Monozygotic twins discordant for trisomy 18

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.

Monozygotic Twins Discordant for Trisomy 13: A Case of Trisomic Rescue Supporting the Continued Need for First-Trimester Ultrasound

Monozygotic twins with discordant karyotypes for trisomy 13 are rare. We report a case of a spontaneously conceived pregnancy who presented with first-trimester ultrasound finding of umbilical cord cyst and increased nuchal translucency in Twin A and no abnormalities in Twin B. Amniocentesis revealed 47,XY,+13 karyotype in Twin A and 46,XY karyotype in Twin B. Selective fetal reduction was performed for Twin A. Twin B was delivered at 32 weeks gestation with normal phenotype. Peripheral blood karyotype revealed 15% mosaicism for trisomy 13 and skin fibroblast revealed 46,XY karyotype. The surviving twin will be monitored for potential complication of uniparental disomy 13 and mosaic trisomy 13. This case reinforces the need for early ultrasound and nuchal translucency measurements, especially in twin gestations.

Massively Parallel Sequencing (MPS) of Cell-Free Fetal DNA (cffDNA) for Trisomies 21, 18, and 13 in Twin Pregnancies

Massively parallel sequencing (MPS) technology has become increasingly available and has been widely used to screen for trisomies 21, 18, and 13 in singleton pregnancies. This study assessed the performance of MPS testing of cell-free fetal DNA (cffDNA) from maternal plasma for trisomies 21, 18, and 13 in twin pregnancies. Ninety-two women with twin pregnancies were recruited. The results were identified through karyotypes of amniocentesis or clinical examination and follow-up of the neonates. Fluorescent in-situ hybridization was used to examine the placentas postnatally in cases of false-positive results. The fetuses with autosomal trisomy 21 (n = 2) and trisomy 15 (n = 1) were successfully detected via MPS testing of cffDNA. There was one false-positive for trisomy 13 (n = 1), and fluorescence in-situ hybridization (FISH) identified confined placental mosaicism in this case. For twin pregnancies undergoing second-trimester screening for trisomy, MPS testing of cffDNA is feasible and can enhance the diagnostic spectrum of non-invasive prenatal testing, which could effectively reduce invasive prenatal diagnostic methods. In addition to screening for trisomy 21, 18, and 13 by cffDNA, MPS can detect fetal additional autosomal trisomy. False-positive results cannot completely exclude confined placental mosaicism.

Phenotypic extremes in liveborn monozygotic twins with mosaic Edwards syndrome

Mosaic trisomy 18 (Edwards syndrome) in monozygotic diamniotic liveborn twins is rare. We describe such a case involving preterm male infants. Although both infants had a low percentage of trisomy 18 cells in peripheral blood leucocytes, their varied phenotypic presentation of mosaic trisomy 18 resulted in one twin surviving, with the other twin's demise at 1 month of age. Despite the presence of trisomy 18 in peripheral leucocytes, further analysis of a buccal smear and skin biopsy of the surviving twin did not show evidence of trisomy 18. Establishing such diagnoses in a timely manner is imperative for the child, parents and clinicians. The clinical course of these twins reflects the unpredictable prognosis associated with the diagnosis of mosaic trisomy 18, and emphasises the challenges that can be encountered when counselling parents.

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.