Cytokines such as interferons (IFNs) activate signal transducers and activators of transcription (STATs) via phosphorylation. Histone deacetylases (HDACs) and the histone acetyltransferase (HAT) CBP dynamically regulate STAT1 acetylation. Here we show that acetylation of STAT1 counteracts IFN-induced STAT1 phosphorylation, nuclear translocation, DNA binding, and target gene expression. Biochemical and genetic experiments altering the HAT/HDAC activity ratio and STAT1 mutants reveal that a phospho-acetyl switch regulates STAT1 signaling via CBP, HDAC3, and the T-cell protein tyrosine phosphatase (TCP45). Strikingly, inhibition of STAT1 signaling via CBP-mediated acetylation is distinct from the functions of this HAT in transcriptional activation. STAT1 acetylation induces binding of TCP45, which catalyzes dephosphorylation and latency of STAT1. Our results provide a deeper understanding of the modulation of STAT1 activity. These findings reveal a new layer of physiologically relevant STAT1 regulation and suggest that a previously unidentified balance between phosphorylation and acetylation affects cytokine signaling.[Keywords: STAT1; acetylation; phosphorylation; histone deacetylase; interferon; HDAC inhibitor; phosphatase TCP45] Supplemental material is available at http://www.genesdev.org.

Nuclear factor-jB (NF-jB) and p53 critically determine cancer development and progression. Defining the cross talk between these transcription factors can expand our knowledge on molecular mechanisms of tumorigenesis. Here, we show that induction of replicational stress activates NF-jB p65 and triggers its interaction with p53 in the nucleus. Experiments with knockout cells show that p65 and p53 are both required for enhanced NF-jB activity during S-phase checkpoint activation involving ataxiatelangiectasia mutated and checkpoint kinase-1. Accordingly, the pro-inflammatory cytokine tumor necrosis factora (TNF-a) also triggers formation of a transcriptionally active complex containing nuclear p65 and p53 on jB response elements. Gene expression analyses revealed that, independent of NF-jB activation in the cytosol, TNFinduced NF-jB-directed gene expression relies on p53. Hence, p53 is unexpectedly necessary for NF-jB-mediated gene expression induced by atypical and classical stimuli. Remarkably, data from gain-and loss-of function approaches argue that anti-apoptotic NF-jB p65 activity is constitutively evoked by a p53 hot-spot mutant frequently found in tumors. Our observations suggest explanations for the outstanding question why p53 mutations rather than p53 deletions arise in tumors of various origins.

Inhibition of Janus-kinase 1/2 (JAK1/2) is a mainstay to treat myeloproliferative neoplasms (MPN). Sporadic observations reported the co-incidence of B-cell non-Hodgkin lymphomas during treatment of MPN with JAK1/2 inhibitors. We assessed 626 patients with MPN, including 69 with myelofibrosis receiving JAK1/2 inhibitors for lymphoma development. B-cell lymphomas evolved in 4 (5.8%) of 69 patients receiving JAK1/2 inhibition compared with 2 (0.36%) of 557 with conventional treatment (16-fold increased risk). A similar 15-fold increase was observed in an independent cohort of 929 patients with MPN. Considering primary myelofibrosis only (N = 216), 3 lymphomas were observed in 31 inhibitor-treated patients (9.7%) vs 1 (0.54%) of 185 control patients. Lymphomas were of aggressive B-cell type, extranodal, or leukemic with high MYC expression in the absence of V617F or other MPN-associated mutations. Median time from initiation of inhibitor therapy to lymphoma diagnosis was 25 months. Clonal immunoglobulin gene rearrangements were already detected in the bone marrow during myelofibrosis in 16.3% of patients. Lymphomas occurring during JAK1/2 inhibitor treatment were preceded by a preexisting B-cell clone in all 3 patients tested. Sequencing verified clonal identity in 2 patients. The effects of JAK1/2 inhibition were mirrored in mice: 16 of 24 mice developed a spontaneous myeloid hyperplasia with the concomitant presence of aberrant B cells. Transplantations of bone marrow from diseased mice unmasked the outgrowth of a malignant B-cell clone evolving into aggressive B-cell leukemia-lymphoma. We conclude that JAK/STAT1 pathway inhibition in myelofibrosis is associated with an elevated frequency of aggressive B-cell lymphomas. Detection of a preexisting B-cell clone may identify individuals at risk.

