Heritability of Longitudinal Measures of Body Mass Index and Lipid and Lipoprotein Levels in Aging Twins

Body-mass index (BMI), total cholesterol (TC), lowdensity lipoprotein cholesterol (LDL-C), high-density lipoprotein cholesterol (HDL-C), and triglyceride (TG) levels are known to be highly heritable. We evaluated the genetic and environmental relationships of these measures over time in an analysis of twin pairs. Monozygotic (235 pairs) and dizygotic (260 pairs) male twins were participants in the National Heart Lung and Blood Institute Veteran Twin Study, and were followed with three clinical exams from mean age 48 years to mean age 63 years. Structural equation modeling (SEM) with adjustment forAPOEgenotype (a significant contributor to TC and LDL-C) was used to assess longitudinal patterns of heritability. Results indicated a contribution of genetic factors to BMI, TC, LDL-C, HLD-C, and TG. Modest increases over time were observed in the heritability of BMI (from 0.48 to 0.61), TC (from 0.46 to 0.57), LDL-C (from 0.49 to 0.64), and HDL-C (from 0.50 to 0.62), but this trend was not present for TG. There was a corresponding decrease in shared environmental influences over time for these traits, although shared environment was a significant contributor only for HDL-C. Moreover, we observed that genetic influences for all measures were significantly correlated over time, and we found no evidence of age-specific genetic effects. In summary, longitudinal analyses of twin data indicate that genetic factors do not account for a significant proportion of the variation in age-related changes of BMI or lipid and lipoprotein levels.

The impact of chorionicity on umbilical cord acid–base values in twin gestations

OBJECTIVE

To evaluate the impact of chorionicity on inter-twin differences in acid-base status at birth.

METHODS

Records for twin pregnancies delivered at > or = 24 weeks' gestation from 1 January 1990 to 31 June 2000 were reviewed. Collected data included maternal demographics, gestational age, fetal presentation, anesthesia, delivery mode, inter-twin interval, umbilical artery (UA) and venous (UV) acid-base values, Apgar scores and birth weights. The influence of chorionicity on umbilical cord biochemistry was evaluated. (p < 0.05 was considered significant.)

RESULTS

Analysis was carried out in 87 twin pairs (29 monochorionic, MC; and 58 dichorionic, DC). MC and DC twins were similar in maternal age (25.5 vs. 28.2 years), estimated gestational age (33.7 vs. 33.6 weeks), Cesarean delivery (55.2 vs. 52.6%), delivery interval (10 vs. 5 min) and respective birth weights (twin A, 1882 vs. 1981; and twin B, 1828 vs. 1872 g). MC first twins had a higher UA pH (7.31 +/- 0.05 vs. 7.26 +/- 0.08; p = 0.0005) than DC first twins. MC first and second twins had higher UA and UV bicarbonate levels than their DC counterparts (DeltapH = 21.7 +/- 5.1 vs. 18.5 +/- 3.1 mmol/l and 22.0 +/- 3.5 vs. 19.6 +/- 2.5 mmol/l, respectively; p = 0.003). MC twins were more discordant in UA pH than DC twins (DeltapH = 0.043 +/- 0.09 vs. 0.003 +/- 0.07; p = 0.009). MC and DC twins had a similar venous pH (DeltapH = 0.01 +/- 0.06 vs. 0.02 +/- 0.06; p = 0.5).

CONCLUSIONS

There is a significant association between placental chorionicity and umbilical cord biochemistry in twins. Although it is possible that the mechanism of this finding is related to placental angioarchitecture, it is unlikely to be a result of simple mixing of blood volumes between twins. The physiology of underlying processes requires further study.

