The influence of zygosity and chorion type on fat distribution in young adult twins consequences for twin studies.

An adverse intra-uterine environment has been associated with abdominal fat distribution in singletons. Twins often have a low birth weight and a short gestation. Therefore, they may have an increased risk to develop abdominal obesity. Furthermore, monozygotic monochorionic twins (MZ MC) have a larger intra-pair birth weight difference compared to monozygotic dichorionic twins (MZ DC). If adult anthropometry is programmed in utero, this may affect the intra-pair correlations in adulthood and, consequently, also the results from the classic twin method to estimate genetic and environmental influences. In the present study, we compared the absolute values, the intra-pair differences, and the intra-pair correlations of body mass, height, BMI, and abdominal fat distribution of 424 MZ MC, MZ DC and dizygotic (DZ) twin pairs (aged 18-34 yrs). DZ, MZ DC and MZ MC twins did not differ for most anthropometric characteristics. Only MZ women tended (p = 0.03) to accumulate more abdominal fat compared to DZ twins. Overall, the contribution of zygosity and chorion type to adult anthropometry was rather low (< or = 1.7%). Although the intra-pair birth weight difference of MZ MC pairs (10.5% in men, 12.3% in women) was significantly larger compared to that of MZ DC pairs (6.9% and 9.2% resp.), the intra-pair differences in adult anthropometry were similar for both MZ twin types. Also the intra-pair correlations of MZ MC and MZ DC pairs were strikingly alike, suggesting no significant influence of the prenatal environment on adult concordance. In conclusion, the substantial difference in the prenatal environment of MZ MC and MZ DC twins did not result in a difference in intra-pair concordance of adult anthropometry and fat distribution. Therefore, we suggest that the chorion type of MZ twins does not bias the twin design and the estimation of the genetic contribution to adult anthropometry.     

Twin concordance for a binary trait. II. Nested analysis of ever-smoking and ex-smoking traits and unnested analysis of a "committed-smoking" trait

Twin concordance rates for a binary trait can provide information about causes of trait variation. However, if trait prevalence varies with age (or birth cohort) or between the sexes, trait concordance rates will be artificially inflated because of the matching within pairs of twins. Our previous paper showed how to minimize the effects of such confounding by using logistic regression to model trait prevalence as a function of age and sex and that the binary correlation coefficient was useful as a measure of concordance that can be adjusted for trait prevalence. This method is extended here to allow for nested analyses and is applied to the smoking habits of a sample of 3,807 pairs of adult twins. For monozygotic (MZ) twins, the correlation coefficients for the binary trait of "ever-smoking" (males: .50 +/- .04; females: .60 +/- .02) were significantly greater than for dizygotic (DZ) twins (males: .37 +/- .05; females: .31 +/- .04; unlike-sex pairs: .21 +/- .03). For "giving-up smoking," given that both twins were previously smokers, the correlations for MZ twins (males: .37 +/- .07; females: .29 +/- .05) were also greater than for DZ twins (males: .11 +/- .09; females: .26 +/- .08; unlike-sex pairs: .13 +/- .06), although the difference was not statistically significant for females. Current smokers who had been smoking for at least 10 years were arbitrarily defined as "committed-smokers." The binary trait of "committed-smoking" was more strongly correlated in MZ twins (males: .41 +/- .06; females: .41 +/- .04) than in DZ twins (males: .22 +/- .08; females: .18 +/- .05; unlike-sex pairs: .16 +/- .05). These observations suggest that as well as depending on socially determined environmental factors, smoking behavior is influenced by genetic factors and/or by environmental factors unique to the MZ twin environment, which are of particular importance as determinants of "committed-smoking." There is a need for further research to investigate the personal characteristics of "committed-smokers" and to seek intervention strategies that are more suited to the needs of individual smokers.     

