J. Cozzi et al., A trisomic germ cell line and precocious chromatid segregation leads to recurrent trisomy 21 conception, HUM GENET, 104(1), 1999, pp. 23-28
A chromosomally normal 37-year-old woman was referred for preimplantation g
enetic diagnosis after having several conceptuses with trisomy 21. Segregat
ion of chromosome 21 was assessed in unfertilised meiosis II oocytes and pr
eimplantation embryos from PGD cycles using fluorescent in situ hybridisati
on (FISH). Of 7 preimplantation embryos, 5 were chromosomally abnormal with
4 having trisomy 21 and one being tetraploid. Of 4 oocytes, 3 had an abnor
mal chromosomal constitution with either an extra chromosome 21 or an extra
chromatid 21. In one oocyte an extra chromatid 21 was detected in both the
metaphase II complement and the first polar body providing the first direc
t evidence of a maternal trisomic germ cell line. Moreover, this result sho
ws that the extra chromosome 21 can precociously divide into its two chroma
tids at the first meiotic division.