SSX2 is a novel DNA-binding protein that antagonizes polycomb group body formation and gene repression

Abstract

Polycomb group (PcG) complexes regulate cellular identity through epigenetic programming of chromatin. Here, we show that SSX2, a germline-specific protein ectopically expressed in melanoma and other types of human cancers, is a chromatin-associated protein that antagonizes BMI1 and EZH2 PcG body formation and derepresses PcG target genes. SSX2 further negatively regulates the level of the PcG-associated histone mark H3K27me3 in melanoma cells, and there is a clear inverse correlation between SSX2/3 expression and H3K27me3 in spermatogenesis. However, SSX2 does not affect the overall composition and stability of PcG complexes, and there is no direct concordance between SSX2 and BMI1/H3K27me3 presence at regulated genes. This suggests that SSX2 antagonizes PcG function through an indirect mechanism, such as modulation of chromatin structure. SSX2 binds double-stranded DNA in a sequence non-specific manner in agreement with the observed widespread association with chromatin. Our results implicate SSX2 in regulation of chromatin structure and function.

Interaction between Polycomb and SSX Proteins in Pericentromeric Heterochromatin Function and Its Implication in Cancer

“…SSX proteins contain a short alpha-helical structure similar to the one that mediates binding to the major groove of DNA in C2H2 zinc finger motifs (C2H2-ZNF) [84]. This is in agreement with the DNA-binding properties of SSX2 [85] and the importance of the SSXRD for the chromatin-association of SSX2 [74]. Thus, the composition of SSX proteins resembles that of the ∼400 known KRAB zinc finger proteins [86] and suggesst that SSX proteins may retain a similar modular function, where the Zinc finger domain mediates DNA binding and the KRAB domain mediates recruitment of repressive chromatin modulating factors (Figure 2).…”

“…In several types of malignant cells, the localization of SSX and SS18-SSX overlaps with a component of the PRC1 complex [74,85,97,98], including BMI, EZH1, RING1A and RING1B. Colocalization with the PRC2 factor EZH2 has also been reported [85]. As mentioned, PcG proteins catalyze the histone modifications H3K27me3 and H2AK119ub, which are instrumental for their repressive function.…”

“…A functional link between SSX molecules and PcG proteins has been proposed in multiple studies. In several types of malignant cells, the localization of SSX and SS18-SSX overlaps with a component of the PRC1 complex [74,85,97,98], including BMI, EZH1, RING1A and RING1B. Colocalization with the PRC2 factor EZH2 has also been reported [85].…”

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

“…The latter is supported by our recent data demonstrating that SSX proteins bind and rearrange chromatin in cancer cells, and it is likely that the effect of the SSX2 protein on chromatin structure and gene expression is what initiates SSX2mediated senescence. Importantly, SSX proteins seem to play an antagonistic role in Polycomb group (PcG) gene repression, suggesting that ectopic expression of SSX molecules may release PcG-repressed genes 17 and facilitate binding of RNA polymerase II preinitiation complex (PIC) to change the transcription landscape. In agreement with this, PcG complexes have been demonstrated to directly inhibit the assembly of the PIC 50 .…”

“…The role of SSX proteins in supporting cell proliferation and their ability to induce senescence suggest that these CT antigens may have oncogenic potential. SSX proteins are DNA-binding factors that regulate chromatin structure and Polycomb function 14,15,17,18 . They contain an SSX repression (SSXRD) domain and a Kruppel-associated box (KRAB), which seem to be important for chromatin-binding and -rearrangement, respectively 14 .…”