The expression of MITF, a member of the MiT family of transcription factors and a master regulator of melanocytic differentiation, is often seen in clear cell sarcoma (CCS). However, neither the mechanism of MITF regulation in CCS nor its function were known. Now, David Fisher and colleagues show that the CCS fusion protein Ewing sarcoma breakpoint region 1 (EWS)–activating transcription factor 1 (ATF1) is oncogenic because it regulates MITF expression.

CCS is a soft-tissue malignancy that is molecularly defined by the EWS–ATF1 translocation. Despite being a sarcoma, CCS contains premelanosomes, expresses markers of melanocytic differentiation and, in some cases, produces melanin.

Fisher and colleagues investigated three human CCS lines that express the melanocyte-specific isoform of MITF (M-MITF). Immunoprecipitation using an EWS-selective antibody revealed that EWS–ATF1 occupied the M-MITF promoter in all three cell lines. So, does EWS–ATF1 regulate transcription of M-MITF? A reporter gene that directs luciferase expression from the M-MITF promoter was constitutively active in all three CCS cell lines. Furthermore, expression of dominant-negative EWS–ATF1 in CCS cells resulted in the selective inhibition of endogenous M-MITF expression. So, EWS–ATF1 is a necessary transactivator of the M-MITF promoter in CCS.

But what is the role of EWS–ATF1-mediated M-MITF expression in CCS? Dominant-negative inhibition of EWS–ATF1 significantly reduced the expression of the M-MITF-target genes, PMEL17 (which encodes the HMB45 antigen used for the pathological identification of CCS) and MLANA (a melanocyte marker). Pigmentation was also diminished in EWS–ATF1-inhibited CCS cells, and this could be rescued by M-MITF expression. Together, these findings indicate that EWS–ATF1 functions through M-MITF to mediate the melanocytic differentiation that is characteristic of CCS. Moreover, M-MITF activity was demonstrated to be necessary for CCS cell survival and proliferation: the inhibition of either EWS–ATF1 or M-MITF abrogated CCS colony formation. Interestingly, the expression of MiT-family members TFEB or TFE3 — oncoproteins that are translocated in alveolar soft-part sarcomas and some papillary renal-cell carcinomas — rescued cell viability in M-MITF-inhibited CCS cells in a dose-dependent manner. This indicates that MiT family members can functionally replace each other.

A xenograft model was used to study CCS tumour growth in vivo. RNA inhibition of EWS–ATF1 potently inhibited in vivo tumour growth, which could be rescued by co-expression of M-MITF. The authors conclude that in CCS, the EWS–ATF1 fusion protein functions by targeting M-MITF to promote tumour cell survival and proliferation.

They propose that CCS could be grouped with melanoma, paediatric renal-cell carcinoma and alveolar soft-part sarcoma to form a family of MiT-associated human cancers, each showing distinct oncogenic deregulation of MiT-family genes. These 'MiT tumours' are clinically and morphologically distinct malignancies that would not otherwise be co-classified, but share characteristics of particular resistance to traditional chemotherapies and radiation therapy. The recognition of a common oncogene family in these seemingly diverse malignancies might aid the discovery of improved therapeutic approaches.