Biology Reference
In-Depth Information
FIGURE 7.10
Complexity of RAD51 regulation. The purpose of this figure is to illustrate the complexity of RAD51 regulation on multiple
levels. Because of the dynamic nature of RAD51 regulation, deregulation at one level may not necessarily affect the ultimate outcome in terms of
HRR activity. Alternatively, deregulation at one level, for example due to impairment of RAD51 foci/filament assembly, could be overcome by
a compensatory mechanism at another level such as increased gene transcription. Please note that this illustration represents an over-
simplification of RAD51 regulation.
phosphorylation, dephosphorylation, acetylation, ubiq-
uitylation, SUMOylation, and neddylation, as well as
regulation on the level of mRNA by RNA-binding
proteins and miRNA. In addition, complex interactions
with signal transduction pathways are beginning to
emerge.
214,369
The complexity of these regulations also
extends to HRR proteins, which ensures proper spatio-
temporal control of their activities.
The concept of highly ordered and regulated protein
machines at DNA damage sites has profound implica-
tions for the development of biomarkers to predict the
functional HRR status of a given cancer. One can readily
envision that the knowledge of mutations in individual
HRR genes will not always be sufficient to predict HRR
status given the possibility of additional genetic or
epigenetic alterations affecting HRR in a cancer cell.
Further, gene expression profiling may not accurately
capture the multitude of post-translational modifica-
tions that regulate the DDR response. An added layer
of complexity comes from amino acid changes in recom-
binational proteins that have been increasingly reported
in both sporadic tumors and in normal cells.
130
Often it
is unclear whether such changes represent polymor-
phisms without functional consequences or hypomor-
phic mutations. Given the highly ordered nature of the
DDR machines, small structural changes introduced by
single amino acid changes in individual proteins may
already alter the activity of the complex significantly.
The complexity of HRR control is illustrated in
Figure 7.10
, depicting the regulation of the RAD51
protein on multiple levels. It becomes readily apparent
that knowledge of the status of individual pathway
components may not necessarily predict the function
of the entire RAD51-dependent pathway.
Even if the entire genome and epigenome of a cancer
was deciphered, a lack of knowledge of the functional
interactions of HRR gene products and other regulatory
components will most likely make an accurate predic-
tion of HRR status and cellular sensitivity to an array
of available anticancer agents impossible. These consid-
erations stress the value of functional repair assays that
may provide a measure of the activity of HRR and oth-
er repair pathways without needing to know the status
of all of their components, many of which remain
unknown.
Expression Biomarkers of HRR Status
Mutations in HRR genes such as BRCA1 or BRCA2
constitute appropriate biomarkers of HRR deficiency
and treatment sensitivity. However, as outlined above,
the fraction of sporadic cancer containing these muta-
tions is very low. Alternatively, array-based strategies
