Poster Presentation 25th Lorne Cancer Conference 2013

Hhex regulates therapeutic resistance of Lmo2-induced pre-Cancerous Stem Cells. (#350)

Benjamin J Shields 1 , Chayanica Nasa 1 , Jacob Jackson 1 , Matthew McCormack 1
  1. Walter and Eliza Hall Institute, Parkville, Vic, Australia

T-cell Acute Lymphoblastic Leukemia (T-ALL) is a common paediatric malignancy that is frequently associated with aberrant expression of transcription factors. One such transcription factor is Lmo2, which is expressed in about 9% of T-ALL cases. In the CD2-Lmo2 transgenic (Lmo2-Tg) mouse model, Lmo2 expression in thymocytes gives rise to self-renewing, radioresistant CD4/CD8 (DN) T cells. These cells express a Haematopoietic Stem Cell (HSC) like transcriptional program and are called pre-Cancerous Stem Cells (pre-CSCs). The homeobox transcription factor, Hhex, is upregulated in Lmo2-induced pre-CSCs and retroviral overexpression of Hhex in T cells phenocopies Lmo2, implying that Hhex is a key component of the Lmo2-induced self renewal program. To assess this possibility, we bred Lmo2-Tg mice with Hhex-targeted (Hhexfl) mice and Lck-Cre transgenic mice to generate CD2-Lmo2:Lck-Cre; Hhex-/∆ (Lmo2-Tg;Hhex-/∆) mice. In this mouse model Hhex is specifically deleted in thymocytes. Hhex deletion in Lmo2-Tg;Hhex-/∆ mice did not prevent accumulation of DN T cells, nor alter the rate of leukemia development. However, loss of Hhex impaired the growth of ex vivo cultured DN T cells on OP9-DL1 feeders and eliminated the transplantation capacity and radioresistance of Lmo2-induced pre-CSCs. We found that c-kit is a target of Lmo2 and Hhex and that c-kit expression is reduced in pre-CSCs lacking Hhex. To determine whether loss of c-kit phenocopies the effects of loss of Hhex in this model, Lmo2-Tg mice were bred with c-kit hypomorphic (c-kitWv/Wv) mice. We found that loss of c-kit signaling in Lmo2-induced pre-CSCs restored radiation sensitivity similarly to loss of Hhex. Taken together, these findings suggest that Hhex is required for therapeutic resistance of Lmo2-induced pre-CSCs and that Hhex mediates its effects via upregulation of c-kit. Therefore targeting Hhex and/or c-kit may be of therapeutic value in the treatment of T-ALL.