Downregulation of internal enhancer activity contributes to abnormally low immunoglobulin expression in the MedB‐1 mediastinal B‐cell lymphoma cell line

Abstract

Primary mediastinal B-cell lymphoma (PMBL) is a highly aggressive tumour with a unique pattern of clinical, morphological, immunological and genetic features distinct from other diffuse large B-cell lymphomas. PMBLs are characterized by a mature B-cell phenotype, but they typically lack immunoglobulin (Ig) gene expression. The PMBL cell line MedB-1 shares many characteristic properties of the primary tumour, including low-level Ig production despite a functionally rearranged IgVH gene and absence of 'crippling' mutations. In this study, a search was undertaken for reasons for downregulated Ig expression. Similar levels of the B-cell-specific transcription factors BOB.1/OBF.1 and PU.1 were found in MedB-1 cells to those in the Ig-producing UM-1 lymphoblastoid cell line. However, MedB-1 lacked the Oct2 transcription factor. Reporter assays showed that Ig-type promoters were active in MedB-1 cells. In contrast, activity of the intronic heavy chain enhancer was dramatically reduced. Ectopic expression of Oct2 was able partially to restore enhancer activity but transcription from the endogenous IgVH gene could not be rescued. Therefore, the role of epigenetic factors in the downregulation of Ig was investigated. Methylated histone 3 lysine 9, a reliable marker of chromatin silencing, was not detected in MedB-1 promoter and enhancer regions. Inhibition of DNA methyltransferase and of histone deacetylases also did not reactivate Ig production. These data suggest the existence of alternative mechanisms of Ig inhibition in MedB-1 cells, different from chromatin silencing and the lack of Oct2.

B-cell lymphomas with features intermediate between distinct pathologic entities. From pathogenesis to pathology

“…An additional consistent feature of this tumor is its lack of IG expression, despite the presence of functionally rearranged IGV H genes. Such defect is not caused by deleterious mutations like in HL but by different molecular mechanisms, one of which consisting of down-regulation of IG internal enhancer activity [24]. In gene expression profiling studies, a PMLBCL molecular signature distinct from that of DLBCL, not otherwise specified (NOS) was delineated and is characterized by low levels of expression of multiple B-cell signaling components and coreceptors and high expression of cytokine pathway components, tumor necrosis factor family members, and extracellular matrix elements [25,26] (Table 1).…”

Primary Mediastinal DLBCL: Evolving Biologic Understanding and Therapeutic Strategies

“…Interestingly, a downregulation of the intronic heavy chain enhancer or post-transcriptional blockage may determine the discordant expression of components of the B-cell receptor (BCR) (ie, CD79a+, sIg-), typical of PMDLBCL [9,16,49].…”

High expression of several tyrosine kinases and activation of the PI3K/AKT pathway in mediastinal large B cell lymphoma reveals further similarities to Hodgkin lymphoma

“…1,2 However, despite the presence of functionally rearranged immunoglobulin V-genes they do not express any immunoglobulin detectable by immunohistochemistry (IHC), 3,4 and downregulation of the internal IgH enhancer activity likely contributes to this phenomenon. 5 Because of the expression of MAL and CD23 in most cases, a derivation from medullary thymic B cells, which express both proteins, is assumed. 6,7 Over the last years, several studies revealed similarities of MBL to the nodular sclerosis (ns) subtype of classical Hodgkin lymphoma (cHL).…”