Epstein-Barr virus and malaria upregulate AID and APOBEC3 enzymes, but only AID seems to play a major mutagenic role in Burkitt lymphoma.

Endemic Burkitt lymphoma (eBL) is characterized by an oncogenic IGH/c-MYC translocation and Epstein-Barr virus (EBV) positivity, and is epidemiologically linked to Plasmodium falciparum malaria. Both EBV and malaria are thought to contribute to eBL by inducing the expression of activation-induced cytidine deaminase (AID), an enzyme involved in the IGH/c-MYC translocation. AID/apolipoprotein B mRNA editing catalytic polypeptide-like (AID/APOBEC) family enzymes have recently emerged as potent mutagenic sources in a variety of cancers, but apart from AID, their involvement in eBL and their regulation by EBV and P. falciparum is unknown. Here, we show that upon inoculation with EBV, human B cells strongly upregulate the expression of enzymatically active APOBEC3B and APOBEC3G. In addition, we found significantly increased levels of APOBEC3A in B cells of malaria patients, which correlated with parasite load. Interestingly, despite the fact that APOBEC3A, APOBEC3B, and APOBEC3G caused c-MYC mutations when overexpressed in HEK293T cells, a mutational enrichment in eBL tumors was only detected in AID motifs. This suggests that even though the EBV- and P. falciparum-directed immune response triggers the expression and activity of several AID/APOBEC members, only the upregulation of AID has oncogenic consequences, while the induction of the APOBEC3 subfamily may primarily have immunoprotective functions.

Role of Dot1L and H3K79 methylation in regulating somatic hypermutation of immunoglobulin genes.

Somatic hypermutation (SHM) and class-switch recombination (CSR) of the immunoglobulin (Ig) genes allow B cells to make antibodies that protect us against a wide variety of pathogens. SHM is mediated by activation-induced deaminase (AID), occurs at a million times higher frequency than other mutations in the mammalian genome, and is largely restricted to the variable (V) and switch (S) regions of Ig genes. Using the Ramos human Burkitt's lymphoma cell line, we find that H3K79me2/3 and its methyltransferase Dot1L are more abundant on the V region than on the constant (C) region, which does not undergo mutation. In primary naïve mouse B cells examined ex vivo, the H3K79me2/3 modification appears constitutively in the donor Sµ and is inducible in the recipient Sγ1 upon CSR stimulation. Knockout and inhibition of Dot1L in Ramos cells significantly reduces V region mutation and the abundance of H3K79me2/3 on the V region and is associated with a decrease of polymerase II (Pol II) and its S2 phosphorylated form at the IgH locus. Knockout of Dot1L also decreases the abundance of BRD4 and CDK9 (a subunit of the P-TEFb complex) on the V region, and this is accompanied by decreased nascent transcripts throughout the IgH gene. Treatment with JQ1 (inhibitor of BRD4) or DRB (inhibitor of CDK9) decreases SHM and the abundance of Pol II S2P at the IgH locus. Since all these factors play a role in transcription elongation, our studies reinforce the idea that the chromatin context and dynamics of transcription are critical for SHM.

Potential Oncogenic Effect of the MERTK-Dependent Apoptotic-Cell Clearance Pathway in Starry-Sky B-Cell Lymphoma.

The histological architecture of certain aggressive B-cell lymphomas (prototypically Burkitt's lymphoma, BL) is characterized by a "starry-sky" (SS) appearance. This is caused by tumor-associated macrophages (TAMs), which appear in standard histological preparations as "stars" in a darkly stained "sky" of lymphoma cells. SS-TAMs accumulate in response to constitutive apoptosis in these tumors and are activated by the apoptotic tumor cells to a pro-oncogenic phenotype. The extent to which SS-TAMs contribute to lymphoma growth through responses generated by interactions with apoptotic tumor cells is unknown. Here, we demonstrate a role for the receptor tyrosine kinase, MERTK, in the oncogenic activity of SS-TAMs. We show that MERTK expression is largely restricted to the macrophages of human BL and of murine models of SS B-cell lymphoma and that it is upregulated in SS-TAMs as compared to the germinal center or paracortical macrophages of normal lymph nodes. Our results further demonstrate that MERTK is active in the phagocytosis of apoptotic lymphoma cells by macrophages and, most significantly, that SS lymphoma growth is markedly inhibited in Mertk-/- mice. These results point toward the MERTK apoptotic-cell clearance/response pathway playing a key role in growth of aggressive B-cell lymphoma and identifies MERTK as a novel potential antilymphoma target.

