'Identical' twins with discordant karyotypes

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.

Determining chorionicity and amnionicity in twin pregnancies: Pitfalls

Although the accuracy of chorioamnionicity determination in multiple pregnancy is nearly 100%, some pitfalls do exist. These pitfalls may arise from some confusing sonographic appearance or because of certain rare variations of twinning going against the general principles. Pitfalls in chorionicity determination include (1) the disappearance of the twin peak sign with the regression of chorion frondosum and thinning of the intertwin membrane with advancing gestation; (2) fake twin peak sign because of other structures creeping into the intertwin membrane-placental junction; (3) intrauterine septum or synechia being mistaken as a thick intertwin membrane; (4) bipartite placenta in monochorionic twin being misinterpreted as two separate placentas of dichorionic twin; (5) erroneous fetal sex determination in sex chromosome mosaicism, monogenic disorders, and malformed genitalia in one fetus; and (6) rare twinning types such as dizygotic monochorionic twin and sesquizygotic twin. Pitfalls in amnionicity determination are (1) the lack of correlation between the number of yolk sacs and amnionicity and (2) failure to visualize the intertwin membrane because of technical issues.

Discordant congenital heart defects in monochorionic twins: Risk factors and proposed pathophysiology

A six-fold increase in congenital heart defects (CHD) exists among monochorionic (MC) twins compared to singleton or dichorionic twin pregnancies. Though MC twins share an identical genotype, discordant phenotypes related to CHD and other malformations have been described, with reported rates of concordance for various congenital anomalies at less than 20%. Our objective was to characterize the frequency and spectrum of CHD in a contemporary cohort of MC twins, coupled with genetic and clinical variables to provide insight into risk factors and pathophysiology of discordant CHD in MC twins. Retrospective analysis of all twins receiving prenatal fetal echocardiography at a single institution from January 2010 –March 2020 (N = 163) yielded 23 MC twin pairs (46 neonates) with CHD (n = 5 concordant CHD, n = 18 discordant CHD). The most common lesions were septal defects (60% and 45.5% in concordant and discordant cohorts, respectively) and right heart lesions (40% and 18.2% in concordant and discordant cohorts, respectively). Diagnostic genetic testing was abnormal for 20% of the concordant and 5.6% of the discordant pairs, with no difference in rate of abnormal genetic results between the groups (p = 0.395). No significant association was found between clinical risk factors and development of discordant CHD (p>0.05). This data demonstrates the possibility of environmental and epigenetic influences versus genotypic factors in the development of discordant CHD in monochorionic twins.

Monozygotic twins discordant for homologous Robertsonian translocation trisomy 21 of 46, XX, + 21, der (21;21) (q10; q10) in a twin-to-twin transfusion syndrome, case report

Background

Monozygotic twins are nearly identical in genotype and phenotype because monozygotic twins arise from one fertilized oocyte. In all cases of discordant karyotype in monozygotic twins, trisomy 21 accounts for about one in 385,000. Monozygotic twins discordant for Robertsonian translocation trisomy 21 of the der (21;21)(q10;q10), in which the additional chromosome originates from the father is rare.

Case presentation

A 28-year-old parous woman, G3P1A0, came to our institution for a dating scan at 8 weeks of gestation. The transvaginal ultrasound examination demonstrated a monochorionic diamniotic pregnancy. She and her husband were healthy, with no family history of trisomy 21 or other congenital diseases. The ultrasound examination of nuchal translucency thickness was discordant in twins at 13 weeks (twin A, NT 1.4 mm with CRL being 65 mm; twin B, NT 7.8 mm with CRL being 69 mm). At 17^+ 4 weeks, twin A was normal, but ventricular septal defect and the hypoplastic left heart was detected in twin B. The deepest vertical pocket was 18 mm in twin A (oligohydramnios) and 102 mm in Twin B (polyhydramnios). The bladder in twin A was absent. Ultrasound findings indicated TTTS Stage II. Amniocentesis was performed for the two fetuses. The karyotyping results revealed 46, XX in twin A but 46,XX,+ 21,der (21;21)(q10;q10) in twin B.

For twin B, the parents opted for selective fetal termination by radiofrequency ablation. The procedure was uneventful. At 40^+ 5 weeks, twin A was born with a birth weight of 4120 g by vaginal delivery.

Conclusions

The early detection of discordant karyotype and twin-to-twin transfusion syndrome is beneficial to the early intervention. In monozygotic twins with a discordant anomaly, the discordant karyotype should be considered.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12884-021-03587-x.

Monozygotic twins discordant for low-level mosaic trisomy 17 at amniocentesis in a pregnancy with a favorable outcome and a literature review of heterokaryotypic monozygotic twins at amniocentesis

OBJECTIVE

We present a set of twins discordant for low-level mosaic trisomy 17 at amniocentesis, and we review the literature of heterokaryotypic monozygotic twins at amniocentesis.

