Comparative Oncology Program and Pediatric Oncology Branch, Center for Cancer Research, National Cancer Institute, Bethesda, MD, USA
The Comparative Oncology Opportunity
The field of comparative oncology is uniquely positioned to take advantage of the completed canine genome to improve our understanding of cancer therapy and biology. Used most often when referring to the study of cancers seen in companion (pet) animals, comparative oncology describes a discipline that integrates the study of naturally occurring cancers in animals into studies of human cancer. Cancers in companion species are well suited to uniquely inform investigations of cancer biology and cancer drug development. The features of cancers in pet dogs that may uniquely contribute to our understanding of cancer pathogenesis, progression and therapy have been recently reviewed. Of most interest to the study of cancer biology is the biological complexity of cancers seen in pet animals. This is based in large part on the intra-tumoral (cell-to-cell) heterogeneity seen in these cancers. A natural consequence of this heterogeneity is the same deadly features of human cancers including acquired resistance to therapy, recurrence, and metastasis. Accordingly, cancers in pet animals capture the "essence" of the problem of cancer seen in human patients. The integration of these naturally occurring cancers into the study of cancer biology and therapy provides an opportunity to answer questions not answered in conventional model systems.
Improving Our Understanding of Cancer Metastasis: Biology and Therapy
The development of metastasis is a universally grave development for cancer patients irrespective of specific cancer histology. Emblematic of this clinical problem is the biology associated with progression in the pediatric solid tumor, osteosarcoma. Indeed, for both human and canine cancer patients afflicted with osteosarcoma, the most common primary tumor of bone, metastasis to the lungs is the most common cause of death. Lung metastases develop in these patients despite highly effective treatment of the primary tumor. Improving our understanding of the biology of metastasis is needed to improve outcomes for these patients. It is important to emphasize that the study of metastasis must include two distinct biologic entities that are both referred to as metastasis. First metastasis is a verb that describes the process of cancer progression from a primary tumor site to a distant secondary site. Second metastasis also refers to the actual metastatic lesion that exists at the secondary site and is responsible for patient mortality. Progress towards understanding metastatic disease (both the verb and noun) and its inherent resistance to conventional treatments is limited by many factors. First, the process of metastasis is believed to begin very early in the course of disease progression and has occurred in most patients at the time of their initial presentation. This provides limited opportunities to study these events in human patients. Second, the genetic aberrations that are responsible for the development of metastasis are complex, heterogeneous and difficult to distinguish from the events responsible for the actual development of cancer. Finally, the access to well described patient samples is limited and often is available only after treatment with cytotoxic chemotherapy. For relatively rare human cancers such as osteosarcoma, these problems are amplified.
Comparative Cross-Species Approach: Identification and Study of Ezrin as a Determinant of Metastasis
To define genes and or proteins that contribute to the metastatic phenotype of metastasis in osteosarcoma, we have utilized a cross-species comparative approach that includes murine, canine, and human systems for gene identification and evaluation. This approach has leveraged the availability of murine, canine, and human genomes, to survey the expression of genes in normal and diseased tissues and then identify either patterns of gene expression of individual genes responsible for or associated with metastasis. As an example, we used this comparative approach to identify and then associate the cytoskeleton linker protein, ezrin, with metastasis. Ezrin is the best characterized of the ERM (Ezrin-Radixin-Moesin) family. ERM proteins exist in the cytoplasm in an inactive "closed conformation" though N-terminal to C-terminal associations within the protein or with other ERM members. The stability and consistency of our findings involving ezrin across species lines has strengthened our belief in this cross-species approach. Ezrin's linkage of the cell membrane to the actin cytoskeleton directly allows cells to interact with its microenvironment and functionally provides an "intracellular scaffolding" that facilitates signal transduction through a number of growth factor receptors and adhesion molecules. We have demonstrated that ezrin is necessary for metastasis in murine transplantable osteosarcoma and genetically engineered rhabdomyosarcoma models, that it is relevant in human-murine sarcoma xenograft models, and that its expression is associated with metastatic progression in pet dogs with naturally occurring osteosarcoma, finally we have found an association between ezrin expression and risk of relapse in pediatric osteosarcoma patients. The value of this comparative approach has been to define biologically relevant motifs that have sustained importance as we cross species lines.
Evaluating Antimetastatic Therapies Based on Ezrin Biology
The comparative approach is uniquely suited to rapidly translate findings from the laboratory to the clinic. With this interest in mind we have linked ezrin expression with the initiation of protein translation by metastatic cells. We have begun to validate these associations in murine, canine and human cancer cells. The connection between ezrin, protein translation and the mTOR pathway has provided a therapeutic opportunity based on the use of mTOR inhibitors. Indeed, we have conducted preclinical studies in mice that support the therapeutic role of inhibiting mTOR using rapamycin (and novel analogs). Novel inhibitors of the mTOR pathway have entered early clinical trials for human osteosarcoma and sarcoma patients. There are many questions that remain unanswered regarding the optimal use of these agents in human patients. Indeed, many of the unanswered questions that exist in the development path of new drugs may be effectively answered by integrating studies that include pet dogs with cancer. To complete the translational effort based on ezrin and mTOR inhibition, we have now completed dose and regimen finding studies in pet dogs with osteosarcoma that will allow the evaluation of the inhibition of translation initiation (by rapamycin) in pet dogs with osteosarcoma. Ongoing studies will now define optimal treatment schedules for the use of rapamycin in children and dogs with osteosarcoma. Efforts to validate reagents and further characterize these models using more sophisticated techniques has been ongoing within several comparative oncology laboratories around the world. Contributing to this effort is Comparative Oncology Program of the US National Cancer Institute's Center for Cancer Research and the not for profit, Canine Comparative Oncology and Genomics Consortium.
This approach exemplifies the values of value of a veterinary and comparative perspective in the study of complex biomedical problems.
1. Paoloni M, Khanna C. Translation of new cancer treatments from pet dogs to humans. Nat Rev Cancer 2008;8:147–156.
2. Khanna C. Novel targets with potential therapeutic applications in osteosarcoma. Curr Oncol Rep 2008;10:350–358.
3. Khanna C, Wan X, Bose S, et al. The membrane-cytoskeleton linker ezrin is necessary for osteosarcoma metastasis. Nat Med 2004;10:182–186.
4. Wan X, Mendoza A, Khanna C, et al. Rapamycin inhibits ezrin-mediated metastatic behavior in a murine model of osteosarcoma. Cancer Res 2005;65:2406–2411.