HDL subspecies in young adult twins: heritability and impact of overweight

The association between abdominal obesity and atherogenic lipid profile emerges from complex interactions of genes and environment. We aimed to explore the heritability and effects of overweight on serum lipid profile (high-density lipoprotein-cholesterol (HDL-C), HDL mean particle size, percentages of HDL(2b, 2a, 3a, 3b, and 3c,) low-density lipoprotein-cholesterol (LDL-C), LDL peak particle size and triglycerides (TGs)) in healthy, young adults. HDL-C, LDL-C, and TG were measured in 52 monozygotic (MZ) and 89 dizygotic (DZ) twin pairs, aged 23-32 years, chosen to represent a wide range of BMIs (17.6-42.9 kg/m2). Of them, 24 MZ and 26 DZ pairs were chosen at random for measurements of HDL mean and LDL peak particle sizes and percentages of HDL subspecies. The heritabilities of the lipid parameters adjusted for BMI were HDL-C 73%, HDL mean particle size 56%, HDL subspecies 46-63%, LDL-C 79%, LDL peak particle size 49%, and TG 64%. Genetic and environmental correlations between BMI and HDL-C, LDL-C, and TG were modest (0.3-0.4). Abdominal overweight (waist circumference>or=94 cm for males and >or=80 cm for females) associated with decreased HDL-C, increased LDL-C, and TG concentrations, smaller HDL mean particle size, lower HDL2b, and higher HDL3c percentages in both genders. Within MZ twins, controlling for genetic influences, within-pair differences in HDL3c percentage were associated with those in waist (r=0.46, P=0.032) and BMI (r=0.51, P=0.013). In conclusion, serum lipid parameters, including LDL peak and HDL mean particle sizes and HDL subspecies distribution are under strong genetic control. Overweight associated with significant lipid profile changes, particularly, small HDL3c increased in overweight independent of genetic influences.

Does having a twin brother make for a bigger brain?

OBJECTIVE

Brain volume of boys is larger than that of girls by approximately 10%. Prenatal exposure to testosterone has been suggested in the masculinization of the brain. For example, in litter-bearing mammals intrauterine position increases prenatal testosterone exposure through adjacent male fetuses, resulting in masculinization of brain morphology.

DESIGN

The influence of intrauterine presence of a male co-twin on masculinization of human brain volume was studied in 9-year old twins.

METHODS

Magnetic resonance imaging brain scans, current testosterone, and estradiol levels were acquired from four groups of dizygotic (DZ) twins: boys from same-sex twin-pairs (SSM), boys from opposite-sex twin-pairs (OSM), girls from opposite-sex twin-pairs (OSF), and girls from same-sex twin-pairs (SSF; n=119 individuals). Data on total brain, cerebellum, gray and white matter volumes were examined.

RESULTS

Irrespective of their own sex, children with a male co-twin as compared to children with a female co-twin had larger total brain (+2.5%) and cerebellum (+5.5%) volumes. SSM, purportedly exposed to the highest prenatal testosterone levels, were found to have the largest volumes, followed by OSM, OSF and SSF children. Birth weight partly explained the effect on brain volumes. Current testosterone and estradiol levels did not account for the volumetric brain differences. However, the effects observed in children did not replicate in adult twins.

CONCLUSIONS

Our study indicates that sharing the uterus with a DZ twin brother increases total brain volume in 9-year olds. The effect may be transient and limited to a critical period in childhood.

Different rates of telomere attrition in peripheral lymphocytes in a pair of dizygotic twins with hematopoietic chimerism

Hematopoietic chimerism in dizygotic twins is due to placental vascular anastomoses and arises when hematopoietic stem cells from one twin home to the bone marrow of the other. We report a case of hematopoietic chimerism in a pair of 27-year-old dizygotic twins who each had a mixture of 46,XX and 46,XY blood lymphocytes, both with 98% male (XY) lymphocytes and 2% female (XX) lymphocytes. Analysis of telomere length by T/C FISH revealed that the female twin generally had longer telomeres than the male twin. Moreover, in the male sibling, the telomeres within the female lymphocytes were shortened to 87% of their original length, while the telomeres within the male lymphocytes were 33% longer in the female sibling. Thus, telomere length attrition in peripheral lymphocytes is determined mainly by the environment of the cell and less by intracellular factors.

