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substitution at residue 48, and thus unable to accept the ubiquitin moiety, results in
centrosome amplification indicating the importance of BRCA1 ubiquitin ligase
activity in the regulation of centrosome duplication (Sankaran et al.
2006
; Starita
et al.
2004
). Interestingly, while inhibition of BRCA1 only affects centrosome
number in cell lines derived from breast tissue, expression of the mutated gamma-
tubulin gene results in centrosome amplification in non-breast cells (Starita et al.
2004
). This result suggests that all cell types need to regulate gamma-tubulin by
modification of residue lysine-48, and in breast cells this process is dependent on
the BRCA1 ubiquitin ligase.
Centrosomes function by nucleating microtubule formation (Moritz et al.
1995
).
An enzymatically functional BRCA1 inhibits the microtubule nucleation function of
the centrosome in vitro (Sankaran et al.
2005
). Thus, BRCA1 E3 ubiquitin ligase
activity inhibits centrosome function but is required for the regulation of centrosome
duplication. The BRCA1 enzymatic activity is also important for the localization of
gamma-tubulin to the centrosome for it has been shown that in addition to ubiqui-
tinating gamma-tubulin itself, BRCA1/BARD1 ubiquitinates a protein in the PCM
that docks gamma-tubulin to the centrosome (Sankaran et al.
2007b
).
Given that BRCA1 is located at the centrosome at all stages of the cell cycle, and
since the BRCA1 enzymatic activity inhibits microtubule nucleation, it needs to be
explained how the centrosomes are capable of establishing the mitotic spindle at all. Ko
et al.
2006
found that BRCA1 function at the centrosome is mostly critical during S-G2
phases of the cell cycle to block the reduplication of the already duplicated centro-
somes. At G2, AURKA localizes to the centrosome and phosphorylates BRCA1 thus
inhibiting its ubiquitin ligase activity and enabling the microtubule nucleation activity
(Sankaran et al.
2007a
)(Fig.
14.1
). AURKA is known to be overexpressed in more
than 60 % of breast cancers (Miyoshi et al.
2001
) and this suggests that the overex-
pression of the protein will cause its localization at the centrosomes during S phase
which results in the inhibition of BRCA1 ubiquitin ligase activity when the BRCA1 is
needed most to inhibit the reduplication of centrosomes. Thus, either overexpression of
AURKA (Fig.
14.1
c) or inhibition of BRCA1 (Fig.
14.1
b) results in the same phe-
notype of supernumerary centrosomes a phenotype that might contribute to chromo-
somal instability, aneuploidy, and tumorigenesis.
Since BRCA1 is associated with familial cases of breast cancer, the likelihood
of a woman to inherit a mutated BRCA1 allele in such a case is high. In such a
scenario, the possibility of having a mutation during the lifetime in the second
BRCA1 allele rendering the protein defective in at least one mammary epithelial
cell is common. Individuals with a family history of breast cancer may get their
BRCA1 gene sequenced. Although many of the characterized mutations result in a
frameshift or a stop codon that results in a truncated protein, some women present
with missense mutations whose disease association is still unclear due to their low
prevalence. Such mutations therefore present a diagnostic dilemma because it is
still unknown how these mutations affect the function of the protein and whether
they render the protein defective or not. A set of BRCA1 point mutants in the
amino terminus of the protein were studied for their effect on the HR process as an
initial step in determining whether a variant of unknown function may predispose