Extranodal marginal zone lymphoma (MZL) of mucosa-associated lymphoid tissue (MALT) and nodal MZL are recognized as distinct entities in the World Health Organization classification of lymphoid tumors. So far, genetic events underlying pathogenesis of both malignancies are incompletely understood. It has been shown, however, that approximately 25% of MALT lymphoma cases are hallmarked by recurrent chromosomal translocations including t(11;18)(q21:q21) leading to an API2 -MALT1 fusion and IGH-mediated t(1;14)(p22;q32), t(14;18)(q32;q21) and t(3;14)(p14;q32) targeting the BCL10, MALT1 and FOXP1 genes, respectively. Notably, the BCL10 and MALT1 proteins play a physiological role in antigen receptor mediated activation of the nuclear factor-κB (NF-κB) signaling pathway, known to be implicated in tumor growth, survival and chemoresistance.
We report here a novel IGH-associated translocation t(X;14)(p11.4;q32.33), that was identified in 2 cases of MALT lymphoma and single cases of nodal MZL and gastric diffuse large B-cell lymphoma (DLBCL). Of note, all 4 patients had an underlying disorder, including Sjögren's syndrome (2 patients with MALT lymphoma), a leukocytoclastic vasculitis and polyneuropathy (MZL) and HP-negative chronic gastritis with intestinal metaplasia (DLBCL). There were 3 female patients and 1 male patient in age ranging from 66 to 82 years (median 75 years). All patients received an initial therapy. Two patients are alive after 42–54 months from diagnosis and 2 died, either from progressive lymphoma, or from unrelated cause (other tumor). In each patient, t(X;14) was identified at the time of lymphoma diagnosis. The translocation occurred as the sole abnormality in 1 case and was accompanied by 2 to 4 additional chromosomal abnormalities in other cases. Mapping of the Xp11.4 breakpoint was performed by fluorescence in situ hybridization (FISH) using tilepath BAC clones. The breakpoint was eventually located in the region of CASK that hosts GPR34 and GPR82, two genes encoding G-protein coupled receptors. Expression of 5 candidate target genes was tested by qRT-PCR. This analysis showed that only 1 of the analyzed genes, GPR34, was upregulated 11–100 fold in 3 studied cases. An aberrant expression of GPR34 protein in these tumors has been demonstrated by immunohistochemistry. GPR34 belongs to the largest family of cell surface molecules involved in signal transmission that play important roles in many physiological and pathological processes, including tumorigenesis.
As constitutive NF-κB activity is frequently associated with B-cell lymphoma development, we examined whether the NF-κB pathway was active in 3 lymphomas with t(X;14) from which material was available. In all of them Western blot analysis showed phosphorylation of the NF-κB inhibitor protein Iκ-Bα, which indicates its degradation and is a hallmark of activation of NF-κB pathway. Jurkat T cells stimulated with PMA/Ionomycin served as a positive control. Quantitative RT-PCR further confirmed upregulation of a number of NF-κB target genes in t(X;14) lymphoma samples, with transcript levels comparable to a t(11;18)-positive MALT lymphoma. To examine the role for GPR34 in NF-κB activation, we overexpressed GPR34 in the B-cell lymphoma cell line BJAB, in which the NF-κB pathway is relatively quiescent. Although GPR34 transcript levels increased 40 to 800 fold, the set of NF-κB target genes was not upregulated, in contrast to its induced upregulation by overexpression of API2-MALT1 or mutant CARD11 (L232LI), known activators of NF-κB signaling in B-cells. These data suggest that activation of NF-κB signaling in MALT lymphomas with t(X;14) is not mediated by GPR34 overexpression and most likely involves other mechanisms.
In conclusion, we identified GPR34 as a new player of B-NHL pathogenesis. The gene is recurrently targeted by the IGH-mediated t(X;14)(p11.4;q32.33) associated with MZ/MALT lymphoma evolving from a previous auto-immune disorder. The functional consequences of t(X;14) remain elusive, but our data indicate that upregulated GPR34 does not activate NF-κB. Although studies are required to determine GPR34 natural ligand(s) and signal pathways, this protein is definitely a very promising novel target for treatment of lymphoma.
No relevant conflicts of interest to declare.
Asterisk with author names denotes non-ASH members.