Gene Expression Profiling of Mantle Cell Lymphoma in the Leukemic Phase Reveals Aberrant Expression of Genes from the TGF-β Signaling Pathway.

Mantle cell lymphoma (MCL) is a distinctive subtype of B-cell lymphoma associated with the t(11;14)(q13;32) and consequent ectopic overexpression of cyclin D1 in the tumor cells. Disease is predominantly disseminated at diagnosis and a frank leukemic phase is detected in one fourth of patients. Ontogenetically, MCL is considered the malignant counterpart of pre-germinal-center naive B-cells. Although the overexpression of cyclin D1 plays a pivotal role on the pathogenesis of MCL, studies with transgenic mice have shown that it is not sufficient by itself to cause lymphoma, and a better understanding of the molecular genetics of this disease may provide insights toward a potentially curable therapy. To address this issue, we compared the gene expression profile of MCL and normal naive B-cells using oligonucleotide microarrays representing 10,000 genes. MCL cells and naive B-cells (IgD+CD38±CD27−) were highly purified, by magnetic activated cell sorting, from the peripheral blood of patients with MCL in the leukemic phase and from tonsils of normal controls, respectively (purity > 95% in all samples). Three individuals were selected for each group and experiments were performed in replicates using the Amersham CodeLink Human UniSet I Bioarrays. For validation purposes, the expression of 10 selected genes (6 overexpressed and 4 underexpressed in lymphoma cells) was quantified by TaqMan real-time RT-PCR in non-purified peripheral blood samples from 25 patients with MCL in the leukemic phase and compared with normal naive B-cells, with fully concordant results. Data mining from our microarray results revealed an aberrant expression of several genes from the TGF-β signaling pathway in MCL (p<0.01): ACVR1 (fold change = 2.5), ACVR2 (2.9), ACVR2B (16.3), BMP4 (11.8), TGIF (4.0), Smad2 (3.4) and Smad6 (0.6). Except for TGIF and Smad6, all other genes induce the TGF-β signaling pathway. Although TGIF was overexpressed, it depends on the relative levels of Smad co-repressors or co-activators to exert its inhibitory activity; whereas Smad6, which is also an inhibitory mediator, was underexpressed. The activin receptors ACVR1, ACVR2 and ACVR2B are receptors of the TGF-β superfamily, which consists of TGF-β, activins, bone morphogenic proteins (BMPs) and others. Upon ligand binding, activin receptors induce anti-proliferative and pro-apoptotic responses, acting as tumor suppressors in early tumorigenesis. In advanced cancer, however, there is a loss of growth-inhibitory responsiveness downstream the core TGF-β signaling pathway, and it may be used as a tumor-progression factor by inducing immune supression, angiogenesis, epithelial-mesenchymal transdifferentiation and increased potential for metastasis. Interestingly, the cyclin D1/TGF-β double transgenic liver model in mice (