Summary MicroRNAs (miRNA, miR) are negative regulators of gene expression that play an important role in diverse biological processes such as development, cell growth, apoptosis and haematopoiesis, suggesting their association with cancer. Here we analysed the expression signatures of 157 miRNAs in 58 diffuse large B‐cell lymphoma (DLBCL), 46 follicular lymphoma (FL) and seven non‐neoplastic lymph nodes (LN). Comparison of the possible combinations of DLBCL‐, FL‐ and LN resulted in specific DLBCL‐ and FL‐signatures, which include miRNAs with previously published function in haematopoiesis (MIRN150 and MIRN155) or tumour development (MIRN210, MIRN10A, MIRN17‐5P and MIRN145). As compared to LN, some miRNAs are differentially regulated in both lymphoma types (MIRN155, MIRN210, MIRN106A, MIRN149 and MIRN139). Conversely, some miRNAs show lymphoma‐specific aberrant expression, such as MIRN9/9*, MIRN301, MIRN338 and MIRN213 in FL and MIRN150, MIRN17‐5P, MIRN145, MIRN328 and others in DLBCL. A classification tree was computed using four miRNAs (MIRN330, MIRN17‐5P, MIRN106a and MIRN210) to correctly identify 98% of all 111 cases that were analysed in this study. Finally, eight miRNAs were found to correlate with event‐free and overall survival in DLBCL including known tumour suppressors (MIRN21, MIRN127 and MIRN34a) and oncogenes (MIRN195 and MIRNLET7G).

A new and powerful tool for classification and understanding the biology of human lymphomas is microRNA (miRNA) expression profiling. MiRNAs comprise a class of approximately 1000 RNA species, which are thought to directly regulate the expression of ∼30% of the transcripts of the human genome on a posttranscriptional level. An important regulatory role of miRNAs has been implicated in biological processes such as cell proliferation and differentiation, fat metabolism, insulin secretion and hematopoiesis. Recent studies indicate that miRNAs are mechanistically involved in the development of various human malignancies and therefore represent a promising new class of biomarkers. We evaluated the miRNA expression profiles of 58 Diffuse Large B-Cell Lymphomas (DLBCL), 46 Follicular Lymphomas (FL) and 7 lymph nodes with chronic lymphadenitis. A quantitative PCR-based method was used to determine the expression levels of 157 miRNA species. The analysis was performed exclusively on FFPE tissues, which are traditionally inapt to molecular analysis. It was possible to clearly distinguish malignant and non-malignant tissues by miRNA expression patterns. We found 34 differentially expressed miRNAs by comparing DLBCL and lymph nodes, 25 miRNAs by comparing FL and lymph nodes and 55 by comparing DLBCL and FL. It was noticeable that the miRNA expression profiles of FL and lymph nodes are more similar compared to miRNA expression profiles of DLBCL. We were also interested to find out whether it is possible to classify the DLBCL cases in germinal center-like DLBCL (GCB-DLBCL) and Non-germinal center-like DLBCL (Non-GCB-DLBCL) based on miRNA expression patterns. We used the immunohistochemical classification, published by Hans et al., to identify both subgroups (Hans CP, Weisenburger DD et al. Confirmation of the molecular classification of diffuse large B-cell lymphoma by immunohistochemistry using a tissue microarray. Blood. 2004; 103:275–282). Comparing miRNA expression profiles of GCB and Non-GCB cases of DLBCL, 8 miRNAs were found to be differentially expressed using the partial least squares discriminant analysis (PLS-DA). However, using a cross-validation approach it was not possible to predict the GCB status on the basis of miRNA expression patterns. Similar to Landgraf et al. we found a reduced miR-150 expression level in DLBCL in comparison to other closely related germinal center malignancies (Landgraf P, Ruscu M et al. A mammalian microRNA expression atlas based on small RNA library sequencing. Cell. 2007; 129:1401–1414). To determine the function of miR-150, Ramos cells were transfected with anti-miR-150 and an analysis of putative target proteins was carried out. It was possible to show a regulation of transcription factor v-myb/c-myc, which plays an important role in the control of proliferation and differentiation of hematopoietic progenitor cells, by miR-150. Moreover, miRNA profiles of DLBCL from a randomized trail will be compared to survival data in the future.