Epithelioid sarcoma (ES) is a very rare (incidence ~0.02–0.05/100,000) mesenchymal tumor characterized by a prevalent epithelioid morphology [1]. It primarily affects adolescents and young adults, accounting for less than 1% of all sarcomas in adults and 4–8% of pediatric non-rhabdomyosarcomatous sarcomas [3,4]. ES is an aggressive tumor, with a high rate of recurrences (from 15% to 60% in different series) and 30–50% of the cases metastasizing at distal sites [6,7,8,9,10,11,12,13,15,16].  A molecular hallmark of ES is the loss of SMARCB1 (SWI/SNF-related matrix-associated actin-dependent regulator of chromatin subfamily B member 1) proteprotein expression.

SMARCB1 (SWI/SNF-related matrix-associated actin-dependent regulator of chromatin subfamily B member 1), a.k.a. INI1 (Integrase Interactor 1), is a subunit of the mammalian SWI/SNF (SWItch/Sucrose Non-Fermentable) ATP-dependent chromatin remodeling complexes, also known as BAF (BRG1/BRM-associated factor) complexes. These complexes are central regulators of nucleosome remodeling and are implicated in different biological processes, including cell cycle control and maintenance of genomic stability [40,41,42]. Mammalian SWI/SNF complexes are classified into three subgroups: canonical BAF (cBAF), polybromo-associated BAF (PBAF), and non-canonical BAF (ncBAF), also called GLTSCR1 or GLTSCR1L-containing and BRD9-containing (GBAF) complexes [36,37] (Figure 1). SMARCB1 participates only into cBAF and PBAF complexes [38,39]. Notably, SWI/SNF subunits are mutated in over 20% of all human cancers [42,43]. In particular, SMARCB1 inactivation is implicated in the pathobiology of ES, malignant rhabdoid tumors (MRT), atypical teratoid/rhabdoid tumors (AT/RT) of the central nervous system, malignant peripheral nerve sheath tumors (MPNST), myoepithelial neoplasms and renal medullary carcinomas (RMC) [19,44,45,46].

Over 90% of ES harbor biallelic SMARCB1 deletions or, more rarely, nonsense frameshift and splice site mutations [56,57,58]. In the ES samples lacking obvious SMARCB1 genetic defects, i.e. SMARCB1-intact ES, epigenetic mechanisms have been proposed to account for SMARCB1 functional inactivation. However, no promoter methylation was detected [61,62], and decitabine treatment failed to restore SMARCB1 expression in these tumors [62]. microRNAs have been also implicated in SMARCB1 silencing, namely miR-193a-5p, miR-206, miR-381, and miR-671-5p [63,64,65], although the actual capacity of these microRNAs to inhibit SMARCB1 in the context of ES cells was not confirmed by independent studies [62]. Therefore, the mechanism underlying the loss of SMARCB1 expression in SMARCB1-intact ES remains to be elucidated.

In SMARCB1-deleted ES cell models, such as the VA-ES-BJ cell line, restoration of SMARCB1 expression induces cell cycle arrest and impairs anchorage-independent growth and cell migration, substantiating its tumor suppressive role [69]. The mechanism of action of SMARCB1 as a tumor suppressor relies on the intersection with several pathways, including cell proliferation and survival  [71] (Figure 2).

SMARCB1 negatively regulates the expression of cyclin D1, E2F, and AURKA and, accordingly, SMARCB1-deleted tumors show up-regulation of these targets and cell cycle perturbation [68,72,73,74]. SMARCB1 was also demonstrated to directly bind MYC and to interfere with MYC-mediated transcriptional regulation [75,76]. Indeed, in SMARCB1-negative cell lines, SMARCB1 loss associates with enhanced MYC activity and increased DNA replication [77]. SMARCB1 also binds and potentiates p53 transactivation activity [79], and favors nucleotide excision repair by interacting with several components of this machinery, including BRCA1, BARD1 and XPC [80,81]. This suggests that SMARCB1 loss may result in impaired control over genome stability. Furthermore, SMARCB1 plays a role in Wnt/β-catenin and sonic hedgehog (SHH) signaling pathways [82,83]. In particular, SMARCB1 has been reported to bind GLI1, a SHH effector, thereby inhibiting the SHH signaling [82]. Finally, a role for SMARCB1 in regulating the expression of IL6 has been recently proposed [84].

Notably, SMARCB1 was documented to repress EZH2 and, accordingly, high levels of EZH2 have been shown in SMARCB1-deficient tumors, including ES [61], AT/RT [85], MRT [86] and chordomas [87]. EZH2 is a core component of PRC2 complexes (Polycomb Repressive Complex 2) and catalyzes the methylation of histone H3 on lysine 27 (H3K27) mediating the transcriptional repression of target genes [86]. PRC2 complexes are functional antagonists of the SWI/SNF complexes, and the balance between the two is key for cellular homeostasis [35,86,88,89] (Figure 3).

Concomitant inactivation of EZH2 and SMARCB1 is synthetic lethal. In vitro and in vivo experiments demonstrated that EZH2 silencing in SMARCB1-deficient MRT cells significantly impaired cell proliferation and triggered cell senescence, and prevented the formation of tumors driven by SMARCB1 loss in mouse models [86]. Also the pharmacological inhibition of EZH2 induced strong anti-proliferative effects in SMARCB1-deleted cells and determined a complete regression of xenografts in mouse [90,91]. Based on these findings, EZH2-inhibitors have been proposed as a potential therapeutic strategy in SMARCB1-deficient tumors. In 2013, an open-label, multicenter, phase I/II trial demonstrated that the EZH2 inhibitor tazemetostat showed antitumor activity in ES [92]. The study was followed by a phase II, multicenter study (NCT02601950) in 2015: of the 62 ES patients enrolled in this trial, 15% (9/62) showed durable objective response after 32 weeks of treatment, and 21% (13/62) remained progression-free at 1 year [58]. These encouraging results led to the accelerated approval by the USA Food and Drug Administration (FDA) of tazemetostat for the treatment of adults and adolescents over 16 years of age with locally advanced or metastatic ES not eligible to complete surgical resection. Based on these promising results, other PRC2 components are currently considered for therapeutic targeting.