P-539 Chaotic blastocysts in preimplantation genetic testing for aneuploidies: prevalence, characterization and re-biopsy results
Authors: Calull Bago A , Víctores Monroy A , González Ortega C , Cancino Villarrel P , Martínez Robles I , Ramírez Macías A  A , Gutiérrez Gutiérrez A  M 
Published: July 2024

Abstract

Study question
Are we excluding blastocysts with favorable reproductive potential when embryos are designated as chaotic by Preimplantation Genetic Testing for Aneuploidies (PGT-A)?

Summary answer
The re-biopsy of chaotic blastocysts can rescue embryos with favorable reproductive potential only when there are ≥5 chromosomes displaying intermediate copy number values (CNV).

What is known already
An embryo is deemed as chaotic when multiple aneuploidies (5, 6 or more) are detected in the biopsied cells of the trophectoderm. Such embryos are classified as aneuploid, therefore their transfer is not recommended. There is evidence that chaotic embryos may result from the biopsy of apoptotic cells, cellular fragments and, more broadly, DNA degraded by multiple factors. Studies involving the re-biopsy and re-testing of a limited number of chaotic embryos have found a relevant percentage of euploid results. Moreover, there has been published a case report of a healthy live birth after the transfer of a chaotic embryo.

Study design, size, duration
This is a retrospective study of 1,442 cycles of PGT-A performed in twenty-two in vitro fertilization (IVF) clinics from November 2017 to December 2023. A total of 5,801 blastocysts were biopsied and analyzed in a single reference laboratory using Next-Generation Sequencing. Ninety-three chaotic blastocysts donated from patients who provided informed consent were thawed for re-biopsy in two IVF clinics. The subsequent re-analysis was performed in the same PGT laboratory using the same methodology.

Participants/materials, setting, methods
This study assessed the prevalence of chaotic embryos in all included cycles. Chaotic embryos were considered those with ≥5 chromosomes showing CNV below 1.75 or above 2.25. We sub-classified these embryos into pure chaotic (≥5 uniform aneuploidies), mixed chaotic (≥5 alterations considering uniform aneuploidies and chromosomes with intermediate CNV) and mosaic chaotic (≥5 chromosomes with intermediate CNV). Re-biopsies of 69 chaotic blastocysts were compared to the initial PGT-A results.

Main results and the role of chance
The prevalence of chaotic embryos was 4.3% (251/5,801); 13.5% (n = 34) pure, 53.0% (n = 133) mixed and 33.5% (n = 84) mosaic. After thawing 93 embryos, the survival rate was 74.2%. The 69 embryos that survived underwent re-biopsy yielding the following PGT-A results: 24.6% (n = 17) euploid, 37.7% (n = 26) chaotic, 26.1% (n = 18) aneuploid non-chaotic, 7.2% (n = 5) mosaic and 4.4% (n = 3) non informative. Nevertheless, outcomes varied substantially when the analysis was conducted based on the sub-category of chaotic embryos. In the pure category (n = 11), 72.7% (n = 8) showed identical results to the initial biopsy while 27.3% (n = 3) were aneuploid non-chaotic confirming at least one aneuploidy identified at the initial biopsy. Among the mixed category (n = 24), 54.2% (n = 13) were also mixed chaotic, and 45.8% (n = 11) were aneuploidy non-chaotic. All re-biopsies in this category also manifested at least one common aneuploidy with the initial biopsy or its opposite (trisomy/monosomy). In the mosaic category (n = 31), 54.8% (n = 17) were euploid, 16.1% (n = 5) were mosaic chaotic, 16.1% (n = 5) were mosaic non-chaotic and 12.9% (n = 4) were aneuploid non-chaotic. The concordance with the initial biopsy in terms of aneuploid result was 100% in the pure and mixed chaotic categories. However, over half of the embryos initially categorized as mosaic chaotic were, in fact, euploid.

Limitations, reasons for caution
Implantation, miscarriage and live birth rates are not explored. These outcomes are crucial for understanding the clinical significance of euploid embryos initially deemed as chaotic. The survival rate of chaotic embryos should be considered as it is lower than the survival rate of vitrified blastocysts reported in most IVF laboratories.

Wider implications of the findings
Previous research has suggested that re-biopsy can rescue embryos from PGT-A cycles given that a chaotic result has reduced predictive value. However, our current study reveals that not all chaotic embryos are suitable for re-biopsy, as 100% of pure and mixed chaotic blastocysts were aneuploid when re-tested.

Trial registration number
Not applicable