MOSAICISM IN HUMAN EMBRYOS

Mosaicism in Early Human Embryos-What Does It Have To Do With Me?

Author: Gary Harton, PhD Market Development Leader, Genetics, PerkinElmer

If you are going through an IVF cycle today, or maybe tomorrow, embryo mosaicism is sure to be a term you will hear.  This does, of course, assume you are planning to take advantage of genetic testing of your embryos while they are in the lab and before you and your clinical team decide on the ‘best’ embryo to transfer.  If you don’t already know about genetic testing of embryos during IVF, officially termed preimplantation genetic testing for aneuploidy (PGT-A) then you might want to read this information first.  Briefly, PGT-A allows you and your IVF team to test each of your embryos for chromosome gains and losses before transferring them back into your uterus to attempt pregnancy.  Ideally each cell in the body has a matching set of 23 chromosome pairs, 22 chromosomes numbered 1 through 22 and a pair of sex chromosomes, either XX if you are genetically female or XY if you are genetically male.  The data is quite clear that embryos created through IVF sometimes have the wrong number of chromosomes.  What happens to these embryos?  Sometimes they can lead to a live birth. You would have heard of Down syndrome (having an extra copy of chromosome 21) or Turner’s syndrome (having only 1 X chromosome and no partner X or Y chromosome).  But did you know embryos have been found with an extra copy (trisomy) and/or a missing copy (monosomy) for every chromosome?  And some embryos have more than one chromosome at the same time.  This is naturally occurring, not specific to IVF, and is thought to be some part of the reason getting pregnant is so hard!

So what is embryo mosaicism then?  I like to describe it in a simple fashion using art, mainly because it is fairly common for people to know what mosaic tiles are and most people have seen mosaic art pieces (those really cool images that it turns out are made up of thousands of other tiny images put together just right).  The other reason I like to speak of this in terms of art is it’s one of the acronyms we use in the field to describe IVF, assisted reproductive technology or ART…I like to be clever whenever I can.  In our blog today however, we aren’t making beautiful art pieces, or making cool patterns at the bottom of our pool, we are talking about all of the individual cells that make up embryos.  The latest technology to assess embryos is really sensitive and has led to three different possible results for an embryo test:

• Euploid – correct number of chromosomes and genetically the best embryos to transfer with the highest chance of pregnancy success
• Mosaic – an in between result where some cells in the test are euploid and some cells are aneuploid from an embryo which has some chance of success but typically less than euploid embryos²
• Aneuploid – incorrect number of chromosomes and an embryo which studies have shown have an almost 0% chance of success¹

Up until about 5 years ago before the new testing methods were broadly adopted, we didn’t know much about mosaicism in early human embryos.  We had some idea it might have existed from early data on embryo testing with the older methods but much of that data has been proven to have been an artefact of a poor test, so we really didn’t understand the true level of mosaic embryos in humans.  We also had some idea mosaicism is real from prenatal testing where about 1% of chorionic villus (CVS) samples are diagnosed as mosaic; however, only a very small percentage of these findings are confirmed in the fetus³.  We shouldn’t be too surprised we find mosaic embryos during PGT-A since we are testing the same tissue from the embryo we test later in pregnancy in CVS and we already know some percentage of these CVS findings are mosaic, so they must have been there in some embryos.  As mentioned above, the move over to the newer next generation sequencing (NGS)-based testing has made testing more sensitive and has led to an increasing ability to discern subtleties between euploid and aneuploid, leading to a new grey category of result, mosaicism.

At this point, you’re probably wondering WHY DO I NEED TO KNOW ALL OF THIS???  Well, I think it is important for everyone to understand what goes on in the genetics lab.  It can be really confusing and scary to go through IVF and that’s without getting a mosaic result for one of your embryos.  My goal here is to explain to you what a mosaic embryo is so you’re prepared, and to explain how we are working with you and your IVF team to give you the best possible outcomes from as many embryos as possible during your IVF cycle.

