ESR1 mutations in metastatic lobular breast cancer patients

Invasive lobular breast cancer (ILC) represents the second most common histology of breast cancer after invasive ductal breast cancer (IDC), accounts for up to 15% of all invasive cases and generally express the estrogen receptor (ER, coded by the ESR1 gene). ESR1 mutations have been associated with resistance to endocrine therapy, however these have not been specifically evaluated in ILC. We assessed the frequency of ESR1 mutations by droplet digital PCR in a retrospective multi-centric series of matched primary tumor and recurrence samples (n = 279) from 80 metastatic ER-positive ILC patients. We further compared ESR1 mutations between IDC and ILC patients in metastatic samples from MSKCC-IMPACT (n = 595 IDC and 116 ILC) and in ctDNA from the SoFEA and PALOMA-3 trials (n = 416 IDC and 76 ILC). In the retrospective series, the metastases from seven patients (9%) harbored ESR1 mutations, which were absent from the interrogated primary samples. Five patients (6%) had a mutation in the primary tumor or axillary metastasis, which could not be detected in the matched distant metastasis. In the MSKCC-IMPACT cohort, as well as in the SoFEA and PALOMA-3 trials, there were no differences in prevalence and distribution of the mutations between IDC and ILC, with D538G being the most frequent mutation in both histological subtypes. To conclude, no patient had an identical ESR1 mutation in the early and metastatic disease in the retrospective ILC series. In the external series, there was no difference in terms of prevalence and type of ESR1 mutations between ILC and IDC.