Pax5 loss imposes a reversible differentiation block in B-progenitor acute lymphoblastic leukemia

Abstract

Loss-of-function mutations in hematopoietic transcription factors including PAX5 occur in most cases of B-progenitor acute lymphoblastic leukemia (B-ALL), a disease characterized by the accumulation of undifferentiated lymphoblasts. Although PAX5 mutation is a critical driver of B-ALL development in mice and humans, it remains unclear how its loss contributes to leukemogenesis and whether ongoing PAX5 deficiency is required for B-ALL maintenance. Here we used transgenic RNAi to reversibly suppress endogenous Pax5 expression in the hematopoietic compartment of mice, which cooperates with activated signal transducer and activator of transcription 5 (STAT5) to induce B-ALL. In this model, restoring endogenous Pax5 expression in established B-ALL triggers immunophenotypic maturation and durable disease remission by engaging a transcriptional program reminiscent of normal B-cell differentiation. Notably, even brief Pax5 restoration in B-ALL cells causes rapid cell cycle exit and disables their leukemia-initiating capacity. These and similar findings in human B-ALL cell lines establish that Pax5 hypomorphism promotes B-ALL self-renewal by impairing a differentiation program that can be re-engaged despite the presence of additional oncogenic lesions. Our results establish a causal relationship between the hallmark genetic and phenotypic features of B-ALL and suggest that engaging the latent differentiation potential of B-ALL cells may provide new therapeutic entry points.

Infection Exposure Is a Causal Factor in B-cell Precursor Acute Lymphoblastic Leukemia as a Result ofPax5-Inherited Susceptibility

“…3A ; Supplementary Table S4), but no differences were detected in earlier developmental compartments of the BM (data not shown). These data therefore suggest that Pax5 heterozygosity favors the appearance of an aberrant B-cell precursor compartment in the BM and that differentiation to mature PB B cells is impaired in vivo in keeping with observations in Pax5 -null mice ( 13 ) and in a stable in vivo Pax5 knockdown model ( 23 ). These observations are in agreement with reports of preleukemic activity in mice bearing pB-ALL-associated oncogenes, e.g., the ETV6-RUNX1 fusion ( 24,25 ).…”

“…Common among these secondary events are alterations disrupting the PAX5 gene, which encodes a master transcriptional regulator of B-cell development ( 13,14 ). In this setting, PAX5 seems to retain driver functions in established leukemia because restoring endogenous PAX5 expression triggers disease remission ( 23 ).…”

Ebf1 heterozygosity results in increased DNA damage in pro-B cells and their synergistic transformation by Pax5 haploinsufficiency

“…16 The finding that the developmental block observed in Pax5-deficient leukemia cells can be reversed on restoration of Pax5 expression suggests that the reduction in Pax5 function results in a reversible disruption of differentiation. 17 A similar mechanism of action has been proposed for Ebf1; however, reduced amounts of Ebf1 in normal cells appear to result in reduced proliferation and expansion of B-cell progenitors, 4,6,18,19 indicating that the involvement of EBF1 in malignant transformation is more complex.…”

Masterminding B Cells

“…Furthermore, hypomorphic mutations of Pax5 have recently been associated with familial B-ALL (31). The inference is that reduced levels of Pax5 impose a differentiation block, thereby putting the cycling progenitor cell pool at increased risk of acquiring frankly oncogenic mutations (32)(33)(34). In strong support of this hypothesis, mice lacking Pax5 in mature B cells develop aggressive progenitor cell lymphomas (19).…”

“…Excitingly, Liu et al (32) have recently used a mouse model to show that, even in established B-ALL, the differentiation program can be re-engaged by elevating Pax5, despite the presence of other mutations. In light of the remarkable success of differentiation therapy for acute promyelocytic leukemia (35), this result encourages a search for comparable agents for treating B-ALL.…”