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

Monochorionic twins with discordant trisomy 21, another case to remind this uncommon condition and how to deal with

Monozygotic twins discordant for trisomy 21 are rare. We present the twelfth reported case of this uncommon condition undergoing invasive prenatal diagnosis. Dealing with discordant fetal anomalies in monochorionic pregnancy can be challenging for physicians and patients; pros and cons of different invasive procedure options must be discussed with the couple, contending with certain specific peculiarities of this type of pregnancy. Although chorionic villi sampling can be performed earlier, higher risk of misdiagnosis of discordant aneuploidy is reported. Indeed, when heterokaryotic twins are suspected, a two-sampling amniocentesis is recommended, to ensure independent sampling of each fetus and to rule out confined placental mosaicism as well. The couple should be informed of the possible management and the risks for the non-affected twin whether selective reduction is required, and likewise if the pregnancy continues without selective termination.

Monochorionic diamniotic twins of discordant external genitalia with 45,X/46,XY mosaicism

Background

Monozygotic twins with 45,X/46,XY mosaicism, discordant for phenotypic sex, are extremely rare.

Methods

This report describes the clinical findings of a rare case of 45,X/46,XY mosaicism in monozygotic twins with different external genitalia. Single nucleotide polymorphism (SNP) array analysis, performed by collecting DNA from each umbilical cord, showed identical SNPs in the autosomal chromosomes of both fetuses.

Results

Chorionic villus sampling of a 37‐year‐old primigravida carrying monozygotic twins revealed a 45,X/46,XY karyotype. Autopsy of the aborted fetuses revealed a penis and testes on one fetus and a vagina, uterus, and ovaries in the other fetus––which also had severe cystic hygroma. Cell counting using fluorescence in situ hybridization with XY probes (XY‐FISH) showed 20% and 80% abundance of 45,X cells in the internal genitalia, liver, heart, lung, adrenal gland, bone marrow, and spine of the male and female fetuses, respectively.

Conclusion

These results indicated that the fetuses were genetically monozygotic twins and their different degrees of mosaicism may have resulted in different genital phenotypes.

Dysmorphism in One of a Set of Male Twins; Could They have been Identical Twins? A Diagnostic Dilemma

Twinning occurs worldwide, but Nigerian women of the southwest extraction record one of the highest rates in the world. Among the notable risk factors for fraternal twinning is advanced maternal age, which is also an independent risk factor for Down syndrome. Even as morphological characteristics can easily be applied to distinguish identical from fraternal twins, in cases of an associated dysmorphism, in any member of the couplet, it becomes a difficult tool to use, as has been observed in the case of a set of twins who were of the same sex, blood group, hemoglobin genotype, and shared the same placenta.

Monozygotic twins discordant for trisomy 13: counselling and management issues

The diagnosis and management of a heterokaryotypic monochorionic pregnancy, in which one of twins had trisomy 13, is presented. Monozygosity and discordant karyotypes were confirmed by amniocentesis of both the sacs. Radiofrequency ablation of the trisomic twin was successfully performed at 18-weeks gestation and the pregnancy ended at term with the birth of a healthy girl who remains well on follow-up at 12 months of age. We reiterate the importance of early amniocentesis of both the sacs in the presence of discordant fetal abnormalities and consideration of selective fetal termination to optimise the outcome of heterokaryotypic monochorionic twin pregnancies.

Discordant Prenatal Phenotype and Karyotype of Monozygotic Twins Characterized by the Unequal Distribution of Two Cell Lines Investigated by Different Methods: A Review

We present the case of a monozygotic twin pregnancy discordant for phenotype and karyotype. A chorionic villus sample was performed at the 11th week of gestation in a primigravida because of cystic hygroma detected by ultrasound in one twin of a monochorionic, biamniotic pregnancy. Rapid testing by means of quantitative fluorescence polymerase chain reaction and conventional karyotyping, obtained by both short- and long-term culture, revealed a homogeneous monosomy X (45,X). Amniocentesis was performed separately for both twins before termination and showed an homogeneous monosomy X in one sample and a 46,X,del(X)(p11.1) karyotype in the other one. Postmortem fetal tissues culture confirmed the discordant karyotype between the two embryos. Placental samples obtained after termination revealed the cell line which was not detected at chorionic villus sampling. Based on this and previous reports, we suggest that in cases of a phenotypic discordance detected at ultrasound in the first trimester, it is advisable to perform a karyotype analysis on amniocytes because it better reflects fetal constitution rather than chorionic villi or lymphocytes in case of heterokaryotipic monosomy X monochorionic twins.

