Original ArticleMutations of the LIM protein AJUBA mediate sensitivity of head and neck squamous cell carcinoma to treatment with cell-cycle inhibitors

- Mutations of AJUBA, SMAD4, and RAS predicted sensitivity of HNSCC to treatment with inhibitors of PLK1, CHK1/2, and WEE1.
- PLK1 inhibition led to apoptosis in vitro and decreased tumor size in vivo in an orthotopic AJUBA-mutant HNSCC model.
- AJUBA-mutant HNSCC has high PLK1 protein expression, leading to a threshold effect for PLK1 inhibition.
- Forced AJUBA expression in AJUBA-mutant HNSCC cells led to increased proliferation.

The genomic alterations identified in head and neck squamous cell carcinoma (HNSCC) tumors have not resulted in any changes in clinical care, making the development of biomarker-driven targeted therapy for HNSCC a major translational gap in knowledge. To fill this gap, we used 59 molecularly characterized HNSCC cell lines and found that mutations of AJUBA, SMAD4 and RAS predicted sensitivity and resistance to treatment with inhibitors of polo-like kinase 1 (PLK1), checkpoint kinases 1 and 2, and WEE1. Inhibition or knockdown of PLK1 led to cell-cycle arrest at the G2/M transition and apoptosis in sensitive cell lines and decreased tumor growth in an orthotopic AJUBA-mutant HNSCC mouse model. AJUBA protein expression was undetectable in most AJUBA-mutant HNSCC cell lines, and total PLK1 and Bora protein expression were decreased. Exogenous expression of wild-type AJUBA in an AJUBA-mutant cell line partially rescued the phenotype of PLK1 inhibitor-induced apoptosis and decreased PLK1 substrate inhibition, suggesting a threshold effect in which higher drug doses are required to affect PLK1 substrate inhibition. PLK1 inhibition was an effective therapy for HNSCC in vitro and in vivo. However, biomarkers to guide such therapy are lacking. We identified AJUBA, SMAD4 and RAS mutations as potential candidate biomarkers of response of HNSCC to treatment with these mitotic inhibitors.