A study of lactose absorption capacity in twins

Summary Lactose absorption capacity was determined by lactose tolerance tests with breath hydrogen determination in 102 healthy, adult, Hungarian pairs of twins in order to test monogenic Mendelian inheritance of the absorptive lactase phenotypes, lactose absorber and lactose malabsorber. Of the total, 52 pairs were monozygous (MZ) and 50 dizygous (DZ) twins of indentical sex. All MZ twins were concordant with respect to lactase phenotype. Among DZ twins, the distribution of lactase phenotypes was in agreement with Hardy-Weinberg expectations derived from the frequencies of the hypolactasia gene in DZ and MZ twins, and in the general Budapest population.In the second part of the study, three commonly used methods of lactose tolerance testing, the blood glucose, the blood galactose, and the breath hydrogen tests, were compared in 49 pairs of twins concordant for lactase phenotype. Blood galactose concentration showed the greatest and only significant difference between the intrapair correlation coefficients of MZ and DZ, and no overlap between lactose absorbers and lactose malabsorbers. The intrapair correlation coefficients of peak breath hydrogen concentration in MZ and DZ twins did not significantly differ from zero, but the resolution of lactase phenotypes was satisfactory. Differences in glucose absorption and concentration in lactose absorbers and malobsorbers overlapped considerably, and among lactose absorbers correlation coefficients in DZ were higher than in MZ twins. In MZ and DZ twins, the difference in concordance and constancy of lactose intolerance symptoms was not significant.     

Does higher concordance in monozygotic twins than in dizygotic twins suggest a genetic component?

It is widely regarded that twins can be used as a natural experiment to subject hypotheses to empirical testing regarding the contributions of genetic factors to phenotypic variability in human traits, especially behavioral traits. In genetic epidemiology, a higher concordance rate in monozygotic (MZ) twins than in dizygotic (DZ) twins is often taken as prima facie evidence for a genetic component. While twins studies have been used to estimate the contributions of genetic factors to phenotypic variability in human traits, the corresponding methodology that allows the estimation entails several crucial assumptions. The most critical is that MZ and DZ twins are equally similar environmentally. Although MZ twins are genetically more similar than DZ twins, they are often environmentally more similar. This paper demonstrates that, even in the complete absence of any genetic factor and of any biases, the greater environmental similarity alone in MZ twins can result in higher concordance rate in MZ twins than in DZ twins. This is especially true when there are multiple environmental factors, which may have multiple exposure levels and/or interact strongly, although each of them may be of low risk. This may serve as a sobering antidote to the uncritical reliance on twin studies without examining the validity of the underlying assumptions.     

Contribution of familial factors to the occurrence of cancer before old age in twin veterans

In an earlier report, we evaluated familial factors in deaths from all causes before age 62 among the 31,848 white male twin veterans who were followed during 1946–1978 through the National Academy of Sciences-National Research Council Twin Registry. We now report data for this group on twin concordances and heritabilities of cancer recorded on the death certificate as an underlying or associated cause. The study subjects have a mortality from cancer 0.88 times, and one from all causes 0.84 times, that of U.S. white males [12], but this is very similar to the mortality of other U.S. veterans [9].

Among 11,350 monozygotic (MZ) and 14,450 dizygotic (DZ) individuals in twin pairs alive on January 1, 1946, 1,162 MZ and 1,646 DZ individuals died before January 1, 1979. Cancer was diagnosed for 223 MZ and 323 DZ twins as an underlying or associated cause of death. Among the latter were 176 MZ and 274 DZ pairs with the only death in the pair a cancer death, 10 MZ and eight DZ pairs concordant for cancer, and 12 MZ and 14 DZ pairs in which the first death in the pair from cancer was followed by death of the other twin from another cause. When account is taken of the three MZ and two DZ pairs concordant for lung cancer, most likely related to cigarette smoking, the twin cancer death concordance rates are very low, and they are not appreciably different between the two zygosity groups.

Genetic factors may be important in some specific forms of cancer. However, these data suggest that genetic factors and early familial environment, generally shared by twin-pair members, do not contribute much to mortality from most cancers between 30 and 60 years of age.

     

Behavioral inhibition as a function of relationship in preschool twins and siblings.

