Heritability of daytime cortisol levels and cortisol reactivity in children

Individuals differ widely in cortisol output over the day and cortisol reactivity to challenge, both of which are relevant to disease risk. There is limited evidence concerning the heritability of these differences, so we evaluated the heritability of cortisol levels in the afternoon and cortisol reactivity using a twin design. The study involved 80 monozygotic (MZ) and 70 dizygotic (DZ) same-sex twin pairs aged 11.2 years on average. Salivary cortisol was measured in the afternoon at home before and after playing a computer game. Ratings of excitement and upset were also obtained, and objective task performance was assessed. Salivary cortisol levels averaged 4.08 (S.D. 2.3) nmol/l at pretask baseline, and declined on average over the session to 3.45 (1.9) nmol/l immediately after the tasks and 2.87 (1.6) nmol/l 10min later. There were, however, marked individual differences, with cortisol reactivity (difference between pretask baseline and post-task 1) ranging from +4.53 to -6.23nmol/l. Intra-class correlations for all the cortisol parameters were substantially greater for MZ (range 0.41-0.57) than for DZ (0.11-0.29) twin pairs. Quantitative genetic modelling confirmed significant heritability for pretask baseline cortisol (58%), the two post-task values (60 and 56%), and cortisol reactivity (44%). The study lacked power for assessing sex differences. Subjective reports of excitement were also somewhat heritable, but there was little covariation of cortisol and subjective responses, so genetic influences on covariation could not be tested. These findings indicate that individual differences in children's cortisol levels recorded before tasks and cortisol reactivity to behavioural challenges are influenced by genetic factors.

Effect of the BDNF V166M polymorphism on working memory in healthy adolescents

Brain-derived neurotrophic factor (BDNF) may play a role in modulating memory function and there is growing evidence that the BDNF V166M polymorphism may influence episodic memory in humans. However, previous association studies examining this polymorphism and working memory are inconsistent. The current study examined this association in a large sample of adolescent twin-pairs and siblings (785 individuals from 439 families). A range of measures (event-related potential, general performance and reaction time) was obtained from a delayed-response working-memory task and total association was examined using the quantitative transmission disequilibrium tests (QTDT) program. Analyses had approximately 93-97% power (alpha= 0.05) to detect an association accounting for as little as 2% of the variance in the phenotypes examined. Results indicated that the BDNF V166M polymorphism is not associated with variation in working memory in healthy adolescents.

Association of beta3 adrenergic receptor and peroxisome proliferator-activated receptor gamma 2 polymorphisms with insulin sensitivity: a twin study

OBJECTIVE

To study the effect of beta3 adrenergic receptor (beta3AR) Trp64Arg and peroxisome proliferator activated receptor gamma 2 (PPARgamma2) Prol2Ala polymorphisms on insulin resistance.

METHODS

One hundred and eight dizygotic twin pairs were enrolled in this study. Microsatellite polymorphism was used to diagnose zygosity of twins. Insulin sensitivity was estimated with logarithm transformed homeostasis model assessment (HOMA). PCR-RFLP analysis was performed to detect the variants. As a supplement to the sib-pair method, identity by state (IBS) was used to analyze the association of polymorphisms with insulin sensitivity.

RESULTS

The genotype frequencies of Trp64Trg, Trp64Arg, and Arg64Arg were 72.3%, 23.8%, and 3.9%, respectively, while the genotype frequencies of Prol2Pro, Prol2Ala, and Alal2Ala were 89.9%, 9.6%, and 0.5%, respectively. For beta3AR Trp64Arg the interclass co-twin correlations of Waist-to-hip ratio (WHR), blood glucose (GLU), and insulin (INS), homeostasis model assessment insulin resistance index (HOMA-IR) of the twin pairs sharing 2 alleles of IBS were greater than those sharing 0-1 allele of IBS, and HOMA-IR had statistic significance. For PPARgamma2 Pro12Ala most traits of twin pairs sharing 2 alleles of IBS had greater correlations and statistic significance in body mass index (BMI), WHR, percent of body fat (PBF) and GLU, but there were low correlations of either insulin or HOMA-IR of twin pairs sharing 1 or 2 alleles of IBS. The combined effects of the two variations showed less squared significant twin-pair differences of INS and HOMA-IR among twins sharing 4 alleles of IBS.

CONCLUSIONS

Beta3AR Trp64Arg and PPARgamma2 Pro12Ala polymorphisms might be associated with insulin resistance and obesity, and there might be slight synergistic effects between this two gene loci, and further studies are necessary to confirm this finding.

Amniocentesis for Twin Pregnancies: Is Alpha-Fetoprotein Useful in Confirming that the Two Sacs Were Sampled?

