Abstract 2692: ETV6-RUNX1 targets a developmentally restricted embryonic human B-myeloid progenitor

Childhood acute lymphoblastic leukemia (cALL) is distinct from that in adults with higher incidence, better prognosis and a distinct mutational spectrum. One hypothesis for this difference is that cALL arises in transient cells unique to early human development. We explored this in ETV6-RUNX1 cALL where evidence from twins and neonatal heel prick testing has shown that this mutation arises in utero and is an initiating event. We characterized B cell development in first trimester human fetal liver (FL) to identify compartments vulnerable to ETV6-RUNX1. Using CD19 as a marker of B lineage commitment we found the first CD19+ B cells emerge in the human FL at Carnegie Stage (CS) 17 and were distinct from adult in that the majority expressed surface IL7 receptor. We used IL7R to identify a CD19-IL7R+ B progenitor compartment that produced B cells in vitro, possessed DJH recombination, but also had monocytic potential. Single cell analysis of CS20 IL7R+ progenitors revealed co-expression of lymphoid and myeloid programmes, whereas at CS17 they were strongly myeloid primed indicating that IL7R+ progenitors acquire lymphoid potential in this developmental window. Some co-expression of lymphoid and myeloid programmes also persisted in CS20 FL B cells. We tested whether FL B cell development could be modeled using human pluripotent stem cells (hPSCs). In vitro B cell differentiation of hPSCs produced IL7R expressing pro and preB cells as well as an IL7R+ progenitor that switched from myeloid to B-myeloid priming during culture. At the global transcriptional level the hPSC lymphoid hierarchy mapped closely with FL, with both separating from adult suggesting that hPSCs provide a developmentally relevant model of early FL B lymphopoiesis. We next used CRIPSR-directed homologous recombination to engineer the expression of ETV6-RUNX1 under the endogenous ETV6 promoter. ETV6-RUNX1 hPSCs displayed a partial block in B cell differentiation at the level of the IL7R+ progenitor. ETV6-RUNX1 expressing B cells co-expressed an abnormal B-myeloid gene expression signature akin to that seen in the IL7R+ progenitor. Both the transcriptional and differentiation phenotypes were dependent on ETV6-RUNX1 as demonstrated by their reversion upon cre-mediated excision of the knock-in cassette. Our data support a model where expression of ETV6-RUNX1 inhibits lymphoid specification in an early FL IL7R+ lymphomyeloid progenitor, arresting B lineage differentiation and resulting in the production of myeloid-primed B cells. This may explain the relatively high levels of myeloid antigen expression lineage promiscuity seen in cALL. ETV6-RUNX1 hPSCs will afford the systematic evaluation of the contribution of additional mutations seen in cALL and may offer a tractable platform for drug screening. In conclusion we propose that a novel IL7R+ lymphomyeloid progenitor in the human FL is a candidate target cell for in utero pre-leukemic initiation in cALL. Citation Format: Simon E. Richardson, Charlotta Boiers, Alya Zriwil, Virginia Turati, John Brown, Dapeng Wang, Javier Herrero, Stefan Karlsson, Andrew J. H. Smith, Sten Erik Jacobsen, Tariq Enver. ETV6-RUNX1 targets a developmentally restricted embryonic human B-myeloid progenitor. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 2692.