Acquired ESR1 Mutations in ER+/HER2− Breast Cancer: A Predictive ctDNA Biomarker

Introduction

The detection of acquired ESR1 (estrogen receptor 1) mutations has become a clinically actionable step in managing estrogen-receptor–positive, HER2-negative (ER+/HER2−) advanced breast cancer. Unlike the germline or tumor-intrinsic alterations often discussed in oncology, the ESR1 mutations of interest here typically emerge under the selective pressure of endocrine therapy—making this a biomarker of acquired resistance. This article explains what the assay measures, how it is performed, how each result is interpreted, and how it informs therapeutic eligibility, with attention to areas that remain in flux.

What the Test Measures

The estrogen receptor alpha, encoded by ESR1, drives proliferation in the majority of breast cancers. Standard endocrine therapies work by lowering estrogen availability (aromatase inhibitors) or by antagonizing/degrading the receptor. Under prolonged endocrine pressure—particularly aromatase inhibitor exposure—tumor subclones can acquire point mutations in the ESR1 ligand-binding domain (LBD). These mutations render the receptor constitutively active, meaning it can signal transcription in a ligand-independent manner. In practical terms, the tumor no longer depends on circulating estrogen to grow, undermining therapies that work by depleting estrogen.

Because these mutations are typically acquired rather than present at diagnosis, they are most informative in the resistance setting—after a patient has already received endocrine therapy for advanced disease. The test asks a focused question: is an activating ESR1 ligand-binding-domain mutation present or absent?

How It Is Tested

Specimen. The preferred specimen is plasma circulating tumor DNA (ctDNA), obtained by a peripheral blood draw. This "liquid biopsy" approach captures tumor-derived DNA fragments shed into the bloodstream and is well suited to the resistance setting, where tissue re-biopsy may be difficult and where ctDNA can sample multiple metastatic clones simultaneously.

Assay. Detection is performed by next-generation sequencing (NGS) or PCR-based methods targeting the ESR1 ligand-binding domain. These platforms are designed to detect low-frequency variants, since resistant subclones may represent only a fraction of total circulating DNA.

Preanalytic constraints. ctDNA testing is exquisitely sensitive to preanalytic handling. Blood should be collected in appropriate cell-stabilizing tubes and processed within validated time windows to prevent dilution of tumor-derived DNA by genomic DNA released from lysed leukocytes. The timing of the draw matters conceptually: because ESR1 mutations are acquired under therapy, testing is most meaningful after prior endocrine exposure, when resistant clones have had the opportunity to emerge. A negative result early in the disease course does not exclude later emergence.

Scoring. The readout is qualitative and binary in clinical application: an activating ESR1 LBD mutation is present versus absent. The two reported result states are ESR1-mutant and ESR1 wild-type.

What Each Result State Means

ESR1-mutant. An activating ligand-binding-domain mutation is detected in plasma ctDNA. This indicates that a resistant clone bearing a constitutively active estrogen receptor is present. Biologically, this predicts reduced benefit from endocrine strategies that rely on lowering estrogen and identifies a tumor whose receptor remains an active driver despite prior therapy.

ESR1 wild-type. No activating LBD mutation is detected. Importantly, "wild-type" in a ctDNA context reflects the sensitivity and clonal representation of the sample. A negative result may mean the mutation is truly absent, or that ctDNA shedding is low or a resistant subclone falls below the assay's limit of detection. The evidence suggests that a negative result should therefore be interpreted in clinical context rather than as absolute proof of absence.

What It Determines for Treatment Eligibility

The predictive value of this biomarker is anchored in the EMERALD trial, which compared elacestrant—an oral selective estrogen receptor degrader (SERD)—against standard endocrine therapy in ER+/HER2− advanced breast cancer that had progressed on prior endocrine therapy [1]. The trial evaluated outcomes both in the overall population and specifically in patients with detectable ESR1 mutations.

In regulatory terms, an acquired ESR1 mutation informs eligibility for the oral SERD drug class (elacestrant) in ER+/HER2− advanced disease following prior endocrine therapy, with ctDNA-based ESR1 mutation detection serving as a companion diagnostic [1]. Framed educationally: the presence of an activating ESR1 mutation is what gates access to this therapeutic option—it defines a molecularly selected population, rather than dictating an individual's treatment.

It is essential to emphasize the boundaries of this statement. This biomarker determines class eligibility, not a personalized directive. Whether any specific therapy is appropriate for a given patient depends on the full clinical picture and is a decision made between patient and treating oncologist. The role of the diagnostic is to establish whether the molecular criterion is met.

Caveats and What Is Evolving

Several issues remain contested or in active development:

• Definition of HER2 status. The eligibility framework rests on ER+/HER2− classification, but the boundary of "HER2-negative" is itself evolving. The emergence of the HER2-low category (tumors with low-level HER2 expression that were historically grouped as HER2-negative) complicates how populations are defined across studies and in practice. How HER2-low status intersects with ESR1-directed treatment selection is an area to watch.

• Sensitivity and clonal dynamics. ctDNA assays vary in analytic sensitivity, and ESR1 mutations can be polyclonal and dynamic, waxing and waning with therapy. Serial testing may reveal mutations not detected on a single draw. The optimal timing and frequency of testing continue to be refined.

• Cutoffs and variant interpretation. Not all ESR1 LBD variants are equally activating, and thresholds for calling a variant "present" at low allele fractions are not fully standardized across platforms.

The evidence base here is genuinely fast-moving. Readers should regard the framework above as reflecting the current predictive rationale grounded in EMERALD and the associated companion diagnostic, while recognizing that classification boundaries and assay standards continue to mature.

References

1. Bidard FC, Kaklamani VG, Neven P, et al. Elacestrant versus Standard Endocrine Therapy in ER+/HER2− Advanced Breast Cancer (EMERALD). J Clin Oncol. 2022. doi:10.1200/JCO.22.00338.