SOX11 expression is restricted to EBV-negative Burkitt lymphoma and associates with molecular genetic features

SRY-related HMG-box gene 11 (SOX11) is a transcription factor overexpressed in mantle cell lymphoma (MCL), a subset of Burkitt lymphomas (BL) and precursor lymphoid cell neoplasms but is absent in normal B-cells and other B-cell lymphomas. SOX11 has an oncogenic role in MCL but its contribution to BL pathogenesis remains uncertain. Here, we observed that the presence of Epstein-Barr virus (EBV) and SOX11 expression were mutually exclusive in BL. SOX11 expression in EBV- BL was associated with an IG∷MYC translocation generated by aberrant class switch recombination, while in EBV-/SOX11- tumors the IG∷MYC translocation was mediated by mistaken somatic hypermutations. Interestingly, EBV- SOX11 expressing BL showed higher frequency of SMARCA4 and ID3 mutations compared to EBV-/SOX11- cases. By RNA-sequencing, we identified a SOX11-associated gene expression profile, with functional annotations showing partial overlap with the SOX11 transcriptional program of MCL. Contrary to MCL, no differences on cell migration or BCR signaling were found between SOX11- and SOX11+ BL cells. However, SOX11+ BL showed higher adhesion to VCAM-1 than SOX11- BL cell lines. Here we demonstrate that EBV- BL comprises two subsets of cases based on SOX11 expression. The mutual exclusion of SOX11 and EBV, and the association of SOX11 with a specific genetic landscape suggest a role of SOX11 in the early pathogenesis of BL.

SOX11/PRDX2 axis modulates redox homeostasis and chemoresistance in aggressive mantle cell lymphoma

Mantle cell lymphoma (MCL) is an incurable B-cell neoplasm characterized by an aggressive behavior, short responses to conventional therapies and SOX11 overexpression, which is associated with aggressive disease features and inferior clinical outcome of patients. Oxidative stress is known to induce tumorigenesis and tumor progression, whereas high expression levels of antioxidant genes have been associated with chemoresistance in different cancers. However, the role of oxidative stress in MCL pathogenesis and the involvement of SOX11 regulating redox homeostasis in MCL cells are largely unknown. Here, by integrating gene set enrichment analysis of two independent series of MCL, we observed that SOX11+ MCL had higher reactive oxygen species (ROS) levels compared to SOX11- MCL primary tumors and increased expression of Peredoxine2 (PRDX2), which upregulation significantly correlated with SOX11 overexpression, higher ROS production and worse overall survival of patients. SOX11 knockout (SOX11KO) significantly reduced PRDX2 expression, and SOX11KO and PRDX2 knockdown (PRDX2KD) had increased ROS levels and ROS-mediated tumor cell death upon treatment with drugs, compared to control MCL cell lines. Our results suggest an aberrant redox homeostasis associated with chemoresistance in aggressive MCL through SOX11-mediated PRDX2 upregulation, highlighting PRDX2 as promising target for new therapeutic strategies to overcome chemoresistance in aggressive MCLs.

Tumorigenic role of Musashi-2 in aggressive mantle cell lymphoma

SOX11 overexpression has been associated with aggressive behavior of mantle cell lymphomas (MCL). SOX11 is overexpressed in embryonic and cancer stem cells (CSC) of some tumors. Although CSC have been isolated from primary MCL, their relationship to SOX11 expression and contribution to MCL pathogenesis and clinical evolution remain unknown. Here, we observed enrichment in leukemic and hematopoietic stem cells gene signatures in SOX11+ compared to SOX11- MCL primary cases. Musashi-2 (MSI2) emerged as one of the most significant upregulated stem cell-related genes in SOX11+ MCLs. SOX11 is directly bound to the MSI2 promoter upregulating its expression in vitro. MSI2 intronic enhancers were strongly activated in SOX11+ MCL cell lines and primary cases. MSI2 upregulation was significantly associated with poor overall survival independently of other high-risk features of MCL. MSI2 knockdown decreased the expression of genes related to apoptosis and stem cell features and significantly reduced clonogenic growth, tumor cell survival and chemoresistance in MCL cells. MSI2-knockdown cells had reduced tumorigenic engraftment into mice bone marrow and spleen compared to control cells in xenotransplanted mouse models. Our results suggest that MSI2 might play a key role in sustaining stemness and tumor cell survival, representing a possible novel target for therapeutic interventions in MCL.

