The contribution of paternal chromatin to chromosome segregation errors in human pre-implantation embryos.

Screening of human embryos for chromosomal aneuploidies before transfer in in
vitro fertilization by us and others has revealed the majority of embryos to be
chromosomally abnormal. The bulk of these errors arise during the first mitotic
divisions of early pre-implantation development, resulting in chromosomally
mosaic embryos. Research into the origin of embryo aneuploidy has so far
focused on meiotic segregation errors and on identifying causative factors in
the oocyte. This study aims to address the paternal contribution to embryo
aneuploidy.
Upon fertilization, the paternal genome, contributed by the sperm, is unpacked
by removing the protamines; small basic proteins that allowed compaction of the
genome in the small sperm head. The protamines are immediately replaced by
maternally provided histones, the basic building blocks of chromatin in all
somatic cells. However, part of the paternal genome was already packaged by
histones when it entered the oocyte, and these histones are not replaced.
Histones may be modified by methylation, acetylation, and other modifcations.
Such *marks* provide epigenetic information that may influence the regulation
and behavior of chromosomes. In the context of cell divisions, the histones and
histone modifications on pericentric heterochromatin are important. Pericentric
heterochromatin is the region on the chromosome that flanks the centromere.
Correct establishment of the epigenetic signature of this region is well known
to be crucial for chromosome segregation in somatic cells. We now have evidence
indicating that sperm-inherited epigenetic marks are required for pericentric
heterochromatin formation in the zygote. Interestingly, variable nucleosome
content is a frequent characteristic of human sperm, particularly of subfertile
men. We hypothesize that paternal and maternal chromosomes differ in the
composition of pericentric heterochromatin which in turn impacts on segregation
behavior of paternal and maternal chromosomes during pre-implantation embryo
development.

We aim to:
1) Characterize differences between paternal and maternal chromatin
organization and pericentric heterochromatin formation after fertilization and
during pre-implantation embryo development.
2) Analyze if this asymmetry affects formation of the kinetochore; a
specialized protein structure that is required for microtubule attachment
during metaphase and cell cycle checkpoint function.
3) To functionally investigate the capacity of paternal and maternal
chromosomes to align properly on the metaphase plate in tripronuclear zygotes.

The first part of the study is descriptive, involving fixed material: human
tripronuclear zygotes and surplus embryos. The second part uses an established
assay to assess chromosome alignment and error correction in tripronuclear
zygotes.

The study will not interfere with the standard IVF and embryo transfer
procedures and will only use surplus embryos. The study will not negatively
affect pregnancy rates, nor will it affect the women*s or children*s health.
Insight gained from this study will help to increase our understanding of
factors contributing to the high rates of embryo aneuploidy in IVF. If our
results show that paternally derived chromosomes are more frequently involved
in alignment failure, and if this can be related to the pericentric
heterochromatin signature, the next step would be to relate this to the
variable nucleosome content in sperm and sperm quality parameters. Finally this
may lead to the development of improved sperm cell selection protocols, which
may aid in lowering the frequency of embryo aneuploidy in embryos from future
IVF patients.