidence interval: 0.820, 0.875). There was a positive association between pancolitis and the TNFR1A36G polymorphism in ulcerative colitis (odds ratio: 5.341, 95% confidence interval: 1.484, 19.39). The biological response to infliximab was lower in patients carrying TNFR1 36G (odds ratio: 0.47, 95% confidence interval: 0.234, 0.946).

CONCLUSION

The TNFR2 587G allele was more frequent in ulcerative colitis. Both single nucleotide polymorphisms were negatively associated with smoking in Crohn's disease. A relation between TNFR1A36G and pancolitis was found in ulcerative colitis. There was no clear effect of the polymorphisms on infliximab response although, the TNFR1 minor was associated with a lower response to infliximab.

Genome wide scan in a Flemish inflammatory bowel disease population: support for the IBD4 locus, population heterogeneity, and epistasis

BACKGROUND AND AIMS

Genome wide scans in inflammatory bowel disease (IBD) have indicated various susceptibility regions with replication of 16cen (IBD1), 12q (IBD2), 6p (IBD3), 14q11 (IBD4), and 3p21. As no linkage was previously found on IBD regions 3, 7, 12, and 16 in Flemish IBD families, a genome wide scan was performed to detect other susceptibility regions in this population.

METHODS

A cohort of 149 IBD affected relative pairs, all recruited from the Northern Flemish part of Belgium, were genotyped using microsatellite markers at 12 cM intervals, and analysed by Genehunter non-parametric linkage software. All families were further genotyped for the three main Crohn's disease associated variants in the NOD2/CARD15 gene.

RESULTS

Nominal evidence for linkage was observed on chromosomes 1 (D1S197: multipoint non-parametric linkage (NPL) score 2.57, p = 0.004; and at D1S305-D1S252: NPL 2.97, p = 0.001), 4q (D4S406: NPL 1.95, p = 0.03), 6q16 (D6S314: NPL 2.44, p = 0.007), 10p12 (D10S197: NPL 2.05, p = 0.02), 11q22 (D11S35-D11S927: NPL 1.95, p = 0.02) 14q11-12 (D14S80: NPL 2.41, p = 0.008), 20p12 (D20S192: NPL 2.7, p = 0.003), and Xq (DXS990: NPL 1.70, p = 0.04). A total of 51.4% of patients carried at least one NOD2/CARD15 variant. Furthermore, epistasis was observed between susceptibility regions 6q/10p and 20p/10p.

CONCLUSION

Genome scanning in a Flemish IBD population found nominal evidence for linkage on 1p, 4q, 10p12, and 14q11, overlapping with other genome scan results, with linkage on 14q11-12 supporting the IBD4 locus. The results further show that epistasis is contributing to the complex model of IBD and indicate that population heterogeneity is not to be underestimated. Finally, NOD2/CARD15 is clearly implicated in the Flemish IBD population.

Linkage of myostatin pathway genes with knee strength in humans

This study was the first to explore the potential role of the myostatin (GDF8) pathway in relation to muscle strength and estimated muscle cross-sectional area in humans using linkage analysis with a candidate gene approach. In young male sibs (n = 329) 11 polymorphic markers in or near 10 candidate genes from the myostatin pathway were genotyped. Muscle mass was estimated by anthropometric measurements, and maximal knee strength was evaluated using isokinetic dynamometers (Cybex NORM). Single-point nonparametric variance components and linear quantitative trait locus regression linkage analysis methods were used. Linkage patterns were observed between knee extension and flexion peak torque with markers D2S118 (GDF8), D6S1051 (CDKN1A), and D11S4138 (MYOD1), and a maximum LOD score of 2.63 (P = 0.0002) was observed with D2S118. The ratios of peak torque over muscle and bone area of the midthigh of the lower contraction velocity (60 degrees/s) showed more frequently significant LOD scores than the torques at high velocity (240 degrees/s). Although myostatin is physiologically more related to muscle mass through possible effects of hyperplasia and hypertrophy than it is to strength, only two estimated muscle cross-sectional areas were marginally linked (LOD 1.06 and 1.07, P = 0.01) with marker D2S118 near GDF8 (2q32.2). The present results gave suggestive evidence that the myostatin pathway might be important for strength phenotypes, and GDF8, CDKN1A, and MYOD1 are potential candidate regions for a further and denser mapping with respect to these phenotypes.

