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

completely separated. Following cleavage furrow ingression, the two daughter cells remain attached
for up to several hours through an intercellular bridge that consists of remnants of the contractile ring,
MTs and more than 100 other proteins 195 . Abscission can be delayed in both budding yeast and human
cells by an Aurora B-dependent checkpoint that senses the presence of chromatin in the intracellular
bridge 196,197 . The persistent presence of active Aurora kinase prevents abscission, so that the intercellular
bridge remains intact. This way, Aurora B can inhibit the formation of tetraploid cells in human cells
196 and inhibit chromatin damage in yeast 197 when chromosomes get trapped in the cleavage furrow.
In the absence of any chromatin in the intercellular bridge, abscission will be induced and result in full
physical separation of the two daughter cells. The daughter cells enter the G1 phase of the subsequent
cell cycle and will, if sufficient growth signals are present, proceed with another round of cell division.
3. The DNA damage response
3.1 Repair mechanisms
Chromatin is continuously being exposed to DNA damaging insults, as a result of cell-endogenous
or exogenous processes. DNA damage can be the result of normal cellular metabolism, while
examples of exogenous insults are cigarette smoke, x-rays or sunlight exposure. To prevent the cell
from progressing through the cell cycle when its DNA is damaged, various evolutionarily conserved
responses are present in cells. These responses can lead to repair of the lesion and cell cycle arrest
or apoptosis when the damage is unrepairable. Several types of DNA damage have been found,
each of which is repaired by a specific DNA repair mechanism. Mispaired DNA bases are repaired
through mismatch repair, intra- and interstrand crosslinks by nucleotide excision repair or interstrand
crosslink repair respectively, while single-strand breaks (SSB) are resolved by single-strand break repair
(reviewed in 198 ). Double-strand breaks (DSB) are in general being repaired by either homologous
recombination (HR) or non-homologous end joining (NHEJ), although at least two other types of DSB
repair pathways have been identified as well, such as alternative-NHEJ and single strand annealing
(reviewed in 199 ). HR is promoted by Cdk activity and is mainly active during the S and G2 phases of
the cell cycle. HR is a relatively error-free repair mechanism because it uses homologous sequences
on, preferably, sister chromatids to repair DSBs. NHEJ is more error prone, since it religates two
broken DNA strands independent of their homology 199 . NHEJ is active throughout the cell cycle and
starts with the recruitment of the Ku heterodimer (Ku70 and Ku80), that can form a ring around the
site of DNA damage 200 . This heterodimer loads and activates the catalytic subunit of DNA-PK (DNA-
PKcs), the kinase activity of which is essential for NHEJ to occur 201,202 . Loading of DNA-PKcs results in
recruitment of the DNA ligase XRCC4/LIG4, and this promotes religation of the two broken ends 203,204 .
Although various DSB repair pathways have been identified, the initial response to DSBs almost always
includes activation of the ATM kinase. Double-strand breaks are first being recognized by sensors,
such as the Mre11-Rad50-Nbs1 (MRN) complex, Ku70/80 and the PARP family of proteins, which poly-
ADP-ribosylate DNA surrounding the break 205 . These sensors on their turn promote the recruitment
and activation of ATM 206 . ATM phosphorylates H2AX on serine 139 to form gH2AX 207,208 , which acts
to recruit and sustain binding of a variety of other repair proteins 209 , such as MDC1, which is a direct
sensor of gH2AX and binds to Serine 139 through its BRCT domain 210,211 . MDC1 indirectly stabilizes
ATM binding and as such allows spreading of gH2AX to form discrete foci of up to 1-2 megabases 207,212 .
53BP1, one of the more downstream components of the DNA damage signaling pathway 213 that
promotes NHEJ through inhibition of HR 214-216 , binds methylated DNA surrounding the DSB 217,218 .
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General Introduction 1