Myogenic transcription is repressed in myoblasts by serum-activated cyclin-dependent kinases, such as cdk2 and cdk4. Serum withdrawal promotes muscle-speci®c gene expression at least in part by down-regulating the activity of these cdks. Unlike the other cdks, cdk9 is not serum-or cell cycle-regulated and is instead involved in the regulation of transcriptional elongation by phosphorylating the carboxyl-terminal domain (CTD) of RNA polymerase II. While ectopic expression of cdk2 together with its regulatory subunits (cyclins E and A) inhibits myogenic transcription, overproduction of cdk9 and its associated cyclin (cyclin T2a) strengthens MyoD-dependent transcription and stimulates myogenic di erentiation in both MyoD-converted ®broblasts and C2C12 muscle cells. Conversely, inhibition of cdk9 activity by a dominant negative form (cdk9-dn) represses the myogenic program. Cdk9, cyclinT2 and MyoD can be detected in a multimeric complex in C2C12 cells, with the minimal cdk9-binding region of MyoD mapping within 101 ± 161 aa of the bHLH region. Finally, cdk9 can phosphorylate MyoD in vitro, suggesting the possibility that cdk9/cycT2a regulation of muscle di erentiation includes the direct enzymatic activity of the kinase on MyoD.

Endemic Burkitt lymphoma (eBL) is primarily found in children in equatorial regions and represents the first historical example of a virus-associated human malignancy. Although Epstein-Barr virus (EBV) infection and MYC translocations are hallmarks of the disease, it is unclear whether other factors may contribute to its development. We performed RNA-Seq on 20 eBL cases from Uganda and showed that the mutational and viral landscape of eBL is more complex than previously reported. First, we found the presence of other herpesviridae family members in 8 cases (40%), in particular human herpesvirus 5 and human herpesvirus 8 and confirmed their presence by immunohistochemistry in the adjacent non-neoplastic tissue. Second, we identified a distinct latency program in EBV involving lytic genes in association with TCF3 activity. Third, by comparing the eBL mutational landscape with published data on sporadic Burkitt lymphoma (sBL), we detected lower frequencies of mutations in MYC, ID3, TCF3 and TP53, and a higher frequency of mutation in ARID1A in eBL samples. Recurrent mutations in two genes not previously associated with eBL were identified in 20% of tumors: RHOA and cyclin F (CCNF). We also observed that polyviral samples showed lower numbers of somatic mutations in common altered genes in comparison to sBL specimens, suggesting dual mechanisms of transformation, mutation versus virus driven in sBL and eBL respectively.

Burkitt lymphoma (BL) is classified into

The molecular feature of Burkitt lymphoma (BL) is the translocation that places c-Myc under the control of immunoglobulin gene regulatory elements. However, there is accumulating evidence that some cases may lack an identifiable MYC translocation. In addition, during the EUROFISH project, aiming at the standardization of FISH procedures in lymphoma diagnosis, we found that five cases out of 35 classic endemic BLs were negative for MYC translocations by using a split-signal as well as a dual-fusion probe. Here we investigated the expression pattern of miRNAs predicted to target c-Myc, in BL cases, to clarify whether alternative pathogenetic mechanisms may be responsible for lymphomagenesis in cases lacking the MYC translocation. miRNAs are a class of small RNAs that are able to regulate gene expression at the post-transcriptional level. Several studies have reported their involvement in cancer and their association with fragile sites in the genome. They have also been shown to control cell growth, differentiation, and apoptosis, suggesting that these molecules could act as tumour suppressors or oncogenes. Our results demonstrated a modulation of specific miRNAs. In particular, down-regulation of hsa-let-7c was observed in BL cases, compared to normal controls. More interestingly, hsa-mir-34b was found to be down-regulated only in BL cases that were negative for MYC translocation, suggesting that this event might be responsible for c-Myc deregulation in such cases. This hypothesis was further confirmed by our in vitro experiments, which demonstrated that increasing doses of synthetic hsa-mir-34b were able to modulate c-Myc expression. These results indicate for the first time that hsa-mir-34b may influence c-Myc expression in Burkitt lymphoma as the more common aberrant control exercised by the immunoglobulin enhancer locus.

IntroductionChronic lymphocytic leukemia (CLL) is the most common B-cell neoplasm in Europe and the United States, characterized by progressive accumulation of monoclonal CD5 ϩ B cells in peripheral blood, bone marrow, and peripheral lymphoid organs. The clinical course of CLL is highly variable, ranging from an indolent disease that may never require treatment to a rapidly progressive disease. 1 One of the principal prognostic features is the mutational status of the immunoglobulin heavy chain variable region genes (IGHV). CLL with poor prognosis has unmutated IGHV genes (U-CLL), whereas CLL with good prognosis carries somatic mutations in these genes (M-CLL). 2,3 Neoplastic CLL cells are typically arrested in the G 0 /G 1 phase of the cell cycle and accumulate in tissues because of prolonged survival. 1 Although extrinsic factors, such as escape from immune surveillance and chemotaxis to a favorable microenvironment, contribute to the extended survival of CLL B cells, 4 CLL is primarily a disease of defective apoptosis, and intrinsic defects in a number of components of the apoptotic circuitry have been identified, including overexpression of antiapoptotic proteins (eg, Bcl-2, Mcl-1, XIAP) and reduction in the expression of proapoptotic proteins (eg, Bax, DAPK-1). [5][6][7] Other apoptosis defects, including abnormalities in the p53 and ATM pathways in the most aggressive subsets, have also been implicated in the prolonged survival of malignant cells. [8][9][10] In this respect, apoptosis is emerging as a key therapeutic target in CLL, as witnessed by the ongoing clinical trials on Bcl-2 inhibitors. 11 p66Shc, a member of the Shc family of protein adapters, acts as antagonist of mitogenic signaling and positive regulator of oxidative stress-induced apoptosis in fibroblasts. 12,13 In T cells, where its expression is epigenetically controlled, 14,15 p66Shc uncouples the T-cell receptor from activation of the Ras/mitogen-activated protein kinase pathway by competitively inhibiting recruitment to the T-cell receptor of the mitogenic isoform, p52Shc. 16 Furthermore, p66Shc enhances T-cell susceptibility to apoptotic stimuli by increasing reactive oxygen species (ROS) production and impairing Ca 2ϩ homeostasis. 17 We have recently reported that p66Shc is expressed in murine B cells and that p66Shc deficiency results in enhanced proliferative responses of mouse B cells to B-cell receptor (BCR) engagement, 18 suggesting that p66Shc may display similar activities in T and B cells.Here, we have investigated the role of p66Shc in B-cell survival. We show that p66Shc promotes B-cell apoptosis by uncoupling the BCR from the survival pathways mediated by Akt and Erk. On the basis of these findings, we have investigated p66Shc expression and function in B cells from patients with CLL. The results identify a role for p66Shc in the imbalance among proapoptotic and antiapoptotic Bcl-2 family members in CLL and An Inside Blood analysis of this article appears at the front of this issue.The online version of this article contains...

