A trisomic germ cell line and precocious chromatid segregation leads to recurrent trisomy 21 conception

A chromosomally normal 37-year-old woman was referred for preimplantation genetic diagnosis after having several conceptuses with trisomy 21. Segregation of chromosome 21 was assessed in unfertilised meiosis II oocytes and preimplantation embryos from PGD cycles using fluorescent in situ hybridisation (FISH). Of 7 preimplantation embryos, 5 were chromosomally abnormal with 4 having trisomy 21 and one being tetraploid. Of 4 oocytes, 3 had an abnormal 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 direct evidence of a maternal trisomic germ cell line. Moreover, this result shows that the extra chromosome 21 can precociously divide into its two chromatids at the first meiotic division.