Notch Pathway Activation Targets Leukemic Stem Cells in Chronic-Phase Chronic Myeloid Leukemia (CP-CML)

The phenomenon of disease persistence in CP-CML under therapy with ABL kinase inhibitors suggests BCR-ABL-independent mechanisms are being exploited to sustain the survival of leukemic stem cells (LSC). Many self-renewal pathways have been implicated in their survival. The role of Notch signalling has yet to be fully elucidated, with conflicting reports suggesting a variety of roles in myeloid disease. The aims of this project were (1) to assess the expression and functional role of the Notch pathway in primary CD34⁺CP-CML cells, and (2) to assess differences in the functional role between stem and progenitor subpopulations in disease maintenance and progression. We hypothesized that Notch activation would have different functional effects dependent on disease phase and cell maturity.

Extensive analysis of existing microarray datasets, GSE47927 and E-MTAB2481, comparing CP-CML LSCs to normal HSCs, showed that the Notch pathway was downregulated in CP-CML (p=0.05, both). To validate these results, using Fluidigm technology, mRNA expression of untreated CD34⁺ CP-CML cells (n=24) was compared to normal bone marrow CD34⁺ cells (n=5). The Notch pathway was silenced in untreated CP-CML, as evidenced by downregulation of Notch target genes, HES1 (p<0.0001) and HIF1A (p=0.0029). The Notch receptor, NOTCH2, was significantly upregulated (p<0.0001) in the same samples. This was confirmed with immunofluorescence (IF) of the full-length receptor (n=5). The activated form of the Notch receptor, val1744, was absent on CD34⁺CP-CML cells by either IF or Western blotting (n=3), nor was there any significant change in functional analyses of CP-CML cells treated with the pharmacological Notch inhibitor, DAPT (n=5). These results support our hypothesis that the pathway is silenced in CP-CML.

As CD34⁺ CP-CML cells abundantly express NOTCH2, we investigated whether the pathway could be reactivated through exogenous stimulation upon ligand binding. An OP9 co-culture system was utilized to allow for overexpression of Notch ligands, DLL1 or JAG1, using OP9GFP as control stroma. Compared to non-stromal conditions, culturing over 7 days on OP9GFP led to a significant increase in growth (p=0.02, n=5). The overexpression of JAG1 led to a further increase compared to OP9GFP (p=0.002). We confirmed that activation of the pathway was through JAG1, and not DLL1, by utilizing DAPT, where we observed a significant decrease in live cell counts within the OP9GFP (p=0.01) and OP9JAG1 (p=0.018) experimental arms. Upregulation of the pathway was confirmed with increased expression of HES1 at the gene level (n=4, p=0.001), andval1744 protein by IF (n=5).

CP-CML samples (n=3) were FACS-sorted into Lin^-CD34⁺38⁺ and Lin^-CD34⁺38^- populations to assess for functional variation in stem and progenitor populations. Following 7 days in culture, live cell counts showed a significant increase in Lin^-CD34⁺38⁺ cells on OP9JAG1 co-culture (p=0.04). There was a decrease in cell growth of Lin^-CD34⁺38^- cells in the same experimental conditions (p=0.02), with an associated increased trend in late apoptotic cells (ns). We hypothesized that Notch activation would have a toxic influence on an immature CML LSC population. LTCIC assays showed a significant decrease in colonies for CD34⁺38^-cells co-cultured with OP9JAG1 for 7 days (p<0.05, n=8).

In myeloid BP (n=4), in similar experiments, sorted Lin^-CD34⁺38⁺ and Lin^-CD34⁺38^- populations did not demonstrate this functional profile of Notch activation, despite an upregulation in NOTCH2 receptor expression compared to normal (n=11) (p=0.0035). To investigate this further, we sorted CP (n=12), myeloid BP (n=11), and lymphoid BP (n=5) into stem and progenitor populations, before undertaking quantitative PCR of 90 self-renewal gene components of Wnt, Notch, Hedgehog, and BMP pathways. The self-renewal pathways were highly deregulated between CP and BP. The differential gene expression in lymphoid BP was comparable to myeloid BP. There was statistically significant upregulation in Wnt components in myeloid BP compared to CP, particularly TCF7(p=0.0011). We hypothesize that Wnt upregulation is preventing Notch activation in myeloid BP-CML.

Taken together, our results identify that Notch activation may be a therapeutic approach to target CP-CML LSCs. However, this cannot be translated to myeloid BP, highlighting the complexity of self-renewal pathway interaction.