Checkpoint kinase 1 is essential for normal B cell development and lymphomagenesis.

Checkpoint kinase 1 (CHK1) is critical for intrinsic cell cycle control and coordination of cell cycle progression in response to DNA damage. Despite its essential function, CHK1 has been identified as a target to kill cancer cells and studies using Chk1 haploinsufficient mice initially suggested a role as tumor suppressor. Here, we report on the key role of CHK1 in normal B-cell development, lymphomagenesis and cell survival. Chemical CHK1 inhibition induces BCL2-regulated apoptosis in primary as well as malignant B-cells and CHK1 expression levels control the timing of lymphomagenesis in mice. Moreover, total ablation of Chk1 in B-cells arrests their development at the pro-B cell stage, a block that, surprisingly, cannot be overcome by inhibition of mitochondrial apoptosis, as cell cycle arrest is initiated as an alternative fate to limit the spread of damaged DNA. Our findings define CHK1 as essential in B-cell development and potent target to treat blood cancer.

Pediatric B-Cell Lymphoma With Lymphoblastic Morphology, TdT Expression, MYC Rearrangement, and Features Overlapping With Burkitt Lymphoma.

Burkitt lymphoma (BL) and B-lymphoblastic lymphoma are subtypes of pediatric non-Hodgkin lymphoma with different presenting features, treatment, and outcomes. This case report documents a 5-year-old female who presented with B-cell lymphoma with lymphoblastic morphology, terminal deoxynucleotidyl transferase expression, MYC rearrangement, and features overlapping with BL. Genomic microarray analysis identified a gain on the long arm of chromosome 1 without other definitive changes. She was treated according to a BL protocol and remains in remission 16-months after initial diagnosis.

Dual inhibition of histone deacetylases and phosphoinositide 3-kinases: effects on Burkitt lymphoma cell growth and migration.

Burkitt lymphoma is a highly aggressive non-Hodgkin lymphoma that is characterized by MYC deregulation. Recently, the PI3K pathway has emerged as a cooperative prosurvival mechanism in Burkitt lymphoma. Despite the highly successful results of treatment that use high-dose chemotherapy regimens in pediatric Burkitt lymphoma patients, the survival rate of pediatric patients with progressive or recurrent disease is low. PI3Ks are also known to regulate cell migration, and abnormal cell migration may contribute to cancer progression and dissemination in Burkitt lymphoma. Little is known about Burkitt lymphoma cell migration, but the cooperation between MYC and PI3K in Burkitt lymphoma pathogenesis suggests that a drug combination could be used to target the different steps involved in Burkitt lymphoma cell dissemination and disease progression. The aim of this study was to investigate the effects of the histone deacetylase inhibitor suberoylanilide hydroxamic acid combined with the PI3K inhibitor LY294002 on Burkitt lymphoma cell growth and migration. The combination enhanced the cell growth inhibition and cell-cycle arrest induced by the PI3K inhibitor or histone deacetylase inhibitor individually. Moreover, histone deacetylase inhibitor/PI3K inhibitor cotreatment suppressed Burkitt lymphoma cell migration and decreased cell polarization, Akt and ERK1/2 phosphorylation, and leads to RhoB induction. In summary, the histone deacetylase inhibitor/PI3Ki combination inhibits cell proliferation and migration via alterations in PI3K signaling and histone deacetylase activity, which is involved in the acetylation of α-tubulin and the regulation of RhoB expression.

