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

Abstract

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 wellcharacterized 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 SSX2expressing 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.

A new machine learning method for cancer mutation analysis

“…SSX proteins are expressed in multiple types of tumors, such as 40% of melanomas and up to 65% of breast cancers. The SSX family comprises nine similar members, most likely redundant in their cellular functions [26].…”

“…It has been shown that the SSX proteins are activated in several critical mitogenic pathways, such as MAPK and Wnt [26].…”

A new machine learning method for cancer mutation analysis

“…It has been shown that the SSX proteins are activated in several critical mitogenic pathways, such as MAPK and Wnt [41].…”

A natural variation-based screen in mouse cells reveals USF2 as a regulator of the DNA damage response and cellular senescence

“…However, given the complexity of the senescence program, many more regulators likely remain to be identified. Indeed, bioinformatic approaches have identified dozens of other transcription factor candidates (Chan et al 2022; Wang et al 2016; Xie et al 2014; Han et al 2018; Martínez-Zamudio et al 2020; Brückmann et al 2019; Tyler et al 2021; Zhang et al 2021), many of which remain unvalidated (but see (Wang et al 2016; Xie et al 2014; Han et al 2018; Martínez-Zamudio et al 2020) for recent discoveries of the roles of DLX2, FOXO3 and AP-1 in senescence).…”

“…However, given the complexity of the senescence program, many more regulators likely remain to be identified. Indeed, bioinformatic approaches have identified dozens of other transcription factor candidates in senescence (17)(18)(19)(20)(21)(22)(23)(24), many of which remain unvalidated (but see (18)(19)(20)(21) for recent discoveries of the roles of DLX2, FOXO3 and AP-1 in senescence).…”