Twin studies and estimates of heritability

The Prenatal Environment in Twin Studies: A Review on Chorionicity

A literature search was conducted to identify articles examining the association of chorionicity (e.g., whether twins share a single chorion and thus placenta or have separate chorions/placentas) and genetics, psychiatry/behavior, and neurological manifestations in humans twins and higher-order multiples. The main aim was to assess how frequently chorionicity has been examined in relation to heritability estimates, and to assess which phenotypes may be most sensitive to, or affected by, bias in heritability estimates because of chorionicity. Consistent with the theory that some chorionicity effects could lead to overestimation and others to underestimation of heritability, there were instances of each across the many phenotypes reviewed. However, firm conclusions should not be drawn since some of the outcomes were only examined in one or few studies and often sample sizes were small. While the evidence for bias due to chorionicity was mixed or null for many outcomes, results do, however, consistently suggest that heritability estimates are underestimated for measures of birth weight and early growth when chorionicity is not taken into account.

Genetic and environmental factors in associations between infant growth and adult cardiometabolic risk profile in twins

BACKGROUND

Accelerated infant growth is associated with an altered, mostly adverse adult cardiometabolic risk profile. The importance of genetic and environmental factors to these associations is unclear.

OBJECTIVE

The objective was to examine the importance of genetic and environmental factors in the associations between infant growth and adult cardiometabolic risk factors (anthropometric characteristics, lipids, insulin sensitivity, leptin, blood pressure, and fibrinogen) in twins.

DESIGN

Cardiometabolic risk factors were assessed in 240 twin pairs (aged 18-34 y) from the East Flanders Prospective Twin Survey. Infant growth was defined as change in weight z score. We regressed intrapair differences in growth during 4 growth windows (0-1, 1-6, 6-12, and 12-24 mo) against intrapair differences in the risk factors in monozygotic and dizygotic twins separately.

RESULTS

Within monozygotic twin pairs only, associations between infant growth and most adult lipids, glucose, leptin, and blood pressure (eg, systolic blood pressure: b = 5.95 mm Hg per change in z score, P = 0.01 in monozygotic twins; b = -1.64, P = 0.82 in dizygotic twins from 12 to 24 mo) were found. Within dizygotic twin pairs only, associations between growth and triglycerides and fibrinogen (eg, fibrinogen: b = 0.07 ln mg/dL per change in z score, P = 0.31 in monozygotic twins; b = 0.79, P = 0.01 in dizygotic twins from 0 to 1 mo) were identified. Most associations showed a detrimental effect of accelerated growth, but beneficial associations were also identified (eg, total-to-high-density-lipoprotein cholesterol ratio: b = -0.22 per change in z score from 1 to 6 mo, P = 0.008 in monozygotic twins).

CONCLUSION

Our data showed that environmental factors play a role in the associations between infant growth and most adult lipids, glucose, leptin, and blood pressure, whereas genetic factors are involved regarding triglycerides and fibrinogen.

The East Flanders Prospective Twin Survey (EFPTS): An Actual Perception

The East Flanders Prospective Twin Survey (EFPTS) is a prospective, population-based registry of multiple births in the province of East-Flanders, Belgium. EFPTS has several unique features: it is population-based and prospective, with the possibility of long-term follow-up; the twins (and higher order multiple births) are recruited at birth; basic perinatal data recorded; chorion type and zygosity established; and since 1969 placental biopsies have been taken and frozen at −20 °C for later determination of genetic markers. The EFPTS is the only large register that includes placental data and allows differentiation of three subtypes of monozygotic (MZ) twins based on the time of the initial zygotic division: the dichorionic–diamnionic pairs (early, with splitting before the fourth day after fertilization), the monochorionic–diamnionic pairs (intermediate, splitting between the fourth and the seventh day post-fertilization), and the monochorionic–monoamnionic pairs (late, splitting after the eighth day post-fertilization). Studies can be initiated taking into account primary biases, those originating ‘in utero’. Such studies could throw new light on the controversy over the validity of the classic twin method, the consequences of early embryological events, and the gene–environment interactions as far as periconceptional and intrauterine environment are concerned.

Can Comparison of MZ- and DZ-Twin Concordance Rates be Used Invariably to Estimate Heritability?

Proponents of the validity of the classical MZ-DZ twin comparison model for calculating heritability claim that the environments influencing MZ and DZ twin individuals are essentially identical. This ‘equal environments assumption’ may or may not be universally true when applied to the analysis of subjective traits. We examined the validity of this assumption as applied to the propensity for smoking cigarettes, reasoning that equality of environments should lead to equal smoking prevalences in MZ and DZ twin individuals. We identified 8 twin populations with data on smoking. We compiled odds ratios (ORs) for ever smoking in MZ and DZ twin individuals in these 8 studies and overall, using a fixed-effects meta-analytic method based on the Mantel-Haenszel procedure. The prevalence of smoking was less in MZ twin individuals than in DZ twin individuals in 7 of 8 studies. The overall OR was 0.86 (95% confidence interval 0.84, 0.89). ORs were virtually unchanged when the analyses were stratified for gender and age, and no differences were found in relation to the location of the study, the date of the study or the birth years of the cohorts. For cigarette smoking, the environments of MZ and DZ twins may not be co-equal. For subjective traits, heritability estimates may be influenced by these unequal environmental factors that differentially affect their development and characteristics in MZ and DZ twins.