MATERIALS AND METHODS

We describe a monozygotic twin pregnancy with discordant karyotypes and structural abnormalities. A 22-year-old, primigravid woman underwent amniocentesis at 21 weeks of gestation because of an abnormal maternal serum screening result for Down syndrome. Prenatal ultrasound revealed twin-twin transfusion syndrome but no detectable fetal structural abnormalities. Conventional cytogenetic analysis was applied on cultured amniocytes and parental bloods. Polymorphic DNA marker analysis by quantitative fluorescent polymerase chain reaction (QF-PCR) testing was performed on the DNAs extracted from cultured amniocytes, parental bloods and peripheral bloods of the twins after birth. Interphase fluorescence in situ hybridization (FISH) analysis was performed on buccal mucosal epithelial cells.

RESULTS

Amniocentesis revealed a karyotype of 47,XX,+17 [3]/46,XX [23] in twin A and a karyotype of 46,XX in twin B. The parental karyotypes were normal. QF-PCR confirmed monozygotic twinning and excluded uniparental disomy (UPD) 17. At 35 weeks of gestation, a 1778-g twin A and a 2396-g twin B were delivered smoothly. Both infants had the karyotype of 46,XX in the peripheral bloods and were phenotypically normal except that twin A had preaxial polydactyly on the right hand. Postnatal QF-PCR testing confirmed monozygotic twinning. The infants were doing well at age 2 years and 7 months at follow-ups with normal physical and psychomotor development. FISH analysis on buccal mucosal epithelial cells showed trisomy 17 signals in 4.16% (4/96) cells, compared with 5% (5/101 cells) in normal control.

CONCLUSIONS

Monozygotic twins discordant for low-level mosaic trisomy 17 at amniocentesis without ultrasound abnormalities can have a favorable outcome. Prenatal diagnosis of twins discordant for structural abnormalities and/or chromosomal aberrations should alert the possibility of monozygotic twinning, and QF-PCR testing is useful for rapid determination of zygosity and exclusion of UPD under such a circumstance.

Prenatal and perinatal outcomes of twin pregnancy discordant for one fetus with nuchal translucency above the 95th percentile

OBJECTIVE

To compare prenatal and perinatal outcomes of twin pregnancies in which one twin the nuchal translucency (NT) was above the 95th percentile in dichorionic twins (DCT) and monochorionic twins (MCT).

METHOD

In this retrospective study, 93 twin pregnancies (186 fetuses) in which one twin the NT was above the 95th percentile and the co-twin had normal NT were analyzed. Results of of G-banding and Chromosomal microarray (CMA), ultrasound findings and pregnancy outcomes were reviewed.

RESULTS

Totally, 57 pregnancies (114 fetuses) were DCT and 36 pregnancies (72 fetuses) were MCT. Karyotyping and CMA results shown that clinically significant chromosomal abnormalities were found in 16 fetuses, including 13 aneuploidies, 2 chromosomal mosaic and 1 pathological Copy number variations (CNVs) (14 were DCT and 2 were MCT). Overall, the incidence of fetal chromosomal abnormalities was 12.3% (14/114) in DCT and 2.8%(2/72) in MCT (χ² = 3.932, p = 0.047). Among the cases with normal prenatal diagnosis result, structural abnormalities were found in four fetuses (4.0%, 4/100) in DCT and two fetuses (2.9%, 2/70) in MCT (p > 0.999). There were one intrauterine fetal demises (IUFD) and two miscarry in DCT. One IUFD, three subsequently developed Twin-to-Twin Transfusion Syndrome (TTTS) and four developed selective intrauterine growth restriction (sIUGR) in MCT. Totally, the overall fetal survival rate was 85.1% (97/114) in DCT and 80.6% (58/72) in MCT (χ² = 0.653, p = 0.419).

CONCLUSION

Compared to MCT, the incidence of chromosomal abnormalities in DCT discordant for one fetus with NT above the 95th percentile was higher. The risk of structural abnormalities and the rate of fetal survival for both MCT and DCT was similar.

Management of monochorionic twins discordant for structural fetal anomalies

OBJECTIVE

To review the perinatal management and outcomes of monochorionic twin pregnancies (MC) discordant for congenital anomalies (DCA).

METHODS

Retrospective, study of all MC DCA cases referred to our tertiary referral center from 1997 to 2018. We excluded cases complicated with twin-to-twin transfusion syndrome, twin anemia-polycythemia sequence, twin reversed arterial perfusion sequence or selective intra-uterine growth restriction. Patients were counseled about the possibility of expectant (EM) or interventional management (selective feticide [SF] or termination of the entire pregnancy [TOP]).

RESULTS

One hundred eight of 4157 (2.6%) MC pregnancies were discordant for anomaly. Fifty two of 108 n(48.1%) underwent SF at a mean gestational age of 31.4 ± 5.9 weeks while 52/108(48.1%) opted for EM. Livebirth rate of the healthy co-twin was similar between the two groups (SF: 88.5% vs EM: 82.7%, P = .87). In the SF group, six healthy co-twins (6/52, 11.5%) died 5.3 ± 3.1 days after SF of the abnormal co-twin. In the EM group, in-utero demise of the abnormal twin occurred in 9 of 52 (17.3%) of the cases and was followed by the spontaneous demise of the healthy co-twin in 4 of 9 (44.4%) of these cases.

CONCLUSION

Selective feticide does not seem to significantly alter survival of the healthy co-twin compared to EM.

