Presence of an extra chromosome alters meiotic double-stranded break repair dynamics and MLH1 foci distribution in human oocytes.

Studies performed on human trisomic 21 oocytes have revealed that during meiosis, the three homologues 21 synapse and, in some cases, achieve what looks like a trivalent. This implies that meiotic recombination takes place among the three homologous chromosomes 21, and to some extent, crossovers form between them. To see how meiotic recombination is in the presence of an extra chromosome 21, we analyzed the distribution of three recombination markers (γH2AX, RPA, and MLH1) on trisomic 21 oocytes at pachynema and, in particular, on chromosomes 21. Results clearly show how the presence of an extra chromosome 21 alters meiotic recombination progression, leading to the presence of a higher number of early recombination markers at pachynema. Moreover, the distribution on these chromosomes 21 of some of these markers is different in aneuploid oocytes. Finally, there is a substantial increase in the number of MLH1 foci, a marker of most crossovers in mammals, which is related to the number of synapsed chromosomes in pachynema. Thus, bivalents 21 had fewer MLH1 foci than partial or total trivalents, suggesting a close relationship between synapsis and crossover designation. All of the data presented suggest that the presence of an extra chromosome alters meiotic recombination globally in aneuploid human oocytes.

Evolution of the meiotic prophase and of the chromosome pairing process during human fetal ovarian development.

BACKGROUND: Studies on human oocytes in prophase I are limited due to the difficulty in obtaining the sample. However, a complete study of meiotic prophase evolution and the homologue pairing process is necessary to try to understand the implication of oogenesis in the origin of human aneuploidy. METHODS: A complete analysis of meiotic prophase progression comprising the long developmental time period during which meiotic prophase takes place, based on the analysis of a total of 8603 oocytes in prophase I from 15 different cases is presented. The pairing process of chromosomes 13 and 18 is also described. RESULTS: The findings significantly relate for the first time the evolution of meiotic prophase to fetal development. Although for both chromosomes 13 and 18 a high pairing efficiency is found, pairing failure at the pachytene stage has been observed in 0.1% of oocytes. However, errors at the diplotene stage are substantially increased, suggesting that complete, premature disjunction of the homologues commonly occurs. Moreover, pre-meiotic errors are also described. CONCLUSIONS: Our findings show that homologous chromosomes pair very efficiently, but the high frequency of complete, premature homologue separation found at diplotene suggests that mechanisms other than the pairing process could be more likely to lead to the high aneuploidy rate observed in human oocytes.

Chromosome 18 pairing behavior in human trisomic oocytes. Presence of an extra chromosome extends bouquet stage.

Little is known about the first meiotic prophase stages in the human female because these occur during fetal life, and only a few studies have addressed aneuploid human oocytes. In this paper, the synaptic process in the meiotic prophase in three 47, XX+18 cases is analyzed. A complete study of the dynamics of centromeres and telomeres, cohesin core and synapsis development in aneuploid female meiosis was performed. Investigation of chromosome dynamics in prophase of trisomy 18 oocytes show that these events follow the major patterns seen earlier in euploid oocytes. However, there is a significant delay in the resolution of bouquet topology which could relate to the presence of a surplus chromosome 18 axial element in zygotene oocytes. Pachytene oocytes displayed normal synapsis among the three chromosome 18s. However, in some oocytes the surplus chromosome 18 core was aligned to the bivalent 18. As ataxia telangiectasia and Rad3 related kinase (ATR) has been described as a marker for late-pairing chromosomes in mice, ATR distribution was analyzed in human meiocytes--spermatocytes, euploid oocytes and trisomic oocytes. In contrast to the observations made in mice, no preferential staining for late-pairing chromosomes was observed in humans. In the cases studied, bivalent synapses progressed as in a normal ovary, contrasting with the hypothesis that a surplus chromosome can modify pairing of other chromosomes.

Female-specific features of recombinational double-stranded DNA repair in relation to synapsis and telomere dynamics in human oocytes.

