PDK1 Mediates <i>NOTCH1</i>-Mutated Head and Neck Squamous Carcinoma Vulnerability to Therapeutic PI3K/mTOR Inhibition.

Abstract

Head and neck squamous cell carcinoma (HNSCC) is driven largely by the loss of tumor suppressor genes, including <i>NOTCH1</i>, but lacks a biomarker-driven targeted therapy. Although the PI3K/mTOR pathway is frequently altered in HNSCC, the disease has modest clinical response rates to PI3K/mTOR inhibitors and lacks validated biomarkers of response. We tested the hypothesis that an unbiased pharmacogenomics approach to PI3K/mTOR pathway inhibitors would identify novel, clinically relevant molecular vulnerabilities in HNSCC with loss of tumor suppressor function.<b>Experimental Design:</b> We assessed the degree to which responses to PI3K/mTOR inhibitors are associated with gene mutations in 59 HNSCC cell lines. Apoptosis in drug-sensitive cell lines was confirmed <i>in vitro</i> and <i>in vivo</i>. NOTCH1 pathway components and PDK1 were manipulated with drugs, gene editing, knockdown, and overexpression. PI3K/mTOR inhibition caused apoptosis and decreased colony numbers in HNSCC cell lines harboring <i>NOTCH1</i> loss-of-function mutations (<i>NOTCH1</i> ^MUT) and reduced tumor size in subcutaneous and orthotopic xenograft models. In all cell lines, <i>NOTCH1</i> ^MUT was strongly associated with sensitivity to six PI3K/mTOR inhibitors. NOTCH1 inhibition or knockout increased <i>NOTCH1</i> ^WT HNSCC sensitivity to PI3K/mTOR inhibition. PDK1 levels dropped following PI3K/mTOR inhibition in <i>NOTCH1</i> ^MUT but not <i>NOTCH1</i> ^WT HNSCC, and PDK1 overexpression rescued apoptosis in <i>NOTCH1</i> ^MUT cells. PDK1 and AKT inhibitors together caused apoptosis in <i>NOTCH1</i> ^WT HNSCC but had little effect as single agents. Our findings suggest that <i>NOTCH1</i> ^MUT predicts response to PI3K/mTOR inhibitors, which may lead to the first biomarker-driven targeted therapy for HNSCC, and that targeting PDK1 sensitizes <i>NOTCH1</i> ^WT HNSCC to PI3K/mTOR pathway inhibitors.