Cooperative Somatic Alterations in XPO1 and TNFAIP3 (A20) Promote Immune Modulation and Tissue Infiltration in Primary Mediastinal B Cell Lymphoma
Authors: Whipp Ethan C. , Walker Janek S. , Gharghabi Mehdi , Cempre Casey , Walker Brandi , Lapalombella Rosa 
Published: December 2023

Background: Primary Mediastinal B cell lymphoma (PMBCL) is an aggressive large B cell non-Hodgkin lymphoma typically presenting as a mass in the anterior mediastinum in early adulthood, disproportionately affecting females (2:1). Previous studies have identified constitutive activation of the NF-κB pathway and immune evasion as hallmark features of PMBCL. The master negative regulator of canonical NF-κB, TNFAIP3 (A20), is inactivated in ~25-60% of PMBCL cases via disruptive mutations or deletions. Gains of the 2p16.1-15 locus ( REL, XPO1, BCL11A) are seen in ~40-75% of PMBCL cases. Interestingly, PMBCL cases with 2p16.1-15 gain/amplification display concomitant increases in XPO1 expression but lack correlative increases in REL or BCL11A. In addition, XPO1 mutations are seen in ~15-33% of PMBCL cases, with the most prevalent mutation (E571K) implicated in NF-KB activation. Despite the established occurrence of genetic events impacting XPO1 and A20 in PMBCL, the cooperative potential of these alterations remains unexplored. Consequently, we conducted an extensive review of literature describing the PMBCL genomic landscape and utilized in vivo models to examine the combined potential of XPO1 and A20 alterations to promote lymphomagenesis.

Methods: Genomic data was obtained from NGS analyses from a total of 233 PMBCL patients across 6 independent published research studies and analyzed for cooccurrence of A20 & XPO1 alterations (Camus et al. 2022; Chapuy et al. 2019; Lacy et al. 2020; Mareschal et al. 2015; Mottock et al. 2019; Tuveri et al 2022). For in vivo analysis, we developed a murine model of B cell specific overexpression of wildtype (WT) or E571K- XPO1 with CD19.cre driven depletion of A20 (Eµ-XPO1xA20 KO). Whole body histopathologic evaluation of FFPE tissue with H&E staining was performed on 6-month-old, age and sex matched littermate mice.

Results: We found co-occurrence of inactivating events in A20 and XPO1 missense mutations impacting 15% of PMBCL evaluated cases, with consistent results seen across all studies (range: 13.8-20%). As expected, A20 mutations/del were more prevalent than XPO1 mutations across all studies (OR 2.46, 95% CI 0.84-7.18, p = 0.02). XPO1 mutated cases trended towards a greater likelihood of having co-occurring A20 mutations (OR 8.51, 95% CI 0.84-86.47, p = 0.08). XPO1 mutations and gain (2p16) were found to be mutually exclusive events with gains co-occurring with A20 mutations/del in ~17% of PMBCL cases evaluated. Analysis of survival of PMBCL patients with cooccurring A20 inactivating alterations and XPO1 mutations is ongoing although preliminarily trend towards reduced mean 2-year survival (p=0.056) compared to all others are observed. Taken together, co-occurring genetic alterations effecting A20 and XPO1 are prevalent in PMBCL the alternative biology stemming from potential cooperative mechanisms may represent an aggressive subgroup of PMBCL. To evaluate concurrent B cell-specific alterations of A20 and XPO1 in vivo we utilized Eµ-XPO1xA20 KO mouse models. Performing histopathology in 6-month-old mice we observed enlarged lymph nodes due to marked increases in lymphocytes within the cortex, paracortex, medulla, and sinuses in mice with loss of functional A20, particularly in Eµ-XPO1xA20 KO mice with overexpression of WT XPO1. Increased lymphocyte populations drove overall enlargement of the node (mandibular, mesenteric, pancreatic, and mediastinal) and distorted normal architecture, featuring small lymphocytes populating sinus regions and larger lymphocytes populating areas resembling follicles. Eµ-XPO1(WT & E571K)xA20 KO mice additionally featured mild mononuclear infiltrates in the lung and salivary glands; all of which were observed with increased severity compared to A20KO, Eµ-XPO1, or non-transgenic littermates.

Significance: We identified co-occurring genetic lesions impacting A20 and XPO1 in PMBCL tumors and show that these alterations synergize in vivo leading to nodal/extranodal lymphocyte infiltration. The immunomodulatory effects appear to be driven by the cooperation of XPO1 overexpression, more so than mutation, and A20 inactivation in young mice. Further investigation of these models will allow elucidation of underlying cooperative mechanisms and reveal targetable pathways in PMBCL.