Mechanism of BCL6-dependent stem cell maintenance in B cell lineage leukemia
Despite significant advances in the treatment of leukemia over the past four decades, the rate of long-term survival has reached
a plateau and still large numbers of leukemia patients die, mostly because of relapse and drug-resistance, which was recently
attributed to the persistence of leukemia stem cells. If a therapy succeeds in eradicating leukemia stem cells, de novo initiation
of the disease (relapse) is no longer possible. Therapeutic progress in recent clinical trials has likely been stalled, partly because
current cytotoxic therapy approaches target proliferating bulk leukemia cells rather than quiescent leukemia stem cells. We now
discovered that BCL6, a factor known to play a central role in B cell lymphomas, also plays a key role in the maintenance of
leukemia stem cells. Since leukemia stem cells represent the origin of relapse and drug-resistance in leukemia in many cases, the
identification of BCL6 as a target for leukemia stem cell eradication holds great promise.
BCL6 is a master regulatory factor that controls the production of many different important genes. BCL6 was not previously
known to be involved in leukemias. In preliminary studies for this proposal, we have discovered aberrant expression of BCL6 as a
central component of a fundamentally novel pathway of leukemia stem cell self-renewal and drug-resistance in a wide array of
human leukemias, some of which are still difficult to treat. In these leukemias, drug-treatment results in aberrant production of
BCL6 by the leukemia cells, which appears to allow leukemia stem cell to self-renew and become resistance against drugtreatment.
Recently a drug has been developed that can attach to BCL6 and block its cancer-causing activities. We found that
this BCL6 inhibitor, which is called RI-BPI, has strong synergistic activity when combined with conventional drug-treatment,
which opens up a powerful new therapeutic strategy for leukemia stem cell eradication through targeted inhibition of BCL6.
Based on the discovery of BCL6 as a key component of a novel pathway of drug-resistance and stem cell self-renewal in a wide
array of leukemias, we propose three Aims to develop these findings towards application in patient care:
(1) To test the hypothesis that aberrant expression of BCL6 in human leukemia cells promotes leukemia stem cell survival,
(2) To determine the frequency and phenotype of BCL6-dependent leukemia stem cells in human B cell ALL and
(3) To validate a the role of the BCL6 inhibitor RI-BPI as a therapy for targeted eradication of leukemia stem cells. Since RI-BPI is
currently going through the process of approval for use in clinical trials, we expect to be able to test the power of this approach
in clinical trials by the end of the funding period.
B cell lineage acute lymphoblastic leukemia (ALL) represents the most frequent malignancy in childhood and is frequent in
adults as well. Thousands of children and adults in California are afflicted with B cell leukemia and a significant portion of these
patients will ultimately die despite the tremendous progress that has been made in leukemia treatment. Compelling evidence
indicates that many leukemias are not curable because currently available chemotherapy target the bulk of the rapidly dividing
leukemia cells but not the rare drug resistant leukemia stem cells that are quiescent and do not divide. For this reason, current
research efforts both by laboratory investigators and clinicians focus on the leukemia stem cells because they are widely
considered as the origin of drug-resistance and recurrence of the disease. Ground-breaking research in other subtypes of
leukemia that are more frequent in adults has recently identified the leukemia stem cell. Additional research even found the
"Achilles heel" of these resilient leukemia cells and potential targets for future drug-therapy. Unfortunately, leukemia stem cells
have not been identified in B cell lineage ALL, nor do we know the mechanism that enables drug-resistance in these leukemia
stem cells. The absence of this information represents a major unsolved problem, because knowledge about the biology of the
leukemia stem cells is required for the development of future drug-therapies that will help to eradicate leukemia stem cells in
this frequent leukemia subset. This seems particularly important, since B cell lineage ALL accounts for about 30% of all childhood
cancers and is by far the most frequent malignancy in children and teenagers.
In summary, the benefits to the citizens of California from the CIRM disease specific grant in leukemia are: (1) direct benefit to the
thousands of patients with B cell lineage leukemia (2) higher quality of life due to definitive and targeted treatments that avoid
severe side-effects and long-term disabilities (3) new partnership between the laboratory and clinical investigators as well as the
pharmaceutical sector in California that leads to new synergies (4) the deployment of a multidisciplinary approach for pathwayspecific
drug-therapy that will be applicable to other types of cancer (5) Realization of the CIRM mandate to develop highly
effective novel therapies within a short time to benefit the health of Californians suffering from B cell lineage ALL.