Lactate dehydrogenase inhibitors sensitize lymphoma cells to cisplatin without enhancing the drug effects on immortalized normal lymphocytes.

Up-regulation of glycolysis, a well recognized hallmark of cancer cells, was also found to be predictive of poor chemotherapy response. This observation suggested the attempt of sensitizing cancer cells to conventional chemotherapeutic agents by inhibiting glucose metabolism. Lactate dehydrogenase (LDH) inhibition can be a way to hinder glycolysis of cancer cells without affecting the metabolism of normal tissues, which usually does not require this enzymatic activity. In this paper, we showed that two LDH inhibitors (oxamate and galloflavin) can increase the efficacy of cisplatin in cultured Burkitt's lymphoma (BL) cells and that this potentiating effect is not exerted in proliferating normal lymphocytes. This result was explained by the finding that in BL cells LDH inhibition induced reactive oxygen species (ROS) generation, which was not evidenced in proliferating normal lymphocytes. In BL cells treated with the association of cisplatin and LDH inhibitors, these ROS can be a further cause of DNA damage, to be added to that produced by cisplatin, leading to the failure of the response repair. At present LDH inhibitors suitable for clinical use are actively searched; our results can allow a better understanding of the potentiality of LDH as a possible target to develop innovative anticancer treatments.

Protein kinase CK2 is widely expressed in follicular, Burkitt and diffuse large B-cell lymphomas and propels malignant B-cell growth.

Serine-threonine kinase CK2 is highly expressed and pivotal for survival and proliferation in multiple myeloma, chronic lymphocytic leukemia and mantle cell lymphoma. Here, we investigated the expression of α catalytic and ß regulatory CK2 subunits by immunohistochemistry in 57 follicular (FL), 18 Burkitt (BL), 52 diffuse large B-cell (DLBCL) non-Hodgkin lymphomas (NHL) and in normal reactive follicles. In silico evaluation of available Gene Expression Profile (GEP) data sets from patients and Western blot (WB) analysis in NHL cell-lines were also performed. Moreover, the novel, clinical-grade, ATP-competitive CK2-inhibitor CX-4945 (Silmitasertib) was assayed on lymphoma cells. CK2 was detected in 98.4% of cases with a trend towards a stronger CK2α immunostain in BL compared to FL and DLBCL. No significant differences were observed between Germinal Center B (GCB) and non-GCB DLBCL types. GEP data and WB confirmed elevated CK2 mRNA and protein levels as well as active phosphorylation of specific targets in NHL cells. CX-4945 caused a dose-dependent growth-arresting effect on GCB, non-GCB DLBCL and BL cell-lines and it efficiently shut off phosphorylation of NF-κB RelA and CDC37 on CK2 target sites. Thus, CK2 is highly expressed and could represent a suitable therapeutic target in BL, FL and DLBCL NHL.

A source of the single-stranded DNA substrate for activation-induced deaminase during somatic hypermutation.

During somatic hypermutation (SHM), activation-induced deaminase (AID) mutates deoxycytidine on single-stranded DNA (ssDNA) generated by the transcription machinery, but the detailed mechanism remains unclear. Here we report a higher abundance of RNA polymerase II (Pol II) at the immunoglobulin heavy-chain variable (Igh-V) region compared with the constant region and partially transcribed Igh RNAs, suggesting a slower Pol II progression at Igh-V that could result in some early/premature transcription termination after prolonged pausing/stalling of Pol II. Knocking down RNA-exosome complexes, which could decrease premature transcription termination, leads to decreased SHM. Knocking down Spt5, which can augment premature transcription termination, leads to increase in both, SHM and the abundance of ssDNA substrates. Collectively, our data support the model that, following the reduction of Pol II progression (pausing or stalling) at the Igh-V, additional steps such as premature transcription termination are involved in providing ssDNA substrates for AID during SHM.

