Biology Reference
In-Depth Information
C. BRCA2
Germline mutations of the BRCA2 gene predispose individuals to highly
penetrant, autosomal-dominant breast and ovarian cancers as well as predis-
position to other types of cancers.
5,174-176
Mutations of BRCA2 in metazoans
lead to gross chromosomal rearrangements, accumulation of chromosomal
breaks, developmental arrest, meiotic defects, and increased hypersensitivity
of DSB and interstrand cross-linking (ICL)-causing agents.
177-182
Biallelic
BRCA2
mutations that lead to expression of truncated forms of BRCA2 pre-
dispose individuals to Fanconi anemia (FA) and the designation as
FANCD1.
183
The similar radiation sensitivity and developmental defective
phenotypes observed in
RAD51
- and
BRCA2
-deficient cell types suggest that
BRCA2 is intricately involved in RAD51-mediated RR repair (
Table I
). These
assertions have been solidified by an observed interaction between BRCA2 and
RAD51 using the yeast two-hybrid system.
181,184
BRCA2 possesses two spatially distinct RAD51-binding regions. The first
region involves repeated sequences of BRC motifs and the second RAD51
interaction motif is located at the C terminus of the protein (C-terminal
RAD51-binding domain (CTRB);
Fig. 5
A; Refs.
85,181,184-188
). Each BRC
repeat consists of about 35 amino acids and several of the residues within each
motif are conserved. This conservation is seen among metazoan BRCA2 ortho-
logs.
189,190
However, the number of repeats in each organism varies. For
instance, humans, mice, and chicken have eight BRC repeats,
Drosophila
has
four,
Caenorhabditis elegans
and
U. maydis
have a single one, and the plant
species rice and
Arabidopsis thaliana
have eight and four repeats, respective-
ly.
191
The BRC repeats are not functionally equivalent.
192
Mutations within
BRC repeats lead to abrogation of RAD51 binding and thus manifest defective
DNA repair.
193
Structural analysis of the human BRCA2 BRC-4 repeat with
the core region of RAD51 revealed an interface on BRC-4 that mimics a
binding motif of RAD51.
194
This surface was suggested to function as an
oligomerization site for RAD51 to facilitate RAD51 NPF formation.
194
Studies
with
U. maydis
Brh2 also suggested a similar recruitment mechanism of
RAD51.
119
All BRC repeats of human BRCA2 bind to RAD51 with variable
affinity but with a binding stoichiometry of 1:1.
195
For example, BRC-1, -2, -3,
and -4 bind to free RAD51 with a higher affinity compared to BRC-5, -6, -7,
and -8.
195
BRC repeats can also modulate the loading of RAD51 onto
DNA.
118,195,196
RAD51 filament formation on dsDNA leads to a dead-end
complex that is recombination-deficient both
in vivo
and
in vitro
.
197,198
BRC-
1, -2, -3, and -4 are able to suppress the ATP turnover rate of RAD51 and
facilitate nucleation on RPA-coated ssDNA, while suppressing RAD51
binding to dsDNA
117,195
(
Fig. 5
B). This in turn leads to enhanced recombinase
activity.
195
The latter group, BRC-5, -6, -7, and -8, however, do not enhance