Heritability of somatotype components from early adolescence into young adulthood: a multivariate analysis on a longitudinal twin study

BACKGROUND

Several studies with different designs have attempted to estimate the heritability of somatotype components. However they often ignore the covariation between the three components as well as possible sex and age effects. Shared environmental factors are not always controlled for.

AIM

This study explores the pattern of genetic and environmental determination of the variation in Heath-Carter somatotype components from early adolescence into young adulthood.

SUBJECTS AND METHODS

Data from the Leuven Longitudinal Twin Study, a longitudinal sample of Belgian same-aged twins followed from 10 to 18 years (n = 105 pairs, equally divided over five zygosity groups), is entered into a multivariate path analysis. Thus the covariation between the somatotype components is taken into account, gender heterogeneity can be tested, common environmental influences can be distinguished from genetic effects and age effects are controlled for.

RESULTS

Heritability estimates from 10 to 18 years range from 0.21 to 0.88, 0.46 to 0.76 and 0.16 to 0.73 for endomorphy, mesomorphy and ectomorphy in boys. In girls, heritability estimates range from 0.76 to 0.89, 0.36 to 0.57 and 0.57 to 0.76 for the respective somatotype components. Sex differences are significant from 14 years onwards. More than half of the variance in all somatotype components for both sexes at all time points is explained by factors the three components have in common.

CONCLUSIONS

The finding of substantial genetic influence on the variability of somatotype components is further supported. The need to consider somatotype as a whole is stressed as well as the need for sex- and perhaps age-specific analyses. Further multivariate analyses are needed to confirm the present findings.

Markers for cardiovascular disease in monozygotic twins discordant for the use of third-generation oral contraceptives

Oral contraceptives (OC) modulate the risk for developing cardiovascular (CV) diseases. The aim of this study was to determine whether the use of third-generation OC has an impact on markers of CV disease in genetically identical women. We performed an intrapair comparison in 27 monozygotic twin pairs, one of whom was taking third-generation OC, whereas the other was not using OC. Biometric parameters were ascertained and conventional and 24-h ambulatory blood pressure (BP) was recorded. A fasting blood sample was taken for the measurement of glucose, insulin, proinsulin, lipids, and insulin-like growth factor binding protein-1 (IGFBP-1). Insulin resistance and beta-cell function were calculated by homeostasis model assessment (HOMA). A 24-h urine sample for cortisol was obtained. Third-generation OC use increased 24-h ambulatory systolic and diastolic BP by 5.2 and 3.9 mmHg, respectively (both P=0.0003). There was no effect on glucose, insulin and proinsulin levels, and on HOMA parameters, but the IGFBP-1 levels were markedly raised (P=0.0009). The lipid profile showed a 34% increase in triglyceride levels (P < 0.0001), but also a 7% increase in HDL-cholesterol levels (P=0.037). Use of third-generation OC impacts on CV disease markers in young-adult genetically identical women. Some changes are beneficial (increased HDL-cholesterol), whereas others may be deleterious (increased BP and triglyceride levels) or have unknown effects at this time (increased IGFBP-1 levels).

