Preimplantation genetic diagnosis for X;autosome translocations: lessons from a case of misdiagnosis

Preimplantation genetic diagnosis (PGD) is offered to couples carrying a reciprocal translocation in an attempt to increase their chance of phenotypically normal offspring. For the selection of embryos that are balanced for the translocation chromosomes, it is critical to use a combination of DNA probes that can take account of all the segregation patterns of the particular translocation. The frequency of the different segregation types differs depending on the chromosomes involved, the location of the breakpoints and the number of chiasmata and the sex of the carrier. We report on a case of misdiagnosis after PGD–fluorescence in situ hybridization in a female translocation 46,X,t(X;5)(q13;p14) carrier. Transfer of two embryos diagnosed as balanced for the translocation chromosomes resulted in a singleton pregnancy that miscarried at 8 weeks' gestational age. The unbalanced karyotype of the fetus was consistent with 3:1 segregation resulting in tertiary trisomy for the derivative chromosome 5: 47,XX,+der(5)t(X;5)(q13;p14)mat. Based on additional molecular cytogenetic studies of fetal tissue and the initially investigated blastomeres, we concluded that the misdiagnosis was most probably due to a technical error, i.e. a partial hybridization failure or co-localization of the Xq/Yq subtelomere probe signals. No evidence for a normal cell line (mosaicism) was found in the fetus, which could have explained the discrepancy. This case demonstrates the importance of using two diagnostic probes or testing 2 cells to detect translocation products with potentially viable imbalance. X;autosome translocations are a special case due to the added complication of X chromosome inactivation and particular caution is advised when designing a PGD strategy. TRIAL REGISTRATION NUMBER not applicable.