Mapping the cellular origin and early evolution of leukemia in Down syndrome

Down with leukemia Down syndrome is a congenital disorder caused by the trisomy of chromosome 21, and it is associated with a greatly increased risk of leukemia with origins in fetal development. Infants with Down syndrome are often born with a preleukemic condition, which later resolves in most cases. By using gene-edited human cells implanted into mouse models, Wagenblast et al. recapitulated the development of preleukemia and leukemia in the context of Down syndrome (see the Perspective by Roberts and Vyas). A specific mutation triggered a preleukemic condition in the context of trisomy 21 as expected, but progression to full-blown leukemia required a different genetic path and was not dependent on trisomy 21. Science, abf6202, this issue p. eabf6202; see also abj3957, p. 155 Initiation and progression of leukemogenesis in Down syndrome involves distinct cellular origins and effects of trisomy 21. INTRODUCTION Leukemia is the most common cancer in children, with the first genetic alterations often occurring during fetal development. These initiating events generate preleukemic cells, which are the evolutionary ancestors of leukemia that arises after birth. Because of our inability to directly access human fetal preleukemia, the identity of the cell of origin and the steps of leukemia evolution remain largely unknown. Down syndrome leukemogenesis represents a disease setting to study human preleukemia and the evolutionary steps that lead to fully transformed leukemia. Up to 30% of children with Down syndrome (trisomy 21) exhibit a preleukemic transient abnormal myelopoiesis (TAM) and, overall, have a 150-fold increased risk of developing myeloid leukemia within the first 5 years of life. However, the mechanism by which an extra copy of chromosome 21 predisposes to preleukemia and leukemia remains unclear. RATIONALE Understanding Down syndrome leukemogenesis requires a humanized model that faithfully recapitulates the full developmental spectrum of premalignant and malignant stages of Down syndrome leukemia. Using CRISPR/Cas9–mediated gene editing in human disomic and trisomic fetal liver–derived hematopoietic stem and progenitor cells and xenotransplantation, we developed a model with which to characterize the genetic events and cellular contexts underlying the preleukemic and leukemic phases of Down syndrome leukemogenesis. RESULTS Trisomy 21 hematopoietic stem and progenitor cells (HSPCs) showed reduced proliferation in vitro and generated smaller grafts in xenotransplanted mice, with reduced serial transplant ability, as compared with that of disomic HSPCs. Preleukemia was initiated in trisomy 21, but not disomic, long-term hematopoietic stem cells (LT-HSCs) when mutations in the erythroid-megakaryocyte transcription factor GATA binding protein 1 (GATA1) were introduced, which led to exclusive expression of the short isoform (GATA1s). Subsequent leukemic progression could occur in multiple stem and progenitor populations, was independent of trisomy 21, and induced through deletion of cohesin genes, including STAG2 (STAG2ko). Serial engraftment in mice showed that GATA1s-induced preleukemia underwent spontaneous resolution, which contrasted with the persistent ability of the GATA1s/STAG2ko–induced leukemia to engraft serially in mice. Leukemic progression was developmentally restricted to fetal and early postnatal stages; adult-derived bone marrow HSPCs were unable to undergo GATA1s/STAG2ko-induced leukemic transformation. We identified a molecular mechanism by which three chromosome 21 microRNAs (miRNAs) contributed to the predisposition toward preleukemia initiation. Simultaneous overexpression of miR-99a, miR-125b-2, and miR-155 in normal disomic LT-HSCs recapitulated a trisomy 21–like hematopoietic state, as assessed through comparable lineage differentiation, reduced self-renewal capacity, and similar gene expression and open chromatin accessibility profile. Removal of these miRNAs in trisomy 21 LT-HSCs inhibited GATA1s-induced preleukemia development. Using secondary xenotransplantations of defined cell populations, we identified CD117+/KIT proto-oncogene (KIT) as a marker of disease-driving cells. Pharmacological KIT inhibition targeted preleukemic stem cells, both in GATA1s-induced preleukemia and in primary Down syndrome preleukemia patient samples. CONCLUSION Collectively, our results provide insight into how human preleukemia and leukemia evolve in fetal life and early childhood. We were able to identify distinct cellular origins and effects of trisomy 21 for preleukemia initiation and leukemia progression. Predisposition to preleukemia in Down syndrome is affected by overexpression of distinct chromosome 21 miRNAs, specifically in the preleukemic LT-HSC cell of origin. Our study reveals the relevance of the cellular and developmental status of the cell of origin during leukemogenesis, which begins to explain why genetic drivers can be distinct between pediatric and adult acute myeloid leukemia. KIT inhibitors targeted preleukemic stem cells, providing proof of principle for early prevention strategies in childhood leukemia that may be able to inhibit leukemia progression, and these results encourage further preclinical and clinical assessment. Cell of origin in Down syndrome leukemogenesis. Down syndrome preleukemia originated in long-term hematopoietic stem cells (LT-HSCs) through mutations in GATA1, leading to the expression of the short isoform GATA1s. Progression toward leukemia occurred in various stem and progenitor cells through mutations in cohesin factors such as STAG2. Predisposition to preleukemia was affected by chromosome 21 miRNAs, and pharmacological inhibition of KIT targeted preleukemic stem cells. Children with Down syndrome have a 150-fold increased risk of developing myeloid leukemia, but the mechanism of predisposition is unclear. Because Down syndrome leukemogenesis initiates during fetal development, we characterized the cellular and developmental context of preleukemic initiation and leukemic progression using gene editing in human disomic and trisomic fetal hematopoietic cells and xenotransplantation. GATA binding protein 1 (GATA1) mutations caused transient preleukemia when introduced into trisomy 21 long-term hematopoietic stem cells, where a subset of chromosome 21 microRNAs affected predisposition to preleukemia. By contrast, progression to leukemia was independent of trisomy 21 and originated in various stem and progenitor cells through additional mutations in cohesin genes. CD117+/KIT proto-oncogene (KIT) cells mediated the propagation of preleukemia and leukemia, and KIT inhibition targeted preleukemic stem cells.