Mutations in the ESR1 gene (ESR1m) are important mechanisms of resistance to endocrine
therapy in
estrogen receptor-positive advanced
breast cancer and have been recognized as a prognostic and predictive
biomarker as well as a potential therapeutic target. However, the prevalence of ESR1m in real-world patients has not been adequately described. Therefore, we sought to evaluate the prevalence of ESR1m in metastatic samples from Brazilian patients with
estrogen receptor-positive (ER+) advanced
breast cancer previously treated with endocrine
therapy. The presence of ESR1m was evaluated in
formalin-fixed
paraffin-embedded (FFPE)
breast cancer tissue using real-time quantitative polymerase chain reaction (RT-qPCR). Mutations in
codons 380, 537, and 538 of the ESR1 gene were analyzed. Out of 77
breast cancer samples, 11 (14.3%) showed mutations in the ESR1 gene. ESR1m were detected in a variety of organs, and the D538G substitution was the most common mutation. In visceral
metastasis, ESR1m were detected in 25% (8/32) of the samples, whereas in nonvisceral
metastasis, ESR1m were detected in 6.7% (3/45) of the samples. The odds of a sample with visceral
metastasis having an ESR1 mutation is 4.66 times the odds of a sample of nonvisceral
metastasis having an ESR1 mutation (95% CI: 1.13-19.27; p value = 0.0333). Our study indicates that the prevalence of ESR1m in samples from Brazilian patients with metastatic ER+
breast cancer is similar to that described in patients included in clinical trials. We observed an association of ESR1m with visceral
metastasis.