Abnormalities in the tumor protein 53 (TP53) gene occur in approximately 5%-10% of patients with de novo acute myeloid leukemia (AML) or myelodysplastic syndrome (MDS).¹ Despite 4 decades of research, the prognosis of patients with TP53-mutated (TP53_mut) cancers remains poor as they often respond poorly to cytotoxic chemotherapy.¹ These patients may have a median survival of 5 to 10 months, irrespective of therapies used.¹ In a recent study, researchers from the University of Hong Kong’s Faculty of Medicine (HKUMed) identified the therapeutic potential of pololike kinase 4 (PLK4) inhibition in AML and the possible mechanisms behind its in vivo efficacy.²

The TP53 gene is a tumor suppressor gene that encodes for the transcription factor p53, but the gene itself is a poor drug target.¹ Instead, mediators along its signaling pathway, including the PLK family, become potential targets of drug design.³ Previous failures with PLK inhibition were attributed to non-specific interactions with similar kinases in the same family, but it has been hypothesized that the structurally distinct PLK4 might offer greater selectivity.³ In this study, in silico analyses of druggable genes in TP53_mut AML were first conducted to identify novel therapeutic targets that were preferentially expressed in this AML subtype.² The results supported the hypothesis that PLK4 was involved in TP53_mut AML.

Following the in silico analyses, the research team conducted in vitro studies to investigate the role of PLK4.¹ It was found that there was elevated PLK4 expression in peripheral blood mononuclear cells with AML when compared with normal cells (p<0.001), as well as in TP53_mut AML samples when compared with TP53-wild type (TP53_WT) AML samples (p<0.01).² These data affirmed the importance of PLK4 in AML pathology, particularly among those with TP53_mut AML.² Furthermore, PLK4 inhibition (with CFI-400945) was shown to induce early inhibitory responses in AML cells regardless of TP53 mutation status.² In TP53_WT AML cells, short-term PLK4 inhibition led to apoptosis as early as day 1 and suppressed cell growth on day 4.² Although similar short-term effects were not seen in TP53_mut AML cells, prolonged PLK4 inhibition was shown to be effective against TP53_mut AML cells.²

In the same series of in vivo studies conducted on TP53_mut AML cells, the relationship between PLK4 inhibition and the p53 signaling pathway was further elucidated to show that PLK4 inhibition additionally induced cytokinesis failure and senescence not seen among TP53_WT AML cells.²  On day 2 of PLK4 inhibition, Immunofluorescence showed significant increases in the number of centrioles and microtubule-organizing centers, which were typical of cytokinesis failure with polyploidy.² Increased polyploidy in TP53_mut AML cells was also associated with cell defects including senescence-associated β-galactosidase, and the formation of cytoplasmic chromatin, which ultimately induced DNA damage in these TP53_mut AML cells.² A late inhibitory response on day 12 was achieved with prolonged inhibition of PLK4 in TP53_mut AML cells, showing further reduction in leukemia growth and abolishment of clonogenic activities.² Notably, despite cytotoxic effects on primary TP53_mut cluster of differentiation 34 positive (CD34+) samples, it did not affect normal cord blood progenitor cells.²

The study also identified two major mechanisms of which PLK4 inhibition may affect TP53_mut AML cells.² Intrinsically, PLK4 inhibition acted via a previously undescribed axis with Protein Arginine Methyltransferase 5 (PRMT5) phosphorylation which increased Enhancer of Zeste Homolog 2 (EZH2) protein expression by stabilizing the process with O-linked-N-acetylglucosaminylation (O-GlcNAcylation), ultimately leading to histone modification in the form of trimethylation of histone H3 at lysine 27 (H3K27me3) to suppress tumor growth.² Extrinsically, the activation of the guanosine monophosphate–adenosine monophosphate Synthase-Stimulator of interferon genes (cGAS-STING) pathway also led to increased secretion of cytokines and chemokines which increased the rate of AML cell destruction by phagocytosis from macrophage or apoptosis induced by T cells.²

Encouragingly, in vivo experiments with transplanted TP53_mut AML cells into mice have already shown to significantly reduce leukemic burden and improve survival (p<0.01) after treatment with CFI-400945, the PLK4 inhibitor used in the captioned study, compared with the chemotherapy regimen of cytarabine 25mg/kg/day and doxorubicin 1.5mg/kg/day.² The research team has since initiated the phase Ib/II trial of CFI-400945 as a monotherapy or combined with azacitidine in treating patients with AML, MDS or chronic myelomonocytic leukemia (CMML), with Queen Mary Hospital as one of the recruitment sites.⁴

In summary, PLK4 inhibition was shown to be an effective strategy for suppressing TP53_mut AML, possibly through both the induction of histone modification and the activation of cGAS-STING pathway, offering the possibility of a novel therapeutic target for patients with TP53_mut AML.²