Reversion of apoptotic resistance of TP53-mutated Burkitt lymphoma B-cells to spindle poisons by exogenous activation of JNK and p38 MAP kinases.

Defects in apoptosis are frequently the cause of cancer emergence, as well as cellular resistance to chemotherapy. These phenotypes may be due to mutations of the tumor suppressor TP53 gene. In this study, we examined the effect of various mitotic spindle poisons, including the new isocombretastatin derivative isoNH2CA-4 (a tubulin-destabilizing molecule, considered to bind to the colchicine site by analogy with combretastatin A-4), on BL (Burkitt lymphoma) cells. We found that resistance to spindle poison-induced apoptosis could be reverted in tumor protein p53 (TP53)-mutated cells by EBV (Epstein Barr virus) infection. This reversion was due to restoration of the intrinsic apoptotic pathway, as assessed by relocation of the pro-apoptotic molecule Bax to mitochondria, loss of mitochondrial integrity and activation of the caspase cascade with PARP (poly ADP ribose polymerase) cleavage. EBV sensitized TP53-mutated BL cells to all spindle poisons tested, including vincristine and taxol, an effect that was systematically downmodulated by pretreatment of cells with inhibitors of p38 and c-Jun N-terminal kinase (JNK) mitogen-activated protein kinases. Exogenous activation of p38 and JNK pathways by dihydrosphingosine reverted resistance of TP53-mutated BL cells to spindle poisons. Dihydrosphingosine treatment of TP53-deficient Jurkat and K562 cell lines was also able to induce cell death. We conclude that activation of p38 and JNK pathways may revert resistance of TP53-mutated cells to spindle poisons. This opens new perspectives for developing alternative therapeutic strategies when the TP53 gene is inactivated.

Inhibition of human mitochondrial peptide deformylase causes apoptosis in c-myc-overexpressing hematopoietic cancers.

Inhibition of human mitochondrial peptide deformylase (HsPDF) depolarizes the mitochondrial membrane, reduces mitochondrial protein translation and causes apoptosis in Burkitt's lymphoma. We showed that HsPDF mRNA and protein levels were overexpressed in cancer cells and primary acute myeloid leukemia samples. Myc regulates mitochondria and metabolism; we also demonstrated c-myc regulated the expression of HsPDF, likely indirectly. Inhibition of HsPDF by actinonin blocked mitochondrial protein translation and caused apoptotic death of myc-positive Burkitt's lymphoma, but not myc-negative B cells. Inhibition of mitochondrial translation by chloramphenicol or tetracycline, structurally different inhibitors of the mitochondrial ribosome, which is upstream of deformylase activity, followed by treatment with actinonin, resulted in reversal of the biochemical events and abrogation of the apoptosis induced by actinonin. This reversal was specific to inhibitors of HsPDF. Inhibition of HsPDF resulted in a mitochondrial unfolded protein response (increased transcription factors CHOP and CEB/P and the mitochondrial protease Lon), which may be a mechanism mediating cell death. Therefore, HsPDF may be a therapeutic target for these hematopoietic cancers, acting via a new mechanism.

Phase II study of alisertib, a selective Aurora A kinase inhibitor, in relapsed and refractory aggressive B- and T-cell non-Hodgkin lymphomas.

