Characterization of IRF4 dependency in melanoma cell lines

Interferon Regulatory Factor 4 (IRF4, also known as MUM1, LSIRF, NFEM5, and ICSAT) is a transcription factor with important regulatory roles on B-cell matura-tion and differentiation. Although the first identification of IRF4 is as lymphocyte spe-cific IRF family member, its expression was later shown in melanocytes, with unknown functional roles, until its recently identified functions in pigmentation. Alongside its key functional roles in lymphocytes, IRF4 expression is shown to be associated with many lymphoid cancers. For example, multiple myeloma (MM) and activated B-cell like dif-fuse large B-cell lymphoma (ABC-DLBCL) cancer cells have high IRF4 expression lev-els and have non-oncogene addiction to IRF4, so that these cells die if IRF4 expression is knocked-down. Surprisingly, immunohistochemistry studies show remarkable IRF4 expression in melanoma samples compared to non-lymphoid cancer types. In addition, several genome wide association studies (GWAS) have also linked IRF4 polymorphisms to melanoma. In the light of these studies, we set out to identify the function of IRF4 in melanoma cell lines. First, by using publicly available gene expression databases we confirmed that IRF4 is highly expressed in melanomas, compared to the non-lymphoid cancers. We also validated that many melanoma cell lines we tested have remarkable endogenous IRF4 expression, while non-lymphoid cancer cell lines have no detectable expression. We then knocked-down IRF4 levels in melanoma cells by introducing short hairpin RNA (shRNA) using a lentiviral infection system and observed reduced com-petitive fitness upon IRF4 knock-down. Our results suggest that melanoma cells have non-oncogene addiction to IRF4 in vitro, similar to the observations in MM and ABC-DLBCL cells.