TP73 As Novel Determinant of Resistance to BCL-2 Inhibition in Acute Myeloid Leukemia
Authors: Nishida Yuki , Ishizawa Jo , Ruvolo Vivian , Wang Feng , Takahashi Koichi , Mak Po Yee , Carter Bing Z , Andreeff Michael 
Published: November 2019

Background. BCL-2 inhibition is a novel and highly effective treatment modality in acute myeloid leukemias (AML). AML patients with IDH1/2 mutations are highly sensitive to BCL-2 inhibition by venetoclax (VEN) (Chen et al Nat Med 2015). High expression levels of the BCL-2 family proteins MCL-1 or BCL-XL, or knockout of TP53 have been reported to confer resistance to BCL-2 inhibition (Pan et al. Cancer Cell 2017, Nechiporuk et al. Cancer Discov 2019). p73 is one of the p53 family transcription factors and generates two isoforms, transactivation p73 (TAp73) and the N-terminally truncated ΔNp73. TAp73 shares a homologous N-terminal activation domain with p53 and has pro-apoptotic function similar to p53. ΔNp73 lacks an activation domain and has a dominant negative effect on the DNA binding of TAp73 and more importantly, of p53.TP73 is expressed in AML except in acute promyelocytic leukemias. However, the associations of TP73 isoforms with clinical and genetic characteristics or sensitivity to BCL-2 inhibition in AML have not been explored.
Results. We determined copy numbers of TAp73 and ΔNp73 mRNA levels in AML samples (N = 78) and normal CD34+ hematopoietic cells (HPC) using droplet digital PCR and investigated their clinical and biological relevance. Both TP73 isoforms were expressed in AML, with TAp73 expression being 50-fold higher in AML than in CD34+ HPC (P = 0.027); no difference seen for ΔNp73 (P = 0.80), suggesting that TAp73 is aberrantly expressed in AML cells. ΔNp73 and TAp73 mRNA levels were highly correlated (R2 = 0.72, P < 0.0001). AML samples had 10-fold more abundant TAp73 than ΔNp73 mRNA levels (P = 0.0017) and isoforms were not associated with disease status (de novo vs relapsed/refractory) or cytogenetic groups, and were mutation-agnostic, except for IDH1/2. IDH1/2 mutant AML showed lower levels of TAp73 and ΔNp73 than those with wild-type IDH1/2 (P = 0.06 and P = 0.007 for TAp73 and ΔNp73, respectively). In a separate dataset, we observed repressed TP73 in IDH1/2 mutant vs. wild-type AML samples (P = 0.073) by RNAseq analysis (N = 47).
Mechanistically, treatment with cell permeable octyl-(R)-2HG, the oncometabolite of mutant IDH1/2, reduced both TAp73 and ΔNp73 and increased susceptibility to VEN. Lentiviral knockdown of p73 in OCI-AML3 cells resulted in enhanced sensitivity to VEN with no significant changes in MCL-1 and p53 protein levels, or TP53 targets (MDM2, CDKN1A, FAS and BBC3). VEN resistant AML cells (MOLM-13 and MV4;11) generated through long-term culture with VEN expressed highly elevated TP73 mRNA and protein levels without significant changes in p53 or TP53 target changes, suggesting that elevated p73 could confer resistance to VEN independent of p53 function (Figure). Knockdown of TP73 showed increased protein levels of SDHB, UQCRC2 and ATP5A, components of mitochondrial respiratory chain complex II, III and V, indicating increased dependency on oxdative phosphorylation by depleting p73. Overexpression of TAp73α by lentiviral gene transfer minimally increased VEN-induced apoptosis, while ΔNp73γ overexpression conferred striking resistance to VEN in MOLM-13 cells, suggesting p73 isoform-specific dependency of VEN sensitivity/resistance. The combination of 5'-azacitidine (5'-aza) and VEN decreased ΔNp73 level by 50%.
Conclusion. Repression of TP73 in IDH1/2 mutant AML, and downregulation of TP73 by the oncometabolite 2-HG were associated with enhanced sensitivity to VEN, suggesting that TP73 determines AML susceptibility to BCL-2 inhibition. VEN resistant cells massively overexpressed TP73, and TP73 knockdown restored sensitivity to VEN. Specifically, overexpression of the ΔNp73γ isoform resulted in induced VEN resistance. ΔNp73 levels were also reduced by combining VEN with 5'-aza. Results may explain the high sensitivity of IDH1/2 mutant AML to VEN as consequence of downregulation of TP73 by 2-HG, and establish the mechanism of synergistic effect by VEN + 5'-aza combination and overexpression of p73 as a novel resistance mechanism to BCL-2 inhibition.

Disclosures
Ishizawa: Daiichi Sankyo: Patents & Royalties: Joint submission with Daiichi Sankyo for a PTC patent titled "Predictive Gene Signature in Acute Myeloid Leukemia for Therapy with the MDM2 Inhibitor DS-3032b," United States, 62/245667, 10/23/2015, Filed. Takahashi:Symbio Pharmaceuticals: Consultancy. Carter:Amgen: Research Funding; AstraZeneca: Research Funding; Ascentage: Research Funding. Andreeff:BiolineRx: Membership on an entity's Board of Directors or advisory committees; Daiichi Sankyo, Inc.: Consultancy, Patents & Royalties: Patents licensed, royalty bearing, Research Funding; Jazz Pharmaceuticals: Consultancy; Celgene: Consultancy; Amgen: Consultancy; AstaZeneca: Consultancy; 6 Dimensions Capital: Consultancy; Reata: Equity Ownership; Aptose: Equity Ownership; Leukemia Lymphoma Society: Membership on an entity's Board of Directors or advisory committees; German Research Council: Membership on an entity's Board of Directors or advisory committees; NCI-CTEP: Membership on an entity's Board of Directors or advisory committees; Cancer UK: Membership on an entity's Board of Directors or advisory committees; Center for Drug Research & Development: Membership on an entity's Board of Directors or advisory committees; CLL Foundation: Membership on an entity's Board of Directors or advisory committees; NCI-RDCRN (Rare Disease Cliln Network): Membership on an entity's Board of Directors or advisory committees; Oncoceutics: Equity Ownership; Breast Cancer Research Foundation: Research Funding; CPRIT: Research Funding; NIH/NCI: Research Funding; Oncolyze: Equity Ownership; Eutropics: Equity Ownership; Senti Bio: Equity Ownership, Membership on an entity's Board of Directors or advisory committees.