Few cancer mechanisms are as devastating as the generation of cancer stem cells, which arise in leukemia from white blood cell precursors. The mechanisms of this transition have been obscure, but the consequences are all too clear. Leukemia stem cells promote an aggressive, therapy-resistant form of disease called blast crisis.
The self-renewal of leukemia stem cells depends on the activation of a microRNA-editing enzyme called ADAR1. According to a new study, ADAR1 activation represents a unique therapeutic vulnerability in leukemia stem cells, which can give rise to blast crisis in chronic myeloid leukemia.
ADAR1 can edit the sequence of microRNAs (miRNAs), small pieces of genetic material. By swapping out just one miRNA building block for another, ADAR1 alters the carefully orchestrated system cells use to control which genes are turned on or off at which times.
ADAR1 is known to promote cancer progression and resistance to therapy. To study ADAR1, the UCSD team used human blast crisis chronic myeloid leukemia (CML) cells in the lab, and mice transplanted with these cells, to determine the enzyme’s role in governing leukemia stem cells.
The scientists, led by Catriona Jamieson, M.D., Ph.D., published their work June 9 in Cell Stem Cell, in an article entitled, “ADAR1 Activation Drives Leukemia Stem Cell Self-Renewal by Impairing Let-7 Biogenesis.” The article presented the first mechanistic link between pro-cancer inflammatory signals and RNA editing–driven reprogramming of precursor cells into leukemia stem cells.
The article describes how ADAR1-mediated A-to-I RNA editing is activated by Janus kinase 2 (JAK2) signaling and BCR-ABL1 signaling. Also, it indicated, in a model of blast crisis (BC) CML, that combined JAK2 and BCR-ABL1 inhibition prevents leukemia stem cell self-renewal commensurate with ADAR1 downregulation.
Essentially, the scientists were able to trace a series of molecular events: First, white blood cells with a leukemia-promoting gene mutation become more sensitive to signs of inflammation. That inflammatory response activates ADAR1. Then, hyper-ADAR1 editing slows down the miRNAs known as let-7. Ultimately, this activity increases cellular regeneration, or self-renewal, turning white blood cell precursors into leukemia stem cells.
“Lentiviral ADAR1 wild-type, but not an editing-defective ADAR1E912A mutant, induces self-renewal gene expression and impairs biogenesis of stem cell regulatory let-7 microRNAs,” wrote the author of the Cell Stem Cell article. “Combined RNA sequencing, qRT-PCR, CLIP-ADAR1, and pri-let-7 mutagenesis data suggest that ADAR1 promotes LSC generation via let-7 pri-microRNA editing and LIN28B upregulation.”
After learning how the ADAR1 system works, Dr. Jamieson’s team looked for a way to stop it. By inhibiting sensitivity to inflammation or inhibiting ADAR1 with a small-molecule tool compound, the researchers were able to counter ADAR1’s effect on leukemia stem cell self-renewal and restore let-7. Self-renewal of blast crisis CML cells was reduced by approximately 40% when treated with the small molecule called 8-Aza as compared to untreated cells.
“A small-molecule tool compound antagonizes ADAR1’s effect on LSC self-renewal in stromal co-cultures and restores let-7 biogenesis,” the study’s authors noted. “Thus, ADAR1 activation represents a unique therapeutic vulnerability in LSCs with active JAK2 signaling.”
“In this study, we showed that cancer stem cells co-opt a RNA editing system to clone themselves. What’s more, we found a method to dial it down,” said Dr. Catriona Jamieson. “Based on this research, we believe that detecting ADAR1 activity will be important for predicting cancer progression.
“In addition, inhibiting this enzyme represents a unique therapeutic vulnerability in cancer stem cells with active inflammatory signaling that may respond to pharmacologic inhibitors of inflammation sensitivity or selective ADAR1 inhibitors that are currently being developed.”