STAT5 antagonism of B cell superenhancer networks initiates progenitor B cell leukemia and predicts patient survival (HEM1P.222)

B cell Acute Lymphoblastic Leukemia (B-ALL) arises from the transformation of progenitor B cells. The transcription factor STAT5 is required for transformation but how STAT5 mediates this effect is unclear. Previous studies suggested that STAT5 only acts to promote survival of progenitor B cells. However, other roles for STAT5 in B cell development and B-ALL have not been explored. Here we show that STAT5 activation drives leukemia in cooperation with defects in a linear signaling pathway emanating from the pre-BCR, including Blnk, Btk, Prkcb, Nfkb1, and Ikzf1. Using microarray analysis and ChIP-Seq we demonstrate that STAT5 disrupts the function of superenhancer binding transcription factor networks that normally promote B cell development. STAT5 versus NFkB or IKAROS binding to these enhancers largely had opposing effects on target gene expression. The antagonism between STAT5 and IKAROS or NFκB has direct clinical relevance as the balance between these transcription factors affects patient outcome; patients with high ratios of active STAT5 to NFκB or IKAROS had more aggressive disease characterized by shorter remission and decreased survival. Thus, our studies illustrate how modest perturbations in two opposing transcriptional programs can have dramatic consequences for B cell development and transformation and how the degree of antagonism between these transcriptional programs ultimately predicts patient outcome to therapy.

STAT5 antagonizes the pre-BCR-dependent NFκB and FOXO tumor suppressor pathways in acute lymphoblastic leukemia (HEM2P.261)

Mice expressing a constitutively active form of STAT5b (Stat5b-CA mice) develop pre-B leukemia with low frequency (~1-2%). Microarray analysis of these leukemias shows decreased expression of genes involved in pre-BCR signaling. Stat5b-CA mice crossed to mice with defects in a coherent signaling pathway downstream of the pre-BCR involving BLNK, BTK, or PKCβ developed pre-B leukemia in > 75% of progeny. These leukemias exhibited reduced expression of a subset of NFκB target genes, which was important for transformation as Stat5b-CA x Nfkb1-/- mice also developed pre-B leukemia. Our results point to an unexpected role for NFκB1 as a tumor suppressor in pre-B cells. Microarrary analysis revealed that STAT5 activation resulted in reduced expression of several NFκB and FOXO target genes, thereby antagonizing these tumor suppressor pathways. To examine this regulation in further detail, we performed ChIP-seq analysis, which showed that STAT5 binds 38% of NFκB target genes that are expressed in pre-B cells. We also observed binding of STAT5 to several FOXO targets including: Foxo1, Cdkn1a, Cdkn2d, Blnk, Vegfa, Cxcr4, Bim, and Dgka. We have validated STAT5 binding to the FOXO targets Foxo1 and Cdkn2d and the NFκB targets Nfkb1, Rel, Irf4, Irf8, Ikzf1, and Ikzf3 by ChIP-qPCR. Our findings demonstrate that STAT5 activation antagonizes the pre-BCR-dependent NFκB and FOXO tumor suppressor pathways that limit pre-B cell proliferation and promote pre-B cell differentiation.

A pre-BCR/NFκB pathway suppresses STAT5-dependent pre-B cell leukemia (P4408)

Mice expressing a constitutively active form of STAT5b (called Stat5b-CA mice) develop pre-B leukemia with low frequency (~1-2%). Microarray analysis of these leukemias shows decreased expression of genes involved in pre-BCR signaling. Stat5b-CA mice crossed to mice with defects in a coherent signaling pathway downstream of the pre-BCR involving BLNK, BTK, or PKCβ developed pre-B leukemia in > 75% of progeny. These leukemias exhibited reduced expression of a subset of NFκB target genes, which was important for transformation as Stat5b-CA x Nfkb1-/- mice also developed pre-B leukemia. Our results point to an unexpected role for NFκB1 as a tumor suppressor in pre-B cells. We found that STAT5 cooperates with defects in this pre-BCR-dependent pathway to repress expression of the potential tumor suppressor genes Foxo1, Foxo3a, Ikzf1, and Ikzf3. Our findings demonstrate that STAT5 activation antagonizes a pre-BCR-dependent pathway that limits pre-B cell proliferation, promotes pre-B cell differentiation, and inhibits pre-B cell transformation.

A Pre-BCR/NFκB Pathways Suppresses STAT5-dependent Pre-B Cell Leukemia (111.18)

Mice expressing a constitutively active form of STAT5b (called Stat5b-CA mice) develop pre-B leukemia with low frequency (~1-2%). Microarray analysis of these leukemias shows decreased expression of genes involved in pre-BCR signaling. Stat5b-CA mice crossed to mice with defects in a coherent signaling pathway downstream of the pre-BCR involving BLNK, BTK, or PKCβ developed pre-B leukemia in > 75% of progeny. These leukemias exhibited reduced expression of a subset of NFκB target genes, which was important for transformation as Stat5b-CA x Nfkb1-/- mice also developed pre-B leukemia. Our results point to an unexpected role for NFκB1 as a tumor suppressor in pre-B cells. We found that STAT5 cooperates with defects in this pre-BCR-dependent pathway to repress expression of the potential tumor suppressor genes Foxo1, Foxo3a, Ikzf1, and Ikzf3. Our findings demonstrate that STAT5 activation antagonizes a pre-BCR-dependent pathway that limits pre-B cell proliferation, promote pre-B cell differentiation, and inhibit pre-B cell transformation.