Epigenetic Silencing of Stk39 in B-Cell Lymphoma Inhibits Apoptosis from Genotoxic Stress

B-cell lymphomas, the most frequent human immune system malignancies, often contain dysregulated TCL1 oncogene expression. TCL1 transgenic (TCL1-tg) mice develop a spectrum of B-cell malignancies, supporting an oncogenic role for TCL1 in B cells. Our prior global survey of DNA methylation patterns in TCL1-tg B-cell lymphomas identified many lymphoma-specific candidate hypermethylated genes, including Stk39. The Stk39 encoded protein, sterile 20-like-related proline-alanine-rich kinase (SPAK), regulates cell stress responses, and microarray studies identified reduced SPAK expression in metastatic prostate and treatment-resistant breast cancers, suggesting that its loss may have a role in cancer progression. Here we identified DNA hypermethylation and SPAK silencing in TCL1-tg B-cell lymphomas and SPAK silencing without DNA methylation in multiple subtypes of human B-cell lymphomas. SPAK knockdown by shRNA protected B cells from caspase-dependent apoptosis induced by DNA double-strand breaks but not apoptosis in response to osmotic or oxidative cell stressors. Caspase 3 activation by cleavage was impaired with SPAK repression in DNA damaged B cells. Interestingly, c-Jun NH2-terminal kinase is potentially activated by SPAK and pharmacological inhibition of c-Jun NH2-terminal kinase in SPAK-expressing B cells recapitulated the cell-protective phenotype of SPAK knockdown. Taken together, these data indicate that SPAK loss in B-cell lymphomas promotes in-creased cell survival with DNA damage and provides a potential mechanism for increased resistance to genotoxic stress in cancer. (Am J Pathol 2009, 175:1653-1661; DOI: 10.2353/ajpath.2009.090091)