The occurrence of relapse in acute myeloid leukaemia (AML) is attributed to the persistence of leukaemic stem cells (LSCs), which possess self-renewal and proliferative ability. Current efforts have been done on developing therapeutic strategies targeting LSCs. Recently, microRNAs (miRNAs), a class of small noncoding RNAs, have been implicated in regulation of cancer stem cells (CSCs). Several studies have shown that depletion of miRNAs leads to cancer and miRNA overexpression represents a promising strategy for cancer treatment. Our data show that miR-101a is supressed in Mixed-Lineage Leukaemia (MLL)-mediated AML stem cells and its forced expression impairs LSC functions via reversing the epigenetic landscape and inhibiting pro-survival pathways, leading to apoptotic cell death.
We performed miRNA microarray analysis of MLL pre-leukemic stem cells (pre-LSCs) compared to control and identified a novel tumour suppressor, miR-101a, that is downregulated in pre-LSCs. qPCR confirmed that miR-101a is suppressed in LSCs compared to normal haematopoietic stem cells. Overexpression of miR-101a in pre-LSCs confers a growth disadvantage in vitro as we have observed 60-70% decrease in colony forming ability of miR-101a. In vivo studies of AML development is ongoing. The marked reduction in cell and colony number of miR-101a overexpressed cells is largely attributed to significant increase in apoptosis and decrease in expression of pro-survival proteins (i.e. Mcl-1 and Bcl-2). Strikingly, our result revealed a significant reduction in Ezh2, a crucial epigenetic regulator in maintenance of MLL AML and a known target of miR-101a. This finding is also accompanied by a global decrease of histone H3K27Me3 mark, implicating that miR-101a may contribute to leukaemogenesis via regulating epigenetic pathways in LSCs.
In summary, our preliminary results suggest that suppression of miR-101a contributes to aberrant epigenetic landscape and activates pro-survival pathways resulting in aggressive AML. Forced expression of miR-101a potentially eliminates leukemia-associated epigenetic marks and thus reverses the epigenetic process that originally altered gene expression in LSCs. This may provide a means to eradicate drug-resistant LSCs.