Targeting polo-like kinase 1 in pediatric leukemia

Abstract

Acute lymphoblastic leukemia (ALL) is one of the most common cancers among children in the world, with almost 90% complete remission rate after the primary treatment. However, some patients are resistant to treatment and some relapse later. Moreover, existing treatments are toxic and induce life-long adverse effects. Potentially, it can be avoided if a more specific drug is used. Therefore, there is a need for new therapeutic targets and reliable validation of the developed drugs to avoid potential off-target effects. The family of serine-threonine polo-like kinases (Plk) is important for cell cycle. Among other family members, Polo-like kinase 1 is a crucial regulator of mitosis, and it is particularly important for the proliferation of cancer cells This thesis aimed to improve understanding of Plk1 as a treatment target in pediatric leukemia.

In Paper I we aimed to identify the potential off-target proteins of Plk1 smallmolecule inhibitors such as BI2536, volasertib and NMS-1286937. The drugs are already involved in the late-stage clinical trials and result in severe side-effects that may affect the survival of the patients. We used cellular thermal shift assay and thermal proteome profiling to identify the proteins that have a change in the thermal stability after treatment. We found that the BI2536 and volasertib bind to Prostaglandin reductase 2 (PTGR2), an enzyme that regulates prostaglandin E2 metabolism and contributes to immune response. Also, we found that volasertib and NMS-1286937 affect the stability of a number of transcriptional coactivators, RNA splicing regulators, and proteins involved in the intracellular transport. In Papers II and III we investigated the potential of Plk1 as a treatment target in primary cells from pediatric T-cell and B-cell ALL. We showed that Plk1 is highly expressed both in patient cells and T/B-cell ALL cell lines, compared to peripheral blood mononuclear cells (PBMCs) from healthy donors. Further we targeted primary patient cells with RNAi prodrugs – short interfering RiboNucleic Neutrals (siRNN). We showed that siRNNs entered leukemic cells and induced specific Plk1 mRNA knockdown followed by double-strand DNA breaks, cell cycle arrest and apoptosis. We also showed that siRNNs did not induce apoptosis in PBMCs compared to Plk1 small molecule inhibitor volasertib.

To conclude, small molecule inhibitors of Plk1 affect high number of proteins apart from Plk1 itself that might explain the side effects of the treatment. Specific RNAi -based drugs may help to overcome this issue in T-cell and B-cell ALL.