Chromosomal abnormalities detected by karyotyping and microarray analysis in twins with structural anomalies

OBJECTIVES

To evaluate the incidence and types of chromosomal abnormalities detected in twins with structural anomalies and compare their distribution according to chorionicity and amnionicity and by structural-anomaly type. The added value of chromosomal microarray analysis (CMA) over conventional karyotyping in twins was also estimated.

METHODS

This was a single-center, retrospective analysis of 534 twin pregnancies seen over an 11-year period, in which one or both fetuses were diagnosed with congenital structural anomalies on ultrasound. The ultrasound findings and invasive prenatal diagnostic results were reviewed. Twin pregnancies were categorized as monochorionic monoamniotic (MCMA), monochorionic diamniotic (MCDA) or dichorionic diamniotic (DCDA). Chromosomal abnormalities detected by G-banding karyotyping and/or CMA were analyzed by chorionicity and amnionicity and by structural-anomaly type.

RESULTS

The 534 twin pairs analyzed comprised 25 pairs of MCMA, 112 pairs of MCDA and 397 pairs of DCDA twins. Of the 549 fetuses affected by structural anomalies, 432 (78.7%) underwent invasive prenatal testing and cytogenetic results were obtained. The incidence of overall chromosomal abnormalities in the DCDA fetuses (25.4%) was higher than that in the MCMA (3.7%) and MCDA (15.3%) fetuses. The incidence of aneuploidy was significantly higher in the DCDA group (22.8%) than in the MCMA (0.0%) and MCDA (12.4%) groups. The incidence of chromosomal abnormalities detected in fetuses, with anomalies of the cardiovascular, faciocervical, musculoskeletal, genitourinary and gastrointestinal systems, was higher in the DCDA group than in the MCDA group. In both the DCDA and MCDA groups, hydrops fetalis was associated with the highest incidence of chromosomal abnormality; of these fetuses, 67.6% had Turner syndrome (45,X). Pathogenic copy-number variations (CNVs) undetectable by karyotyping were identified by CMA in five (2.0%; 95% CI, 0.3-3.7%) DCDA fetuses. No pathogenic CNVs were found in MCMA and MCDA twins.

CONCLUSIONS

Dichorionic twins with structural anomalies have a higher risk of chromosomal abnormalities, especially aneuploidies, than do monochorionic twins. The incremental diagnostic yield of CMA over karyotyping seems to be lower (2.0%) in twins than that reported in singleton pregnancy. Copyright © 2019 ISUOG. Published by John Wiley & Sons Ltd.

Gene expression profiling of amniotic fluid mesenchymal stem cells of monozygotic twins discordant for trisomy 21

Down syndrome is one of the most common chromosomal disorders and yet our understanding about the dysregulated genes in this disease is limited. Through this case study, we investigated the gene expression profile of primary amniotic fluid mesenchymal stem cells (AFMSCs) isolated from the amniotic sac of monozygotic twins discordant for trisomy 21 with one fetal hydrops at 17 weeks of gestation. AFMSCs were cultured to analyze the gene expression profiles for the human transcriptome array. Gene ontology was used to evaluate dysregulated gene functions. Total 25,799 genes were identified such that 65 were up-regulated (0.25%) and 111 were down-regulated (0.43%) with a log₂ fold change trisomy 21/euploidy (log₂ [FC]) > 1, p < 0.01). 16 genes were selected and verified by qRT-PCR, which showed compatible result with transcriptome array. At the chromosome level, chromosome 21 was found to carry the highest percentage of up-regulated genes (2.13%, 7/329 genes) with the highest mean log₂ [FC] (0.23, p < 10^-5), particularly on 21q22.3. There were eight segments with significant mean log₂ [FC] on chromosomes 1, 6, 11, and 21 for upregulation, and on chromosomes 16, 17, and 19 for downregulation, indicating a pattern of dysregulated genes clustering in domains along the genome. Gene ontology showed the identified genes associated with extracellular matrix organization (11 genes, p = 5.1 × 10^-6) and central nervous system development (8 genes, p = 6.0 × 10^-5). Using transcriptome analysis of the AFMSCs of monozygotic twins discordant for trisomy 21, we report the dysregulated genes involved in Down syndrome, their predominance on chromosome 21, and the cluster pattern on the whole genome.

Pervasive Inter-Individual Variation in Allele-Specific Expression in Monozygotic Twins

Despite being developed from one zygote, heterokaryotypic monozygotic (MZ) co-twins exhibit discordant karyotypes. Epigenomic studies in biological samples from heterokaryotypic MZ co-twins are of the most significant value for assessing the effects on gene- and allele-specific expression of an extranumerary chromosomal copy or structural chromosomal disparities in otherwise nearly identical germline genetic contributions. Here, we use RNA-Seq data from existing repositories to establish within-pair correlations for the breadth and magnitude of allele-specific expression (ASE) in heterokaryotypic MZ co-twins discordant for trisomy 21 and maternal 21q inheritance, as well as homokaryotypic co-twins. We show that there is a genome-wide disparity at ASE sites between the heterokaryotypic MZ co-twins. Although most of the disparity corresponds to changes in the magnitude of biallelic imbalance, ASE sites switching from either strictly monoallelic to biallelic imbalance or the reverse occur in few genes that are known or predicted to be imprinted, subject to X-chromosome inactivation or A-to-I(G) RNA edited. We also uncovered comparable ASE differences between homokaryotypic MZ twins. The extent of ASE discordance in MZ twins (2.7%) was about 10-fold lower than the expected between pairs of unrelated, non-twin males or females. The results indicate that the observed within-pair dissimilarities in breadth and magnitude of ASE sites in the heterokaryotypic MZ co-twins could not solely be attributable to the aneuploidy and the missing allelic heritability at 21q.