Influence of weight-bearing exercises on bone acquisition in prepubertal monozygotic female twins: a randomized controlled prospective study

The aim of this study was to determine whether high-impact exercises have an osteogenic effect in 21 prepubertal female monozygotic twins aged 8.7 (SD 0.7) years. Bone mineral density (aBMD), bone mineral content (BMC), bone area, and body composition were derived from DXA. Skeletal maturity was assessed by the Tanner-Whitehouse technique. Anthropometric dimensions (28 dimensions) were measured and also used to derive adiposity and muscularity indices, and information about physical activity was obtained by questionnaire. These measurements were taken before and after the exercise period. The exercise program consisted of high-impact exercises. During 9 months, one girl of each twin pair performed the exercises 3 times a week under supervision of a teacher while their sisters served as control group. At baseline there were no differences between the groups. After 9 months, exercisers (Ex) and controls (Con) had similar increases in height and weight. Significant lower adiposity was observed in the exercise group vs the control group. None of the bone indices differed significantly between the two groups. When the analyses were done for a subgroup of twin pairs (n = 12) who did not participate in high-impact sports during their leisure time, significant differences were obtained for aBMD and BMC of the proximal femur in favor of the exercise group. These results indicate that for prepubertal girls who are not involved in sport activities or who are only involved in low-impact sport activities this exercise program has an osteogenic effect on the proximal femur, but for girls who are already involved in high-impact sports this protocol does not have any additional effect on the bone status.

Prenatal life and post-natal psychopathology: evidence for negative gene-birth weight interaction

BACKGROUND

Many studies suggest that pregnancy and birth complications (PBCs) are environmental risk factors for child psychopathology. However, it is not known whether the effects of PBCs occur independently of genetic predisposition. The current study examined the possibility of gene-environment interaction in a twin design.

METHOD

The East Flanders Prospective Twin Survey prospectively records the births of all twin pairs born in East Flanders, Belgium. The current study included 760 twin pairs aged 6-17 years. Multilevel regression analysis was used to assess the effects of several PBCs collected around the time of birth. Using structural equation modelling, ACE models assuming additive genetic (A), shared environmental (C) and unique environmental (E) influences, were compared in order to examine whether the contribution of genetic factors to parent-rated child problem behaviour varied as a function of exposure to dichotomously and continuously defined PBCs.

RESULTS

A main independent effect of lower birth weight, corrected for gestational age (small for gestational age--SGA), on child problem behaviour was found. In addition, there was an interaction between genetic influence and SGA, in that being smaller for gestational age resulted in less influence of additive genetic factors on individual differences in problem behaviour.

CONCLUSIONS

Results are suggestive of negative gene-birth weight interaction. Children who are SGA are less sensitive to the genetic effects, and those with high genetic vulnerability are less sensitive to the effects of being SGA in bringing about post-natal mental health effects.

Child psychopathology and lower cognitive ability: a general population twin study of the causes of association

Previous work has demonstrated associations between lower cognitive ability and childhood and adult non-psychotic psychopathology. As both cognitive ability (CA) and child psychopathology (CP) are influenced by genetic factors, one explanation for the association is that they are the pleiotropic manifestations of the same underlying genetic factors. The present paper examines three possible causes of the association: additive genetic factors, common environmental factors and individual-specific environmental factors. Three hundred and seventy-six twin pairs from the East Flanders Prospective Twin Survey were examined with the Child Behaviour Checklist and the Wechsler Intelligence Scale for Children-Revised. The cross-twin within-variable, within-twin cross-variable and cross-twin cross-variable correlations were calculated. Using structural equation modelling, bivariate models were fitted. The best fitting model was chosen, based on likelihood and parsimony. The observed phenotypic correlation between CP and CA was -0.19 (95% CI: -0.09, -0.27), with genetic factors accounting for about 84% of the observed correlation. Bivariate model fitting quantified the genetic correlation between CP and CA at -0.27 (95% CI: -0.12, -0.42) and the individual-specific environmental correlation at -0.17 (95% CI: -0.03, -0.31). In children, three different genetic factors may exist: one that solely affects the liability to CP, one that has only an effect on CA and one that influences both CP and CA. While individual-specific environmental factors can influence the liability to both traits, our results suggest that most of the environmental factors that increase the risk of CP do not influence CA and vice versa.