Increasing Melanoma Incidence: Putatively Explainable by Retrotransposons Experimental Contributions of the Xiphophorine Gordon‐Kosswig Melanoma System

Abstract

The worldwide accelerating increase of neoplasia in humans is difficult to explain. We use the Xiphophorus tumor model to approach this problem by melanoma provocation with X-rays. Melanoma develops following inappropriate expression of x-erb B-conducted developmental genes and their controllers. These oncodeterminants are inherited according to Mendelian rules. We detected a new type of oncodeterminants that, following a single treatment of embryos with X-rays, generates a self-generating non-Mendelian melanoma transmission and accelerating increase of its incidence in succeeding generations (e.g., 0-->18-->33-->52%). To localize these oncodeterminants, we crossed nonirradiated fish having half of their chromosomes irradiated with nonirradiated fish having none of, half of, or all of their chromosomes irradiated. Because tumor rate and expression in the following generations correspond to the rates of treated chromosomes, we conclude that the new oncodeterminants are distributed over the chromosomes of the fish, where they may increase in the changing generations. By means of xiphophorine-specific retroviral DNA, we isolated two retrotransposons that behave hereditarily like the new transgenerational oncodeterminants. Sequence analysis revealed three ORFs flanked by LTRs containing motives of regulatory sequences typical for known retroviral and retrotransposal LTRs. Pol- and env-resembling sequences are lacking. Southern and in situ hybridization showed their multiple and repetitive nature distributed throughout the chromosomes and indications for their capability to increase in number without further treatment. Their transcripts are expressed in concert with those of most of the other known xiphophorine tumor determinants. Their expression is extremely high in cell cultures from tumorous embryos derived from ancestors treated as embryos with X-rays.

Amplification of a long terminal repeat-like element on the Y chromosome of the platyfish, Xiphophorus maculatus

“…4a). This pattern is different from the one reported from radioactive in situ hybridization analysis with the original XIR-1 probe (Anders et al 1994), indicating some structural differences between XIR-1 and the XIR LTR-like sequence that we isolated from cosmid D18 097. The FISH signal was conspicuously located at the terminal region of the long arm and at least 20 metaphase spreads were evaluated to confirm the near-terminal location of the XIR probe.…”

“…Southern blot analysis using low-stringency as well as high-stringency washes revealed that the XIR repeat is highly reiterated within the genome of both males and females of X. maculatus from Rio Jamapa and from Rio Usumacinta (Fig. 2a) as described for the original XIR LTR (Anders et al 1994). …”

“…1) from the ONCXmrk locus on the Y chromosome were isolated and cosmid D18 097 was partially sequenced. A sequence presenting 88% nucleotide identity with the 5′ long-terminal-repeat (LTR) of the Xiphophorus XIR retrotransposon (GenBank accession number S77502, Anders et al 1994;Roushdy et al 1999) was identified about 45 kb upstream from ONC-Xmrk. Other than this no associated sequence was found with significant similarities to typical retroelement genes such as reverse transcriptase.…”

Transformation-Associated Recombination (TAR) Cloning of Tumor-Inducing Xmrk2 Gene from Xiphophorus maculatus

“…Because we wished to generate a representative genomic library, the use of highly repeated DNA elements as hooks in our cloning vectors was critical. DNA sequencing of some regions of X. maculatus revealed the presence of several different and varied repetitive elements (Anders et al, 1994;Nanda et al, 2000;Froschauer et al, this issue). For our first library construction we chose the XIR-1 repeat, originally characterized by Anders et al (1994), that is the 5Ј region of a retroviral long terminal repeat (LTR); characterization of the repeat indicated this element was widely distributed over the fish genome.…”

Divergent patterns of selection on the DAB and DXB MHC class II loci in Xiphophorus fishes

“…Xiphophorus fishes have been used in research of the immune system for well over fifty years (Kallman 1958(Kallman , 1964. Backcrosses between certain strains of helleri and maculatus can result in fish that develop malignant melanoma, among other diseases (Anders et al 1994;Kazianis et al 2001). The availability of inbred strains and the initial characterization of MHC DAB and DXB genes in Xiphophorus make the genus an excellent system in which to study evolutionary dynamics of MHC class II genes.…”