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

Since it has been shown that replication stress can induce chromosome segregation
errors 379,380 , testing the hypothesis that cleavage furrow ingression in the presence of
chromosome segregation errors results in fragile site breakage, could reveal a feedback loop
which provides a genetically unstable cancer cell with even more possibilities to evolve.
Finally, another cause of missegregation-induced damage could be the presence of an increased,
centromere localized force imposed by microtubules coming from the two spindle poles. This has
also been suggested by an earlier study, in which the authors observed centromere-localized DNA
damage foci in interphase CIN cells 622 . To address this point, future studies should determine whether
inhibition of this centromere-localized force reduces the DNA damage observed following mitotic
exit. In addition, we are currently performing Chromatin-IP experiments using DNA damage specific
antibodies to assess where the DSBs are specifically induced at for example centromeres or fragile
sites following chromosome segregation errors.
1.3 Chromosome segregation errors & Chromothripsis
The recent discovery of a chromosome shattering phenomenon induced by double-stranded breaks,
termed chromothripsis, in 2-3% of all cancer genomes 623 , has resulted in the emergence of various
hypotheses explaining this striking phenotype. Chromothripsis is defined by the presence of multiple
and complex rearrangements, which typically produce a single chromosome, chromosome arm
or region that is made up out of fragments from multiple chromosomes 623 . Interestingly, the copy
number state in these regions of chromothripsis alternates between one or two copies and does
not include multiple copies. It is therefore thought that chromothripsis occurs in one single cellular
event, since progressive events would most likely lead to multiple copy number changes 623 . Moreover,
chromothripsis is defined by the presence of a high level of breakpoint clustering at one specific
chromosomal locus, while progressive, consecutive events would be expected to cause randomly
distributed breakpoints 624 . Chromothripsis has been shown to be causative in the development
of congenital defects 625 . Moreover, it has been shown to be linked to tumors harboring mutations
in the tumor suppressor p53 and is associated with poor prognosis in patients with acute myeloid
leukemia and neuroblastoma 626 . In general, chromothripsis seems to be yet another mechanism of
creating heterogeneity in cancer cells 624 by for example disruption of tumor suppressor genes 623 .
Several underlying mechanisms for chromothripsis have been proposed, such as replication fork
stalling or exposure to irradiation during mitosis 624 , but direct causes of chromothripsis are still
unknown. The finding that micronuclei undergo massive replication-induced DNA damage and that
chromatin present in these micronuclei can remain compartmentalized for several cell divisions,
led to the hypothesis that chromosome segregation errors could cause chromothripsis through
the formation of micronuclei 390 . The induction of multiple DSBs in replication-defective micronuclei
and subsequent reintegration of the chromatin from this micronucleus into the main nuclear
mass, could lead to the formation of highly localized breaks on only one single chromosome 390 .
Alternatively, the presence of highly clustered DSBs has also led to the hypothesis that chromothripsis
is induced due to DNA damaging events on condensed chromatin 623 . Since lagging chromosomes can
get damaged during cytokinesis while being in a condensed state 389 , we propose another hypothesis
in which cleavage furrow ingression could cause chromothripsis by inducing multiple breaks on the
lagging chromatin. In line with this hypothesis, genome sequencing of patients with congenital disease
has revealed that the highly clustered DSBs associated with chromothripsis are often repaired by NHEJ
627 , as was to be the preferred mode of repair of cytokinesis-induced damage as well (Chapter 2). To
test our hypothesis we have generated single clones following a single chromosome missegregation
event and are currently evaluating the presence of chromothripsis in these clones.
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Discussion
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