Abstract PD7-10: Neoadjuvant trial with letrozole identifies PRR11 in 17q21-23 amplicon as a resistance mechanism to endocrine therapy in ER-positive breast cancer
Authors: Lee K-M , Guerrero-Zotano A , Formisano L , Jansen V , Gonzalez Ericsson P , Arteaga C 
Published: May 2019

Abstract
Approximately 20% of patients with early ER+ breast cancer (BC) treated with adjuvant antiestrogen therapy relapse with metastatic disease. Previously, we identified 3 amplicons (11q11.3, 8p11.23, and 17q21-23) associated with endocrine-resistance (Giltnane et al. Sci Transl Med 2017). The 17q21-23 amplicon has been associated with highly proliferative luminal B tumors and cancers with high genomic instability. A causal role of this region in endocrine resistance is unclear. We performed whole transcriptome analysis on RNA extracted from 58 ER+ breast cancers of patients treated with letrozole for 5.4-9.2 months (median 7.2 months). PRR11 (Proline rich 11), located in 17q21-23, was significantly upregulated in non-responding tumors as defined by cancer relapse after a median follow up of 5 years and/or a preoperative endocrine prognostic index (PEPI) ≥4. Differential gene expression analysis between tumors expressing low vs high PRR11 mRNA showed that BC signatures associated with proliferation, cell cycle, IGF-1 and PI3K signaling were enriched in tumors with high PRR11 expression. In the Metastatic Breast Cancer Project and TCGA, PRR11 amplification was higher in metastatic vs. primary BCs (16.5% and 8.5%, respectively; Fisher's p=0.0088). Gene Set Enrichment Analysis of mRNA expression in METABRIC and TCGA revealed significant enrichment of hallmark gene sets associated with proliferation in PRR11 amplified ER+ BCs. Genome-scale RNAi screening in Project Achilles showed that among all genes in the 17q21-23 amplicon, PRR11 knockdown results in the 4th strongest anti-proliferative effect in MCF7 cells. PRR11 knockdown with siRNA inhibited proliferation, cell cycle progression, and RB phosphorylation in HCC1428 LTED (long-term estrogen deprived), MCF7 LTED, and fulvestrant-resistant MCF7 cells. Using a PCR array with 84-cell cycle genes, we identified SKP2, CDKN1A, CCNB2, CCNA2, CKS2 and CCNB1 as genes downregulated by PRR11 knockdown. Except for CDKN1A, expression of all those genes was elevated ER+ BCs with PRR11 gain or amplification in TCGA. PRR11 associates with the p85 regulatory subunit of PI3K via its SH3 domain. We speculated this association would suppress p85 homodimers, thus permitting binding of PI3Kα (p110α)-p85 dimers to IRS1 and, hence, activating PI3K/AKT. To test this, we co-transfected HEK293T cells with HA-p85 and FLAG-p85. Transfection of PRR11 into these HEK293T cells reduced HA-p85 and FLAG-p85 homodimers as shown by HA and FLAG pulldowns followed by FLAG and HA immunoblots, respectively. Finally, PRR11 knockdown resulted in a reduction of p110a and S473 P-AKT levels and inhibition of IGF-1/2 stimulated P-AKT. Not inconsistent with these data, PRR11 amplification and PIK3CA mutations in METABRIC and TCGA are exclusive of each other, suggesting these alterations are functionally linked with the same signaling pathway. These data support a role of PRR11 in PI3K/AKT activation that may be causal to resistance to estrogen deprivation. We propose PRR11, located in the 17q21-23 amplicon, is a potential mediator of resistance to antiestrogen therapy by amplifying PI3K/AKT signaling, suggesting PI3K may be a therapeutic target in ER+ BCs harboring PRR11 amplification.
Citation Format: Lee K-M, Guerrero-Zotano A, Formisano L, Jansen V, Gonzalez Ericsson P, Arteaga C. Neoadjuvant trial with letrozole identifies PRR11 in 17q21-23 amplicon as a resistance mechanism to endocrine therapy in ER-positive breast cancer [abstract]. In: Proceedings of the 2018 San Antonio Breast Cancer Symposium; 2018 Dec 4-8; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2019;79(4 Suppl):Abstract nr PD7-10.