Identification of a transforming ras oncogene in an ultraviolet radiation-induced corneal tumor of Monodelphis domestica

The role of ultraviolet radiation in melanomagenesis

The role of ultraviolet radiation (UV) in the pathogenesis has been discussed controversially for many decades. Studies in mice (SCID, HGF/SF, SV40T) which develop malignant melanoma, show a role of UVB in melanomagenesis. In contrast to this, the role of UVA is less clear. We will review the recent in vitro and in vivo data in support of the hypothesis that UVA is also involved in the development of malignant melanoma. The role of UVA in p53 activation, apoptosis, cell cycle arrest and photoproduct formation is discussed.

Animal models of melanoma: an HGF/SF transgenic mouse model may facilitate experimental access to UV initiating events

Cutaneous malignant melanoma, the most lethal of the skin cancers, known for its intractability to current therapies, continues to increase in incidence, providing a significant public health challenge. There is a consensus that skin cancer is initiated by sunlight exposure. For non-melanoma skin cancer there is substantial evidence that chronic exposure to the ultraviolet B radiation (UVB) (280-320 nm) portion of the sunlight spectrum is responsible. Experimentally, UVB is mutagenic and chronic UVB exposure can cause non-melanoma skin cancer in laboratory animals. Non-melanoma tumors in animals and in humans show characteristic UVB signature lesions in the tumor suppressor p53 and/or in the patched (PTCH) gene. An action spectrum or wavelength dependence for squamous cell carcinoma in the mouse shows a major peak of efficacy in the UVB. For malignant melanoma, however, the situation is unclear and the critical direct target(s) of sunlight in initiating melanoma and even the wavelengths responsible are as yet unidentified. This lack of information is in major part a result of a paucity of animal models for melanoma which recapitulate the role of sunlight in initiating this disease. The epidemiology of melanoma differs significantly from non-melanoma skin cancer. Intense sporadic sunlight exposure in childhood, probably exacerbated by additional adult exposure, is associated with elevated melanoma risk. Melanoma is also a disease of gene-environment interactions with underlying genetic factors playing a significant role. These major differences indicate that extrapolation from information for non-melanoma skin cancer to melanoma is unlikely to be useful. We summarize in this review the experimental information available on the role of UV radiation in melanoma and give an overview of animal melanoma models. A new model derived by neonatal UV irradiation of hepatocyte growth factor/scatter factor (HGF/SF) transgenic mice is described which recapitulates the etiology, the histopathology and molecular pathogenesis of human disease. It is anticipated that the HGF/SF transgenic model will provide a means to access the mechanism(s) by which sunlight initiates this lethal disease and provide an appropriate vehicle for derivation of appropriate therapeutic and preventive strategies.

H-ras oncogene activation in invasive UVR-induced corneal sarcomas of the opossum Monodelphis domestica

Chronic exposure to ultraviolet radiation (UVR) induces corneal sarcomas in the South American opossum Monodelphis domestica. Cell lines are readily established from these tumors. Northern blotting of mRNA from six such cell lines revealed high expression of the H-ras oncogene. H-ras cDNA from an eye tumor cell line was cloned and characterized; the germline sequence of codons 12, 13, and 61 was confirmed by examination of H-ras sequences amplified from liver DNA by the polymerase chain reaction. The Monodelphis H-ras coding sequence is 84-89% identical to that of other vertebrates at the nucleotide level, and the predicted 189-amino-acid sequence differs by 2-12 amino acids from that of other vertebrates. Analysis of 12 primary invasive corneal sarcomas induced by chronic UVR exposure revealed no evidence of H-ras gene amplification or rearrangement. One tumor was heterozygous for an activating point mutation in codon 61 of the H-ras gene; the tumor was also homozygous for a point mutation at an adjacent site in codon 62. These results provide additional evidence for the functional importance and consequent evolutionary conservation of the ras oncogenes.

