Pre-B cell receptor signaling in acute lymphoblastic leukemia. B lymphocytes are not only the cells that produce antibodies as part of the human immune system, they are also the cell of origin in most cases of acute lymphoblastic leukemia (ALL). ALL represents by far the most frequent type of cancer in children and is also a common disease in adults. For many years, patients with ALL are treated with chemotherapy and current treatment protocols lead to cure rates of 80 percent for children and 55 percent for adult patients with ALL. Our goal is to better understand the biology of human ALL, namely as a catastrophic aberration of normal B lymphocyte development. During normal B lymphocyte development, the pre-B cell receptor represents a critical signaling unit that guides early B lymphocyte precursors on their path of maturation. If signaling from the pre-B cell receptor is compromised, as for instance in patients with innate immune defects, the B lymphocyte precursors are arrested in their development at a primitive stage -as in ALL cells. Therefore, we propose to investigate the function of the pre-B cell receptor signaling unit (1) as a potential target to disrupt aberrant cell signaling that promotes leukemic growth and (2) to restore normal pre-B cell receptor signaling in the leukemia cells.
Pre-B cell receptor signaling in acute lymphoblastic leukemia
1. The ubiquitin-ligase CBL targets SYK for proteasomal degradation: Rao N, Ghosh AK, Ota S, Zhou P, Reddi AL, Hakezi K, Druker BK, Wu J, Band H. The non-receptor tyrosine kinase Syk is a target of Cbl-mediated ubiquitylation upon B-cell receptor stimulation. EMBO J. 2001; 20: 7085-95.
2. The SRC family kinase LYN activates CBL: Nishizumi H, Taniuchi I, Yamanashi Y, Kitamura D, Ilic D, Mori S, Watanabe T, Yamamoto T. Impaired proliferation of peripheral B cells and indication of autoimmune disease in Lyn-deficient mice. Immunity. 1995; 3: 549-60.
3. BCR-ABL1 activates STAT5/JAK3: Ilaria RL Jr, Van Etten RA. BCR/ABL induces the tyrosine phosphorylation and DNA binding activity of multiple specific STAT family members. J Biol Chem. 1996; 271: 31704-10.
4. BCR-ABL1 activates BTK but neither SYK nor SLP65: Feldhahn N, Klein F, Mooster JL, Hadweh P, Sprangers M, Wartenberg M, Bekhite MM, Hofmann WK, Herzog S, Jumaa H, Rowley JD, Müschen M. Mimicry of a constitutively active pre-B cell receptor in acute lymphoblastic leukemia cells. J Exp Med. 2005; 201: 1837-52.
5. Activation of SRC family kinases by BCR-ABL1 is critical for leukemic transformation of pre-B cells: Hu Y, Liu Y, Pelletier S, Buchdunger E, Warmuth M, Van Etten RA, Li S. Requirement of Src kinases Lyn, Hck and Fgr for BCR-ABL1-induced B-lymphoblastic leukemia but not chronic myeloid leukemia. Nat Genet. 2004; 36: 453-61.
6. SRC family kinases drive survival and proliferation of pre-B cells through activation of NF-κB: Saijo K, Schmedt C, Su IH, Karasuyama H, Lowell CA, Reth M, Adachi T, Patke A, Santana A, Tarakhovsky A. Essential role of Src-family protein tyrosine kinases in NF-?B activation during B cell development. Nat Immunol. 2003; 4: 274-9.
7. Tyrosine-phosphorylated STAT5 induces upregulation of MYC, CCND2 and increases pre-B cell proliferation: Tsuruyama T, Nakamura T, Jin G, Ozeki M, Yamada Y, Hiai H. Constitutive activation of Stat5a by retrovirus integration in early pre-B lymphomas of SL/Kh strain mice. Proc Natl Acad Sci U S A. 2002; 99: 8253-8.
8. Pre-B cell receptor signaling results in phosphorylation of SLP65 Y96: Patterson HC, Kraus M, Kim YM, Ploegh H, Rajewsky K. The B cell receptor promotes B cell activation and proliferation through a non-ITAM tyrosine in the Igα cytoplasmic domain. Immunity. 2006; 25: 55-65. Reconstitution of IKAROS expression redirects BCR-ABL1 to phosphorylate SLP65 Y96: Figure 7 C, this study.
