Abstract
Translocations or mutations of FUS, EWSR1, and TAF15 (FET) result in distinct genetic diseases. N-terminal translocations of any FET protein to a series of transcription factors yields chimeric proteins that contribute to sarcomagenesis, whereas mutations in the conserved COOH-terminal domain of wild-type FUS were recently shown to cause familial amyotrophic lateral sclerosis. We thus investigated whether the loss of one FUS allele by translocation in liposarcoma may be followed by mutations in either the remaining FUS allele or the paralogous EWSR1. Furthermore, we investigated the strength of the FET promoters and their contributions to sarcomagenesis given the proteins' frequent involvement in oncogenic translocations. We sequenced the respective genomic regions of both FUS and EWSR1 in 96 liposarcoma samples. Additionally, we determined FET transcript and protein levels in several liposarcoma cell lines. We did not observe sequence variations in either FUS or EWSR1. However, protein copy numbers reached an impressive 0.9 and 5.5 Mio of FUS and EWSR1 per tumor cell, respectively. Compared with adipose-derived stem cells, FUS and EWSR1 protein expression levels were elevated on average 28.6-fold and 7.3-fold, respectively. TAF15 mRNA levels were elevated on average 3.9-fold, although with a larger variation between samples. Interestingly, elevated TAF15 mRNA levels did not translate to strongly elevated protein levels, consistent with its infrequent occurrence as translocation partner in tumors. These results suggest that the powerful promoters of FET genes are predominantly responsible for the oncogenic effect of transcription factor translocations in sarcomas.