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See related article by Jeselsohn et al., p. 1757
Estrogen Receptor Mutations in Breast Cancer—New Focus
on an Old Target
Corrinne V. Segal1,2 and Mitch Dowsett1,2
Recent studies have provided strong evidence for the emergence of substantial numbers of constitutively
active ESR1 mutations in estrogen receptor–positive metastatic breast cancer that are undetected in primary
disease. Some of these mutants remain partially sensitive to current anti-estrogen therapies but effective
therapeutics targeted at them may require new approaches. Clin Cancer Res; 20(7); 1–3. 2014 AACR.
In this issue of Clinical Cancer Research, Jeselsohn and
colleagues (1) report an overall 12% frequency of mutations
in the coding sequence of ESR1 in metastatic estrogen
receptor–positive (ERþ) breast cancers but the lack of their
detection in primary disease.
About 80% of primary breast cancers express estrogen
receptor a (ERa), the product of the ESR1 gene, and are
described as ERþ. Estrogen is the primary stimulant of the
development and continued growth of ERþ tumors: withdrawal of estrogen from primary ERþ breast tumors
decreases proliferation in >90%, although this reduction
may be modest in some (2). The mainstay of treatment of
ERþ breast cancer is endocrine therapy, in the form of drugs
that antagonize the ER (e.g., tamoxifen), therapies that
result in estrogen deprivation (e.g., aromatase inhibitors),
or fulvestrant, a drug that destabilizes and antagonizes the
receptor. Most patients receive endocrine therapy for 5 years
starting shortly after surgery (adjuvant therapy).
Endocrine therapy for breast cancer has resulted in
markedly reduced recurrence and mortality rates; however,
a significant proportion of patients relapse with metastatic
disease. If the recurrence has not arisen within a few months
of starting adjuvant therapy, these patients will be treated
with an alternative endocrine treatment (first-line metastatic treatment). Resultant shrinkage or stasis of metastatic
lesions is achieved in many patients and those showing a
response have a good chance of responding to subsequent
lines of endocrine treatments: patients whose cancer has
progressed on tamoxifen often respond to fulvestrant or an
aromatase inhibitor and vice versa (3). The lack of crossresistance suggests that the mechanisms by which this
Authors' Affiliations: 1Breakthrough Breast Cancer Research Centre at
The Institute of Cancer Research and 2Academic Department of Biochemistry, The Royal Marsden Hospital, London, United Kingdom
Corresponding Author: Mitch Dowsett, Academic Department of Biochemistry, Royal Marsden Hospital, London SW3 6JJ, United Kingdom.
Phone: 020-7808-2885; Fax: 020-7376-3918; E-mail:
[email protected]
doi: 10.1158/1078-0432.CCR-14-0067
2014 American Association for Cancer Research.
www.aacrjournals.org
occurs may vary depending upon the prior treatment.
Eventually, pan-endocrine resistance develops.
Many mechanisms have been elucidated in model systems of resistance to tamoxifen, and more recently to
estrogen deprivation and fulvestrant. These include altered
expression of growth factor receptors, such as HER2, or
aberrant expression of ER coregulators leading to either ERindependent growth or ER-dependent/ligand-independent
growth (4). However, there has been limited confirmation
of these findings in the clinic. Although some patients with
acquired resistance to tamoxifen show loss of ER in metastases, an obvious route to loss of sensitivity, the majority
continue to express ER and there is less evidence for ER loss
with resistance to aromatase inhibitors (4, 5). Thus, resistance mechanisms remain incompletely explained and
there is little biomarker-related guidance of therapy for
metastatic disease.
Despite the central role of estrogens in the development
of breast cancer, ERa mutations in primary disease have
thus far proved rare (Fig. 1). For example, no ESR1
mutations were identified in a study of 390 ERþ primary
tumors (6). In metastatic disease, the presence of an ESR1
mutation unique to the metastases was first noted 20 years
ago (7), but until very recently, surprisingly few further
data accumulated. However, three groups have recently
reported substantial numbers of ESR1 mutations in metastatic breast cancer (8–10). The current work of Jeselsohn
and colleagues (1) is consistent with and extends these
reports. They sequenced the coding region of ESR1, and
182 additional cancer-related genes, in 58 treatment-na€ve
primary and 76 metastatic ERþ HER2 breast tumors and
set out to describe better the functional relevance of the
observed ESR1 mutations. None of the primaries contained detectable ESR1 mutations but 11 (14.5%) of the
metastatic samples contained previously reported mutations (8, 9), clustered in the ligand-binding domain
(LBD), predominantly at amino acids 537 or 538, as well
as a novel 344insC mutation. Notably, a direct correlation
was apparent between the number of endocrine treatments and mutation frequency, which was 5/25 (25%)
in tumors from patients receiving an average of seven lines
of treatment. All of the other 182 cancer-related genes
OF1
Segal and Dowsett
Activation
function1
(AF1)
ERa
1
DBD
180
L536R
Y537N/C
D538G
Nuclear
localization
signal
(NLS)
E380Q
V392I
E352V
Primary disease-frequency 1%*
LBD
595
263 302
Figure 1. Schematic of ESR1
mutations identified in primary and
þ
metastatic advanced ER breast
cancer and factors that may
influence their development. The
majority of the mutations affect the
ligand-binding domain in helix 12
(H12). Collated from refs. (1, 6–12).
