PARP Inhibition Enhances Radiotherapy of SMAD4-Deficient Human Head and Neck Squamous Cell Carcinomas in Experimental Models.

Abstract

<i>SMAD4</i> loss causes genomic instability and the initiation/progression of head and neck squamous cell carcinoma (HNSCC). Here, we study whether <i>SMAD4</i> loss sensitizes HNSCCs to olaparib (PARP inhibitor) in combination with radiotherapy (RT). We analyzed HNSCC The Cancer Genome Atlas data for <i>SMAD4</i> expression in association with <i>FANC/BRCA</i> family gene expression. Human HNSCC cell lines were screened for sensitivity to olaparib. Isogenic HNSCC cell lines were generated to restore or reduce SMAD4 expression and treated with olaparib, radiation, or the combination. HNSCC pretreatment specimens from a phase I trial investigating olaparib were analyzed. <i>SMAD4</i> levels correlated with levels of <i>FANC/BRCA</i> genes in HNSCC. HNSCC cell lines with <i>SMAD4</i> homozygous deletion were sensitive to olaparib. <i>In vivo</i>, olaparib or RT monotherapy reduced tumor volumes in <i>SMAD4</i>-mutant but not <i>SMAD4</i>-positive tumors. Olaparib with RT dual therapy sustained tumor volume reduction in <i>SMAD4</i>-deficient (mutant or knockdown) xenografts, which exhibited increased DNA damage and cell death compared with vehicle-treated tumors. <i>In vitro</i>, olaparib alone or in combination with radiation caused lower clonogenic survival, more DNA damage-associated cell death, and less proliferation in <i>SMAD4</i>-deficient cells than in <i>SMAD4</i>-positive (endogenous SMAD4 or transduced SMAD4) cells. Applicable to clinic, 5 out of 6 SMAD4-negative HNSCCs and 4 out of 8 SMAD4-positive HNSCCs responded to a standard treatment plus olaparib in a phase I clinical trial, and SMAD4 protein levels inversely correlated with DNA damage. <i>SMAD4</i> levels are causal in determining sensitivity to PARP inhibition in combination with RT in HNSCCs.