If spontaneous or chemically induced DNA damage is not repaired, cells either sustain mutations or die. To prevent this, all organisms have evolved biochemical pathways that repair specific types of DNA damage. Over the past 15 years, researchers have demonstrated a strong connection between cellular DNA repair processes and tumor development. Genes that code for activities leading to reversal of DNA damage are termed DNA repair genes. Despite the known involvement of DNA repair genes in important cellular functions and in human disease pathology, comparative analyses of DNA repair gene structure have been performed only among organisms having estimated evolutionary divergence times of over a billion years (yeasts and humans). We have initiated studies of DNA repair gene evolution by characterizing human DNA. repair gene homologues of Xiphophorus fish (platyfish and swordtails). Divergence times between fishes and the common ancestor leading to tetrapods is estimated to be about 450 million years; thus comparative data concerning gene map location and genetic structure derived from analyses of Xiphophorus fish provide insight into the events leading to derivation of the mammalian (human) condition.

It would enhance our understanding of DNA repair mechanisms in vertebrates if we had available data showing the origin and relatedness of the DNA repair genes. Are these genes members of highly conserved gene families? What functional constraints are placed on a gene solely due to its map location? Which domains of the encoded proteins are conserved? Do cells silence these genes during developmental differentiation: These are some questions that need to be answered for us to understand the relationships between DNA repair processes and biological phenomena such as evolutionary rates, embryonic development, and oncogenic transformation.

Our nucleotide sequence determinations of four fish DNA repair genes have shown that very strong conservation of amino acid sequence (>85% identity; >95% similarity) is evident when the fish genes are compared with human gene homologues. For these genes, genetic fine structure (exon number, exon lengths, and protein domain positions) appears to be virtually identical in both fishes and humans. These data suggest the gene products serve similar or identical cellular functions in all vertebrates.

In interrelated studies, our laboratory is constructing fish stocks that carry genes experimentally introduced into the DNA of adult animals (e.g., transgenic). The introduced genes allow study of the associated trait in a common genetic background. We utilize the killifish, medaka (Oryzias latipes), as our transgenic host. The Xiphophorus genes we have characterized will be used to produce transgenic medaka stocks, which will allow us to evaluate the contribution of each introduced gene to DNA repair events after the host organism is subjected to DNA-damaging agents. Transgenic fish enable novel experiments to be performed with intact whole animals in order to further our understanding of DNA repair and tumor induction.