title: Monozygotic twins concordant for blood karyotype, but phenotypically discordant: a case of "mosaic chimerism".
abstract: 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.
authors.affiliation: Department of Histology, Limoges University Hospital, Limoges, France.
authors.forename: Sylvie
authors.initials: S
authors.lastname: Bourthoumieu
authors.forename: Catherine
authors.initials: C
authors.lastname: Yardin
authors.forename: Faraj
authors.initials: F
authors.lastname: Terro
authors.forename: Brigitte
authors.initials: B
authors.lastname: Gilbert
authors.forename: Cécile
authors.initials: C
authors.lastname: Laroche
authors.forename: Robert
authors.initials: R
authors.lastname: Saura
authors.forename: Marie-Claire
authors.initials: MC
authors.lastname: Vincent
authors.forename: Françoise
authors.initials: F
authors.lastname: Esclaire
mesh_terms: D000328:Adult; D002869:Chromosome Aberrations; D002871:Chromosome Banding; D002880:Chromosomes, Human, Pair 11 / Q000235:genetics; D004200:Diseases in Twins / Q000235:genetics / Q000473:pathology; D005260:Female; D006801:Humans; D017404:In Situ Hybridization, Fluorescence; D008607:Intellectual Disability / Q000473:pathology; D007621:Karyotyping; D009030:Mosaicism; D010641:Phenotype; D014430:Twins, Monozygotic / Q000097:blood / Q000235:genetics*