A functional genetic screen identifies the Mediator complex as essential for SSX2-induced senescence

• Published in: Cell death & disease 2019-11, Vol.10 (11), p.841-12, Article 841
• Main Authors: Brückmann, Nadine H., Bennedsen, Sofie N., Duijf, Pascal H. G., Terp, Mikkel G., Thomassen, Mads, Larsen, Martin, Pedersen, Christina B., Kruse, Torben, Alcaraz, Nicolas, Ditzel, Henrik J., Gjerstorff, Morten F.
• Format: Article
• Language: English
• Subjects: 38/23; 42/41; 42/44; 631/67/395; 631/80/509; Antibodies; Biochemistry; Biomedical and Life Sciences; Cell Biology; Cell Culture; Cell Line, Tumor; Cell Nucleus - genetics; Cell proliferation; Cellular Senescence - genetics; Chromatin; DNA-directed RNA polymerase; Epirubicin; Gene Expression Regulation, Neoplastic; Genetic screening; Genetic transformation; Humans; Immunology; Life Sciences; Mediator Complex - genetics; Mediator Complex Subunit 1 - genetics; Melanoma; Melanoma - genetics; Melanoma - pathology; Neoplasm Proteins - genetics; Protein Kinases - genetics; Raf protein; Repressor Proteins - genetics; RNA polymerase; Sarcoma, Synovial - genetics; Sarcoma, Synovial - pathology; Senescence; Transcription; Transcription Factors - genetics; Transcription, Genetic; Transcriptional Activation - genetics; Tumor cells; Tumorigenesis; Tumors
• ISSN: 2041-4889; 2041-4889
• DOI: 10.1038/s41419-019-2068-1

The senescence response to oncogenes is believed to be a barrier to oncogenic transformation in premalignant lesions, and describing the mechanisms by which tumor cells evade this response is important for early diagnosis and treatment. The male germ cell-associated protein SSX2 is ectopically expressed in many types of cancer and is functionally involved in regulating chromatin structure and supporting cell proliferation. Similar to many well-characterized oncogenes, SSX2 has the ability to induce senescence in cells. In this study, we performed a functional genetic screen to identify proteins implicated in SSX2-induced senescence and identified several subunits of the Mediator complex, which is central in regulating RNA polymerase-mediated transcription. Further experiments showed that reduced levels of MED1, MED4, and MED14 perturbed the development of senescence in SSX2 -expressing cells. In contrast, knockdown of MED1 did not prevent development of B-Raf- and Epirubicin-induced senescence, suggesting that Mediator may be specifically linked to the cellular functions of SSX2 that may lead to development of senescence or be central in a SSX2-specific senescence response. Indeed, immunostaining of melanoma tumors, which often express SSX proteins, exhibited altered levels of MED1 compared to benign nevi. Similarly, RNA-seq analysis suggested that MED1, MED4, and MED14 were downregulated in some tumors, while upregulated in others. In conclusion, our study reveals the Mediator complex as essential for SSX2-induced senescence and suggests that changes in Mediator activity could be instrumental for tumorigenesis.