Mosaic Embryo: What to Expect After the Transfer?

Mosaic Embryo: Should You Give This Little One the Chance? Many implantation embryos designed during IVF Treatment cycles contain two or more cytogenetically distinct cell lines. This phenomenon, known as chromosomal mosaicism, can involve the presence of cells with different types of abnormality [aneuploidy], the absence of any normal embryonic cells or a mixture of normal [euploid] and abnormal [aneuploid] embryonic cells.

Recent studies have represented that among tiny mosaic embryo–dudes and embryo–ladies, or embryos with a mix of normal (euploid) and abnormal (aneuploid) cells, many still have the potential to implant in the womb and develop into chromosomally healthy newborns.

The interpretation of mosaicism among preimplantation embryos is complicated thing both for doctors and couples. Doctors should identify those mosaic embryos that won’t have the developmental abnormalities. Couples should make the serious decision concerning the Preimplantation Genetic Screening (PGS) testing that reveals the abnormalities in the genetic content. And this decision should be made only after the counselling.

Genetic Counselling is exclusively designed for giving the couples the most transparent and inclusive information about whether or not to pursue PGS. The pre–test (pre–PGS) counseling should include a discussion about the frequency of mosaic results, the challenges associated with the interpretation of these results, the possibility of a false positive diagnosis of embryonic mosaicism, and the limited predictive data available. The Genetic Counsellor should also say that the priority for Embryo Transfer should be given to mosaic embryos with low mosaicism levels.

1. What does the Mosaic Embryo look like inside?

Metaphorically saying, have you ever seen the pearl rondelle beads accurately covered ‘curled up’ in translucent package or envelope? If you have never even envisioned that, Google the rondelle pearl beads ‘curled up’ in translucent package or envelope. This is how your handsome embryo–dude or gorgeous embryo–lady looks like inside. If we are talking about normal [healthy] embryo, this pearl content is accurately shaped, ivory–colored and has no ‘dark–blue’ [abnormal] pearls.

The mosaic embryo is a phenomenon. This tiny bundle can be envisioned as mixed pearls: ivory and dark–blue pears in the same line accurately covered ‘curled up’ in translucent package or envelope. It doesn’t have the pure chromosomal content. It has the affected [or even mixed] chromosomal content. But this affected content has its own categories as, in future, if the mosaic embryo corrects itself, the affected mosaic cells can be ‘TRANSFERRED’ to the placenta and won’t affect the tiny fetus, or, in the other case, they can turn the tiny embryo in ‘MOSAIC FETUS’ with many abnormalities. And the hardest question is should you give this little one the chance?

2. Does the mosaicism affect all embryonic cells?

Mosaic embryos can be categorized according to the percentage of abnormal cells in the biopsy, the chromosomes involved, and the types of abnormalities (full chromosome, the segment of a chromosome, single chromosome involved or multiple).

The mosaic embryo–dudes and embryo–ladies that have several chromosomes affected by mosaic abnormality [mosaic aneuploidy] have significantly lower implantation rates than any other category of mosaic embryos. In contrast, embryos with a mosaic segmental abnormality have a capacity to implant.

3. How do they ‘REVEAL’ the mosaic embryos? 

Mosaic preimplantation embryos contain two or more cell lines with a different chromosome content, the consequence of errors in chromosome segregation occurring during mitotic divisions. In other words, their chromosome content is mixed. The scientists have designed advanced techniques to identify those tiny mosaic embryos. Wondering which ones?

In the past, to identify the embryonic mosaicism, the experts used Fluorescence In Situ Hybridization (FISH), a method favored because it represented the information on the chromosomal content of each cell. Using this method, the experts usually examined only about one–third to one–half of the chromosomes in each cell.

At present, the experts use the most advanced and accurate techniques for detecting the embryonic mosaicism.

Molecular cytogenetic techniques (for example, array Comparative Genome Hybridization [aCGH], Single–Nucleotide Polymorphism [SNP] array, quantitative Polymerase Chain Reaction [qPCR], Next–Generation Sequencing [NGS]) have the significant advantage over FISH technique. Wondering which one? Molecular cytogenetic techniques represent the information on the copy number of all 24 types of the chromosome.

Alternatively, the experts may use comprehensive chromosome screening technologies. These technologies involve the analysis of blastocyst biopsy samples, typically composed of 5 embryonic cells, which are not separated but instead are analyzed as a single entity. Although the presence of a mixture of normal and aneuploid cells in the testing sample can sometimes be detected with the use of methods such as aCGH, qPCR, and SNP array, they are relatively insensitive for this purpose. If the ideal results are obtained, mosaicism associated with proportions of aneuploid cells ranging from 40% to 60% can be detected.

