Hhex induces promyelocyte self-renewal and cooperates with growth factor independence to cause myeloid leukemia in mice.

The hematopoietically expressed homeobox (Hhex) transcription factor is overexpressed in human myeloid leukemias. Conditional knockout models of murine acute myeloid leukemia indicate that maintains leukemia stem cell self-renewal by enabling Polycomb-mediated epigenetic repression of the Cdkn2a tumor suppressor locus, encoding p16 and p19 However, whether overexpression also affects hematopoietic differentiation is unknown. To study this, we retrovirally overexpressed in hematopoietic progenitors. This enabled serial replating of myeloid progenitors, leading to the rapid establishment of interleukin-3 (IL-3)-dependent promyelocytic cell lines. Use of a Hhex-ERT2 fusion protein demonstrated that continuous nuclear Hhex is required for transformation, and structure function analysis demonstrated a requirement of the DNA-binding and N-terminal-repressive domains of Hhex for promyelocytic transformation. This included the N-terminal promyelocytic leukemia protein (Pml) interaction domain, although deletion of Pml failed to prevent -induced promyelocyte transformation, implying other critical partners. Furthermore, deletion of p16 or p19 did not promote promyelocyte transformation, indicating that repression of distinct Hhex target genes is required for this process. Indeed, transcriptome analysis showed that overexpression resulted in repression of several myeloid developmental genes. To test the potential for overexpression to contribute to leukemic transformation, -transformed promyelocyte lines were rendered growth factor-independent using a constitutively active IL-3 receptor common β subunit (βcV449E). The resultant cell lines resulted in a rapid promyelocytic leukemia in vivo. Thus, overexpression can contribute to myeloid leukemia via multiple mechanisms including differentiation blockade and enabling epigenetic repression of the locus.