DNA polymerase θ protects leukemia cells from metabolically induced DNA damage.

Leukemia cells accumulate DNA damage, but altered DNA repair mechanisms protect them from apoptosis. We showed here that formaldehyde generated by serine/1-carbon cycle metabolism contributed to the accumulation of toxic DNA-protein crosslinks (DPCs) in leukemia cells, especially in driver clones harboring oncogenic tyrosine kinases (OTKs: FLT3(internal tandem duplication [ITD]), JAK2(V617F), BCR-ABL1). To counteract this effect, OTKs enhanced the expression of DNA polymerase theta (POLθ) via ERK1/2 serine/threonine kinase-dependent inhibition of c-CBL E3 ligase-mediated ubiquitination of POLθ and its proteasomal degradation. Overexpression of POLθ in OTK-positive cells resulted in the efficient repair of DPC-containing DNA double-strand breaks by POLθ-mediated end-joining. The transforming activities of OTKs and other leukemia-inducing oncogenes, especially of those causing the inhibition of BRCA1/2-mediated homologous recombination with and without concomitant inhibition of DNA-PK-dependent nonhomologous end-joining, was abrogated in Polq-/- murine bone marrow cells. Genetic and pharmacological targeting of POLθ polymerase and helicase activities revealed that both activities are promising targets in leukemia cells. Moreover, OTK inhibitors or DPC-inducing drug etoposide enhanced the antileukemia effect of POLθ inhibitor in vitro and in vivo. In conclusion, we demonstrated that POLθ plays an essential role in protecting leukemia cells from metabolically induced toxic DNA lesions triggered by formaldehyde, and it can be targeted to achieve a therapeutic effect.

Inactivation of p21-Activated Kinase 2 (Pak2) Inhibits the Development of Nf2-Deficient Tumors by Restricting Downstream Hedgehog and Wnt Signaling.

Because loss of the NF2 tumor suppressor gene results in p21-activated kinase (Pak) activation, PAK inhibitors hold promise for the treatment of NF2-deficient tumors. To test this possibility, we asked if loss of Pak2, a highly expressed group I PAK member, affects the development of malignant mesothelioma in Nf2;Cdkn2a-deficient (NC) mice and the growth properties of NC mesothelioma cells in culture. In vivo, deletion of Pak2 resulted in a markedly decreased incidence and delayed onset of both pleural and peritoneal malignant mesotheliomas in NC mice. In vitro, Pak2 deletion decreased malignant mesothelioma cell viability, migration, clonogenicity, and spheroid formation. RNA-sequencing analysis demonstrated downregulated expression of Hedgehog and Wnt pathway genes in NC;Pak2-/- mesothelioma cells versus NC;Pak2+/+ mesothelioma cells. Targeting of the Hedgehog signaling component Gli1 or its target gene Myc inhibited cell viability and spheroid formation in NC;P+/+ mesothelioma cells. Kinome profiling uncovered kinase changes indicative of EMT in NC;Pak2-/- mesothelioma cells, suggesting that Pak2-deficient malignant mesotheliomas can adapt by reprogramming their kinome in the absence of Pak activity. The identification of such compensatory pathways offers opportunities for rational combination therapies to circumvent resistance to anti-PAK drugs. IMPLICATIONS: We provide evidence supporting a role for PAK inhibitors in treating NF2-deficient tumors. NF2-deficient tumors lacking Pak2 eventually adapt by kinome reprogramming, presenting opportunities for combination therapies to bypass anti-PAK drug resistance.

Somatic Epigenetic Silencing of RIPK3 Inactivates Necroptosis and Contributes to Chemoresistance in Malignant Mesothelioma.