Pitfalls in assessing chorioamnionicity: novel observations and literature review

Accurate diagnosis of chorioamnionicity in multiple pregnancies is the key to appropriate clinical management of multiple gestation. Although prenatal ultrasound assessment of chorioamnionicity is well established and highly accurate if performed in early pregnancy, exceptions and artifacts arise from anatomic variations in multiple pregnancies and unusual sonographic features do exist. We have summarized our own experiences and reports from the literature on these pitfalls as follows: (1) discordant fetal sex in monochorionic pregnancies due to sex chromosome abnormalities, genital malformation in 1 fetus, or dizygotic twins forming a monochorionic placenta; (2) separate placental masses in monochorionic pregnancies due to bipartite placenta; (3) false-negative and false-positive λ sign can arise for various reasons, and in partial monochorionic/dichorionic placentas both T and λ sign may co-exist; (4) intrauterine synechia appearing as a thick and echogenic intrauterine septum may lead to erroneous diagnosis of dichorionic twins; and (5) errors in ascertaining amnionicity by the visualization of thin intertwin amniotic membranes and the number of yolk sacs. The ultrasound techniques to reduce inaccuracy in prenatal determination of chorioamnionicity and the use of single nucleotide polymorphisms based on noninvasive prenatal test to determine zygosity are also reviewed.

Monozygotic twins discordant for trisomy 21: Discussion of etiological events involved

OBJECTIVE

To elucidate the etiologies of discordant trisomy 21 in monozygotic twin pregnancy.

CASE REPORT

A monochorionic diamniotic twin pregnancy with hydrops and cleft lip (twin 1) found in one fetus presented at gestational age of 17 weeks. Amniotic fluid karyotyping showed nonmosaic trisomy 21 in twin 1 (47, XY, +21 [20]) and a normal karyotype in twin 2 (46, XY [20]). Short tandem repeat (STR) polymorphism markers revealed that the two fetuses were monozygotic, and the two chromosomes 21 were maternal isodisomy in the trisomy fetus. The chromosomal constitution of placentas in the territory of trisomy 21 cotwin was 47, XY, +21 [20] and was a mosaic 47, XY+21 [12]/46, XY [8] in the normal karyotyped twin.

CONCLUSION

Our case of monozygotic twin with discordant trisomy 21 might start with a prezygotic maternal meiosis II non-disjunction error-caused trisomy 21 zygote, and after twinning, one remained trisomy 21, and the other twin underwent trisomy rescue and became a mosaic trisomy 21 in morula or early blastocyst stage before the formation of pre-embryo, which subsequently resulted in mosaic trisomy 21 of the placental tissue and normal chromosomal constitution of the fetus.

Whole-exome sequencing analysis in twin sibling males with an anterior cruciate ligament rupture

Familial predisposition is among the major genetic risk factors for non-contact musculoskeletal tissue injuries. Personal genome sequence shows that different polymorphism profiles may account for the number and the degree of injuries and the recovery time. Genotyping studies allow investigation into genome factors with potential impact on pathogenesis of non-contact ligament injuries. We have studied a family with twin sibling males surgically diagnosed of an anterior cruciate ligament non-contact rupture and non-affected progenitors (father and mother) were subjected to whole exome sequencing (WES) analysis. WES analysis previously carried out on 16 individuals, without ACL injury medical records, were also included in this study for single nucleotide variants (SNVs) and small insertions and deletions detection (indels), variant filtering and to prioritize variants relative to the disease. WES analysis to identify SNVs and indels was performed using open web-based bioinformatics tools. A set of 11 new variants shared by family members can be associated to ACL non-contact injury, including SerpinA11, ARSI, NOCHT4, EPB41, FDFT1, POMC, KIF26A, OLFML2B, ATG7, FAH and WDR6. All of them, except ATG7 and WDR6, have shown a damaging predictive pattern by combinatorial standard predictive scores. In combination to the identified SNVs of EPB41 and SerpinA11 genes, ACTL7A gene showed a predicted deleterious variant reinforcing the idea these variants impact on of fibroblast-like cells deformability and ECM misbalance, Differential gene expression and RNA sequencing analysis will help to understand the combined participation of these protein coding genes in ACL non-contact injuries.

Screening and Testing in Multiples

The choice of screening or invasive procedure in twin pregnancies is a personal choice of whether the patient wishes to take a small risk of having a baby with a serious disorder versus a small risk of having a complication because she wishes to avoid that. How to interpret such risks has profound effects on the perceived value of techniques, either leading to a decision to screening or going directly to chorionic villus sampling. There are profound issues surrounding the data and the interpretation of the data. No single short review can exhaustively examine all of the issues.