Monozygotic (MZ) twins spend more time with each other and are more genetically alike than are dizygotic (DZ) twins or nontwin siblings and therefore probably experience less diversity in their playmates than DZ twins, who in turn may experience less diversity than other-age siblings. Thus MZ twins may be more inhibited than DZ twins, who may in turn be more inhibited than nontwin siblings. To test this, 205 children (42 MZ twins, 94 DZ twins, and 69 nontwin siblings) were assessed in a playroom laboratory during free play with an unfamiliar peer. Children's inhibition was rated based on latency to touch a toy for the first time and amount of time spent interacting with the other child. Additionally, the Child Behavior Checklist (CBCL) withdrawn scale was used to assess inhibition according to parent report. Behavioral ratings showed that MZ and DZ twins did not significantly differ on the inhibition or withdrawn ratings. DZ twins were significantly more inhibited than were nontwin siblings, according to laboratory ratings. Greater inhibition of twins during the free peer play situation can be explained by their relative lack of experience in playing with children who are less genetically and phenotypically like themselves. Parental ratings showed exactly the opposite pattern, suggesting a rater bias effect.     

Genetic control of variation in human gingival fibroblast proliferation rate

Summary Logarithmic proliferation rate (Days 1 to 6) of gingival fibroblasts derived from 15 pairs of monozygotic (MZ) and 9 pairs of dizygotic (DZ) human twins was compared under optimal and suboptimal growth conditions. Cell proliferation rates exhibited considerable variability among strains. For Caucasian donors (13 MZ, 6 DZ pairs) DZ twins demonstrated significantly greater (P<0.01) within-pair variance in cell proliferation rate compared to MZ twins when evaluated under optimal growth conditions. Heritability analysis indicated strong genetic control of proliferation rate of human gingival fibroblasts (HGF) under optimal growth conditions (1.0±0.67), whereas proliferation rate of HGF under suboptimal growth conditions revealed less genetic control (0.42±0.61). These findings emphasize the importance of carefully matching control and test HGF in assays dependent on cellular proliferation. This work was supported by grants DE-06671 and DE-07841 from the National Institutes of Health.     

Sex and genetic differences in hair color changes during early childhood.

Hair color was assessed routinely from three months to six years for children participating in a longitudinal study of twins: 169 female twin pairs, 161 male pairs, and 60 opposite-sex pairs. Age trends, established by sampling only one number of every pair, showed marked changes in hair color for both sexes, but there was a consistent excess of light-haired males and dark-haired females. Within-pair concordance rates were calculated for same-sex pairs whose zygosity had been determined independently through bloodtyping. A high rate of concordance was found for MZ twins at every age in spite of the general change in hair color, indicating a strong genetic influence in the timing of color changes. The results are discussed in terms of accelerated maturation of females, and the need for genetic models of the inheritance of hair color which are age- and sex-specific.     

Twinning and heteropaternity in chimpanzees (Pan troglodytes)

Unlike monozygotic (MZ) twins, dizygotic (DZ) twins develop from separate ova. The resulting twins can have different sires if the fertilizing sperm comes from different males. Routine paternity testing of a pair of same-sexed chimpanzee twins born to a female housed with two males indicated that the twins were sired by two different males. DNA typing of 22 short-tandem repeat (STR) loci demonstrated that these twins were not MZ twins but heteropaternal DZ twins. Reproductive data from 1926-2002 at five domestic chimpanzee colonies, including 52 twins and two triplets in 1,865 maternities, were used to estimate total twinning rates and the MZ and DZ components. The average chimpanzee MZ twinning rate (0.43%) equaled the average human MZ rate (0.48%). However, the chimpanzee DZ twinning rate (2.36%) was over twice the human average, and higher than all but the fertility-enhanced human populations of Nigeria. Similarly high twinning rates among African chimpanzees indicated that these estimates were not artifacts of captivity. Log-linear analyses of maternal and paternal effects on recurrent twinning indicated that females who twinned previously had recurrence risks five times greater than average, while evidence for a paternal twinning effect was weak. Chimpanzee twinning rates appear to be elevated relative to corresponding estimated human rates, making twinning and possibly heteropaternity more important features of chimpanzee reproductive biology than previously recognized.     

