Functional test of a naturally occurred tumor modifier gene provides insights to melanoma development

Occurrence of degenerative interactions is thought to serve as a mechanism underlying hybrid unfitness in most animal systems. However, the molecular mechanisms underpinning the genetic interaction and how they contribute to overall hybrid incompatibilities are limited to only a handful of examples. A vertebrate model organism, Xiphophorus, is used to study hybrid dysfunction, and it has been shown from this model that diseases, such as melanoma, can occur in certain interspecies hybrids. Melanoma development is due to hybrid inheritance of an oncogene, xmrk, and loss of a co-evolved tumor modifier. It was recently found that adgre5, a G protein-coupled receptor involved in cell adhesion, is a tumor regulator gene in naturally hybridizing Xiphophorus species Xiphophorus birchmanni (X. birchmanni) and Xiphophorus malinche (X. malinche). We hypothesized that 1 of the 2 parental alleles of adgre5 is involved in regulation of cell growth, migration, and melanomagenesis. Accordingly, we assessed the function of adgre5 alleles from each parental species of the melanoma-bearing hybrids using in vitro cell growth and migration assays. In addition, we expressed each adgre5 allele with the xmrk oncogene in transgenic medaka. We found that cells transfected with the X. birchmanni adgre5 exhibited decreased growth and migration compared to those with the X. malinche allele. Moreover, X. birchmanni allele of adgre5 completely inhibited melanoma development in xmrk-transgenic medaka, while X. malinche adgre5 expression did not exhibit melanoma suppressive activity in medaka. These findings provide evidence that adgre5 is a natural melanoma suppressor and provide new insight in melanoma etiology.