Background
ESR1 ligand-binding domain (LBD) mutations that induce endocrine therapy (ET) resistance in breast cancer (BC) were first reported almost two decades ago [
1‐
4] and novel developments of sensitive technologies such as next-generation sequencing (NGS) confirmed that
ESR1 LBD mutations act as drivers of ET resistance [
5‐
8]. In addition, recent developments of digital genomic technologies revealed that plasma cell-free DNA (cfDNA) is an useful source to quickly assess the mutational profiles and monitor the molecular changes under treatment [
9]. It has been reported the clinical significance of monitoring
ESR1 LBD mutations in plasma cfDNA [
10‐
12]. In the BOLERO-2 study, Chandarlapaty and colleagues found that a total of 155 (28.8%) of 541 ER-positive MBC patients had the D538G and/ or the Y537S
ESR1 mutation in plasma cfDNA, which are the representative
ESR1 LBD mutations, and each of them was associated with shorter overall survival [
13]. Interestingly, they also demonstrated an additional benefit of the use of mTOR (mammalian target of rapamycin) inhibitor depending on the
ESR1 LBD mutation present; the D538G
ESR1 mutation derived a large benefit from treatment with an mTOR inhibitor, whereas those with the Y537S mutation did not. These data suggest that each
ESR1 LBD mutation may play a different role and more work is needed to confirm this.
Pakdel and colleagues [
14] found that mutation of the one charged amino acid, E380Q
, resulted in a requirement for less estradiol than wild-type (WT)
ESR1 to achieve maximal activity and this mutation also showed high trans-activation activity in the absence of added hormone. They suggested that this
ESR1 LBD mutation may be important in DNA binding and protein–protein interactions that modulate transcriptional activity of the estrogen receptor (ER). After 20 years, the presence of the E380Q
ESR1 mutation came to be reported in both tumor tissue DNA (ttDNA) and plasma cfDNA [
10,
14‐
19]. In the recent phase 2 clinical trial for plasma cfDNA of aromatase inhibitor (AI) resistant metastatic BC (MBC) patients, this mutation was found in 26% (15/57) of
ESR1 mutant plasma samples [
18]. Fribbens and colleagues reported that the E380Q
ESR1 mutation was found in 9.5% (6/63) in the SoFEA study for hormone receptor (HR)-positive BC patients who had demonstrated prior sensitivity to AIs, but it was found in 24.4% (22/91) in the PALOMA3 trial for HR–positive BC patients who had progressed during prior ET [
19]. These results suggest that E380Q
ESR1 mutation may be a marker for screen of ET-resistant BC like the other representative
ESR1 LBD mutations (Y537S, Y537N, Y537C, and D538G) [
10‐
12,
20]. However, the literature contains little information regarding the E380Q
ESR1 mutation in Japanese BC patients. Thus, the present study screened for the presence of the
ESR1 E380Q mutation in ttDNA and plasma cfDNA of 62 ER-positive Japanese BC patients using droplet digital polymerase chain reaction (ddPCR) and compared the frequency with the representative
ESR1 LBD mutations (Y537S, Y537N, Y537C, and D538G). To our knowledge, this is the only precise study to use ddPCR to examine the presence of the E380Q
ESR1 mutations in a series of tumor tissue and plasma samples of Japanese BC patients.
Discussion
In this study, we investigated the frequencies of the E380Q
ESR1 mutation in comparison with the other
ESR1 LBD mutations, Y537S, Y537N, Y537C, and D538G in tumor tissue and plasma DNA. In Vitro, Toy and colleagues showed differences in the ligand-independent activity among
ESR1 LBD mutations [
6]. More recently, they also found tumors driven by D538G, E380Q or S463P were effectively inhibited by fulvestrant, but, Y537S mutants were not fully inhibited by fulvestrant despite dosing to higher levels than are achieved in the hospital [
24]. Therefore, identification of the frequency and characteristics of each
ESR1 LBD mutation will deepen our knowledge and understanding of acquired ET resistance.
The raw data of E380Q
ESR1 mutation is shown in Additional file
1: Table S1. The measurement results of two experiments were processed to one value with QuantaSoft
™ software (Bio-Rad Laboratories). Four early representative studies on
ESR1 mutations in ttDNA reported a total of one (2.5%) of E380Q in comparison with a total of 14 (35.8%) of D538G, 10 (25.6%) of Y537S, four (10.2%) of Y537N, three (7.6%) of Y537C, and seven other
ESR1 mutations among a total of 39
ESR1 LBD mutation-positive ER-positive MBC patients [
5‐
8]. More recently, deep sequencing of 929 breast tumor biopsies (including ER-positive, HER2-positive and ER-negative tumors) indicated 95 patients (10.2%) having somatic mutations in
ESR1, which consisted of 20 (21.1%) of E380Q, 34 (35.8%) of D538G in comparison with 13 (13.7%) of Y537S, 6 (6.3%) of Y537C, and 5 (5.3%) of Y537N [
24]. Meanwhile, in the recent clinical trials for plasma cfDNA of ET resistance MBC patients, E380Q
ESR1 mutation was found in 26% (15/57) [
18] and 24.2% (22/91) of
ESR1 mutant plasma samples whose frequency was more than that of one of the major
ESR1 LBD mutations, Y537N [
19].
The frequency of the E380Q
ESR1 mutation in our study seems to be rare among
ESR1 LBD mutations. We found a total of two (16.6%) E380Q
ESR1 mutation out of 12 MBC with
ESR1 LBD mutations and we did not find the E380Q
ESR1 mutation in plasma cfDNA (Table
2). Plasma cfDNA has the possibility to integrate
ESR1 mutations from distinct populations of cells which are caused by inter- and/or intra-tumoral heterogeneity [
11,
25]. However, the E380Q
ESR1 mutation was not identified in any of our 69 analyzed plasma cfDNA samples. In another small cohort of HR-positive Japanese MBC patients, whole exon sequencing of the
ESR1 gene using NGS did not identify E380Q
ESR1 mutation in their recurrent tumor samples and plasma samples [
26].
Identifying associations between the status of the E380Q
ESR1 mutation and response to ET will help to use ET more effectively. Li and colleagues detected the E380Q
ESR1 mutation in an ER-positive patient-derived xenograft that reacted to tamoxifen, but was resistant to AI
. [
15]. De Mattos-Arruda and colleagues reported that the MAF of the E380Q
ESR1 mutation in plasma cfDNA increased from 46 to 58% under disease progression [
16]. However, in this study, there were an insufficient number of samples to formally analyze a predicted association between the E380Q
ESR1 mutation and the patient’s prognosis. The present study has limitations. This was a retrospective and single-institute study. Since this was a selected mutation-based study, not all the
ESR1 LBD mutations were investigated. The number of patients with the E380Q
ESR1 mutation was small due to the selection criteria. Although the appearance of
ESR1 LBD mutations is closely associated with medical history of ET, this studied population is heterogeneously treated and we could not investigate whether or not the presence of
ESR1 LBD mutations is dependent on specific hormone therapies.