Familial factors in early deaths: Twins followed 30 years to ages 51–61 in 1978

Summary Subjects in the National Academy of Sciences-National Research Council Twin Registry of 31,848 male twin veterans were followed for mortality from 1 January 1946, or from the date of entry into military service if that was later, to 31 December 1978. During this time 3,573 deaths occurred among them, 837 due to trauma and 2,712 due to disease.Mortality from all causes for the entire follow-up period was 10.2% among 11,350 monozygotic (MZ) twins and 11.4% among 14,450 dizygotic (DZ) twins. Mortality of veterans is known to be favorable compared to U.S. males. Among U.S. males of the same ages as the two respective twin zygosity groups, a mortality of 13.9% would have been expected during this time period. Observed mortality from trauma was 2.3% for MZ twins and 2.5% for DZ twins, with 3.0% expected in either group. Observed mortality from all disease was 7.9% for MZ twins and 8.8% for DZ twins, with 10.9% expected in either group.For total mortality, the case twin concordance rates, based on individual deaths, were 28.2% among MZ twins and 17.7% among DZ twins. For trauma, respectively by zygosity, these concordance rates were 6.9% and 3.9%. In this sample, familial factors appear to be of little consequence in trauma deaths. For all disease the concordance rates were 30.1% and 17.4%. Estimating heritability of liability to death from disease, as proposed by Edwards (1969), provides values of h ²=r=0.51 for MZ twins, h ²=2r=0.48 for DZ twins, and h ₂=2(r _MZ–r _DZ=0.54 using data for the two zygosity groups combined.Presented in abbreviated form at The Third International Congress of Twin Studies, Jerusalem, Israel, 19 June 1980     

Inheritance of hypodontia in twins

The aim of this study was to establish prevalence of hypodontia in the twin sample and to assess the degree of its heritability. A study was performed in a sample of 96 twin pairs, 38 pairs being monozygotic (MZ) and 58 pairs dizygotic (DZ), from north-west Croatia. The sample included 25.82% of all twins born in the region during a ten-year period. The twin zygosity was determined according to the WHO recommendations (1996). A revised Holzinger's index (Hc') according to Allen was applied to calculate the degree of heritability. Hypodontia was found in 22 out of the total of 192 twins analyzed (11.5%). Among 96 pairs of twins, hypodontia was observed in 17 pairs (7 MZ and 10 DZ pairs). Among the MZ, 4 pairs were found to be concordant for hypodontia, among the DZ one pair only. The heritability index was 0825. A prevalence of hypodontia in twins observed in this study is significantly higher than in the general population. A high index of heritability (Hc' = 0.825) points to a high genetic determination.     

Potential bias regarding birth weight in historical and contemporary twin data bases.

In this study we examine the hypothesis that monozygotic (MZ) twins in historical databases are less discordant for birth weight due to negative selection of severely discordant MZ twins. Furthermore, we test the hypothesis that MZ twins are less discordant for birth weight when comparing a volunteer based twin registry with a population based twin registry, due to selective registration. Data were available on 3927 twin pairs from the volunteer Australian Twin Registry born before 1964, 3059 volunteer twin pairs from the Netherlands Twin Register born 1987-1989 and 454 Belgian twin pairs from The East Flanders Prospective Twin Survey born 1987-1989. Intrapair relative birth weight differences (RBWD) were computed for MZ and dizygotic (DZ) twins from each twin registry. Comparing birth weight differences between MZ and DZ twins provides support for the hypothesis that MZ twins are subject to a negative selection in historical databases. Furthermore, Australian MZ twins have a lower RBWD compared to Dutch MZ twins when corrected for the RBWD of Australian and Dutch DZ twins, indicating circumstances which only affect MZ twins. Our hypothesis that MZ twins are less discordant for birth weight in a volunteer based twin registry compared to a population based twin registry had to be rejected. We suggest that investigators using historical databases to test the fetal origins hypothesis should be aware of this increased likelihood of selective exclusion of individuals with extreme morphometric parameters at time of birth.     

