Allen Discovery Center and Department of Pathology, University of Washington School of Medicine, Seattle, Washington, United States of America.
University of Colorado School of Medicine, Aurora, Colorado, United States of America.
PLoS Genet. 2018 Sep 14;14(9):e1007642. doi: 10.1371/journal.pgen.1007642. eCollection 2018 Sep.
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.
PAX5 是哺乳动物配对盒 (PAX) 转录因子家族的九个成员之一，在 B 细胞发育中发挥重要作用。大约三分之一的前 B 急性淋巴细胞白血病 (ALL) 患者在恶性细胞中获得 PAX5 的杂合失活突变，杂合性胚系 PAX5 功能丧失突变导致常染色体显性 ALL 易感性。至少在小鼠中，Pax5 是前 B 细胞成熟所必需的，当在 ALL 的 Pax5 缺陷小鼠模型中恢复 Pax5 表达时，白血病会缓解。这些观察结果表明，PAX5 缺乏可逆转地驱动白血病发生。PAX5 及其两个最密切相关的同源物 PAX2 和 PAX8 在 ALL 中未发生突变，在胚胎发育过程中表现出重叠的表达和功能冗余。然而，只有 PAX5 在淋巴细胞中表达，而 PAX2 和 PAX8 分别主要特异性表达于肾脏和甲状腺。我们表明，在 ALL 细胞中，强制表达 PAX2 或 PAX8 可补充 PAX5 功能丧失突变，如通过调节 PAX5 靶基因、恢复免疫表型和形态分化以及最终降低复制潜力来确定。因此，激活在淋巴细胞中通常沉默的 PAX5 同源物 PAX2 或 PAX8 可能代表治疗 PAX5 缺陷 ALL 的一种新方法。在追求这种策略的过程中，我们利用了这样一个事实，即在肾脏中，PAX2 被细胞外高渗上调。我们发现，在潜在的临床可实现水平的高渗条件下，通过一种依赖于 NFAT5 的机制，NFAT5 是协调对高渗反应的转录因子，可在 ALL 细胞中转录激活内源性 PAX2。我们还发现，高渗条件上调 ALL 细胞中残留的野生型 PAX5 表达，并调节基因表达，包括 PAX5 突变的原发性 ALL 细胞。这些发现特别表明，渗透压感应途径可能是 ALL 的一个新的治疗靶点，并更广泛地指出利用基因同源物来挽救驱动癌症和其他疾病的突变的可能性。
