Gain of function of ASXL1 truncating protein in the pathogenesis of myeloid malignancies

Additional Sex Combs-Like 1 (ASXL1) is mutated at a high frequency in all forms of myeloid malignancies associated with poor prognosis. We generated a Vav1 promoter-driven Flag-Asxl1(Y588X) transgenic mouse model, Asxl1(Y588X)Tg, to express a truncated FLAGASXL1(aa1- 587) protein in the hematopoietic system. The Asxl1(Y588X)Tg mice had an enlarged hematopoietic stem cell (HSC) pool, shortened survival, and predisposition to a spectrum of myeloid malignancies, thereby recapitulating the characteristics of myeloid malignancy patients with ASXL1 mutations. ATAC-and RNA-sequencing analyses revealed that the ASXL1(aa1-587) truncating protein expression results in more open chromatin in cKit1 cells compared with wild-type cells, accompanied by dysregulated expression of genes critical for HSC self-renewal and differentiation. Liquid chromatography-tandem mass spectrometry and coimmunoprecipitation experiments showed that ASXL1(aa1-587) acquired an interaction with BRD4. An epigenetic drug screening demonstrated a hypersensitivity of Asxl1(Y588X)Tg bone marrow cells to BET bromodomain inhibitors. This study demonstrates that ASXL1(aa1-587) plays a gain-of-function role in promoting myeloid malignancies. Our model provides a powerful platform to test therapeutic approaches of targeting the ASXL1 truncation mutations in myeloid malignancies.