PURPOSE: Aurora A kinase (AAK) is overexpressed in aggressive lymphomas and can correlate with more histologically aggressive forms of disease. We therefore designed a phase II study of alisertib, a selective AAK inhibitor, in patients with relapsed and refractory aggressive non-Hodgkin lymphomas. PATIENTS AND METHODS: Patients age ≥ 18 years were eligible if they had relapsed or refractory diffuse large B-cell lymphoma (DLBCL), mantle-cell lymphoma (MCL), transformed follicular lymphoma, Burkitt's lymphoma, or noncutaneous T-cell lymphoma. Alisertib was administered orally at 50 mg twice daily for 7 days in 21-day cycles. RESULTS: We enrolled 48 patients. Histologies included DLBCL (n = 21), MCL (n = 13), peripheral T-cell lymphoma (n = 8), transformed follicular lymphoma (n = 5), and Burkitt's (n = 1). Most common grade 3 to 4 adverse events were neutropenia (63%), leukopenia (54%), anemia (35%), thrombocytopenia (33%), stomatitis (15%), febrile neutropenia (13%), and fatigue (6%). Four deaths during the study were attributed to progressive non-Hodgkin lymphoma (n = 2), treatment-related sepsis (n = 1), and unknown cause (n = 1). The overall response rate was 27%, including responses in three of 21 patients with DLBCL, three of 13 with MCL, one of one with Burkitt's lymphoma, two of five with transformed follicular lymphoma, and four of eight with noncutaneous T-cell lymphoma. The alisertib steady-state trough concentration (n = 25) revealed the expected pharmacokinetic variability, with a trend for higher incidence of adverse event-related dose reductions at higher trough concentrations. Analysis for AAK gene amplification and total AAK protein revealed no differences between histologies or correlation with clinical response. CONCLUSION: The novel AAK inhibitor alisertib seems clinically active in both B- and T-cell aggressive lymphomas. On the basis of these results, confirmatory single-agent and combination studies have been initiated.

Loss of the proteins Bak and Bax prevents apoptosis mediated by histone deacetylase inhibitors.

Burkitt lymphoma is characterized by deregulation of c-myc, and therapies targeting c-myc are under investigation as treatments. Histone deacetylase inhibitors are known to abrogate c-myc expression, leading us to examine their effect in a series of Burkitt lymphoma cell lines. While treatment with romidepsin, panobinostat, vorinostat, or belinostat for 48 h resulted in complete cell death in the Ramos and ST486 lines, CA46 and DG75 cells were resistant. In parallel studies, CA46 and DG75 cells were also insensitive to 48 h treatment with the Aurora kinase inhibitors (AKIs) MLN8237 (alisertib), VX-680 (tozasertib), or ZM447439. Bax knockdown is known to lead to HDI resistance, and we found that loss of Bax or both Bak and Bax correlated with resistance to both AKIs and HDIs in the Burkitt cell lines. As proof-of-concept to evaluate the contribution of Bax and Bak to HDI-mediated apoptosis, we found that apoptosis was unaffected in HCT-116 colon carcinoma cells lacking Bak, blunted in cells lacking Bax, and nearly completely abrogated in cells lacking both Bak and Bax compared with wild-type cells. To explore potential clinical variations in Bak and Bax expression, a series of samples from 16 patients diagnosed with Burkitt lymphoma was examined. While the majority of samples were positive for both Bak and Bax, some (3/16) expressed low levels of both proteins. We thus conclude that HDI-mediated and AKI-mediated apoptosis requires mitochondrial engagement, and that baseline Bax and Bak expression may serve as biomarkers for patients with Burkitt lymphoma likely to respond to HDI treatment.

Downregulation of the polyamine regulator spermidine/spermine N(1)-acetyltransferase by Epstein-Barr virus in a Burkitt's lymphoma cell line.

Transition of Akata Burkitt's lymphoma (BL) from a malignant to nonmalignant phenotype upon loss of Epstein-Barr virus (EBV) is evidence for a viral contribution to tumorigenesis despite the tight restriction of EBV gene expression in BL. Examination of global cellular gene expression in Akata subclones that retained or lost EBV identified spermidine/spermine N(1)-acetyltransferase (SAT1), an inducible enzyme whose catabolism of polyamines affects both apoptosis and cell growth, as one of a limited number of cellular genes downregulated by EBV. Re-infection of the EBV-negative Akata clone reduced SAT1 mRNA to a level comparable with the parental EBV-positive Akata. EBV-positive Akata cells demonstrated decreased SAT1 enzyme activity concomitant with altered intracellular polyamine constituents. Reduction of SAT1 in EBV-positive BL was a transcriptional effect. Forced expression of the viral BCL2 homologue, BHRF1, in an EBV-negative Akata clone reduced SAT1 mRNA. Thus, EBV repression of polyamine catabolism becomes a complementary alteration to dysregulated c-myc enhancement of polyamine synthesis in BL and favorable to BL lymphomagenesis.