Let’s get back to your PGT-A results and how you’re going to decide what to do with that one embryo that is mosaic.  To be honest, most laboratories that use NGS for PGT-A, and report mosaic results, find only a small percentage of all embryos tested are mosaic.  Hopefully you have a few euploid embryos to transfer and you’ll have a successful pregnancy and you can leave the worry about your mosaic embryo for another day.  At this moment, the recommendation from professional societies, like the Preimplantation Genetic Diagnosis International Society (PGD-IS) is to transfer euploid embryos first, then, when there are no more euploid embryos, discuss with the patient whether to transfer a mosaic embryo.  In 2019, this society published a guideline document that can be found here⁴.  This society is also working on some new initiatives that might help everyone better understand mosaic embryo transfer and potential outcomes from these embryos.  It’s a very active area of clinical investigation throughout the clinics that use PGT-A. It should be noted there have been a few hundred transfers of mosaic embryos to date, and many babies born from those embryos with no sign of poor health outcomes.  However, a recent case study⁵ was published from a clinic in Istanbul, Turkey detailing the analysis, transfer and outcome of a mosaic embryo that resulted in the birth of a child that is mosaic for the same chromosome that was found to be mosaic in the embryo. This is the first description of detailed follow-up of mosaic embryo transfer and a detected mosaicism in a live born child.  This report should be taken into account when thinking about the pros and cons of transferring mosaic embryos.  This child, while mosaic for a single chromosome, appears healthy at this stage, but I’m sure we’ll all be cautiously watching for an update.  This report clearly shows the transfer of mosaic embryos comes with some risk of leading to a mosaic live birth.

Some doctors in the field have decided they don’t understand enough about mosaic embryos to counsel their patients appropriately and have therefore decided not to have mosaic embryos reported to them on their genetic report.  The mosaic embryos are reported as aneuploid instead.  For me I think this is a reasonable decision at the time because we really don’t know enough about mosaic embryos.  In time, and with continued work together, more information will be available to help guide decision making around mosaic embryo transfer.  It should be noted here that the majority of IVF cycles in the world do not utilize PGT-A. Embryos from these non-PGT-A cycles are just as often euploid, aneuploid and/or mosaic and are transferred blindly every day.  Let that one sink in for a moment…

The IVF and Reproductive Genetics field continues to work together to better understand the genetics of embryos, the factors that can lead to a mosaic embryo and how to interpret a mosaic embryo result.  Eventually physicians, nurses and embryologists will have all of the tools they need to counsel couples before an IVF cycle about genetic testing and if the couple decides on PGT-A, they can better counsel the couple on the results of genetic testing.  If you are interested, and can wait a bit longer for more information, I have heard about an excellent paper from the Genetic Counseling Special Interest Group of the American Society of Reproductive Medicine (ASRM) which should be published any day.  This paper reinforces the critical role that genetic counselors bring to the field and why this specialty is so important in reproductive genetics going forward.  The more we look for during PGT-A, the more we find and the more we need their help to explain the results and help couples decide on their best path forward during IVF.

References

1. Tiegs AW, Xin T, Whitehead C, Neal S, Osman EK, Kim JG, Hanson B, Seli E, Patounakis G, Gutmann J, Castlebaum AJ, Scott RT. Does preimplantation genetic testing for aneuploidy (PGT-A) harm embryos?  No-a multi-center, prospective, blinded, non-selection study evaluating the predictive value of an aneuploid diagnosis and impact of biopsy.  Fertil Stert. 2019. Sept;112(3):Supplement e31.
2. Spinella F, Fiorentino F, Biricik A, Bono S, Ruberti A, Cotroneo E, Baldi M, Cursio E, Minasi MG, Greco E. Extent of chromosomal mosaicism influences the clinical outcome of in vitro fertilization treatments.  Fertil Steril. 2018.Jan;109(1):77-83.
3. Hahnemann JM, Vejerslev LO. Accuracy of cytogenetic findings on chorionic villus sampling (CVS)-diagnostic consequences of CVS mosaicism and non-mosaic discrepancy in centres contributing to EUCROMIC 1986-1992.  Prenat Diagn. 1997 Sep;17(9):801-20.
4. Cram DS, Leigh D, Handyside A, Rechitsky L, Xu K, Harton G, Grifo J, Rubio C, Fragouli E, Kahraman S, Forman E, Katz-Jaffe M, Tempest H, Thornhill A, Strom C, Escudero T, Qiao J, Munne S, Simpson JL, Kuliev A. PGD IS Position Statement on the Transfer of Mosaic Embryos 2019. RBM Online. 2019 Aug;39(1):e1-4.
5. Kahraman S, Cetinkaya M, Yuksel B, Yesil M, Pirkevi Cetinkaya C. The birth of a baby with mosaicism resulting from a know mosaic embryo transfer: a case report. Hum Reprod. 2020 Mar 27;35(3):727-733.