Structural Basis for the SUMO2 Isoform Specificity of SENP7

SUMO proteases or deSUMOylases regulate the lifetime of SUMO-conjugated targets in the cell by cleaving off the isopetidic bond between the substrate and the SUMO modifier, thus reversing the conjugation activity of the SUMO E3 ligases. In humans the deSUMOylating activity is mainly conducted by the SENP/ULP protease family, which is constituted of six members sharing a homologous catalytic globular domain. SENP6 and SENP7 are the most divergent members of the family and they show a unique SUMO2/3 isoform preference and a particular activity for dismantling polySUMO2 chains. Here, we present the crystal structure of the catalytic domain of human SENP7 bound to SUMO2, revealing structural key elements for the SUMO2 isoform specificity of SENP7. In particular, we describe the specific contacts between SUMO2 and a unique insertion in SENP7 (named Loop1) that is responsible for the SUMO2 isoform specificity. All the other interface contacts between SENP7 and SUMO2, including the SUMO2 C-terminal tail interaction, are conserved among members of the SENP/ULP family. Our data give insight into an evolutionary adaptation to restrict the deSUMOylating activity in SENP6 and SENP7 for the SUMO2/3 isoforms.

Abstract 777: Deciphering the role of MSI2 as a regulator of mantle cell lymphoma stem-like properties

Mantle cell lymphoma (MCL) is a mature B cell neoplasm with two distinct biological subtypes regarding clinical, molecular and pathological features. Conventional MCL (cMCL) subtype is characterized by high aggressiveness and poor outcome with frequent relapses after initial response to treatment, suggesting the presence of a population of cancer stem cells (CSC) able to self-renew and responsible for drug resistance. SOX11 embryonic transcription factor is aberrantly overexpressed mostly in cMCL and has an oncogenic role in MCL. MCL-CSC have been isolated from MCL cases by different groups. However, its relationship to SOX11 expression and contribution to MCL clinical features and aggressive behavior remains unknown. We hypothesized that SOX11 may promote stem cell-like properties through the activation of stem cell-related genes, leading to an aggressive and incurable tumor in MCL patients. Here, we integrated the differential gene expression profile (GEP) between SOX11+ and SOX11- MCL primary cases with stem cell-related genes, and specific SOX11 ChIP-chip and epigenetic data in MCL cell lines and primary cases, to identify potential mediators of CSC directly regulated by SOX11 in MCL. We observed an enrichment of hematopoietic (HSC) and leukemic stem cells (LSC) gene signatures in SOX11+ compared to SOX11- MCL primary cases. MSI2, an RNA binding protein that maintains self-renewal and prevents differentiation in HSC, emerged as one of the most significant upregulated CSC-related gene in SOX11+ compared to SOX11- MCLs, positively correlating with SOX11 expression in MCL primary cases. Our in vitro experiments showed that SOX11 binds to MSI2 regulatory regions increasing its expression in MCL cells. MSI2 enhancers located in intronic regions were activated in SOX11+ but not in SOX11- MCL cell lines and primary cases. MSI2 upregulation significantly associated with poor overall survival, independently of common risk factors (SOX11 expression, high copy number alterations, 17p/TP53, 11q/ATM and 9p/CDKN2A alterations), in MCL patients. MSI2 knockdown MCL cell lines had a gene expression profile enriched in proapoptotic-related genes; whereas HSC and LSC gene signatures were downregulated compared to its control cells. MSI2 knockdown or MSI2 inhibition with Ro 08-2750 treatment decreased clonogenic growth, tumor survival and chemoresistance in MCL cells. SOX11+/MSI2 high MCL primary leukemic samples showed higher percentage of ALDH+ cells compared to SOX11-/MSI2 low samples, which decreased after Ro 08-2750 treatment. Besides, MSI2 silencing delayed tumor growth in vivo in MCL xenograft mice models. Overall, our results suggest that SOX11+ MCL cases acquire stemness features through the upregulation of MSI2 expression, which promotes chemoresistance, self-renewal and tumor survival. MSI2 represents a novel biomarker for MCL-CSC and therapeutic target for relapsed MCL.

Citation Format: Marta Sureda-Gómez, Patricia Balsas, Marta Leonor Rodríguez, Ferran Nadeu, Anna De Bolòs, Álvaro Eguileor, Marta Kulis, Giancarlo Castellano, José Ignacio Martin-Subero, Santiago Demajo, Pedro Jares, Eva Giné, Elias Campo, Virginia Amador. Deciphering the role of MSI2 as a regulator of mantle cell lymphoma stem-like properties [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 777.