Mosaicism does not affect accuracy of 24 chromosomes preimplantation genetic screening on cleavage stage embryos.

Biricik A.1, Fiorentino F.1, Kokkali G.2, Rienzi L.3, Spizzichino L.1, Bono S.1, Gordon A.4, Arizzi, L.5, Greco E. 5, Ubaldi F.M.3, Pantos K.2

1 “GENOMA”- Molecular Genetics Laboratory, Via Po, 102 00198 Rome – Italy

2 Centre for Human Reproduction, Genesis Athens Hospital, Athens, Greece

3 G.EN.E.R.A Centre for Reproductive Medicine, Clinica Valle Giulia, Via G. De Notaris 2, 00197 Rome, Italy.

4 Bluegnome Ltd, Cambridge CB22 5LD, UK

5 Assisted Reproduction Centre, European Hospital, Rome, Italy


Introduction: Different randomized clinical trials (RCTs) have shown that preimplantation genetic screening (PGS), as it was practiced, has not provided the expected benefits. The possible explanation for this poor clinical performance has been mainly attributed to the embryonic chromosomal mosaicism, that is present on day-3 of development, which may decrease the chances of a live birth by prematurely labeling an embryo as abnormal. In this study we aimed to evaluate the accuracy of the 24-chromosomes PGS performed on cleavage stage embryos, in order to ascertain if the tested blastomeres were representative for the whole embryo.

Methods: embryos biopsy was carried out at day-3. Single cell DNA was amplified by whole genome amplification (WGA) and processed by Array-CGH according to the 24sure protocol, BlueGnome. Euploid embryos were then selected for transfer on day-5 or day-6 of the same cycle. In order to verify the results obtained following day-3 PGS, chromosomally abnormal embryos that developed to blastocyst stage were re-biopsied on day-6 and reanalyzed.

Results: 111 PGS cycles were carried out for 104 couples. The mean maternal age was 39.0±3.7 years. A total of 838 embryos were biopsied on day 3. Overall, 768 (91.6%) embryos were successfully diagnosed, 553 (72.0%) of which resulted aneuploid. Embryos suitable for transfer where identified in 75 cycles (67.6%). Following transfer of 124 embryos, 50 women (mean maternal age 38.1±3.2 years) had a clinical pregnancy (66.7% pregnancy rate/ET). A total of 63 embryos implanted (50.8% implantation rate/ET), for 60 of which heart beat was also detected. After the clinical cases, 218 non-transferred embryos from 64 PGS cycles were successfully reanalyzed. Aneuploidy mosaicism was detected in 100/218 (45.9%) embryos. Despite high levels of mosaicism found, all day-3 aneuploid embryos followed-up were again diagnosed as abnormal after re-analysis on trofectoderm cells, confirming at the end the previous results regardless of the actual abnormal genotype.

Conclusions: Post-zygotic errors leading to mosaicism were common. However, mosaic embryos were confirmed as chromosomally abnormal after re-analysis at blastocyst stage. Although a larger follow-up study are required in order to confirm the above findings, array-CGH analysis on single blastomeres has demonstrated an accurate aneuploidy detection tool and may assist in identifying abnormal embryos at cleavage stage.