Whole genome and exome sequencing of monozygotic twins discordant for Crohn's disease

Background

Crohn’s disease (CD) is an inflammatory bowel disease caused by genetic and environmental factors. More than 160 susceptibility loci have been identified for IBD, yet a large part of the genetic variance remains unexplained. Recent studies have demonstrated genetic differences between monozygotic twins, who were long thought to be genetically completely identical.

Results

We aimed to test if somatic mutations play a role in CD etiology by sequencing the genomes and exomes of directly affected tissue from the bowel and blood samples of one and the blood-derived exomes of two further monozygotic discordant twin pairs. Our goal was the identification of mutations present only in the affected twins, pointing to novel candidates for CD susceptibility loci. We present a thorough genetic characterization of the sequenced individuals but detected no consistent differences within the twin pairs. An estimate of the CD susceptibility based on known CD loci however hinted at a higher mutational load in all three twin pairs compared to 1,920 healthy individuals.

Conclusion

Somatic mosaicism does not seem to play a role in the discordance of monozygotic CD twins. Our study constitutes the first to perform whole genome sequencing for CD twins and therefore provides a valuable reference dataset for future studies. We present an example framework for mosaicism detection and point to the challenges in these types of analyses.

Electronic supplementary material

The online version of this article (doi:10.1186/1471-2164-15-564) contains supplementary material, which is available to authorized users.

Monozygotic twins discordant for 18q21.2qter deletion detected by array CGH in amniotic fluid

Discordant chromosomal anomalies in monozygotic twins may be caused by various timing issues of erroneous mitosis and twinning events. Here, we report a prenatal diagnosis of heterokaryotypic monozygotic twins discordant for phenotype. In a 28-year-old woman, ultrasound examination performed at 26 weeks of gestation, detected intrauterine growth restriction and unilateral cleft lip and palate in twin B, whereas twin A had normal fluid, growth and anatomy. Molecular karyotyping in twin B identified a 18q21.2qter deletion, further confirmed by FISH analysis on amniocytes. Interestingly, in twin A, cytogenetic studies (FISH analysis and karyotype) on amniocytes were normal. Genotyping with microsatellite markers confirmed the monozygosity of the twins. At 32 weeks of gestation, selective termination of twin B was performed by umbilical cord coagulation and fetal blood samples were taken from the umbilical cord in both twins. FISH analyses detected mosaicism in both twins with 75% of cells being normal and 25% harboring the 18qter deletion. After genetic counseling, the parents elected to terminate the second twin at 36 weeks of gestation. In postmortem studies, FISH analyses revealed mosaicism on several tissues in both twins. Taking into account this observation, we discuss the difficulties of genetic counseling and management concerning heterokaryotypic monozygotic twins.

Behavior genetics and postgenomics

The science of genetics is undergoing a paradigm shift. Recent discoveries, including the activity of retrotransposons, the extent of copy number variations, somatic and chromosomal mosaicism, and the nature of the epigenome as a regulator of DNA expressivity, are challenging a series of dogmas concerning the nature of the genome and the relationship between genotype and phenotype. According to three widely held dogmas, DNA is the unchanging template of heredity, is identical in all the cells and tissues of the body, and is the sole agent of inheritance. Rather than being an unchanging template, DNA appears subject to a good deal of environmentally induced change. Instead of identical DNA in all the cells of the body, somatic mosaicism appears to be the normal human condition. And DNA can no longer be considered the sole agent of inheritance. We now know that the epigenome, which regulates gene expressivity, can be inherited via the germline. These developments are particularly significant for behavior genetics for at least three reasons: First, epigenetic regulation, DNA variability, and somatic mosaicism appear to be particularly prevalent in the human brain and probably are involved in much of human behavior; second, they have important implications for the validity of heritability and gene association studies, the methodologies that largely define the discipline of behavior genetics; and third, they appear to play a critical role in development during the perinatal period and, in particular, in enabling phenotypic plasticity in offspring. I examine one of the central claims to emerge from the use of heritability studies in the behavioral sciences, the principle of minimal shared maternal effects, in light of the growing awareness that the maternal perinatal environment is a critical venue for the exercise of adaptive phenotypic plasticity. This consideration has important implications for both developmental and evolutionary biology.

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.

Monozygotic twins discordant for trisomy 13

Monozygotic twins with discordant karyotypes are rare. We report a case of monozygotic twins discordant for trisomy 13 by amniocyte karyotypes. Ultrasound revealed multiple congenital anomalies in Twin A (47,XY,+13), none in Twin B (46,XY), and monochorionic-diamniotic placentation. Zygosity testing performed both prenatally and after birth supported monozygosity. Twin A died in the neontal period. Twin B survived and had normal physical examination, but peripheral blood karyotype revealed 20% mosaicism for trisomy 13. Monochorionic-diamniontic placentation with vascular anastomoses was confirmed by pathological examination. In this paper, we discuss the various mechanisms by which monozygotic twins may have discordant karyotypes. The surviving twin, structurally and developmentally normal at 6 months of age, will be monitored for potential complications of uniparental disomy of chromosome 13 and trisomy 13 mosaicism.

