Understanding Pre-Leukemia in Trisomy 21 Human HSC and Modeling Progression Towards Down Syndrome Associated Leukemia Using CRISPR/Cas9 at Single Cell Resolution

Leukemia is the most common cancer in children. Sequencing data from identical twins suggests that the first genetic alterations in childhood leukemia occur in utero. Children with Down syndrome (Trisomy 21, T21) have an increased risk of childhood leukemia. In 30% of newborns with Down syndrome, a transient pre-leukemia disease occurs, which is characterized by a clonal proliferation of immature megakaryocytes carrying somatic mutations in the GATA1 transcription factor. These acquired GATA1 mutations lead to the expression of an N-terminal truncated protein (GATA1-Short). In 20% of the cases, acute megakaryoblastic leukemia (AMKL) evolves from the pre-leukemia by acquisition of additional genetic mutations in the transient leukemia clone, predominantly in genes of the cohesin complex. It is hypothesized that this represents a multi-step process of leukemogenesis with three distinct genetic events: T21, GATA1-Short and additional cohesin mutations. Yet, it remains unclear how an extra copy of chromosome 21 predisposes towards leukemia, the mechanisms of leukemic transformation and the interplay between each genetic component. Therefore, we wanted to establish a tractable human model system to investigate the initiation and evolution of transient leukemia and AMKL using CRISPR/Cas9 genome editing in primary human hematopoietic stem cells (HSCs).