Biology Reference
In-Depth Information
“template switching” or “fork reversal”. Once the
invading DNA strand is extended beyond the site of
DNA lesion on the original template, it may switch
back to the original template with the help of proteins
promoting branch migration and HJ dissolution.
96
When replication forks encounter SSBs, one-ended
DSBs could be generated. If one-ended DSBs have suffi-
cient ssDNA overhangs or are resected, the 3
0
ends of
these breaks could invade the opposite arm of replica-
tion forks and form D loops. However, unlike typical
two-ended DSBs, one-ended DSBs cannot form double
HJs. It has been proposed that the migration of D loops
may help to re-establish lagging strand synthesis, and
replication forks could be regenerated by dissolution
of a single HJ.
91
HRR proteins may be used in many
different ways at stalled replication forks depending
on the nature of blockage and the DNA structures
arising at these forks.
A specific role in HRR has been proposed for a putative
replication restart complex consisting of FANCD1/
BRCA2, FANCD2, FANCG, and XRCC3.
112,113
The
HRR proteins FANCD1/BRCA2, FANCJ/BRIP1, and
FANCN/PALB2 require DNA replication for their asso-
ciation with ICL while the other FA pathway members,
including FANCI and FANCD2 do not.
114
Altogether,
these data illustrate the increasing evidence that the FA
pathway is not linear but rather composed of multifunc-
tional proteins that form different subcomplexes with
specific functions.
113,115
e
122
For a discussion of the influ-
ence of the FA pathway on RAD51 foci formation, see
“Human Cancer Predisposition Syndromes with HRR
Defects” below. A detailed review of the non-HRR func-
tions of this pathway is beyond the scope of this text.
ALTERATIONS OF HRR IN
HUMAN CANCER
Role of the Fanconi Anemia (FA) Pathway in HRR
Stalled replication forks activate the FA pathway,
which has, among other functions, a poorly defined
role in the promotion of HRR (reviewed in
97,98
). This
pathway is composed of at least 14 genes, FANCA
through FANCP, known to cause FA in patients when
mutated in both alleles (see also “The DNA Damage
Response and HRR as an Anticancer Barrier” below
for a description of the syndrome).
99
e
102
The FA proteins
together with BRCA1 cooperate in a common biochem-
ical “FA/BRCA” pathway, which is believed to function
mainly in the detection, stabilization, and repair of
stalled DNA replication forks.
99,103
A multiprotein
nuclear core complex is required for baseline and
damage-induced mono-ubiquitination of the down-
stream effectors FANCD2 and FANCI, called the “ID”
complex.
101
In response to DNA damage such as replica-
tion fork-blocking ICLs, mono-ubiquitinated FANCD2
relocates into chromatin and co-localizes with BRCA2/
FANCD1, RAD51, and other DNA damage response
proteins; and these protein accumulations can be visual-
ized as subnuclear foci.
104
e
106
Disruption of the nuclear
core complex or mutation of FANCD2's mono-ubiquiti-
nation site at K561 abrogates loading of FANCD2 into
chromatin, foci formation, and DNA repair.
105
In
response to ICL, this complex pathway promotes nucle-
olytic incisions near ICL, TLS, and subsequently HRR
and NER to restore replication forks and remove ICLs
(reviewed in
97,98
).
The FA pathway is closely linked to HRR via its func-
tional interaction with BRCA1 as well as the discovery
that BRCA2, BRIP1, PALB2, and possibly RAD51C repre-
sent FA gene products FANCD1, FANCJ, FANCN, and
FANCO, respectively.
100,107
e
110
In addition, using genetic
analysis in the chicken DT40 model, fancc was epistatic
with xrcc3 with regard to crosslinker sensitivity.
111
The DNA Damage Response and HRR
as an Anticancer Barrier
The DNA Damage Response (DDR)
The information stored in the genome encodes for the
entire make-up of a cell and ultimately of any organism.
Thus, the genome is the most valuable component of
a cell. Maintaining integrity of the genetic information
encoded by the DNA is therefore of the utmost impor-
tance to ensure faithful transmission to further genera-
tions. However this endeavor is constantly challenged,
as the DNA is exposed to various damaging threats.
These can arise both exogenously or endogenously.
Ionizing radiation, UV radiation, and various chemicals
are examples for exogenously occurring threats,
whereas reactive oxygen species (ROS) for instance are
produced during normal metabolism.
1
To cope with
these challenges a highly regulated network of signaling
pathways has evolved which orchestrates the cellular
response to DNA damage. This network, the DDR,
senses the damage, transduces the signal to downstream
players, eventually leading to removal of the lesion.
Alternatively, if the damage is too severe, the affected
cell is removed from the proliferating pool by either
apoptosis or senescence.
1,123
The DNA Damage Response: A Barrier to Cancer
Development
As DNA damage occurs, the DDR becomes activated.
Cells that continue to proliferate are not only those that
have successfully repaired the damage, but also the ones
that fail to trigger cell death or senescence programs or
that exhibit faulty DNA repair. In the latter scenario,
the damaged DNA is passed on to further generations.
Failure to activate death/senescence programs or faulty