Sporadic Burkitt lymphoma (sBL) can be delineated from diffuse large B-cell lymphoma (DLBCL) by a very homogeneous mRNA expression signature. However, it remained unclear whether all three BL variantsFsBL, endemic BL (eBL) and human immunodeficiency virus-associated BL (HIV-BL)F represent a uniform biological entity despite their differences in geographical occurrence, association with immunodeficiency and/or incidence of Epstein-Barr virus (EBV) infection. To address this issue, we generated micro RNA (miRNA) profiles from 18 eBL, 31 sBL and 15 HIV-BL cases. In addition, we analyzed the miRNA expression of 86 DLBCL to determine whether miRNA profiles recapitulate the molecular differences between BL and DLBCL evidenced by mRNA profiling. A signature of 38 miRNAs containing MYC regulated and nuclear factor-kB pathway-associated miRNAs was obtained that differentiated BL from DLBCL. The miRNA profiles of sBL and eBL displayed only six differentially expressed miRNAs, whereas HIV and EBV infection had no impact on the miRNA profile of BL. In conclusion, miRNA profiling confirms that BL and DLBCL represent distinct lymphoma categories and demonstrates that the three BL variants are representatives of the same biological entity with only marginal miRNA expression differences between eBL and sBL.

CDK9 is a member of the CDC2-like family of kinases. Its cyclin partners are members of the CYCLIN T family (T1, T2a, and T2b) and CYCLIN K. The CDK9/CYCLIN T1 complex is very important in the differentiation programme of several cell types, controlling specific differentiation pathways. Limited data are available regarding the expression of CDK9/CYCLIN T1 in haematopoietic and lymphoid tissues. The aim of this study was to analyse the expression of the CDK9/CYCLIN T1 complex in lymphoid tissue, in order to assess its role in B- and T-cell differentiation and lymphomagenesis. CDK9/CYCLIN T1 expression was found by immunohistochemistry in precursor B and T cells. In peripheral lymphoid tissues, germinal centre cells and scattered B- and T-cell blasts in interfollicular areas expressed CDK9/CYCLIN T1, while mantle cells, plasma cells, and small resting T-lymphocytes displayed no expression of either molecule. CDK9/CYCLIN T1 expression therefore appears to be related to particular stages of lymphoid differentiation/activation. CDK9 and CYCLIN T1 were highly expressed in lymphomas derived from precursor B and T cells, from germinal centre cells, such as follicular lymphomas, and from activated T cells (ie anaplastic large cell lymphomas). Hodgkin and Reed-Sternberg cells of classical Hodgkin's lymphoma also showed strong nuclear staining. Diffuse large B-cell, Burkitt's lymphomas, and peripheral T-cell lymphomas, among T-cell lymphoproliferative disorders, showed a wide range of values. No expression of CDK9 or CYCLIN T1 was detected in mantle cell and marginal zone lymphomas. However, at the mRNA level, an imbalance in the CDK9/CYCLIN T1 ratio was found in follicular lymphoma and diffuse large B-cell lymphomas with germinal centre phenotype, and in the cell lines of classical Hodgkin's lymphomas, Burkitt's lymphomas, and anaplastic large cell lymphoma, in comparison with reactive lymph nodes. These results suggest that the CDK9/CYCLIN T1 complex may affect the activation and differentiation programme of lymphoid cells. The molecular mechanism through which the CDK9/CYCLIN T1 complex is altered in malignant transformation needs to be elucidated.

The protein product of the retinoblastoma (RB) gene is necessary for the completion of the muscle differentiation program and for myogenic basic helix-loop-helixdependent transcription. In fact, in addition to induction and maintenance of permanent cell cycle withdrawal through negative regulation of E2F-responsive genes involved in proliferation, pRb also plays a positive role in the activation of muscle-specific genes. In pRbÀ/À myocytes, the expression of late myogenic markers is defective and myoblast fusion into myotubes occurs without irreversible cell cycle exit. This evidence demonstrates only a partial functional redundancy between pRb and its relatives p107 and pRb2/p130, as these pRbÀ/À multinucleated cells, which display p107 levels higher than normal myotubes, respond to mitogens with cell cycle re-entry and DNA synthesis. At the molecular level, pRb myogenic functions are mediated by cooperation with MyoD, Myocyte enhancer factor 2 (MEF2), High mobility group box protein-1 (HBP1) and histone deacetylase1, affecting chromatin configuration and tissue-specific transcription, and by post-translational modification in response to intracellular signaling cascades.