Genomic and Clinical Landscape of Metastatic Hormone Receptors-positive Breast Cancers Carrying ESR1 Alterations

Background: Somatic genetic alterations of the estrogen receptor 1 gene (ESR1) are enriched in endocrine therapy-resistant, estrogen receptor-positive (ER+) metastatic breast cancer (mBC). Herein, we investigated and compared the clinical and genomic landscape of ESR1-mutant (ESR1) and ESR1 wild type (ESR1) ER+/ human epidermal growth factor receptor 2 (HER2)- mBCs.

Methods: Clinical and genomic data were retrieved from cBioPortal using the publicly-available MSK MetTropism dataset. Metastatic, ER+/HER2- mBC samples were included in the analysis. Only oncogenic and likely oncogenic alterations according to OncoKB were included. Statistical analyses were carried out using alpha level of 0.05, with a false discovery rate threshold of 10% for multiple comparisons using the Benjamini-Hochberg method.

Results: Among 679 samples, 136 ESR1 among 131 tumors were found (19.2%). The frequency of ESR1 was higher in ductal versus lobular mBC (21.2% versus 13.8%, P = 0.052) and enriched in liver metastasis compared with other sites (22.5% versus 12.7%; q = 0.02). Compared with ESR1 mBC, ESR1 tumors showed higher fraction of genome altered (FGA) {[0.28 interquartile range (IQR), 0.15-0.43] versus 0.22 (0.11-0.38); P = 0.04} and tumor mutational burden (TMB) [4.89 (IQR 3.46-6.85) versus 3.92 (2.59-6.05) mut/Mb; P = 0.001]. Tumors harboring p.E380X alterations showed higher TMB compared with those with H11-12 alterations [8.24 (IQR 5.06-15.3) versus 4.89 (IQR 3.46-6.75) mut/Mb; P = 0.01]. Genetic alterations of TP53 were enriched in ESR1 tumors (36% versus 14%) [odds ratio (OR) 3.17, 95% confidence interval (CI) 1.88-5.64, q = 0.001]. Considering signaling pathways, ESR1 tumors showed a lower occurrence of TP53 (OR 0.48, 95% CI 0.30-0.74; q = 0.003) and MAPK (OR 0.29, 95% CI 0.11-0.65; q = 0.009) alterations. TP53 (q < 0.001), CDH1 (q < 0.001), and ERBB2 (q < 0.001) demonstrated mutual exclusivity with ESR1.

Conclusions: ER+/HER2- mBCs carrying ESR1 exhibit a divergent genomic background, characterized by a lower prevalence of TP53 and MAPK pathway alterations. Less common ESR1 alterations falling outside the H11-H12 region seem to occur in tumors with higher TMB, deserving further investigation to understand their potential actionability.