The East Flanders Prospective Twin Survey (EFPTS)

The East Flanders Prospective Twin Survey (EFPTS) is a prospective, population-based registry of multiple births in the province of East Flanders, Belgium. EFPTS has several unique features: it is population based and prospective, with the possibility of long-term follow-up; the twins (and higher order multiple births) are ascertained at birth; basic perinatal data recorded; chorion type and zygosity established; and since 1969 placental biopsies have been taken and frozen at −20 °C for later determination of genetic markers. The EFPTS is the only large register that includes placental data and allows differentiation of 3 subtypes of monozygotic (MZ) twins based on the time of the initial zygotic division: the dichorionic–diamnionic pairs (early, before the 4th day after fertilization), the monochorionic–diamnionic pairs (intermediate, between the 4th and the 7th day post fertilization), and the monochorionic–monoamnionic pairs (late, after the 8-day post fertilization). This added a new dimension to didymology (the science of twins; didymos is the Greek word for twin): the timing of MZ twinning. Studies can be initiated taking into account primary biases, those originating in utero. Such studies could throw new light on the controversy over the validity of the classic twin method, the consequences of early embryological events (before and just after implantation of the embryo), the origin of congenital malformations, the sex proportion of multiples, the gene–environment interactions as far as intrauterine environment is concerned, to name but a few.

Digital Dermatoglyphic Heritability Differences as Evidenced by a Female Twin Study

The genetic and environmental contributions to determine digital dermatoglyphic traits were investigated by using female dizygotic and monozygotic twin pairs to estimate heritability indexes (h2). The evaluated sample was composed by 20 monozygotic twin pairs and 13 dizygotic twin pairs. A significant heritability (h2 = 0.65 to 0.96) was observed for 12 dermatoglyphic characteristics (delta indexes and ridge counts for right hand, left hand and both hands, and ridge counts for most individual fingers). A negative correlation between the ridge counts and heritability indexes from individual fingers was found for the left hand, which appears to be associated to a higher arch pattern frequency in most left-hand fingers, since this frequency was negatively correlated with ridge counts and positively correlated with heritability indexes. Heritability indexes of right-hand fingers were positively correlated with loop pattern frequency and negatively correlated with whorl pattern frequency. The low heritability of ridge counts from left thumb, ring and little fingers (h2 = 0.11 to 0.32) indicates a higher chance that the chorion type had an influence in the intra-pair variance of monozygotic twins. Results confirmed the predominant genetic influence on the total ridge count. The heritability indexes varied in up to 8 times between different fingers and its association to ridge counts and pattern frequency was very variable between hands, evidencing that the use of dermatoglyphic traits from individual fingers as indicators of genetic influences to other human traits should consider this variability.

Sequencing genomes: from individuals to populations

The whole genome sequences of Jim Watson and Craig Venter are early examples of personalized genomics, which promises to change how we approach healthcare in the future. Before personal sequencing can have practical medical benefits, however, and before it should be advocated for implementation at the population-scale, there needs to be a better understanding of which genetic variants influence which traits and how their effects are modified by epigenetic factors. Nonetheless, for forging links between DNA sequence and phenotype, efforts to sequence the genomes of individuals need to continue; this includes sequencing sub-populations for association studies which analyse the difference in sequence between disease affected and unaffected individuals. Such studies can only be applied on a large enough scale to be effective if the massive strides in sequencing technology that have recently occurred also continue.

Influence of chorionicity on the heritability estimates of blood pressure: a study in twins

BACKGROUND

A basic assumption of the twin design is that environmental influences including prenatal experiences are equal across twin types. However, the intra-uterine environment may differ according to the chorionicity of the monozygotic twins, which may have biased previous heritability estimates of blood pressure.

OBJECTIVE

The aim of the present study was to assess whether the heritability of blood pressure, derived from measurements in monozygotic and dizygotic twins, differs according to the chorionicity of the monozygotic twins.

METHODS

Conventional and 24-h ambulatory blood pressures were measured in 125 dizygotic twin pairs and in 97 dichorionic and 128 monochorionic monozygotic twin pairs at the age of 18-34 years. The twin sample was drawn from the East Flanders Prospective Twin Survey, in which perinatal data were collected at birth. Intra-pair correlation coefficients were calculated and compared between both types of monozygotic twin pairs. Heritability was estimated from model-fitting and path analysis, based on the dizygotic twins and, respectively, all monozygotic twins and the two subtypes.

RESULTS

Intra-pair correlation coefficients for the various blood pressures, after adjustment for body mass index, ranged from 0.45 to 0.71 in the monozygotic twin pairs and did not differ significantly according to chorionicity. Heritability estimates of blood pressure were between 52 and 64%, and were similar when calculated from dizygotic twins and, respectively, dichorionic and monochorionic monozygotic twins.

CONCLUSIONS

Heritability estimates of conventional and ambulatory blood pressure do not differ significantly according to the chorionicity of the monozygotic twins.

The design, execution, and interpretation of genetic association studies to decipher complex diseases

Genetic association studies are becoming increasingly frequent in the obstetric and gynecologic literature and they are considered central to the deciphering of the genetic basis of complex disease. The purpose, design, execution, analysis, and interpretation of genetic association studies in reproduction are discussed. Frequently used terms are defined (eg, genotype, haplotype, polymorphism, single nucleotide polymorphism, linkage disequilibrium). Guidelines are proposed for the evaluation of reports of genetic association studies (including selection of polymorphisms for study, study design, assay characteristics, sample size, multiple testing, and multivariable analysis). The potential value of this type of investigation in elucidating the mechanisms of disease in reproduction is illustrated.