High Expression of Micro-RNA BIC / miR155 in All Subtypes of Hodgkin Lymphoma.

We recently demonstrated that the noncoding BIC gene was highly expressed in Hodgkin Reed-Sternberg (HRS) cells of the vast majority of both classical and nodular lymphocyte predominant (NLP) Hodgkin lymphoma (HL). The BIC gene belongs to the family of noncoding mRNA-like molecules and was suggested to function through its noncoding RNA transcript. The human BIC gene shows a high degree of sequence identity to the Mouse and Gallus gallus BIC genes, which was most pronounced in a region containing a ~70 nucleotide stem-loop structure. Recently published data revealed expression of miR155 micro-RNA in murine tissue. The miR155 sequence was identical to the first 22 nucleotides of the ~70 nucleotide stem-loop structure of the BIC gene indicating that BIC is a primary micro-RNA transcript. Micro-RNAs (miRNA) are endogenous small RNAs (20–22 nucleotides) that inhibit protein translation by paring to the messages of protein-coding mRNAs. Based on recent data, it is clear that deregulation of miRNA expression, processing or stability may lead to deregulation of important cellular processes and thereby contribute to the development of cancer.

In this study we analyzed the expression of miR155 in HL and NHL samples and studied the regulation of BIC/miR155 expression. Northern blot analysis revealed that HL cell lines, which all have a high level of BIC transcripts, also have high levels of the functionally mature miR155. NHL samples with low levels of BIC transcripts also have low levels of miR155. These data provide experimental support for processing of primary BIC transcripts to the functionally mature miR155 fragments and indicate that miR155 expression is specific for HL. To analyze whether BIC expression could be induced in NHL cell lines we studied several stimuli in the wild type Burkitt lymphoma derived cell line Ramos and a NF-κB deficient variant cell line. Real-time RT-PCR analysis revealed a high level of BIC and of the 70 nucleotide miR155 precursor in anti-IgM stimulated Ramos cells. Anti-IgM stimulation of the NF-κB deficient Ramos cell line revealed only low levels of BIC and pre-miR155. These data demonstrate that transactivation of NF-κB is required for continuous upregulation of BIC in Ramos cells. Stimulation of Ramos cells with PMA/calcium-ionophore, which directly activates PKC, significantly upregulated the level of BIC and pre-miR155. This suggests that activation of PKC and its downstream signaling components, including NF-κB are involved in the regulation of BIC expression. Thus, whereas HRS cells typically do not express surface immunoglobulin, upregulation of BIC in these cells may be induced by constitutively activated NF-κB.