Targeting casein kinase 1α-mediated FADD phosphorylation restores sensitivity in select CDK4/6 inhibitor-resistant breast cancer models

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Abstract

Resistance to CDK4/6 inhibitors (CDK4/6i) combined with endocrine therapy presents a major barrier to improving outcomes in ER+ breast cancer. We identified FADD phosphorylation at Ser194 (phospho-FADD) as a mediator of CDK4/6i resistance in the models examined. Phospho-FADD acted as a pseudosubstrate inhibitor of the APC/C-Cdh1 complex, promoting G1/S transition and bypassing the canonical CDK4/6-Rb-E2F pathway. This CDK4/6-independent pathway was associated with PI3K hyperactivation. Clinical relevance of this bypass pathway was supported by increased phospho-FADD and pAKT in 73% of paired patient biopsies at post-treatment recurrence, while the remaining cases exhibited high baseline phospho-FADD and pAKT with intrinsic non-response. Inhibition of FADD phosphorylation with the CK1α degrader DEG-77, or PI3K inhibition, restored CDK4/6i sensitivity in resistant cells. In CDK4/6i-refractory xenografts, CK1α degradation combined with CDK4/6i resulted in profound tumor regressions and increased progression-free survival compared with either single agent, including complete tumor regressions in 92% of tumors. These findings support CK1α-mediated FADD phosphorylation as a targetable resistance mechanism in a subset of CDK4/6i-resistant ER+/HER2– breast cancer.

Authors

Sahezeel Awadia, Elizabeth K. Ziemke, Nicole M. Curnutt, Emily Kirk, Julianne Thomas, Anna Zimmerman, Maya J. Mileski, Sundaresh Ram, Reine Abou Zeidane, Craig Galban, Christina M. Woo, Corey W. Speers, Judith Leopold, Alnawaz Rehemtulla