Interleukin 4-induced gene 1 is activated in primary mediastinal large B-cell lymphoma

The molecular markers that distinguish primary mediastinal large B-cell lymphoma (PMBL) from nonmediastinal diffuse large B-cell lymphomas (NM-DLBLs) remain to be identified. Using cDNA representational difference analysis to compare PMBL and NM-DLBL transcripts, we isolated a cDNA fragment homologous to the mouse B-cell interleukin 4 (IL-4)-inducible gene FIG1 (interleukin 4-induced gene 1) transcript. The human FIG1 mRNA encodes a 567 amino acid protein that comprises a signal peptide and a large flavin-binding amino oxidase domain, and shares significant homology with secreted apoptosis-inducing L-amino acid oxidases. Northern blot studies showed that FIG1 mRNA expression is mainly restricted to lymphoid tissues. It is expressed at low levels in thymus, spleen, tonsils, and reactive lymph nodes, and is highly up-regulated in IL-4+CD40-activated tonsillar B cells. Interestingly, in human B-cell lines, FIG1 mRNA expression appeared restricted to the PMBL-derived MedB-1 and Karpas 1106 cell lines. Using real-time reverse transcriptase-polymerase chain reaction (RT-PCR), we demonstrated that all but one PMBL (16/17) displayed high FIG1 mRNA levels, whereas most NM-DLBLs (12/18) and all low-grade B-cell lymphomas tested (8/8) exhibited low FIG1 mRNA levels. The difference between PMBLs and NM-DLBLs was statistically significant (Fisher test; P =.0003). Southern blot studies did not show rearrangement of the FIG1 gene. FIG1 gene expression might be due to a constitutive activation of a cytokine signaling pathway in PMBL.