A NOVEL EFFECTOR DOMAIN FROM THE RNA-BINDING PROTEIN TLS OR EWS IS REQUIRED FOR ONCOGENIC TRANSFORMATION BY CHOP

In human myxoid liposarcoma, a chromosomal rearrangement leads to fusi on of the growth-arresting and DNA-damage inducible transcription fact or CHOP (GADD153) to a peptide fragment encoded by the TLS gene. We ha ve found that wild-type TLS and a closely related sarcoma-associated p rotein, EWS, are both abundant nuclear proteins that associate in vivo with products of RNA polymerase II transcription. This association le ads to the formation of a ternary complex with other heterogeneous RNA -binding proteins (hnRNPs), such as Al and C1/C2. An NIH-3T3-based tra nsformation assay was used to study the oncogenic role of the sarcoma- associated domain of these RNA-binding proteins. Transduction of the T LS-CHOP oncogene into cells by means of a retroviral expression vector leads to loss of contact inhibition, acquisition of the ability to gr ow as colonies in soft agar, and tumor formation in nude mice. Mutatio ns that interfere with the function of the leucine zipper dimerization domain or the adjacent basic region of CHOP abolish transformation. T he essential role of the TLS component was revealed by the inability o f truncated forms to fully transform cells. Domain swap between TLS- a nd EWS associated oncogenes demonstrated that the component contribute d by the RNA-binding proteins are functionally interchangeable, wherea s the transcription factor component specifies tumor phenotype. The sa rcoma-associated component of TLS and EWS contribute a strong transcri ptional activation domain to the fusion proteins; however, transformin g activity cannot be fully substituted by fusion of CHOP to other stro ng trans-activators. The juxtaposition of a novel effector domain from sarcoma-associated RNA-binding proteins to the targeting domain of tr anscription factors such as CHOP leads to the creation of a potent onc ogene.