DNA methylation profiling: a new tool for evaluating hematologic malignancies

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 NH(2)-terminal kinase is potentially activated by SPAK and pharmacological inhibition of c-Jun NH(2)-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 increased cell survival with DNA damage and provides a potential mechanism for increased resistance to genotoxic stress in cancer.

Expression of sprouty2 inhibits B-cell proliferation and is epigenetically silenced in mouse and human B-cell lymphomas

B-cell lymphoma is the most common immune system malignancy. TCL1 transgenic mice (TCL1-tg), in which TCL1 is ectopically expressed in mature lymphocytes, develop multiple B- and T-cell leukemia and lymphoma subtypes, supporting an oncogenic role for TCL1 that probably involves AKT and MAPK-ERK signaling pathway augmentation. Additional, largely unknown genetic and epigenetic alterations cooperate with TCL1 during lymphoma progression. We examined DNA methylation patterns in TCL1-tg B-cell tumors to discover tumor-associated epigenetic changes, and identified hypermethylation of sprouty2 (Spry2). Sprouty proteins are context-dependent negative or positive regulators of MAPK-ERK pathway signaling, but their role(s) in B-cell physiology or pathology are unknown. Here we show that repression of Spry2 expression in TCL1-tg mouse and human B-cell lymphomas and cell lines is associated with dense DNA hypermethylation and was reversed by inhibition of DNA methylation. Spry2 expression was induced in normal splenic B cells by CD40/B-cell receptor costimulation and regulated a negative feedback loop that repressed MAPK-ERK signaling and decreased B-cell viability. Conversely, loss of Spry2 function hyperactivated MAPK-ERK signaling and caused increased B-cell proliferation. Combined, these results implicate epigenetic silencing of Spry2 expression in B lymphoma progression and suggest it as a companion lesion to ectopic TCL1 expression in enhancing MAPK-ERK pathway signaling.

Methylation of tumor suppressor genes p16(INK4a), p27(Kip1) and E-cadherin in carcinogenesis

Not only genomic mutations but also abnormal epigenetic methylation can significantly contribute to gene silencing and carcinogenesis. Methylation is particularly often observed in the CpG islands of the promoter regions in the regulatory genes. However, there are considerable differences in the incidence of methylation e.g. in the tumor suppressor genes, so that aberrant methylation of p16(INK4a) is relatively frequently observed in tumors, p27(Kip1) methylation is rare, and the incidence of E-cadherin methylation occurs at an intermediate rate. Although true genomic defects are generally much less common than methylation, parallel tendencies for both are often observed, probably reflecting the different levels of evolutionary advantage for tumor cells from inactivation of different genes. This also suggests that loss of p27 expression could be more a consequence of carcinogenesis, while lost p16 expression is a true oncogenic event. Due to the role of p27 in maintaining cellular quiescence, however, loss of its expression can still be a useful partial indicator of the aggressiveness of cancer. Loss of E-cadherin or its catenin partners of cellular adhesion will result in increasing invasiveness and metastatic potential of neoplastic cells but, because of several alternative routes to the same effect, incidence of lost expression for one component gene like E-cadherin does not need to be very high. Similarly, there must be a relatively high number of genes with modest or low incidence of aberrant silencing by methylation, to reflect multiple alternatives for the multistep process of carcinogenesis. Nevertheless, methylation of different genes also shows characteristic differences between different cancer and tumor types, and the epigenetic methylation patterns therefore have considerable diagnostic and prognostic potential. Realising this potential requires efficient methods for profiling the status of methylation. Such profiling methods have only recently become available and are still under relatively rapid development.

An anti-apoptotic role for galectin-3 in diffuse large B-cell lymphomas

Increased resistance to apoptosis promotes lymphomagenesis with aberrant expression of cell survival proteins such as BCL-2 and c-MYC occurring in distinct lymphoma subtypes. Galectin-3 is an anti-apoptotic protein that protects T cells, macrophages, and breast carcinoma cells from death triggered by a variety of agents. We have found high levels of galectin-3 protein expression in a subset of B-cell neoplasms including diffuse large B-cell lymphoma (DLBCL), primary effusion lymphoma (PEL), and multiple myeloma (MM), in both cell lines and patient samples. However, we failed to detect galectin-3 in Burkitt lymphoma (BL), follicular lymphoma (FL), marginal zone lymphoma (MZL), MALT lymphoma or B-small lymphocytic lymphoma (B-SLL) cell lines or patient samples. To determine whether galectin-3 expression protects B cells from apoptosis, galectin-3-negative BL cells were transfected with a galectin-3 expressing plasmid, which resulted in markedly increased resistance to anti-Fas-induced cell death. In contrast, galectin-3-positive PEL cells transfected with an amino-terminal truncated galectin-3 vector showed increased sensitivity to anti-Fas induced apoptosis. During normal B-cell development, galectin-3 expression was lowest in germinal center and plasma B cells, from which DLBCL, PEL, and MM derive, and highest in long-lived naïve and memory B cells. This pattern of expression suggests that aberrantly increased galectin-3 levels in specific B-cell populations may yield a protective advantage during transformation and/or progression of certain B-cell neoplasms.

Transdifferentiation and nuclear reprogramming in hematopoietic development and neoplasia

Cell transplantation and tissue regeneration studies indicate a surprisingly broad developmental potential for lineage-committed hematopoietic stem cells (HSCs). Under these conditions HSCs transition into myocytes, neurons, hepatocytes or other types of nonhematopoietic effector cells. Equally impressive is the progression of committed neuronal stem cells (NSCs) to functional blood elements. Although critical cell-of-origin issues remain unresolved, the possibility of lineage switching is strengthened by a few well-controlled examples of cell-type conversion. At the molecular level, switching probably initiates from environmental signals that induce epigenetic modifications, resulting in changes in chromatin configuration. In turn, these changes affect patterns of gene expression that mediate divergent developmental programs. This review examines recent findings in nuclear reprogramming and cell fusion as potential causative mechanisms for transdifferentiation during normal and malignant hematopoiesis.