Functions of Novel Ligand-independent Flt3 Alleles and RANKL in Promoting Dissemination of Murine B-Cell Leukemias to the Central Nervous System
Survival rates for pediatric B-cell acute lymphoblastic leukemia (B-ALL) have improved dramatically, but outcomes for the 15% who relapse and for adults with B-ALL remain poor. Up to 40% of pediatric B-ALL patients require central nervous system (CNS) prophylaxis that causes significant treatment-related morbidities. p53-/- Rag-2-/- Prkdcscid;scid triple mutant (TM) mice spontaneously develop B-ALL that disseminates to the CNS. We used this model to investigate molecular mechanisms that drive CNS dissemination of leukemic B-cells. We show that CNS-disseminating B-ALLs had recurrent genomic rearrangements that replaced N-terminal Fms-like tyrosine kinase 3 (Flt3) exons with endogenous retrovirus (ERV) transcriptional control elements. ERV-Flt3 fusion genes encoded truncated FLT3 (trFLT3) proteins that induced ligand-independent STAT5 phosphorylation and proliferation of hematopoietic progenitor cells. Furthermore, trFLT3 promoted de novo development of CNS-disseminating B-ALL from hematopoietic progenitors. Thus, a novel mutational mechanism involving ERV-mediated FLT3 activation can drive the development of B-ALL characterized by high degree of CNS-invasion. Ectopic expression of trFlt3 suggested that TM B-ALLs initiate prior to B-cell commitment, since Flt3 is normally repressed by PAX5 upon B-cell commitment, co-incident with Cd19 expression. In support of this idea, we report evidence of Flt3 amplification in a rare subset of CD19- progenitors, and we show that CD19- FLT3+ cells from leukemic TM mice contain leukemia-initiating cells. Finally, we compared gene expression profiles of trFl3+ and trFl3- B-ALLs to identify potential Flt3 effectors important for CNS dissemination. TM B-ALLs uniquely expressed RANKL, a key regulator of osteoclast differentiation and normal B-cell development. FLT3 inhibition decreased RANKL expression, suggesting at least partial dependence on trFLT3 signaling. RANKL-expressing TM B-ALLs decreased trabecular bone density after adoptive transfer to normal mice, demonstrating a role for RANKL in leukemia-associated bone pathology. Importantly, a RANKL biologic antagonist inhibited CNS dissemination of TM B-ALLs in adoptive transfer experiments. Thus, my studies identified novel ligand-independent Flt3 mutations that arise prior to B-cell commitment and promote development of CNS-disseminating B-ALLs. Furthermore, I identified RANKL as a potential therapeutic target that may limit leukemia CNS dissemination and leukemia-associated bone pathology.
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