Acute myeloid leukaemia (AML) is the most common and aggressive form of acute leukaemia in adults. The most common driver mutations in AML are activating mutations in the FMS-like tyrosine kinase 3 (FLT3) gene, occurring in approximately one third of cases [1]. Internal tandem duplication (FLT3-ITD) mutations in the FLT3 juxtamembrane domain are the most common (~ 25%), and are associated with genomic instability, and intermediateadverse prognosis [2]. Point mutations in the FLT3 activation loop, most commonly at amino acid D835, occur in~ 8% of AML patients and their prognostic effect remains to be defined.

The landscape of treatment options for AML is rapidly changing, however there remains limited durable treatment options for molecular subtypes such as mutant-FLT3 AML. To identify novel therapeutic targets, we undertook quantitative phosphoproteomic profiling of primary AML blasts (Supplementary Methods)[3]. Across seven patient samples (3× wildtype-FLT3, 4× mutant-FLT3; Tables S1 and S2), 4345 unique phosphosites were identified from 1994 proteins, with expected ratios of serine, threonine, and tyrosine sites [pS: pT: pY 88.9%: 10.6%: 0.6%]. The top pathways phosphorylated in AML blasts were growth and survival signalling pathways (ERK/MAPK, AMPK signalling, Fig. S1) and DNA damage repair signalling pathways (ATM signalling, DNA double strand break (DSB) repair, Fig. S1). Kinase substrate enrichment analysis (KSEA) revealed activation of the cell cycle and apoptosis regulator, Casein Kinase 2 (CK2-A1), in all blast samples (Figs. 1 a and S2). Increased CK2-A1 expression has been demonstrated in a range of haematological cancers, including AML; driving cell proliferation, survival, and drug resistance [4]. Further