Skeletal development in monozygotic and dizygotic twins]

The maturation and development of 27 monozygotic and 23 dizygotic twins were studied over a 10 year period with data collected at one year intervals from age 9 to adulthood. In this manner the ossification process was recorded based on X-ray films of the carpal bones. Further information was acquired through anthropometric and somatoscopic data reproduced with standard photography. Sex and phase specific genetic factors influencing the maturation process are presented and analysed.     

Primary osteoarthritis of the hip in monozygotic and dizygotic male twins.

Population and total hip replacement surveys show that primary osteoarthritis of the hip is uncommon in African Americans and rare in Asians, suggesting a genetic basis for this disease. We studied genetic influences on primary osteoarthritis of the hip by estimating monozygotic (MZ) and dizygotic (DZ) twin correlations using a two-stage data collection. A total of 6419 male veteran twins of the NAS-NRC Twin Registry, born between 1917 and 1927, were contacted by telephone (first stage). Telephone interview determined that 2% reported a total hip replacement for arthritis rather than fracture. X-rays of twin pairs in which one twin had undergone total hip replacement were sought and reviewed (second stage), and concordance for primary arthritis was determined based on x-ray diagnosis. Heritability of primary osteoarthritis, Kellgren & Lawrence Grade II and higher, was estimated using a covariance structure analysis for 2-stage data. The best-fitting model included only components for additive genetics and for unique environment. Additive genetics accounted for 53% (95% confidence interval 30-72%) in the liability for self reported hip replacement surgery and unique environment for the remaining 47% (95% confidence interval 28-70%). Additive genetics accounted for 61% (95% confidence interval 18-86%) of the variance in liability for x-ray determined primary osteoarthritis with unique environment accounting for the remaining 39%. These data establish a genetic influence on primary osteoarthritis of the hip in male twins and suggest that further work is indicated to isolate the genes responsible for this disease.     

Differences and similarities in the intra-uterine behaviour of monozygotic and dizygotic twins.

Diagnostic advances have made it possible to use ultrasonograph to assess placentation and therefore zygosity in utero in the case of monochorionic-monozygotic twins. Foetal behaviour of 15 monozygotic and 15 unlike-sexed dizygotic twin pairs was studied serially with ultrasounds from 10 to 22 weeks gestational age. Each twin, regardless of its zygosity, showed individualised behavioural styles. One twin was found to be 'dominant' in the sense of being more active, but less reactive, possibly due to the fewer stimuli being generated by its co-twin. Monozygotic twins, as opposed to dizygotic twins, showed greater similarities in activity and reactivity levels, but were never behaviourally identical and decreased in likeness with increasing age. Our data suggest that so-called identical twins are very similar, but not behaviourally identical, from very early in pregnancy. The unequally shared intrauterine environment contributes to putting each monozygotic twin on a progressively distinct behavioural path.     

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.     

Comparisons of dermatoglyphic patterns in monochorionic and dichorionic monozygotic twins

The data presented here indicate that different influences affect dermatoglyphic pattern development in MC-MZ and DC-MZ twins. Only five of 84 variables had significant mean differences but their clustering suggested a real difference in mean placement of the atd angle. Nineteen of 84 variables had significantly different within-pair mean squares for the two twin types. Larger numbers of twins will be required to obtain accurate estimates of the magnitude of the dermatoglyphic differences between MC-MZ and DC-MZ twins. Studies of dermatoglyphics in MC-MZ and DC-MZ twins are important to the understanding of factors which influence early embryonic development and when better documented may provide a mechanism for retrospectively diagnosing placental type of MZ twins.     

Effects of exercise on excretion rates of urinary free cortisol.

Excretion rates of urinary free cortisol were studied in 20 men assigned to four treadmill exercise groups: walking at 3 mph for 10 min or 30 min, or running at 7.5 mph for 10 min or 30 min. Free cortisol in urine was measured before and 30, 60, and 90 min after exercise, and again on a control day. Patterns of free-cortisol excretions after exercise at 7.5 mph for 10 and 30 min were significantly different from the control day (P less than 0.05) with the largest changes occurring in the 30-min group. Exercise and control patterns were not different for the other two conditions (P greater than 0.05). Within the 7.5 mph-30 min group the postexercise cortisol excretion rates were directly related to the relative intensity of exercise (%VO2max) and the respiratory exchange ratio. It is concluded that changes in free cortisol excretion rates depend on the duration as well as the intensity of exercise.     

Sirenomelia and anencephaly in one of dizygotic twins

The combination of sirenomelia and anencephaly was observed in a stillborn dizygotic twin. A review of the literature revealed no other patients reported to have both conditions. Various explanations concerning the genesis of sirenomelia, and also the combination with anencephaly, are discussed.     

Interaction between the ABO and Rh-Hr blood group systems in pairs of monozygotic (MZ) and dizygotic (DZ) twins

Previous studies have revealed significant deviations of twin pairs' blood group distributions. A comparison between monozygotic (MZ) and dizygotic (DZ) pairs in a sample of 688 twin pairs as to interaction between ABO and Rh indicates that the main contribution to total deviation comes from the MZ pairs, thus confirming a different behaviour of the two twin types in this respect.     

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.