ER⁺ Breast Cancers Resistant to Prolonged Neoadjuvant Letrozole Exhibit an E2F4 Transcriptional Program Sensitive to CDK4/6 Inhibitors.

Abstract

<b>Purpose:</b> This study aimed to identify biomarkers of resistance to endocrine therapy in estrogen receptor-positive (ER⁺) breast cancers treated with prolonged neoadjuvant letrozole.<b>Experimental Design:</b> We performed targeted DNA and RNA sequencing in 68 ER⁺ breast cancers from patients treated with preoperative letrozole (median, 7 months).<b>Results:</b> Twenty-four tumors (35%) exhibited a PEPI score ≥4 and/or recurred after a median of 58 months and were considered endocrine resistant. Integration of the 47 most upregulated genes (log FC > 1, FDR < 0.03) in letrozole-resistant tumors with transcription-binding data showed significant overlap with 20 E2F4-regulated genes (<i>P</i> = 2.56E-15). In patients treated with the CDK4/6 inhibitor palbociclib before surgery, treatment significantly decreased expression of 24 of the 47 most upregulated genes in letrozole-resistant tumors, including 18 of the 20 E2F4 target genes. In long-term estrogen-deprived ER⁺ breast cancer cells, palbociclib also downregulated all 20 E2F4 target genes and P-RB levels, whereas the ER downregulator fulvestrant or paclitaxel only partially suppressed expression of this set of genes and had no effect on P-RB. Finally, an E2F4 activation signature was strongly associated with resistance to aromatase inhibitors in the ACOSOG Z1031B neoadjuvant trial and with an increased risk of relapse in adjuvant-treated ER⁺ tumors in METABRIC.<b>Conclusions:</b> In tumors resistant to prolonged neoadjuvant letrozole, we identified a gene expression signature of E2F4 target activation. CDK4/6 inhibition suppressed E2F4 target gene expression in estrogen-deprived ER⁺ breast cancer cells and in patients' ER⁺ tumors, suggesting a potential benefit of adjuvant CDK4/6 inhibitors in patients with ER⁺ breast cancer who fail to respond to preoperative estrogen deprivation. <i>Clin Cancer Res; 24(11); 2517-29. ©2018 AACR</i>.