True inter-tissue chromosomal mosaicism in a human blastocyst: problems of interpretation of preimplantation genetic testing results

The results of preimplantation genetic testing for aneuploidies (PGT-A) do not always coincide with the true chromosomal constitution of the blastocyst, especially in cases of complex chromosomal rearrangements. This can be explained by truly biological reasons, such as the self-correction of chromosomally abnormal embryos and mosaicism, as well as by PGT-A procedural specificity. In this report, we analyzed the trophectoderm (TE) and the inner cell mass (ICM) cells from one blastocyst using array comparative genomic hybridization (aCGH) and fluorescent in situ hybridization (FISH) methods and compared the obtained results with the result of PGT-A. PGT-A by aCGH revealed tetrasomy of the short arm of chromosome X in the TE cells of the blastocyst advocating for its unsuitability for uterine transfer. A re-biopsy of TE was performed, followed by aCGH, and then the remaining part of the blastocyst was divided into TE and ICM for analysis by FISH with DNA probes for the short arm and pericentromeric region of chromosome X. In each of the 10 examined TE cell nuclei, four signals corresponding to chromosome X were detected, and in each of the 7 examined ICM cell nuclei, two signals were detected. To summarize, in TE, both aCGH and FISH detected tetrasomy for the short arm of chromosome X. In contrast, ICM showed no chromosomal imbalance for the short arms of chromosome X. Thus, true inter-tissue mosaicism for tetrasomy of the short arm of chromosome X was revealed.

1. Harper J.C., Delhanty J.D. Preimplantation genetic diagnosis. Curr Opin Obstet Gynecol. 2000;12(2):67-72. doi: 10.1097/00001703-200004000-00002.

2. Barbash-Hazan S., Frumkin T., Malcov M., et al. Preimplantation aneuploid embryos undergo self-correction in correlation with their developmental potential. Fertil Steril. 2009;92(3):890-896. doi: 10.1016/j.fertnstert.2008.07.1761.

3. Capalbo A., Rienzi L. Mosaicism between trophectoderm and inner cell mass. Fertil Steril. 2017;107(5):1098-1106. doi: 10.1016/j.fertnstert.2017.03.023.

4. McCoy R.C. Mosaicism in preimplantation human embryos: when chromosomal abnormalities are the norm. Trends Genet. 2017;33:448–463. doi: 10.1016/j.tig.2017.04.001.

5. Griffin D.K., Brezina P.R., Tobler K., et al. The human embryonic genome is karyotypically complex, with chromosomally abnormal cells preferentially located away from the developing fetus. Hum Reprod. 2023;38(1):180-188. doi: 10.1093/humrep/deac238.

6. Kubicek D., Hornak M., Horak J., et al. Incidence and origin of meiotic whole and segmental chromosomal aneuploidies detected by karyomapping. Reprod. Biomed. Online. 2019;38,330–339. doi: 10.1016/j.rbmo.2018.11.023.

7. Coll L., Parriego M., Carrasco B., et al. The effect of trophectoderm biopsy technique and sample handling on artefactual mosaicism. J Assist Reprod Genet. 2022;39:1333–1340. doi: 10.1007/s10815-022-02453-9.

8. Popovic M., Borot L., Lorenzon A.R., et al. Implicit bias in diagnosing mosaicism amongst preimplantation genetic testing providers: results from a multicenter study of 36 395 blastocysts. Hum Reprod. 2024;39(1):258-274. doi: 10.1093/humrep/dead213.

9. Treff N.R., Marin D. The «mosaic» embryo: Misconceptions and misinterpretations in preimplantation genetic testing for aneuploidy. Fertil. Steril. 2021;116:1205–1211. doi: 10.1016/j.fertnstert.2021.06.027.

10. Johnson D.S., Cinnioglu C., Ross R., et al. Comprehensive analysis of karyotypic mosaicism between trophectoderm and inner cell mass. Mol Hum Reprod. 2010;16(12):944-9. doi: 10.1093/molehr/gaq062.

11. Capalbo A., Wright G., Elliott T., et al. FISH reanalysis of inner cell mass and trophectoderm samples of previously array-CGH screened blastocysts shows high accuracy of diagnosis and no major diagnostic impact of mosaicism at the blastocyst stage. Hum Reprod. 2013;28(8):2298-307. doi: 10.1093/humrep/det245.

12. Victor A.R., Griffin D.K., Brake A.J., et al. Assessment of aneuploidy concordance between clinical trophectoderm biopsy and blastocyst. Hum Reprod. 2019;34:181–192. doi: 10.1093/humrep/dey327.

13. Vera-Rodríguez M., Michel C.E., Mercader A., et al. Distribution patterns of segmental aneuploidies in human blastocysts identified by next-generation sequencing. Fertil Steril. 2016;105(4):1047-1055. e2. doi: 10.1016/j.fertnstert.2015.12.022.

14. Babariya D., Fragouli E., Alfarawati S., et al. The incidence and origin of segmental aneuploidy in human oocytes and preimplantation embryos. Hum Reprod. 2017;32(12):2549-2560. doi: 10.1093/humrep/dex324.

15. Fragouli E., Lenzi M., Ross R., et al. Comprehensive molecular cytogenetic analysis of the human blastocyst stage. Hum Reprod. 2008;23(11):2596-608. doi: 10.1093/humrep/den287.

16. Huang J., Yan L., Lu S., et al. Re-analysis of aneuploidy blastocysts with an inner cell mass and different regional trophectoderm cells. J Assist Reprod Genet. 2017;34:487–493. doi: 10.1007/s10815-017-0875-9.

17. Capalbo A., Poli M., Rienzi L., et al. Mosaic human preimplantation embryos and their developmental potential in a prospective, nonselection clinical trial. Am J Hum Genet. 2021;108:2238–2247. doi: 10.1016/j.ajhg.2021.11.002.

18. Tikhonov A.V., Krapivin M.I., Malysheva O.V., et al. ReExamination of PGT-A Detected Genetic Pathology in Compartments of Human Blastocysts: A Series of 23 Cases. J Clin Med. 2024;13(11):3289. doi: 10.3390/jcm13113289.