Impact of the genome on the epigenome is manifested in DNA methylation patterns of imprinted regions in monozygotic and dizygotic twins

One of the best studied read-outs of epigenetic change is the differential expression of imprinted genes, controlled by differential methylation of imprinted control regions (ICRs). To address the impact of genotype on the epigenome, we performed a detailed study in 128 pairs of monozygotic (MZ) and 128 pairs of dizygotic (DZ) twins, interrogating the DNA methylation status of the ICRs of IGF2, H19, KCNQ1, GNAS and the non-imprinted gene RUNX1. While we found a similar overall pattern of methylation between MZ and DZ twins, we also observed a high degree of variability in individual CpG methylation levels, notably at the H19/IGF2 loci. A degree of methylation plasticity independent of the genome sequence was observed, with both local and regional CpG methylation changes, discordant between MZ and DZ individual pairs. However, concordant gains or losses of methylation, within individual twin pairs were more common in MZ than DZ twin pairs, indicating that de novo and/or maintenance methylation is influenced by the underlying DNA sequence. Specifically, for the first time we showed that the rs10732516 [A] polymorphism, located in a critical CTCF binding site in the H19 ICR locus, is strongly associated with increased hypermethylation of specific CpG sites in the maternal H19 allele. Together, our results highlight the impact of the genome on the epigenome and demonstrate that while DNA methylation states are tightly maintained between genetically identical and related individuals, there remains considerable epigenetic variation that may contribute to disease susceptibility.     

The Host Genotype and Environment Affect Strain Types of Bifidobacterium longum subsp. longum Inhabiting the Intestinal Tracts of Twins

To investigate the influences of host genotype and environment on Bifidobacterium longum subsp. longum inhabiting human intestines at the strain level, six pairs of twins, divided into two groups (children and adults), were recruited. Each group consisted of two monozygotic (MZ) twin pairs and one dizygotic (DZ) twin pair. Child twins had been living together from birth, while adult twins had been living separately for 5 to 10 years. A total of 345 B. longum subsp. longum isolates obtained from 60 fecal samples from these twins were analyzed by multilocus sequence typing (MLST), and 35 sequence types (STs) were finally acquired. Comparison of strains within and between the twin pairs showed that no strains with identical STs were observed between unrelated individuals or within adult DZ twin pairs. Eight STs were found to be monophyletic, existing within MZ twins and child DZ twins. The similarity of strain types within child cotwins was significantly higher than that within adult cotwins, which indicated that environment was one of the important determinants in B. longum subsp. longum strain types inhabiting human intestines. However, although these differences between MZ and DZ twins were observed, it is still difficult to reach an exact conclusion about the impact of host genotype. This is mainly because of the limited number of subjects tested in the present study and the lack of strain types tracing in the same twin pairs from birth until adulthood.     

Psychophysiologic test performance in normal twins and in a pair of identical twins with essential tremor that is suppressed by alcohol

Summary In a sample of 52 adult male twin pairs (26 MZ, 26 DZ) performance in a series of psychophysiologic tests was measured. The intraindividual variability was determined by repetitive tests in the same subjects. In a number of cases the measured parameters proved relatively stable within the same person; identical twins, however, were no more similar to one another than fraternal twins. In a pair of identical twins with autosomal-dominant essential tremor the degree of the tremor could be effectively reduced by alcohol application.This work was supported by the Deutsche Forschungsgemeinschaft within the SFB 35.     

