Monozygotic twins discordant for Ullrich-Turner syndrome

Phenotypically Discordant Anomalies in Conjoined Twins: Quirks of Nature Governed by Molecular Pathways?

A multitude of additional anomalies can be observed in virtually all types of symmetrical conjoined twins. These concomitant defects can be divided into different dysmorphological patterns. Some of these patterns reveal their etiological origin through their topographical location. The so-called shared anomalies are traceable to embryological adjustments and directly linked to the conjoined-twinning mechanism itself, inherently located within the boundaries of the coalescence area. In contrast, discordant patterns are anomalies present in only one of the twin members, intrinsically distant from the area of union. These dysmorphological entities are much more difficult to place in a developmental perspective, as it is presumed that conjoined twins share identical intra-uterine environments and intra-embryonic molecular and genetic footprints. However, their existence testifies that certain developmental fields and their respective developmental pathways take different routes in members of conjoined twins. This observation remains a poorly understood phenomenon. This article describes 69 cases of external discordant patterns within different types of otherwise symmetrical mono-umbilical conjoined twins and places them in a developmental perspective and a molecular framework. Gaining insights into the phenotypes and underlying (biochemical) mechanisms could potentially pave the way and generate novel etiological visions in the formation of conjoined twins itself.

Neurofibromatosis type 2 discordance in monozygous twins

Neurofibromatosis type 2 (NF2) is an autosomal dominant condition caused by pathogenic variants in the NF2 gene. The pathogenic variant is either inherited or obtained by de novo mutation, characterised by the presence of schwannomas, meningiomas and ependymomas. Here we report the presence of NF2 in one twin, with bilateral vestibular schwannomas and a pathogenic variant of the NF2 gene identified in both tumour and lymphocytes, while his monozygous brother remains asymptomatic. Imaging of the unaffected twin showed no tumour load and genetic testing via Sanger sequencing and Amplification Refractory Mutation System assay demonstrated low levels of expression of the NF2 variant in lymphocytes. Further testing on non-haemopoietic tissue showed little expression or absence of the pathogenic variant. Given there is no family history and the low level of the variant, we assume the pathogenic variant is a de novo mutation during embryogenesis. De novo mutations have been described as occurring at three possible time points in the creation of monozygous twins with different genetic make-up; prior to the twinning event, as a cause of the event, or after the twinning event. Of these options, we hypothesise that the discordance in the expression of the NF2 variant between these twins is likely due to a mutational event that occurred as a result of either of the latter two possibilities, between which we cannot determine. The pathogenic variant in lymphocytes was likely transferred between the twins through a shared blood supply in utero, and the non-haemopoietic samples that showed low levels of expression, were likely due to the presence of lymphocytic cells. Therefore, we have a discordance between monozygous twins at the NF2 gene.

Placentation in Multiple Births

Outcomes of multifetal pregnancy in prenatal life are markedly affected by chorionicity. Several disease processes are found in monochorionic (MC) twins that do not occur in dichorionic (DC) twins. Improvements in prenatal outcomes will depend on reliable first trimester diagnosis of chorionicity, allowing early monitoring for complications of MC placentation. Particular structures and functions of MC twin placentas affect outcomes and can be targeted for specific treatments, especially in twin-twin transfusion. The causes of severe DC twin fetal growth discordance are clarified. In post-natal life, zygosity is a determining effect in genetic predisposition to many chronic diseases, including neoplasia. Few MC twins know that they are monozygotic (MZ). Few twin researchers realize that MZ twins may be genetically discordant. Abandonment of the word “identical” for MZ twins would assist in clarifying these issues of zygosity, concordance and discordance.

Blood ties: chimerism can mask twin discordance in high-throughput sequencing

Twin studies have long provided a means to separate the contributions of genetic and environmental factors. A recent pioneering report by Baranzini et al. presented an analysis of the complete genomes and epigenomes of a monozygotic (MZ) twin pair discordant for multiple sclerosis. This failed to find any difference between the twins, raising doubts regarding the value of whole-genome twin studies for defining disease susceptibility alleles. However, the study was carried out with DNA extracted from blood. In many cases, the hematopoietic lineages of MZ twins are chimeric due to twin-to-twin exchange of hematopoietic stem cells during embryogenesis. We therefore wondered how chimerism might impact the ability to identify genetic differences. We inferred the blood chimerism rates and profiles of more than 30 discordant twin cases from a wide variety of medical conditions. We found that the genotype compositions of the twins were highly similar. We then benchmarked the performance of SNP callers to detect discordant variations using high-throughput sequencing data. Our analysis revealed that chimerism patterns, well within the range normally observed in MZ twins, greatly reduce the sensitivity of SNP calls. This raises questions regarding any conclusions of genomic homogeneity that might be drawn from studies of blood-derived twin DNA.

