| dc.description.abstract |
<p>In order to maintain genome integrity cells employ a set of well conserved DNA
damage checkpoints. DNA damage checkpoints are active during interphase and serve
to prevent mitosis with broken DNA. Mitosis with broken DNA is associated with DNA
segregation errors, genome instability and even cell death in resulting daughter cells.
It has recently has been appreciated that cells can compensate for damaged DNA during
mitosis. However, little is known about this mitotic DNA damage response.</p><p>In
this work, I have utilized a genetically tractable system to study mitotic DNA damage
responses in Drosophila. During development, Drosophila rectal papillar cells undergo
developmentally programmed inactivation of DNA damage responses. Following inactivation,
papillar cells undergo two rounds of mitosis. We find that papillar cells fail to
undergo cell death or high-fidelity DNA repair prior to mitosis and instead enter
mitosis with DNA double stranded breaks (DSBs). Remarkably, papillar cells segregate
acentric DNA fragments into daughter cells during mitosis resulting in viable daughter
cells, normal organ development and function. Proper segregation and organ formation
is dependent on the FANCONI Anemia gene FANCD2. Loss of FANCD2 results in unaligned
acentric fragments and mis-segregation of broken DNA resulting acentric micronuclei
formation. Mis-segregation of acentric DNA results in cell death and failure to form
a developmentally normal and functional organ. Thus, we have uncovered a role for
FANCD2 in mitotic DNA damage responses.</p><p>Additionally, we find that single-stranded
DNA (ssDNA) is present during papillar cell mitosis following DNA DSB induction.
ssDNA is present on both the edge of segregating and lagging DNA as well as spanning
short regions between fragments of lagging DNA. The observation that ssDNA is present
suggests that while papillar cells do not initiate complete repair, some level of
DNA resection must occur following DNA DSB induction. In line with this reasoning,
we find a role for the DNA damage sensor complex, the MRN complex, in papillar cell
survival following I-Cre induction. The MRN complex consists of three components,
Mre11, Rad50 and NBS1. Loss of Mre11 or NBS1 results in reduced papillar cell survival
following I-Cre induction. Furthermore, Mre11 is a nuclease. Thus, we propose that
MRE11 acts at sites of DNA DSBs in papillar cells to create ssDNA. We hypothesize
that formation of ssDNA is sufficient to form a DNA/protein bridge between segregating
and lagging DNA to enable proper DNA segregation. Interestingly, resistance to DNA
damage is also observed in many cancers. We speculate that such DNA damage resistant
cancer cells may utilize similar mechanisms to compensate for DNA breaks during mitosis.</p>
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