Preimplantation genetic diagnosis for a known cryptic translocation: follow-up clinical report and implication of segregation products

This report describes preimplantation genetic diagnosis (PGD) of a couple with a known paternally-derived balanced cryptic translocation 46,XY.ish t(2q;17q)(210E14-,B37c1+;B37c1-,210E14+) in embryos from a couple who previously had a child with severe mental retardation and was previously described in this journal [Bacino et al., 2000]. This child inherited the unbalanced product of translocation from her father: 46,XX.ish der(2)t(2q;17q)pat(210E14-,B37c1+). The couple desired a normal offspring and sought PGD to avoid clinical pregnancy termination. They were treated three times with in vitro fertilization followed by PGD. Two sequential FISH hybridizations were performed. In the first hybridization, telomeric probes to 2q and 17q and a chromosome 17 centromere probe were employed. The second hybridization screened for maternal age-related aneuploidy (X,Y,13,18,21). Of the 18 informative embryos, only 4 (22%) were normal. The remaining 12 (67%) were abnormal; most with unbalanced products (10/12) from the paternally-derived rearrangement. The most frequent mode of segregation observed for this cryptic translocation was adjacent-1 (7/18, 39%). This suggests cryptic translocations are amenable to PGD and, as are traditional translocations, demonstrate higher frequencies of unbalanced segregants than the empiric risk of 10-15% observed at amniocentesis or chorionic villus sampling. Thus, cryptic translocations presumably behave like overt translocations, in that PGD must be performed on a relatively large number of embryos to assure even 2-3 transferable embryos.