Fourier analysis of facial profiles of young twins

Twins studies provide a powerful approach to determining the relative contribution of genetics and environment to observed variation. Such studies assume trait differences in monozygous (MZ) twins are due to environmental factors and those in dizygous (DZ) twins are due to both genetic and environmental factors. This study quantitated facial profiles of twins using Fourier equations, determining their value in profile analysis and the assessment of the genetic contribution to facial shape. Standardized profile slide photographs of 79 pairs of 4-6 year-old twins (37 MZ pairs, 42 DZ pairs) were scanned and x and y coordinates were extracted from each profile using sellion and Camper's plane as references. The coordinates were subjected to Fourier analysis and the normalised vertex projection coefficients were studied. The means of the differences between coefficients for MZ co-twins did not differ significantly from that of DZ co-twins, although the DZ group showed higher mean differences in the higher harmonics. Subjective examination of superimposed reconstructions showed wider variation between DZ co-twins than MZ co-twins. Correct classification of twins by discriminant function analysis using Fourier coefficients was similar for both groups (MZ: 70.3%; DZ: 73.8%). Fourier analysis could quantitate facial profiles of young children and differentiate some details, but was unable to discriminate between genetic and environmental influences, and any possible interactions between these influences, on their overall facial profiles at this developmental stage.     

Genetic determination of NOR activity in human lymphocytes from twins

Summary Activity of nucleolar organizer regions (NORs) was studied in cultured blood lymphocytes from 20 monozygotic (MZ) and 20 dizygotic (DZ) twin pairs. The number of Agstained NORs, the degree of staining, and the frequency of acrocentric associations were used as criteria of the NOR activity, the acrocentric chromosomes being identified by G-banding. Analysis of intrapair concordance as well as of intrapair variance showed the number of Ag+NORs and the size of Ag-deposits to be highly heritable traits. Intrapair differences in acrocentric association frequency were not significantly higher in DZ compared with MZ twins.     

A study in twins of the urinary excretion of vanilmandelic acid, 17-keto-steroids, 17-hydroxycorticosteroids, and creatinine]

The 24-hour urinary excretion rates of creatinine, vanilmandelic acid (VMA), 17-hydroxycorticosteroids, 17-ketosteroids were analysed in 30 female twin pairs. For each of these variables, a comparative study based on the breakdown of total variance into interpair and intrapair variance was undertaken, first on identical (MZ) and fraternal (DZ) twins and then on twins who live together or separately. The comparison of test results shows a strong likeness in the MZ group and in the living together group with respect to the urinary excretion of 17-OH. The variability of VMA excretion can be explained by genetic factors while the variability of 17-ketosteroids and creatinine can be explained with reference to environmental factors.     

Utilizing Twins as Controls for Non-Twin Case-Materials in Genome Wide Association Studies

Twin registries around the globe have collected DNA samples from large numbers of monozygotic and dizygotic twins. The twin sample collections are frequently used as controls in disease-specific studies together with non-twins. This approach is unbiased under the hypothesis that twins and singletons are comparable in terms of allele frequencies; i.e. there are no genetic variants associated with being a twin per se. To test this hypothesis we performed a genome-wide association study comparing the allele frequency of 572,352 single nucleotide polymorphisms (SNPs) in 1,413 monozygotic (MZ) and 5,451 dizygotic (DZ) twins with 3,720 healthy singletons. Twins and singletons have been genotyped using the same platform. SNPs showing association with being a twin at P-value < 1 × 10^-5 were selected for replication analysis in 1,492 twins (463 MZ and 1,029 DZ) and 1,880 singletons from Finland. No SNPs reached genome-wide significance (P-value < 5 × 10^-8) in the main analysis combining MZ and DZ twins. In a secondary analysis including only DZ twins two SNPs (rs2033541 close to ADAMTSL1 and rs4149283 close to ABCA1) were genome-wide significant after meta-analysis with the Finnish population. The estimated proportion of variance on the liability scale explained by all SNPs was 0.08 (P-value=0.003) when MZ and DZ were considered together and smaller for MZ (0.06, P-value=0.10) compared to DZ (0.09, P-value=0.003) when analyzed separately. In conclusion, twins and singletons can be used in genetic studies together with general population samples without introducing large bias. Further research is needed to explore genetic variances associated with DZ twinning.