Chromosome instability is a hallmark of tumorigenesis. This study establishes that chromosome instability is also common during early human embryogenesis. A new array-based method allowed screening of genome-wide copy number and loss of heterozygosity in single cells. This revealed not only mosaicism for whole-chromosome aneuploidies and uniparental disomies in most cleavage-stage embryos but also frequent segmental deletions, duplications and amplifications that were reciprocal in sister blastomeres, implying the occurrence of breakage-fusion-bridge cycles. This explains the low human fecundity and identifies post-zygotic chromosome instability as a leading cause of constitutional chromosomal disorders.

Genomic imbalances are a major cause of constitutional and acquired disorders. Therefore, aneuploidy screening has become the cornerstone of preimplantation, prenatal and postnatal genetic diagnosis, as well as a routine aspect of the diagnostic workup of many acquired disorders. Recently, array comparative genomic hybridization (array CGH) has been introduced as a rapid and high-resolution method for the detection of both benign and disease-causing genomic copy-number variations. Until now, array CGH has been performed using a significant quantity of DNA derived from a pool of cells. Here, we present an array CGH method that accurately detects chromosomal imbalances from a single lymphoblast, fibroblast and blastomere within a single day. Trisomy 13, 18, 21 and monosomy X, as well as normal ploidy levels of all other chromosomes, were accurately determined from single fibroblasts. Moreover, we showed that a segmental deletion as small as 34 Mb could be detected. Finally, we demonstrated the possibility to detect aneuploidies in single blastomeres derived from preimplantation embryos. This technique offers new possibilities for genetic analysis of single cells in general and opens the route towards aneuploidy screening and detection of unbalanced translocations in preimplantation embryos in particular.

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The serpin plasminogen activator inhibitor-1 (PAI-1) slowly converts to an inactive latent form by inserting a major part of its reactive center loop (RCL) into its ␤-sheet A. A murine monoclonal antibody (MA-33B8), raised against the human plasminogen activator (tPA)⅐PAI-1 complex, rapidly inactivates PAI-1. Results presented here indicate that MA-33B8 induces acceleration of the active-to-latent conversion. The antibodyinduced inactivation of PAI-1 labeled with the fluorescent probe N,N-dimethyl-N-(acetyl)-N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl) ethylene diamine (NBD) at P9 in the RCL caused a fluorescence enhancement and shift identical to those accompanying the spontaneous conversion of the P9⅐NBD PAI-1 to the latent form. Like latent PAI-1, antibody-inactivated PAI-1 was protected from cleavage by elastase. The rate constants for MA-33B8 binding, measured by NBD fluorescence or inactivation, were similar (1.3-1.8 ؋ 10 4 M ؊1 s ؊1 ), resulting in a 4000-fold faster inactivation at 4.2 M antibody binding sites. The apparent antibody binding rate constant, at least 1000 times slower than one limited by diffusion, indicates that exposure of its epitope depends on an unfavorable equilibrium of PAI-1. Our observations are consistent with this idea and suggest that the equilibrium involves partial insertion of the RCL into sheet A: latent, RCL-cleaved, and tPA-complexed PAI-1, which are inactive loop-inserted forms, bound much faster than active PAI-1 to MA-33B8, whereas two loop-extracted forms of PAI-1, modified to prevent loop insertion, did not bind or bound much more weakly to the antibody. Plasminogen activator inhibitor-1 (PAI-1)1 is a serine proteinase inhibitor of the serpin family, a class of inhibitory and noninhibitory proteins sharing a common tertiary structure. Its most prominent features are the major, 5-stranded ␤-sheet A, which can accommodate an additional strand in position 4, and a flexible reactive center loop (RCL) of about 20 residues denoted P15-P5Ј (1, 2). Inhibitory serpins function as suicide substrate inhibitors and acylate their target proteinases (3, 4). The acyl-enzyme complex is stabilized by insertion of the cleaved RCL as strand 4 of ␤-sheet A, accompanied by a translocation of the acylated enzyme from its initial binding site to its "locking" site. If RCL insertion is hindered by mutating the P14 threonine to an arginine (5), or by blocking position 4 of ␤-sheet A with a peptide analog of the P14-P7 part of the RCL (6), PAI-1 becomes a substrate that is completely hydrolyzed at the scissile bond by its target proteinases.PAI-1 is the only serpin known to fold spontaneously to an inactive latent form (7), and the t1 ⁄2 for this conversion is 9.5 h at 25°C and pH 7. 4 (6, 8). In this process, the P15-P3 portion of the RCL, which is normally exposed at the apex of the molecule, is inserted as strand 4 in ␤-sheet A, forcing the remainder of the loop, including the reactive center, to adopt an extended conformation alongside the protein scaffold. The term "latent" was introduced becaus...