Expression analysis of Mll3 knockdown p53-/- HSPC vs. control p53-/- HSPC

Recurring deletions of chromosome 7 and 7q [-7/del(7q)] occur in myelodysplastic syndromes and acute myeloid leukemia (AML) and are associated with poor prognosis. However, the identity of specific tumor suppressors on 7q remains elusive. Using RNAi and CRISPR/Cas9 approaches, we show that a ~50% reduction in gene dosage of the mixed lineage leukemia 3 (MLL3) gene, located on 7q36.1, cooperates with other events occurring in -7/del(7q) AMLs to promote leukemogenesis. Mll3 suppression impairs the differentiation of HSPC. Interestingly, Mll3 suppressed leukemias, like human -7/del(7q) AMLs, are refractory to conventional chemotherapy but sensitive to the BET inhibitor JQ1. Thus, our mouse model functionally validates MLL3 as a haploinsufficient 7q tumor suppressor, and suggests a therapeutic option for this aggressive disease. Total RNA obtained from sorted lin-ckit+ hematopoietic stem and progenitor cells of recipient mice transplanted with shRen;p53-/- or shMll3;p53-/- cells at 6 weeks after transplant