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Embryo Mosaicism: What You Need to Know

by Dr. Geoffrey Sher on June 28, 2017

In 2005, my associate Levent Keskintepe PhD and I introduced Preimplantation Genetic Screening (PGS) with the ability to identify all chromosomes in the embryo’s cells, into the field of IVF.

This approach, which is now widely used throughout the world, permits selection of those embryos that are most likely to be competent, and has dramatically improved IVF success rates.

However, some abnormal (or aneuploid) embryos are capable of autocorrecting and reverting to a normal karyotype (euploid) during intrauterine development and of then propagating healthy babies. This is because some embryos can harbor both aneuploid AND euploid cells. This combination of aneuploid plus euploid cells in the same embryo is referred to as “mosaicism.”

It is an indisputable fact that many mosaic embryos further cell replication can result in the euploid cell component predominating ultimately resulting in a healthy conceptus. In many cases it is not possible to identify embryo “mosaicism”. Accordingly, we tend to preserve certain aneuploid embryos and recommend that they be transferred.

Once pregnant chorionic villus sampling (CVS) or amniocentesis should be done to determine the normalcy of the pregnancy, providing the patient(s) with the opportunity to terminate such pregnancies if they so choose. Join me tomorrow at 1:00PM PST on my Facebook page, as I address the pros and cons of preserving versus discarding all aneuploid embryos and define my policy in advising such patients.

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  • Justyna Akyondem - September 14, 2017 reply

    Dr. Sher:

    Thank you so much for posting this video and article. This is incredibly helpful!

    Are there specific questions that I would need to ask my lab in order to get a good report of the PGS results? I have found that detailed information is given only when asked for.

    Also what are your thoughts about the accuracy of a single 6-10 cell Trophoderm biopsy on a day 5 blastocyst. I have read a number of papers suggesting that such a small biopsy sample size is not adequate to accurately identify the existence of multiple cell lines in a day 5 blastocyst.

    Dr. Geoffrey Sher

    Dr. Geoffrey Sher - September 15, 2017 reply

    Just asked for a comprehensive report…That should do it. I think the 6-10 cell biopsy is adequate.

    Geoff Sher

  • Chrissy - July 31, 2017 reply

    What are you thoughts on transferring a monosomy 20 embryo? The first time it was tested it came back as “no result” so the clinic did a re-biopsy and sent back in and it came back with the above abnormality. Would you say it’s likely too abnormal?

    Dr. Geoffrey Sher

    Dr. Geoffrey Sher - July 31, 2017 reply

    I would do the transfer. It could be a mosaic.

    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 pre-implantation embryo development, and represents a major cause of early pregnancy loss. About a decade ago, I and an associate, Levent Keskintepe Ph.D 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.” As stated, some mosaic embryos will In the process of subsequent cell replication convert to the normal euploid state (i.e. autocorrect)
    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:
    1. 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).
    2. “Mitotic aneuploidy” occurs when following fertilization and subsequent cell replication (cleavage), some cells (blastomeres) of a meiotically 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 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 differentiate between these two varieties of aneuploidy would be of considerable clinical value. And would provide a strong argument in favor of preserving certain aneuploid embryos for future dispensation.
    Aneuploidy, involves the addition (trisomy) or subtraction (monosomy) of one chromosome in a 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 more than a single pair (i.e. complex aneuploidy). Aside from monosomy involving absence of the y-sex chromosome (i.e. XO) which can resulting in a live birth (Turner syndrome) all monosomies involving autosomes (non-sex chromosomes) are lethal and will not result in viable offspring). Some autosomal meiotic aneuploidies, especially trisomies 13, 18, 21, can progress to viable, but severely chromosomally defective babies. All 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 mitotic aneuploidy (“mosaicism) often autocorrects in the uterus. Most complex aneuploidies are meiotic in origin and will almost invariably fail to propagate viable pregnancies.
    There is presently no microscopic or genetic test that can reliable differentiate between meiotic and mitotic aneuploidy. Notwithstanding this, the fact that some “mosaic” embryos can autocorrect in the uterus, makes a strong argument in favor of transferring aneuploid of embryos in the hope that the one(s) transferred might be “mosaic” and might propagate viable healthy pregnancies. On the other hand, it is the fear that embryo aneuploidy might result in a chromosomally abnormal baby that has led many IVF physicians to strongly oppose the transfer of any aneuploid embryos to the uterus.
    While certain meiotic aneuploid trisomies (e.g. trisomies 13, 18, & 21) can and sometimes do result in chromosomally defective babies, no other meiotic autosomal trisomies can do so. Thus the transfer of trisomic embryos in the hope that one or more might be mosaic, should exclude the use of embryos with trisomies 13, 18 or 21. Conversely, no autosomal monosomic embryos are believed to be capable of resulting in viable pregnancies, thereby making the transfer of autosomally monosomic embryos, in the hope that they are “mosaic”, a far less risky proposition. Needless to say, if such action is being contemplated, 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 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.

    Geoff Sher

  • Brianna - June 29, 2017 reply

    What are your thoughts/opinions on transferring an XXY or an XYY embryo? Thanks!

    Dr. Geoffrey Sher

    Dr. Geoffrey Sher - June 29, 2017 reply

    I would not knowingly do so!

    Geoff Sher

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