, total number of patients with
mutations in primary disease (7/674).
0/58, ref. (1); 0/390, ref. (6); 1/35, ref.
(7); 6/183, ref. (8); 0/3, ref. (9); 0/5, ref.
(10). x, total number of patients
with mutations in metastatic disease
(43/222). 11/76, ref. (1); 2/5, ref. (7);
14/80, ref (8); 6/11, ref. (9); 5/13,
þ
ref. (10); 2/7, ref. (11); 3/30 (NB ER
status unconfirmed), ref (12).
Activation
function1
(AF1)
ERa
1
S432fs
439fs
S463P
K531E
V534E
P535H
L536R/Q
LBD
DBD
180
E380Q
S47T
Nuclear
localization
signal
(NLS)
344insC
D538G
Metastatic disease-frequency 19% §
Y537N/C/S
Potential factors
Time, treatment, metastasis, mutational drivers
263 302
595
© 2014 American Association for Cancer Research
sequenced exhibited similar mutation frequencies between
the primary and metastasis.
In the largest of the other recent reports, Toy and colleagues (8) found nine LBD mutations in 36 cases enrolled in a
metastatic collection program. In the two cases in which they
could also test the primary tumor, the mutation was absent.
They also found that 5 of 44 cases from the BOLERO-2
clinical trial of letrozole everolimus harbored ESR1 mutations with the frequency enriched in cases with a long
duration of endocrine therapy (8). Similarly, Robinson and
colleagues (9) and Merenbakh-Lamin and colleagues (10)
reported that 6 of 11 and 5 of 13 cases, respectively, of breast
cancer metastases contained LBD-localized ESR1 mutations.
In each of these reports, mutations at 537 and 538 were the
most frequent, but others at 534 or 536 were also noted.
Given the consistent finding of mutations in the LBD of ERa,
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Clin Cancer Res; 20(7) April 1, 2014
elucidation of their functional significance and susceptibility
to therapeutic targeting is of paramount importance.
Functional studies by Jeselsohn and colleagues (1) and
others (8–12) provide evidence that many of the mutants
are strong promoters of ER signaling and, importantly, are
constitutively active in the absence of estradiol and therefore likely to elicit resistance to estrogen deprivation strategies. They stimulate increased expression of ER-regulated
genes, and increase proliferation and migration in vitro
compared with wild-type ESR1. The ESR1 mutants have
also generally been found to confer reduced sensitivity to
anti-estrogens compared with wild-type ESR1. Although
signaling is reduced by 4-hydroxytamoxifen and fulvestrant,
much higher doses are required to reach a similar level of
inhibition observed in the wild type (1, 8–12). These in vitro
findings are supported by molecular modeling that indicate
Clinical Cancer Research
Estrogen Receptor Mutations in Breast Cancer
that the mutants favor the agonist conformation and mediate their effects through influence of coactivator or corepressor binding (8). It is also notable that in ER-Y537N–
mutant cells, fulvestrant induces much more modest ER
degradation compared with wild type (1) and therefore
reduced ability to inhibit ligand-independent activity of ER.
This relative resistance to degradation seems less apparent
with ER-Y537S (11).
Interestingly, while it has been difficult to sustain patientderived xenografts (PDX) from ERþ breast cancer tissues,
aberrations in the ESR1 gene were found in 4 of 5 luminal
breast cancer metastases. This included one case with ERY537S that was found in a further PDX in which the ER
status was not available. A fusion gene (ESR1/YAP1) was
also discovered in an ERþ PDX that, like the point mutations, induced estradiol-independent growth. It is plausible
that the drive provided by certain constitutively active ESR1
alterations is a major advantage in promoting survival and
growth in the low-estrogen environment of the PDX system.
These PDXs are likely to be particularly valuable in elucidation of the best therapeutic approaches for tumors with
specific ESR1 alterations.
In summary, there is now substantial evidence for the
emergence of functionally aberrant ESR1 mutations, particularly in the LBD, in metastatic ERþ breast cancer, which
are undetectable in primary tumors. Correlative evidence is
consistent with this emergence being due to selective pres-
sure of multiple endocrine therapies. The genomic instability associated with advanced disease may contribute to
the prevalence of the mutations, for example through selfperpetuating defects in DNA-repair mechanisms. ESR1
mutations might also promote the success of the metastatic
process and be apparently enriched as a result, although this
hypothesis requires their presence below the detection
limits of the high-depth sequencing conducted to date. To
confirm the role of the mutations as resistance mechanisms
and their potential as targets for new therapies requires
careful prospective study of metastatic disease aligned with
comprehensive genomic and functional analyses.
Disclosure of Potential Conflicts of Interest
No potential conflicts of interest were disclosed.
Authors' Contributions
Conception and design: C.V. Segal, M. Dowsett
Analysis and interpretation of data (e.g., statistical analysis, biostatistics, computational analysis): C.V. Segal, M. Dowsett
Writing, review, and/or revision of the manuscript: C.V. Segal, M. Dowsett
Acknowledgments
The authors thank the Mary-Jean Mitchell Green Foundation and Breakthrough Breast Cancer for funding. They also thank NHS for providing
funding to the NIHR Royal Marsden Hospital, and the Institute of Cancer
Research Biomedical Research Centre.
Received January 31, 2014; accepted February 7, 2014; published
OnlineFirst February 28, 2014.
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