ESR1 Mutation Testing in Metastatic Breast Cancer

Three types of ESR1 alterations have been identified at varying prevalences: ESR1 amplifications, ESR1 mutations, and ESR1 gene fusions [3]. Few studies have addressed the functional significance of ESR1 amplifications and ESR1 gene fusions; in contrast, ESR1 mutations have been well studied. The prevalence of ESR1 mutations in metastatic breast cancer varies and differs by specific sites of metastasis and history of endocrine therapy.

ESR1 mutations are found in approximately 20–40% of individuals who have received aromatase inhibition treatment. However, the prevalence decreases significantly in recurrent breast cancer after adjuvant aromatase inhibitor therapy, to around 4–5%. In cases where aromatase inhibitors are administered before surgery (neoadjuvant), the prevalence further decreases to a range of 1.5–7%.

Additionally, ESR1 mutations are rare, occurring in less than 1% of metastatic breast cancer cases where endocrine therapy has not been administered [1]. Consequently, ESR1 mutations primarily emerge post-aromatase inhibition and are uncommon in the primary tumors [4].

The most common mechanism of acquired resistance is activating missense mutations of ESR1 [5]. While genomic studies have identified over 50 mutations in ESR1, the majority with pathogenic significance are located within codons 536–538, a region within the ligand binding domain [5]. The gain-of-function mutations p.Y537C/N and p.D538G are the most common leading to constitutive ERα activity, independent of estrogen binding [2,4] (Figure 2).

Biomarkers serve various purposes in breast cancer, involving the analysis of both primary and metastatic tumors, utilizing various technologies for their identification and evaluation. Hormone receptor (HR) status (estrogen receptor and progesterone receptor) and HER2 expression rely on immunohistochemistry, which can be performed on the primary tumor, at the site where the cancer originated.

Detection of BRCA1/2 mutations relies on molecular techniques such as polymerase chain reaction (PCR) or next-generation sequencing (NGS) on DNA extracted from tumor tissue or from white blood cells in a blood sample. In the context of metastatic or unresectable breast cancer, biomarker testing may be performed at recurrence to understand the change in biology of the disease, including its evolving molecular profile.

Certain relevant molecular biomarkers, such as AKT1, ERBB2, microsatellite instability (MSI), NTRK1/2/3, and PTEN, may be identified from tissue of primary tumors. However, for ESR1 mutation research, tissue from metastatic tumors is preferred due to its rarity in primary tumors. Liquid biopsies can be employed to capture tumor heterogeneity over time.

Liquid biopsies have emerged as an invaluable tool in oncology research, offering a minimally invasive method for detecting biomarkers while providing an alternative sample type when tumor biopsies may not be feasible. Since ESR1 mutations are acquired and sub-clonal, liquid biopsies offer a promising approach for assessing and monitoring the dynamic changes in mutation status while capturing tumor heterogeneity [3,4].

Technologies used for liquid biopsies, such as digital PCR (dPCR) or NGS, require adequate sensitivity to detect low levels of circulating tumor nucleic acids. dPCR excels at identifying a smaller number of previously identified genomic alterations, while NGS offers the capability to identify a broader range of mutations per sample.

NGS technology can identify many more mutations, enabling the detection of rare mutation sites and polyclonal mutations while simultaneously analyzing multiple genes, thereby providing a more comprehensive view of the tumors’ mutational landscape.

Oncomine Solutions are complete end-to-end NGS workflows, including bioinformatics for precision oncology research. Requiring as little as 10 ng of DNA or RNA, Oncomine Solutions can generate results from limited tissue and small biopsies in as little as 24 hours.

The Oncomine Precision Assay enables evaluation of relevant biomarkers of breast cancer from tissue and liquid biopsy specimens (Table 1). Simultaneous analysis of relevant biomarkers and relevant co-mutations is essential to understanding the molecular mechanisms underlying ESR1 mutations. Research involving ESR1 mutations can elucidate its role in disease progression and metastasis. 

Oncomine Solutions provide an ideal method for molecular profiling in metastatic breast cancer research because of the ability to identify diverse mutations simultaneously with other relevant genes, with low input requirements and a short turnaround time.