Mir-21 Is Over Expressed in Aggressive SMZL

Within the group of indolent B cells lymphomas derived from cells of the marginal zone, the pathogenesis of Splenic Marginal Zone Lymphoma (SMZL) remains incompletely characterized, in contrast to MALT lymphoma. However numerous findings recently contributed to a better description of this lymphoma, including its possible association with hepatitis C and the recurrence of 7q deletion. Furthermore, the peculiar pattern of Ig genes mutations and the biased usage of some segments (IGHV1-2) is suggestive of a T-independent Ag driven proliferation, at least at initial steps. The clinical heterogeneity is also marked with a lack of reproducible prognostic factors able to characterize the cases with a more aggressive course or histological transformation. MicroRNA (miRNA) are small non-coding RNA that have a post transcriptional regulation role by inhibiting translation of mRNA in protein and their abnormal expression was already found to contribute to the oncogenesis of various leukemias and lymphomas. The aim of the study was then to identify a miRNA profile of SMZL and to assess if specific miRNAs were associated with biological or clinical characteristics. MiRNA expression profile of SMZL was obtained by quantitative RT-PCR technology (Taqman microRNA Assays Human Panel, Applied Biosystems) which enabled to analyze 365 miRNAs. Six frozen spleen specimens of SMZL (including 2 cases with histological transformation) and 5 frozen specimens of non tumoral spleen (traumatic) were used to compare miRNA levels of expression (2^−□□Ct method). Out of these 365 miRNAs, 95 were found to be reproducibly detectable in those samples and were further analyzed. Three miRNA were uniformly overexpressed in SMZL samples as compared to non tumor samples: miR-155 (mean fold change = 3.1), miR-451 (mean = 2.37), miR-486 (mean = 2.7). Conversly, 4 miRNAs were uniformly underexpressed: miR-127 (mean = 0.32), miR-139 (mean = 0.32), miR-335 (mean = 0.26), miR-411 (mean = 0.25). Interestingly, 1 miRNA, miR-21 was specifically found overexpressed only in the 2 transformed cases of SMZL (mean = 3.49) in comparison with the 4 non transformed cases (mean = 0.94). Those results were confirmed in a larger cohort of SMZL patients using a simplex real time PCR (Taqman microRNA Assays Human, Applied Biosystems) on frozen or paraffin embedded samples with a specific overexpression of miR-21 in 10 transformed cases (mean = 14.4), absent in the 6 non transformed cases (mean = 0.9) (Mann Whitney test: p =.0022). MiR-155 plays a key role in B cell differentiation and was already found overexpressed in other lymphomas subtypes. Little is know about the other miR overexpressed except for MiR- 21. MiR-21 is commonly overexpressed in solid tumors and the level of its expression is correlated in tumor growth and metastatic dissemination. It was also found overexpressed in chronic lymphocytic leukaemia and DLBCL (diffuse large B cell lymphoma). In opposite to DLBCL, our results seem to link miR-21’s overexpression with aggressive forms of SMZL. MiR-21 regulates multiple genes associated with apoptosis, migration and invasiveness. Altogether, these data points towards a specific miRNA expression pattern in SMZL and suggests that miR-21 could regulate important oncogenic pathways in transformed SMZL lymphomas.