What next for preimplantation genetic screening? High mitotic chromosome instability rate provides the biological basis for the low success rate

Abstract

Preimplantation genetic screening is being scrutinized, as recent randomized clinical trials failed to observe the expected significant increase in live birth rates following fluorescence in situ hybridization (FISH)-based screening. Although these randomized clinical trials are criticized on their design, skills or premature stop, it is generally believed that well-designed and well-executed randomized clinical trials would resolve the debate about the potential benefit of preimplantation genetic screening. Since FISH can analyze only a limited number of chromosomal loci, some of the embryos transferred might be diagnosed as 'normal' but in fact be aneuploid for one or more chromosomes not tested. Hence, genome-wide array comparative genome hybridization screening enabling aneuploidy detection of all chromosomes was thought to be a first step toward a better design. We recently showed array screening indeed enables accurate determination of the copy number state of all chromosomes in a single cell. Surprisingly, however, this genome-wide array screening revealed a much higher frequency and complexity of chromosomal aberrations in early embryos than anticipated, with imbalances in a staggering 90% of all embryos. The mitotic error rate in cleavage stage embryos was proven to be higher than the meiotic aneuploidy rate and as a consequence, the genome of a single blastomere is not representative for the genome of the other cells of the embryo. Hence, potentially viable embryos will be discarded upon screening a single blastomere. This observation provides a biological basis for the failure of the randomized clinical trials to increase baby-take-home rates using FISH on cleavage stage embryos.

Figures

Figure 1

The karyogram of embryo 19 is shown. The karyogram is composed of color bars that each represent the chromosome copy number state based on the results of the BAC (bacterial artificial chromosomes) array and the SNP (single nucleotide polymorphism) copy number and genotyping data in a specific blastomere. Black represents a normal region, red a hemizygous deletion, green a duplication, dark green an amplification and gray discordance between the analyses or unreliable aberrations. In this male embryo, whole chromosome imbalances of mitotic origin were detected in chromosomes 1 and 14, whereas chromosome 20 showed a monosomy in all sister blastomeres suggesting a meiotic non-disjunction. Moreover, 4q and 10q terminal deletions with a reciprocal 4q duplication and 10q amplification respectively were detected in a proportion of its sister blastomeres. Finally, a 5q terminal deletion and reciprocal 5q duplication were detected in two blastomeres, whereas the remaining part of the chromosome proximal to the 5q deletion was trisomic. Two sister blastomeres contained a monosomy for chromosome 5. In addition, a remaining sister blastomere contained three copies of chromosome 5, whereas a fifth lacked a 5q terminal part of which the size was equal to the partial deletion and duplication in its sister blastomeres.