title: Monozygotic twins discordant for trisomy 21 and maternal 21q inheritance: a complex series of events.
abstract: 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.
authors.affiliation: Genetic Medicine, University Hospitals of Geneva, Geneva, Switzerland. sophie.dahoun@hcuge.ch
authors.forename: Sophie
authors.initials: S
authors.lastname: Dahoun
authors.forename: Sarantis
authors.initials: S
authors.lastname: Gagos
authors.forename: Maryline
authors.initials: M
authors.lastname: Gagnebin
authors.forename: Corinne
authors.initials: C
authors.lastname: Gehrig
authors.forename: Carole
authors.initials: C
authors.lastname: Burgi
authors.forename: Fabienne
authors.initials: F
authors.lastname: Simon
authors.forename: Chantal
authors.initials: C
authors.lastname: Vieux
authors.forename: Philippe
authors.initials: P
authors.lastname: Extermann
authors.forename: Robert
authors.initials: R
authors.lastname: Lyle
authors.forename: Michael A
authors.initials: MA
authors.lastname: Morris
authors.forename: Stylianos E
authors.initials: SE
authors.lastname: Antonarakis
authors.forename: Frédérique
authors.initials: F
authors.lastname: Béna
authors.forename: Jean-Louis
authors.initials: JL
authors.lastname: Blouin
mesh_terms: D000649:Amniocentesis; D002891:Chromosomes, Human, Pair 21 / Q000235:genetics*; D004200:Diseases in Twins; D004314:Down Syndrome / Q000235:genetics*; D005260:Female; D005819:Genetic Markers; D005838:Genotype; D006801:Humans; D007621:Karyotyping; D018895:Microsatellite Repeats; D009630:Nondisjunction, Genetic; D011110:Polymorphism, Genetic; D011247:Pregnancy; D011296:Prenatal Diagnosis; D014430:Twins, Monozygotic
chemical_list: D005819:Genetic Markers