The role of beta-catenin in development of origin-specific leukaemia stem cells

Abstract

While we and others have prospectively identified potential origins of leukaemic stem cells (LSCs) resulting in phenotypically identical myeloid leukaemia, the functional differences among these origin-specific LSCs have not been vigorously investigated. Recently, our lab has shown that -catenin, which is dispensable for normal HSCs, is essential for MLL LSCs, highlighting it as a potential therapeutic target for selective eradication of LSCs while sparing
normal HSCs. Here, I investigated the role of -catenin in LSCs of distinctive cellular origins using both RTTA (in vitro) and bone marrow transplantation (in vivo) assays. I demonstrated that conditional deletion of -catenin abolished the leukaemogenic potential of LSK-Meis1- Hoxa9 pre-LSCs whereas CMP-Meis1-Hoxa9 pre-LSCs were able to develop into LSCs in vivo regardless of -catenin status. Interestingly, conditional inactivation of -catenin abolished the in
vivo leukaemogenic property of GMP-MLL pre-LSCs whereas LSK-MLL pre-LSCs were still able to induce leukaemia in vivo in the absence of -catenin, revealing functional differences in origin-specific LSCs in spite of their ability to induce phenotypically identical leukaemia. Since a major function of -catenin is to mediate stem cell self-renewal, I hypothesized the presence of
multiple alternative self-renewal pathways in normal HSCs that may allow the LSK/HSC-origin specific LSCs overcoming inactivation of -catenin. Hox genes have been previously shown by others and us as key players in mediating self-renewal in normal and malignant haematopoiesis. Consistently, Hoxa9 exhibited functions as -catenin in mediating developing of MLL pre-LSCs originated from GMP but not HSCs. To test if Hoxa9 may compensate the loss of -catenin in LSK-MLL pre-LSCs, I have created a conditional -catenin/Hoxa9 knockout
model where a combination of these two molecules can be inactivated in normal HSCs and various myeloid progenitors. As a result, I demonstrated that specific inactivation of -catenin or Hoxa9 abolished the oncogenic potentials of GMP-MLL pre-LSCs, but not LSK-MLL pre- LSCs. Strikingly, inactivation of both pathways suppressed oncogenic transformation by LSKMLL pre-LSCs and resulted in down-regulation of several Meis1-Hoxa9 target genes. Among
them is Prmt1, which has been identified as a critical epigenetic modifying enzyme associated with an oncogenic MLL fusion complex. Prmt1 knockdown inhibited clonogenic activity of LSKMLL-ENL LSC with Hoxa9 or -catenin knockout, suggesting Prmt1 as a key modulator for transformation of LSK-MLL-ENL LSC. Together, I have demonstrated, for the first time, specific
molecular and functional differences among origin-specific LSCs, in which crosstalk between multiple self-renewal pathways inherited from the cell of origins may determine the biology of the disease and mediates resistance to potential targeted therapies.

Date of Award	2015
Original language	English
Awarding Institution	King's College London
Supervisor	Eric So (Supervisor) & Farzin Farzaneh (Supervisor)