Histone deacetylase inhibitor potentiated the ability of MTOR inhibitor to induce autophagic cell death in Burkitt leukemia/lymphoma.

BACKGROUND: Burkitt leukemia/lymphoma is a major subtype of aggressive B-cell lymphoma. Biological targeted therapies on this disease need to be further investigated and may help to improve the clinical outcome of the patients. METHODS: This study examined the anti-tumor activity of the histone deacetylases (HDAC) inhibitor valproic acid (VPA) combined with the mammalian target of rapamycin (MTOR) inhibitor temsirolimus in Burkitt leukemia/lymphoma cell lines, as well as in primary tumor cells and a murine xenograft model. RESULTS: Co-treatment of VPA and temsirolimus synergistically inhibited the tumor cell growth and triggered the autophagic cell death, with a significant inhibition of MTOR signaling and MYC oncoprotein. Functioned as a class I HDAC inhibitor, VPA potentiated the effect of temsirolimus on autophagy through inhibiting HDAC1. Molecular silencing of HDAC1 using small interfering RNA (siRNA) attenuated VPA-mediated regulation of CDKN1A, CDKN1B and LC3-I/II, regression of tumor cell growth and induction of autophagy. Meanwhile, VPA counteracted temsirolimus-induced AKT activation via HDAC3 inhibition. HDAC3 siRNA abrogated the ability of VPA to modulate AKT phosphorylation, to suppress tumor cell growth and to induce autophagy. Strong antitumor effect was also observed on primary tumor cells while sparing normal hematopoiesis ex vivo. In a murine xenograft model established with subcutaneous injection of Namalwa cells, dual treatment efficiently blocked tumor growth, inhibited MYC and induced in situ autophagy. CONCLUSIONS: These findings confirmed the synergistic effect of the HDAC and MTOR inhibitors on Burkitt leukemia/lymphoma, and provided an insight into clinical application of targeting autophagy in treating MYC-associated lymphoid malignancies.

Galloflavin suppresses lactate dehydrogenase activity and causes MYC downregulation in Burkitt lymphoma cells through NAD/NADH-dependent inhibition of sirtuin-1.

Activation of the myc oncogene in cancer cells upregulates lactate dehydrogenase A (LDH-A) expression, leading to a sustained glycolytic flux that is needed to produce ATP under hypoxic conditions. We studied the effects of galloflavin (GF), a recently identified LDH inhibitor, on myc overexpressing Burkitt lymphoma (BL) cells. Epstein-Barr virus-infected lymphoblasts were used as a non-neoplastic control. Our results showed that myc overactivation induced a two- to seven-fold increase in LDH-A expression in BL cells compared with non-neoplastic lymphoblasts; this result is consistent with previously reported data. Moreover, GF treatment suppressed LDH activity and inhibited BL cell replication but did not affect lymphoblast viability. Surprisingly, we found that increased levels of the MYC and LDH-A proteins did not lead to a metabolic shift in BL cells toward glycolytic ATP generation. BL cells were treated with GF at doses that achieved 50% inhibition of cell growth and lactate production, and ATP levels were scarcely affected after GF treatment. The same results were also obtained by suppressing LDH activity with oxamate, an LDH specific inhibitor. Our data suggest that LDH activity is important for maintaining a correct NAD/NADH balance in BL cells. LDH inhibition led to decreased NAD cellular levels, which resulted in sirtuin-1 inhibition. Confirming previous studies, sirtuin-1 inhibition caused a reduction in MYC protein levels, depriving BL cells of their most important survival signal. This study further describes the biological functions of the LDH enzyme and suggests that LDH inhibition could be useful for the treatment of cancer.

Carnitine-acyltransferase system inhibition, cancer cell death, and prevention of myc-induced lymphomagenesis.

