MEDB-08. Inhibition of different mitotic targets demonstrated distinct DNA damage and cell death response in p53-mutant medulloblastoma

BACKGROUND: In normal cells, cell cycle is tightly regulated by mitotic proteins to ensure smooth transition through each phase of cell division. Here, we examine two proteins – KIF11, a mitotic kinesin, responsible for assembly and maintenance of mitotic spindle during mitosis; and MELK, a serine/threonine kinase, essential for mitotic progression. Cancer cells can upregulate MELK and KIF11 to promote uncontrolled cell division and protect the cells from apoptotic cell death, leading to tumorigenesis. AIMS: We investigated the response of p53-mutant medulloblastoma (MB) by inhibiting KIF11 and MELK separately to study the effects on cell cycle progression and cell death mechanisms. RESULTS: Cell proliferation was suppressed by inhibition of either KIF11 or MELK in MB, independent of p53-mutant status. Regardless of p53-mutant status, inhibiting KIF11 induced cell cycle arrest at G2/M. In contrast, inhibiting MELK (IC(50) dose) induced more prominent G2/M arrest in p53-mutant cells compared to p53-wildtype cells. In p53-mutant MB, arrested cells during MELK inhibition subsequently underwent apoptotic cell death at 24h and 48h. With KIF11 inhibition, p53-mutant cells at 24h were already in necrotic stage. p53-mutant cells reached necrotic stage in a shorter time with KIF11 inhibition than MELK inhibition. On immunoblotting, independent of p53-mutant status, KIF11 inhibition produces more significant increase in DNA damage marker and c-PARP indicative of apoptosis, compared to MELK inhibition. Treatment with KIF11 or MELK inhibitor increased p53 protein expression in p53-wildtype (normal stress response). However, in p53-mutant cells, p53 protein expression decreased post-KIF11-inhibition, but remained unchanged post-MELK inhibition. In-vivo, inhibiting KIF11 was less tolerable in a patient-derived orthotopic xenograft model with p53-mutation. CONCLUSION: Inhibition of either mitotic target KIF11 or MELK, can induce anti-proliferative effects in MB. In p53-mutant MB, DNA damage and cell death response with KIF11-inhibition are more marked.