To explain the excess cancer rate in males, several candidates for “escape from X-inactivation tumor-suppressor” (EXITS) were recently identified. In this report we provide direct experimental evidence supporting UTX’s role as an EXITS gene. Using a mouse lymphoma model, we show clear dosage effect of UTX copy number during tumorigenesis, which strongly supports the EXITS theory. Importantly, UTX deletion not only accelerates lymphomagenesis, it also strongly promotes tumor progression. UTX-knockout tumors are more aggressive, showing enhanced brain dissemination and formation of blood vessels. Efnb1 is overexpressed in UTX KO tumors and can lead to such phenotypes. In human patients, lymphomas with low UTX expression also express high levels of Efnb1, and cause significantly poor survival. Lastly, we show that UTX deficiency renders lymphoma sensitive to cytarabine treatment. Taken together, these data highlight UTX loss’s profound impacts on tumor initiation and drug response.

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Recurrent somatic loss-of-function mutations in histone demethylases are frequently detected in cancer. However, whether loss of a histone demethylase can cause cancer has not been determined. Here, we report that knockout of the histone demethylase Utx in mice causes a chronic myelomonocytic leukemia (CMML)-like disease with splenomegaly, monocytosis, and extramedullary hematopoiesis. Mutational analysis of patient data indicated that UTX mutations occur simultaneously with TP53 mutations in myeloid malignancies, and combined inactivation of Utx and Trp53 accelerated the development of CMML in a cell-autonomous manner. Utx loss caused increased self-renewal of hematopoietic stem cells and predisposed hematopoietic stem cells to differentiate into myeloid-derived lineages. Transcriptome and chromatin immunoprecipitation analyses revealed that Utx activates key transcriptional factors required for erythroid differentiation by modulating histone H3 lysine 27 and lysine 4 trimethylation. Our results suggest that Utx suppresses CMML formation by controlling hematopoietic stem cell self-renewal and differentiation.