Molecular Basis and Stem Cell Biology of Human Philadelphia Chromosome-positive Leukemias
The human Philadelphia chromosome arises from a translocation between chromosomes 9 and 22, and results in formation of the chimeric and constitutively activated BCR-ABL tyrosine kinase. Philadelphia chromosome-positive (Ph+) leukemias induced by the BCR-ABL oncogene include chronic myeloid leukemia (CML) and B-cell acute lymphoblastic leukemia (B-ALL). CML often initiates in a chronic phase and eventually progresses to a terminal blastic phase, in which either acute myeloid or acute B-lymphoid leukemia develops. Some Ph+ leukemia patients, however, have B-ALL as their first clinical appearance. It is generally believed that shutting down the kinase activity of BCR-ABL will completely inhibit its functions, leading to inactivation of its downstream signaling pathways. Therefore, current therapeutic efforts have focused on targeting BCR-ABL kinase activity using kinase inhibitors.
The BCR-ABL tyrosine kinase inhibitor imatinib mesylate (Gleevec) is the standard of care for Ph+ leukemia. Imatinib induces a complete hematologic response in chronic phase CML patients. However, imatinib does not completely eliminate BCR-ABL-expressing leukemia cells, and some patients present with drug resistance. Imatinib prolongs survival of mice with BCR-ABL-induced CML, but does not cure the disease. Other BCR-ABL kinase inhibitors, such as dasatinib, nilotinib, etc., have been shown to inhibit almost all imatinib-resistant BCR-ABL mutants; the exception is the T315I mutant, which is present in 15-20% of imatinib-resistant patients. Dasatinib is also a potent inhibitor of SRC family kinases, but the role of the anti-SRC activity of this compound in Ph+ leukemia therapy is unclear. In general, these BCR-ABL kinase inhibitors are less effective in treating CML blastic phase patients and patients with Ph+ B-ALL. Recently, ponatinib, which has an activity against the BCR-ABLT315I, has been approved by FDA for treating CML patients with BCR-ABL mutations including the T315I, and future clinical trials will show the effectiveness of this new BCR-ABL kinase inhibitor. Although BCR-ABL kinase inhibitors are effective in controlling CML, it is generally believed that they will not cure the disease because a small group of BCR-ABL-expressing leukemia stem cells (LSCs) are insensitive to inhibition of BCR-ABL kinase activity. Now it becomes clear that LSCs are basically resistant to inhibition by BCR-ABL kinase inhibitors. Therefore, we decided to study molecular mechanisms for survival and self-renewal of LSCs in CML.
It is feasible to specifically target LSCs
To identify CML stem cells, we tested whether BCR-ABL-expressing hematopoietic stem cells (HSCs) function as the stem cells. To do so, we isolated bone marrow cells from CML mice, and sorted the BCR-ABL-expressing HSCs (GFP+Lin- Sca-1+c-Kit+) by FACS. The sorted cells were then transferred into recipient mice, and the mice developed and died of CML, indicating that BCR-ABL expressing HSCs function as CML stem cells. With our method of isolating LSCs in CML mice, we decided to further understand the biology of LSCs and to identify selective and effective target genes that play key roles in survival and self-renewal of these LSCs. Based on our DNA microarray data and genetic validation of candidate genes in our leukemia mouse models, we have identified a group of genes that are essential for the functions of LSCs, shedding light on developing an anti-stem cell therapy for CML. For example, we have shown that the survival and self-renewal of LSCs but not normal hematopoietic stem cells require the arachidonate 5-lipoxygenase (5-LO) gene (Alox5) and that Alox5 is essential for CML development (Chen et al. Loss of the Alox5 gene impairs leukemia stem cells and prevents chronic myeloid leukemia. Nature Genetics 41:783-792, 2009). We have also identified several other LSC-specific genes such as Msr1 (Chen et al. A tumor suppressor function of the Msr1 gene in leukemia stem cells of chronic myeloid leukemia. Blood 118:390-400, 2011), Blk (Zhang et al. The Blk pathway functions as a tumor suppressor in chronic myeloid leukemia stem cells. Nature Genetics 44:861-871, 2012), etc.
BCR-ABL activates some pathways in a kinas-independent manner
If BCR-ABL kinase inhibitors efficiently inhibit BCR-ABL kinase activity, we wonder why these inhibitors would not shut down all pathways activated by BCR-ABL, as mentioned above. Our hypothesis is that BCR-ABL activates its downstream pathways through both kinase activity-dependent and independent mechanisms. In fact, we have identified a group of genes whose expression is alter by BCR-ABL, but BCR-ABL kinase inhibitors cannot restore their expression; examples of these genes include Alox5, Msr1, Blk, etc. Regulation of expression of these BCR-ABL kinase activity-independent genes provides a novel mechanism explaining why LSCs are resistant to BCR-ABL kinase inhibitors and why these inhibitors are unlikely to cure CML. We plan to further explore this BCR-ABL kinase activity-independent mechanism in LSCs of CML. Our effort will help to develop novel anti-stem cell therapeutic strategies for curing CML, and knowledge learned from studying LSCs of CML will help to understand the biology of cancer stem cells in other malignant diseases.