The unresolved origin of uniparental diploid cell lines

LETTER TO THE EDITOR The unresolved origin of uniparental diploid cell lines To the Editor: I should like to comment on the interesting report of Morales et al. (1) describing prenatally detected cases of androgenetic/biparental mosaicism. Two mechanisms of origin have been discussed for case 1 that represents a mixture of androgenetic 46,XX and normal biparental 46,XY cells. One alternative involves dispermic fertilization of an oocyte and formation of a triploid zygote followed by a ‘‘division without replication’’ and missegregation of the paternal pronuclei. In the resulting 2 n/n mosaic, endoreduplication of the haploid paternal genome would finally create the diploid androgenetic cell line. This example resembles our observation of an oocyte with three pronuclei that underwent a premature irregular cytokinesis including one of the pronuclei in the smaller half of the divided cytoplasm (2). We surmised that isolation of the supernumerary pronucleus by premature cytokinesis might be relevant for diploidization of triploid zygotes and also for the development of haploid/ diploid mosaics or diploid uniparental cell lines. Although the oocyte described by us obviously had two maternal pronuclei, the corresponding mechanism is also applicable to a dispermic oocyte. Here, one half of the divided ooplasm could receive the maternal pronucleus and one of the paternal pronuclei, giving rise to a diploid biparental cell line. The second paternal pronucleus within the other half of the ooplasm could undergo endoreduplication and create an androgenetic cell line. This concept differs from the model suggested by Morales et al. (1) in one decisive point, that is, a triploid one-cell zygote will not be formed because there is a cytoplasmic division before (not without) replication. The alternative mechanism of origin for case 1 is based on the assumption that a normally fertilized oocyte fuses with another fertilized but ‘‘empty’’ oocyte. In other words, the second cell is supposed to contain no maternal chromosomes. To the best of my knowledge,

there has been no case without chromosomes among the thousands of female gametes that became available for analysis in the course of assisted reproduction (3), and I agree with Golubovsky (4), who already stated that convincing evidence for the existence of anuclear oocytes does not exist. Morales et al. (1) further mention fertilization of a normal oocyte by a diploid spermatozoon as a third but unlikely possibility. In a previous review I have compiled supportive evidence for this event as a mechanism of origin for diandric triploids (5). A possible restriction, however, is the fact that formation of a diploid paternal pronucleus may mainly occur after intracytoplasmic sperm injection because diploid spermatozoa are particularly found in oligozoospermic men. Therefore, I wonder whether the cases presented by Morales et al. (1) have resulted from natural conceptions or from assisted reproduction, and in addition, whether a diploid spermatozoon could have played a role in their second case. This would eliminate the necessity of postulating a failure of DNA replication in the female pronucleus followed by normal replication in the embryo. I appreciate the efforts of Morales et al. (1), who tried to clarify the origin of the observed abnormalities, and believe that information obtained from assisted reproduction will serve as a useful contribution. Bernd E. Rosenbusch, Ph.D. Department of Gynaecology and Obstetrics University of Ulm Ulm, Germany October 6, 2009 B.E.R. has nothing to disclose. doi:10.1016/j.fertnstert.2009.10.055

REFERENCES 1. Morales C, Soler A, Badenas C, RodrıguezRevenga L, Nadal A, Martınez JM, et al. Reproductive consequences of genome-wide paternal uniparental disomy mosaicism: description of two cases with different mechanisms of origin and pregnancy outcomes. Fertil Steril 2009;92:393. e5–9.

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2. Rosenbusch BE, Schneider M. Separation of a pronucleus by premature cytokinesis: a mechanism for immediate diploidization of tripronuclear oocytes? Fertil Steril 2009;92:394. e5–8. 3. Rosenbusch B. The incidence of aneuploidy in human oocytes assessed by conventional cytogenetic analysis. Hereditas 2004;141:97–105.

4. Golubovsky MD. Postzygotic diploidization of triploids as a source of unusual cases of mosaicism, chimerism and twinning. Hum Reprod 2003;18: 236–42. 5. Rosenbusch B. Mechanisms giving rise to triploid zygotes during assisted reproduction. Fertil Steril 2008;90:49–55.

Fertility and Sterility Vol. 93, No. 1, January 2010 Copyright ª2010 American Society for Reproductive Medicine, Published by Elsevier Inc.

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