Chromosome segregation errors: a double-edged sword - TI Pharma

cytoplasms and pinches off the membrane in between the two daughter cells during abscission.
In order to align chromosomes in such a way that each daughter cell will receive the same
amount of DNA, the mitotic spindle has evolved, which is a structure mainly originating from two
microtubule organizing centers at opposite sides of the cell. The mitotic spindle has a bipolar
conformation and primarily consists of dynamic a- and b-tubulin polymers, called microtubules
(MTs), which are associated with a variety of microtubule-associated proteins (reviewed in 4 ).
To create chromosome-microtubule attachments and to ensure congression of the chromosomes to
the metaphase plate, MTs rapidly polymerize and depolymerize during prophase and prometaphase
to ultimately achieve attachments with their plus-ends at specific, highly conserved sites on the
chromosomes, called kinetochores. The initial proposed model of ‘search and capture’ 5 , in which
kinetochores randomly capture MTs originating from centrosomes (the mammalian microtubule
organizing centers), has been extended with the knowledge that MT growth is strongly biased towards
chromatin due to the presence of a Ran-GTP gradient 6 and the fact that MTs can also be directly
nucleated from chromatin 7 and kinetochores 8 . On top of that, a gradient of Aurora B kinase activity
around the chromatin is thought to also help in stabilizing MTs in the vicinity of chromatin 9 .
2. Molecular mechanisms controlling mitotic progression
2.1 Centromeres, Kinetochores and Microtubule attachments
Kinetochores are big proteinaceous structures 10 , of which most of the proteins are not present
during interphase and are only being assembled into the kinetochore upon mitotic entry and
disassembled upon mitotic exit (Fig.2). Kinetochores are built up at specific loci on the chromatin,
called centromeres, which, in humans, are long stretches of repetitive heterochromatic a-satellite
DNA 11 . Centromeres contain non-coding DNA sequences ranging in size from ~125 base pairs in the
budding yeast Saccharomyces cerevisiae to several tens of megabases in higher eukaryotes, such as
humans 11 . Although eukaryotic centromeric DNA does not show any sequence or size conservation
between species, the functionality has been conserved, i.e. marking the region where a Histone H3
variant, named CENP-A in vertebrates 12 , incorporates into the nucleosomes. CENP-A is thought to
be the epigenetic mark that determines centromere identity 13 and forms specific nucleosomes that
serve as a building block for kinetochore formation. CENP-A nucleosomes are essential and, in some
organisms, sufficient for formation of a proper centromere and kinetochore 14-17 . Since centromeric
DNA was found to also consist of Histone H3 containing nucleosomes 18 , a model originated in
which centromeric DNA adapts a 3D conformation where only CENP-A nucleosomes are present on
the side of the sister chromatid facing the spindle pole and as such create an interface for innerkinetochore
formation only on the outside of the chromatid which is facing the spindle poles 18 .
The vertebrate inner kinetochore consists of multiple proteins that are closely associated to centromeric
DNA and CENP-A throughout the cell cycle. Together these proteins form the Constitutive Centromere
Associated Network (CCAN), which is required for proper kinetochore formation (reviewed in 19,20 ).
CENP-C and the CENP-W/T complex, which are part of the CCAN, have been shown to be able to
form ectopic kinetochores in human cells, indicating that they function as core kinetochore assembly
factors 21 . Recently, budding yeast homologues of the CCAN proteins CENP-T and CENP-W were
identified, revealing that the proteins that make up the inner kinetochore structure are evolutionary
conserved 22,23 .
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General Introduction 1