Tyrosine kinase inhibitors (TKIs) have revolutionised the treatment of chronic myeloid
leukemia (CML), however, fail to cure the disease due to the persistence of a refractory
fraction of stem/progenitor cells. Autophagy is a recycling mechanism utilised by the cell
as a survival mechanism under stressful conditions, and its induction has been suggested to
have a cytoprotective role in cancer cells.
In this study we demonstrate that autophagy is triggered in CML upon TKI-mediated
inhibition of BCR-ABL, and protects from cell death. In order to evaluate if specific
autophagy inhibition enhances TKI effects, we stably transduced primary CML
stem/progenitor cells with a vector carrying a short-hairpin against the key autophagy gene
ATG7. Knock-down of basal ATG7/autophagy levels in CML stem/progenitor cells
inhibited by approximately 50% the survival of the cells in a clonogenic assay, and
reduced by 75% their erythroid differentiation potential. Furthermore, ATG7 knock-down
enhanced the effects of TKIs imatinib (IM; 1st), dasatinib (DAS; 2nd), nilotinib (NIL; 2nd)
and ponatinib (PON; 3rd generation), reducing by 92-98% the survival of these cells in a
clonogenic assay. In contrast, ATG7 knock-down in normal stem cells, with or without
TKI treatment, did not have a significant effect on survival and proliferation.
ATG7 was also knocked-down in final disease stage, blast crisis (BC), patient-derived
K562 and KCL22 cell lines. Both cell lines appeared to depend significantly on autophagy
for survival as indicated by high apoptosis levels (70-100%) after ATG7 knock-down.
Interestingly, ATG7 knock-down cells appeared to be more differentiated compared to the
control (scrambled shRNA).
Our findings suggest a role for basal autophagy in the survival, differentiation decisions
and clonogenicity of CML cells, and support the combined use of autophagy inhibition
with TKIs for the eradication of CML stem/progenitor cells. This could be partially
attributed to a bypass of the differentiation block upon autophagy inhibition, which
facilitates TKI-targeting. We underline the necessity for the development of specific
autophagy inhibitors that in combination with TKIs could potentially eradicate the fraction
of persistent CML stem/progenitor cells and offer a curative option for CML patients
