Leukemia is the most frequent form of cancer in children and teenagers, but is also common in adults. Chemotherapy has vastly improved the outcome of leukemia over the past four decades. However, many patients still die because of recurrence of the disease and development of drug-resistance in leukemia cells. In preliminary studies for this proposal we discovered that in most if not all leukemia subtypes, the malignant cells can switch between an “expansion phase” and a “dormancy phase”. The “expansion phase” is often driven by oncogenic tyrosine kinases (e. g. FLT3, JAK2, PDGFR, BCR-ABL1, SRC kinases) and is characterized by vigorous proliferation of leukemia cells. In this phase, leukemia cells not only rapidly divide, they are also highly susceptible to undergo programmed cell death and to age prematurely. In contrast, leukemia cells in “quiescence phase” divide only rarely. At the same time, however, leukemia cells in “domancy phase” are highly drug-resistant. These cells are also called ‘leukemia stem cells’ because they exhibit a high degree of self-renewal capacity and hence, the ability to initiate leukemia.
Progress during Year 1: During the first year of this project, we discovered that the BCL6 factor is required to maintain leukemia stem cells in this well-protected safe haven. Our findings during year 1 demonstrate that the “dormancy phase” is strictly dependent on BCL6, which allows them to evade cell death during chemotherapy treatment. Once chemotherapy treatment has ceased, persisting leukemia stem cells give rise to leukemia clones that reenter “proliferation phase” and hence initiate recurrence of the disease. Pharmacological inhibition of BCL6 using inhibitory peptides or blocking molecules leads to selective loss of leukemia stem cells, which can no longer persist in a “dormancy phase” .
In year 1, we have performed screening procedures to identify novel therapeutic BCL6 inhibitors to eradicate leukemia stem cells: We have found that dual targeting of oncogenic tyrosine kinases (“expansion phase” ) and BCL6 (“dormancy phase”) represents a powerful strategy to eradicate drug-resistant leukemia stem cells and prevent the acquisition of drug-resistance and recurrence of the disease.
Goal for years 2-3: Targeting of BCL6-dependent leukemia stem cells may reduce the risk of leukemia relapse and may limit the duration of tyrosine kinase inhibitor treatment in some leukemias, which is currently life-long.