Discordance in Nuchal Translucency Measurements in Monochorionic Diamniotic Twins as Predictor of Twin-to-Twin Transfusion Syndrome

Objective:Discordance in nuchal translucency measurements in monochorionic diamniotic twin pregnancies was assessed as predictor of Twin-to-Twin Transfusion Syndrome.Study Design:A total of 61 monochorionic diamniotic twins were enrolled. Nuchal translucency (NT) and crown-rump-length (CRL) discordance was calculated as the percentage of delta NT and CRL (absolute difference NT/CRL fetus 1 and fetus 2) of the largest measurement, and correlated with subsequent development of Twin-to-Twin Transfusion Syndrome.Results:Twin-to-Twin Transfusion Syndrome developed in 14/61 (23%) of cases. A NT discordance of 20% or more best predicted Twin-to-Twin Transfusion Syndrome development, with a sensitivity of 64% and a specificity of 78%. We found a positive predictive value of 50% and negative predicted value of 86% for Twin-to-Twin Transfusion Syndrome development if NT discordance was 20% or more.Conclusion:NT discordance of more than 20% in monochorionic diamniotic twins is associated with an increased risk for subsequent development of Twin-to-Twin Transfusion Syndrome, and with earlier presentation of symptoms.

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.

Monozygotic twins discordant for trisomy 18

We report on the pre- and postnatal cytogenetic, molecular genetic and clinical findings in monochorionic-diamniotic twins discordant for trisomy 18. Structural anomalies were identified in one of the twins on prenatal ultrasound examination at 20 weeks' gestation and sampling of amniotic fluid from both sacs was performed for karyotyping. This revealed trisomy 18 in the twin with abnormalities and a normal karyotype in the other twin. Elective Cesarean section was performed at 31 + 5 weeks and the aneuploid twin died shortly after delivery. The surviving twin showed low-grade mosaicism for trisomy 18 on postnatal analysis but has shown normal development. For prenatal diagnosis in monochorionic-diamniotic twin pregnancy the sampling of both amniotic sacs is recommended, especially if one twin has structural anomalies on ultrasound scan.

Why are monozygotic twins different?

Although popularly designated as "identical", monozygotic (MZ) twins are rarely identical. Much has been speculated on the origin of MZ twins and several theories have been proposed. Post-fertilization events, such as chromosomal mosaicism, skewed X-inactivation and imprinting mechanisms, as well as other epigenetic mechanisms are responsible for the differences between MZ twins. Numerous discordant MZ twins have been reported including discordance for lateral asymmetry, major malformation, growth and intrauterine death of the co-twin. This discrepancy may have long-term implications on complex diseases and their predisposition, organ transplantation and interpretation of twin-based studies. We reviewed the genotypic and phenotypic differences between MZ twins and discuss their main causes.

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.

Monozygotic twins with discordant karyotypes following preimplantation genetic screening and single embryo transfer: case report

PURPOSE

to report a case of monozygotic monochorial diamniotic twins with discordant karyotypes.

METHODS AND RESULTS

the pregnancy was achieved following a treatment cycle with intracytoplasmic sperm injection (ICSI) and preimplantation genetic screening (PGS) for chromosomes X, Y, 13, 16, 18, 21, 22. One embryo euploid for studied chromosomes was transferred. Prenatal ultrasonography revealed monozygotic twins. One fetus had growth retardation, multiple organ abnormalities and polyhydramnion. The other twin had normal ultrasound appearance. Delivery on week 29 of gestation resulted in the birth of two females, a stillborn twin with karyotype 45,XX,-13[12]/46,XX,r(13)[3] and a healthy twin with normal karyotype.

CONCLUSIONS

the discordance in the twins' karyotypes originated from a mosaic embryo. Structural chromosomal abnormality of the affected twin could not be revealed using standard PGS investigation. Embryo splitting occurred probably due to apoptotic process in an early stage of embryo development. Apoptosis represents one of the possible mechanisms which can explain the embryo twinning process globally.

Difficult diagnosis and management of an heterokaryotypic monochorionic twin pregnancy with discordant fetal sex and 45,X/47,XYY karyotypes

We report twins for whom ultrasound examinations revealed a Turner syndrome in the female fetus and a normal male fetus. A selective pregnancy termination was decided on the female fetus with hydrops. The death of both twins called in question the chorionic diagnosis. Amniotic fluid cytogenetic analysis revealed a 45,X karyotype in the female twin and a 47,XYY karyotype in the male twin. Molecular cytogenetic analysis on genital and renal cells showed different levels of 45,X/47,XYY mosaicism in both twins; molecular analysis on the amniocytes showed monozygosity. Monozygotic twins with discordant sex are very rare. This study showed the difficult diagnosis and management of a monochorionic twin pregnancy with discordant fetal sex.

