Activating PAX gene family paralogs to complement PAX5 leukemia driver mutations

PAX5, one of nine members of the mammalian paired box (PAX) family of transcription factors, plays an important role in B cell development. Approximately one-third of individuals with pre-B acute lymphoblastic leukemia (ALL) acquire heterozygous inactivating mutations of PAX5 in malignant cells, and heterozygous germline loss-of-function PAX5 mutations cause autosomal dominant predisposition to ALL. At least in mice, Pax5 is required for pre-B cell maturation, and leukemic remission occurs when Pax5 expression is restored in a Pax5-deficient mouse model of ALL. Together, these observations indicate that PAX5 deficiency reversibly drives leukemogenesis. PAX5 and its two most closely related paralogs, PAX2 and PAX8, which are not mutated in ALL, exhibit overlapping expression and function redundantly during embryonic development. However, PAX5 alone is expressed in lymphocytes, while PAX2 and PAX8 are predominantly specific to kidney and thyroid, respectively. We show that forced expression of PAX2 or PAX8 complements PAX5 loss-of-function mutation in ALL cells as determined by modulation of PAX5 target genes, restoration of immunophenotypic and morphological differentiation, and, ultimately, reduction of replicative potential. Activation of PAX5 paralogs, PAX2 or PAX8, ordinarily silenced in lymphocytes, may therefore represent a novel approach for treating PAX5-deficient ALL. In pursuit of this strategy, we took advantage of the fact that, in kidney, PAX2 is upregulated by extracellular hyperosmolarity. We found that hyperosmolarity, at potentially clinically achievable levels, transcriptionally activates endogenous PAX2 in ALL cells via a mechanism dependent on NFAT5, a transcription factor coordinating response to hyperosmolarity. We also found that hyperosmolarity upregulates residual wild type PAX5 expression in ALL cells and modulates gene expression, including in PAX5-mutant primary ALL cells. These findings specifically demonstrate that osmosensing pathways may represent a new therapeutic target for ALL and more broadly point toward the possibility of using gene paralogs to rescue mutations driving cancer and other diseases.
Author summary Mutations inactivating PAX5 disrupt B cell differentiation and occur frequently in ALL. Others have previously shown that restoring PAX5 expression normalizes B cell differentiation and leads to disease remission in a mouse model of ALL. We found that exogenous expression of PAX5’s intact and closely related gene family members, PAX2 or PAX8, which are ordinarily silent in lymphocytes but expressed in kidney and other tissues, can substitute for PAX5 and restore differentiation in ALL cells. A new approach for treating ALL might therefore be to discover ways to activate expression of PAX2 or PAX8 in leukemic cells. In the kidney, PAX2 expression is activated by changes in extracellular osmolarity. We found that PAX2 retains the capacity for osmotic activation in ALL cells and that wild type PAX5 expression also increases when ALL cells are osmotically stressed. Adjustment of serum osmolarity—or treatment with drugs targeting pathways responding to osmotic stress—may offer a potential new avenue for ALL therapy by elevating expression of PAX gene family members. More generally, our studies point toward a novel strategy of recruiting paralogs to complement mutations in genes responsible for cancer and other diseases.