Splicing factor Ybx1 mediates persistence of Jak2-mutated neoplasms

Janus kinases (Jak) mediate cytokine, hormone and growth factor responses in hematopoietic cells. Jak2 is one of the most frequently mutated genes in the aging hematopoietic system and in hematopoietic cancers. Mutations in Jak constitutively activate downstream signaling and are drivers of myeloproliferative neoplasms (MPN). In clinical use, Jak-inhibitors have incomplete effects on overall disease burden of Jak2 mutated clones prompting us to investigate the mechanism underlying disease persistence. By in-depth phospho-proteome profiling we here identify proteins involved in mRNA processing as targets of mutant Jak2. Inactivation of the post-translationally modified Jak2-target Ybx1 sensitizes Jak-inhibitor persistent cells to apoptosis and results in RNA mis-splicing, retained intron enrichment and disruption of the transcriptional control of extracellular signal-regulated kinase (ERK) signaling. In combination with pharmacological Jak-inhibition it induces apoptosis in Jak2-dependent murine and primary human cells, leading to in vivo regression of the malignant clones and inducing remission. This identifies and validates a novel cell-intrinsic mechanism how differential protein phosphorylation results in splicing-dependent alterations of Jak2-ERK-signaling and the maintenance of Jak2V617F malignant clones. Therapeutic targeting of Ybx1 dependent ERK-signaling in combination with Jak2-inhibition may eradicate Jak2-mutated cells. Examination of Ybx1 mRNA processing and splicing events in BaF3 V617F cells with shNT, sh1Ybx1 and sh2Ybx1