Gender differences in plasma ghrelin and its relations to body composition and bone - an opposite-sex twin study

BACKGROUND

Ghrelin, a peptide hormone that plays a role in the regulation of appetite and body adiposity, may also play a role in bone metabolism.

OBJECTIVES

We used the opposite-sex twin model to study associations of plasma ghrelin levels with measures of bone mass and body composition, and determine how such associations were influenced by gender and age.

PATIENTS AND MEASUREMENTS

We measured total plasma ghrelin by radioimmunoassay (RIA) and bone mass/body composition parameters by dual energy X-ray absorptiometry in 79 pairs of opposite sex twins (n = 158 subjects). To examine the effect of age, the study population was divided by median age into two groups: under 51.2 years (38 pairs) and over 51.2 years (41 pairs).

RESULTS

Women had higher plasma ghrelin levels than men (median 1063 vs. 869 ng/l, P < 0.01). Age was a significant predictor of plasma ghrelin levels after adjustment for gender, fat mass and body size. In the older age group, plasma ghrelin levels were inversely associated with fat mass measures in men and women, but there were gender differences in the nature of these associations. In women, plasma ghrelin correlated inversely with body mass index (BMI, r = -0.32), total fat mass (r = -0.30) and fat mass/lean mass ratio (r = -0.42), whereas in men associations with abdominal fat mass (r = -0.31) and fat distribution index (r = -0.33) were observed. Plasma ghrelin was associated with alcohol consumption in older men and women. In the obese subgroup (BMI > 30) no significant gender differences in plasma ghrelin were found. Plasma ghrelin levels were not significantly associated with bone mineral density (BMD) generally, except for hip BMD in younger women (r = -0.39).

CONCLUSION

Plasma ghrelin levels are associated with age, gender, alcohol intake and fat mass measures but only weakly to bone mass measures.

The influence of birth weight and genetic factors on lipid levels: a study in adult twins

Twins can be used to investigate the biological basis for observed associations between birth weight and later disease risk, as they experiencein uterogrowth restriction compared with singletons, which can differ in magnitude within twin pairs despite partial or total genetic identity. In the present study, sixty monozygotic and seventy-one dizygotic same-sex twin pairs aged 19–50 years and eighty-nine singleton controls matched for age, gestational age, sex, maternal age and parity were recruited from an obstetric database. Associations between fasting lipid levels and birth weight were assessed by linear regression with adjustment for possible confounding factors. Twins were significantly lighter at birth but were not significantly different in adult height, weight or lipid levels from the singleton controls. There was a significant inverse association between birth weight and both total and LDL-cholesterol levels among singleton controls (−0·53mmol/l per kg (95% CI −0·97, −0·09),P=0·02 and −0·39mmol/l per kg (95% CI −0·76, −0·02),P=0·04, respectively), but there was no significant association between birth weight and lipid levels in either unpaired or within-pair analysis of twins. The results suggest that thein uterogrowth restriction and early catch-up growth experienced by twins does not increase the risk of an atherogenic lipid profile in adult life.

Confined blood chimerism in monochorionic dizygous (MCDZ) twins

OBJECTIVE

To present the risk factors, prenatal and postnatal implications of monochorionic dyzygotic twinning.

METHODS

We present a case of monochorionic twins diagnosed on first-trimester ultrasound following a 2-embryo transfer in vitro fertilisation pregnancy. Postnatal buccal smears performed as part of a twin research study revealed the twins to be dizygous, and DNA testing on peripheral blood lymphocytes confirmed the presence of blood chimerism.

RESULTS

Despite the monochorionic placentation, buccal swabs revealed discordant results for all four microsatellite loci tested, consistent with dizygosity. Zygosity testing on peripheral blood lymphocytes revealed that, of the nine DNA microsatellite loci tested, three had all four parental alleles represented. This result was concordant between the twins. The diagnosis of blood chimerism was thus confirmed, presumably due to the inter-twin vascular anastomoses of the monochorionic placenta.

