CEBPA-Driven Expression of the Transcriptionally Inactive deltaTP73 Isoform Phenocopies TP53mutated Poor Risk and Drug-Resistant Acute Myeloid Leukemia

Mutations in TP53 ( TP53mut) are present in roughly 10% of acute myeloid leukemia (AML) patients and represent a unique subgroup of patients with very poor prognosis. While the exact mechanisms of TP53mut-driven leukemogenesis remain elusive, transcriptional changes associated with suppression of the TP53 signaling pathway are considered to confer a dismal prognosis on TP53mut patients. Using molecular profiling of a large cohort of AML patients (n=823) we identify a new poor prognostic subgroup in AML that is TP53 WT but shares strong similarities with TP53mut patients. Patients included in this new group present increased expression of the oncogenic TP73 gene isoform ΔNp73,which lacks the transactivation domain but still binds to chromatin. We find that the transcriptional and metabolic program of ΔNp73high AMLs strongly resembles that of TP53mut AML, being enriched for stemness signatures like “17LSC” and “LSC UP”. In line with TP53mut AML, ΔNp73high/ TP53WT frequently co-occurs with U2AF1, SRSF2, TET2, and RUNX1 mutations . Lentiviral overexpression (OE) of ΔNp73 in healthy CD34 + cells increased cell proliferation and stemness. ΔNp73-OE in TP53WT AML models, imposed drug resistance against several standard-of-care cytotoxic therapies in AML (e.g., AraC, venetoclax (VEN) ± azacytidine (AZA), and FLT3 inhibitors), while drug resistance was not further enhanced upon ΔNp73-OE in TP53mut AML models. These results were validated using ex vivo treated primary AML samples (n=80), whereby ΔNp73 high samples were more resistant to several cytotoxic therapies, including VEN+AZA . Lentiviral ΔNp73-OE in primary APL samples (n=10) allowed efficient engraftment in vivo using xenograft mouse models with colonization of secondary organs and splenomegaly, which was not observed in the empty vector controls. To unravel mechanisms via which ΔNp73controls oncogenic transformation we performed RNA-seq and ChIP-seq to identify direct transcriptional targets of TP53, ΔNp73, and the transcriptionally active isoform TAp73. These studies revealed that ΔNp73 and TP53 compete for the same target genes, whereby ΔNp73-OE resulted in complete loss of TP53 binding and strong downregulation of these loci, including downregulation of known TP53 target genes CDKN1A, BBC3, and GAS6, but also of TP53 itself. Gene set enrichment analyses of molecular programs in ΔNp73-OE cells revealed that these were associated with the terms “LSC UP”, “CEBPA targets”, and “SIGNAL TRANSDUCTION BY TP53”,