Activation of the Xmrk proto-oncogene of Xiphophorus by overexpression and mutational alterations.

Xmrk is a receptor tyrosine kinase closely related to the human EGF receptor. In the teleost fish Xiphophorus two versions of the Xmrk gene exist, an oncogene (ONC) and a proto-oncogene (INV). While ONC-Xmrk is the melanoma-inducing gene, INV-Xmrk appears not to be involved in transformation of pigment cells. To elucidate the mechanism that converts the proto-oncogene into a transforming oncogene a comparative analysis of the structure, expression and function of both versions of the gene was performed. In contrast to ONC-Xmrk which is expressed at high levels in melanoma cells, the proto-oncogene INV-Xmrk is ubiquitously expressed at very low levels indicating overexpression as one possible reason for tumorigenicity by ONC-Xmrk. As sequence comparison of the proto-oncogene and the oncogene revealed a number of amino acid changes, a possible effect of these mutations on the activation of the ONC-Xmrk receptor was determined. A constitutive activation of the oncogenic receptor was found and ectopic expression of INV-Xmrk after microinjection into medakafish embryos did not lead to the high tumour rate in transgenic fish as observed for the oncogene. Our data therefore suggest that overexpression of the receptor alone is not sufficient for melanoma induction, but that in addition activating mutations in ONC-Xmrk are responsible for its full tumorigenic potential.