Human ASXL1-Mutant Hematopoiesis Is Driven by a Truncated Protein Associated with Aberrant Deubiquitination of H2AK119

Mutations in additional sex combs like 1 (ASXL1) confer poor prognosis in myeloid malignancies and are also found in pre-malignant clonal hematopoiesis of indeterminate potential (CHIP). However, the mechanisms by which these mutations contribute to disease initiation remain unresolved and therapeutic targeting has remained elusive. Here, we demonstrate that ASXL1 mutations are frequently founding lesions in acute myeloid leukemia (AML) and lead to the expression of a functional, truncated protein. We developed a human disease model that faithfully recapitulates ASXL1-mutant CHIP with increased self-renewal and in vivo competitive advantage that can spontaneously progress to a lethal myeloid malignancy. We determined that truncated ASXL1 leads to global redistribution of the repressive chromatin mark H2AK119Ub, increased chromatin accessibility, and activation of both myeloid and stem cell gene expression programs. Finally, we identified dysregulation of H2AK119Ub as a therapeutic vulnerability and demonstrated that ASXL1-mutant cells are vulnerable to inhibition of the PRC1 complex. RNA-Seq, ATAC-Seq and CUT&RUN Sequencing of H2AK119Ub in cord-blood derived human HSPCs