Susceptibility to ultraviolet-induced corneal sarcomas is highly heritable in a laboratory opossum model

The laboratory opossum, Monodelphis domestica, develops hyperplasia and neoplasia of the corneal stroma after repeated exposure to low doses of ultraviolet radiation (UVR) (non-erythemic in skin). We exposed adult animals from genetically diverse families within our colony to determine whether there are any heritable components to the risk of this form of eye cancer. From about 5 months of age, animals were exposed 3 times a week to a dose of about 125 J/m2 of UVB (spectral peak = 302 nm). Thirty-three sibships (151 individuals) completed at least 30 weeks of the protocol and 137 individuals completed 45 weeks. For genetic analysis, each animal was classified at 30 and 45 weeks as affected with corneal sarcoma or not. Heritabilities were estimated for the dichotomous eye-tumor trait which was highly heritable at both time points. This eye-cancer model system is valuable as a source of material for in vitro studies of angiogenesis and neoplastic transformation, for in vivo studies of tumor therapy and prevention, and for further research on the genetic determinants of cancer.

Expression of fibroblast growth factors in ultraviolet radiation-induced corneal tumors and corneal tumor cell lines from Monodelphis domestica

Chronic exposure of the gray, short-tailed opossum, Monodelphis domestica, to ultraviolet radiation (UVR) induces highly vascularized mesenchymal tumors of the cornea. Cell lines derived from these UVR-induced corneal tumors and the corneal tumors themselves were examined for the presence of mRNA coding for basic and acidic fibroblast growth factors (FGF), transforming growth factors-beta and -alpha (TGF-beta and TGF-alpha), epidermal growth factor (EGF), and tumor necrosis factor-alpha (TNF-alpha). Basic FGF was expressed in the cell lines derived from corneal tumors and in the corneal tumors. Expression of basic FGF was high in one corneal tumor. Transcripts for acidic FGF were detected only in the corneal tumor cell lines, not in primary tumors. TGF-beta expression was detected in the corneal tumors and tumor-derived cell lines. TGF-alpha, EGF, and TNF-alpha transcripts were not detectable in any opossum material; however, homologous gene sequences for TGF-alpha and EGF were detected on Southern blots of opossum genomic DNA. Southern blot analysis revealed no evidence of amplification or rearrangement of the genes for basic FGF or acidic FGF in the UVR-induced corneal tumor that expressed high levels of basic FGF. Opossum basic FGF, which stimulated the proliferation of fetal bovine heart endothelial cells, was purified by heparin affinity chromatography from a UVR-induced corneal tumor and a corneal tumor cell line. Immunoblotting of opossum basic FGF from a corneal tumor cell line using antiserum to bovine basic FGF showed two prominent immunoreactive bands of 17.5 and 18.5 kDa. Expression of basic FGF and acidic FGF may play a role in the development and progression of UVR-induced corneal tumors in M. domestica.

Characterization of the K-ras gene of the marsupial Monodelphis domestica

A cDNA clone isolated from a plasmid library contains the complete coding sequence for the K-ras gene of the marsupial Monodelphis domestica, a South American opossum. The nucleotide sequence of the coding region of the opossum K-ras gene is very similar to the K-ras coding sequences of placental mammals. The coding region of the opossum gene is 95% identical to the human gene at the nucleotide level; the human and opossum genes are 99% identical at the level of encoded amino acids. Transcribed but untranslated regions of the opossum gene 3' and 5' to the coding region are similar to corresponding regions of the human and mouse genes, but are less highly conserved than translated sequences. Based on the nucleotide sequence of the opossum K-ras cDNA clone, primers were designed that allowed amplification of exons 1 and 2 of the gene from opossum genomic DNA by the polymerase chain reaction. When exons 1 and 2 of K-ras were amplified from DNA isolated from an ultraviolet radiation-induced eye tumor of M. domestica and the nucleotide sequence of amplified material was determined, a heterozygous mutation in codon 61 of the gene was detected. This T to A transversion resulted in a change in the amino acid encoded by the codon. The tumor from which DNA was isolated had previously been shown to contain a transforming K-ras oncogene. Thus, the opossum K-ras gene can be mutationally activated in a manner similar to the K-ras genes of placental mammals.

Purification and characterization of fibroblast growth factors from the opossum, Monodelphis domestica, brain

An extract from the brain of the opossum Monodelphis domestica was fractionated by heparin affinity chromatography. A major peak of mitogenic activity (heparin binding growth factor 2, HBGF-2) eluted from heparin-Sepharose between 1.7 and 2.0 M NaCl. Antisera specific for bovine bFGF detected four polypeptides of 17.5-23 kDa in opossum brain HBGF-2 preparations. Opossum brain heparin binding growth factor 1 (HBGF-1), a minor peak of activity, eluted from heparin-Sepharose at 1.1 NaCl and contained a 16.2 kDa protein that cross-reacted with antiserum against bovine aFGF.