Is zygosity or chorionicity the main determinant of fetal outcome in twin pregnancies?

OBJECTIVE

The purpose of this study was to examine whether fetal outcome in twin pregnancies is dependent on zygosity or chorionicity.

STUDY DESIGN

This was a prospective observational study comprised of women with twin pregnancies who attended the fetal medicine unit at St Michael's Hospital, Bristol, Ireland, during the years 1998 to 2000 and who were delivered in hospitals in south west England. After delivery, zygosity was determined with umbilical cord blood with the use of microsatellite markers that were amplified by polymerase chain reaction. Placentae were examined histologically for chorionic type. The perinatal outcomes of 3 groups of monozygotic monochorionic, monozygotic dichorionic, and dizygotic pregnancies were compared with the use of the Mann-Whitney U test and the Fisher's exact test.

RESULTS

All 92 dizygotic and 15 monozygotic dichorionic pregnancies resulted in live births. In 7 of the 39 cases in the monozygotic monochorionic group, either both twins were not live born or delivery occurred <24 weeks of gestation. The gestational age at delivery and birth weight were significantly lower, and there were a greater number of cases with birth weight discordancy of >25% in the monochorionic pregnancies compared with the other 2 groups (P < .05). There were no significant differences in any of the study parameters between the monozygotic dichorionic and dizygotic groups.

CONCLUSION

Fetal outcome in twin pregnancies is related to chorionicity rather than zygosity.

Monozygotic twins concordant for blood karyotype, but phenotypically discordant: A case of “mosaic chimerism”

We report on 23 years old discordant monozygotic (MZ) twins, one with minor anomalies and mental delay, the other one being normal. Both had 46,XX,dup(11)(p12p15)/46,XX mosaicism in blood, with a similar proportion of abnormal cells (respectively, 16% and 17%). However, interphase fluorescence in situ hybridization (FISH) analysis performed on buccal smear and urinary sediment using specific probes located at the duplicated region showed that mosaicism was only present in the abnormal twin, with 68% abnormal cells. We hypothesize that the postzygotic chromosomal rearrangement may have occurred early in one embryo after the twinning event, and the blood mosaicism observed in both twins would have resulted from blood exchanges via placental anastomoses. This hypothesis of chimerism is strongly supported by twin-to-twin transfusion syndrome observed during fetal life of our twins. This case and those previously reported lead us to suggest that blood is particularly unsuitable for cytogenetic investigations of twins.

Invasive diagnostic procedures in twin pregnancies

The rising rate of multiple pregnancies and its association with advanced maternal age has expanded the need for prenatal diagnosis in twins and higher order gestations. The complexity of the invasive diagnostic procedures and the risk of loss of an unaffected twin raise significant clinical, technical and ethical issues. In this review we discuss the specific issues of early scanning, counseling and determination of chorionicity prior to invasive procedures in twins. We present the available data describing the risk associated with these procedures in twins and compare data of fetal loss rate from different studies. We also discuss the issues of fetal blood sampling and late karyotyping in twin pregnancies.

Monozygotic twins with discordant karyotypes: a case report

A monochorionic twin pregnancy had normal male karyotype on chorionic villous sampling. At delivery, one twin presented as morphologically normal, the other as trisomy 21. A twinning event and chromosome division error shortly after conception resulted in monozygotic twins with discordant tissue karyotypes and blood chromosome chimerism for trisomy 21.

Growth of heterokaryotic monozygotic twins discordant for Ullrich-Turner syndrome during the first years of life

The rare observation of different karyotypes in monozygotic (MZ) twins, i.e., heterokaryotic monozygosity, occurs due to chromosomal aberration in one of the twins after separation of the embryos. We report on the differences of heterokaryotic MZ Turkish twins who are discordant for Ullrich-Turner syndrome. Chromosomal analyses from peripheral lymphocytes revealed a 45,X/46,XX mosaicism in both twins. FISH analyses of buccal smears showed 99% of nuclei 45,X in twin A and 98% of nuclei 46,XX in twin B. These results are consistent with a non-mosaic 45,X and 46,XX karyotype, respectively. The girls showed a different growth pattern in the first years. As their genotype should be identical except for the number of X chromosomes, the difference in phenotype may be a pure result of loss of one X chromosome in the affected girl. Special interest is set on the spontaneous and growth hormone induced growth of the twins.

