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Biology of Osteosarcoma

Biology studies are becoming increasingly important in osteosarcoma research and even in clinical research, such as in clinical trials conducted by the Children's Oncology Group. Understanding the biology of osteosarcoma is important because unlike many other adolescent cancers, osteosarcomas in one person to the next do not share a single specific molecular abnormality. Most osteosarcomas display complex genetic mutations and a variety of nonspecific alterations in different molecular pathways, such as the RB pathway and the p53 pathway which control cell growth.

Osteosarcoma is thought to be a tumor of the osteoprogenitor cell (early bone cell). These are stromal cells in the bone marrow (tissue that fills most bone cavities) and periosteum (bone membrane) capable of differentiating into many different cells. Published studies suggest that many of the altered molecular pathways in osteosarcoma have a role in the normal development and growth of the bone. One goal of our basic research program is to translate new discoveries from research on basic bone biology to the understanding of the development and treatment of osteosarcoma.

Tumor suppressor pathway alteration

Most osteosarcomas have some type of combined inactivation of the retinoblastoma and p53 pathway.

RB pathway

The RB pathway is a major regulator of cell growth including tumor growth, and a alteration in any one of the genes of this pathway may play a role in the development of osteosarcoma. Alterations of the genes involved in the RB pathways have been found frequently in osteosarcoma samples, 70% of the samples had alterations in the RB gene itself, 10% in the p16 INK4a and 10% in the CDK4 and Cyclin D1 gene.

P53 Pathway

The p53 gene is the most commonly mutated gene in human cancer, and in osteosarcoma the overall frequency of mutations ranges from 15% to 30%. The p53 gene has multiple roles as a central regulator of a cells response to stress, and is linked to the RB pathway. p53 also appears to play a role in the normal development and function of bone.

Because almost all osteosarcomas have alterations in the retinoblastoma and p53 pathway, these alterations are believed to occur early in tumor development and therefore they might not be useful markers to distinguish osteosarcomas that are likely to be cured with current therapy from those that are not.

Telomere maintenance

One of the explanations for the complex karyotypes might be the dysfunction of the telomeres. Telomeres are long DNA sequences at the end of chromosomes. One of the functions of telomeres is to regulate how often cells can divide. Many cancers evade regulation of growth by being able to lengthen telomeres. Unlike many other cancers which predominately use telomerase more than 50 % of osteosarcomas use the alternative lengthening of telomeres pathway. Work is in progress to examine the utility of the different mechanism of telomere maintenance in osteosarcoma as prognostic markers.

Drug resistance pathways

Recurrence of osteosarcoma is at least partially due to intrinsic (without treatment) or acquired (after treatment) drug resistance. There are many possible mechanisms of drug resistance in osteosarcoma. One of the best studied is p-glycoprotein expression, which can result in resistance to doxorubicin and etoposide, two drugs used in the treatment of osteosarcoma. However measurements of p-glycoprotein expression failed to predict the prognosis in patients with localized osteosarcoma treated with standard multi drug chemotherapy. Studies on methotrexate resistance have been performed and have shown that the intrinsic resistance is due to decreased expression of the reduced folate carrier gene, responsible for transporting methotrexate into the cell; in contrast acquired resistance seems to be due to mutations in target protein of methotrexate, dihydrofolate reductase. Trials based on trimetraxate, which does not require a functional reduced folate carrier gene are based on this observation.

Gene and protein expression

Development of high throughput methodology to examine gene and protein expression allows to look at as many as thousands gene and proteins at the same time. These methods are used to define new subgroups of osteosarcoma that can not be defined clinically or by looking under the microscope. The information gained from gene expression profiling of tumor tissue will be used to improve the diagnostic categorization of tumors, to provide useful prognostic markers for outcome and therapeutic response.

In the largest project of its kind, Swedish scientists are studying normal and cancerous tissues to discover not only the location and abundance of all human proteins. Instead of testing each tissue sample on its own slide, the protein profiling is done using tissue microarrays generated by assembling large number of patient biopsies onto a single glass slide. So far 718 antibodies have been tested in 48 different normal tisssues and 20 different cancer types. The goal is to make a comprehensive protein atlas for every protein, which is expected to be completed in 2015.

However for routine clinical use and to follow the response of osteosarcoma to chemotherapy markers that can be measured in the blood are needed. Recent advances in mass spectrometry instrumentation, protein and peptide separation methods, and informatics tools have fueled the rapid growth of clinical proteomics for the discovery and identification of new biomarkers in the blood to aid in the monitoring of therapy. However, the integration of proteomics approaching clinical research is not trivial and requires close collaborations between analytical chemists, statisticians, and clinical researchers. One of the goals of the clinical research program is to identify these biomarkers using serum samples collected by the Children's Oncology Group. In addition, clinical trials are in progress at Riley Hospital for Children that will allow us to follow these biomarkers through the treatment phase and for several months afterwards.