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Page 10 of 16 Lei et al. J Cancer Metastasis Treat 2019;5:38 I http://dx.doi.org/10.20517/2394-4722.2019.12

ESR1 structural rearrangements and ESR1 fusions
In contrast to well-studied ESR1 point mutations, structural rearrangements involving ESR1 are under-
studied. A variety of ESR1 gene fusion transcripts have been identified in luminal breast tumors [72,73] .
Analysis of RNA-seq data from 990 primary TCGA breast samples revealed that 21 of these tumors (2.1%),
all of the luminal B subtype, contained recurrent fusion transcripts involving the first two non-coding exons
of ESR1 fused to various C-termini sequences from the coiled-coil domain containing 170 gene, CCDC170
[73]
(ESR1-e2>CCDC170) . These fusion transcripts do not provide sufficient coding sequences to generate
chimeric ER fusion proteins but instead generate truncated forms of CCDC170 proteins (DCCDC170).
Exogenous expression of DCCDC170 in ER+ breast cancer cells led to enhanced growth and reduced
[73]
sensitivity to tamoxifen suggesting a role for ESR1-e2>CCDC170 in endocrine therapy resistance. Another
independent study that examined early stage and non-metastatic ER+ breast samples also identified two
ESR1-e2>CCDC170 fusion transcripts as well as ESR1-e2>C6orf211 and another fusion containing the first
[72]
6 exons of ESR1 fused to AKAP12 (ESR1-e6>AKAP12) . These ESR1 fusions were identified in 4 out of
62 surgical samples (6.5%) that were resistant to letrozole aromatase inhibitor treatment 10-21 days post
[74]
treatment as defined by Ki67 labeling , suggesting a higher frequency for these ESR1 fusions gene events
in endocrine-refractory tumors compared to primary, untreated samples. However, detailed functional
characterization and evidence demonstrating a causal role for ESR1 fusions in endocrine therapy resistance
has been lacking and the incidence of ESR1 fusions from late-stage ER+ breast cancer still remains unclear.
Furthermore, therapeutic strategies to treat ESR1 translocated tumors remains poorly understood.

Using a PDX model to better understand endocrine therapy resistance, we previously reported a somatic
gain-of-function event in the form of a chromosomal translocation identified in a patient presenting with
aggressive endocrine therapy resistant, metastatic ER+ disease. This translocation produced an in-frame
fusion gene consisting of exons 1-6 of ESR1 (ESR1-e6) and the C-terminus of the Hippo pathway coactivator
gene, YAP1 (ESR1-e6>YAP1), thereby generating a stable ESR1 fusion protein that was a highly active
[47]
constitutive transcription factor [Figure 1D]. Our group more recently discovered another in-frame
ESR1 fusion gene involving the protocadherin 11 X-linked gene, PCDH11X (ESR1-e6>PCDH11X) provided
by inter-chromosomal translocation that also produced stable ESR1 fusion protein identified in a patient
[75]
with endocrine-refractory, metastatic ER+ breast cancer . In both ESR1-e6>YAP1 and ESR1-e6>PCDH11X
fusions, the LBD of ESR1 is replaced with in-frame sequences from another gene, and therefore the drug
binding domain that endocrine therapies recognize is absent. These two fusions promoted endocrine
therapy resistant cell proliferation and constitutively activated ER target genes. Interestingly, both fusions
also upregulated an epithelial-to-mesenchymal transition (EMT)-like transcriptional signature, induced cell
[75]
motility, and increased lung metastatic frequency . These results suggest that ESR1 fusions are able to drive
not only endocrine therapy resistance, but also drive metastasis, linking these two lethal processes together.

Importantly, ESR1 fusion-driven growth could be suppressed by CDK4/6 inhibition. This suggests that
targeting downstream kinases of ER could be a potential therapeutic strategy to treat ESR1 translocated
tumors and further suggests that ESR1 fusion status may be a potential biomarker to stratify patients to
CDK4/6 inhibitor therapy. To further explore therapeutic strategies to target ESR1 fusions, a collaborative
[66]
study was performed to examine interacting proteins with ESR1 fusion transcriptional complexes . Results
from that study showed enhanced recruitment of 26S proteasomal subunits to ESR1-e6>YAP1 driving
transcriptional activation and cell proliferation. Subsequent pharmacological inhibition with a broad-
spectrum proteasome inhibitor, MG132, blocked ESR1-e6>YAP1-mediated activation of an ERE-luciferase
reporter. Furthermore, bortezomib, a specific 26S proteasome inhibitor in phase II clinical trial used to treat
[76]
endocrine-refractory, metastatic ER+ breast cancer in combination with fulvestrant suppressed growth
driven by ESR1-e6>YAP1. Taken together, these results suggest that downstream ER kinases such as CDK4/6
as well as transcriptional coregulators such as the 26S proteasome are attractive therapeutic targets to treat
ESR1 fusion positive, metastatic breast tumors.