A New Remedy for Blast-Crisis CML?

Wnt proteins are extracellular ligands that activate a variety of signaling pathways important in cellular functions, including proliferation and differentiation. The so-called “canonical” pathway involves their binding to cell surface co-receptors from the LRP and Frizzled families, which then allow β-catenin proteins to enter the nucleus, where they associate with the transcription co-activator Lef-Tcf to activate specific genetic programs. Experimental evidence suggests that aberrant Wnt/catenin activation occurs in many cancers. In CML, β-catenin activation in the granulocytemacrophage progenitor (GMP) fraction was seen in samples from blastcrisis patients. Furthermore, alternative splicing of mRNA for an important regulator of the β-catenin pathway, GSK3β, was shown to occur with progression-facilitated β-catenin activation. Moreover, gene expression studies have demonstrated activation of the Wnt/catenin pathway, including aberrant expression of frizzled family members, in CML progression. Several pharmaceutical companies have been working on targeting this pathway, either directly or by influencing activity of pathways with crosstalk to the Wnt/catenin pathway (e.g., Hedgehog).

Wnt also signals through “non-canonical” pathways, including Ca²⁺/NFAT. In this pathway, Wnt-mediated dephosphorylation of NFAT by calcineurin causes NFAT to translocate to the nucleus, where it promotes transcription of various cytokine genes (Figure 1). While the role of the pathway has been unclear in cancer, the recent work by Gregory et al. suggests it may be involved in the survival of Bcr-Abl leukemia despite TKI therapy.

The authors used an RNAi-based “synthetic lethal” screen to identify pathways whose inhibition sensitized CML cells to TKIs. A large shRNA library was transduced into CML cell lines, and shRNAs associated with resistance to imatinib were identified. Among these were the Wnt receptor Frizzled-8 (FZD-8) as well as components of the Ca²⁺/NFAT pathway. Transduction of shRNA targeting FZD-8 verified that inhibition sensitized CML cells to imatinib and that this caused a decrease in NFAT activation. The authors next targeted the control of NFAT by calcineurin and demonstrated that the calcineurin inhibitor cyclosporin A (CsA) inhibited NFAT activation. They then used a Ph⁺ ALL mouse model to demonstrate that treatment with the TKI dasatinib and CsA was associated with a dramatic survival advantage compared with either drug alone (Figure 2). The data thus suggest a role of the Wnt/Ca²⁺/NFAT pathway in TKI resistance and suggest a novel method to induce sensitivity to resistant Bcr-Abl leukemia.

Curiously, a variation of this mouse model may have already been done in humans. Several transplant groups have given TKI to patients who have undergone allogeneic transplant for advanced-phase CML or Ph⁺ ALL. Preliminary data suggest this is very effective in preventing relapse. Since most of these patients are already on a calcineurin inhibitor, one wonders if the efficacy is not just from blocking Bcr-Abl activity but promoting apoptosis by blocking the Wnt/Ca²⁺/NFAT pathway.