Topoisomerase II is differentially required for meiotic chromosome morphology in spermatogenesis and oogenesis
Date
2024
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Publisher
University of Delaware
Abstract
Infertility is a common and world-wide health issue that can be devastating to affected couples. The rate of infertility is between 5% to 15% in both developed and under-developed nations. People in every socioeconomic status, both males and females, and every race and ethnicity are affected. Aneuploidy, when a cell has an incorrect number of chromosomes, is a leading cause of infertility in both females and males. Not only is aneuploidy a key contributing factor in infertility, but it is also the leading cause of miscarriages and birth defects. Aneuploidy occurs as a result of perturbations in the specialized cell division of meiosis. For meiosis to be successful, it is dependent on the accurate separation of homologous chromosomes during meiosis I, and the successive segregation of sister chromatids at meiosis II resulting in haploid gametes from diploid precursor cells. There are many crucial events that need to be properly regulated for this to occur, including homolog pairing, crossing over, recombination, and segregation of homologous chromosomes and sister chromatids. Differences in male and female meiosis have been observed at each of these stages, however the mechanisms that result in this sexual dimorphism have yet to be fully characterized. Elucidating the sexually dimorphic meiotic programs will provide us with the fundamental knowledge needed to treat infertility in both males and females. ☐ Many meiotic events are sex-specific including chromosome structure. In particular, the degree of chromosome compaction in spermatogenesis is much greater than in oogenesis. Differential chromosome compaction is coordinated by several proteins including sperm-specific histone variants and post-translational modifications, condensins, cohesins, and DNA topoisomerases. Previously a loss-of-function mutation of C. elegans Topoisomerase II, top-2(it7) was identified in which homologous chromosomes fail to segregate resulting in aneuploid sperm and embryonic lethality after fertilization. I found that top-2 plays sex-specific roles in the localization of meiotic chromosome structural components and on chromosome structure during late meiotic prophase. During spermatogenesis, late prophase chromosomes are significantly compromised in their ability to condense and individualize after loss of top-2 function. During oogenesis, chromosome structure appears to be unaffected in top-2(it7), however, diakinesis bivalent length is elongated. These sex-specific differences are likely due to differences in the temporal regulation of late meiotic prophase events. ☐ As top-2(it7) changes the chromosome morphology of meiotic chromosomes in both spermatogenesis and oogenesis, I next determined if top-2(it7) changes the recombination landscape. I examined meiotic double-strand break repair by the assembly and disassembly of RAD-51 foci during spermatogenesis and oogenesis. I found fewer RAD-51 foci in the transition zone through late pachytene in top-2(it7) spermatogenic and oogeneic germlines. Even though fewer RAD-51 foci were observed, crossover formation was not perturbed in top-2(it7) spermatogenesis nor oogenesis as five and six GFP::COSA-1 foci, respectively, were found per nucleus in the mutant germlines. However, top-2(it7) appears to alter the position of crossovers in spermatogenic germlines. In addition, I have preliminary data showing that there may be a lack of individualization in top-2(it7) spermatocytes at late diakinesis which is visualized using a DNA fluorescence in situ hybridization (FISH) technique called oligopaint that labels individual chromosomes. ☐ The sexually dimorphic role of topoisomerase II during meiosis presents a unique opportunity to elucidate a novel role for a ubiquitous enzyme. I have determined that TOP-2 is required for the timely release of sister chromatid cohesion during spermatogenesis. I have also shown that TOP-2 is crucial for meiotic chromosome structure in both oogenesis and spermatogenesis, even though loss of TOP-2 is more detrimental to spermatogenesis. These data provide insight into the disassembly of chromosome structural components and chromosome segregation and reveal how these events are regulated by sex-specific mechanisms of meiosis and their role in aneuploidy and infertility.
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Keywords
Infertility, Meiosis, Oogenesis, Spermatogenesis, Topoisomerase II