Modeling Transient Myeloproliferative Disorder Using Induced Pluripotent Stem Cells and CRISPR/Cas9 Technology

Down syndrome (DS) is recognized as one of the most important leukemia-predisposing syndromes. Specifically, 1-2% of DS children develop acute myeloid leukemia (AML) prior to age 5. AML in DS children (DS-AML) is characterized by the pathognomonic mutation in the gene encoding the essential hematopoietic transcription factor GATA1, resulting in N-terminally truncated mutant GATA1 (GATA1s). Trisomy 21 and GATA1s together induce a transient myeloproliferative disorder (TMD) exhibiting pre-leukemic characteristics. Approximately thirty percent of these cases progress into DS-AML by acquisition of additional somatic mutations in a step-wise manner.

We employed disease modeling in vitro by the use of customizable induced pluripotent stem cells (iPSCs) (7, 8) to generate a TMD model. Isogenic iPSC lines derived from the fibroblasts of a DS patient with trisomy 21 and with disomy 21 were used. We also obtained DS2-iPS10 (iPSCs derived from DS patient fibroblast) from Prof. George Daley, Children's Hospital, Harvard University (Boston, MA). CRISPR (Clustered Regularly Interspaced Short Palindromic Repeats)/Cas9 system with the indicated guide sequence (Fig. 1A) was used to introduce clinically relevant GATA1 mutation in both disomic and trisomic iPSC lines. A representative plot of TIDE (Tracking of Indels by Decomposition) analysis showing 98% allelic mutation frequency of a clone with 2 bp deletion at chromosomal level (Fig. 1B) correlated with sequence analysis using Basic Local Sequence Alignment Tool (BLAST) and Sanger sequencing chromatogram (Fig. 1C). This mutation resulted in the disruption of first initiation codon and thus prevented the production of full length GATA1 protein, while allowing the usage of second initiation codon at 84^th position to generate GATA1s. GATA1 and GATA1s are not expressed in iPSCs. To determine the expression of GATA1s, we differentiated these mutant iPSC lines into hematopoietic stem cell progenitors (HSPCs) using hematopoietic differentiation kit (StemCell Technologies) following a protocol depicted in Fig. 1D. The HSPCs derived from two distinct clones of trisomic iPSCs showed expression of full-length GATA1 protein and GATA1 mutant HSPCs lacked the expression of full-length GATA1 as expected (Fig. 1E). These HSPCs expressed GATA1s. Given that trisomy 21 promotes hematopoietic differentiation, an increase in the percentage of erythroid, megakaryoid and myeloid population was observed in trisomy 21 HSPCs with full length GATA1 (Fig. 1F, compare bars 1 and 3 in each category). The expression of GATA1s reduced erythroid lineage cells whereas it augmented megakaryoid and myeloid lineages in both disomy 21 (compare red and blue bars 1 and 2) and trisomy 21 backgrounds (compare bars 3 and 4). HSPCs derived from trisomy 21 iPSCs with GATA1s exhibited more megakaryoid expansion compared to the GATA1s in disomy 21 background (Fig. 1F, compare bars 2 and 4), in agreement with the synergistic function of trisomy 21 and GATA1s in promoting TMD. Transplantation of HSPCs derived from GATA1 mutated trisomic iPSCS into NSG-SGM3 mice showed the presence of human CD45+ cells in peripheral blood at 12 weeks post cell injection (Fig. 1G). In conclusion, we have developed a model system representing TMD, which can be used for step-wise modeling of Down-syndrome AML by introducing additional mutations.

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