<p>In eukaryotic systems, the
genetic material of the cell –DNA– is packed into a protein-dense structure
called chromatin. Chromatin structure is critical for preservation of the
genetic material as well as coordination of vital processes such as DNA replication,
transcription and DNA damage repair. The fundamental repeating unit of
chromatin is nucleosome which is composed of an octamer of small alkaline
proteins called histones and the DNA wrapped around this octamer. The
nucleosomes are then packed into higher-order structures leading to formation
of 3D chromatin architecture. The
chromatin is a dynamic structure; the spacing between nucleosomes, or the
folding of the larger chromatin segments is subjected to alterations during
embryonic development, tissue specifications or <i>simply during any event that require gene expression changes</i>.
Failure in proper regulation of chromatin structure has been associated with
embryonic defects and disease such as cancer. </p>
<p>This work has focused on a class
of ATP-dependent chromatin remodeling complexes known as
switch/sucrose-non-fermentable (SWI/SNF) or BRG-associated factors (BAF)
complex. This family of complexes act on chromatin and alter its physical
structure by mobilizing histones or nucleosome particles through the activity
of its ATPase –BRG1 or BRM, enabling more accessible DNA for the other factors
such as transcription factors to localize and recruit transcription machinery.
In particular, we discovered and biochemically defined a novel version of this
family of chromatin complexes that we named as GLTSCR1/1L-BAF (GBAF). GLTSCR1
and GLTSCR1L are two uncharacterized paralogous proteins that have been
identified as BRG1-interacting proteins. Biochemically surveying the essence of
this interaction, we realized that these proteins incorporates into a
previously unknown SWI/SNF family complex that lacks well-characterized SWI/SNF
subunits such as ARID1/2, BAF170, BAF47; instead, uniquely comprise GLTSCR1/1L
and bromodomain-containing protein BRD9. Focusing on the GLTSCR1 subunit, we
observed that its absence is well-tolerated by many different cell types except
slight growth retardation by prostate cancer cells. Expanding the cohort of
prostate cancer cells, we realized that not the paralogous subunits GLTSCR1 or
GLTSCR1L but unique and non-redundant subunit BRD9 is the major GBAF-dependence
in prostate cancer cells. We observed that especially the androgen-receptor
positive cell lines have severe growth defects upon <i>BRD9 </i>knockdown or inhibition. <i>In
vivo, </i>we showed that xenografts with <i>BRD9
</i>knockdown prostate cancer cells (LNCaP) have smaller tumor size. We
demonstrated that BRD9 inhibition can block the expression of androgen-receptor
targets. Similarly, <i>BRD9 </i>knockdown
and treatment with antiandrogen drug (enzalutamide) has overlapping
transcriptional effects.
Mechanistically, we showed that BRD9 interacts with AR and it
colocalizes with AR in subset of AR -binding sites. Surprisingly, we realized
that BRD9 depletion has similar transcriptional and phenotypic effects as BET
protein inhibitors. BET protein family contains 4 bromodomain containing
proteins (BRD2, BRD3, BRD4, BRDT). These proteins were previously shown to be
critical for AR-dependent gene expression. We detected interaction between BRD9
and BRD2/4. We demonstrated that BRD4 and BRD9 had shared binding sites on
genome, a fraction of which are co-bound by AR.
At particular target sites we showed that BRD9 localization is dependent
on BET proteins, but not the other way around. Taking together, we provided
some evidences that GBAF targeting through BRD9 can be a novel therapeutic
approach for prostate cancer. Growing body of reports suggested that current
therapy options targeting the androgen receptor is failing due to acquired
resistance. Therefore, targeting the AR pathways via its coregulators such as
BET proteins or SWI/SNF complexes can serve as potent alternative
approaches. Further research is needed
to elucidate the roles of GBAF and BET proteins in androgen receptor
independent prostate cancer cells, which are still responsive to GBAF or BET
manipulations although to a lesser extent.</p>
| Identifer | oai:union.ndltd.org:purdue.edu/oai:figshare.com:article/12135837 |
| Date | 27 April 2020 |
| Creators | Aktan Alpsoy (8715333) |
| Source Sets | Purdue University |
| Detected Language | English |
| Type | Text, Thesis |
| Rights | CC BY 4.0 |
| Relation | https://figshare.com/articles/CHARACTERIZATION_OF_NOVEL_SWI_SNF_CHROMATIN_REMODELING_COMPLEX_GBAF_IN_HEALTH_AND_DISEASE/12135837 |
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