CONCLUSION

Monochorionic dizygous twinning is rare, but appears to be more common after assisted reproduction. It is presumed that outer cell mass fusion may occur when two embryos are replaced in close proximity. These pregnancies are not only at risk for the usual complications of monochorionicity, but also have the potential to be heterokaryotypic. Postnatally, the twins may have long-term blood chimerism.

Type 1 collagen marker of bone turnover, insulin-like growth factor, and leptin in dichorionic twins with discordant birth weight

OBJECTIVE

We investigated the relationship between IGF-I-IGF binding protein (IGFBP)-1 and leptin levels with type 1 collagen markers of bone turnover in dichorionic twins with or without discordant birth weight of 20% or greater.

METHODS

Maternal and cord bloods were collected from gestational age-matched dichorionic twins with (n = 16) or without (n = 16) discordant birth weight. The samples were assayed for cross-linked carboxyl terminal telopeptide (ICTP, a marker of bone resorption) and propeptide (PICP, a marker of bone formation) of type I collagen, leptin, IGF-I, and IGFBP-1 by RIA.

RESULTS

The intrauterine growth-restricted (IUGR) twins of the discordant group had higher fetal ICTP (P < 0.001) and IGFBP-1 (P < 0.001) levels, whereas PICP (P < 0.001), IGF-I (P < 0.001), and leptin (P < 0.001) were lower than the cotwins with normal weight (AGA). In contrast, concentrations of IGF-I, IGFBP-1, ICTP, PICP, and leptin were comparable between concordant twin pairs. Leptin levels were positively correlated with PICP (r = 0.61; P < 0.001) and negatively with ICTP (r = -0.57; P < 0.001) in concordant and AGA twins but not in IUGR twins. In IUGR twins, IGF-I had positive association with PICP (r = 0.76; P < 0.001) and negative association with ICTP (r= -0.76; P < 0.001), whereas IGFBP-1 was negatively correlated with PICP levels (r = -0.65; P < 0.01). No such association was found in concordant and AGA twins.

CONCLUSION

These data suggest that IUGR twins had high bone turnover, which is independent of maternal factors and perhaps may be due to altered IGF axis.

Determination of zygosity by questionnaire and physical features comparison in Chinese adult twins

This study reports on the determination of zygosity in Chinese adult twins by simple questionnaire and physical features comparison. The subjects were 511 twin pairs from two cities and their town areas, consisting of 371 monozygotic (MZ) and 140 same-sex dizygotic (DZ) pairs, identified by ABO blood group and multiplex polymerase chain reaction of several polymorphic short tandem repeat markers. The twins themselves responded to 8 questionnaire items, 4 items on twin similarity, and 4 items on the frequency of mistaking one twin for another by parents, relatives, teachers and strangers when they were 6 to 13 years old. Research assistants responded to 20 items regarding twins' physical features at the moment of interview. A parsimonious model established using stepwise logistic regression analysis of the 28 items showed that the total accuracy of zygosity diagnosis was 90.1%. The accuracy was 89.2% when using only the items dealing with the confusion of twins and 85.4% using only similarity. In the questionnaire, 'facial appearance', 'mistaken by teachers' and 'mistaken by strangers' had stronger discriminating power between MZ and DZ twins. Two physical features--'eyelid' and 'middigital hair'--were informative to some extent. There was no statistically significant sex and area difference in the validity of such questionnaire and physical features comparison-based classification. In conclusion, questionnaire-based zygosity assessment in this Chinese adult twin sample could still be regarded as a valid and valuable classification method. Physical features comparison, however, could only provide limited information for zygosity determination.

The heritability of melatonin secretion and sensitivity to bright nocturnal light in twins

The super-sensitivity of the neurohormone melatonin to light in patients with bipolar disorder provides evidence of the circadian nature of the disorder. This response has been proposed as an endophenotype for identifying people at risk of the disorder and guiding investigations of molecular genetic targets. However, before this response is used as an endophenotypic marker, the heritable nature of melatonin sensitivity in the normal population must be established. The aim of this study was to investigate the heritability of nocturnal melatonin secretion and sensitivity to light in monozygotic and dizygotic twins with no psychiatric history. This study investigated overall melatonin levels (between 2000 and 2400 h) and suppression by 500 lx of light (between 2400 and 0100 h) in 20 pairs of twins (nine monozygotic, 11 dizygotic). The results indicate that melatonin secretion is highly heritable with secretion in one twin being a significant predictor of secretion in their twin in both monozygotic and dizygotic pairs. In relation to light sensitivity, genetic loading appears to play a significant role with the greatest concordance between monozygotic twins, followed by dizygotic twins and finally low concordance in unrelated individuals. This provides additional support for the usefulness of melatonin sensitivity to light as a potential endophenotypic marker of bipolar affective disorder.

