Abstract PD6-6: Estrogen receptor (ESR1) mutations confer resistance to hormone therapy using a common mechanism
Authors: Gelsomino Luca , Gu Guowei , Rechoum Yassine , Ando' Sebastiano , Fuqua Suzanne AW 
Published: May 2015

Background: The idea that somatic estrogen receptor gene (ESR1) mutations could play an important role in the evolution of hormone-responsive breast cancers was proposed by us with our original identification of two ESR1 mutations at residues K303 and Y537. Technical issues with ESR1 mutation detection and the resulting paucity of reports in tumors led many to assume that ESR1 mutations were not there. However, with recent Next Generation Sequencing of metastatic tumors, mutation of ESR1 is now an accepted certainty. ESR1 mutant allele frequencies vary over a wide dynamic range, and are usually a minority population within tumors. Therefore how does a minor subclonal tumor population drive resistance in metastatic tumors?

Methods: MCF-7 cells expressing endogeous wild-type ER was transduced with ESR1 mutants K303R, Y537N, Y537S, and D538G lentivirus and stable clones selected. ER transcriptional assays and growth in soft agar were performed. Digital drop (dd) PCR and primer extension snp detection were used to ascertain mutant:WT ESR1 allele frequency in cell lines, 200 primary tumors from patients treated with tamoxifen monotherapy, and 20 metastatic breast tumors.

Results: Mutant ER constitutive transcriptional activity was fully antagonized by the antiestrogens tamoxifen or fulvestrant in MCF-7 stable transfectants. In contrast, soft agar growth of all ESR1 mutant-expressing cells was unexpectedly and completely resistant to the growth inhibitory effects of tamoxifen, although mutant-expressing cells were a minority subpopulation in the stable clones. Therefore, in cells with WT ER co-expression, the mutant resistant phenotype dominates. We found that phosphorylation of IGF1Rß; was constitutively increased in all ESR1-mutant expressing cells. Treatment with a specific IGF1Rß inhibitor in combination with tamoxifen drastically restored hormone sensitivity in cells expressing the ESR1 mutations. These results suggest a convergence in resistance mechanisms between the K303R and Y537 ESR1 mutation hot spots. We are exploring whether the dominant mutant ESR1 resistant phenotype occurs via activation of paracrine mediators, and have identified altered IGF-1 and interleukin 6 signaling in mutant-expressing cells. Mutation detection in a retrospective cohort of primary and metastatic breast tumors is ongoing and will be presented.

Conclusions: We hypothesize that the selection of dominant-acting ESR1 mutations in tumors is a key event in breast cancer progression, potentially due to the selective pressure of antiestrogens. The dominant-resistant phenotype of ESR1 mutants in a majority WT background supports the subclonal evolution of ESR1 mutations in breast cancer recurrence. A common resistance mechanism (like consitutive IGF1Rß activation) should enable biologic targeting of ESR1 mutation-positive metastatic patients a feasible clinical goal.

Support: NIH/NCI R01 CA72038 and CPRIT RP1210732 to SAWF.

Citation Format: Luca Gelsomino, Guowei Gu, Yassine Rechoum, Sebastiano Ando', Suzanne AW Fuqua. Estrogen receptor (ESR1) mutations confer resistance to hormone therapy using a common mechanism [abstract]. In: Proceedings of the Thirty-Seventh Annual CTRC-AACR San Antonio Breast Cancer Symposium: 2014 Dec 9-13; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2015;75(9 Suppl):Abstract nr PD6-6.