Abstract P1-06-11: A targetable EGFR-driven tumor-initiating program in breast cancer
Authors: Savage P , Saleh SMI , Wang Y-C , Revil T , Badescu D , Liu L , Iacucci E , Zuo D , Bertos N , Munoz-Ramos V , Asselah J , Meterissian S , Omeroglu A , Hébert S , Kleinman C , Park M , Ragoussis J 
Published: March 2017

Abstract
Background: Inter- and intra-tumour heterogeneity underlies variability in therapeutic response. Although targeting of the epidermal growth factor receptor (EGFR) in breast cancer has failed to demonstrate clinical efficacy at the population level, complete and durable responses have been reported at low frequencies. The molecular determinants of these responses are unknown, but are of importance in the era of precision medicine.
Results: We performed a patient-derived xenograft (PDX) clinical trial with gefitinib in a breast cancer PDX cohort. Consistent with clinical trial data, gefitinib exhibited limited efficacy across most models. One PDX, however, demonstrated a complete and durable (&gt;6 months) clinical response, and was subject to deep molecular profiling to identify determinants of response. Exome sequencing revealed no single nucleotide variants or copy number alterations in EGFR pathway members. EGFR was differentially expressed between the two major cellular subpopulations identified by single-cell RNAseq and this cellular heterogeneity in EGFR expression was validated immunohistochemically. Fluorescence-activated cell sorting of the EGFRhi subpopulation revealed cells with enhanced stem-like properties, including ALDH activity, sphere-forming capacity in vitro, ability to form tumours in vivo and seeding lung micrometastases from orthotopically transplanted tumours. Tumourspheres derived from EGFRhi cells developed into mixed EGFRhi and EGFRlo subpopulations, as did macrometastases, supporting that EGFRhi subpopulation can self-renew and re-populate. Analysis of expressed SNVs in the single-cell RNAseq data, filtered by variants identified from exome sequencing, showed no clonal segregation, supporting a non-clonal origin of the functionally distinct EGFRhi and EGFRlo subpopulations. This EGFR-driven tumour initiating cell program was observed in independent PDX models, some which showed growth inhibition in response to gefitinib.
Conclusions: Using bulk and single-cell genomic profiling, we identified and functionally validated an EGFR-driven tumour-initiating program in a subset of aggressive breast tumours, which may be predictive of gefitinib sensitivity. This contradicts traditional beliefs that good therapeutic targets are homogenously expressed, in that we show that a target displaying intra-tumour heterogeneity can be effective so long that it is expressed in the tumour-initiating population.
Citation Format: Savage P, Saleh SMI, Wang Y-C, Revil T, Badescu D, Liu L, Iacucci E, Zuo D, Bertos N, Munoz-Ramos V, Asselah J, Meterissian S, Omeroglu A, Hébert S, Kleinman C, Park M, Ragoussis J. A targetable EGFR-driven tumor-initiating program in breast cancer [abstract]. In: Proceedings of the 2016 San Antonio Breast Cancer Symposium; 2016 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2017;77(4 Suppl):Abstract nr P1-06-11.