P-516 Minimizing mosaicism: impact of trophectoderm (TE) biopsy specimen size on mosaic blastocysts in preimplantation genetic testing for aneuploidy (PGT-A)
Authors: Han J , Yoon S , Jeong J , Lee G , Kim C , Kim T  H , Eum J  H , Lee H  J , Han E  J , Park J  E , Kim S  J , Lee W  S 
Published: July 2024

Abstract

Study question
Is the size of TE biopsy specimens related to the chromosomal status of the blastocysts in PGT-A?

Summary answer
Smaller biopsy specimens are associated with higher proportion and altered composition of mosaic blastocysts in PGT-A.

What is known already
Concordance between TE biopsy and inner cell mass or whole embryos is influenced by mosaicism type, degree, biopsy location, number of cells biopsied, and cell loading. Particularly, mosaic embryos and those with structural rearrangements have a higher level of inconsistency. Recent studies have demonstrated the potential reclassification of chaotic embryos as euploid, leading to successful live births. This study aimed to investigate the potential influence of TE biopsy specimen size on the chromosomal status of blastocysts undergoing PGT-A.

Study design, size, duration
This retrospective study, conducted from October, 2023, to January, 2024, included 246 blastocysts from patients undergoing TE biopsy and PGT-A at the blastocyst stage on days 5, 6, or 7. Next-generation sequencing (NGS) was used to obtain PGT-A results, categorizing blastocysts into euploid, aneuploid, and mosaic based on their genetic composition.

Participants/materials, setting, methods
The biopsy specimen area was measured using the Image J program. Mosaic blastocysts were categorized into either a chaotic mosaic group or another group, which included blastocysts with fewer than three chromosomal aneuploidies and segmental mosaicism. Further analysis was conducted by classifying blastocysts into high-level mosaic (&gt;30% aneuploidy) and low-level mosaic (≤30% aneuploidy) groups. Group differences were analyzed using t-tests and chi-square, with a significance threshold set at p&lt;0.05.

Main results and the role of chance
In the analysis of expanded mosaic blastocysts, the chaotic mosaic group had smaller biopsy specimens than those in the comparison group (p&lt;0.05). The high-level mosaic group also had significantly smaller biopsy specimens than the low-level mosaic group (p&lt;0.05). When categorizing the biopsied sample size into two groups—below 1350 µM2 (n = 98) and above 1500 µM2 (n = 97)—the rates of mosaic embryos were 37% (36/98) and 30% (29/97), respectively, showing a numerical difference without statistical significance. These findings suggest a correlation between the size of biopsy specimens and the chromosomal status of mosaic blastocysts, particularly indicating that a smaller sample size is associated with an elevated chance of mosaic embryos showing chaotic or high-level mosaic characteristics.

Limitations, reasons for caution
The biopsy specimens were photographed, and their areas were measured using the Image J program, lacking a comprehensive representation of the three-dimensional sample morphology. This constraint could potentially affect the accuracy and in-depth understanding of the spatial characteristics of the biopsy specimens.

Wider implications of the findings
The study emphasizes the crucial role of optimizing biopsy specimen size in PGT-A. The identified correlation with the chromosomal status in mosaic embryos, especially the association with chaotic or high-level mosaic embryo outcomes in smaller samples, highlights the need for careful size considerations.

Trial registration number
not applicable