Human embryo development occurs through a process that encompasses reprogramming, sequential cleavage divisions and mitotic chromosome segregation and embryonic genome activation. Chromosomal abnormalities may arise during germ cell and/or preimplantation embryo development and represents a major cause of early pregnancy loss. About a decade ago, I and my associate, Levent Keskintepe PhD were the first to introduce full embryo karyotyping (identification of all 46 chromosomes) through preimplantation genetic sampling (PGS) as a method by which to selectively transfer only euploid embryos (i.e. those that have a full component of chromosomes) to the uterus. We subsequently reported on a 2-3-fold improvement in implantation and birth rates as well as a significant reduction in early pregnancy loss, following IVF. Since then PGS has grown dramatically in popularity such that it is now widely used throughout the world.
Most IVF programs that offer PGS services, require that all participating patients consent to all their aneuploid embryos (i.e. those with an irregular quota of chromosomes) be disposed of. However, there is now growing evidence to suggest that following embryo transfer, some aneuploid embryos will in the process of ongoing development, convert to the euploid state (i.e. “autocorrection”) and then go on to develop into chromosomally normal offspring. In fact, I am personally aware of several such cases occurring within our IVF network. So clearly, summarily discarding all aneuploid embryos as a matter of routine we are sometimes destroying some embryos that might otherwise have “autocorrected” and gone on to develop into normal offspring.
Thus by discarding aneuploid embryos the possibility exists that we could be denying some women the opportunity of having a baby. This creates a major ethical and moral dilemma for those of us that provide the option of PGS to our patients. On the one hand, we strive “to avoid knowingly doing harm” (the Hippocratic Oath) and as such would prefer to avoid or minimize the risk of miscarriage and/or chromosomal birth defects and on the other hand we would not wish to deny patients with aneuploid embryos, the opportunity to have a baby.
The basis for such embryo “autocorrection” lies in the fact that some embryos found through PGS-karyotyping to harbor one or more aneuploid cells (blastomeres) will often also harbor chromosomally normal (euploid) cells (blastomeres). The coexistence of both aneuploid and euploid cells coexisting in the same embryo is referred to as “mosaicism.”
It is against this background, that an ever-increasing number of IVF practitioners, rather than summarily discard PGS-identified aneuploid embryos are now choosing to cryobanking (freeze-store) certain of them, to leave open the possibility of ultimately transferring them to the uterus. In order to best understand the complexity of the factors involved in such decision making, it is essential to understand the causes of embryo aneuploidy of which there are two varieties:
- Meiotic aneuploidy” results from aberrations in chromosomal numerical configuration that originate in either the egg (most commonly) and/or in sperm, during preconceptual maturational division (meiosis). Since meiosis occurs in the pre-fertilized egg or in and sperm, it follows that when aneuploidy occurs due to defective meiosis, all subsequent cells in the developing embryo/blastocyst/conceptus inevitably will be aneuploid, precluding subsequent “autocorrection”. Meiotic aneuploidy will thus invariably be perpetuated in all the cells of the embryo as they replicate. It is a permanent phenomenon and is irreversible. All embryos so affected are thus fatally damaged. Most will fail to implant and those that do implant will either be lost in early pregnancy or develop into chromosomally defective offspring (e.g. Down syndrome, Edward syndrome, Turner syndrome).
- “Mitotic aneuploidy” occurs when following fertilization and subsequent cell replication (cleavage), some cells (blastomeres) of a meiotically normal (euploid) early embryo mutate and become aneuploid. This is referred to as “mosaicism”. Thereupon, with continued subsequent cell replication (mitosis) the chromosomal make-up (karyotype) of the embryo might either comprise of predominantly aneuploid cells or euploid cells. The subsequent viability or competency of the conceptus will thereupon depend on whether euploid or aneuploid cells predominate. If in such mosaic embryos aneuploid cells predominate, the embryo will be “incompetent”). If (as is frequently the case) euploid cells prevail, the mosaic embryo will likely be “competent” and capable of propagating a normal conceptus.
Since some mitotically aneuploid (“mosaic”) embryos can, and indeed do “autocorrect’ while meiotically aneuploid embryos cannot, it follows that an ability to reliably differentiate between these two varieties of aneuploidy would potentially be of considerable clinical value. The recent introduction of a variety of preimplantation genetic screening (PGS) known as next generation gene sequencing (NGS) has vastly improved the ability to reliably and accurately karyotype embryos and thus to diagnose embryo “mosaicism”.
The ability of mosaic embryos to autocorrect is influenced by the stage at which the condition is diagnosed as well as the percentage of mosaic cells. Many embryos diagnosed as being mosaic while in the earlier cleaved state of development, subsequently undergo autocorrection to the euploid state (normal numerical chromosomal configuration) during the process of undergoing subsequent mitotic cell to the blastocyst stage. Similarly, mosaic blastocysts can also undergo autocorrection after being transferred to the uterus. The lower the percentage of mosaic cells in the blastocyst the greater the propensity to autocorrect and propagate chromosomally normal (euploid) offspring. By comparison, a blastocyst with 10% mosaicism could yield a 30% healthy baby rate with 10-15% miscarriage rate, while with >50% mosaicism the baby rate is roughly halved and the miscarriage rate double.
Aneuploidy involves the addition (trisomy) or subtraction (monosomy) of one or part of one chromosome in any given pair. As previously stated, some aneuploidies are meiotic in origin while others are mitotic “mosaics”. Certain aneuploidies involve only a single, chromosome pair (simple aneuploidy) while others involve several pairs (i.e. complex aneuploidy). Aside from monosomy involving the absence of the y-sex chromosome (i.e. XO) which can result in a live birth (Turner syndrome) of a compromised baby, virtually all monosomies involving autosomes (non-sex chromosomes) are likely to be lethal and will rarely result in viable offspring. Some autosomal meiotic aneuploidies, especially trisomies 13, 18, 21, can propagate viable and severely chromosomally defective babies. Other meiotic autosomal trisomies will almost invariably, either not attach to the uterine lining or upon attachment, will soon be rejected. All forms of meiotic aneuploidy are irreversible while as stated, mitotic aneuploidy (“mosaicism) can autocorrect, yielding healthy offspring. Most complex aneuploidies are meiotic in origin and will thus almost invariably fail to propagate viable pregnancies.
Since certain “mosaic” meiotic aneuploid trisomy embryos (e.g. trisomies 13, 18, & 21) can potentially result in aneuploid concepti. For this reason, it is my opinion that unless the woman/couple receiving such embryos is willing to commit to terminating a resulting pregnancy found through amniocentesis or chorionic villus sampling (CVS) to be so affected, she/they are probably best advised not to transfer have them transferred to the uterus. Embryos harboring other autosomal mosaic trisomic embryos, should they not autocorrect in-utero will hardly ever produce a baby and as such there is hardly any risk at all…in transferring such embryos. However, it is my opinion that in the event of an ongoing pregnancy, amniocentesis or CVS should be performed to make certain that the baby is euploid. Conversely, when it comes to mosaic autosomal monosomy, given that virtually no autosomal monosomy embryos are likely to propagate viable pregnancies, the transfer of such mosaic embryos is virtually risk free. Needless to say, in any such cases , it is absolutely essential to make full disclosure to the patient (s) , and to insure the completion of a detailed informed consent agreement which would include a commitment by the patient (s) to undergo prenatal genetic testing (amniocentesis/CVS) aimed at excluding a chromosomal defect in the developing baby and/or a willingness to terminate the pregnancy should a serious birth defect be diagnosed.