The PAX3-FKHR Fusion Protein Created by the t(2;13) Translocation in Alveolar Rhabdomyosarcomas Is A More Potent Transcriptional Activator than PAX3

Abstract

Tumor-specific translocations that result in altered expression of cellular genes occur frequently enough to be considered a general mechanism of oncogenesis (reviewed in references 18, 28, and 53). Chromosomal translocations may lead to oncogenesis via insertional activation in which one gene is placed under the control of an active expression element of another gene, typically an immunoglobulin or T-cell receptor locus, and thus inappropriately expressed. Translocation-mediated oncogenesis may also involve the recombination of portions of two genes to produce a chimeric gene encoding a fusion protein with aberrant functional properties. Several examples of tumor-specific translocations which generate chimeric transcription factors with altered regulatory function have been identified, and their oncogenic potentials have been demonstrated. It appears that functional modules from each factor collaborate in the fusion, e.g., a DNA-binding domain is fused to a transcriptional regulation or protein association domain, and the gain of the function results in neoplastic transformation.Many of the examples of translocation-generated fusion proteins have been found in the hematopoietic system (reviewed in references 28 and 66). A prototype is the BCR-ABL fusion associated with the t(9;22) Philadelphia chromosome of chronic myelocytic leukemia (7,22). In acute lymphoblastic leukemia, there are two different translocations of chromosome 19 involving the E2A gene. A t(1;19) translocation fuses the E2A activation domain to the DNA-binding domain of the homeobox gene PBX-1, thus creating an activator of PBX-1-regulated target genes (50,62,84). The product of the fusion of E2A and HLF genes, associated with the t(17;19) translocation, exhibits altered DNA-binding properties attributed to a restricted protein interaction potential (46, 47). In acute promyelocytic leukemia, the t(15;17) translocation fuses the retinoic acid receptor to the putative transcription factor PML and results in a functionally altered receptor (23,36,49).Only a few examples of translocation-generated fusion proteins in human solid tumors have been characterized. The Ewing's sarcoma EWS-FLI1 fusion protein associated with the t(11;22) translocation has greater transactivating and transforming capacity than its wild-type counterpart FLI1 (1, 57). In myxoid liposarcomas, the t(12;16) translocation fuses the NH 2 terminus of the TLS (FUS) protein with the COOH-terminal DNA-binding and leucine zipper dimerization regions of the CHOP protein (20,67). In clear-cell sarcoma, the t(12;22) translocation joins the NH 2 -terminal EWS transactivation domain to the ATF1 DNA-binding and protein interaction domain (88). For all of the above examples, it is significant that the fusion gene and/or protein(s) potentially represents unique tumor-specific targets for therapy. A common goal is to determine how these chimeric transcription factors alter normal gene expression.Alveolar rhabdomyosarcomas are pediatric malignant tumors that characteristically exhibit a t(2;13)...

Cloning and characterization of AFX, the gene that fuses to MLL in acute leukemias with a t(X;11)(q13;q23)

“…Moreover, in accordance with the AFX gene, the FKHR gene contains also a QIYEWM peptide motif upstream of the breakpoint. The PAX3/FKHR fusion protein is a more potent transcriptional activator than the normal PAX3 protein (Fredericks et al, 1995). Although the transcriptional potential of MLL might be augmented by the forkhead protein of AFX in a similar fashion, this possiblity is in contrast to current beliefs which suggest that the various MLL rearrangements diminish or destroy the function of the MLL protein.…”

PAX-FKHR function as pangenes by simultaneously inducing and inhibiting myogenesis

“…Antibodies used were: HA (Covance, Richmond, CA, USA); PAX3 (Fredericks et al, 1995); MyoD, myogenin, cyclin D1, cyclin D3, Rb, p21 (BD Pharmingen, San Jose, CA, USA); Myf5, cyclin A, cyclin E (Santa Cruz, Santa Cruz, CA, USA); b-actin (Sigma, St Louis, MO, USA); MHC (Developmental Studies Hybridoma Bank, Iowa City, IA, USA). Immunofluorescence was carried out on cells grown on collagen-coated glass coverslips.…”

“…The consequence of these translocations is the expression of the PAX3-FKHR and PAX7-FKHR (collectively, PAX-FKHR) chimeric transcription factors containing the PAX DNA-binding domains and the potent FKHR transactivation domain. The functions of PAX-FKHR are modified compared to the wild-type PAX proteins due to changes in their abundance (Davis and Barr, 1997), transcriptional activity (Fredericks et al, 1995) and target gene recognition (Epstein et al, 1998). PAX-FKHR are therefore thought to contribute to the phenotype and malignancy of ARMS by aberrantly regulating PAX-specific target genes, signaling pathways and biological processes.…”

Rhabdomyosarcoma – working out the pathways

“…The majority of these structural rearrangements translocate the PAX3 gene at 2q35 to the FKHR gene at 13q14 (t(2;13)(q35;q14)); less frequently, a t(1;3)(q36;q14) translocation fuses PAX7 to FKHR. The PAX3-FKHR chimera is a more potent transcriptional activator than wild type pax3 (Fredericks et al, 1995), and presumably disrupts the normal regulation of target genes downstream of PAX3. Recently, it was reported that it is the PAX3 homeodomain recognition helix, and not the pairedbox DNA binding domain, that is required for transformation by the PAX3-FKHR chimera (Lam et al, 1999).…”