A His-X-His pseudotripeptide zinc complex (X is a N-alkyl glycine derivative) similar to the catalytic center of the carbonic anhydrase was computer designed and experimentally synthesized. Using 2D-NMR techniques, all proton, carbon chemical shifts and nuclear overhauser effect signals were assigned. The three-dimensional structure of the complex was determined with the COSMOS (computer simulation of molecular structures) force field by applying 13C bond polarization theory chemical shift pseudo forces and restrictions for NOE distances. From molecular dynamics, simulated annealing simulations and geometry optimizations, the three best force field structures were taken for a final investigation by density functional theory calculations.

Mastocytosis is characterized by abnormal expansion and accumulation of neoplastic mast cells in one or more organ systems. Traditionally, mastocytosis is divided into cutaneous mastocytosis (CM) and systemic mastocytosis (SM). In most patients with SM, the transforming somatic mutation KIT D816V is detected. However, only few studies have quantified the KIT D816V allele burden in CM and SM. The aim of the present study was to quantify KIT D816V in various forms of mastocytosis and to correlate the allele burden of KIT D816V with the disease category, serum tryptase levels and clinical outcomes. KIT D816V was quantified in bone marrow (BM) and peripheral blood (PB) cells by a real-time PCR (qPCR) assay based on allele-specific primers. In addition, BM and PB cells were also examined for the presence of KIT D816V by melting curve analysis after PCR clamping in all patients. Overall, 225 DNA samples (BM, n=112; PB, n=113) from 107 patients with mastocytosis (females: n=57; males, n=50; median age 49 years; range 18-73 years) were analyzed. Based on WHO criteria, 14 patients had CM, 3 the provisional diagnosis of mastocytosis in the skin (MIS), 66 indolent SM (ISM), 6 smouldering SM (SSM), 7 aggressive SM (ASM), one mast cell leukemia (MCL) and 10 patients SM with an associated hematologic non-mast cell lineage disorder (SM-AHNMD). KIT D816V was found in in 76/107 patients (71%) by melting curve analysis after PCR clamping, and in 92/107 patients (86%) by qPCR (p<0.005). In paired BM and PB samples of 43 patients an excellent correlation of the KIT D816V burden with almost identical results was found (r=0.98, p<0.001). When examining the KIT D816V allele burden in KIT D816V+ patients (n=92) in various categories of the disease, significant differences were found between CM (median KIT D816V allele fraction: 0.042%), MIS (median: 0.084%), ISM (median: 0.286%), SSM (median: 3.012%), ASM (median: 9.346%) and SM-AHNMD (median: 3.761%) (p<0.001). Moreover, we found that the KIT D816V allele burden correlates significantly with the serum tryptase level in our patients (r=0.50, p<0.005). Consecutive studies revealed that the KIT D816V allele fraction is of prognostic significance concerning survival as determined by Cox regression (p=0.015). As assessed by Kaplan Meier estimates and log rank testing, patients with a KIT D816V allele burden of ≥2% were found to have a significantly shorter survival than those with an allele burden of less than 2% (p=0.001) (Figure 1). Thirty patients were evaluated at diagnosis and during the follow up. In untreated patients with stable disease, the KIT D816V allele burden remained within a constant range. By contrast, in patients with disease progression, an increase in the KIT D816V burden over time was detectable. In patients responding to cytoreductive agents (cladribine n=4; hydroxyurea n=1) a significant decrease in the median KIT D816V allele burden (by 91.6%) after therapy compared to pre-therapeutic samples was observed (p=0.027). In summary, our data show that qPCR is a highly sensitive approach for the detection and quantification of KIT D816V in patients with mastocytosis and that the KIT D816V mutation burden differs significantly among patients in different WHO subtypes. Moreover, the KIT D816V allele burden correlates with serum tryptase levels and is of prognostic significance concerning survival in patients with mastocytosis. Finally, quantification of KIT D816V may serve as follow up parameter useful for determining the natural course and treatment responses in patients with mastocytosis. We recommend that the KIT D816V mutation burden is included as a novel parameter in daily practice and clinical trials in advanced SM.Figure 1Overall survival of patients with KIT D816V+ mastocytosis. Patients were split into those with a KIT D816V allele burden of<2% and those with an allelic burden of ≥2%. Survival was estimated by the method of Kaplan and Meier (p=0.001).Figure 1. Overall survival of patients with KIT D816V+ mastocytosis. Patients were split into those with a KIT D816V allele burden of<2% and those with an allelic burden of ≥2%. Survival was estimated by the method of Kaplan and Meier (p=0.001). Disclosures: Valent: Novartis: Consultancy, Honoraria, Research Funding.