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

Abstract

Xmrk is a receptor tyrosine kinase closely related to the human EGF receptor. In the teleost ®sh 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 protooncogene 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 eect of these mutations on the activation of the ONCXmrk receptor was determined. A constitutive activation of the oncogenic receptor was found and ectopic expression of INV-Xmrk after microinjection into medaka®sh embryos did not lead to the high tumour rate in transgenic ®sh as observed for the oncogene. Our data therefore suggest that overexpression of the receptor alone is not sucient for melanoma induction, but that in addition activating mutations in ONC-Xmrk are responsible for its full tumorigenic potential.

Comparative structure and characterization of a CDKN2 gene in a Xiphophorus fish melanoma model

“…Xmrk-2 has been shown to be highly expressed in melanized tissues, with maximal expression being observed in melanoma (Schartl, 1995;Wittbrodt et al, 1989;Woolcock et al, 1994) and is believed to act as a mediator of proliferation in melanotic cells (Dimitrijevic et al, 1998;Wellbrock et al, 1998). Here, we have analysed a similar melanoma model in which the melanomas are based on the Sp pigment pattern and have similarly detected gross Xmrk-2 overexpression in all melanized tissues including melanomas.…”

Role of Epidermal Growth Factor Receptor Signaling in RAS-Driven Melanoma

“…Our results suggest that these pathways are operative in melanocyte transformation. It is noteworthy that the contribution of EGFR signaling to melanoma is evolutionarily conserved, since activating mutations in the EGFR homologue, Xmrk, lead to melanoma susceptibility in Xiphophorus fish (17).…”

Ligand-independent Dimerization and Activation of the Oncogenic Xmrk Receptor by Two Mutations in the Extracellular Domain

“…However, the ectopic overexpression of INV-and ONC-Xmrk in embryos of transgenic fish showed that exclusively those fish expressing ONC-Xmrk were developing tumors with high incidence, short latency periods, and a specific pattern of affected tissues, whereas only a basal rate of tumor induction appeared in the case of INV-Xmrk-expressing fish, comparable to the rate obtained with the expression of another, nonactivated receptor (6,11). Besides that, INV-Xmrk was shown to be not phosphorylated when transiently expressed in human cells (HEK293), in contrast to the strong autophosphorylation of ONC-Xmrk.…”

“…2 Two copies of the Xmrk gene have been found. One of them, INV-Xmrk, is a gene invariably present in all fish (6). 3 It is ubiquitously expressed at low levels, and, although its physiological role is still unknown, it appears not to be involved in melanoma formation.…”