Activating mutations in the extracellular domain of the melanoma inducing receptor Xmrk are tumorigenic in vivo

Abstract

Mutated versions or overexpression of receptor tyrosine kinases such as the epidermal growth factor receptor are found frequently in various cancers. In Xiphophorus the formation of hereditary melanoma is caused by the overexpression of the Xmrk oncogene locus. Xmrk is a mutationally altered version of the epidermal growth factor receptor. Two amino acid changes in the extracellular domain of the receptor were shown in vitro to be responsible for a constitutive, ligand-independent activity of Xmrk. To analyze whether these two mutations are indeed responsible for the in vivo oncogenic activity of the receptor, both were independently introduced into the wild-type, non-oncogenic Xiphophorus EGF-receptor and tested in Medaka embryos for their tumorigenic capacity. Both mutations were sufficient to induce tumors after short latency periods and at a comparable frequency as the native Xmrk oncogene. The G359R mutation led to a significantly higher tumor rate than the C578S mutation. Our study shows that subtle point mutations of the EGF-receptor can lead to a highly tumorigenic oncoprotein.

Evolution of Xmrk: an oncogene, but also a speciation gene?

“…(26) In the transgenic fishtumor formation assay, only EGF receptors with either both or one of the two dimerising amino acid changes are tumorigenic, while those without the mutation do not induce cancer. (22,27) The ligand-independent activity of Xmrk is consistent with one of the hallmarks of cancer cells, namely unlimited cell proliferation, which is not controlled by the absence or presence of growth regulating factors. Besides the activating amino acid changes, there is obviously a second precondition for Xmrk-induced melanomagenesis, namely transcriptional activation.…”

“…Transgenic expression of the platyfish Xmrk gene in the closely related medakafish (Oryzias latipes), which are normally not cancer-prone leads to tumors at high frequency and after short latency periods (22,23) (Wilde, Meierjohann, Laisney and Schartl, unpublished). While the EGF receptor and related growth factor receptor proteins are inactive in the absence of the respective ligand and signal only after the growth factor has bound to the extracellular domain, Xmrk is constitutively active and leads to a ligand-independent activation of the intracellular signal transduction network.…”

A Mutated EGFR Is Sufficient to Induce Malignant Melanoma with Genetic Background-Dependent Histopathologies

“…However, previous analyses have shown that xmrk is a potent oncogene that can also transform a variety of other cell types (Winkler et al, 1994;Winnemoeller et al, 2005). Owing to this, the generation of a transgenic melanoma model critically depended on a pigment cell-specific promoter to drive xmrk expression.…”

Malignant melanoma: genetics and therapeutics in the genomic era

“…Thus, in addition to providing experimental evidence that EGFR activation is biologically relevant, the abovementioned study in the inducible model also points out the possibility that EGFR or its ligands may constitute alternative point(s) of therapeutic intervention in RAS-activated melanoma. It should be mentioned that the contribution of EGFR signaling to melanoma development and possibly progression is evolutionarily conserved, as activating mutations in the EGFR homolog, Xmrk, increase melanoma susceptibility in Xiphophorus fish (Wittbrodt et al 1992;Winnemoeller et al 2005;Meierjohann et al 2006; for review, see Bardeesy et al 2000). It therefore remains possible that similar activating mutations exist in human melanoma, although systematic resequencing of large cohorts of melanomas from different ethnic and/or molecular subclasses will be required to uncover such examples.…”