Replacement of FISH by genome-wide single cell array analysis for preimplantation genetic diagnosis of translocation carriers

Preimplantation genetic diagnosis (PGD) is the genetic analysis of a single (or two) blastomere(s) from
an in vitro fertilized embryo and is offered to couples to avoid the transmission of heritable genetic
disorders to their potential offspring. Fluorescence in situ hybridization (FISH) is the current standard technique to detect chromosomal imbalances, however it requires a long, labor-intensive and
indication-specific preparation. Moreover, the number of fluorescent labels is limited and couples
carrying complex chromosomal aberrations cannot be helped. Array-based PGD can overcome all these
shortcomings and will increase the accuracy.
As proof-of-principle, we applied the genome-wide 1Mb resolution BAC array in the PGD-clinic for a
couple that suffered multiple miscarriages and could not be helped using the current FISH technique
since one of the partners carried a complex chromosomal aberration including a reciprocal translocation
between chromosomes 6 and 14 and an insertion between chromosomes 2 and 3. In concurrence with
the ethical committee, we decided to check only the copy number status of the chromosomes involved
in the translocation and insertion. Consequently, whole chromosome or segmental imbalances of other
autosomes did not affect the transfer policy. The chromosomal imbalances detected following array
screening were confirmed on the remaining blastomeres using locus-specific FISH.
Two array-based PGD cycles with single embryo transfer were performed, which resulted in a clinical
pregnancy. The embryo that give rise to the pregnancy was normal diploid (or balanced) for both the
translocation and the insertion region, but carried however a trisomy 8 and nullisomy 9 in one
blastomere, while the second blastomere was completely normal diploid. Besides the successful
implementation of single cell arrays into the fertility clinic, we thus proved that at least some mosaic embryos, which are currently all discarded, can result in a clinical pregnancy. Finally, the application of single cell arrays on human cleavage stage IVF embryos revealed new insights in the segregation patterns during early embryogenesis. Whereas mosaic FISH results would have been classified as potential FISH errors, whole-chromosome array analysis shows that despite the occurrence of meiotic aberrations, mosaic results can be explained by additional mitotic non-disjunctions.
In conclusions, FISH can be replaced by genome-wide single cell array analysis in PGD for the detection
of chromosomal imbalances as small as 10 Mb.