Monozygotic twins discordant for trisomy 21 and maternal 21q inheritance: a complex series of events

We report on a monochorionic/diamniotic twin pregnancy discordant for trisomy 21. Amniocentesis (at 13(5/7) weeks) was performed following ultrasound signs of hydrops and cystic hygroma in twin 1 (T1). Prenatal karyotype showed non-mosaic trisomy 21 in T1 (47,XX,+21[7]), and low-grade mosaic trisomy 21 in twin 2 (T2) (47,XX,+21[2]/46,XX[19]). Post mortem examination of fetal skin, kidneys and lungs confirmed trisomy 21 in T1 (47,XX,+21[548]) and the placenta (47,XX,+21[200]). T2 had a normal karyotype (46,XX[648]). Analysis of microsatellite polymorphisms in multiple samples from the placenta, hand, lungs, kidneys and the umbilical cords of both twins confirmed monozygosity for all loci tested, and trisomy 21 in T1. Unexpectedly, T1 and T2 inherited different maternal alleles for markers of the most distal 4 Mbp of 21q. At least four successive events are needed to explain the genetic status of both twins and include maternal MI premature chromatids separation or maternal II meiotic nondisjunction and post-zygotic events such as, chromosome rescue, nondisjunction, an/or recombination.

Dichorionic triamniotic triplet pregnancy with monozygotic twins discordant for trisomy 13 after preimplantation genetic screening: case report

OBJECTIVE

To report the first dichorionic triamniotic triplet pregnancy discordant for trisomy 13 after in vitro fertilization (IVF) treatment with preimplantation genetic screening (PGS).

DESIGN

Case report.

SETTING

Private IVF center.

PATIENT(S)

A 40-year-old para 1+6 woman.

INTERVENTION(S)

IVF combined with PGS for chromosomes 13, 16, 18, 21, and 22, resulting in the transfer of two embryos.

MAIN OUTCOME MEASURE(S)

Prenatal fetal ultrasonography revealed a dichorionic triamniotic triplet pregnancy. An amniocentesis, performed at 15-weeks' gestation, confirmed that the singleton and one monozygotic twin were normal but the other monozygotic twin was trisomy 13.

RESULT(S)

After diagnosis and counseling, selective termination of the trisomy 13 monozygotic twin was performed at 16 weeks and 4 days. At 18 weeks and 4 days the co-twin died. A healthy boy was delivered by elective caesarean section at 36-weeks' gestation.

CONCLUSION(S)

Assisted reproductive techniques that breach the embryo's zona pellucida such as assisted hatching and PGS embryo biopsy increase the incidence of monozygotic twins. Due to high levels of mosaicism in human preimplantation embryos, PGS cannot ensure that embryos diagnosed as normal and selected for transfer do not contain abnormal cells. Hence, further reports of discordant monozygotic twins following PGS are expected, emphasizing the need for appropriate counseling of patients wishing to embark on an IVF/PGS treatment cycle.

Application of QF-PCR for the prenatal assessment of discordant monozygotic twins for fetal sex

OBJECTIVE

To establish the utility of quantitative fluorescent polymerase chain reaction (QF-PCR) in order to determine the zygosity of multiple pregnancies, as well as to define the origin of the most frequent aneuploidies in amniotic fluid samples.

METHODS

We describe the case of a monochorionic (MC) diamniotic (DA) pregnancy with phenotypically discordant twins (nuchal cystic hygroma and non-immune hydrops in twin A and no anomalies in twin B). QF-PCR was performed for rapid prenatal diagnosis in uncultured amniocytes and subsequently in cultured cells. Polymorphic markers for chromosomes X, Y, 13, 18 and 21 were used for determination of zygosity as well as sex chromosome aneuploidy.

RESULTS

Twin A showed a Turner Syndrome (TS) mosaicism pattern by QF-PCR in uncultured amniocytes. The monozygotic origin of the pregnancy was determined. Interphase fluorescence in situ hybridization (I-FISH) in this sample showed a mosaicism X0/XY (83/17%). Cytogenetic analysis revealed a 45,X0 karyotype in twin A and a 46,XY karyotype in twin B.

CONCLUSIONS

QF-PCR is a reliable tool for the determination of the zygosity independently of the chorionicity and the fetal sex in case of twin pregnancy. Testing both direct and cultured cells can provide useful results for genetic counselling in chromosomal mosaicisms.

Prenatal diagnosis in monozygotic twins with Down syndrome who had different phenotypes

We report the case of monozygotic (MZ) male twin fetuses with different Down syndrome (DS) phenotypes. Prenatal fetal sonography showed a bichorial biamniotic pregnancy with increased nuchal translucency in twin A and a cervical cystic hygroma and heart defect in twin B. Cytogenetic analysis performed after double amniocentesis showed free and homogeneous trisomy 21 in both twins. Monozygosity was confirmed by molecular analysis. The pregnancy was terminated at 17 weeks of gestation (WG). Postmortem analysis confirmed the phenotypic discordance. To our knowledge, this is the first reported prenatal diagnosis of MZ male twins with different Down syndrome phenotypes but identical karyotypes. We discuss the mechanisms involved in phenotypic discordance of monozygotic twins and particularly the role of environmental factors.

Monozygotic twins discordant for monosomy 21 detected by first-trimester nuchal translucency screening

BACKGROUND

Chromosomal abnormality in one fetus of a monozygotic twin pregnancy is rare, and discussion of prenatal detection of such a case offers some insight into this clinical problem.

