Abstract A24: BTG1 mutations confer a fitness advantage and promote aggressive B cell lymphoma development by lowering the threshold for MYC induction

BTG1 somatic mutations are exclusively found in mature B cell malignancies, ~12% diffuse large B cell lymphoma (DLBCL) and are particularly enriched in the MCD/cluster 5 subtype of ABC-DLBCL, characterized by extranodal dissemination and poor clinical outcome. However, the mechanism of action and clinical relevance of BTG1 mutations remain unknown. We find that BTG1 mutations score among the top mutations with DLBCL driver potential, using a rigorous genomic and epigenomic covariates analysis. Most notably, BTG1 mutant patients presented with inferior clinical outcome (p=0.0011) in ABC-DLBCL cases from 5 cohorts and BTG1 mutation was independently associated with lower overall survival in a multivariable Cox regression analysis (p=0.0190) DLBCL originates from mature B cells having experienced the germinal center (GC) reaction. We therefore generated a conditional knockin mouse model to express the most frequent Btg1 Q36H in B cells. Btg1 Q36H GC B cells showed a dramatic fitness advantage in in vivo competitive assays. This effect was specific to the GC compartment and was dependent on T cells. RNAseq showed that Btg1Q36H GC B cells were markedly enriched for genes normally transiently induced upon positive selection by T cells, including MYC targets. The same signatures were enriched in BTG1 mutant DLBCL patients and isogenic BTG1Q36H vs BTG1WT human DLBCL cell lines. We observed a higher proportion of MYC-expressing cells in Btg1Q36H GCs without an increase in maximal MYC levels per cell, also confirmed in human DLBCL lines and primary human tonsillar B cells, suggesting a lowered threshold for MYC induction in mutant cells. Mechanistically, ~800 transcripts associated with BTG1WT, but not BTG1Q36H. These belong to the same gene sets that characterize positively selected GC B cell, including MYC itself. BTG1Q36H therefore showed reduced association with the MYC mRNA. We found that this associated with facilitated MYC protein synthesis. Polysome profiling also showed higher translation capacity in BTG1Q36H DLBCL cells. Crossing our Btg1 Q36H mice to the VavP-Bcl2 model showed that VavP-Bcl2+Btg1Q36H mice displayed shorter survival, earlier onset of lymphoma and dysplastic B cell infiltration into non lymphoid organs. Immunoglobulin genes sequencing showed that VavP-Bcl2+Btg1Q36H lymphoma B cells were highly clonal, extensively mutated and selected. Importantly, the lack of BTG1 deletions suggested that BTG1 missense mutations do not cause a full loss-of-function of the protein. Indeed, we observed that shRNA-mediated knockdown of BTG1 resulted in apoptosis. Collectively, BTG1 mutations contribute to the formation of aggressive lymphomas through an entirely novel mechanism, by lowering the threshold to MYC induction in response to T cell selection signals, conferring dramatic fitness. This effect appears to correspond to a partial loss of function disrupting a novel GC context-specific check point, whereby BTG1 normally attenuates spurious MYC translation to tightly restrict fitness potential.

Citation Format: Coraline Mlynarczyk, Matt Teater, Juhee Pae, Ling Wang, Jonatan Ersching, Antonin Papin, Darko Barisic, Ersilia Barin, Kenneth B. Hoehn, Zhengming Chen, Diu T. T. Nguyen, Chiara Evans, Ashley S. Doane, Michael G. Kharas, David W. Scott, Gabriel Victora, Ari Melnick. BTG1 mutations confer a fitness advantage and promote aggressive B cell lymphoma development by lowering the threshold for MYC induction [abstract]. In: Proceedings of the Third AACR International Meeting: Advances in Malignant Lymphoma: Maximizing the Basic-Translational Interface for Clinical Application; 2022 Jun 23-26; Boston, MA. Philadelphia (PA): AACR; Blood Cancer Discov 2022;3(5_Suppl):Abstract nr A24.

