Tet2-mutated myeloid progenitors possess aberrant in vitro self-renewal capacity

To the editor:

Ten-Eleven-Translocation 2 (TET2) is one of the most frequently mutated genes in various hematologic malignancies.^1,2  Li et al have previously reported that the deletion of Tet2 in mice led to dysregulated hematopoietic stem cells (HSCs) and subsequent development of myeloid malignancies.³  We and others have also reported similar findings using different strains of Tet2-mutant mice.^4-7  These observations led us to speculate that enhanced HSC function of Tet2-mutant mice sets a critical background for malignant transformation, and such dysregulated HSCs are the cell of origin for myeloid malignancies. However, leukemic stem cells (LSCs) of myeloid leukemia have been shown to emerge from committed progenitors, such as common myeloid progenitors (CMPs) or granulocyte-monocyte progenitors with enhanced self-renewal capacity conferred by leukemia-associated oncogenes.⁸  Because Tet2 mutation enhances self-renewal of HSCs and causes expansion of myeloid progenitors, it is reasonably speculated that committed myeloid progenitors may also aberrantly acquire stem cell function by Tet2 mutation, and this in collaboration with their inherent high proliferative capacity leads to the development of LSCs. However, none of the reports so far have tested stem cell capacity of myeloid progenitors in Tet2-deficient mice.

To address this issue, we examined HSCs and myeloid progenitors in fetal livers (FLs) from hypomorphic Tet2 gene-trap (gt) mice.⁴  Serial replating colony formation assay showed increased replating capacity of Tet2^gt/gt FL-Lineage⁻Sca-1⁺c-Kit⁺ (LSK) cells as compared with wild-type (WT) ones as reported previously (Figure 1A, upper panel). Surprisingly, Tet2^gt/gt FL-CMPs also showed enhanced serial replating capacity to the same level as Tet2^gt/gt FL-LSKs, whereas WT FL-CMPs could not be replated more than 4 times (Figure 1A, lower panel). These data clearly indicate that functional loss of Tet2 confers aberrant in vitro self-renewal capacity, one of the hallmarks of LSCs, to FL-CMPs.

Next, we assessed long-term repopulation and self-renewal capacity of FL-CMPs in vivo by serial transplantation assays. Consistent with previous reports, Tet2^gt/gt CD150⁺LSK cells, highly purified FL-HSC fraction, showed significantly higher engraftment in the secondary and the tertiary recipients as compared with WT (Figure 1B, upper panel). In contrast, however, both WT and Tet2^gt/gt FL-CMPs did not engraft for long-term in the primary recipients and completely lost repopulation in the secondary recipients (Figure 1B, lower panel). These data indicate that Tet2 loss did not confer FL-CMPs with in vivo stem cell capacity.

Taken together, these data seem to support the current notion that abnormal HSCs with TET2 mutation are the cell of origin for myeloid malignancies.^9,10  However, they also raise a possibility that TET2-mutated myeloid progenitors are prone to additional mutations because of their enhanced self-renewal and behave as premalignant cells. In addition, enhanced self-renewal potential may contribute to the progression of myeloid disease by expanding TET2-mutated progenitors. It should be mentioned that our Tet2 gt mice are slightly different from other published models, and that we examined cells from FL, not from BM. Nonetheless, our observation calls for further investigations on the role of TET2-mutated myeloid progenitors in the development of myeloid disease.