Characterization of the Xiphophorus fish (Teleostei: Poeciliidae) ERCC2/XPD locus

Cloning of JunA and JunB and comparison of mRNA expression levels in two Xiphophorus melanoma models

The cloning and mRNA expression analysis of Xiphophorus maculatus JunA and JunB proto-oncogenes (designated X-JunA and X-JunB, respectively) is described. In mammals, JunA and JunB proteins make up the activator protein-1 (AP-1) transcription factor with related Fos proteins. The deduced amino acid sequences of X-JunA and X-JunB exhibit moderate degrees of similarity when compared to their human homologues, while the regions considered functionally critical, namely, the transactivation domains, DNA-binding domain, and the leucine zipper, are highly conserved. X-JunA and X-JunB mRNA expression levels in six X. maculatus Jp 163 A tissues were assayed by real-time RT-PCR. In addition, X-JunA and X-JunB mRNA levels are compared in skin and tumor tissues derived from two distinct Xiphophorus backcross hybrid tumor models, one of which develops melanoma spontaneously, whereas the other requires induction via UVB exposure for melanoma development. X-JunB mRNA expression was higher than X-JunA expression in tissues from X. maculatus parental animals. X-JunB was also more highly expressed than X-JunA in both spontaneous and induced melanoma tissue and nonmelanotic skin tissue. However, X-JunA mRNA levels were significantly higher in the spontaneous melanomas compared to melanomas induced by UVB exposure. The authors speculate that these findings may indicate that JunA regulation is affected by regulatory differences between the two melanoma model systems.

Perturbation of DNA repair gene expression due to interspecies hybridization

The effect of interspecies hybridization on gene regulation was examined using real-time polymerase chain reaction (RT-PCR) to measure the expression of five base-excision repair genes in brain, eye, gill, liver, and tailfin tissues from Xiphophorus parental species and F(1) hybrids. Relative mRNA levels of uracil N-glycosylase (Ung), Apurinic/apyrimidinic endonuclease (Ape1), polymerase-beta (Polb), flap endonuclease (Fen1), and DNA ligase (Lig1) were measured in three parental Xiphophorus species (X. maculatus Jp 163 B, X. helleri Sarabia, and X. andersi andC) and in two interspecies F(1) hybrids, the Sp-helleri hybrid (X. maculatus Jp 163 BxX. helleri Sarabia) and the Sp-andersi hybrid (X. maculatus Jp 163 BxX. andersi) to identify genes that undergo changes in expression levels upon interspecies hybridization. Significant differences in gene expression were observed between parental animals and their respective F(1) hybrids in both interspecies crosses. Generally, marked increases in DNA repair gene mRNA levels were observed across all tissues in F(1) hybrid animals from the Sp-helleri cross compared to either X. maculatus or X. helleri parents. In contrast, the Sp-andersi F(1) hybrid animals generally exhibited decreased base-excision repair gene expression, although this trend was more specific to individual tissues than observed for Sp-helleri hybrids.

Parental 5-methylcytosine methylation patterns are stable upon inter-species hybridization of Xiphophorus (Teleostei: Poeciliidae) fish

Cytosine methylation appears to be established as an important DNA base modification involved in regulation of gene expression but is poorly understood from an evolutionary viewpoint. Xiphophorus progeny from inter-species crosses and backcrosses that are utilized in contemporary tumor induction studies were analyzed for cytosine methylation pattern inheritance using Southern blot analyses. Methylation patterns at CCGG sequences of 411 independent chromosomes in three distinct inter-species crosses were analyzed. In every case the non-recurrent parental methylation pattern remained unaltered for each of the genes studied, once introduced into the recurrent parental genetic background. Through F(1) inter-species hybridization and succeeding meiosises leading to first generation (BC(1)) and second generation (BC(2)) backcross hybrid progeny, we demonstrate that parental species methylation patterns are stable.

Xiphophorus interspecies hybrids as genetic models of induced neoplasia

Fishes of the genus Xiphophorus (platyfishes and swordtails) are small, internally fertilizing, livebearing, and derived from freshwater habitats in Mexico, Guatemala, Belize, and Honduras. Scientists have used these fishes in cancer research studies for more than 70 yr. The genus is presently composed of 22 species that are quite divergent in their external morphology. Most cancer studies using Xiphophorus use hybrids, which can be easily produced by artificial insemination. Phenotypic traits, such as macromelanophore pigment patterns, are often drastically altered as a result of lack of gene regulation within hybrid fishes. These fish can develop large exophytic melanomas as a result of upregulated expression of these pigment patterns. Because backcross hybrid fish are susceptible to the development of melanoma and other neoplasms, they can be subjected to potentially deleterious chemical and physical agents. It is thus possible to use gene mapping and cloning methodologies to identify and characterize oncogenes and tumor suppressors implicated in spontaneous or induced neoplasia. This article reviews the history of cancer research using Xiphophorus and recent developments regarding DNA repair capabilities, mapping, and cloning of candidate genes involved in neoplastic phenotypes. The particular genetic complexity of melanoma in these fishes is analyzed and reviewed.

