PATH-68. Susceptibility of p53-mutant medulloblastomas (MB) to polo-like kinase 4 (PLK4) inhibition

Abstract BACKGROUND PLK4 is a serine / threonine protein kinase that regulates centriole duplication and progression of the cell cycle. It is upregulated in cancers including MB, the most common childhood malignant brain tumour. Inhibitors of PLK4 are a potential novel cancer therapeutic agent that is currently undergoing clinical trials in leukemia. p53 is a tumour suppressor gene that plays a role in multiple cellular pathways including cell cycle arrest, apoptosis and DNA repair. It is frequently mutated in many different types of cancer, including the SHH-activated, p53-mutant subgroup of MB which is the subgroup of MB with the worst prognosis. RESULTS We show that inhibition of PLK4 inhibits proliferation of different MB cell lines independent of p53-mutational status. Similarly, in both p53-WT and p53-mutant MB cells, immunoblot staining shows upregulation of both DNA damage marker, pH2Ax, as well as apoptosis marker, c-PARP. However, upon treatment with a PLK-inhibitor, p53-mutant MB cells demonstrate a much more prominent cell cycle arrest at G2/M phase after 24 hours, as well as apoptosis at 24 and 48 hours compared to the p53-WT MB cells. Furthermore, the p53-mutant MB cells demonstrate polyploidy on flow cytometry, and abnormal multinucleated nuclear morphology with Hoechst staining upon PLK4 inhibition which is not present in p53-WT MB cells. Normal human brain expresses low PLK4. Across 4 patient MB cohorts (>1000 tumors), PLK4 gene expression is significantly higher in SHH group compared to WNT, Group 3 or 4 MBs (adjusted p<0.05, Omnibus Kruskal-Wallis). Preclinical drug testing with PLK-inhibitor in a p53-mutant SHH patient-derived mouse model of MB is planned. CONCLUSION PLK4-inhibitors are a potential novel cancer therapeutic agent in the treatment of MB, and may be more efficacious in patients with p53-mutations or in combination with p53-inhibitors. SHH-MBs are frequently p53-mutants and potentially a good subtype to target with PLK4-inhibitors.