Mutant p53 Drives the Development of Pre-Leukemic Hematopoietic Stem Cells through Modulating Epigenetic Regulators

AML is thought to arise from leukemia stem cells (LSCs); however, recent evidence suggests that the transforming events may initially give rise to pre-leukemic hematopoietic stem cells (pre-leukemic HSCs), preceding the formation of fully transformed LSCs. Pre-leukemic HSCs have been shown to contribute to normal blood development and harbor a selective growth advantage compared to normal HSCs. Pre-leukemic HSCs can acquire subsequent mutations, and once differentiation capacity is impaired, leukemia emerges. Recently, acquired somatic TP53 mutations, including p53^R248W and p53^R273H, were identified in healthy individuals as well as AML patients, suggesting that TP53 mutations may be early events in the pathogenesis of AML. We found that p53^R248W HSCs showed a multi-lineage repopulation advantage over WT HSCs in transplantation experiments, demonstrating that mutant p53 confers a pre-leukemic phenotype in murine HSCs.

Although TP53 mutations are limited in AML, TP53 mutations do co-exist with mutations of epigenetic regulator, ASXL-1, or receptor tyrosine kinase, FLT3, in AML. Mutations in Asxl-1 are present in ~10-30% of patients with myeloid malignancies and confer poor prognosis.  Loss of Asxl-1 in the hematopoietic compartment leads to a myelodysplastic-like syndrome in mice and reduced stem cell self-renewal. Internal tandem duplications in Flt3 (Flt3-ITD) occur in ~30% of AML patients and are associated with adverse clinical outcome.  Flt3-ITD-positive mice develop a myeloproliferative neoplasm (MPN) and HSCs expressing Flt3-ITD have decreased self-renewal capabilities. We hypothesize that mutant p53 drives the development of pre-leukemic HSCs with enhanced self-renewal capability, allowing clonal expansion and subsequent acquisition of Asxl-1 or Flt3 mutations leading to the formation of fully transformed leukemia stem cells. To define the role of mutant p53 in Asxl-1^+/- HSCs, we generated p53^R248W/+Asxl-1^+/- mice and performed in vitro serial replating assays as well as in vivo competitive bone marrow transplantation experiments.  We found that p53^R248W significantly enhanced the serial replating ability of Asxl-1-deficient bone marrow cells.  Interestingly, while bone marrow from Asxl-1^+/- mice had very poor engraftment compared to wild type bone marrow cells 16 weeks post-transplantation, the expression of p53^R248W in Asxl-1^+/- bone marrow rescued the defect.  To examine the role of mutant p53 in Flt3-ITD-positive HSCs, we generated p53^R248W/+Flt3^ITD/+ mice.  We found that p53^R248W enhanced the replating ability of Flt3^ITD/+ bone marrow cells.  Despite the fact that Flt3^ITD/+ bone marrow cells displayed decreased repopulating ability compared to wild type cells 16 weeks post-transplant, expression of p53^R248W in Flt3^ITD/+ cells rescued the defect.  We are monitoring leukemia development in primary and secondary transplant recipients as well as in de novo p53^R248W/+Asxl-1^+/- and p53^R248W/+Flt3^ITD/+ animals and predict that mutant p53 may cooperate with Asxl-1 deficiency or Flt3-ITD in the formation of LSCs to accelerate leukemia development in Asxl-1 deficient or Flt-ITD-positive neoplasms. 

Mechanistically, dysregulated epigenetic control underlies the pathogenesis of AML and we discovered that mutant p53 regulates epigenetic regulators, including Ezh1, Ezh2, Kdm2a, and Setd2, in HSCs. H3K27me3 is catalyzed by EZH1 or EZH2 of the Polycomb repressing complex 2 (PRC2). Both Ezh1 and Ezh2 are important for HSC self-renewal. SETD2 is a histone H3K36 methyltransferase and mutations in SETD2 have been identified in 6% of patients with AML. SETD2 deficiency resulted in a global loss of H3K36me3 and increased self-renewal capability of leukemia stem cells. We found that there were increased levels of H3K27me3 and decreased levels of H3K36me3 in p53^R248W/+ HSCs compared to that of the WT HSCs. In ChIP experiments, we found that p53^R248W, but not WT p53, was associated with the promoter region of Ezh2 in mouse myeloid progenitor cells, suggesting that p53^R248W may directly activate Ezh2 expression in hematopoietic cells. Given that Asxl-1 has been shown to regulate H3K27me3 in HSCs, the synergy between mutant p53 and Asxl-1 deficiency on LSC self-renewal could be due to changes in histone modifications.

Overall, we demonstrate that mutant p53 promotes the development of pre-leukemic HSCs by a novel mechanism involving dysregulation of the epigenetic pathways.