Role of Ubiquitin E3 Ligase UBR5 in B-Cell Development and Lymphoma

Mantle Cell Lymphoma (MCL) is a non-Hodgkin's lymphoma (NHL) that typically affects older adults. In MCL, there is uncontrolled growth of B cells within the mantle zone of lymph node and spleen germinal centers. The vast majority of MCL cases have a translocation of chromosome 11 and 14 which juxtaposes the cyclin D1 gene, CCND1, with the immunoglobin heavy chain gene promoter (Eµ) and leads to the overexpression of CCND1. This translocation of CCND1 is not enough to cause MCL by itself and additional mutations are needed for MCL to develop. Although MCL only represents ~5% of NHL patients, it has the poorest survival rate among NHL sub-types due to a lack of successful therapeutic treatments. As such, it is important to identify potential targets for treatment. Recently published data shows that in 18% of all MCL patients have a mutation in the gene encoding the E3 ubiquitin ligase, UBR5. UBR5 is part of the ubiquitin proteasome system, a degradation/recycling pathway in which proteins are tagged with a ubiquitin protein and degraded by the proteasome. Of the identified UBR5 mutations, over 60% of the mutations are truncations at the carboxy terminus that cut off the cysteine residue linked to ubiquitin transfer in exon 59 suggesting a catalytic dead mutant protein is produced. Interestingly, UBR5HECT mutations are specific to MCL and are not found in other sub-types of NHL. By studying UBR5, we can determine the role of UBR5 mutations in MCL, elucidate molecular mechanism of UBR5 in B cell development, and identify potential therapeutic targets. In order to study the role of UBR5 in B cell development and MCL, we generated a conditional mouse model targeting exon 58 similar to mutations in MCL patients and crossed the mice with Mb1^CRE/WT mice to delete exon 58 specifically in B cells at the pro-pre B cell stage of B cell development. Mb1^CRE/WT; Ubr5^fl/fl mice shows that mice lacking the carboxy terminus of UBR5 have a block in B cell differentiation at the mature B cell IgM⁺ IgD⁺ stage within the spleen. Mb1^CRE/WT; Ubr5^fl/fl mice showed a marked decrease of mature IgM⁺ IgD⁺ B cells in the BM and spleen. Specifically, within the spleen, Mb1^CRE/WT; Ubr5^fl/fl mice produce abnormal follicular B cells (higher IgM and CD23 expression and lower IgD and CD22 expression) and significant reductions in marginal zone B cells, plasma cells, size of germinal centers, and number of germinal centers. Mass spectrometry comparing mouse B220+ splenocytes in Mb1^CRE/WT; Ubr5^fl/fl compared to Mb1^CRE/WT; Ubr5^+/+ mice showed that UBR5 in B cells in Mb1^CRE/WT; Ubr5^fl/fl mice has over two-fold more expression. Additionally, IgD was the most downregulated protein, B cell specific proteins were downregulated, and proteins involved with mRNA splicing were upregulated within the mass spectrometry. The loss of the UBR5 HECT domain leads to increased UBR5 half-life and upregulation of spliceosome proteins. Coupled together, this suggests that the loss of the carboxy terminus of UBR5 impedes B cell maturation by disrupting IgD expression, potentially by interfering with mRNA splicing. Finally, we have crossed and are currently aging our Mb1^CRE/WT; Ubr5^fl/fl mice with an Eµ^CCND1 mouse model to determine if Ubr5 mutations lead to tumorigenesis. These studies aim to identify the role of UBR5 in the context of normal B cell development and lymphomagenesis with the goal of identifying therapeutic targets for drug discovery.