CASE

A 28-year-old gravida 1 had ultrasound screening at 11 weeks of gestation that revealed a monochorionic, diamniotic twin pregnancy with increased nuchal translucency (7.7 mm) in one fetus. Subsequent evaluation showed one 45,XY,-21 karyotype and one normal male karyotype. The pregnancy was monozygous by DNA analysis. The affected neonate died 5 minutes after delivery of both twins by cesarean.

CONCLUSION

Monochorionic twins discordant for fetal abnormalities can be evaluated with molecular analysis. Study of such cases may reveal the extent to which an early diagnosis can lead to therapeutic interventions to support survival of the viable twin.

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.

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.

Screening for trisomy 21 in monochorionic twins by measurement of fetal nuchal translucency thickness

OBJECTIVE

To determine whether in screening for trisomy 21 by measurement of fetal nuchal translucency (NT) thickness in monochorionic twin pregnancies it is preferable to use the higher, smaller or average NT.

METHODS

We retrospectively examined 769 monochorionic twin pregnancies that had undergone NT screening. The selection criteria were that first, in each pregnancy both fetuses were alive at the 11 to 13 + 6-week scan and second, the fetal karyotype had been determined by prenatal invasive testing or the pregnancy outcome was known. In each pregnancy the risk for trisomy 21 was calculated by a combination of maternal age and fetal NT for crown-rump length (CRL). Three estimates of risk for each pregnancy were made using the higher, smaller and average NT and these were compared for detection and false positive rates.

RESULTS

The median maternal age was 33 (range, 16-45) years, the CRL was 62 (range, 45-84) mm and gestational age was 12 (range, 11 to 13 + 6) weeks. Either the fetal karyotype was normal, or phenotypically normal babies were born, in 761 cases. The karyotype was abnormal in eight cases, including six with trisomy 21. The estimated risk using the higher, smaller and average NT was 1 in 300 or more in 6 (100%), 4 (66.7%) and 6 (100%) of the trisomy 21 pregnancies and in 148 (19.4%), 57 (7.5%) and 106 (13.9%) of the normal pregnancies. For a detection rate of 100%, the false positive rates using the higher, smaller and average NT would be 5.1%, 45.9% and 4.2%, respectively.

CONCLUSION

In monochorionic twins, effective screening for trisomy 21 is best provided by using the average NT measured in the two fetuses.

Invasive Antenatal Interventions in Complicated Multiple Pregnancies

Multiple pregnancies still pose challenging problems for modern medicine. Complications in monochorionic multiple pregnancies have sparked a revival of fetoscopy as a means surgery on the placenta and umbilical cord. As such, fetoscopic laser coagulation is currently the best first-line treatment for severe twin-to-twin transfusion syndrome. Also, fetoscopic or ultrasound-guided cord coagulation seems to be an effective technique for selective feticide in monochorionic twins, albeit with still considerably higher fetal loss rates compared with selective feticide by potassium chloride injection in dichorionic twins.

Managing twins discordant for fetal anomaly

An excess of structural anomalies is observed in twins compared to singletons. Approximately 1-2% of twin pregnancies may face the dilemma of expectant management versus selective termination following diagnosis of an anomaly affecting only one fetus. If the option of selective fetocide is considered, the main variable determining the technique to achieve this aim is chorionicity. In a dichorionic pregnancy, passage of substances from one twin into the circulation of the co-twin is unlikely due to the lack of placental anastomoses, hence KCl can be injected safely into the circulation of the affected twin to produce fetal asystole. In monochorionic twin pregnancies, selective termination needs to be performed by ensuring complete and permanent occlusion of both the arterial and venous flows in the umbilical cord of the affected twin, in order to avoid acute haemorrhage from the co-twin into the dying fetus, which may lead to death or organ damage. Bipolar cord coagulation under ultrasound guidance is associated with approximately 70-80% survival rates.

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.

Monochorionic diamniotic twins: complications and management options

PURPOSE OF REVIEW

Monochorionic compared with dichorionic twins have disproportionately high fetal loss rates, perinatal mortality and morbidity. This is because of the unpredictable vascular anastomoses and the often asymmetrical distribution of the single placenta between both twins.

RECENT FINDINGS

The pathophysiology of twin-to-twin transfusion syndrome is usually explained on an angioarchitectural basis, although certain hemodynamic and hormonal factors also may be involved. The results of the large randomized trials on amnioreduction, fetoscopic laser coagulation and septostomy are still awaited. An update is given on hardware and instruments required for fetoscopic laser. Subsequently, the problem of the monochorionic twin set with severe early discordant growth is addressed. Several etiological mechanisms have been proposed, but little is known of its natural history. Also, umbilical artery Doppler waveforms may not have the same predictive value as in singletons. Prophylactic laser coagulation of the vascular anastomoses to protect against the adverse effects of single intrauterine demise, has so far not been shown to confer any benefit in outcome. Finally, pathophysiology and management of discordant structural and chromosomal anomalies in monochorionic twins are discussed. Laser and monopolar coagulation, which can be introduced through a needle, may be used for selective feticide in early pregnancy or low hemodynamic conditions. Bipolar coagulation seems more effective at later gestational ages and normal hemodynamic conditions.

SUMMARY

Our insight into the complications associated with monochorionic twins has increased in recent years. It is hoped that this will lead to better surveillance and ultimately an improved outcome for these high-risk pregnancies.

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.