Polo-Like Kinase 1 (PLK1) Inhibition Reduces Cell Proliferation and Induces Apoptosis in Childhood Acute Lymphoblastic Leukemia

Abstract 3529

Polo-like Kinase 1 (PLK1) is an essential regulator of mitosis. It is often overexpressed and a predictor of poor prognosis in different types of solid tumours and adult hematopoietic malignancies, making PLK1 an interesting therapeutic target in these cancer types. No PLK1 inhibitor has entered clinical trials in pediatric malignancies yet. This study therefore aimed to determine the potential of PLK1 as a new target in the treatment of childhood acute lymphoblastic leukemia (ALL), the most common type of childhood cancer.

Reverse phase protein arrays were performed to analyze levels of PLK1 protein expression and phosphorylation at T210, the major activating phosphorylation site, in ALL patient samples (n=174) and normal bone marrow mononuclear cells (nBM) (n=11). Both PLK1 expression and T210 phosphorylation were elevated (2.6-fold and 1.8-fold, respectively) in patients compared to nBM (p<0.0001). Furthermore, PLK1 protein expression levels were 1.3-fold higher in ALL cells carrying an E2A-PBX1 translocation than in other ALL genetic subtypes (p<0.0001). Knockdown of PLK1 expression by lentivirally delivered short hairpins led to inhibition of cell proliferation and induction of apoptosis in three precursor B-ALL cell lines and one T-ALL cell line, as indicated by decreased cell numbers and increased cleavage of PARP. This suggests an essential role for PLK1 in maintenance of the leukemic cell. An MTS cell viability assay was used to test the efficacy of the PLK1-specific inhibitor NMS-P937 (NMS-1286937) in primary ALL samples (n=15). When exposed to 120nM of NMS-P937 for 96 hours, there was a significant reverse correlation between PLK1 protein expression and leukemic cell survival (r_s=−0.58; p=0.024): patient cells with a high expression were more sensitive than cells with a low expression, whereas nBM cells were resistant up to concentrations in the micromolar range. Cumulative incidence of relapse and non-response to therapy (CIR) did not significantly differ between ALL patients with high and low levels of PLK1 protein (CIR=25.9% vs 15.7%; p=0.215). Patients with a high PLK1 T210 phosphorylation level, however, showed a trend towards a higher CIR than those with a low expression (CIR=27.5% vs 14.0%; p=0.078). Finally, Sanger sequencing of PLK1 mutational hot-spots led to the identification of a new non-synonymous mutation in 1 out of 38 patients. This mutation was located in exon 5 of the PLK1 gene, resulting in Ser335Arg, and did not lead to unusual levels of PLK1 protein or T210 phosphorylation, nor to NMS-P937 sensitivity.

Taken together, these results show that PLK1 is overexpressed in pediatric ALL and plays a pivotal role in the proliferation and survival of pediatric ALL cells. Moreover, they underline the potency of PLK1-inhibiting drugs as a valuable addition to current ALL treatment strategies, especially for cases expressing high levels of PLK1 protein.