Trisomy 15 mosaic derived from trisomic conceptus: report of a case and a review

We report on a fetus with 47,XX,+15 chromosome abnormality detected on chorionic villus sampling (CVS). The pregnancy was terminated at 15.5 weeks of gestation and chromosome analysis done on amniocytes and fetal tissues showed a karyotype 46,XX/47,XX,+15. Autopsy showed multiple abnormalities. Short-arm polymorphisms of the three number 15 chromosomes were highly informative in the delineation of parental origin and the stage of meiotic error. Using fluorescent in situ hybridization (FISH) with D15Z1 and a chromosome 15 painting probe, in addition to DA/DAPI and G-banding, we were able to show that the trisomic conceptus was derived through maternal meiosis I error. The trisomic state was then partially corrected by the loss of one of the two maternal 15s resulting in mosaicism without uniparental disomy (UPD). Striking differences in the proportion of trisomic cells in kidneys, blood, intestine, and skin, and lower proportions of trisomic cells in transformed and frozen than in fresh tissues, illustrate the continuing cell selection in this fetus in favour of the normal cell line. Trisomy 15 conceptions are usually aborted spontaneously in the first trimester of pregnancy. The longer survival of this fetus is most probably the result of a chromosome 15 loss from the trisomic zygote. To the best of our knowledge, the presence of this lethal trisomy has been reported in only five live-born infants, and in five fetuses including the present case, it was detected prenatally and the pregnancies were terminated.