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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 7 of 11

displacement of histone H1 contribute to unfolding of the chromatin fibers and also to largely reversible
uncoiling of nucleosomal DNA (nucleosome unfolding). Since FACT is tightly bound to (trapped by) and
unfolds nucleosomes genome-wide, FACT-dependent gene expression is globally affected, preferentially
decreasing the viability of cancer cells that have a higher requirement for FACT. After a longer curaxin
treatment, FACT-bound unfolded/destabilized nucleosomes are prone to disruption, possibly evicting core
histones from the DNA. Nucleosome unfolding and histone eviction are likely to result in the release of
negative DNA supercoiling that was constrained in intact nucleosomes and in formation of alternative
DNA structures, such as Z-DNA, resulting in further FACT trapping.

In addition to affecting the structures of nucleosomes and chromatin fibers, curaxins globally alter genome
topology in cells. Curaxins induce eviction of CTCF from DNA, thus disrupting domain boundaries
and dramatically decreasing the number and type of chromatin loops that are characteristic of cancer
cells. The latter effect is likely explained by the global unfolding of the chromatin fiber due to the altered
geometry and topology caused by curaxin treatment. In turn, the changes in chromatin topology result
in less efficient EPC, and in preferentially reduced expression levels of enhancer-dependent genes (e.g.,
Myc) and global alteration of the profiles of gene expression. The suppression of enhancer-dependent
oncogenes strongly decreases the efficiency of cancer cell survival. This combined mechanism of curaxin
action explains the high efficiency of the compounds and suggests novel therapeutic strategies targeting
chromatin structure and spatial organization of the genome to alter gene expression profiles, thereby
suppressing cancer cell growth.

PERSPECTIVES
Mutations in enzymes that “write”, “read”, or “erase” chromatin marks have emerged as a recurring theme
[88]
in multiple types of cancer . However, the mechanisms through which epigenetic changes benefit cancer
cells remain poorly understood. Existing theories tend to focus on specific genes or pathways; that is, the
effects of altering global properties of chromatin are usually attributed to altered expression of individual
genes or groups of genes. However, discoveries such as “oncohistones”, where mutations in core histones
[89]
are associated with cancer , raise the possibility that carcinogenesis results from a broad combination of
defects occurring genome-wide.

The role of FACT in cancer, and the use of curaxins to combat cancers, face similar questions. Neither
FACT nor curaxins have clear gene-specific effects, and yet the former preferentially supports the viability
of aggressive cancer cells and the latter has clear anti-cancer activity. In one scenario, FACT and curaxins
could function through a common mechanism by destabilizing nucleosomes. In this model, FACT
might enhance global expression of genes, including a set that is important for maintaining excessive
proliferation, and curaxins extend this activity past the point where it is advantageous. However, recent
results suggest instead that FACT and curaxins might have opposing effects on nucleosomes, with curaxins
[34]
promoting unwrapping of DNA from the core , while FACT promotes survival of nucleosomes by
[29]
tethering the components together and promoting reassembly . As FACT is capable of both destabilizing
and stabilizing nucleosomes, understanding how it promotes survival of cancer cells and how curaxins
oppose this activity will ultimately require better understanding of how global nucleosome stability
contributes to cancer cell progression and viability.

Pervasive cryptic transcription has been detected in multiple types of cancer [90-92] , suggesting that the
DNA is more accessible to transcription machinery in tumor cells than it is in normal cells, as expected
if the nucleosomal barrier is globally weakened. In this case, the role of FACT in stabilizing and restoring
nucleosomes would be critical to allowing tumor cells to maintain chromatin. Curaxins would then
interfere with this process, creating an excessive burden of disrupted chromatin, eventually leading to
cell death. Then, the next questions to answer are: Why are nucleosomes less stable in tumors, thereby