OBJECTIVE

To assess if amniotic fluid alpha-fetoprotein (AFAFP) could be useful to determine if both sacs are sampled during an amniocentesis for twin pregnancies.

METHOD

We reviewed all amniocenteses performed on twin pregnancies over a 5-year period. Inclusion criteria were restricted to pregnancies where both karyotypes and AFAFP were available on each fetus. Pregnancies complicated by fetal anomalies were excluded. The following information was obtained: maternal age, gestational age at the procedure, karyotypes, AFAFP values, pregnancy and neonatal outcome. Placental pathology reports were used to confirm chorionicity. Analysis was performed to evaluate the impact of the fetal gender and chorionicity on the AFAFP values.

RESULTS

260 pregnancies were reviewed. Mean maternal age was 36.9 years (33.6, 40.1). Gestational age at the time of the procedure was 16.2 weeks (14.5, 17.9). Complications included 1.8% of misdiagnosis (discrepancy between karyotype and gender). The difference of AFAFP values between the two fetuses was statistically larger in dichorionic pregnancies than in monochorionic gestations. Fetal gender had no influence on the AFAFP.

CONCLUSION

Amniocentesis in twin pregnancies is associated with a 1.8% risk of misdiagnosis. AFAFP can help to assess the chorionicity of a twin pregnancy. When the difference between the two values is <0.2 MoM and the chorionicity was thought to be dichorionic and the two karyotypes are similar, then failure to sample both sacs should be suspected.

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.

Heritability of testosterone levels in 12-year-old twins and its relation to pubertal development

The aim of this study was to estimate the heritability of variation in testosterone levels in 12-year-old children, and to explore the overlap in genetic and environmental influences on circulating testosterone levels and androgen-dependent pubertal development. Midday salivary testosterone samples were collected on 2 consecutive days in a sample of 183 unselected twin pairs. Androgen-induced pubertal development was assessed using self-report Tanner scales of pubic hair development (boys and girls) and genital development (boys). A significant contribution of genetic effects to the variance in testosterone levels was found. Heritability was approximately 50% in both boys and girls. The remaining proportion of the variance in testosterone levels could be explained by nonshared environmental influences. The relatively high correlation between testosterone levels of opposite-sex dizygotic twins suggests that sex differences in genes influencing variation in testosterone levels have not yet developed in pre- and early puberty. Variance in pubertal development was explained by a large genetic component, moderate shared environmental influences, and a small nonshared environmental effect. Testosterone levels correlated moderately (r = .31) with pubertal development; the covariance between testosterone levels and pubertal development was entirely accounted for by genetic influences.

Environmental influences on family similarity in afternoon cortisol levels: twin and parent-offspring designs

Modest genetic effects on morning, but not late-day, cortisol levels have been established. Environmental demands may influence basal cortisol levels later in the day. Thus, we anticipated that individuals in the same family would have similar afternoon cortisol levels to the extent that they share aspects of their environment. We examined afternoon basal cortisol levels measured across 3 consecutive days in mothers and fathers and in multiple offspring in two separate large and longitudinal studies. Study I involved 321 families with singletons while study II involved 233 families with twins. Modest family similarity was apparent for afternoon basal cortisol levels in both studies. Spouses' cortisol levels were also correlated. Data from study II demonstrated that family resemblance in afternoon cortisol was accounted for by underlying shared environmental factors, but not underlying genetic factors. Shared environment accounted for 62% of the variation in twin afternoon basal cortisol levels and 14% of the variation in parent afternoon basal cortisol levels. We used pooled data from the two studies to examine whether parental depression, socioeconomic status (SES), and offspring sex and age impacted cortisol levels. Female offspring had higher cortisol levels than males, and cortisol decreased with age until about 9 years of age, after which cortisol increased with age. Family similarity persisted after accounting for parental depression, SES, time of day, and offspring sex and age, which suggests that the shared family environment influences parent and offspring stress hormone levels throughout the childhood years.

The use of digit ratios as markers for perinatal androgen action

Since the ratio of the second-to-fourth finger length was first proposed as a marker for prenatal androgen action in 1998, over 100 studies have been published that have either further tested the association between the digit ratio and prenatal androgens, or employed digit ratios as a marker to investigate the association between prenatal androgens and a variety of outcomes, including behavior, fertility, and disease risks. Despite the clear demand for an adult marker of prenatal androgen action and increased use of digit ratios as such a marker, its validity remains controversial. This review (1) evaluates current evidence for the relationship between digit ratios and prenatal androgens (using experimentation with animal models, amniotic testosterone, and congenital adrenal hyperplasia case-control studies), (2) describes opportunities for future validation tests, and (3) compares the potential advantages and disadvantages of digit ratio measures with more established methods for studying the effects of prenatal androgens.