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Gooding et al. J Cancer Metastasis Treat 2019;5:41 I http://dx.doi.org/10.20517/2394-4722.2019.11 Page 5 of 14

evidence of an open reading frame and carries multiple conserved repeat sequence elements of unclear
[50]
significance . Collectively, these features implicate BORG as a lncRNA, whose primary sequence has been
subject to several functional analyses. For instance, BORG houses several novel pentamer motifs that are
essential in facilitating its strict residence in the nucleus, representing the first demonstration of sequence-
[51]
based determinants operant in dictating the subcellular localization of lncRNAs . Indeed, the nuclear
localization of lncRNAs directly impacts their ability to elicit widespread alterations in transcriptional
networks by: (1) localizing transcription factors to specific genomic loci [52-54] ; and (2) exerting gross changes
[55]
in the nuclear architecture of cells . Likewise, BORG oversees a host of cellular functions that are readily
harnessed by breast cancer cells to enhance their tumorigenic behaviors. As will be discussed in the
succeeding sections, these BORG-dependent events play an essential role in promoting breast cancer cell
proliferation, chemoresistance, and survival.

Control of proliferation
In undertaking a combination of in silico and cell biological analyses, we recently determined that
the expression of BORG directly correlates with aggressive breast cancer phenotypes, and with their
metastatic competence and recurrence. Specifically, BORG liberates D2.OR cells from a state of dormancy
[56]
in 3D-organotypic cultures by conferring a proliferative shift in the cell cycle from G0/G1 to G2/S .
Importantly, this proliferative stimulus is sufficient to enable BORG-expressing D2.OR cells to form overt
[56]
metastases in the lungs of BALB/c mice . Interestingly, the mitogenic properties of BORG are highly
context-dependent, as they only emerge in D2.OR cells propagated in microenvironments that mimic
primary and/or metastatic tumor sites (e.g., 3D-cultures). Such context-dependent activities of BORG
imply that this lncRNA confers malleable phenotypes to DTCs, thus compelling them to activate adaptive
signaling programs that enable their survival and outgrowth within diverse metastatic niches.

BORG as a manipulator of protein function
In searching for mechanistic insights into how BORG induces DTCs to escape from dormancy, we
performed mass spectrometry analyses on proteins captured by the pulldown of biotinylated, anti-sense
BORG transcripts. These analyses identified the E3 SUMO ligase TRIM28 (KAP1) as a strong binding
[57]
partner of BORG . TRIM28 functions as a transcriptional co-repressor and scaffolding protein for histone
and DNA modifying enzymes that enhance breast cancer cell proliferation, doing so in part by suppressing
the transcription of senescence promoting genes, especially p21 and Gadd45a [58-61] . Interestingly, elevating
BORG expression in D2.OR cells to levels that approximate those detected in their metastatic D2.A1
counterparts dramatically downregulated the expression p21 and Gadd45a, indicating that BORG may rely
upon TRIM28 to confer proliferative states to dormant DTCs. Indeed, CRISPR/Cas9-mediated knockout
of TRIM28 restores a dormant phenotype in BORG-expressing D2.OR cells, as does expression of mutant
[57]
BORG transcripts that can no longer bind TRIM28 . Thus, the oncogenic activities of BORG depend upon
its physical interaction with TRIM28, an event that serves as a proliferative stimulus to dormant DTCs.

The diverse range of functions elicited by lncRNAs is thought to be promoted by their unique structural
diversity. Their inherent length and nucleic acid structure allow the formation of flexible, complex secondary
[40]
and higher order structures that facilitate their interactions with macromolecular complexes . Indeed,
lncRNAs can acquire behaviors analogous to ligands, as their binding to proteins can trigger conformational
[62]
changes and/or modify protein: protein interactions that dramatically impact protein activation states .
Accordingly, BORG enhances the function of TRIM28 to inhibit transcription by regulating the pause
and release of RNA Polymerase II (Pol II) [63,64] . For instance, heterologous expression of mutant BORG
transcripts that retain their capacity to bind TRIM28 remain competent to elicit Pol II promoter pausing
at the p21 and Gadd45a loci, whereas those BORG mutants incapable of binding TRIM28 fail to impact
[57]
the pausing index of Pol II at these loci . Furthermore, widespread evidence indicates that lncRNAs can
function as molecular scaffolds for proteins, thereby: (1) tethering cooperative proteins together to enhance
their functions; or (2) localizing RNA-protein complexes to specific genomic regions through base-pair and