Differences in Copy Number Variation between Discordant Monozygotic Twins as a Model for Exploring Chromosomal Mosaicism in Congenital Heart Defects

Studies addressing the role of somatic copy number variation (CNV) in the genesis of congenital heart defects (CHDs) are scarce, as cardiac tissue is difficult to obtain, especially in non-affected individuals. We explored the occurrence of copy number differences in monozygotic (MZ) twins discordant for the presence of a CHD, as an illustrative model for chromosomal mosaicism in CHDs. Array comparative genomic hybridization was performed on peripheral blood-derived DNA obtained from 6 discordant MZ twin pairs and on sex-matched reference samples. To identify CNV differences between both twin members as well as potential CNVs in both twins contributing to the phenotype, DNA from each twin was hybridized against its co-twin, and against a normal control. Three copy number differences in 1 out of 6 MZ twin pairs were detected, confirming the occurrence of somatic CNV events in MZ twins. Further investigation by copy number and (epi)genome sequencing analyses in MZ twins, discordant for the presence of CHDs, is required to improve our knowledge on how postzygotic genetic, environmental and stochastic factors can affect human heart development.

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.

Identical but not the same: the value of discordant monozygotic twins in genetic research

Monozygotic (MZ) twins show remarkable resemblance in many aspects of behavior, health, and disease. Until recently, MZ twins were usually called "genetically identical"; however, evidence for genetic and epigenetic differences within rare MZ twin pairs has accumulated. Here, we summarize the literature on MZ twins discordant for Mendelian inherited disorders and chromosomal abnormalities. A systematic literature search for English articles on discordant MZ twin pairs was performed in Web of Science and PubMed. A total number of 2,016 publications were retrieved and reviewed and 439 reports were retained. Discordant MZ twin pairs are informative in respect to variability of phenotypic expression, pathogenetic mechanisms, epigenetics, and post-zygotic mutagenesis and may serve as a model for research on genetic defects. The analysis of single discordant MZ twin pairs may represent an elegant approach to identify genes in inherited disorders.

Lessons from BWS twins: complex maternal and paternal hypomethylation and a common source of haematopoietic stem cells

The Beckwith-Wiedemann syndrome (BWS) is a growth disorder for which an increased frequency of monozygotic (MZ) twinning has been reported. With few exceptions, these twins are discordant for BWS and for females. Here, we describe the molecular and phenotypic analysis of 12 BWS twins and a triplet; seven twins are MZ, monochorionic and diamniotic, three twins are MZ, dichorionic and diamniotic and three twins are dizygotic. Twelve twins are female. In the majority of the twin pairs (11 of 13), the defect on chromosome 11p15 was hypomethylation of the paternal allele of DMR2. In 5 of 10 twins, there was additional hypomethylation of imprinted loci; in most cases, the loci affected were maternally methylated, but in two cases, hypomethylation of the paternally methylated DLK1 and H19 DMRs was detected, a novel finding in BWS. In buccal swabs of the MZ twins who share a placenta, the defect was present only in the affected twin; comparable hypomethylation in lymphocytes was detected in both the twins. The level of hypomethylation reached levels below 25%. The exchange of blood cells through vascular connections cannot fully explain the degree of hypomethylation found in the blood cell of the non-affected twin. We propose an additional mechanism through which sharing of aberrant methylation patterns in discordant twins, limited to blood cells, might occur. In a BWS-discordant MZ triplet, an intermediate level of demethylation was found in one of the non-affected sibs; this child showed mild signs of BWS. This finding supports the theory that a methylation error proceeds and possibly triggers the twinning process.

Variant Klinefelter syndrome 47,X,i(X)(q10),Y and normal 46,XY karyotype in monozygotic adult twins

Klinefelter syndrome (KS; 47, XXY) is characterized by increased body height, hypergonadotrophic hypogonadism, and infertility. We describe a patient with a variant KS (47,X,i(Xq),Y) who has a twin brother with a 46,XY karyotype. Molecular studies showed that the twins were monozygotic. The presence of an isochromosome Xq in one of two monozygotic twins allows precise investigation of its phenotypic effect. The patient was somewhat shorter (3.5 cm) and had a smaller volume of the testes (8 vs. 18 ml) as compared to his twin brother. Furthermore he had increased gonadotrophin levels and an extreme oligoasthenoteratozoospermia (OAT). These data support the view that genes on Xp cause increased body height and genes on Xq cause infertility in KS. To our knowledge this is the first report on a heterokaryotypic monozygotic twin with a variant KS.

Diagnosis and management of heterokaryotypic monochorionic twins

The diagnosis, management, and outcome of six consecutive heterokaryotypic monochorionic twins were evaluated. All suspected cases, based on discordant ultrasound findings, underwent amniocentesis of both sacs. Two cases also had chorionic villous sampling (CVS). Dual amniocentesis was superior to CVS in diagnosing heterokaryotypic monochorionic twins. In four cases, the X-chromosome was involved and autosomal aneuploidy was noted in the others. In five cases, the anomalous twin was selectively reduced by cord coagulation. All pregnancies ended with a phenotypically normal liveborn and all children are developing normally at 1-7 years of age.