January 13, 2012 — Childhood retinoblastoma is a rare and aggressive cancer that can be fatal if diagnosed at an advanced stage. Researchers have now shed light on the mechanisms underlying the rapid progression of the disease, and have identified a potential treatment target and promising therapeutic approaches.
The findings are published online January 11 in Nature.
The researchers, from St. Jude Children's Research Hospital in Memphis, Tennessee, found that the expression of the proto-oncogene spleen tyrosine kinase (SYK) is upregulated in retinoblastoma and is required for tumor cell survival.
In fact, they note that their "key discovery" was that SYK is important in retinoblastoma. SYK, previously thought to be primarily expressed in hematopoietic tissues, has been implicated in several hematologic malignancies.
The researchers showed that the inhibition of SYK with a small-molecule inhibitor caused cell death in retinoblastoma cells in culture and in vivo. However, progenitor cells and neurons in the retina normally express little or no SYK, and SYK has no known function in normal eye development.
Experimental drugs targeting the SYK protein are already in clinical trials for adults with leukemia and rheumatoid arthritis; this discovery suggests that they might also be applicable in retinoblastoma.
Different Applications for the Same Drug
"One theme is really emerging in my view, and that is finding new uses for drugs developed for other applications," said study author Michael Dyer, PhD, from the Department of Developmental Neurobiology at St. Jude Children's Research Hospital. "This is a striking example of a drug being developed for adult rheumatoid arthritis possibly having an application and benefit for a pediatric eye cancer. You can't get further apart than that."
Dr. Dyer told Medscape Medical News that as these types of discoveries move forward, oncologists and physicians are going to start noticing new applications for different drugs.
I think this is where cancer genome efforts are going to take us.
"I think this is where cancer genome efforts are going to take us," he added. "In 5 or 10 years from now, I don't think we will be surprised at all when we have these dramatic departures from a drug being developed for one disease and having applications for another."
There are already examples of that here and there, but genomic sequencing "is really going to give us a lot of insight in that area," he added. "Drug development in the past decade or so has been focusing on molecularly targeted therapeutics. Instead of targeting cell proliferation or division, they are targeting a particular site on a particular protein for a particular disease."
"But now the genome efforts have caught up, so to speak," Dr. Dyer continued. "There are data coming out that will allow us to begin to match up those great targeted drugs with particular cancers."
There are some who don't feel that there is a great deal of benefit from doing a whole-gene analysis, he pointed out. "They feel we already know a lot about these cancers and we should just be focusing on the genes and proteins we know about. But I think this is an example of something completely unexpected being discovered, and I think this is happening more and more."
Very Few Mutations
Retinoblastoma is initiated by the loss of the RB1 gene. Although tumors progress very rapidly after the inactivation of RB1, the underlying mechanism is not known. In this study, the researchers found that the retinoblastoma genome is stable, but that multiple cancer pathways "can be epigenetically deregulated." In other words, modifications to gene expression can occur without actually changing the DNA sequence itself.
The dogma in the field has been that once RB1 is mutated, the genome of the affected cell becomes unstable, chromosomes begin to break and recombine, and mutations quickly develop in the pathways that are essential for cancer progression, explained Dr. Dyer. But what they found was the opposite — that these tumors contain very few mutations or chromosomal rearrangements.
To identify mutations that cooperate with RB1 loss, Dr. Dyer and colleagues performed whole-genome sequencing of retinoblastomas. The sequencing was part of the 3-year Pediatric Cancer Genome Project (PCGP), conducted by St. Jude Children's Research Hospital and Washington University in St. Louis, Missouri, which was launched in 2010. The PCGP will complete whole-genome sequencing of normal and tumor DNA from 600 children and adolescents with a number of cancers.
Whole-genome sequencing was done on 4 retinoblastomas and on matched normal tissue. The paired germline DNA samples of the retinoblastomas showed no genetic lesions in known tumor-suppressor genes or oncogenes, aside from RB1 and MYCN. Of greater importance, note the authors, is an orthotopic xenograft drawn from a primary tumor; it showed no evidence of clonal variation or new coding-region mutations. This finding, they say, suggests that the retinoblastoma genome is more stable than previously believed.
The researchers next assessed the role of RB1 in genome stability and in nongenetic mechanisms of cancer pathway deregulation. They found that instead of genetic mutations in the cancer genes, they were being epigenetically regulated differently than normal cells.
To determine whether SYK was expressed in primary human retinoblastomas, they performed immunohistochemistry on a retinoblastoma tissue microarray or whole-eye sections. High levels of SYK were expressed in 100% of the 82 tumor samples but were absent in normal tissue.
Currently, small-molecule inhibitors of SYK have been developed to treat autoimmune disorders. In addition, 2 of these agents, BAY 61-3606 and R406, have demonstrated efficacy in preclinical studies of leukemia.
In this study, these experimental small-molecule inhibitors induced retinoblastoma tumor cell death in vitro and in vivo, and Dr. Dyer notes that they are currently reformulating R406. "Retinoblastoma patients are very young — 5 years or younger," he said. "The pharmacology and toxicity need to be worked out. We need to be able to deliver the right dose with low side effects. We are optimistic that it can be achieved."
The researchers hope to begin a phase 1 trial in retinoblastoma patients soon, although Dr. Dyer said it is too early to estimate a start date.
Improving Treatment
About 95% of retinoblastomas can be cured if they are treated before the disease metastasizes, Dr. Dyer noted.
The prognosis tends to be worse for children in developing nations, where the cancer is often advanced by the time it is diagnosed. "The focus of clinical trials in developing nations is saving lives and vision, as about 50% of children lose one or both eyes," said Dr. Dyer. "Even though the curative rate is high in countries like the United States, current therapies can have both short- and long-term side effects, such as secondary leukemias, neuropathy, or damage to the brain."
Thus, it would be advantageous to develop therapies with fewer adverse effects, he said. "In developing nations, an oral drug that could be easily administered to children could greatly improve survival."
The study was funded in part by the PCGP, the National Cancer Institute, the National Institutes of Health, the American Cancer Society, the Research to Prevent Blindness Foundation, and ALSAC.
Nature. Published online January 11, 2012. Abstract
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