Impact of Growth Factor Independence 1 in Human T-Cell Lymphomas; Pathogenic Potential Identified by Insertional Mutagenesis in a Murine T-Cell Lymphoma Model.

Abstract 5047 Introduction The transcriptional repressor and oncogene Growth factor independence 1 (Gfi1) has a major oncogenic potential and is aberrantly expressed in murine lymphomas and several human cancers. Gfi1 is a key regulator of stem cell quiescence and plays a significant role in T-cell development, lineage commitment, and influences development of maturate granulocytes and monocytes. The genomic locus on murine chromosome 5 encoding Gfi1 is a frequent integration locus activated in T-cell lymphomas induced by the SL3-3 Murine Leukemia Virus (MLV) as well as other MLVs, indicating that Gfi1 is essential in development of these tumors. In the SL3-3 induced T-cell lymphoma model, retroviral insertions in the Gfi1 3'UTR have been demonstrated to decouple microRNA-mediated posttranscriptional regulation of protein expression (Dabrowska et al, 2009) further supporting its role in lymphomagenesis. In human cancers, Gfi1 protein expression has been observed in HTLV-1 induced ATLL and SCLC but no knowledge exists on how Gfi1 contributes to initiating and maintaining human T-cell lymphomas. Methods Gfi1 gene and protein expression patterns were determined in precursor and mature human T-cell lymphomas by real time PCR and Western blot analysis. Furthermore, localization and expression patterns of the Gfi1 protein was determined in the human T-cell lymphomas by immunohistochemical staining with Gfi1 antibodies and compared to similar staining of MLV induced tumors. Results Our results demonstrated that Gfi1 mRNA and protein levels vary significantly among the human T-cell lymphomas, and do not always show a direct proportional pattern. Thus, Gfi1 mRNA expression can be relatively high without resulting in a corresponding high protein expression, suggesting that a microRNA mediated posttranscriptional regulation exists in some tumors but may be disrupted in others. Furthermore, an additional Gfi1 protein variant was identified in one of the T-cell lymphoma entities. Immunohistochemical staining demonstrated varying Gfi1 protein expression in both nucleus and cytoplasm in the T-cell lymphomas and different distributions of the protein within the tumor and tumor cells were observed among samples. Staining of normal human tonsil demonstrated Gfi1 protein to be localized in the cytoplasm. We hypothesise that regulation of Gfi1 may include shuttling between cytoplasm and nucleus and that lymphomagenesis enables unlimited nuclear access. Conclusion Our data shows that deregulated Gfi1 expression plays a major role in the development of MLV induced lymphomas and strongly indicates that retroviral insertional mutagenesis in murine models of human NHLs can be used to identify new genes involved in lymphomagenesis and, by use of functional assays, their impact on human lymphomas can be evaluated. Disclosures No relevant conflicts of interest to declare.