Ligand-dependent tumor induction in medakafish embryos by a Xmrk receptor tyrosine kinase transgene.

Xmrk encodes a subclass I receptor tyrosine kinase (RTK) which has been cloned from the melanoma-inducing locus Tu of the poeciliid fish Xiphophorus. To demonstrate a high oncogenic potential in vivo we transferred the gene into early embryos of the closely related medakafish. Ectopic expression of the Xmrk oncogene under the control of a strong, constitutive promoter (CMVTk) led to the induction of embryonic tumors with high incidence, after short latency periods, and with a specific pattern of affected tissues. We demonstrate ligand-dependent transformation in vivo using a chimeric receptor consisting of the extracellular and transmembrane domains of the human EGF receptor (HER) and the cytoplasmatic domain of Xmrk. Expression of the chimeric receptor alone does not lead to kinase activation or induction of tumors. Coexpression of the chimera with its corresponding ligand, human transforming growth factor alpha (hTGF alpha), however, results in the activation of the chimeric RTK. In injected fish embryos the induction of the neoplastic growth is observed with similar incidence and tissue distribution as in embryos carrying the native Xmrk oncogene suggesting that the ligand as well as factors downstream of the RTK are required for tumor formation. In this study we show single-step induction of tumors by ectopic expression of RTKs in vivo substantiating the significance of autocrine stimulation in RTK induced tumors in vertebrates.