The small FOXP1 isoform predominantly expressed in activated B cell-like diffuse large B-cell lymphoma and full-length FOXP1 exert similar oncogenic and transcriptional activity in human B cells.

The forkhead transcription factor FOXP1 is generally regarded as an oncogene in activated B cell-like diffuse large B-cell lymphoma. Previous studies have suggested that a small isoform of FOXP1 rather than full-length FOXP1, may possess this oncogenic activity. Corroborating those studies, we herein show that activated B cell-like diffuse large B-cell lymphoma cell lines and primary activated B cell-like diffuse large B-cell lymphoma cells predominantly express a small FOXP1 isoform, and that the 5'-end of the Foxp1 gene is a common insertion site in murine lymphomas in leukemia virus- and transposon-mediated insertional mutagenesis screens. By combined mass spectrometry, (quantative) reverse transcription polymerase chain reaction/sequencing, and small interfering ribonucleic acid-mediated gene silencing, we determined that the small FOXP1 isoform predominantly expressed in activated B cell-like diffuse large B-cell lymphoma lacks the N-terminal 100 amino acids of full-length FOXP1. Aberrant overexpression of this FOXP1 isoform (ΔN100) in primary human B cells revealed its oncogenic capacity; it repressed apoptosis and plasma cell differentiation. However, no difference in potency was found between this small FOXP1 isoform and full-length FOXP1. Furthermore, overexpression of full-length FOXP1 or this small FOXP1 isoform in primary B cells and diffuse large B-cell lymphoma cell lines resulted in similar gene regulation. Taken together, our data indicate that this small FOXP1 isoform and full-length FOXP1 have comparable oncogenic and transcriptional activity in human B cells, suggesting that aberrant expression or overexpression of FOXP1, irrespective of the specific isoform, contributes to lymphomagenesis. These novel insights further enhance the value of FOXP1 for the diagnostics, prognostics, and treatment of diffuse large B-cell lymphoma patients.

B-Lymphoblastic Lymphomas Evolving from Follicular Lymphomas Co-Express Surrogate Light Chains and Mutated Gamma Heavy Chains.

Follicular lymphoma (FL) is an indolent B-cell non-Hodgkin lymphoma able to transform into germinal center-type diffuse large B-cell lymphoma. We describe four extraordinary cases of FL, which progressed to TdT+CD20- precursor B-lymphoblastic lymphoma (B-LBL). Fluorescence in situ hybridization analysis showed that all four B-LBLs had acquired a MYC translocation on transformation. Comparative genomic hybridization analysis of one case demonstrated that in addition to 26 numerical aberrations that were shared between the FL and B-LBL, deletion of CDKN2A/B and 17q11, 14q32 amplification, and copy-neutral loss of heterozygosity of 9p were gained in the B-LBL cells. Whole-exome sequencing revealed mutations in FMN2, NEB, and SYNE1 and a nonsense mutation in KMT2D, all shared by the FL and B-LBL, and TNFRSF14, SMARCA2, CCND3 mutations uniquely present in the B-LBL. Remarkably, all four FL-B-LBL pairs expressed IgG. In two B-LBLs, evidence was obtained for ongoing rearrangement of IG light chain variable genes and expression of the surrogate light chain. IGHV mutation analysis showed that all FL-B-LBL pairs harbored identical or near-identical somatic mutations. From the somatic gene alterations found in the IG and non-IG genes, we conclude that the FLs and B-LBLs did not develop in parallel from early t(14;18)-positive IG-unmutated precursors, but that the B-LBLs developed from preexistent FL subclones that accumulated additional genetic damage.

Illegitimate WNT pathway activation by beta-catenin mutation or autocrine stimulation in T-cell malignancies.

Recent studies in mice have shown a role for the canonical WNT pathway in lymphocyte development. Because cancers often arise as a result of aberrant activation of signaling cascades that normally promote the self-renewal and expansion of their progenitor cells, we hypothesized that activation of the WNT pathway might contribute to the pathogenesis of lymphoproliferative disease. Therefore, we screened a large panel (n = 162) of non-Hodgkin lymphomas (NHL), including all major WHO categories, for nuclear expression of beta-catenin, a hallmark of "active" WNT signaling. In 16 lymphomas, mostly of T-lineage origin, nuclear localization of beta-catenin was detected. Interestingly, some of these tumors contained established gain-of-function mutations in the gene encoding beta-catenin (CTNNB1); however, in the majority, mutations in either CTNNB1 or APC were not detected. Functional analysis of WNT signaling in precursor T-lymphoblastic lymphomas/leukemias, the NHL subset in which beta-catenin accumulation was most prevalent (33% positive), revealed a constitutively activated, but still responsive, WNT pathway, which controlled T-cell factor-mediated gene transcription and cell growth. Our data indicate that activation of the WNT pathway, either by CTNNB1 mutation or autocrine stimulation, plays a role in the pathogenesis of a subset of NHLs, in particular, those of T-cell origin.

Strong and selective glomerular localization of CD134 ligand and TNF receptor-1 in proliferative lupus nephritis.

CD134 (OX40) is a member of the tumor necrosis factor (TNF) receptor (TNFR) family that can be expressed on activated T lymphocytes. Interaction between CD134 and its ligand (CD134L) is involved in costimulation of T and B lymphocyte activation, and in T cell adhesion to endothelium. To examine the possible role of this interaction in the pathogenesis of systemic lupus erythematosus (SLE), expression of CD134 and CD134L on peripheral blood leukocytes was studied, and no significant differences between SLE patients and control individuals were found. Immunohistology on renal biopsies from patients with lupus nephritis or other renal disorders, using a recombinant human CD134-containing chimeric molecule to detect CD134L, demonstrated the abundant presence of CD134L in all cases of proliferative lupus nephritis in a granular distribution predominantly along the epithelial side of the glomerular capillary wall. Confocal laser scanning microscopy indicated colocalization with subepithelial immune deposits. In none of the other renal disorders examined, including nonproliferative forms of lupus nephritis, was glomerular staining for CD134L detected in a similar pattern. Endothelial CD134L expression was frequently observed in different types of vasculitis. CD134 was detected on perivascular infiltrating leukocytes and on part of the tubular epithelium, but not on glomerular resident cells. Immunohistology for several other TNF(R) family members revealed in proliferative lupus nephritis a similar distribution for TNFR1 as was observed for CD134L. In contrast, glomerular expression of TNFR2 was similar in all cases examined. The glomerular presence of CD134L and TNFR1 in proliferative lupus nephritis in association with subepithelial immune deposits may be of pathogenetic significance and have diagnostic value.