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Chang et al. J Cancer Metastasis Treat 2019;5:78 I http://dx.doi.org/10.20517/2394-4722.2019.31 Page 3 of 11
[9]
dimers . However, depletion of FACT only minimally affects transcript elongation rates in vivo, while
numerous studies have shown that FACT is critically involved in maintaining the nucleosomal structure
of the genome [6,9,55-59] . Thus, depletion of FACT in yeast causes accumulation of short transcripts from
normal transcription start sites and cryptic transcripts initiated intragenically [56-59] . Yeast FACT also plays
important roles in facilitating efficient replication and nucleosome assembly/disassembly during this
process [14,15] . FACT occupancy is increased at DNA damage sites after different stresses, including UV-
[19]
induced damage [17,60,61] , oxidative damage , and single- and double-strand breaks [62,63] . FACT also plays
[64]
[65]
[61]
roles in recovery from transcription arrest , replication fork stalling , and R-loop formation . The
FACT subunit SSRP1 is able to detect non-B form DNA structures, and this activity might be important for
[34]
inducing cell signaling cascades in response to these perturbations .
FACT is essential for normal embryonic development. Ssrp1 knockout mice demonstrated early embryonic
lethality at the blastocyst stage (3.5 dpc), and ES cells derived from the Ssrp1-null blastocysts could not be
[67]
[66]
propagated in vitro . Defects in development were also observed in zebrafish with a mutant Ssrp1 and
in plants with reduced levels of SSRP1 and SPT16 proteins [68,69] . Several studies also revealed that FACT
subunits play specific roles in mammalian cell differentiation and considerably modify gene expression
[70]
profiles during muscle cell differentiation and in vitro differentiation of human mesenchymal stem
[71]
cells . Although FACT can be involved in gene transcription, it is not ubiquitously expressed and
therefore is not a required core component of the transcription machinery. The levels of FACT expression
are higher in undifferentiated and proliferating cells, but are very low in most differentiated mammalian
cells [72-75] . Importantly, both curaxin treatment and (CRISPR)-mediated deletion of the FACT subunit Spt16
[76]
block induction of pluripotency without affecting the viability or proliferation of fibroblasts . In tumor
cells, both FACT subunits are highly expressed, and their higher levels correlate with poor prognosis in
several tumor types [35,74,77-80] . Thus, FACT plays a more important role in cancer cells than in normal cells,
making it a target for cancer therapy. The FACT complex is also involved in HIV integration into the host
[81]
genome and viral gene transcription [82,83] , suggesting that FACT could serve as a target for HIV therapy
as well. The mechanistic studies of FACT activity described below therefore provide insights that could
have clinical significance.

FACT-NUCLEOSOME INTERACTIONS
FACT does not bind stably to intact nucleosomes and does not affect nucleosome structure unless it is
assisted by additional factors. The HMGB1-domain protein Nhp6 provides this nucleosome-destabilizing
activity in vitro and in vivo for yeast FACT, and also supports nucleosome reorganization by human FACT
[2]
in vitro . The HMGB1 domain of SSRP1 does not appear to be adequate to promote uncoiling of the DNA
from intact nucleosomes, suggesting that it may function primarily during nucleosome assembly [29,31,55] .
Curaxins can distort DNA shape and this distortion promotes trapping of FACT in chromatin and
changes chromatin compaction [31,46] . We propose that either HMGB proteins or curaxins weaken the
histone-DNA contacts near the entry/exit sites of nucleosomal DNA and thus initiate a series of binding
events by FACT domains that can lead to full reorganization of the nucleosome [Figure 1]. In this model,
DNA distortion exposes the initial set of binding sites for the CTDs of each subunit of FACT, resulting in
similar reorganization of the nucleosome independent of the initiating factor (curaxin or HMGB binding).
In the reverse reaction leading to nucleosome assembly, Nhp6 or the intrinsic HMGB domain of SSRP1 can
assist in stabilizing bent forms of the DNA to promote formation of the canonical nucleosome, resulting
in release of FACT. Curaxins do not provide this assistance, instead leading to persistent binding of FACT
and trapping it in chromatin (see below).

CURAXIN-INDUCED TRAPPING OF FACT ON CHROMATIN
Computational modeling and circular dichroism studies suggest that curaxins intercalate between
DNA base pairs and the side chains of the molecule bind to both major and minor grooves of DNA [34,39] .
Intercalation of curaxins in DNA causes an increase in the distance between the DNA base pairs, resulting