Chromosomal translocations resulting in the expression of chimaeric
transcription factors are frequently observed in tumour cells, and have been suggested to be a common mechanism in human
carcinogenesis.
Ewing sarcoma and related peripheral primitive neuroectodermal tumours share recurrent translocations that fuse the gene EWSR1 (formerly EWS) from 22q-12 to FLI1 and genes encoding other
ETS transcription factors (which bind
DNA through the conserved ETS domain). It has been shown that transduction of the gene EWSR1-FLI1 (encoding EWS-FLI1
protein) can transform NIH3T3 cells, and that mutants containing a deletion in either the EWS domain or the
DNA-binding domain in FLI1 lose this ability. This indicates that the
EWS-FLI1 fusion protein may act as an aberrant
transcription factor, but the exact mechanism of
oncogenesis remains unknown. Because
ETS transcription factors regulate expression of
TGFBR2 (encoding the
TGF-beta type II receptor,
TGF-beta RII; Refs 9,14), a putative tumour suppressor gene, we hypothesized that
TGFBR2 may be a target of the
EWS-FLI1 fusion protein. We show here that
Ewing sarcoma [corrected] (ES) cell lines with the EWSR1-FLI1 fusion have reduced
TGF-beta sensitivity, and that fusion-positive ES cells and primary tumours both express low or undetectable levels of
TGFBR2 mRNA and
protein product. Co-transfection of FLI1 and the
TGFBR2 promoter induces promoter activity, whereas EWSR1-FLI1 leads to suppression of
TGFBR2 promoter activity and FLI1-induced promoter activity. Introduction of EWSR1-FLI1 into cells lacking the EWSR1-FLI1 fusion suppresses
TGF-beta RII expression, whereas antisense to EWSR1-FLI1 in ES cell lines positive for this gene fusion restores
TGF-beta RII expression. Furthermore, introduction of normal
TGF-beta RII into ES cell lines restores
TGF-beta sensitivity and blocks tumorigenicity. Our results implicate
TGF-beta RII as a direct target of
EWS-FLI1.