Balancing Act: Equalizing Sex Specific Differences in CENP-A levels is Critical for Mammalian Embryogenesis - PROJECT SUMMARY Abnormal chromosome numbers (aneuploidy) are common in early embryos, particularly mammals. Embryonic aneuploidies can arise during gamete differentiation (inherited) or during mitotic divisions in the embryo (acquired). While previous studies have primarily focused understanding how inherited aneuploidies occur, the molecular origins of acquired aneuploidies are not well understood. Substantial literature has implicated specialized regions of chromosomes, termed centromeres, as critical for mediating proper chromosome segregation during cell division. Centromeres are epigenetically defined by the presence of nucleosomes containing the histone H3 variant CENP-A. Work conducted in somatic model systems has demonstrated that the quantitative maintenance of CENP-A nucleosomes at centromeres is essential for faithful chromosome segregation. Thus, somatic CENP-A levels are quite stable. However, our lab and others have shown that CENP- A is highly dynamic in the germline, and is regulated in a sex-specific manner, resulting in the generation of terminally differentiated gametes (sperm and MII oocytes) with drastically different levels of CENP-A. Despite this drastic difference in CENP-A levels, we have shown that parental CENP-A levels approximately equalize in the zygote, prior to entry into the first mitosis. The overarching objective of this proposal is to investigate how CENP-A levels between maternal and paternal centromeres equalize during the first cell cycle. Our preliminary data indicates that maternally inherited CENP-A protein is deposited onto paternal centromeres. Additionally, analysis of early post-fertilization stage zygotes indicates that CENP-A deposition machinery is preferentially recruited to paternal centromeres. Furthermore, earlier literature has revealed that chromatin remodeling is differentially regulated between the maternal and paternal genomes. Thus, my central hypothesis is that CENP- A equalization is essential for proper early embryonic development and is controlled by centromeric and pericentromeric chromatin factors that differ between the two parental genomes, leading to asymmetric deposition of CENP-A prior to gametic nuclear fusion. In Aim 1 I will define the source of CENP-A protein used during zygotic equalization. For Aim 2, I will determine the chromatin factors involved in regulating CENP-A zygotic equalization. Finally, in Aim 3 I will test the control mechanism for zygotic CENP-A levels. Completion of this project will contribute knowledge to how zygotic CENP-A regulation mediates proper chromosome segregation in early mammalian embryos.