9. Phosphorylation of SLP65 Y96 is required for SLP65-mediated tumor suppression and the ability of SLP65 to bind and inhibit JAK3: Nakayama J, Yamamoto M, Hayashi K, Satoh H, Bundo K, Kubo M, Goitsuka R, Farrar MA, Kitamura D. BLNK suppresses pre-B cell leukemogenesis through inhibition of JAK3. Blood. 2009; 113: 1483-1492.
10. SLP65 Y96-mediated inhibition of STAT5/JAK3 leads upregulation of p27 and induces cell cycle arrest: Nakayama et al. Blood. 2009; 113: 1483-1492.
11. Pre-B cell receptor/SLP65 signaling induce upregulation of IKAROS: Thompson EC, Cobb BS, Sabbattini P, Meixlsperger S, Parelho V, Liberg D, Taylor B, Dillon N, Georgopoulos K, Jumaa H, Smale ST, Fisher AG, Merkenschlager M. Ikaros DNA-binding proteins as integral components of B cell developmental-stage-specific regulatory circuits. Immunity. 2007; 26: 335-44; Ma S, Pathak S, Trinh L, Lu R. Interferon regulatory factors 4 and 8 induce the expression of Ikaros and Aiolos to down-regulate pre-B-cell receptor and promote cell-cycle withdrawal in pre-B-cell development. Blood. 2008; 111: 1396-403.
12. IKAROS induces upregulation of p27 and cell cycle arrest: Kathrein KL, Lorenz R, Innes AM, Griffiths E, Winandy S. Ikaros induces quiescence and T-cell differentiation in a leukemia cell line. Mol Cell Biol. 2005; 25: 1645-54.
Recent work by our group demonstrated that the oncogenic BCR-ABL1 kinase in Ph+ ALL and blast crisis, but not chronic phase CML, induces expression of a mutator enzyme, termed AID (Activation-induced Cytidine Deaminase). The mutations that confer drug-resistance in Ph+ ALL and blast crisis CML can indeed be explained by activity of the AID enzyme. Additional experiments showed that engineered expression of AID in AID-negative chronic phase CML cells introduces the same mutations that cause drug-resistance in patients with AID-positive Ph+ ALL and CML blast crisis. Based on these observations, we propose three series of experiments to address the following questions: (1) Is the AID enzyme required for drug-resistance in AID-positive Ph+ ALL? To test this hypothesis, we will investigate whether BCR-ABL1-induced leukemia cells from mice carrying a deletion of the AID-gene fail to develop drug-resistance. (2) Does the AID enzyme play a critical role in the progression of chronic phase CML into blast crisis? Chronic phase CML can be treated very successfully for many years, whereas blast crisis represents a final and often fatal stage of the disease. (3) Which are the factors that induce aberrant expression of the AID enzyme in Ph+ ALL and blast crisis CML? The identification of such factors will likely help to understand how expression of this deleterious mutator enzyme can be prevented.
Childhood leukemia typically arises from a prenatally established pre-leukemic clone (Greaves and Wiemels, 2003). Genetic abnormalities that were acquired in utero are detected in cord blood samples, Guthrie cards and concordant twins who developed leukemia after a variable postnatal latency of up to 14 years. This led to a scenario, in which the initial prenatal lesion represents a first, albeit insufficient hit, which is followed by a series of additional transforming events, which ultimately cause leukemia (Greaves, 2009). Strikingly, some of these prenatal lesions define childhood leukemia subtypes yet are commonly detected in cord blood and on Guthrie cards from healthy individuals who will never develop disease. For instance, the TEL-AML1 gene rearrangement was detected in 6 of 567 (~1%) cord blood samples but leads to overt leukemia in less than 1% of these cases (with a cumulative risk of TEL- AML1 ALL at 1:14,000; Mori et al., 2004). These findings strongly support the notion that covert pre-leukemic clones are frequent but only a small minority of these pre-leukemic clones develop into frank leukemia after they acquired critical secondary genetic lesions. However, the postnatal mechanism(s) that drive the evolution of the fetal pre-leukemic clone towards childhood leukemia remain elusive and are at the center of this proposal. Since only a small minority of fetal pre-leukemic clones give rise to leukemia after birth, it appears critical to understand which mechanism(s) influence the clonal evolution of these pre-leukemic cells. This information will be highly significant, because control over these mechanisms could make childhood ALL a largely preventable disease.
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 drug-treatment. 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.