TET1 Drives Myelodysplastic Syndromes and Represents a Promising Therapeutic Target

Myelodysplastic syndromes (MDS) are a group of heterogeneous clonal hematopoietic disorders characterized by the abnormally enhanced self-renewal of hematopoietic stem cells (HSCs), ineffective hematopoiesis, and dysplastic changes in the bone marrow. The TET (Ten-Eleven Translocation) protein family, consisting of TET1, TET2, and TET3, is critical for DNA demethylation and plays essential roles in epigenetic regulation and gene expression. While TET2 mutations are common and well-studied in blood cancers, including MDS, TET1 is rarely mutated but instead, its aberrant overexpression has been suggested to play a critical role in the development of acute myeloid leukemia (AML) and B-cell acute lymphoblastic leukemia (B-ALL) in our previous studies. However, the expression pattern and functional significance of TET1 in MDS remain unknown. We analyzed TET1 expression levels in multiple gene expression databases containing samples collected from normal healthy controls and MDS patients, and observed a significant and consistent increase in TET1 expression levels across different MDS subtypes, suggesting a strong clinical relevance of high TET1 expression in MDS. We next explored the function of TET1 in MDS through a series of in vitro and in vivo studies. Depletion of TET1 significantly inhibited cell growth/proliferation, cell cycle and colony-forming ability while promoting apoptosis and myeloid differentiation in MDS-L cells, an MDS stem cell line established from a MDS patient. Interestingly, TET1 depletion could be fully rescued by the restoration of both wildtype (WT) and enzymatic mutant (Mut) TET1, indicating that TET1's function in MDS is not dependent on its DNA demethylation activity, differing from its pathological roles in other cancers. Moreover, TET1 knockdown greatly inhibited the engraftment of MDS-L cells in vivo, alleviated MDS syndromes and extended survival in mice. To further elucidate TET1's role in MDS pathogenesis, we retrovirally overexpressed WT or Mut TET1 in normal lineage-negative hematopoietic stem/progenitor cells (HSPCs) and transplanted them into lethally irradiated recipient mice. Strikingly, at day 54 post bone marrow (BM) transplantation (BMT), the hematopoietic system derived from WT or Mut TET1 overexpressed HSPCs showed significant skewing towards myeloid populations with a marked decrease in B cell lineage in peripheral blood (PB), BM and spleen, exhibiting typical MDS-like syndromes in mice. These data clearly demonstrated that TET1 overexpression drives MDS in vivo. To test if TET1 is an effective therapeutic target in MDS, we applied a TET1 inhibitor developed by our group recently, which inhibits TET1 expression, to treat MDS in vitro and in vivo. As expected, TET1 inhibitor treatment significantly inhibited MDS-L cell growth/proliferation, cell cycle and colony-forming ability while promoting cell apoptosis, myeloid differentiation, perfectly phenocopying TET1 depletion in MDS. Notably, the TET1 inhibitor didn't affect cell viability/growth of normal human CD34+ HSPCs, suggesting its specific effects on MDS stem cells. Next, we transplanted MDS-L cells into NRGS mice and then treated the mice with TET1 inhibitor, decitabine (a first-line treatment for MDS patients) and their combination starting from day 10 post-transplantation (TET1 inhibitor, five times per week, ten times; Decitabine, three times per week, six times). Strikingly, TET1 inhibitor significantly prolonged mice survival, demonstrating similar therapeutic effect to decitabine. The combination of the TET1 inhibitor and decitabine further extended mice survival, indicating great therapeutic potential of the combination in treating MDS. In conclusion, our study reveals TET1's crucial role in MDS pathogenesis and suggests that targeting TET1 alone and especially in combination with other drugs like decitabine represent a novel and promising therapy for MDS.