BCR-ABL kinase is dead; long live the CML stem cell

Abstract

Chronic myeloid leukemia (CML) is a hematopoietic disease characterized by expansion of myeloid blood cells. It is caused by the t(9;22) chromosomal translocation that results in the expression of the fusion tyrosine kinase BCR-ABL. Tyrosine kinase inhibitor (TKI) therapy has led to long-term remissions, but patients remain BCR-ABL+. There is agreement that TKIs do not kill CML stem cells; however, it is controversial whether this is because of a lack of BCR-ABL kinase inhibition in CML stem cells or because CML stem cells do not require BCR-ABL for survival. In this issue of the JCI, Corbin and colleagues provide definitive evidence that BCR-ABL is kinase active in CML stem cells and that TKIs inhibit this kinase activity without affecting CML stem cell survival. Rather, CML stem cells revert to a normal dependence on cytokines for survival and proliferation. These results demonstrate that the CML stem cell is not BCR-ABL addicted and have important implications for developing curative therapeutic approaches to CML.

Figure 1. Summary of BCR-ABL effects in different classes of CML.

In untreated patients with chronic phase CML, BCR-ABL is widely expressed and activated in both the stem and progenitor cell compartment. During effective treatment with imatinib or other TKIs, BCR-ABL is inactivated. This allows BCR-ABL^– cells to repopulate the progenitor cell compartment while cells that express inactive BCR-ABL persist in the stem cell compartment. The relative level of expression is likely low; the figure is not meant to represent quantitative levels of BCR-ABL persistence. In patients demonstrating imatinib resistance, a mutant BCR-ABL is present in the stem cell compartment that gives rise to progenitor cells with mutant BCR-ABL. The figure represents a steady state, as some reports suggest mutant BCR-ABL may first develop in a progenitor cell and repopulate the stem cell compartment.