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.
Many 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, growing evidence suggests that following embryo transfer, some aneuploid embryos will in the process of ongoing development, convert to the euploid state (i.e. “autocorrect”) and then go on to develop into chromosomally normal offspring. In fact, I am personally aware of several such cases having occurred in my own practice. 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 (“Mosaicism”) 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”.
Most complex aneuploidies are meiotic in origin and will thus almost invariably fail to propagate viable pregnancies.The ability of mosaic embryos to autocorrect is influenced by stage of embryo development in which the diagnosis is made, which chromosomes are affected, whether the aneuploidy involves a single chromosome (simple) or involves 3 or more chromosomes (complex),and the percentage of cells that are aneuploid. Many embryos diagnosed as being mosaic prior to their development into blastocysts (in the cleaved state), subsequently undergo autocorrection to the euploid state (normal numerical chromosomal configuration) as they develop to blastocysts in the Petri dish. This is one reason why “mosaicism” is more commonly detected in early embryos than in blastocysts. Embryos with segmental mosaic aneuploidies, i.e.the addition (duplication) or subtraction (deletion), are also more likely to autocorrect. Finally, the lower the percentage of mitotically aneuploid (mosaic) cells in the blastocyst the greater the propensity for autocorrection and propagation of chromosomally normal (euploid) offspring. A blastocyst with <30% mosaicism could yield a 30% likelihood of a healthy baby rate with 10-15% miscarriage rate, while with >50% mosaicism the baby rate is roughly halved and the miscarriage rate double.
As stated, the transfer of embryos with autosomal meiotic trisomy,will invariably result in failed implantation, early miscarriage or the birth of a defective child. Those with autosomal mitotic ( “mosaic”) trisomies , while having the ability to autocorrect in-utero and result in the birth of a healthy baby can, depending on the percentage of mosaic (mitotically aneuploid) cells present, the number of aneuploid chromosomes and the type of mosaicism (single or segmental) either autocorrect and propagate a normal baby, result in failed implantation, miscarry or cause a birth defect (especially with trisomies 13, 18 or 21). This is why when it comes to giving consideration to transferring trisomic embryos, suspected of being “mosaic”, I advise patients to undergo prenatal genetic testing once pregnant and to be willing to undergo termination of pregnancy in the event of the baby being affected. Conversely, when it comes to meiotic autosomal monosomy, there is almost no chance of a viable pregnancy. in most cases implantation will fail to occur and if it does, the pregnancy will with rare exceptions, miscarry. “Mosaic” (mitotically aneuploid) autosomally monosomic embryos where a chromosome is missing), can and often will “autocorrect” in-utero and propagate a viable pregnancy. It is for this reason that I readily recommend the transfer of such embryos, while still (for safety sake) advising prenatal genetic testing in the event that a pregnancy results.
Given our ability to recognize “mosaicism” through karyotyping of embryos, the question arrases as to which “mosaic” embryos are capable of auto-correcting in-utero and propagating viable pregnancies. Research suggests that that virtually no autosomal monosomy embryos will propagate viable pregnancies. Thus, the transfer of such mosaic embryos is virtually risk free. Needless to say however, in any such cases , it is 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.