The heritable determinants of cartilage oligomeric matrix protein

OBJECTIVE

Cartilage oligomeric matrix protein (COMP) is a cartilage matrix macromolecule. The protein is detectable in serum and has been investigated as a biomarker of osteoarthritis (OA). An association between COMP and OA has been shown, yet the precise factors governing serum levels of COMP remain unclear. The aim of this study was to determine whether genetic factors influence serum levels of COMP.

METHODS

A classic twin study was conducted using COMP levels in serum obtained from healthy female twin volunteers. COMP levels were determined by an inhibition enzyme-linked immunosorbent assay method. The heritability of COMP was determined by comparing correlation among 160 monozygotic and 349 dizygotic twin pairs. Data on potential confounding factors, including age, body mass index, and the presence of OA as assessed by hand, hip, and knee radiographs, were included in the analysis.

RESULTS

Serum levels of COMP showed a correlation of 0.72 (95% confidence interval [95% CI] 0.65-0.80) among monozygotic twin pairs and 0.47 (95% CI 0.39-0.55) in dizygotic pairs. This equated to an estimated heritability for COMP of 40% (95% CI 20-60%). Although age and body mass index were found to be significantly associated with COMP in regression analysis, taking the effects of these factors into account did not influence the estimate of heritability.

CONCLUSION

This study showed that heritable factors influence serum levels of the cartilage matrix biomarker COMP. Together with other published data, the results suggest that genetic factors operate at an early stage in the etiologic pathways that influence the development of radiographically discernible OA.

Comparison of serum FSH and Inhibin B levels between adult male dizygotic and monozygotic twins

BACKGROUND

FSH hypersecretion occurs in mothers of dizygotic (DZ) twins. Twinning is inherited via both sexes and transmitted through the female. FSH hypersecretion may thus occur in male DZ twins.

METHODS

We assayed FSH and its counter-regulatory hormone, Inhibin B, in 108 adult male DZ and 100 monozygotic (MZ) twins (as controls) and compared our results to published norms.

RESULTS

Inhibin B was elevated and higher in DZ compared with MZ twins with similar FSH.

CONCLUSION

The normal FSH: Inhibin B endocrine feedback axis is different in adult male DZ twins. This contributes to the theory that the answer to human DZ twinning lies in the actions of FSH and Inhibin, and in their mutual interaction.

Heritability of free and receptor-bound leptin in normal twins

Free and receptor-bound leptin may be regulated by different mechanisms. Genes that influence the concentration of these fractions may have an important functional bearing. We determined circulating leptin receptor concentrations, bound as well as free leptin concentrations, and body composition in 24 monozygotic (MZ) and in 22 dizygotic (DZ) twin pairs. Bound leptin and leptin receptor concentrations were inversely correlated with body fat content. Free leptin concentrations were directly correlated with body fat content. The correlations in age- and sex-adjusted free leptin, bound leptin, and leptin receptor concentrations were higher between MZ twins than between DZ twins. Adjusted heritability (h2) estimates were 0.28 for free leptin, 0.73 for bound leptin, and 0.55 for leptin receptor. The genetic correlation with body fat was −0.58 for the leptin receptor, −0.20 for bound leptin, and 0.93 for free leptin. Our data are consistent with a strong genetic influence on leptin receptor and bound leptin and a weaker genetic influence on free leptin concentrations. The same genes that lower bound leptin and leptin receptor concentrations may increase fat mass or vice versa.