BACKGROUND: The metabolic alterations of cancer cells represent an opportunity for developing selective antineoplastic treatments. We investigated the therapeutic potential of ST1326, an inhibitor of carnitine-palmitoyl transferase 1A (CPT1A), the rate-limiting enzyme for fatty acid (FA) import into mitochondria. METHODS: ST1326 was tested on in vitro and in vivo models of Burkitt's lymphoma, in which c-myc, which drives cellular demand for FA metabolism, is highly overexpressed. We performed assays to evaluate the effect of ST1326 on proliferation, FA oxidation, and FA mitochondrial channeling in Raji cells. The therapeutic efficacy of ST1326 was tested by treating Eµ-myc mice (control: n = 29; treatment: n = 24 per group), an established model of c-myc-mediated lymphomagenesis. Experiments were performed on spleen-derived c-myc-overexpressing B cells to clarify the role of c-myc in conferring sensitivity to ST1326. Survival was evaluated with Kaplan-Meier analyses. All statistical tests were two-sided. RESULTS: ST1326 blocked both long- and short-chain FA oxidation and showed a strong cytotoxic effect on Burkitt's lymphoma cells (on Raji cells at 72 hours: half maximal inhibitory concentration = 8.6 µM). ST1326 treatment induced massive cytoplasmic lipid accumulation, impairment of proper mitochondrial FA channeling, and reduced availability of cytosolic acetyl coenzyme A, a fundamental substrate for de novo lipogenesis. Moreover, treatment with ST1326 in Eµ-myc transgenic mice prevented tumor formation (P = .01), by selectively impairing the growth of spleen-derived primary B cells overexpressing c-myc (wild-type cells + ST1326 vs. Eµ-myc cells + ST1326: 99.75% vs. 57.5%, difference = 42.25, 95% confidence interval of difference = 14% to 70%; P = .01). CONCLUSIONS: Our data indicate that it is possible to tackle c-myc-driven tumorigenesis by altering lipid metabolism and exploiting the neoplastic cell addiction to FA oxidation.

High levels of AID cause strand bias of mutations at A versus T in Burkitt's lymphoma cells.

Ig gene somatic hypermutation in the germinal center (GC) B cells occurs at C and G at roughly the same frequency. In contrast, there is a 2-fold increase of mutations at A relative to T on the non-transcribed strand of the V genes but it is unclear what triggers such strand bias. Using an efficient mutagenesis system that recapitulates characteristic features of Ig gene hypermutation in the GC B cells, we found that low levels of AID induced similar frequency of mutations at A and T. However, high levels of AID specifically increased mutations at A, but not T, leading to strand bias. These results explain why strand bias of A:T mutations is observed only in the highly mutated V genes but not in the less mutated switch region or the BCL-6 gene. High levels of AID also increased the proportion of transversions at G relative to transversions at C. Our results identify a clue to the strand bias of A:T mutations and provide an in vitro model to elucidate this unsolved mystery in the hypermutation field.

Decitabine represses translocated MYC oncogene in Burkitt lymphoma.

Burkitt lymphoma (BL) is caused by translocation of the MYC gene to an immunoglobulin locus resulting in its constitutive expression depending on the activity of the immunoglobulin (Ig) enhancer elements. Treatment of BL cell lines with epigenetic modifiers is known to repress B-cell-specific genes and to up-regulate B-cell-inappropriate genes including the transcription repressor ID2 expression. We found that the DNA methyltransferase inhibitor decitabine/5-aza-2-deoxycytidine (5-aza-dC) represses the MYC oncogene on RNA and protein levels by inducing ID2. Down-regulation of MYC was associated with repression of transcriptional activity of the Ig locus and with inhibition of proliferation. The induction of ID2 can be in part explained by activation of the transcription factor NF-κB. We conclude that up-regulation of ID2 contributes to anti-tumour activity of 5-aza-dC via repression of Ig locus activity and consequently MYC expression.