PURPOSE: Receptor-interacting protein kinase 3 (RIPK3) phosphorylates effector molecule MLKL to trigger necroptosis. Although RIPK3 loss is seen in several human cancers, its role in malignant mesothelioma is unknown. This study aimed to determine whether RIPK3 functions as a potential tumor suppressor to limit development of malignant mesothelioma. EXPERIMENTAL DESIGN: RIPK3 expression was examined in 66 malignant mesothelioma tumors and cell lines. Promoter methylation and DNMT1 siRNA studies were performed to assess the mode of RIPK3 silencing in RIPK3-deficient malignant mesothelioma cells. Restoration of RIPK3 expression in RIPK3-negative malignant mesothelioma cells, either by treatment with 5-aza-2'-deoxycytidine or lentiviral expression of cDNA, was performed to assess effects on cell viability, necrosis, and chemosensitization. RESULTS: Loss of RIPK3 expression was observed in 42/66 (63%) primary malignant mesotheliomas and malignant mesothelioma cell lines, and RT-PCR analysis demonstrated that downregulation occurs at the transcriptional level, consistent with epigenetic silencing. RIPK3-negative malignant mesothelioma cells treated with 5-aza-2'-deoxycytidine resulted in reexpression of RIPK3 and chemosensitization. Ectopic expression of RIPK3 also resulted in chemosensitization and led to necroptosis, the latter demonstrated by phosphorylation of downstream target MLKL and confirmed by rescue experiments. Mining of RIPK3 expression and survival outcomes among patients with malignant mesothelioma available from The Cancer Genome Atlas repository revealed that promoter methylation of RIPK3 is associated with reduced RIPK3 expression and poor prognosis. CONCLUSIONS: These data suggest that RIPK3 acts as a tumor suppressor in malignant mesothelioma by triggering necroptosis and that epigenetic silencing of RIPK3 by DNA methylation impairs necroptosis and contributes to chemoresistance and poor survival in this incurable disease.

Inactivation of Bap1 Cooperates with Losses of Nf2 and Cdkn2a to Drive the Development of Pleural Malignant Mesothelioma in Conditional Mouse Models.

Pleural malignant mesothelioma is a therapy-resistant cancer affecting the serosal lining of the thoracic cavity. Mutations/deletions of BAP1, CDKN2A, and NF2 are the most frequent genetic lesions in human malignant mesothelioma. We introduced various combinations of these deletions in the pleura of conditional knockout (CKO) mice, focusing on the contribution of Bap1 loss. While homozygous CKO of Bap1, Cdkn2a, or Nf2 alone gave rise to few or no malignant mesotheliomas, inactivation of Bap1 cooperated with loss of either Nf2 or Cdkn2a to drive development of malignant mesothelioma in approximately 20% of double-CKO mice, and a high incidence (22/26, 85%) of malignant mesotheliomas was observed in Bap1;Nf2;Cdkn2a (triple)-CKO mice. Malignant mesothelioma onset was rapid in triple-CKO mice, with a median survival of only 12 weeks, and malignant mesotheliomas from these mice were consistently high-grade and invasive. Adenoviral-Cre treatment of normal mesothelial cells from Bap1;Nf2;Cdkn2a CKO mice, but not from mice with knockout of one or any two of these genes, resulted in robust spheroid formation in vitro, suggesting that mesothelial cells from Bap1;Nf2;Cdkn2a mice have stem cell-like potential. RNA-seq analysis of malignant mesotheliomas from triple-CKO mice revealed enrichment of genes transcriptionally regulated by the polycomb repressive complex 2 (PRC2) and others previously implicated in known Bap1-related cellular processes. These data demonstrate that somatic inactivation of Bap1, Nf2, and Cdkn2a results in rapid, aggressive malignant mesotheliomas, and that deletion of Bap1 contributes to tumor development, in part, by loss of PRC2-mediated repression of tumorigenic target genes and by acquisition of stem cell potential, suggesting a potential avenue for therapeutic intervention. SIGNIFICANCE: Combinatorial deletions of Bap1, Nf2, and Cdkn2a result in aggressive mesotheliomas, with Bap1 loss contributing to tumorigenesis by circumventing PRC2-mediated repression of oncogenic target genes.