The most accurate method to detect embryonic mosaicism is the new advanced technique of high–resolution Next–Generation Sequencing (hr–NGS). hr–NGS technique accurately detects embryonic mosaicism (aneuploid embryonic cells) from 20% to 80%. This method is utterly sensitive for detecting minor lines in mixed cell populations compared with aCGH (but several experts suggest aCGH as it has 100% specificity and sensitivity).

4. Why should they validate the ‘Embryonic Mosaicism’? 

Embryonic Mosaicism is closely associated with the pregnancy loss. It is proven that the embryos with a mosaic biopsy miscarry more often and implant less frequently, although it is also clear that some of the mosaic embryos can correct themselves and turn into the viable fetuses.

Currently, there is no technique that detects the embryonic mosaicism with 100% specificity and sensitivity. To confirm that the embryo is ‘mosaic’ or ‘normal’, sometimes more than one technique should be used. The embryos categorized by hr–NGS as abnormal [aneuploid] or normal [euploid] are recommended to be confirmed by the aCGH technique. Mosaic embryos (20%–80% abnormal cells estimated by hr–NGS technique) may show the different results. On the other hand, hr–NGS technique detects many more mosaics than the aCGH technique.

Embryonic mosaicism can be revalidated with the use of the aCGH and the mosaicism may be not reliably detected. On the contrary, those embryos that are categorized as ‘NORMAL’ with the use of hr–NGS technique, after the revalidation with the use of the aCGH technique, may have the mosaic chromosomal content.

The fact that the most of mosaic embryos detected by hr–NGS are classified as ‘NORMAL’ when analyzed using the aCGH technique, indicates that validation of the embryonic mosaicism should be done.

Some of the experts may say that validation would result in fewer embryos receiving a “normal” diagnosis after PGS–A, but, this strong criterion may also result in transferring the most viable embryos with accurate chromosomal content and decreasing the miscarriage rates.

5. Embryo Selection: which Mosaic Embryos are better to transfer?

Chromosomal abnormalities that occur in Mosaic Embryos can be categorized as ‘monosomies’ and ‘trisomies’ Normally, there are 46 chromosomes arranged in 23 pairs. Twenty–two chromosomal pairs are called ‘autosomes’, and one pair, number 23, is the sex chromosomes. Any variation from this gold pattern causes chromosomal abnormalities.

A chromosome from any of the pairs may be duplicated (trisomy) or absent (monosomy); an entire set of 23 chromosome pairs can be duplicated three (triploidy) or more (polyploidy) times; or one arm or part of one arm of a single chromosome may be missing (deletion).

The experts recommend that the percentage of mosaicism be considered in embryo selection decisions. It must be also noted that the proportion of aneuploidy should be less than 20 percent because the higher percentage of aneuploidy is associated with the adverse outcomes. The higher percentage of aneuploidy not only disturbs but also may disrupt the delicate balance of genetic content.

They also recommend the preferential transfer of embryos showing mosaic monosomies over mosaic trisomies. The embryos showing mosaic euploid/monosomy are preferable to euploid/trisomy, given that monosomic embryos (excepting 45,X) are not viable. 

Another recommendation given by the experts is to transfer some mosaic embryos in preference to others, depending on the type of chromosome involved. The most problematic mosaic embryos that shouldn’t be transferred are those that have affected chromosomes 14 and 15; chromosomes 2, 7, and 16; and chromosomes 13, 18, and 21.

Mosaic embryos involving chromosomes 14 and 15 will cause the genetic disorder, called ‘disomy’. It is the condition of having a chromosome represented twice in a chromosomal complement. Mosaic embryos that have abnormalities in chromosomes 2, 7, and 16 are associated with intrauterine growth retardation. And mosaic embryos with affected chromosomes 13, 18, and 21 are considered to be problematic because in trisomic form these abnormalities can potentially reach term and the abnormalities they cause are beyond the imagination.

CONCLUSION: 

The Mosaic Embryo is a phenomenon as it contains both normal and abnormal embryonic cells. And it is difficult to say, which embryonic cells will prevail. This mystery hides either miracle or frustration. Therefore, prior to transfer the tiny Mosaic Embryo, it should be accurately validated.

The use of only one advanced technique for the detection of mosaicism and/or the application of diagnostic criteria that categorize most Mosaic Embryos as either normal or abnormal has negative clinical consequences. Mosaic Embryos may be categorized as abnormal [aneuploidy], leading to potentially viable embryos being discarded, or as entirely normal, carrying an elevated risk of aneuploid pregnancy.

Every chromosome can be associated with an abnormal phenotype when in mosaic form, the spectrum extending from apparently normal to severely affected/lethal. However, as mentioned above, the types of mosaicism observed during preimplantation embryonic development, and those that affect the fetus or the newborn might represent different phenomena. This is probably one of the most important questions remaining to be answered by the experts.