Social distancing patterns in nine municipalities of Rio Grande do Sul, Brazil: the Epicovid19/RS study

OBJECTIVE To describe social distancing practices in nine municipalities of the state of Rio Grande do Sul, Brazil, stratified by gender, age, and educational attainment. METHODS Two sequential cross-sectional studies were conducted in the municipalities of Canoas, Caxias do Sul, Ijuí, Passo Fundo, Pelotas, Porto Alegre, Santa Cruz do Sul, Santa Maria, and Uruguaiana to estimate the population prevalence of COVID-19. The study was designed to be representative of the urban population of these municipalities. A questionnaire including three questions about social distancing was also administered to the participants. Here, we present descriptive analyses of social distancing practices by subgroups and use chi-square tests for comparisons. RESULTS In terms of degree of social distancing, 25.8% of the interviewees reported being essentially isolated and 41.1% reported being quite isolated. 20.1% of respondents reported staying at home all the time, while 44.5% left only for essential activities. More than half of households reported receiving no visits from non-residents. Adults aged 20 to 59 reported the least social distancing, while more than 80% of participants aged 60 years or older reported being essentially isolated or quite isolated. Women reported more stringent distancing than men. Groups with higher educational attainment reported going out for daily activities more frequently. CONCLUSIONS The extremes of age are more protected by social distancing, but some groups remain highly exposed. This can be an important limiting factor in controlling progression of the COVID-19 pandemic.

Cytotoxic Escherichia coli strains encoding colibactin isolated from immunocompromised mice with urosepsis and meningitis

Immune-compromised mouse models allow for testing the preclinical efficacy of human cell transplantations and gene therapy strategies before moving forward to clinical trials. However, CRISPR/Cas9 gene editing of the W^sh/W^sh mouse strain to create an immune-compromised model lacking function of Rag2 and Il2rγ led to unexpected morbidity and mortality. This warranted an investigation to ascertain the cause and predisposing factors associated with the outbreak. Postmortem examination was performed on 15 moribund mice. The main lesions observed in these mice consisted of ascending urogenital tract infections, suppurative otitis media, pneumonia, myocarditis, and meningoencephalomyelitis. As Escherichia coli strains harboring polyketide synthase (pks) genomic island were recently isolated from laboratory mice, the tissue sections from the urogenital tract, heart, and middle ear were subjected to E. coli specific PNA-FISH assay that revealed discrete colonies of E. coli associated with the lesions. Microbiological examination and 16S rRNA sequencing confirmed E. coli-induced infection and septicemia in the affected mice. Further characterization by clb gene analysis and colibactin toxicity assays of the pks+ E. coli revealed colibactin-associated cytotoxicity. Rederivation of the transgenic mice using embryo transfer produced mice with an intestinal flora devoid of pks+ E. coli. Importantly, these barrier-maintained rederived mice have produced multiple litters without adverse health effects. This report is the first to describe acute morbidity and mortality associated with pks+ E. coli urosepsis and meningitis in immunocompromised mice, and highlights the importance of monitoring and exclusion of colibactin-producing pks+ E. coli.

The use of 18F-2-fluorodeoxyglucose (FDG) to label antibody fragments for immuno-PET of pancreatic cancer

We generated ¹⁸F-labeled antibody fragments for PET imaging using a sortase-mediated reaction to install a transcyclooctene (TCO)-functionalized short peptide onto proteins of interest, followed by reaction with a tetrazine-labeled-¹⁸F-2-deoxyfluoroglucose (FDG). The method is rapid, robust, and site-specific (radiochemical yields >25%, not decay corrected). The availability of ¹⁸F-2-deoxyfluoroglucose avoids the need for more complicated chemistries used to generate carbon-fluorine bonds. We demonstrate the utility of the method by detecting heterotopic pancreatic tumors in mice by PET, using anti-Class II MHC single domain antibodies. We correlate macroscopic PET images with microscopic two-photon visualization of the tumor. Our approach provides easy access to ¹⁸F-labeled antibodies and their fragments at a level of molecular specificity that complements conventional¹⁸F-FDG imaging.

Collecting lymphatic vessel permeability facilitates adipose tissue inflammation and distribution of antigen to lymph node-homing adipose tissue dendritic cells

Collecting lymphatic vessels (CLVs), surrounded by fat and endowed with contractile muscle and valves, transport lymph from tissues after it is absorbed into lymphatic capillaries. CLVs are not known to participate in immune responses. In this study, we observed that the inherent permeability of CLVs allowed broad distribution of lymph components within surrounding fat for uptake by adjacent macrophages and dendritic cells (DCs) that actively interacted with CLVs. Endocytosis of lymph-derived Ags by these cells supported recall T cell responses in the fat and also generated Ag-bearing DCs for emigration into adjacent lymph nodes (LNs). Enhanced recruitment of DCs to inflammation-reactive LNs significantly relied on adipose tissue DCs to maintain sufficient numbers of Ag-bearing DCs as the LN expanded. Thus, CLVs coordinate inflammation and immunity within adipose depots and foster the generation of an unexpected pool of APCs for Ag transport into the adjacent LN.