Individuals with Down syndrome (DS) are highly predisposed to develop hematopoietic malignancies, especially acute megakaryoblastic leukemia (DS-AMKL). Generally, genes located on chromosome 21 tend to be increased in Down syndrome patients, but how these individual genes contribute to development of DS-AMKL is still unclear. SON, a gene located on chromosome 21, is upregulated in Down syndrome patients, and it has been reported that DS-AMKL cells have higher level of SON expression. The SON protein functions as a splicing factor and a transcription regulator in leukemia but the role of SON upregulation in DS-AMKL pathogenesis is still not fully elucidated. In addition, runt-related transcription factor 1 (RUNX1) is also located on chromosome 21, but its expression level is downregulated in DS-AMKL patients despite the increased gene dosage. We show that SON represses RUNX1 expression and blocks terminal megakaryocytic differentiation in DS-AMKL. Knockdown of SON in DS-AMKL cell lines leads to morphologic changes and increased cell adhesion. Moreover, cell surface marker analyses demonstrate that the depletion of SON induces increased CD41+ CD61+ populations as well as polyploidy cells, indicating megakaryocytic differentiation. Consistent with these results, SON deletion causes upregulation of RUNX1, which is a critical factor in the megakaryocytic differentiation. Furthermore, our data show that knockdown of SON increases several megakaryocytic transcription factors and decreases erythropoietic transcription factors, facilitating terminal megakaryocytic differentiation of leukemic blasts. Mechanistically, SON binds to the proximal promoter region as well as two intronic enhancer regions upstream of the proximal promoter at the RUNX1 gene locus, and suppresses transcription of RUNX1 by lowering the level of H3K4me3. Taken together, we propose that the increased genomic dosage of SON in Down syndrome impairs terminal differentiation of megakaryo-erythroblasts into mature megakaryocytes through binding to the RUNX1 promoter/enhancer to repress its transcription, which contributes to the higher incidence of AMKL in Down syndrome patients. Our results reveal a novel connection of SON to DS-AMKL pathogenesis and provide clinical importance of SON upregulation in DS. These findings suggest that SON could be a promising therapeutic target for DS-AMKL patients.

Citation Format: Lana Vukadin, Eun young Park, Jung Hyun Kim, Erin Eun Young Ahn. SON represses RUNX1 expression and impairs megakaryocytic differentiation in Down syndrome acute megakaryoblastic leukemia (DS-AMKL) [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 1482.