Preimplantation genetic testing (PGT) to exclude transfer of an aneuploid embryo (PGT-A) has not always been possible. When only fluorescent in situ hybridization with chromosome specific probes were available, fewer than half of all chromosomes could be tested. By 2009–2010 all 24 chromosomes could be tested, initially using array comparative genome hybridization. Predictive value of an embryo known to be aneuploid or euploid was quickly determined. The livebirth rate following transfer of 133 euploid embryos was 41% (55/133), compared with 4% (4/99) if the embryo was aneuploid by array comparative genome hybridization1.
The present standard methodology for PGT-A is based on next-generation sequencing (NGS). Chromosomal status is based not on individual cells, but on quantity of DNA. In trophectoderm biopsy the number of cells is generally considered to be 5–10, but the exact number is actually not known in any given case. Laboratory results are based on the relative quantity of normal (euploid) versus abnormal (aneuploid) DNA. It would naturally be more straight forward for communication if outcomes could be reported (stratified) into either 100% euploid or 100% aneuploid DNA. However, biopsy of a truly euploid embryo may frequently show some aneuploid DNA; similarly, biopsy of an aneuploid embryo may frequently show some euploid DNA. Spurious explanations include damage to some of the cells during blastocyst biopsy, failure to complete S-phase replication and thus showing an amount of DNA consistent with deletion or duplication, or vicissitudes of bioinformatics. Given this reality, an embryo is defined as nonmosaic euploid if DNA content is 20% or less aneuploid; an embryo is defined as aneuploid if DNA content is at least 80% or more aneuploid. Aneuploidy content between 20% and 80% is defined as mosaicism.
Figure 1 shows our results of embryo biopsies, now totaling over 15,000. Embryos underwent NGS following trophectoderm biopsy either for PGT-A alone or for combined PGT-A/PGT-M. Mean maternal age was 36.7 years. These samples came from over 200 referral centers, with variable frequencies of mosaicism per center. Of the total embryos biopsied:
Forty-four percent were euploid as defined by <20% aneuploid DNA.
Twenty-four percent were aneuploid as defined by >80% aneuploid DNA (11% trisomy; 13% monosomy).
Eight percent involved more than 1 chromosome error (complex).
Eight percent were mosaic for a whole chromosome (numerical mosaicism).
Fourteen percent showed duplication or deletion of a segment of a chromosome (segmental mosaicism). Two percent failed amplification.
Figure 2 shows that chromosomes involved in numerical (whole chromosome) mosaicism predominately involved smaller chromosomes. Probably this reflects increased likelihood of survival to the blastocyst stage compared with mosaicism involving larger chromosomes. This would be consistent with liveborn mosaic autosomal monosomy observed only in infants monosomic for smaller chromosomes2.
Figure 3 shows that chromosomes involved in segmental mosaicism (duplication/deletion) predominantly involve larger chromosomes (1–12). S-phase replication is more likely to be incomplete in larger chromosome than in smaller chromosomes.
Trophectoderm, from which biopsy is taken, differentiates into placenta, thus, if biopsied, avoiding damage to the inner cell mass (ICM) destined to differentiate into embryonic organ systems. However, blastocyst biopsy results are representative of the embryo, as there is a high concordance between amount of aneuploid or euploid DNA in the inner cell mass and DNA in trophectoderm for whole chromosome aneuploidy or euploidy. If trophectoderm DNA content shows euploidy, ~90% of ICM is concordant, also showing whole chromosomal euploidy. This is based on studies of nontransferred, donated, aneuploid and euploid blastocysts3. In contrast, concordance between ICM and trophectoderm is only 50% in segmental aneuploidy.
While the transfer of mosaic embryos should ideally be avoided, it was also demonstrated that the transfer of monosomy or trisomy mosaic embryos can result in livebirth4. The portion of the trophectoderm from which biopsy was taken was presumably not representative of the inner cell mass. The (nonbiopsied) inner cell mass was deduced to be fully euploid or if mosaic having little aneuploid DNA that did not persist. The experience of outcome following transfer of >500 NGS-detected mosaic embryos has now been accumulated5. Live births with anomalies have not been reported, although robust assessment is lacking. If a mosaic embryo containing 20%–40% aneuploid DNA is transferred, results may nearly be as favorable as transferring an embryo with 0%–20% aneuploid DNA. However, if a mosaic embryo containing 40%–80% aneuploid DNA is transferred, pregnancy rates are lower and miscarriages rates higher.
The dilemma of whether to transfer a mosaic embryo is not common. In only a few cycles (1%) will the only potentially viable embryo be mosaic. If transfer of a mosaic embryo is envisioned, guidelines that maximize the pregnancy rates at lowest risks are available. Preimplantation Genetic Diagnosis International Society (PGDIS)6 provides guidelines.
- If a euploid embryo is available, it should be transferred in lieu of any mosaic embryo.
- If a nonmosaic euploid embryo is not available, a mosaic embryo can be considered for transfer.
- Prioritization can identify the more preferable mosaic embryo should multiple mosaic embryos exist.
- Least logical would be transfer of a mosaic aneuploid embryo (trisomic or monosomic) for a chromosome known to result in liveborn trisomy (chromosomes 13, 18, 21).
- Mosaic aneuploid embryos involving larger chromosomes (1–12) are paradoxically preferable to those mosaic for smaller chromosomes2. Recognized pregnancies with mosaic autosomal monosomy have been reported only for the smaller chromosomes, such as monosomy 21. Apparent lethality exists for mosaic monosomy involving larger chromosomes (1–12); thus, if an embryo showing autosomal monosomy for 1–12 were transferred and genuine the rationale is that even if mosaicism persists there will not be survival until livebirth.
- Transfer of an embryo monosomic or trisomic for a chromosome subject to uniparental disomy should be avoided. Chromosomes 7, 8, 11, 14, 15, 21, and 22 are of concern. Overall, the least hazardous embryos should be 1, 2, 3, 4, 5, 6, 9, 10, and 12.
- Prioritization may be considered based on percent aneuploidy, but only based on general ranges. No data exist showing predictive outcome correlated with a linear increase or decrease in DNA amount. If the amount of aneuploid DNA is 20%–40%, outcome is relatively favorable. As the percent of aneuploid DNA increases, for example to 60%–80%, pregnancy rates decrease and miscarriage rates increase.
- If pregnancy results from transfer of a mosaic PGT embryo, chorionic villus sampling or amniocentesis is strongly recommended irrespective of maternal age. This recommendation differs from merely offering an invasive procedure to woman of advanced material age whose conception was natural.
PGT-A is highly accurate and can be useful, increasing the pregnancy rate 15%–20% compared with transfer of morphologically normal embryos whose chromosomal status was not known. Predictive value of a nonmosaic euploid embryo (<20% aneuploid DNA) resulting in a pregnancy is 10-fold greater than for a nonmosaic aneuploid embryo (>80% aneuploid DNA). Outcome for a mosaic embryo containing 20%–40% aneuploid DNA seems more favorable than for a mosaic embryo containing 40%–80% aneuploid DNA.
Conflict of interest disclosures
The authors declare that they have no financial conflict of interest with regard to the content of this report.
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