Chromosome segregation errors are a significant cause of aneuploidy among human neonates and often result from errors in female meiosis that occur during fetal life. For the latter reason, little is known about chromosome dynamics during female prophase I. Here, we analyzed chromosome reorganization, and centromere and telomere dynamics in meiosis in the human female by immunofluorescent staining of the SYCP3 and SYCP1 synaptonemal complex proteins and the course of recombinational DNA repair by IF of phospho-histone H2A.X (gamma-H2AX), RPA and MLH1 recombination proteins. We found that SYCP3, but not SYCP1, aggregates appear in the preleptotene nucleus and some persist up to pachytene. Telomere clustering (bouquet stage) in oocytes lasted from late-leptotene to early pachytene-significantly longer than in the male. Leptotene and zygotene oocytes and spermatocytes showed strong gamma-H2AX labeling, while gamma-H2AX patches, which colocalized with RPA, were present on SYCP1-tagged pachytene SCs. This was rarely seen in the male and may suggest that synapsis installs faster with respect to progression of recombinational double-strand break repair or that the latter is slower in the female. It is speculated that the presence of gamma-H2AX into pachytene highlights female-specific peculiarities of recombination, chromosome behavior and checkpoint control that may contribute to female susceptibility for aneuploidy.

The use of foetal ovarian stromal cell culture for cytogenetic diagnosis. Stromal ovarian culture cytogenetic diagnosis.

Some studies have been carried out to analyze human female first meiotic prophase. Most of them use samples from foetuses collected after legal interruption of pregnancy. In some cases, a control population is needed and foetuses aborted for non-chromosomal reasons are used. The assumption of these samples as being euploids could perhaps represent an error. In this article, we describe an easy methodology to certify the euploidy of foetal ovarian tissue using an one-week somatic culture. Using this protocol, we have obtained a primary culture in 88.2% of the studied cases, material usable for being karyotyped in 93.3% of the cases, and a cytogenetic diagnosis was performed in 100% of these cases. Finding the same karyotype in cultured cells in cases in which we had a prenatal cytogenetic diagnosis has validated the technique, and in applying this protocol we have been able to check our prophase meiotic-study control population.

Oclusión fetoscópica del cordón umbilical en gestación gemelar monocorial / Fetoscopic cord occlusion in discordant monochorionic pregnancies

Objetivos: La enfermedad o muerte de un feto amenaza gravemente a su gemelo en la gestación gemelar monocorial. En estos casos la cirugía endoscópica intrauterina, mediante la oclusión de cordón, puede ser la única solución para mejorar el pronóstico. Este estudio evalúa la experiencia y los resultados perinatales con la oclusión fetoscópica de cordón umbilical en gestación monocorial discordante en los primeros 14 casos realizados en nuestro centro. Material y métodos: Catorce casos de gestación monocorial biamniótica con indicación de oclusión de cordón tratados mediante oclusión fetoscópica con láser (n = 2), coagulación bipolar (n = 6) o una combinación de los dos (n = 6). Se analizan los resultados perinatales y la presencia de alteraciones en ecografía cerebral al mes de vida. Resultados: El procedimiento se pudo completar en todos los casos. No hubo complicaciones intraoperatorias remarcables. Se produjo muerte intrauterina del feto normal en dos casos (14 per cent). La incidencia global de rotura prematura de membranas fue del 14 per cent (2/14), a las 28 y 36 semanas. La edad gestacional media de parto fue 34,7 (límites, 28-40) semanas, y el peso medio fetal de 2.335 g (límites, 890-3.300 g). La supervivencia neonatal del feto normal fue del 86 per cent (12/14). Un feto nacido a las 28 semanas desarrolló un cuadro de leucomalacia de grado II. En el resto de casos no hubo complicaciones neonatales graves y la ecografía neonatal fue normal. Conclusión: La oclusión fetoscópica de cordón umbilical puede mejorar significativamente el pronóstico del feto normal en la gestación monocorial discordante, y es una técnica consolidada dentro del arsenal terapéutico de la cirugía endoscópica intrauterina. Los resultados presentados en esta serie son comparables a los reportados en series previas, y se sitúan en el rango alto de supervivencia descrito (AU)