Different perceptions of similar observations

Different perceptions of similar observations To the Editor: Having read with interest the manuscript by Li et al. (1), we feel it warrants mentioning that this paper highlights the need for a firm understanding of the biological principles of both normal and abnormal embryo development when interpreting fluorescence in situ hybridization (FISH) results. This study examined chromosomes 13, 18, 21, X, and Y on day 3 of development by preimplantation genetic diagnosis (PGD). Embryos determined to be aneuploid were cultured until day 6 of development, and nuclei derived either from the inner-cell mass alone or from both trophectoderm and inner-cell mass were reexamined with the same five chromosome probes. We note that Figure 3 of Li et al. (1) demonstrates a fluorescent-labeled image the authors describe as “micronucleus extrusion.” The image shows regular background staining of DNA and what they characterize as an “extra” copy of each chromosome probe they used, except one, the Y-chromosome. Micronucleus extrusion has classically been used to describe the expulsion of nuclear fragments from cells exposed to environmental toxins (2). The expulsion is described as irregular and containing fragments of chromosomes at random. This description does not correspond with the well-formed, organized structural appearance of the image offered by Li et al. (1). At the same time, we were amazed by the similarity of the image presented to normal images of the second polar body after FISH. The second polar body is the result of meiosis and therefore is haploid with an X-chromosome having been derived from the female zygote (3). Other possible sources of the “extruded micronucleus” described by the authors were not discussed in the manuscript. The authors present Figure 3 as an example of the embryo’s potential for “self-correction” of early embryonic aneuploidy. We find it difficult to understand how this hypothesis integrates with the extensive knowledge in the literature on micronucleus extrusion as well as the evidence available in the literature on the fate of cells with aneuploid nuclei (4). Keeping in mind the well studied principles of mitosis and the remarkable similarity of the “extruded micronucleus” of Figure 3 in Li et al. (1) to the intact nuclei of a second polar body, we propose that the embryo was misdiagnosed as being triploid because of contamination of their blastomere biopsy with the second polar body, which was inadvertently coaspirated at the time of biopsy.

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The hypothesis offered by Li et al. (1) further implies that a mechanism is in place in an embryonic blastomere to identify extranumeric chromosomes and to eliminate them from the nucleus in one body. As the mechanism of mitosis is well characterized in human cells (5), current knowledge fails to provide an explanation for which component(s) of mitosis would enable such a process to occur. More frequently seen in the literature and a far more plausible explanation of the discordant results presented in Figure 2 than “self-correction” of a mosaic embryo as postulated by the authors is the possibility of the inadvertent biopsy of an aneuploid cell undergoing apoptosis whose genetic composition will ultimately fail to contribute to the whole embryo (4). We applaud the authors for their recognition of the potential for error with PGD. We read with understanding their efforts to minimize these concerns by applying scientifically valid analysis and thought when unexpected aberrations presented themselves in their program. We would, however, lend caution to and take exception with their liberalization of the use of terms “extruded micronucleus” and embryonic “self-correction” when such use occurs to the exclusion of the obvious. Christine Briton-Jones, Ph.D. Queenie Yeung, B.Sc. Jeffrey Steinberg, M.D. The Fertility Institutes Encino, California November 28, 2005

REFERENCES 1. Li M, DeUgarte CM, Surrey M, Danzer H, DeCherney A, Hill DL. Fluorescence in situ hybridization reanalysis of day-6 human blastocysts diagnosed with aneuploidy on day 3. Fertil Steril 2005;84:1395– 400. 2. Dopp E, Hartmann LM, Florea AM, von Recklinghausen U, Pieper R, Shokouhi B, et al. Uptake of inorganic and organic derivatives of arsenic associated with induced cytotoxic and genotoxic effects in Chinese hamster ovary (CHO) cells. Toxicol Appl Pharmacol 2004;201:156 – 65. 3. Verlinsky Y, Kuliev A. Atlas of preimplantation genetic diagnosis. 2d rev. ed. London: Taylor Francis, 2004:157–72. 4. Evsikov S, Verlinsky Y. Mosaicism in the inner cell mass of human blastocysts. Hum Reprod 1998;13:3151–5. 5. Taylor SS, Scott MI, Holland AJ. The spindle checkpoint: a quality control mechanism which ensures accurate chromosome segregation. Chromosome Res 2004;12:599 – 616.

doi:10.1016/j.fertnstert.2006.01.001

Fertility and Sterility姞 Vol. 85, No. 4, April 2006 Copyright ©2006 American Society for Reproductive Medicine, Published by Elsevier Inc.

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