Prenatal diagnosis of female monozygotic twins discordant for Turner syndrome: implications for prenatal genetic counselling

We describe a set of monozygotic (MZ) female twins, one of whom presented with a typical Turner syndrome (TS) phenotype and the other a normal female phenotype. Prenatal fetal ultrasonographic examination showed a monochorial diamniotic pregnancy with a hygroma colli and growth delay in Twin A and no anomalies in Twin B. Karyotypic analysis performed on fetal blood samples demonstrated a 46,XX/45,X (23/2) mosaicism in Twin A and a normal 46,XX chromosome constitution in Twin B. At birth, Twin A presented with a typical TS and Twin B had a normal female phenotype. Postnatal cytogenetic investigation of blood lymphocytes showed the same 46,XX/45,X mosaicism in both twins: 46,XX/45,X (40/7) in Twin A and 46,XX/45,X (40/5) in Twin B. Further investigations at the age of 10 months showed in Twin A a 46,XX/45,X (98/2) mosaicism in lymphocytes and 100% of 45,X (50 analysed cells) in fibroblasts, and in Twin B a normal 46,XX (100 analysed cells) chromosome constitution in lymphocytes but a mild 46,XX/45,X (78/2) mosaicism in fibroblasts. Monozygosity was confirmed by molecular analysis. To our knowledge, this is the first report of prenatal diagnosis of MZ female twins discordant for TS. Review of reported sets of MZ female twins (eight cases) or triplets (one case) discordant for TS shows, as in the present case, that the phenotype correlates better with the chromosomal distribution of mosaicism in fibroblasts than in lymphocytes. In the blood of MZ twins chimerism may modify the initial allocation of the mosaicism. These results suggest that, in cases of prenatal diagnosis of MZ female twins discordant for TS, the phenotype of each twin would be better predicted from karyotype analysis of cells from amniotic fluid than from fetal blood.

Some causes of genotypic and phenotypic discordance in monozygotic twin pairs

The use of the adjective "identical" rather than monozygotic leads to misunderstandings about the biology of monozygotic twinning. Most monozygotic twin pairs are not identical; there may be major discordance for birth weight, genetic disease, and congenital anomalies. These indicate that postzygotic events may lead to the formation of two or more cell clones in the inner cell mass and early embryo that actually stimulate the monozygotic twinning event. There is also evidence that there may be unequal allocation of numbers of cells to the monozygotic twins; this may have widespread implications for the cascade of developmental events during embryogenesis, formation, and vascularization of the placenta. Large-scale zygosity testing at birth could be the template for analysis of twin outcomes and their biologic causes.

Monozygotic twins discordant for the Russell-Silver syndrome

Russell-Silver syndrome (RSS) is a pattern of malformation characterized by intrauterine and postnatal growth retardation, limb asymmetry, triangular face, and hypospadias. We report on a patient, from a triplet pregnancy, who was one of identical male twins discordant for RSS. R.B. was a 710-g male born at 33 weeks of gestation, with hypospadias, chordee, and undescended testes. He had a normal 46,XY karyotype and no renal abnormalities. Female triplet A weighed 1,843 g, and male triplet B weighed 1,920 g. Both had normal physical findings and neonatal period. R.B. was first seen by us at age 6 7/12 years with short stature, triangular and asymmetric face, lower limb length discrepancy, and surgically repaired genital anomalies. Growth hormone testing results were normal. At age 8 7/12 years the brothers appeared physically identical except for size, with a height differential of 114.25 vs. 121.5 cm. Testing to establish biological zygosity was performed using VNTR (variable number tandem repeat) DNA probes YNH24 (D2S44), CMM101 (D14S13), EFD52 (D17S26), TBQ7 (D10S28), and 3'HVR (D16S85), PCR loci MCT118 (D1S80), and HLA-DQ alpha. These data indicate a > 99.99% probability of triplets B and C being monozygotic twins. While most occurrences of RSS are sporadic, familial cases suggesting autosomal dominance have been reported. Three other cases of probable monozygotic twins with RSS have been described. The significance of this confirmation of discordance in determining the cause of RSS is discussed.

Mirror-image dental fusion and discordance in monozygotic twins

A pair of monozygotic twins had similar but not identical dental anomalies. One twin had fusion of deciduous mandibular lateral incisor and canine on the left, with normal dentition on the right; the co-twin had right mandibular incisor/canine fusion, with aplasia of the lateral incisor on the left. These findings are discussed in the context of the related phenomena of situs inversus, mirror-imaging in twins, and gradients of severity of anomalies in the four copies of the mandibular developmental dental field.