Dr. Harbour is the winner of the 2005 Cogan Award
Fort Lauderdale, FL--Research in retinoblastoma and uveal melanoma, two relatively rare ocular cancers, extends beyond ocular oncology to all areas of oncology. Discoveries concerning the mechanisms in those two cancers are shedding light on how other tumors function and may aid in the development of therapies and means of predicting metastasis, according to J. William Harbour, MD, who delivered the Cogan Lecture at the annual meeting of the Association for Research in Vision and Ophthalmology.
"These eye cancers, despite their rarity, have an influence well beyond their prevalence. We started with clinical observations that lead to basic science discoveries biological insights, and exponential cross fertilization with other fields that have already lead to clinical impact. In the future there will be more clinical impact. The research that we conduct extends far beyond ocular oncology to all areas of oncology," he stated.
Dr. Harbour, associate professor, and director of the ocular oncology service at Washington University School of Medicine, St. Louis, is the winner of the 2005 Cogan Award for his significant contributions to the understanding of the molecular regulation of the cell cycle in ocular tumors. The Cogan Award recognizes a researcher, 40 years or younger at the time of nomination, who has made important contributions to research in ophthalmology and visual science directly related to disorders of the human eye or the visual system.
Rb Gene Mutation
Dr. Harbour described recent discoveries in oncology research including the activity of the retinoblastoma (Rb) gene mutation, which is present in many other cancers. Retinoblastoma, while rare, is the most common ocular cancer in children. In the US, 95% of children with retinoblastoma survive, but most die worldwide.
"Scientifically, the genetics of retinoblastoma have provided the clue to the revolution in cancer research 20 years ago. The basic observation was that there were two forms of retinoblastoma. One form develops in only one eye and is not passed on to children, and the other form develops in both eyes and it is passed on to children," Dr. Harbour explained. This accounted for the different inheritance pattern based on the presence of a recessive cancer gene, which was a revolutionary idea at the time. This resulted in the discovery that the retinoblastoma gene (RB gene mutation) had different patterns; either it was partly or completely missing or there were gross rearrangements. A parallel discovery was that the RB gene mutation also appears in small cell lung cancer, other lung cancer, and breast cancer.
Rb Protein
The real revolution started, he pointed out, when investigators began looking at the Rb protein. Virtually every cancer studied has some defect that inactivates the Rb protein. The important factors associated with the Rb protein is that there are 16 sites where it can be phosphorylated and the "business end" of the molecule has a pocket with two boxes that allows multiple proteins to bind to it. "Binding at the molecular level represses gene expression, according to Dr. Harbour. Interaction with proteins that interact with DNA and once bound to DNA different remodeling enzymes can be recruited that ultimately suppress gene expression.
Further research showed that the Rb protein is regulated differently. Uveal melanoma, the most common primary cancer in adults, and most solid tumors partially inactivate Rb and the tumors proliferate slowly with a low rate of apoptosis; conversely, retinoblastomas, small cell lung cancer, and high-grade melanomas delete Rb and there is rapid proliferation of the tumor with a high rate of apoptosis.
"There are at least two tumor suppressor mechanisms by which Rb blocks tumor formation. The primary one is regulation of the cell cycle. The apoptotic response we think is the 'last-ditch' tumor suppressor mechanism when a tumor cell escapes the primary mechanism before becoming cancerous," he said.
Tumors that partially inactivate Rb do so by inactivating p16, a tumor suppressor gene, or by overexpressing cyclin D. This usually leads to slow proliferation, and the advantage for the tumor cell is that the apoptotic response is not triggered.
"We believe that this is why most melanomas in adults do not grow rapidly like leukemia or retinoblastoma cells. The melanomas grow slowly but avoid the apoptotic tendency. In contrast, when Rb is inactivated completely or phosphorylation is affected, there is more rapid proliferation because of the release of a transcription factor (E2F) that results in rapid tumor growth but with a tendency to apoptosis. This may partially explain what we see in melanoma and most solid tumors," Dr. Harbour explained. The high apoptotic rate may be a factor in why retinoblastoma and small cell lung cancer may be more sensitive to chemotherapy and radiotherapy.
Melanomas and the Rb Pathway
This raised the question of why melanomas can target the Rb pathway. Dr. Harbour and colleagues discovered that the melanocytes are involved. In normal melanocytes from the choroid, E2f can drive gene expression. In a dividing normal melanoblast, the Rb is still partially active and blocks the apoptotic genes from being expressed. They found that the melanocyte differentiation factor binds to p16 promoter and activates p16 expression.
"This links melanocyte differentiation to the Rb pathway," he stated. When p16 is activated, Rb becomes dephosphorylated, and remodeling enzymes can be recruited and they inactivate the cell cycle genes. The cells stop growing and differentiate. Melanocytes are required in the pathway for differentiation and for cell cycle activity.
"This is important in melanoma," Dr. Harbour emphasized, "because the pathway creates a selective pressure to inactive p16. If cells are driven into melanocyte differentiation, there is occasionally an escape clone that starts to grow rapidly and inactivates p16 and methylate the p16 promoter, as uveal melanoma does in vivo. When p16 is missing, Rb is rephosphylated and the cells reenter the cell cycle. This is one of the earliest events in malignant transformation."
He contrasted this to the events in retinoblastoma and in other tumors in which Rb is inactivated completely. The Rb pathway is irrelevant; there is cell cycle gene activation but activation of apoptotic genes.
Dr. Harbour posed the question of whether apoptosis can be induced in melanoma by driving Rb into a hyperphosphylated state. He and his colleagues did that by overexpressed cyclins D and E, which resulted in apoptosis of the cancer cells.
Clinical relevance: Class 1 and Class 2 Tumors
About half of patients with uveal melanoma die of metastatic disease and the metastasis likely occurs before uveal melanoma is diagnosed. However, the disruption of the Rb pathway does not predict metastatic death. The disruption of the pathway happens early in the development of uveal melanoma.
Dr. Harbour investigated gene expression profiling to find out what causes uveal melanoma to metastasize in enucleated eyes with melanoma. RNA analysis showed that the tumors clustered into class 1 and class 2. The class 2 had a high number of epitheloid cells, which carry a poor prognosis in melanoma, and in fact all deaths occurred in patients with class 2 tumors in Dr. Harbour's series. Monosomy 3 was also found to be a high risk factor for metastasis.
"We believe that early in tumor progression, there are relatively subtle changes in the Rb pathway and the other pathways that lead to a class 1 melanoma. The class 1 melanoma cells can persist for years, cause local ocular destruction, but never result in patient death. The change to a class 2 melanoma is a huge genetic switch, a malignant progression, during which the cells undergo an epitheloid reversion," he said.
"Retinoblastoma and uveal melanoma provide unique important insights into molecular angiogenesis and have implications far beyond their incidence for cancer biology, developmental biology, and other areas of vision research. Rb research, I believe, will continue to have a profound impact on clinical care both in the area of cancer diagnosis and treatment, hopefully to improve survival, if we can harness the Rb pathway and use it in a controlled way," he concluded.
Dr. Harbour has no proprietary interest in any aspect of this report.
Contact Information
Harbour email: harbour@vision.wustl.edu
Take-Home Message
Research in retinoblastoma and uveal melanoma, two relatively rare ocular cancers, extends beyond ocular oncology to all areas of oncology. Discoveries concerning the mechanisms in those two cancers are shedding light on how other tumors function and may aid in the development of therapies and means of predicting metastasis.