Analysis of mutations in the XPD gene in Italian patients with trichothiodystrophy: site of mutation correlates with repair deficiency, but gene dosage appears to determine clinical severity

Xeroderma pigmentosum (XP) complementation group D is a heterogeneous group, containing patients with XP alone, rare cases with both XP and Cockayne syndrome, and patients with trichothiodystrophy (TTD). TTD is a rare autosomal recessive multisystem disorder associated, in many patients, with a defect in nucleotide-excision repair; but in contrast to XP patients, TTD patients are not cancer prone. In most of the repair-deficient TTD patients, the defect has been assigned to the XPD gene. The XPD gene product is a subunit of transcription factor TFIIH, which is involved in both DNA repair and transcription. We have determined the mutations and the pattern of inheritance of the XPD alleles in the 11 cases identified in Italy so far, in which the hair abnormalities diagnostic for TTD are associated with different disease severity but similar cellular photosensitivity. We have identified eight causative mutations, of which four have not been described before, either in TTD or XP cases, supporting the hypothesis that the mutations responsible for TTD are different from those found in other pathological phenotypes. Arg112his was the most common alteration in the Italian patients, of whom five were homozygotes and two were heterozygotes, for this mutation. The presence of a specifically mutated XPD allele, irrespective of its homozygous, hemizygous, or heterozygous condition, was always associated with the same degree of cellular UV hypersensitivity. Surprisingly, however, the severity of the clinical symptoms did not correlate with the magnitude of the DNA-repair defect. The most severe clinical features were found in patients who appear to be functionally hemizygous for the mutated allele.

Mapping of tyrosine kinase gene family members in a Xiphophorus melanoma model

Xiphophorus fish have been the subject of intensive genetic research for more than 60 yr, primarily because of the availability of a number of interspecific hybrids that are malignant melanoma models with apparently simple oncogene and tumor suppressor gene determinants. The gene map of Xiphophorus is one of the most extensive among nonhuman vertebrates, with about 100 genes assigned to at least 20 independently assorting linkage groups (LGs), as well as more than 250 anonymous DNA sequence markers, providing coverage for most of the genome for genetic mapping studies. This characteristic has resulted in the mapping of a tumor suppressor locus, DIFF, which is one of two genetic determinants of melanoma formation in the best-studied hybrid melanoma, the Gordon-Kosswig melanoma model. The other gene responsible for melanoma formation in this model is a sex-linked tyrosine kinase gene related to EGFR and called Xiphophorus melanoma receptor kinase (Xmrk). The cellular oncogene homologues of the non-receptor tyrosine kinase family orthologous toyes and fyn have also been found to be overexpressed in malignant melanomas of Xiphophorus and may be involved in tumor progression. We report here the map location of a Xiphophorus yes gene, YES1, in LG VI, closest to the EGFR gene and the assignment of a fyn gene homologue to newly designated LG XV, linked to the gene for cytosolic alpha-galactosidase. We also confirmed that an EGFR-related sequence (EGFRL1) that we previously assigned to Xiphophorus LG VI by cross-hybridization to a viral erbB probe was the EGFR orthologue. Our results suggest that the presence of expressed duplicates of members of the tyrosine kinase gene family in teleost fishes may increase the potential number of targets in oncogenic cascades in fish tumor models.

Construction of a functional cDNA clone of the hamster ERCC2 DNA repair and transcription gene

The complete hamster ERCC2 cDNA was constructed in a plasmid vector from clones of three overlapping reverse transcribed/polymerase chain reaction amplified fragments using unique restriction enzyme recognition sites within the regions of overlap. This complete cDNA insert was then cloned into a mammalian expression vector, pcD2E, and tested for function by the ability to confer UV resistance to the ERCC2 mutant CHO cell line UV5. Site-specific mutagenesis was used to introduce the G347-->A and G1844-->A changes resulting in the Cys116-->Tyr and Gly615-->Glu mutations previously identified in UV5 and UVL-13 (also an ERCC2 mutant CHO cell line), respectively. The 116Tyr and 615Glu plasmids each failed to confer UV resistance to UV5 or UVL-13 cells, respectively, demonstrating that the changes identified are indeed the causative mutations in UV5 and UVL-13.

Assignment of the TP53 orthologue to a new linkage group (LG XIV) in fish of the genus Xiphophorus (Teleostei: Poeciliidae)

Using a p53 encoding cDNA fragment of rainbow trout (Oncorhynchus mykiss) as probe, a lambda clone from a platyfish (Xiphophorus maculatus) genomic library was isolated. DNA sequencing of the insert from this clone revealed that it contained the highly conserved domains IV and V of the p53 polypeptide. To map the Xiphophorus p53 gene, joint segregation analysis of the inheritance of a PstI-generated DNA restriction fragment length polymorphism (RFLP) and the inheritance of 36 polymorphic protein and DNA markers was performed in backcross hybrids of X. clemenciae x (X. clemenciae x X. milleri) and X. helleri x X. (helleri x X. maculatus Jp 163 B) using Oncorhynchus cDNA and Xiphophorus genomic p53 probes, respectively. The p53-hybridizing sequence (TP53) was linked to the ACO1 (cytosolic aconitase) locus in both crosses, and defines a new Xiphophorus linkage group, designated LG XIV. This is the first mapping assignment of a known human tumor suppressor gene in fish. Since ACO1 is not linked with melanoma severity in X. helleri x X. maculatus Jp 163 A backcross hybrids, these data indicate that homozygosity for the X. helleri TP53 genotype in backcross hybrids of the cross type is not associated with genetically regulated malignant melanoma formation in the Gordon-Kosswig hybrid melanoma model.