Commonly Discarded Mosaic Embryos Might Lead to Live Births

(Reuters Health) – Many human embryos discarded prior to implantation might have the potential to result in a successful in vitro fertilization procedure and a live birth, a recent study suggests.

Researchers examined the prevalence and distribution of aneuploid cells within unselected preimplantation embryos. They disaggregated 73 blastocysts down to five portions, including the inner cell mass (ICM) and four trophectoderm (TE) biopsies, to investigate how abnormal cells are distributed throughout the embryo.

When mosaicism impacted fewer than 50% of cells in one TE biopsy, indicating low to medium mosaicism, only 1% of aneuploidies affected other portions of the embryo, the study team found.

“We found that abnormal cells are localized to a small area of the trophectoderm and rarely affect other TE sections or the inner cell mass,” said lead author Antonio Capalbo, a clinical geneticist and embryologist and scientific director at Igenomix Italy in Vicenza.

“This is not surprising as some low level of mosaicism can be a normal feature of developing placental cells and they are expected to be well tolerated in this tissue,” Capalbo said by email.

An alternative and equally plausible explanation is that mosaicism-consistent findings in a TE biopsy are indeed coming as a consequence of technical variation, due to the well-known experimental noise of single cell analysis, Capalbo said. “Both are valid explanations of the reason why these embryos have equivalent reproductive competence to fully euploid embryos,” Capalbo added.

The primary outcome of the trial was defined as live birth rate, and the secondary outcome was miscarriage rate.

Overall, researchers found that across 484 euploid, 282 low-grade mosaic, and 131 medium-grade mosaic embryos, the rates of live births and miscarriages were similar, as were obstetrical and neonatal outcomes.

“Based on evidence, these embryos are similar in terms of reproductive competence and do not increase the risk for chromosomal abnormalities including mosaicism in the fetus or newborns,” Capalbo said.

It’s still not clear, however, whether aneuploid cells arrest, become senescent or apoptotic, or are precursors of placental mosaicism, the study team notes in American Journal of Human Genetics.

Even so, the results should help clinicians and patients make more informed decisions on the transfer of potentially mosaic embryos that in current practice might be discarded, Capalbo said.

“Furthermore, it seems reasonable genetic laboratories stop reporting these findings that have no clinical utility for patients,” Capalbo added. “In clinical genetics there is wide consensus that genomic findings should be reported only if they bring clear elements of clinical validity and utility.”

SOURCE: American Journal of Human Genetics, online November 18, 2021.

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